
==== Front
Biomicrofluidics
Biomicrofluidics
BIOMGB
Biomicrofluidics
1932-1058
AIP Publishing LLC

10.1063/5.0192295
5.0192295
BMF23-RV-00342
Review Articles
Microfluidic paper analytic device (μPAD) technology for food safety applications
https://orcid.org/0000-0002-7389-2674
Soman Soja Saghar 1
https://orcid.org/0009-0002-1094-3857
Samad Shafeek Abdul 1
https://orcid.org/0000-0002-0118-9613
Venugopalan Priyamvada 1
https://orcid.org/0000-0003-1634-437X
Kumawat Nityanand 1a)

https://orcid.org/0000-0002-7470-4225
Kumar Sunil 1,2
1 Division of Engineering, New York University Abu Dhabi, Abu Dhabi, P.O. Box 129188, UAE
2 Department of Mechanical Engineering, New York University, Brooklyn, New York 11201, USA
a) Author to whom correspondence should be addressed: nk67@nyu.edu
02 5 2024
5 2024
02 5 2024
18 3 03150119 12 2023
15 4 2024
© 2024 Author(s).
2024
Author(s)
https://creativecommons.org/licenses/by/4.0/ All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).
Foodborne pathogens, food adulterants, allergens, and toxic chemicals in food can cause major health hazards to humans and animals. Stringent quality control measures at all stages of food processing are required to ensure food safety. There is, therefore, a global need for affordable, reliable, and rapid tests that can be conducted at different process steps and processing sites, spanning the range from the sourcing of food to the end-product acquired by the consumer. Current laboratory-based food quality control tests are well established, but many are not suitable for rapid on-site investigations and are costly. Microfluidic paper analytical devices (μPADs) are a fast-growing field in medical diagnostics that can fill these gaps. In this review, we describe the latest developments in the applications of microfluidic paper analytic device (μPAD) technology in the food safety sector. State-of-the-art μPAD designs and fabrication methods, microfluidic assay principles, and various types of μPAD devices with food-specific applications are discussed. We have identified the prominent research and development trends and future directions for maximizing the value of microfluidic technology in the food sector and have highlighted key areas for improvement. We conclude that the μPAD technology is promising in food safety applications by using novel materials and improved methods to enhance the sensitivity and specificity of the assays, with low cost.

New York University Abu Dhabi 10.13039/100012025 crossmark
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pmcI. INTRODUCTION

The food we consume today is the product of local and global ecosystems where fresh and prepared food items undergo multiple processing, storage, and/or transportation steps. Keeping the food safe from pathogens, allergens, toxic chemicals, and adulterants is essential for maintaining a healthy population, and quality control at each step of food production and storage is critical. Unsafe food is a leading cause of malnutrition, especially in children, elderly, and the sick.1 As per a report by WHO, around 600 × 106 incidents of illnesses and 420 000 deaths are recorded every year due to the consumption of unsafe food.2 In the United States alone, annual loss of USD 17.6 billion is attributed to medical costs, productivity losses, and death due to foodborne illnesses.3 Foodborne pathogens, toxins, allergens, and food contaminants contribute to illnesses, and in severe cases, even death. It is a challenge to detect and analyze these harmful components of food in a rapid, sensitive, and user-friendly way.

Food hygiene and safety from production to consumption can be ensured by management systems such as Hazard Analysis and Critical Control Point (HACCP), an international alliance.4 Here, food safety is addressed through the analysis and control of biological, chemical, and physical hazards from raw material production, procurement, and handling, to manufacturing, distribution, and consumption of the finished product. According to the Food and Agriculture Organization (FAO) of United Nations and the International Commission on Microbiological Specifications for Foods (ICMSF), the principles of HACCP are applicable to all phases of food production, including basic husbandry practices, food preparation and handling, food processing, food service, distribution systems, consumer food handling, and consumption. The most important concept underlying the HACCP system is that of the prevention of food hazards rather than inspection. The control of processes and safety conditions comprises the critical control point (CCP) elements with methodical, flexible, and systematic application of the appropriate science and technology for planning, controlling, and documenting food safety.5 HACCP protocols contain risk analysis, risk assessment, and risk management of agricultural products and food materials.6 In general, the food safety protocols follow the traditional and highly standardized laboratory methods for testing prescribed by FAO, US Food and Drug Administration (FDA), United States Department of Agriculture (USDA), and other national and regional agencies, but they also implement newer detection techniques as approved.7

First conceptualized in 2007, paper-based microfluidics continue to be an expanding research field, providing an innovative method for fluid handling and sample analysis for a variety of applications, including clinical diagnosis and environmental monitoring.8,9 In recent years, researchers have recognized the growing potential of microfluidic paper-based analytical devices (μPADs) and have developed promising devices for food analysis. They found that the μPADs allow easy, rapid, and cost-effective point-of-need screening of food materials. The fundamental working principle of microfluidic paper analytic device (μPAD) technology is to miniaturize the analytical device on a paper substrate and conduct the analysis using microunits of liquid reagents while maintaining the accuracy of detection. In food analysis, the testing reliability of μPADs could reach up to a correlation coefficient of R2 = 0.99.10

μPADs are made up of different types of natural papers and modified papers. Paper is a ubiquitously available cellulose material that helps in economizing the μPAD production. μPADs possess several advantages over traditional microfluidics. They are compatible with commonly used biological and chemical reagents for food analysis. μPADs use capillary and gravitational forces for the absorption and flow of liquid samples without any complex additional active flow control devices, and the white background of paper helps in clear visualization of the results in colorimetric assays. Compared to other analytical device manufacturing, techniques for fabricating paper-based microfluidic devices are relatively simpler, such as plotting with an analog plotter, ink jet etching, plasma treatment, paper cutting, wax printing, ink jet printing, flexography printing, screen printing, 3D printing, and laser treatment.8 Based on the application, the fluid flow in a μPAD is controlled and guided by fabricating microfluidic channels or wells on the paper. The microfluidic channels or wells are made by creating patterns of hydrophilic and hydrophobic contrasted regions on the paper.11 The highly porous nature of paper offers high surface to volume ratio, which allows the absorption of more reagents and samples, aiding in better contact between reagent molecules while mixing. Compared to other food analytical devices and machines, μPADs require less foot space and they are easy to transport with less cold chain maintenance. Moreover, μPADs offer easy waste disposal options as most of them can be disposed off by incineration.

This review focuses on the applications of μPADs in food safety. A brief account on the different μPADs fabrication techniques and designs, along with their advantages and limitations, is presented. Subsequently, the fluid flow control techniques and readout mechanisms, such as colorimetric, electrochemical, fluorescence, chemiluminescence (CL), and electrochemiluminescence, are described. This is depicted in Fig. 1. In the end, we have concluded with the future research areas and opportunities for highlighting the importance of μPAD-based microfluidic technology in the food safety sector.

FIG. 1. μPADs offer easy, affordable, reliable, and quick detection of food hazards such as pathogens, adulterants, allergens, and toxic chemicals. μPADs use methods such as colorimetry, fluorescence, electrochemical reactions, chemiluminescence, spectroscopy, distance measuring, and electrochemiluminescence for the detection and analysis.

II. FOOD SAFETY

Food safety remains one of the most important health concerns globally and is particularly challenging in resource limited areas. Considering the risks associated, timely detection of food hazards plays a critical role in controlling foodborne health threats. Quick, easy-to-use, and economical analytical tools are essential to detect harmful materials in food. The common food hazards adversely affecting the organ systems are foodborne pathogens and microbial toxins, food allergens, antibiotics and hormonal residues, food preservatives and additives, chemical toxicants, pesticides, and herbicides (Fig. 2).

FIG. 2. Foodborne pathogens, microbial toxins, food allergens, antibiotics and hormonal residues, food preservatives and additives, chemical toxicants, pesticides, and herbicides are the common food hazards. They cause acute and chronic organ toxicities and diseases affecting different body systems. Diarrhea, vomiting, and gastrointestinal disturbances are the most common acute symptoms observed in food poisoning, while different types of cancers are the chronic conditions caused by unsafe food. Illustrations created with BioRender.com.

A. Foodborne pathogens

The major pathogens associated with food poisoning are bacteria, viruses, parasites, and fungi. A few representative pathogens causing severe foodborne illnesses are Bacillus cereus, Staphylococcus aureus, Proteus vulgaris, Escherichia coli O157:H7, Coliforms, Campylobacter, Listeria monocytogenes, Salmonella spp., Cronobacter sakazakii, Vibrio spp., Aspergillus spp., Fusarium spp., Penicillium spp., etc.12,13 The traditional culture techniques for the detection and identification of foodborne pathogens require 5–7 days to complete. Molecular detection techniques, such as polymerase chain reaction (PCR) and MALDI-TOF to identify the pathogens, are tedious and require well equipped laboratories and trained personnel. This demands a more effective method for pathogen detection, where μPADs become relevant.

Similar to other testing methods, pathogen detection in μPADs includes sampling, treatment, assay, signal detection, and analysis. Most of the bacterial species produce enzymes that can be detected using a suitable substrate to develop specific color reaction in the paper matrix. Using this principle, the wax printing method was used on filter paper to develop a spot assay for the detection of Escherichia coli O157:H7, Salmonella enterica, and Listeria monocytogenes to analyze food samples.14 Three enzyme–substrate pairs were used for the detection of the above pathogens. Escherichia coli was detected using β-galactosidase with chlorophenol red β-galactopyranoside (CPRG), producing a red-violet color in positive samples.15 Salmonella enterica was detected by the reaction of bacterial esterase with 5-bromo-6-chloro-3-indolyl caprylate (MC) substrate giving a purple color. The enzymatic reaction of phosphatidylinositol-specific phospholipase C from Listeria monocytogenes with 5-bromo-4-chloro-3-indolyl-myo-inositol phosphate (X-InP) developed a blue color.16,17 This detection method reduced the bacterial enrichment time in media before conducting the assay to 12 h or less. The assay could detect as low as 101 colony-forming units/cm2 bacteria in meat samples18 (Fig. 3).

FIG. 3. Detection of three potent foodborne pathogens using a chromogenic assay on μPADs. Bacterial specific enzymes and specific substrates were used for the detection. Cross-reactivity study tested the selectivity of each bacterial enzyme–substrate pair. The color change is specific to the enzyme and the substrate.

Artificially synthesized single-stranded nucleic acid (DNA or RNA) probes are known as Aptasensors. Aptasensors are created using a method called the Systematic Evolution of Ligands by Exponential Enrichment (SELEX). SELEX employs a library of random oligonucleotide sequences and involve multiple rounds of amplification in an exponential way, with the goal of isolating at least one aptamer with high affinity to the target pathogen.19,20 Aptasensors directed to a specific bacteria can interact with the surface of the bacterial cells directly to enable the detection without complex sample preparation procedures. Aptasensors incorporated in μPADs could detect two food poisoning pathogens; Escherichia coli O157:H7 and Salmonella typhimurium simultaneously.21 Polystyrene (PS) microparticles assembled with gold nanoparticles (Au NPs) were used for fabricating μPADs. A salt-based nanoparticle aggregation mechanism could produce stable colorimetric signals after the sample addition and drying. Images were captured using an iPhone camera and the greyscale images were analyzed using a python algorithm. The colorimetric results showed linearity over a wide concentration range of cultures of Escherichia coli O157:H7 and Salmonella typhimurium (102–108 CFU/ml). This technique shows the ability of multiplexing and rapid testing on μPADs while maintaining high assay sensitivity.

A gold-based sensing platform integrated with magnetic nanobead–peptide probes in a paper strip was developed to detect the presence of Staphylococcus aureus in food samples.22 The reaction detects the cleavage orchestrated by the Staphylococcus aureus specific proteases and dissociation of the magnetic nanobeads from the sensor surface. The gold sensing platform allows the detection of bacteria in food products such as ground beef, turkey sausage, lettuce, and milk spiked with pure bacterial broth culture. The test resulted in a limit of detection of 7–100 CFU ml–1 of bacteria. This assay is simple to perform, label-free, economical, quick to carry out within minutes, and suitable for other types of bacteria and stable for 6 months when kept cold.23,24

Food materials contain a mixture of pathogenic and nonpathogenic bacteria. A new version of μPAD-based chromogenic array, named paper chromogenic array (PCA) to spot individual pathogens (Listeria monocytogenes, Salmonella enteritidis, and Escherichia coli O157:H7) or multiple pathogens in the presence of nonpathogenic background microflora on food was developed.25 This array integrates a machine learning approach for pathogen detection based on the emitted volatile organic compounds (VOCs) by individual pathogen.26,27 Grade 1 cellulose chromatography paper PCA was fabricated into which 22 chromogenic dye spots were infused along with standard color dots (Fig. 4). When the array is exposed to volatile organic compounds emitted by the pathogens of interest, the dye spots exhibited remarkable color changes and pattern shifts. The pattern was analyzed digitally and used to construct an advanced deep feedforward neural network. After training, the network demonstrated excellent performance in identifying pathogens with 93% accuracy.

FIG. 4. A μPAD paper chromogenic array (PCA) to spot individual pathogens in the presence of nonpathogenic background microflora in food materials was developed based on the production of volatile organic compounds (VOCs) by individual pathogen. The color pattern shift was analyzed digitally. The pattern was used to construct a deep feedforward neural network, which showed 93% accuracy to detect individual bacteria. Illustrations created with BioRender.com.

Coliforms are notorious pathogens responsible for water contamination. If consumed, they can cause extreme illness to fatality. An open-channel μPAD was created by direct printing of omniphilic channels on an omniphobic, fluorinated paper, capable of lysing and detecting coliforms. The lysing step helps in the disruption of bacterial cell wall to release its content to mix with the detection reagents. They used lysing cell mixture with high and low surface tension liquids. This μPAD device demonstrated the flow and control of both high and low surface tension liquids such as cell lysing agents. The μPAD device could detect Escherichia coli, at a concentration as low as ∼104 CFU ml−1, using the bacteria specific, β-galactosidase enzyme.28

B. Microbial toxins

The presence of microbial toxins in food materials is a serious problem worldwide. Toxins produced by bacteria, fungi, algae, and marine organisms can be extensively present in food materials processed and stored in unsafe conditions. Foodborne toxins include mycotoxins, marine biotoxins, cyanogenic glycosides, and toxins occurring in poisonous mushrooms. Aflatoxins and ochratoxins are the most important mycotoxins that are present in food. Aflatoxins are produced by molds, especially by Aspergillus flavus and Aspergillus parasiticus.29 There are four aflatoxins—B1, B2, G1, and G2—as well as two metabolites of aflatoxins M1 and M2.30 Aflatoxin B1 is the most common and potent of all the aflatoxins and is a common milk contaminant and carcinogen, which can cause liver diseases, mutations, and cancer.31,32 The standard method to identify mycotoxins is by analyzing via labor intense methods like high-performance liquid chromatography (HPLC) and high-performance liquid chromatography-tandem mass spectrometry (HPLC–MS/MS) or through regular ELISA kits.33,34 However, most of these methods could not be used directly for complex food matrixes and demands to develop robust techniques to detect food toxins.29,35

An automated μPAD-based competitive enzyme-linked immunosorbent assay (ELISA) to detect aflatoxin B1, with a detection limit of 60 femtograms or 0.1 ng/ml was developed. The μPAD used a dissolvable sucrose valve for fluid flow control and used only submicroliters of samples (0.6 μl).36 The test achieved high sensitivity and minimized sample volume simultaneously by using a new sample-loading method: directly applying the sample solution at the zones on the device that had been prepared with an antibody-conjugated enzyme before immersion in a running buffer. The method provides high sensitivity, small sample volume, equipment-free measurements, low-cost operation, and user-friendliness. This approach could be adopted to analyze other small-sized toxin molecules in different types of food materials.37

A colorimetric assay on a microfluidic paper device was developed for the rapid detection of aflatoxin B1, using specific aptasensors. Aptameric–gold nanoparticle conjugate was physically adsorbed on a μPAD and sample containing aflatoxin B1 was allowed to flow over the μPAD. The nanoconjugate was characterized using UV-vis spectroscopy, and dynamic light scattering for measuring hydrodynamic diameter and zeta potential. The assay could detect 1 μM to 1 pM of aflatoxin B1 with a limit of detection of 10 nM in standard samples.38

In another study, a paper-based microfluidics chip to measure the mycotoxin, deoxynivalenol was devised using a colorimetric competitive immunoassay, using gold nanoparticles, with a detection range of 0.01–20 ppm.39 A portable paper-based microfluidic aptasensor is established to visually detect mycotoxins, zearalenone, and ochratoxin A, simultaneously. The hydrophobic paper matrix is made using laser printing and heat treatment. In this device, the analytes at the sample zone can migrate into separate detection zones through dual-channels. The recognition of toxins is possible by the specific aptamers that destroy fluorescence resonance energy transfer (FRET) from dual-color upconversion nanoparticles (UCNPs) to Cu-TCPP nanosheets and result in green and blue fluorescence recovery. Zearalenone and ochratoxin A could be measured in the sample by capturing fluorescent images and analyzing the corresponding RGB value via a smartphone, with limits of detection down to 0.44 and 0.098 ng/ml, respectively.40

C. Food allergens

Food allergy is an important health concern affecting upto 10% of the population. Food allergy can cause mild to severe symptoms, and in extreme cases, food allergy can lead to anaphylaxis, which is a life-threatening allergic condition. Currently, there is no cure for food allergy. Management of food allergy includes allergen avoidance, quick detection, and emergency treatment. Rapid and accurate detection of allergens in food is crucial for effective curbing of grave outcomes. The most common food allergens are eggs, milk, peanuts, tree nuts, soy, wheat, crustacean shellfish, and fish. Most of the available allergen tests are time consuming, costly, and available only in specialized facilities. μPADs serve as a promising technology to address these challenges.

A three color multiplex lateral flow immunoassay (xLFIA) to detect common allergenic milk casein, ovalbumin, and hazelnut proteins was devised on μPADs. The antibodies against the allergens were individually adsorbed onto gold and silver nanoparticles to produce specific-colored probes on nitrocellulose membrane (Hi-flow plus 180) strips. The strips were inserted in a LFIA device comprising of three lines, each line representing for one allergen. The xLFIA could identify allergens in commercial biscuits as low as 0.1 mg/l.41 This microfluidic paper-based immunoassay for allergen detection shows good fluid control and have multiplexing capabilities.

A lateral flow immunoassay (LFI) μPAD for quick detection of allergic protein in food samples was fabricated using a modified cellulose material. The device could detect the allergen in 15 min, including the sample preparation time. The sample flow rate was optimized by adjusting the geometrical patterns on μPAD. This μPAD could detect as low as 1 ppm ovalbumin—a major egg allergen, in different food samples.42 A sandwich immunoassay strip test with nitrocellulose membrane was developed to detect Bowman–Birk inhibitor, a type of antinutritional factor present in soybean, causing indigestion and stunted growth in human and animals.43 The test could detect allergen concentrations up to 0.5 μg/ml visually, and 0.23 μg/ml with a TSR3000 Membrane Strip Reader (BioDot, USA).

D. Food additives and preservatives

Food additives and preservatives are mostly chemical substances, widely used for enhancing the color, flavor, texture, and shelf life of food. These chemicals are non-toxic when used in limited quantities, but may lead to toxicity and ill health if consumed in large quantities. In this section, we describe the μPADs developed for testing food additives and preservatives.

Sugary beverages are the single largest source of calories and added sugar in the U.S. diet.44,45 In developing countries, sugary drink consumption is rising exponentially due to widespread urbanization and beverage marketing. Added sugar in foods serves as a sweetener, preservative, texture modifier, fermentation substrate, flavoring and coloring agent, and bulking agent. Close monitoring of the sugar content is required in commercial drinks, as these sugar-loaded beverages can increase the risk of type 2 diabetes, heart disease, and, thus, premature death.46 low-cost, sustainable biosensors using cellulose paper substrate have been developed to measure glucose in commercial beverage samples. The device was made up of hydrophilic cellulose paper disk impregnated with immobilized glucose oxidase enzyme placed on top of a screen-printed carbon electrode. Amperometric biosensing was used for the detection here. An amperometric biosensor functions based on the oxidation and reduction of an electroactive species on a biosensor surface using immobilized enzymes. Amperometry measures the electric current vs time (i–t) when a constant electric potential is maintained. The Glucose biosensor could accurately analyze very low sample volumes (5 μl) of commercial beverages with a limit of detection comparable to high-performance liquid chromatography.47 The reaction chemistry is illustrated in Fig. 5.

FIG. 5. (a) Amperometric biosensing was used for the detection of glucose from commercial beverages. An amperometric biosensor works based on the oxidation and reduction of an electroactive species on a biosensor surface using immobilized enzymes such as glucose oxidase. (b) A paper-based chip was used for visual determination of food additives at different concentrations. Fluorescent carbon dots (CDs), namely, red-CDs, blue-CDs, and yellow-CDs were visualized under UV spectrum to detect nitrite, vitamin C, and sunset yellow in a sample.

Inkjet printing is a simple and commonly used method to fabricate μPADs. Researchers have developed inkjet printed paper sensors to analyze food additives, such as food coloring agents like sunset yellow, food preservatives, e.g., nitrite and vitamins, e.g., vitamin C. The device used fluorescent carbon dots (CDs) for the detection. CDs are nanomaterials, which can emit or quench florescence when binding with specific molecules.48,49 In this method, an optimized ink is prepared using organic solvents (e.g., absolute ethanol), polyethylene glycol to slow the drying, and surfactants like FS3100 and SE-F to control the viscosity. The optimized ink was filled into a clean printer cartridge (Hewlett Packard or HP) connected to a computer, and a pre-designed pattern was printed using an HP Deskjet 2628 printer on the No. 2 medium-speed flow qualitative filter paper. After patterning, the microfluidic chips were dried in the temperature range of 60–100 °C. The method demonstrates the fluorescence-based qualitative detection of food additives at various concentrations using a UV lamp at 365 nm [Fig. 5(b)]. The performance of the inkjet printing is highly reliant on the optimal formulation of the printing inks.

Benzoic acid is a frequently used preservative in pickled food and beverages. A Whatman qualitative filter paper-based microfluidic chip device was fabricated for the detection of benzoic acid in food using the Janovsky reaction theory. Janovsky reaction was performed in the following manner in the reaction zones of the μPAD. The circular reaction zones of the μPADs were implanted with 5 N sodium hydroxide and dried at 30 °C for 20 min. The benzoic acid sample was converted to 3,5-dinitrobenzoic acid using KNO3 and H2SO4 reagents at 40 °C for 40 min and was added on the reaction zones. Then, the μPAD device was transferred to a portable detection system and heated at a temperature of 45 °C for 20 min on a hot plate to carry out the chemical reaction. The resulting color change from light to dark brownish-orange shade in the detection zone is imaged using a Complementary Metal Oxide Semiconductor (CMOS) camera. The color change was analyzed using a RGB analysis software after transferring to a smartphone. The color change was proportional to the benzoic acid concentration in the sample.10,50 Researchers have analyzed 21 different types of commercial food samples, including sauces, processed fruits, and dried and pickled vegetables using this method. The standard deviation of the results was not more than 6.6% compared to the HPLC method and the coefficient of correlation across the tests was equal to R2 = 0.9953.

Nitrite is a meat preservative used to extend the shelf life and to maintain the fresh appearance. High nitrate content in meat and vegetables can cause methemoglobinemia and gastric cancers. The colorimetric determination of nitrite is possible through the Griess reaction carried out on μPADs.51,52 In this reaction, nitrite reacts with sulfanilamide and produces positively charged diazonium salt. This salt couples with N-α-naphthyl-ethylenediamine to produce a magenta azo dye compound. The formation of azo compound is directly proportional to the nitrite concentration in the sample. The image of the color developed was captured using a phone camera and analyzed using ImageJ. The test can be carried out quickly within 15 min, with a sensitivity of 1.1 mg kg−1 (Fig. 6). The microfluidic reaction on μPADs could detect nitrite in meat samples including processed meat, pork, ham, sausages. and drinking water.53,54

FIG. 6. Nitrite is a food preservative. A schematic of colorimetric determination of nitrite. The detection was performed through Griess reaction carried out on μPADs. The intensity of color developed was scanned and analyzed using a mobile device.

E. Chemical toxicants and adulterants

The most common method of food adulteration in solid food is the addition of substances, such as sand, ground stones, and pebbles to food grains and used tea leaves addition to tea. Metallic compounds, such as mercury, lead, arsenic, cobalt, and cadmium, are also found as common food adulterants. Moreover, deliberate contamination of pure forms of food products with low quality foreign substances, such as rancid oils, and altered species of meat are emerging concerns. In the last decade, numerous μPADs have been developed to detect food adulterants such as chemical toxicants in food safety sector. We will discuss some of them in this section.

Melamine is a nitrogen containing substance illegally added to milk, infant formula, and pet food to artificially increase the protein value of the food.55–57 The addition of 1% melamine in food causes an artificial elevation of food protein content to 4%.58 Melamine in excess amount can form insoluble melamine-cyanurate crystals in the kidney and cause renal damage.59–61 Colorimetric detection of melamine adulteration in milk was detected on a Whatman filter paper-based visual sensor using Triton X-100 modified, stabilized gold nanoparticles (AuNP). The chemical reaction happens between the Melamine and the AuNPs through the ligand exchange with citrate ions on the surface of AuNPs, leading to the Triton X-100 being removed. The AuNPs aggregate and produce a color change from a wine red to blue depends upon the concentration of the Melamine. This color change causes a shift in absorption peak while measuring using UV-vis absorption spectroscopy. The method could accurately detect melamine as low as 5.1 nM in milk samples, which is a lower value of Melamine concentration as per food safety regulations.62 The naked eye could detect 1.0 μM Melamine in milk samples. The same technique was modified to suit the rapid testing in field conditions using smartphone-based detection of color change.

Researchers developed μPAD devices to detect Mercury poisoning in salmon fish. For the detection, the μPADs were coated with modified gold nanoparticles (AuNPs). The AuNPs have been modified with N,N′-bis (2-dihydroxyethyl) dithiooxamide (HEDTO) and coated on a hydrophobic-hydrophilic barrier created by Triethoxymethylsilane. Upon adding mercury containing samples, HEDTO-AuNPs formed aggregates and showed a color change from red to blue on μPADs. The detection was facilitated by capturing the images of color change using a smartphone camera. The images were analyzed using Adobe Photoshop CS6 image processing software. The technique could offer a limit of detection of 15 nM of mercury from food samples.63

μPADs were fabricated and tested for 10%–50% (v/v) palm oil adulteration in sunflower oil. The devices were prepared in the form of circular discs and rectangular channel strips using simple cutting and crafting technique for visible colorimetric detection. The images were captured using UV-vis spectrophotometry and the average grayscale intensity data were analyzed. The coefficient of determination of the tests was 0.9464, indicating a good empirical fit and suitability of the method for quantitative detection. The assay device was evaluated for stability of signal over a time span of six days after the test. The images were captured every day and analyzed for grayscale intensity values using ImageJ software. The signal remained fairly constant over the test duration, allowing the time flexibility of data analysis after the test.64

One modification in the μPADs technology is the development of electrochemical microfluidic paper-based analytical devices integrated with nanotechnology, called EμPADs. EμPADs report the detection and quantification of anesthetic drugs, such as ketamine in alcoholic and non-alcoholic drinks, using electrochemical sensing. Ketamine is a criminally abused drug in party beverages, and in incidences of robbery to sedate the victims. In this method, the EμPAD surface was coated with zeolite-nanoflakes and graphene oxide nanocrystals (Zeo-GO). When the test beverage (alcoholic drink or fruit juice) was applied to the circular working zone of the EμPADs, the nanocrystals of ZeO-GO EμPAD showed electro-oxidation of ketamine presented in the beverages, thus causing a change in the signal intensity while sensing. The change in the intensity was correlated to the drug concentration in the beverages. This method showed a swift response time of 2 s, with a limit of detection of 0.001 nM/ml and a wide range of detection from 0.001 to 5 nM/ml.65 Similar types of μPADs were devised for the detection of misused drugs such as Estazolam and Clenbuterol.66,67

World Health Organization (WHO) classifies lead (Pb) as a toxic metal of greatest public health concern.68 Pb exposure accounts for more than 1 × 106 deaths each year and causes the loss of 24.4 × 106 disability-adjusted-life-years. Pb causes irreversible acute and chronic neurotoxicity and immune dysregulations.69,70 Pb toxicity occurs through food crops, preserved eggs, and drinking water. A wax printed distance measuring paper-based analytical device (dPAD) has been developed for the detection of Pb in preserved century eggs. The assay uses the competitive binding chemistry between Carminic Acid (CA) and polyethyleneimine (PEI) to detect Pb in food samples. The assay principle is based on the binding property of CA to Pb under appropriate pH conditions. The CA–Pb complex would be trapped in the sample-loading area, allowing only free CA to wick through the hydrophilic detection channel toward the absorption area (Fig. 7). The device was patterned with a sample reservoir and a hydrophilic channel containing a colorimetric indicator reactive to the sample. When the sample is added and flows through the channel, the indicator generates a colored band, whose length can be measured by keeping a ruler along the length of the channel. The length of the colored bands was inversely proportional to the concentration of the analyte added. The method allows us to read and interpret the results without using any sophisticated readout system. dPAD showed good linear correlation of R2 value 0.974 for measuring Pb, within the ranges of 5–100 μg ml−1. The results from the dPAD were comparable to the measurement obtained by atomic absorption spectroscopy.71 dPAD has the potential to be applied in food processing in developing countries, and it does not require laborious instruments and procedures.

FIG. 7. Here, it shows a prototype of distance paper-based analytical device (dPAD) for analyzing lead (Pb) in food samples. The decrease in the color distance on the immobilized dPAD observed when introducing Pb at concentrations ranging from 0 to 100 μg ml−1 with 0.8 mmol l−1 carminic acid in 0.1 M hydroxyethyl piperazine ethane sulfonic acid (HEPES) buffer. The reactions were incubated at ambient temperature for 5 min before analysis.

F. Pesticides and herbicides

The use of organophosphorus (OP) pesticides, such as carbamates, is common in agriculture and forms a major chemical hazard in food. Carbamate is listed as an endocrine disruptor compound, capable of causing hormonal abnormalities if consumed.72 Analyzing these compounds in food materials is important in the food processing step. The use of μPADs technology has been developed for the colorimetric determination of carbamate pesticides based on the inhibition of acetylcholinesterase (AChE) enzyme by the pesticide while reacting with a specific chemical substrate.73–75 The reaction chemistry is as follows: acetylthiocholine iodide substrate is hydrolyzed into thiocholine and acetic acid by AChE. The thiocholine base reacts with 5,5′-dithiobis-(2-nitrobenzoic acid) (DTNB) to generate a yellow color detectable at 405 nm. The intensity of the yellow color developed is inversely proportional to the pesticide concentration. The images were captured using a desktop scanner and analyzed for color intensity using ImageJ software. The device was designed as a spot-array assay with a well diameter of 10 mm detection zone on the patterned paper. The device was distinguished with color codes on hydrophobic and hydrophilic zones, blue color was identified as the hydrophobic zone, and yellow for the reaction developing zone. The design was printed on the sheet of Whatman chromatography Paper using a wax printer. The hydrophobic barrier was created on the paper by heating the paper at 150 °C for 2 min in an oven to melt the wax to impregnate the wax throughout the thickness of the paper in the defined design. The AChE inhibition was determined by plotting a calibration curve of the pesticide concentration against percentage inhibition.76,77 The results were compared with no pesticide controls and different concentrations of pesticides. Another important point to be noted in assay using enzyme kinetics is that high concentrations of pesticides may inhibit the complete activity of AChE, shown as the absence of yellow color. Therefore, a standard detection range is to be followed while conducting these assays.

A sensitive surface-enhanced Raman scattering (SERS) method using gold nanoparticles (Au NPs) is developed to detect methyl parathion on fruit surface. The filter paper substrate was immersed in the prepared Au NP solution. The spectroscopic probe molecule 4-mercaptobenzoic acid (4-MBA) was used to evaluate the performance of the paper substrate for an optimized signal using a portable Raman spectrometer coupled with 785 nm laser. Then, the substrate was applied to detect methyl parathion standard solutions with a linear range between 0.018 and 0.354 μg/cm2 with a limit of detection of 0.011 μg/cm2. After standardization, the actual fruit peel sample spiked with methyl parathion was used to verify the test. The test recovery rate was 94.09%–98.72%, indicating high reliability in testing fruit samples without rigorous pretreatment. The method showed excellent reproducibility and stability.73

A colorimetric determination method for analyzing the widely used herbicide, glyphosate in food grains was developed on μPADs. Glyphosate is linked to disease conditions such as organ toxicity and cancer.78 The glyphosate detecting system showed improved selectivity and sensitivity by using Mn–ZnS quantum dot (QD) embedded molecularly imprinted polymers (MIPs) on μPADs. The detection of glyphosate is based on the oxidation of 2,2′-azino-bis(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) by H2O2 in the presence of Mn–ZnS QD-MIP. Glyphosate non-binding-Mn–ZnS QD-MIP generates •OH from H2O2, producing a dark green color of the test zone. The binding of glyphosate to the Mn–ZnS QD-MIP turns off the generation of •OH, resulting in light green color.79

G. Hormonal and antibiotic residues in food

17β-estradiol is a potent anabolic steroid hormone derivative, illegally used to promote the growth of animals. The residues of 17β-estradiol in foods and milk can cause endocrine disruption through the food chain accumulation.80,81 Molecularly imprinted polymer (MIP) grafted paper-based devices were fabricated for the detection of 17β-estradiol in milk, with high reliability.82 The MIP's optimum synthetic conditions were optimized with the following reagent mixture: acetonitrile as the solvent, 17β-estradiol as the template molecule, (3-aminopropyl) triethoxysilane (APTES) as the functional monomer and tetraethyl orthosilicat (TEOS as the cross linker in the ratio of 1:12:12. This assay showed a limit of detection of 0.25 μg l−1 for spiked milk samples. These paper-based devices and methods can develop into a reliable new platform for high-throughput, sensitive, specific, and multiplex assay in food monitoring.

Antimicrobial resistance is a global threat due to the nonjudicious use of antibiotics to treat bacterial infections in food producing animals. Predictive statistical models estimated 4.95 × 106 deaths associated with bacterial antimicrobial resistance in 2019.24 A number of cost-effective μPAD-based diagnostic devices have been developed to detect antibiotics residues in milk, egg, and meat.67,83,84 The presence of oxytetracycline and norfloxacin residues in pork was determined using metal complexation on microfluidic paper-based analytical devices (μPADs).84

The μPAD technology showed breakthrough platforms to revolutionize food quality analysis with the advantage of equipment-free detection and analysis, at low cost. In Secs. III–V, we will look into the μPAD fabrication techniques, flow control methods, and detection techniques.

III. FABRICATION OF μPADs

The process of fabrication of paper-based microanalytical devices includes the selection and preparation of hydrophilic porous medium and the formation of isolated hydrophilic micro-zones. This could be achieved either by patterning hydrophobic boundaries to enclose hydrophilic zones or by cutting and physically separating the required zones from the rest of the porous medium. The different fabrication techniques can be broadly classified into indirect or direct patterning of hydrophobic barriers, or physical isolation of hydrophilic zones.

A. Indirect patterning methods

In this type of patterning, the designs are first made on an intermediate masking substrate and are subsequently transferred to the porous paper. Fabrication techniques where a pre-built mask is required to selectively shadow the paper substrate are photolithography, screen printing, chemical vapor deposition (CVD), plasma treatment, wax coating, and spraying.

1. Lithography

In Whiteside's revolutionary innovation of modern paper-based analytical devices for micro-fluidic applications, the devices were fabricated using conventional photolithography process.85 Chromatographic paper coated with SU-8 2010 grade photoresist was exposed to UV light. A quartz photomask with the patterns was placed over the paper for the lithography step. Barriers and channels with sub-micrometer resolution were reported using this method. The photolithography procedure involves multiple process steps, use of organic solvents, and needs expensive, pre-prepared masks, and cleanroom environment. Further, the wet chemical treatment and multiple baking steps result in corrosion of the porous medium and compromise its natural wettability.86 To relax the sophistications associated with conventional photolithography process, a Fast Laser Activation of Sheets (FLASH) method, employing custom-formulated inexpensive photoresist, instantaneously printed transparent sheet masks, and sustainable low-cost UV sources was developed.87 Further, several lithography-assisted, hybrid techniques have been invented to avoid the wet chemical processing of paper sheets. Yu and Shi developed μPADs by embossing a lithographically created parafilm master on paper sheets.86 In another approach, He et al. used OTS-coupled silanized paper and exposed it to DUV (254 nm) lithography in an O3 ambience. The silane bonds in the unmasked regions were, thus, selectively decomposed by the UV radiation.88 The instrumentation required for the process makes the lithography-based approaches not feasible for the rapid and mass production of μPADs within a budget and resource limited setup.

2. Screen printing

Screen printing is a simple, rapid, and inexpensive method used for the fabrication of μPADs, first introduced by Dungchai et al., as wax screen printing.89 Figure 8(a) shows the steps involved in the process of screen printing of μPADs. In this method, first, a stencil screen with the desired patterns is prepared.90 Holding the stencil tightly over the paper substrate, paraffine wax is rubbed over the screen, to transfer patterns to the paper. The excess wax is then squeegeed off. In a later heating step, the wax is melted and impregnated into the paper matrix to create hydrophobic barriers. Typically, a mesh of nylon on an aluminum or wood frame is used as the screen. Apart from wax, a variety of hydrophobic materials such as polystyrene, poly(methyl methacrylate) (PMMA), polydimethylsiloxane (PDMS), polycaprolactone, polylactic acid, varnish, and rubber-latex derivatives have been experimented as the ink for screen printing.93–97 Although screen printing is a simple, effortless, and universal method, there are certain downsides, such as poor resolution of the printed patterns, lack of automation, and requirement of multiple screens.98

FIG. 8. Schematic of different techniques used in fabrication of μPADs. (a) Screen printing using a custom-built stencil as a mask. Reproduced with permission from Wang et al., Biosens. Bioelectron. 31(1), 212–218 (2012).90 Copyright 2012 Elsevier. (b) Wax printing process for direct patterning of hydrophobic barriers. Reproduced with permission from Yao et al., Anal. Chem. 82(1), 329–335 (2010).91 Copyright 2010 American Chemical Society. (c) Laser cutting, as a direct patterning method for barrier creation. Reproduced with permission from Nie et al., Analyst 138(2), 671–676 (2013).92 Copyright 2013 Royal Society of Chemistry. (d) Cut and heat plastic lamination process, as a hybrid method for the hydrophobic barrier creation. Reproduced with permission from Kumawat et al., Lab Chip 22(18), 3377–3389 (2022).11 Copyright 2022 Author(s), licensed under a Creative Commons Attribution (CC BY) license.

3. Etching/deposition/plasma treatment

Selective wet etching of silanized paper is a technique that involves low-sophistication, being free of expensive equipment and reagents. To achieve the chemical etching, Cai et al. used a tetramethyl orthosilicate (TMOS)-treated paper and aligned a cheap paper mask penetrated with NaOH solution (30% glycerol). The regions of the hydrophobic silanized paper, in contact with the masked regions with etching reagents, turned hydrophilic.99,100 In contrast, dry patterning techniques involve the surface modification of paper substrate by vapor phase deposition of hydrophobic materials or plasma assisted treatment. Chemical vapor deposition (CVD) is an additive process, which is useful for the solvent-free deposition of functional polymers, pure polymers, and inorganic compounds on paper substrate.101–104 The chemical precursors in gaseous form react at the surface of the paper and form a solid thin hydrophobic film. The masking for the deposition is attained either via patterns created through lithography or via physical blocking with metal or vinyl sheet. Selective plasma treatment is a dry, chemical-free method to alter the hydrophilic nature of the paper structure. μPADs are fabricated by selectively exposing the paper to a plasma discharge through metal masks.105,106 The plasma generation is accomplished by vacuum plasma reactors or portable corona generators. With the combination of plasma assisted fluorocarbon deposition and O2 plasma assisted etching, fully enclosed μPADs have been achieved.107 Both CVD and plasma-based methods allow solventless, substrate-independent way of the creation of hydrophobic barriers. However, the fabrication process involved in these techniques is relatively complex and requires costly reagents, equipment, and multiple masks.108 Other indirect patterning methods include dipping, lacquer spraying, embossing, or stamping of hydrophobic materials.86,109–115

B. Direct patterning methods

In this type of hydrophobic barrier creation, designs defined using CAD software are directly printed on the paper surface using commercially available printers. The requirement for multiple masks, expensive reagents, and sophisticated equipment are thereby generally excluded.

1. Wax printing

Wax printing is arguably the most extensively used technique by researchers for the fabrication of μPADs for food safety applications, including the detection of pathogens, neurotoxic residues, biotoxins, additives, and adulterants in food. In wax printing, the designs are directly printed on the paper surface using a wax printer, as shown in Fig. 8(b). With a subsequent heating step, the wax is melted and impregnated into the porous medium to form hydrophobic barriers.91,116,117 The overall process is rapid, minimal effort, and solvent-free. The limited resolution, the requirement of expensive wax, and the extra heating step are a few limitations of wax printing.118 Apart from these, the discontinuation of commercial wax printers from the market is the main challenge for the continued device fabrication using the technique.

2. Inkjet printing

In the inkjet patterning method, the commercial inkjet printer is filled with a solvent ink, and the inkjet is used to define the boundaries enclosing hydrophilic zones.119,120 Alkyl ketene dimer (ADK), UV-curable ink, and hydrophobic solgel are some of the commonly used inks.121 The inkjet printing can be accomplished in two modes: (i) to impregnate hydrophobic ink barriers in the hydrophilic paper or (ii) to selectively etch and induce hydrophilic zones in a hydrophobized paper. Apart from the barrier creation, the inkjet method can be used to simultaneously print biomolecules, biomarkers, and immunoassay reagents on the paper microzones, thereby creating complete biosensing platforms.122 Hossain et al. first demonstrated the development of bioinks and bioactive papers for rapid detection of neurotoxins and pesticides in food.123,124 Later, omniphobic flouroalkylated (RF) papers printed with high resolution conductive patterns were developed by Lessing et al.125 Inkjet printing has become a powerful technique for defining immuno-chemical and conductive circuits directly on paper. The scalable, foldable, low-cost, and disposable printed circuits fabricated using inkjet printing extend their applications from healthcare to diagnostics and to high-performance electronics. One of the major drawbacks of this method is the slow process due to its inherent dot-by-dot printing approach, which is not suitable for rapid, mass production. Further, the involvement of hazardous solvents, the difficulty in instrumentation due to limited mechanical properties of the bio-inks, and multiple printing steps are some of the factors that need improvement.121

3. Flexographic printing

Flexography is a direct roll-to-roll printing technique used for μPAD fabrication. An anilox roller is used to transfer a thin, uniform layer of hydrophobic ink to a flexible printing plate roller, which has the required designs in the form of relief patterns. Then, the ink from the raised portions of the printing plate is transferred to the paper substrate, fastened to an impression roller. Polystyrene or PDMS, dissolved in organic solvents, is used as hydrophobic ink for flexographic or roll-to-roll printing of μPADs.126–128 The hydrophobic property is decided by the number of layers printed and the penetration depth of ink is limited by the viscosity of solvent, vapor pressure, and solute content. The requirement of complex reagents and multiple roller plates are the drawbacks of this method.

4. Laser printing

In the laser printing method, solid toner ink from a laser printer is used to create hydrophobic patterns on paper. The two-step fabrication process involves the automated printing of the design on paper and a post-baking step.129,130 The high temperature heating step and longer baking time required for the melting and impregnation of the toner ink is a drawback associated with this method.

5. Laser direct-writing

Laser-based direct patterning is another fabrication technique, which employs photopolymerization of hydrophobized paper. Here, the paper is pre-treated with a photopolymer and then the laser beam is used to direct-write patterns on the paper, without any mask. The radiation-exposed regions undergo hardening by photopolymerization, thus forming the hydrophobic barrier, and the unexposed polymers are washed away using a solvent.131

C. Physical isolation methods

Rather than the usage of hydrophobic materials, this approach relies on the physical isolation of the required hydrophilic zone from the rest of the porous medium by cutting boundaries using sharp tools or laser cutter. Paper sheets cut using a knife or scissors is the simplest way of creating a dipstick or chromatographic device. Laser-based cutting of paper sheets offers the speed, precision, scalability, and automation required for mass production capabilities [Fig. 8(c)]. CO2 laser is often employed for the controlled ablation of the cellulose structure and cut through the paper matrix.92,132,133 The physical isolation techniques create stand-alone hydrophilic patterns, which often require a supporting platform to provide mechanical strength. Kumawat et al. developed a method in which CO2 laser cutter is used for cutting features in plastic lamination sheets.11 A filter paper sheet is then placed between the lamination sheets and heated on the hot plate. The melting of the EVA from lamination sheets and its impregnation to the porous medium results in the barrier formation as outlined in Fig. 8(d). The fabricated devices have high mechanical strength against bending, folding, and tearing and are highly robust against strong acids, bases, and solvents. The technique presents a simple and low-cost fabrication process using widely available consumables and commercial tools for automated mass fabrication of μPADs. Table I shows the summary of some of the recent research based on the applications of μPADs for food safety.

TABLE I. Table showing the different fabrication techniques used for developing μPADs for food safety applications.

Year	Fabrication method	Detection scheme	Application in food safety	
2009	Inkjet printing	Colorimetric assays	Neurotoxic pesticides—bendiocarb, carbaryl, malathion 124	
Inkjet printing	Colorimetric assays	Neurotoxic pesticides—paraoxon, aflatoxin B1 123	
2011	Inkjet printing	Colorimetric assays	Heavy metal ions—mercury, silver, copper, cadmium, lead, chromium, and nickel 134	
2012	Wax printing	Colorimetric assays	Foodborne pathogens—Escherichia coli, Salmonella spp., and Listeria monocytogenes14	
Wax printing	Electrochemical	Quality control—food additives—ascorbic acid and sunset yellow in beverages 135	
2013	DUV lithography	Colorimetric assays	Quality control—food additives—nitrite in food 136	
2014	Stamping	Colorimetric assays	Quality control—food additives—nitrite 137	
Inkjet printing	Colorimetric assays	Quality control—food additives—nitrite and nitrate in water 54	
Screen printing	Colorimetric assays	Foodborne pathogens—Escherichia coli 94	
2015	Hybrid—wax/inkjet printing	Colorimetric assays	Neurotoxic pesticides—paraoxon, malathion 138	
Wax printing	Colorimetric assays	Quality control—iodate in salt 139	
Plasma treatment	Colorimetric assays	Quality control—amylose in rice 140	
Wax printing	Colorimetric assays	Foodborne pathogens—Salmonella 141	
Cutting/masking with adhesive tapes	Chemiluminescence	Pesticide residue—DDV in vegetables 142	
Stamping	Colorimetric assays	Quality control—preservative additives—nitrite in ham, sausage, water 51	
Wax printing	Fluorescence	Heavy metal ions/antibiotic—silver/mercury, neomycin in water 143	
Wax dipping	Colorimetric assays	Heavy metals—copper, nickel, chromium in water 144	
Screen printing	Colorimetric assays	Heavy metal ions—copper in water, food, and blood 145	
2016	Screen printing	Colorimetric assays	Neurotoxic pesticides—MPO and CPO in cabbage and green mussel 146	
Wax printing	Chemiluminescence	Heavy metal ions—chromium in natural water samples 147	
2017	Wax printing	Fluorescence	Foodborne pathogens—E. coli 148	
Wax printing	Colorimetric assays	Adulteration—caramel in whiskey 149	
Screen printing	Colorimetric assays	Chemical residue—bisphenol A in food packets 150	
Wax printing	Fluorescence	Biotoxin—phycocyanin in water 151	
Inkjet printing	Colorimetric assays	Neurotoxic pesticides—paraoxon, trichlorfon 152	
Wax printing	Electrochemical	Adulteration—ketamine in beverages 153	
2018	Wax printing	Colorimetric assays	Quality control—benzoic acid in food 10	
Wax printing	Angle based readout and colorimetry	Neurotoxic pesticides—DMMP 154	
Screen printing	SERS	Pesticide residue—thiram, thiabendazole, methyl parathion in fruits and vegetables 155	
Hybrid—cutting /NP aggregation	Colorimetric assays	Adulteration—melamine in milk 156	
Photolithography	Chemiluminescence	Mycotoxin—DON, ZEN, T-2, and HT-2 in cereals 157	
Wax printing	Colorimetric assays	Drug residue—clenbuterol in milk 158	
Hybrid—wax/inkjet printing	Colorimetric assays	Quality control/adulteration—protein, urea, and nitrite in milk 159	
Photolithography	Fluorescence	Food allergens and toxins—egg white lysozyme, ß-conglutin lupin, okadaic acid, brevetoxin 160	
Hybrid—cutting/lamination	Colorimetric assays	Adulteration—metamizole, paracetamol and midazolam maleate in whiskey 161	
2019	Wax printing	Chemiluminescence	Quality control—antioxidants in food—gallic acid, quercetin, catechin, kaempferol, caffeic acid 162	
Inkjet printing	Colorimetric assays	Heavy metals—calcium in water 163	
Wax printing	Hybrid/colorimetric assays	Mycotoxin—DON in food/feed 39	
Wax printing	Colorimetric assays	Adulteration—neutralizers, urea, and detergents in milk 164	
Wax printing	Distance based	Adulteration—potassium iodate in salt/Milk 165	
Wax rubbing	Colorimetric assays	Adulteration—starch in milk 166	
Screen printing	Colorimetric assays	Quality control—food additives—nitrite and nitrate 167	
Inkjet printing	SERS	Quality control—food additives—colorants from the skin of dals and vegetables 168	
2020	Hybrid—cutting/silanization	Colorimetric assays	Heavy metals—mercury in fish/water 169	
Screen printing	Colorimetric assays	Quality control—food additives—nitrite and nitrate 170	
Wax printing	Colorimetric assays	Quality control—food additives—borax, salicylic acid, nitrite and nitrate 171	
Wax printing	Hybrid/colorimetric assays	Adulteration—ketamine in beverages 172	
Laser printing	Colorimetric assays	Quality control—food additives—tartrazine and indigo carmine 173	
Wax printing	Colorimetric assays	Neurotoxic pesticides—phoxim, carbaryl, carbofuran, methamidophos, chlorpyrifos, triazophos 174	
Wax printing	Colorimetric assays	Antibiotic residue—Norfloxacin in meat 175	
Wax printing	Colorimetric assays	Quality control—food additives—nitrite in pork 176	
Photolithography	Colorimetric assays	Mycotoxin—aflatoxin B1 in milk 38	
Wax printing	Chemiluminescence	Drug Residue—beta-agonists in meat 177	
2021	Wax printing	Distance based	Chemical residue—bromide and bromate in water 178	
Screen printing	Colorimetric assays	Heavy metals—Cr3+ water 179	
2022	Wax printing	Colorimetric assays	Biotoxin—microcystin in water 180	
2023	Hybrid—cutting /lamination	Hybrid/colorimetric assays	Antioxidant residue—gallic acid or oenotannin in fruits 181	
2024	Laser printing	Colorimetric assays	Adulteration—syrups in natural honey 182	
Wax printing	Colorimetric assays	Pesticide residue—butachlor in mung beans 183	

IV. μPAD FLUID FLOW CONTROL TECHNIQUES

In this section, our review is focused on the progress that has been made in the area of fluid flow control methods in μPADs and the different tools that are developed to integrate the methods into paper devices.

A. Theory

For the development of accurate and predictable μPADs, understanding the flow behavior is important. Several studies have been reported incorporating the flow control functionality in μPADs. Fluid flow in a porous medium depends on a number of parameters including the pore size, the liquid-air surface tension, and the liquid–solid contact angle. The capillary action in paper-based devices can be formulated by Lucas–Washburn's equation,184,185 L=γDtcosθ4μ,(1)

where L is the distance traveled by the fluid under capillary force in paper channel, t is the time, D is the average pore radius of the paper matrix, μ is the viscosity of the fluid, γ is the surface tension, and θ is the contact angle. This equation assumes one-dimensional flow in a homogenous porous membrane and cannot be used to accurately predict the flow rate in real devices.

Darcy's law can be used to describe the fluid flow in a paper fluidic network containing multiple channels and multiple porous substrates,186 Q=kAμLΔP,(2)

where Q is the volumetric flow rate, k is the material permeability, A is the cross-sectional area of the paper substrate, and P is the pressure difference over the medium length.

Effects of evaporation play an important role in liquid flow in paper substrates and, thus, leading to a new expression for flow length as developed by Liu et al.,187 hev=meρεwδ,(3)

where me is the predicted wicking liquid mass with evaporation, ρ is the density of the fluid, ε is the substrate's effective porosity, w is the channel width, and δ is the thickness of the substrate. Enclosed paper channel devices have a faster flow rate compared to open devices, due to the effects of evaporation.188 Depending on the applications, a flow rate control in paper devices is essential for many applications and several methods were developed that permitted a controlled flow. These methods can be categorized into three major ones, according to the operation modes, as explained below.

B. Mechanical-based methods

Physical motion of components is used to achieve connection or disconnection to the channel surface thereby controlling the flow of the fluid. Research has been carried out on temperature-controlled valve system, in which the fluid flow is guided by melting wax in the patterned channel by heating.189 Kong et al.190 developed a reconfigurable actuator device made out of a folded chromatography paper, which was activated by fluid addition either at the crest or trough, engaging or breaking the fluidic contact between channels.

Rotational valves on paper-based analytical devices have been implemented using hollow rivets,191 plastic comb binding spines,192 and rotational paper-based microfluidic chips193 as ways to control the connection or disconnection between the detecting zones and fluid channel, as shown in Fig. 9(a). Expandable material (sponge) has been demonstrated to control the fluid low as demonstrated by Toley et al.195 When the sponge gets wet with the fluid flow, it expands resulting in either stopping the flow or transferring the flow to another channel, in turn acting as a switch. An electromagnetic valve on μPADs, made by applying ferromagnetic nanoparticles, has been studied. Here, electromagnets are used to operate the connection and disconnection of the valves.196 Kong et al.190 proposed an actuator device made out of a folded chromatography paper, which was actuated by fluid addition either at the crest or trough, engaging or breaking the fluidic contact between the channels.

FIG. 9. (a) Schematics of the hollow-rivet-assisted movable valve paper device, reprinted from Ref. 191. (b) Preparation steps of delayed strip using sugar solution—experimental images of the flow test with varying concentrations of sugar solution, the dashed line, and strip “B” indicate the finish line and an untreated strip, respectively, reprinted from Ref. 36. (c) Sequential delivery of three colored fluids using cellulose shunts, reprinted from Ref. 194.

C. Chemical-based methods

The wicking properties of the paper can be varied by simply embedding various chemicals in the paper channel. A fluid delay effect is generated by injecting dissoluble solutions like sucrose36 in the paper strip and letting it dry, resulting in a viscosity change [see Fig. 9(b)]. Programmable flow delays can also be generated by dissolvable bridges, making use of the dissolving property of pullulan films.197 An increased flow rate is demonstrated by printing toner with hydrophobic properties on the top and bottom of the paper device, thereby preventing the fluid evaporation.198

Coating hydrophobic paraffin wax on fluid channels can control fluid flow by stacking each micropatterned layer and interposing paraffin wax-patterned layers between them.199 Studies on fluid manipulation technology using surfactants have also been developed. Chen et al.200 developed a fluid diode technology in which dried surfactants have been used to bridge a hydrophobic gap in the flow path. However, using dissolving materials in paper channels may affect samples or downstream reactions. Strong et al.201 attempted to overcome this limitation by introducing wax printed fluid time delays on the top and bottom of pre-fabricated μPAD channels.

D. Geometry-based methods

In this method, flow rate control is achieved by changing the channel length, width, or flow path. A lot of research has been done on controlling the flow rate by changing the width and length of the paper channel. Fu et al.202 showed that the flow rate decreases as the width of the paper becomes wider. This is because the longer the width and length of the paper, the greater the resistance and the slower the fluid velocity. The baffle design by Apilux et al.203 increased the flow length thereby creating a time delay resulting in the sequential reagent flow to the detection region. Fu et al.202 placed different reagents at different distances from the detection zone, thereby generating sequential multiple flows to the detection zone.

Applying pressure on specific regions of the paper is another reported way of controlling the fluid flow. The pressure adjusts the flow rate by reducing the pore size and cross-sectional area on the paper, thereby increasing the fluid resistance creating a fluid delay.204 In another method, the use of an absorption pad generates the delay in the fluid flow by placing it in positions where the fluid is to be delayed. It has been shown the delay time can be varied by controlling the dimensions of the pad,194 as shown in Fig. 9(c). The sandwiching paper channel between two flexible films prevents sample evaporation thereby accelerating the flow rate.188 The advantage of this method is that the increased velocity of the fluid can reduce the diagnostic time.

V. DETECTION METHODS USING μPADS

Food safety analysis is one of the major applications of μPADs that requires the development of low cost, rapid, and portable detection methods for rapid monitoring incessant. This section reviews the recently reported μPADS that employed different detection methods, such as colorimetric, electrochemical, chemiluminescence, fluorescence, nanoparticle, and SERS (surface-enhanced Raman scattering) based detection techniques for food and water analysis (Fig. 10).

FIG. 10. Different detection methods using paper-based microfluidic devices for food safety analysis.

A. Colorimetric detection

Colorimetric detection is the most widely used instrument-free detection method for μPADs.205 It relies on the observation of color change during a chemical reaction between the desired analyte and an indicator agent, where the results can be qualitatively analyzed by visual monitoring or quantitatively by using tools like handheld scanners, cell phone,206 or digital cameras207 to measure the intensity of the color produced. Morbioli et al.208 addressed the different methods to improve the color generation using nanoparticles, dyes, enzymes, etc.

A proof of concept study for a simple paper-based test was demonstrated for the specific detection of Escherichia coli (E. coli) bacteria in environmental samples using colorimetric detection.209 A patterned paper chip was used for the realization of a visible immune assay by immobilizing the antibodies for capturing E. coli in the detection zones of the paper chip.94,138,210 Numerous studies have reported the use of μPADs associated with colorimetric detection for the determination of food additives like nitrite and nitrate in food samples.54,137,211 Cardoso et al.51 performed the colorimetric detection of nitrite through the modified Griess reaction.212 Here, the μPAD devices were fabricated by stamping process in a geometry containing eight circular detection zones with immobilized reagents to produce selective color changes when the sample is placed and one central zone as sample inlet. The presence of nitrite in meat samples was detected using μPADs imprinted with wax material, reaction between Griess reagent in the test zone and nitrite in meat samples produced colored complexes.53

μPADs based on colorimetric detection have been demonstrated for the detection of the presence of pesticide residues in food products.213 Nouanthavong et al. reported the use of μPAD with nanoceria for the detection of organophosphate (OP) pesticides using an enzyme inhibition assay with acetylcholinesterase (AChE) and choline oxidase (ChOX).146 A significant interest was developed in analyzing milk and milk products for quality testing. A microfluidic paper-based ELISA platform was used to detect clenbuterol, an illicitly used feed additive for animals,67 by measuring the intensity of color change that was proportional to the analyte concentration. Au nanoparticle (NP) coated paper substrates were used for the detection of melamine in milk samples, the presence of which can cause a variety of health risk issues to infants. The interaction between melamine and the NPs produced a wine red to blue color change, measured using UV-VIS spectroscopy.55 A study was conducted by Salve et al. for the detection of urea, starch, salt, and detergent in milk samples using μPADs fabricated with polydimethylsiloxane (PDMS) as a hydrophobic barrier to confine reagents specific to the target analyte.214

Several works were reported toward the detection of heavy metals in water, using a colorimetric paper sensor. A novel approach for rapid and sensitive detection of heavy metals using μPAD was demonstrated by Hossain and Brennan with solgel entrapped reagents to allow colorimetric visualization of the enzymatic activity of beta-galactosidase (B-GAL).134 A smart gold nanosensor based μPAD was designed to detect very low concentrations of Cu2+ and Pb2+. A visible blue color was developed due to the formation of NP aggregates upon binding with metal ions.215 A novel, highly sensitive and selective μPAD for the detection of Cu2+ in groundwater, drinking water, rice, etc. was achieved by Chaiyo et al.145 The presence of Hg (II) in tap and commercial bottled water was detected by a paper-based device using silver nanoplates, the color of the nanosilver in the test area changes in the presence of Hg(II).216

In spite of all these demonstrated applications, achieving accurate detection with the naked eye is challenging due to the inhomogeneity of the color distribution on the paper assay.217 High background noise of the paper or the sample is another disadvantage of colorimetric detection resulting in low detection limits.

B. Electrochemical detection

Electrochemical μPADs (ePADs) using electrochemical detection is another commonly used method for the detection of food adulterants that involves the direct conversion of a biological or chemical signal to an electrical one to enhance the analytical performance of μPADs. This method comprises a three-electrode system: a counter, working, and reference electrode, which are deposited in the form of conductive inks (silver or carbon inks) on the paper matrix.218 Samples are added to the hydrophilic region of the paper surrounded by the hydrophobic barrier. The wicking property of the paper is used to flow the sample to the sensing zones with electrodes (modified with some reagents) and a precise signal is obtained from the redox reaction involving the sample and the reagents.

A low-cost green biosensor consisting of a hydrophilic paper disk with immobilized glucose oxidase, placed in screen-printed carbon electrode, was used for glucose concentration determination.47 Microwire electrodes, as an alternative to screen-printed electrodes, are employed in ePADs for the non-enzymatic detection of glucose, fructose, and sucrose in aqueous solutions.219 Direct-writing of electrode on paper was demonstrated by Li et al.,220 using a pressure-based ball pen to form ePAD to detect melamine in food samples. Pencil drawn electrodes were employed for the detection of analgesics and sedation drugs in whiskey samples, using ePADs.161 Spiking whiskey with these drugs has been a common practice to prevent hangover and unconsciousness. The integration of μPADs with zeolite-nanoflakes and graphene oxide nanocrystals was reported to be used for the electrochemical sensing of ketamine in alcoholic and non-alcoholic drinks.221

Most electrochemical detections are still performed in the lab due to the need for heavy equipment. Even though handheld-potentiostat equipment are available, they need to be miniaturized and integrated into lab-on-a-chip platforms for use in limited resource setting areas.222

C. Chemiluminescence detection

The chemiluminescence (CL) mechanism is based on the emission of light as a result of a chemical reaction. The luminescence in this method is controlled by the mixing of fluid flow reagents. CL based detection method uses inexpensive reagents and is characterized by a high signal to noise ratio and low limits of detection.223 However, the measurement using this method needs to be done in the dark making the detection process complicated.224

A common CL system being employed in μPADs is the luminol-H2O2 reaction system. In this system, transition metals, enzymes, and NPs have a good catalytic effect making it useful for the detection of various target analytes.225 Li et al.226 developed a double layered 3D μPAD for the detection of glucose, lactate, cholesterol, and choline, based on the appearance of temporally resolved peaks corresponding to the reactions between luminol and H2O2 upon the addition of different analytes. Alahmad et al.147 developed a device to detect Cr(III) in natural water samples based on luminol oxidation by H2O2 in the presence of Cr(III). Wang et al.227 fabricated paper-based molecular imprinted polymer (MIP) grafted multi-disk micro-disk plate that can provide the possibility of performing analytical assays, using MIP as a recognition element and enzyme catalyzed CL as the detection method to determine the presence of pesticides in liquid food samples. A paper-based chemiluminescence device was fabricated by Liu et al.142 for quantifying the level of dichlorvos (a type of insecticide) in vegetables without complicated sample pretreatment.142 Chemiluminescence assay has also been employed for the determination of food quality. For example, Hassanzadeh et al. developed a paper-based chemiluminescence device for the estimation of total phenolic antioxidant capacity in molasses and honey samples.228

Even though CL based detection method has great potential in food safety analysis, the intensive sample pre-treatments, and the need for selective CL system specific to both analytes and samples, etc. are not currently suitable for paper-based ASSURED testing.225

D. Fluorescence detection

μPADs based on fluorometric detection exploit the interaction between the target molecules and fluorescent dyes and measures the emission intensity during analysis under an excitation illumination. This technique has some advantages over other detection methods including low limit of detection, high specificity and sensitivity, and rapid and multiple detection of analytes in a cost-effective manner.225

Numerous fluorescent-based μPAD methods were reported for the detection of trace level amounts of heavy metal ions.229–231 An apparent fluorescent quenching of the test paper was observed when exposed to Hg2+, while a significant fluorescence enhancement was observed in the presence of Pb2+. Qi et al.232 used an ion imprinting technique to fabricate a 3D paper-based microfluidic device for the multiplexed detection of Cu2+ and Hg2+ ions based on the fluorescence quenching of CdTe quantum dots (QDs) and the results revealed good and reproducible performances for the analysis of real samples in local lake water and seawater. Fluorescence-based technique is a suitable method for the detection of allergens in food samples, as demonstrated by Weng and Neethirajan.233 A hybrid PDMS/μPAD aptasensor provided a rapid cost-effective and accurate determination of food allergens (egg white lysozyme, β-conglutin lupine, and brevetoxins in egg, sausage, and mussel, respectively). They used a specific aptamer-graphene oxide sensor that was coupled to QDs as the fluorescence label. Molecular-based fluorescence assay was employed by Ali et al.234 for the detection of E. coli in food products by employing a novel approach for isolating selective RNA cleaving fluoregenic DNAzymes (RFDs) on paper. A polydimethylsiloxane (PDMS)/paper/glass hybrid microfluidic system integrated with aptamer-functionalized graphene oxide was developed for multiplexed pathogen detection.235

The need for additional instrumentation, the significant background noise from the additives (to improve the whiteness in commercial paper), extensive sample preparation steps by trained personnel, etc. make the fluorescence detection method less suitable for paper-based devices that aim to be facile and low cost.236 Even though fluorescence sensing provides high specificity and selectivity, further improvement in the technique can be done with the cost-effective size reduction of fluorescence readers.

E. Nanoparticle-based detection

Another sensing method for the detection of μPADs makes use of nanoparticles (NPs). The optical properties of NPs, due to their high plasmonic and catalytic efficiency,237 enable naked eye observation of any bio-recognition event. Many studies have proved that NPs are effective in enhancing the analytical performance of paper-based device.

Kudo et al.238 demonstrated the colorimetric detection of metal ions such as Zn2+ in environmental water, using a paper-based analytical device with water soluble cationic polymer (PDDA) NPs coated on the detection zone. The detection limit was found to be 0.53 μM and, thus, represented a significant improvement over that achieved using commercial colorimetric Zn2+ test paper (9.7 μM).238 There have been numerous other works239–241 that employed paper-based devices with NPs to enhance the detection efficiency for ion analysis. Gold NPs were used to functionalize μPADs for the detection of melamine in milk by observing a color change upon the addition of samples with contaminant residues.55 Kasoju et al.38 used an aptamer based colorimetric assay followed by salt induced aggregation of NPs for the detection of aflatoxin B1 in milk and animal feed. This device was shown to be an efficient tool for on-site detection of food toxins in less than a minute. Figueredo et al.242 used different types of nanomaterials, such as Fe3O4 NPs, multiwalled carbon nanotubes (MWCNT), and graphene oxide, to enhance the analytical performance of μPADs, thus solving the drawback of using additives avoiding possible interaction with possible enzyme activity. The modified μPADs allowed the visual detection of glucose at low concentrations.242 Kumar et al.243 and Núnez-Bajo et al.244 have also employed NPs for enhancing the detection performance of paper-based devices in glucose analysis.

Even though NP-based detection methods result in rapid detection even at low concentrations of analytes compared to the colorimetric method, it has some limitations. NPs can aggregate in colloidal solutions creating false negative results. The affinity of NPs for many proteins can cause false labeling. Also, this technique requires extensive sample preparations and needs a large number of solvents and reagents. These drawbacks make the NP-based detection method not very suitable for food safety applications using paper devices.245

F. SERS based detection

Raman spectroscopy is an analytical technique based on inelastic light scattering and has many uses in determining the chemical and structural properties of different molecules. SERS enhances the Raman scattering signal of the analytes that are near or adsorbed on the surface of metal nanoparticles and offers high sensitivity, portability, and rapid detection up to a single molecular level.246,247 It is important to develop simple, flexible, and cost-effective substrates for broadening the SERS application window. Integrating SERS to mechanically stable and flexible paper substrates can provide a continuous flow condition for reproducible SERS measurements showing great potential for on-site analysis of food contaminants.73

One of the early studies based on this approach developed a paper-based device with Ag NPs and functionalized with negatively charged poly (sodium 4-styrene sulfonate) or PSS for the detection of common food dyes, such as sunset yellow and lemon yellow in drinks.248 Another research group used robust nanostructured-silver as flexible, paper-based SERS swabs for the direct detection of Metanil Yellow (MY) from toor dal (yellow split pigeon peas) samples and Malachite Green (MG) from green peas and green chilies. MY was efficiently detected in “spiked” dal samples with characteristics Raman peaks at 1148 and 1404 cm−1 with a detection limit of 1 μM. Green peas and green chilies were also used to detect MG that showed a prominent peak at 1370 cm−1.168 Paper-based SERS strategy was also used for the detection of drugs in beverages such as estazolam (EST), where in the latter prepared aqueous solutions showed distinct peaks at 687 and 1000 cm−1.249 In a similar attempt, Ma et al.250 quantified the concentrations of pesticide residues like thiram, thiabendazole, and methyl parathion on apples, oranges, and tomatoes' surfaces using high density Ag NPs/GO on cellulose paper as SERS substrate. Various pathogens in food products have also been quantified using paper-based SERS. For instance, Tian et al. fabricated a SERS swab by functionalizing nitrocellulose membrane using gold nanorods for the determination of E. coli spread in spinach leaves.251

Although there are numerous research works reported using paper-based SERS technology in food analysis, there are still some challenges associated with it. For example, Ag-based SERS substrates have a limited shelf life due to the degeneration of the activity of silver.168 The SERS method has significant difficulties in excluding interference and target the compound of interest when analyzing complex samples.249 Developing cost-effective, target-specific, long-lasting, and reliable substrates, using specific target capture agents and removing the interreference are some possible strategies to increase the sensitivity and selectivity of SERS method for food safety analysis.

VI. CONCLUSIONS AND FUTURE DIRECTIONS

The screening of hazardous materials in food is a major concern in the food safety sector. If the food quality control is not followed stringently, these harmful materials may lead to severe illness and mortality. Conventional food analytical systems rely on tedious, time consuming procedures for food hazard detection, in order to obtain reliable results. Hence, they are not always suitable for on-site rapid surveillance. World Health Organization standards for point-of-need diagnostic assay criteria are stated as: it should be Affordable, Sensitive, Specific, User-friendly, Rapid and Robust, Equipment-free, and Deliverable to end-users (ASSURED). Based on these criteria, every new assay system will be evaluated critically and a five-star scoring system will be used to assess new devices to be used in food safety testing systems.252 The recent advances in μPAD-based diagnostic devices and microfluidic technology potentially offer quick and dependable ASSURED surveillance options in food safety practices.

Detecting hazardous substances and pathogens in food is quite different from detecting those substances in liquid clinical samples such as blood or urine. The following refinements will be required for adopting this technology with maximum efficiency in food safety applications. For example, the food sample could be a solid piece of vegetable containing hard plant layers or a piece of processed meat fibrous muscle steak with raw and denatured proteins, containing hydrophobic lipids, insoluble fat, meat juices, salts, and spices added during cooking, also rich in many chemicals produced in the Maillard reaction during cooking.253,254 This complex nature of the food matrix restricts the availability of the target analyte to interact with the detection reagents. Also, depending upon the nature of the analyte in the food matrix and the μPAD matrix, strong physical and chemical interactions may interfere with the availability of the analyte to the detecting agent. Therefore, a typical food sample preparation involves mixing the food with a solvent, an extraction step to free the analyte from the food matrix, followed by a clean-up step to remove the interfering substances, and finally a concentration step or dilution step to be added if required. So, developments of easy methods for conversion of complex food samples into μPAD compatible liquid form will be a requirement for optimum μPAD assay performance.

Another significant area for improvement is to develop more sensitive detection and analysis systems for μPADs. Current detection methods mostly depend on traditional techniques and instruments such as scanners and grayscale image analysis. However, the conversion of colored image to grayscale hinders the advantages of colorimetry while multiplexing and reduces the assay sensitivity. In multiplexing, grayscale analysis of two different colors may detect with similar intensity. All the other methods, such as spectrometry and fluorescence-based assays, require expensive reagents and equipment limiting their potential for mass commercialization. For the advancement of μ-PAD field, the development of systems free of complex detection equipment is necessary.

We cannot conclude this review without mentioning some of the commercially available μPADs for food safety analysis and related applications. Elabscience based in Texas, USA has developed a number of lateral flow assays for detecting fungal toxins, toxic chemicals, pesticides, and antibiotics residues in food.255 R-biopharm, located in Germany, developed lateral flow devices (LFDs) consisting of immunochromatographic rapid test strips for allergen detection. Currently, they have LFDs for detecting peanut, hazelnut, coconut, almond, and mustard allergens. Also, they have dipstick (EZ PANGASIUS™ Pangasius Species Rapid Kit) tests for species detection in Pangasius fish. Their RIDA®QUICK CIS is an immunochromatographic test developed for the detection cow's milk in milk or cheese of other species (sheep and goat). They have LFDs, and paper-based screening cards for mycotoxin analysis.256 Merck and Australasian medical and scientific Ltd. and develop lateral flow tests for immunological detection of pathogens, such as Salmonella, Listeria, E. coli, and Campylobacter in food and environmental samples.257,258 Paperdrop Diagnostics S.L. based in Spain develops rapid diagnostic tests based on paper microfluidics. They have developed an assay called, Resistgene™ that offers a testing solution to detect multiple antimicrobial resistance genes from blood samples, focusing on the enzyme carbapenemase, a critical antibacterial resistance mechanism.259 Another important development in μPAD technology is I-Corps: A Paper-based Microfluidic Viral Diagnostic Device project by United States Department of Agriculture (USDA). I-Corps is based on the concept of using spectrum-shifting ability of gold nanoparticles as detection molecules on a passive paper flow assay.260 I-Corp can be used in many industries ranging from public health, veterinary medicine, food processing, and water testing.

The developments in the research field and commercial front show that the microfluidic paper analytic device (μPAD) technology is promising in food safety applications, especially as a simple, low-cost point-of-use testing platform. With continuing advances in fabrication techniques that enable mass production at lower costs, development of assays with enhanced sensitivity and specificity, further improvements in detection techniques that reduce or eliminate complex equipment, and commensurate progress in ancillary techniques for preparing μPAD-ready samples, μPADs are well positioned for widespread adoption in food safety industry.

ACKNOWLEDGMENTS

This work was supported by the New York University Abu Dhabi.

AUTHOR DECLARATIONS

Conflict of Interest

The authors have no conflicts to disclose.

Author Contributions

Soja Saghar Soman: Supervision (equal); Writing – original draft (lead); Writing – review & editing (lead). Shafeek Abdul Samad: Writing – original draft (supporting); Writing – review & editing (supporting). Priyamvada Venugopalan: Writing – original draft (supporting); Writing – review & editing (supporting). Nityanand Kumawat: Supervision (equal); Writing – original draft (supporting); Writing – review & editing (supporting). Sunil Kumar: Investigation (lead); Writing – original draft (supporting); Writing – review & editing (supporting).

DATA AVAILABILITY

The data that support the findings of this study are available from the corresponding author upon reasonable request.
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REFERENCES

1. C. J. Savelli et al. , “The FAO/WHO international food safety authorities network in review, 2004–2018: Learning from the past and looking to the future,” Foodborne Pathog. Dis. 16 (7 ), 480–488 (2019). 10.1089/fpd.2018.2582 30932688
2. B. Executive, Accelerating Efforts on Food Safety: Report by the Director-General (World Health Organization, Geneva, 2020).
3. R. Simpson et al. , “Analysing foodborne illness outbreak severity in the USA, 2009–19,” Lancet Global Health 10 , S5 (2022). 10.1016/S2214-109X(22)00134-6
4. T. Chiba, “Management of food hygiene and safety by hazard analysis and critical control point (HACCAP),” Yakugaku Zasshi 142 (1 ), 27–31 (2022). 10.1248/yakushi.21-00161-3 34980748
5. A. G. Herrera, “The hazard analysis and critical control point system in food safety,” Methods Mol. Biol. 268 , 235–280 (2004). 10.1385/1-59259-766-1:235 15156035
6. R. A. Williams and D. J. Zorn, “Hazard analysis and critical control point systems applied to public health risks: The example of seafood,” Rev. Sci. Tech. 16 (2 ), 349–358 (1997). 10.20506/rst.16.2.1028 9501348
7. T. P. Labuza and W. Baisier, “The role of the federal government in food safety,” Crit. Rev. Food Sci. Nutr. 31 (3 ), 165–176 (1992). 10.1080/10408399209527566 1571081
8. X. Li, D. R. Ballerini, and W. Shen, “A perspective on paper-based microfluidics: Current status and future trends,” Biomicrofluidics 6 (1 ), 11301–1130113 (2012). 10.1063/1.3687398 22662067
9. A. W. Martinez et al. , “Patterned paper as a platform for inexpensive, low-volume, portable bioassays,” Angew. Chem., Int. Ed. Engl. 46 (8 ), 1318–1320 (2007). 10.1002/anie.200603817 17211899
10. C. C. Liu et al. , “Microfluidic paper-based chip platform for benzoic acid detection in food,” Food Chem. 249 , 162–167 (2018). 10.1016/j.foodchem.2018.01.004 29407920
11. N. Kumawat et al. , “Rapid and inexpensive process to fabricate paper based microfluidic devices using a cut and heat plastic lamination process,” Lab Chip 22 (18 ), 3377–3389 (2022). 10.1039/D2LC00452F 35801817
12. J. Bai et al. , “Biocontrol and rapid detection of food-borne pathogens using bacteriophages and endolysins,” Front. Microbiol. 7 , 474 (2016). 10.3389/fmicb.2016.00474 27092128
13. S. Umesha and H. M. Manukumar, “Advanced molecular diagnostic techniques for detection of food-borne pathogens: Current applications and future challenges,” Crit. Rev. Food Sci. Nutr. 58 (1 ), 84–104 (2018). 10.1080/10408398.2015.1126701 26745757
14. J. C. Jokerst et al. , “Development of a paper-based analytical device for colorimetric detection of select foodborne pathogens,” Anal. Chem. 84 (6 ), 2900–2907 (2012). 10.1021/ac203466y 22320200
15. I. Tryland and L. Fiksdal, “Enzyme characteristics of beta-D-galactosidase- and beta-D-glucuronidase-positive bacteria and their interference in rapid methods for detection of waterborne coliforms and Escherichia coli,” Appl. Environ. Microbiol. 64 (3 ), 1018–1023 (1998). 10.1128/AEM.64.3.1018-1023.1998 9501441
16. S. H. Notermans et al. , “Phosphatidylinositol-specific phospholipase C activity as a marker to distinguish between pathogenic and nonpathogenic listeria species,” Appl. Environ. Microbiol. 57 (9 ), 2666–2670 (1991). 10.1128/aem.57.9.2666-2670.1991 1662937
17. Z. Wei, L. A. Zenewicz, and H. Goldfine, “Listeria monocytogenes phosphatidylinositol-specific phospholipase C has evolved for virulence by greatly reduced activity on GPI anchors,” Proc. Natl. Acad. Sci. U.S.A. 102 (36 ), 12927–12931 (2005). 10.1073/pnas.0501725102 16118276
18. P. Goullet and B. Picard, “Characterization of enterobacteria by esterase specific-activity profiles,” J. Gen. Microbiol. 136 (3 ), 431–440 (1990). 10.1099/00221287-136-3-431 2202780
19. R. Green, A. D. Ellington, and J. W. Szostak, “In vitro genetic analysis of the Tetrahymena self-splicing intron,” Nature 347 (6291 ), 406–408 (1990). 10.1038/347406a0 2215650
20. C. Lyu, I. M. Khan, and Z. Wang, “Capture-SELEX for aptamer selection: A short review,” Talanta 229 , 122274 (2021). 10.1016/j.talanta.2021.122274 33838776
21. S. Somvanshi et al. , “Microfluidic paper-based aptasensor devices for multiplexed detection of pathogenic bacteria,” Biosens. Bioelectron. 207 , 114214 (2022). 10.1016/j.bios.2022.114214 35349894
22. G. A. Suaifan, S. Alhogail, and M. Zourob, “Rapid and low-cost biosensor for the detection of Staphylococcus aureus,” Biosens. Bioelectron. 90 , 230–237 (2017). 10.1016/j.bios.2016.11.047 27914366
23. G. Suaifan, S. Alhogail, and M. Zourob, “Paper-based magnetic nanoparticle-peptide probe for rapid and quantitative colorimetric detection of Escherichia coli O157:H7,” Biosens. Bioelectron. 92 , 702–708 (2017). 10.1016/j.bios.2016.10.023 27839734
24. F. Mazur et al. , “Paper-based sensors for bacteria detection,” Nat. Rev. Bioeng. 1 (3 ), 180–192 (2023). 10.1038/s44222-023-00024-w 36937095
25. Z. Jia et al. , “Nondestructive multiplex detection of foodborne pathogens with background microflora and symbiosis using a paper chromogenic array and advanced neural network,” Biosens. Bioelectron. 183 , 113209 (2021). 10.1016/j.bios.2021.113209 33836430
26. M. C. Janzen et al. , “Colorimetric sensor arrays for volatile organic compounds,” Anal. Chem. 78 (11 ), 3591–3600 (2006). 10.1021/ac052111s 16737212
27. M. Yang et al. , “Machine learning-enabled non-destructive paper chromogenic array detection of multiplexed viable pathogens on food,” Nat. Food 2 (2 ), 110–117 (2021). 10.1038/s43016-021-00229-5 37117406
28. S. A. Snyder et al. , “Lysis and direct detection of coliforms on printed paper-based microfluidic devices,” Lab Chip 20 (23 ), 4413–4419 (2020). 10.1039/D0LC00665C 32959038
29. N. Ünüsan and C. Ünüsan, “Aflatoxins,” in Encyclopedia of Food Safety, 2nd ed., edited by G. W. Smithers (Academic Press, Oxford, 2024), pp. 470–475.
30. R. Ziółkowski et al. , “From small molecules toward whole cells detection: Application of electrochemical aptasensors in modern medical diagnostics,” Sensors (Basel) 21 (3), 724 (2021). 10.3390/s21030724 33494499
31. S. H. Henry et al. , “Policy forum: Public health: Reducing liver cancer–global control of aflatoxin,” Science 286 (5449 ), 2453–2454 (1999). 10.1126/science.286.5449.2453 10636808
32. J. D. Groopman, T. W. Kensler, and C. P. Wild, “Protective interventions to prevent aflatoxin-induced carcinogenesis in developing countries,” Annu. Rev. Public Health 29 , 187–203 (2008). 10.1146/annurev.publhealth.29.020907.090859 17914931
33. C. Brera et al. , “Simultaneous determination of aflatoxins and ochratoxin A in baby foods and paprika by HPLC with fluorescence detection: A single-laboratory validation study,” Talanta 83 (5 ), 1442–1446 (2011). 10.1016/j.talanta.2010.11.031 21238734
34. H. Xue et al. , “New method for the simultaneous analysis of types A and B trichothecenes by ultrahigh-performance liquid chromatography coupled with tandem mass spectrometry in potato tubers inoculated with fusarium sulphureum,” J. Agric. Food Chem. 61 (39 ), 9333–9338 (2013). 10.1021/jf402997t 24010413
35. G. Miklós et al. , “Detection of aflatoxins in different matrices and food-chain positions,” Front. Microbiol. 11 , 1916 (2020). 10.3389/fmicb.2020.01916 32983001
36. B. Lutz et al. , “Dissolvable fluidic time delays for programming multi-step assays in instrument-free paper diagnostics,” Lab Chip 13 (14 ), 2840–2847 (2013). 10.1039/c3lc50178g 23685876
37. S. Charernchai et al. , “Automated paper-based femtogram sensing device for competitive enzyme-linked immunosorbent assay of aflatoxin B1 using submicroliter samples,” Anal. Chem. 94 (12 ), 5099–5105 (2022). 10.1021/acs.analchem.1c05401 35302345
38. A. Kasoju et al. , “Microfluidic paper device for rapid detection of aflatoxin B1 using an aptamer based colorimetric assay,” RSC Adv. 10 (20 ), 11843–11850 (2020). 10.1039/D0RA00062K 35496625
39. Q. Jiang et al. , “Paper-Based microfluidic device (DON-chip) for rapid and Low-cost deoxynivalenol quantification in food, feed, and feed ingredients,” ACS Sens. 4 (11 ), 3072–3079 (2019). 10.1021/acssensors.9b01895 31713421
40. X. Lin et al. , “A portable paper-based aptasensor for simultaneous visual detection of two mycotoxins in corn flour using dual-color upconversion nanoparticles and Cu-TCPP nanosheets,” Food Chem. 404 (Pt B ), 134750 (2023). 10.1016/j.foodchem.2022.134750 36444087
41. L. Anfossi et al. , “Silver and gold nanoparticles as multi-chromatic lateral flow assay probes for the detection of food allergens,” Anal. Bioanal. Chem. 411 (9 ), 1905–1913 (2019). 10.1007/s00216-018-1451-6 30397760
42. M. Z. Hua and X. Lu, “Development of a microfluidic paper-based immunoassay for rapid detection of allergic protein in foods,” ACS Sens. 5 (12 ), 4048–4056 (2020). 10.1021/acssensors.0c02044 33267576
43. Y. Wang et al. , “Development of an immunochromatographic strip test for the rapid detection of soybean Bowman-Birk inhibitor,” Food Agric. Immunol. 30 (1 ), 1202–1211 (2019). 10.1080/09540105.2019.1680613
44. F. B. Hu and V. S. Malik, “Sugar-sweetened beverages and risk of obesity and type 2 diabetes: Epidemiologic evidence,” Physiol. Behav. 100 (1 ), 47–54 (2010). 10.1016/j.physbeh.2010.01.036 20138901
45. J. Reedy and S. M. Krebs-Smith, “Dietary sources of energy, solid fats, and added sugars among children and adolescents in the United States,” J. Am. Diet Assoc. 110 (10 ), 1477–1484 (2010). 10.1016/j.jada.2010.07.010 20869486
46. V. S. Malik et al. , “Long-Term consumption of sugar-sweetened and artificially sweetened beverages and risk of mortality in US adults,” Circulation 139 (18 ), 2113–2125 (2019). 10.1161/CIRCULATIONAHA.118.037401 30882235
47. C. S. Kuek Lawrence, S. N. Tan, and C. Z. Floresca, “A “green” cellulose paper based glucose amperometric biosensor,” Sens. Actuators, B 193 , 536–541 (2014). 10.1016/j.snb.2013.11.054
48. Y. Deng et al. , “Fully inkjet printing preparation of a carbon dots multichannel microfluidic paper-based sensor and Its application in food additive detection,” ACS Appl. Mater. Interfaces 13 (48 ), 57084–57091 (2021). 10.1021/acsami.1c14435 34797049
49. L. Gan et al. , “Exploration of pH-responsive carbon dots for detecting nitrite and ascorbic acid,” Appl. Surf. Sci. 530 , 147269 (2020). 10.1016/j.apsusc.2020.147269
50. S. Bansal et al. , “Food adulteration: Sources, health risks, and detection methods,” Crit. Rev. Food Sci. Nutr. 57 (6 ), 1174–1189 (2017). 10.1080/10408398.2014.967834 26054861
51. T. M. G. Cardoso, P. T. Garcia, and W. K. T. Coltro, “Colorimetric determination of nitrite in clinical, food and environmental samples using microfluidic devices stamped in paper platforms,” Anal. Methods 7 (17 ), 7311–7317 (2015). 10.1039/C5AY00466G
52. D. Giustarini et al. , “Adaptation of the Griess reaction for detection of nitrite in human plasma,” Free Radic. Res. 38 (11 ), 1235–1240 (2004). 10.1080/10715760400017327 15621701
53. E. Trofimchuk et al. , “Development of paper-based microfluidic device for the determination of nitrite in meat,” Food Chem. 316 , 126396 (2020). 10.1016/j.foodchem.2020.126396 32066068
54. B. M. Jayawardane et al. , “Microfluidic paper-based analytical device for the determination of nitrite and nitrate,” Anal. Chem. 86 (15 ), 7274–7279 (2014). 10.1021/ac5013249 25001619
55. N. Gao, P. Huang, and F. Wu, “Colorimetric detection of melamine in milk based on triton X-100 modified gold nanoparticles and its paper-based application,” Spectrochim. Acta, Part A 192 , 174–180 (2018). 10.1016/j.saa.2017.11.022
56. J. Devries et al. , “Non-protein nitrogen determination: A screening tool for nitrogenous compound adulteration of milk powder,” Int. Dairy J. 68 (2016). 10.1016/j.idairyj.2016.12.003
57. M. F. Alam et al. , “Colorimetric method for the detection of melamine using in-situ formed silver nanoparticles via tannic acid,” Spectrochim. Acta, Part A 183 , 17–22 (2017). 10.1016/j.saa.2017.04.021
58. J. Song et al. , “Colorimetric detection of melamine in pretreated milk using silver nanoparticles functionalized with sulfanilic acid,” Food Control 50 , 356–361 (2015). 10.1016/j.foodcont.2014.08.049
59. N. Kumar, R. Seth, and H. Kumar, “Colorimetric detection of melamine in milk by citrate stabilized gold nanoparticles,” Anal. Biochem. 456 , 43 (2014). 10.1016/j.ab.2014.04.002 24727351
60. J. Manzoori, M. Amjadi, and J. Hasanzadeh, “Enhancement of the chemiluminescence of permanganate-formaldehyde system by gold/silver nanoalloys and its application to trace determination of melamine,” Microchim. Acta 175 , 47–54 (2011). 10.1007/s00604-011-0651-y
61. C. Wang et al. , “Hemolysis of human erythrocytes induced by melamine-cyanurate complex,” Biochem. Biophys. Res. Commun. 402 (4 ), 773–777 (2010). 10.1016/j.bbrc.2010.10.108 21036151
62. L. Trapiella-Alfonso et al. , “Synthesis and characterization of hapten-quantum dots bioconjugates: Application to development of a melamine fluorescentimmunoassay,” Talanta 106 , 243–248 (2013). 10.1016/j.talanta.2013.01.027 23598123
63. S. Shariati and G. Khayatian, “Microfluidic paper-based analytical device using gold nanoparticles modified with N,N′-bis(2-hydroxyethyl)dithiooxamide for detection of Hg(ii) in air, fish and water samples,” New J. Chem. 44 (43 ), 18662–18667 (2020). 10.1039/D0NJ03986A
64. R. Muthukumar et al. , “Detection of adulteration in sunflower oil using paper-based microfluidic lab-on-a-chip devices,” Mater. Today: Proc. 34 , 496–501 (2021). 10.1016/j.matpr.2020.03.099
65. J. Narang et al. , “Point of care with micro fluidic paper based device integrated with nano zeolite—Graphene oxide nanoflakes for electrochemical sensing of ketamine,” Biosens. Bioelectron. 88 (2016). 10.1016/j.bios.2016.08.043
66. X. Sha et al. , “A rapid detection method for xc5bOn-site screening of Estazolam in beverages with Au@Ag core-shell nanoparticles paper-based SERS substrate,” Anal. Sci. 36 (6 ), 667–674 (2020). 10.2116/analsci.19P361 31787667
67. L. Ma et al. , “Rapid detection of clenbuterol in milk using microfluidic paper-based ELISA,” Food Chem. 246 , 437–441 (2018). 10.1016/j.foodchem.2017.12.022 29291870
68. D. Lacerda et al. , “Global decrease in blood lead concentrations due to the removal of leaded gasoline,” Chemosphere 324 , 138207 (2023). 10.1016/j.chemosphere.2023.138207 36822521
69. K. P. Mishra, “Lead exposure and its impact on immune system: A review,” Toxicol. In Vitro 23 (6 ), 969–972 (2009). 10.1016/j.tiv.2009.06.014 19540334
70. D. O'Connor et al. , “The effects of iniquitous lead exposure on health,” Nat. Sustaina. 3 , 77–79 (2020). 10.1038/s41893-020-0475-z
71. K. Katelakha et al. , “A simple distance paper-based analytical device for the screening of lead in food matrices,” Biosensors 11 (3 ), 90 (2021). 10.3390/bios11030090 33809868
72. S. Moreira et al. , “Carbamate pesticides: Shedding light on their impact on the male reproductive system,” Int. J. Mol. Sci. 23 (15 (2022). 10.3390/ijms23158206
73. J. Xie et al. , “Flexible paper-based SERS substrate strategy for rapid detection of methyl parathion on the surface of fruit,” Spectrochim. Acta, Part A 231 , 118104 (2020). 10.1016/j.saa.2020.118104
74. M. Aidil et al. , “Assay for heavy metals using an inhibitive assay based on the acetylcholinesterase from pangasius hypophthalmus (Sauvage, 1878),” Fresenius Environ. Bull. 22 , 3572–3576 (2013).
75. G. L. Ellman et al. , “A new and rapid colorimetric determination of acetylcholinesterase activity,” Biochem. Pharmacol. 7 , 88–95 (1961). 10.1016/0006-2952(61)90145-9 13726518
76. S. Beshana et al. , “Microfluidic paper based analytical devices for the detection of carbamate pesticides,” Bull. Environ. Contam. Toxicol. 109 , 1–8 (2022). 10.1007/s00128-022-03533-3 35789281
77. S. Nouanthavong et al. , “Pesticide analysis using nanoceria-coated paper-based devices as a detection platform,” Analyst 141 , 1837 (2016). 10.1039/C5AN02403J 26842266
78. M. Marino et al. , “Pleiotropic outcomes of glyphosate exposure: From organ damage to effects on inflammation, cancer, reproduction and development,” Int. J. Mol. Sci. 22 (22 ), 12606 (2021). 10.3390/ijms222212606 34830483
79. P. Sawetwong et al. , “Enhanced selectivity and sensitivity for colorimetric determination of glyphosate using Mn–ZnS quantum dot embedded molecularly imprinted polymers combined with a 3D-microfluidic paper-based analytical device,” Talanta 225 , 122077 (2021). 10.1016/j.talanta.2020.122077 33592801
80. H. Noppe et al. , “Novel analytical methods for the determination of steroid hormones in edible matrices,” Anal. Chim. Acta 611 (1 ), 1–16 (2008). 10.1016/j.aca.2008.01.066 18298962
81. B. B. Hirpessa, B. H. Ulusoy, and C. Hecer, “Hormones and hormonal anabolics: Residues in animal source food, potential public health impacts, and methods of analysis,” J. Food Qual. 2020 , 5065386 (2020). 10.1155/2020/5065386
82. L. Xiao et al. , “Molecularly imprinted polymer grafted paper-based method for the detection of 17β-estradiol,” Food Chem. 221 , 82–86 (2017). 10.1016/j.foodchem.2016.10.062 27979279
83. M. Taghizadeh-Behbahani, M. Shamsipur, and B. Hemmateenejad, “Detection and discrimination of antibiotics in food samples using a microfluidic paper-based optical tongue,” Talanta 241 , 123242 (2022). 10.1016/j.talanta.2022.123242 35085991
84. A. Nilghaz and X. Lu, “Detection of antibiotic residues in pork using paper-based microfluidic device coupled with filtration and concentration,” Anal. Chim. Acta 1046 , 163–169 (2019). 10.1016/j.aca.2018.09.041 30482295
85. A. W. Martinez et al. , “Patterned paper as a platform for inexpensive, low-volume, portable bioassays,” Angew. Chem. Int. Ed. 46 (8 ), 1318–1320 (2007). 10.1002/anie.200603817
86. L. Yu and Z. Z. Shi, “Microfluidic paper-based analytical devices fabricated by low-cost photolithography and embossing of parafilm (R),” Lab Chip 15 (7 ), 1642–1645 (2015). 10.1039/C5LC00044K 25710591
87. A. W. Martinez et al. , “FLASH: A rapid method for prototyping paper-based microfluidic devices,” Lab Chip 8 (12 ), 2146–2150 (2008). 10.1039/b811135a 19023478
88. Q. H. He et al. , “Method for fabrication of paper-based microfluidic devices by alkylsilane self-assembling and UV/O-3-patterning,” Anal. Chem. 85 (3 ), 1327–1331 (2013). 10.1021/ac303138x 23244032
89. W. Dungchai, O. Chailapakul, and C. S. Henry, “A low-cost, simple, and rapid fabrication method for paper-based microfluidics using wax screen-printing,” Analyst 136 (1 ), 77–82 (2011). 10.1039/C0AN00406E 20871884
90. S. M. Wang et al. , “Paper-based chemiluminescence ELISA: Lab-on-paper based on chitosan modified paper device and wax-screen-printing,” Biosens. Bioelectron. 31 (1 ), 212–218 (2012). 10.1016/j.bios.2011.10.019 22051546
91. Y. Lu et al. , “Fabrication and characterization of paper-based microfluidics prepared in nitrocellulose membrane By Wax printing,” Anal. Chem. 82 (1 ), 329–335 (2010). 10.1021/ac9020193 20000582
92. J. F. Nie et al. , “One-step patterning of hollow microstructures in paper by laser cutting to create microfluidic analytical devices,” Analyst 138 (2 ), 671–676 (2013). 10.1039/C2AN36219H 23183392
93. Y. Sameenoi et al. , “One-step polymer screen-printing for microfluidic paper-based analytical device (mu PAD) fabrication,” Analyst 139 (24 ), 6580–6588 (2014). 10.1039/C4AN01624F 25360590
94. S. Ma et al. , “Visible paper chip immunoassay for rapid determination of bacteria in water distribution system,” Talanta 120 , 135–140 (2014). 10.1016/j.talanta.2013.12.007 24468352
95. J. Y. Sun, C. M. Cheng, and Y. C. Liao, “Screen printed paper-based diagnostic devices with polymeric inks,” Anal. Sci. 31 (3 ), 145–151 (2015). 10.2116/analsci.31.145 25765267
96. Y. J. Juang, W. S. Li, and P. S. Chen, “Fabrication of microfluidic paper-based analytical devices by filtration-assisted screen printing,” J. Taiwan Inst. Chemi. Eng. 80 , 71–75 (2017). 10.1016/j.jtice.2017.08.007
97. P. Jarujamrus et al. , “Screen-printed microfluidic paper-based analytical device (mu PAD) as a barcode sensor for magnesium detection using rubber latex waste as a novel hydrophobic reagent,” Anal. Chim. Acta 1082 , 66–77 (2019). 10.1016/j.aca.2019.06.058 31472714
98. T. Akyazi, L. Basabe-Desmonts, and F. Benito-Lopez, “Review on microfluidic paper-based analytical devices towards commercialisation,” Anal. Chim. Acta 1001 , 1–17 (2018). 10.1016/j.aca.2017.11.010 29291790
99. L. F. Cai et al. , “Fabrication of a microfluidic paper-based analytical device by silanization of filter cellulose using a paper mask for glucose assay,” Analyst 139 (18 ), 4593–4598 (2014). 10.1039/C4AN00988F 25045759
100. L. F. Cai et al. , “A simple paper-based sensor fabricated by selective wet etching of silanized filter paper using a paper mask,” Biomicrofluidics 8 (5 ), 056504 (2014). 10.1063/1.4898096
101. P. Kwong and M. Gupta, “Vapor phase deposition of functional polymers onto paper-based microfluidic devices for advanced unit operations,” Anal. Chem. 84 (22 ), 10129–10135 (2012). 10.1021/ac302861v 23113699
102. B. Chen, P. Kwong, and M. Gupta, “Patterned fluoropolymer barriers for containment of organic solvents within paper-based microfluidic devices,” ACS Appl. Mater. Interfaces 5 (23 ), 12701–12707 (2013). 10.1021/am404049x 24283374
103. G. Demirel and E. Babur, “Vapor-phase deposition of polymers as a simple and versatile technique to generate paper-based microfluidic platforms for bioassay applications,” Analyst 139 (10 ), 2326–2331 (2014). 10.1039/C4AN00022F 24699534
104. T. Lam et al. , “A chemically patterned microfluidic paper-based analytical device (C-mu PAD) for point-of-care diagnostics,” Sci. Rep. 7 (2017). 10.1038/s41598-017-01343-w
105. P. K. Kao and C. C. Hsu, “One-step rapid fabrication of paper-based microfluidic devices using fluorocarbon plasma polymerization,” Microfluid. Nanofluid. 16 (5 ), 811–818 (2014). 10.1007/s10404-014-1347-5
106. Y. Jiang et al. , “A simple method for fabrication of microfluidic paper-based analytical devices and on-device fluid control with a portable corona generator,” RSC Adv. 6 (4 ), 2888–2894 (2016). 10.1039/C5RA23470K
107. N. Raj, V. Breedveld, and D. W. Hess, “Fabrication of fully enclosed paper microfluidic devices using plasma deposition and etching,” Lab Chip 19 (19 ), 3337–3343 (2019). 10.1039/C9LC00746F 31501838
108. Y. He et al. , “Fabrication of paper-based microfluidic analysis devices: A review,” RSC Adv. 5 (95 ), 78109–78127 (2015). 10.1039/C5RA09188H
109. T. Songjaroen et al. , “Novel, simple and low-cost alternative method for fabrication of paper-based microfluidics by wax dipping,” Talanta 85 (5 ), 2587–2593 (2011). 10.1016/j.talanta.2011.08.024 21962687
110. T. Nurak, N. Praphairaksit, and O. Chailapakul, “Fabrication of paper-based devices by lacquer spraying method for the determination of nickel (II) ion in waste water,” Talanta 114 , 291–296 (2013). 10.1016/j.talanta.2013.05.037 23953473
111. M. M. Thuo et al. , “Fabrication of Low-cost paper-based microfluidic devices by embossing or cut-and-stack methods,” Chem. Mater. 26 (14 ), 4230–4237 (2014). 10.1021/cm501596s
112. D. Gosselin et al. , “Low-cost embossed-paper micro-channels for spontaneous capillary flow,” Sens. Actuators, B 248 , 395–401 (2017). 10.1016/j.snb.2017.03.144
113. V. F. Curto et al. , “Fast prototyping of paper-based microfluidic devices by contact stamping using indelible ink,” RSC Adv. 3 (41 ), 18811–18816 (2013). 10.1039/c3ra43825b
114. P. D. Garcia et al. , “A handheld stamping process to fabricate microfluidic paper-based analytical devices with chemically modified surface for clinical assays,” RSC Adv. 4 (71 ), 37637–37644 (2014). 10.1039/C4RA07112C
115. K. L. Dornelas, N. Dossi, and E. Piccin, “A simple method for patterning poly(dimethylsiloxane) barriers in paper using contact-printing with low-cost rubber stamps,” Anal. Chim. Acta 858 , 82–90 (2015). 10.1016/j.aca.2014.11.025 25597806
116. Y. Lu et al. , “Rapid prototyping of paper-based microfluidics with wax for low-cost, portable bioassay,” Electrophoresis 30 (9 ), 1497–1500 (2009). 10.1002/elps.200800563 19340829
117. E. Carrilho et al. , “Paper microzone plates,” Anal. Chem. 81 (15 ), 5990–5998 (2009). 10.1021/ac900847g 19572563
118. D. M. Cate et al. , “Recent developments in paper-based microfluidic devices,” Anal. Chem. 87 (1 ), 19–41 (2015). 10.1021/ac503968p 25375292
119. K. Abe, K. Suzuki, and D. Citterio, “Inkjet-printed microfluidic multianalyte chemical sensing paper,” Anal. Chem. 80 (18 ), 6928–6934 (2008). 10.1021/ac800604v 18698798
120. K. Abe et al. , “Inkjet-printed paperfluidic immuno-chemical sensing device,” Anal. Bioanal. Chem. 398 (2 ), 885–893 (2010). 10.1007/s00216-010-4011-2 20652543
121. K. Yamada et al. , “Paper-based inkjet-printed microfluidic analytical devices,” Angew. Chem. Int. Ed. 54 (18 ), 5294–5310 (2015). 10.1002/anie.201411508
122. M. Aghababaie et al. , “Recent advances in the development of enzymatic paper-based microfluidic biosensors,” Biosens. Bioelectron. 226 , 115131 (2023). 10.1016/j.bios.2023.115131 36804663
123. S. M. Z. Hossain et al. , “Reagentless bidirectional lateral flow bioactive paper sensors for detection of pesticides in beverage and food samples,” Anal. Chem. 81 (21 ), 9055–9064 (2009). 10.1021/ac901714h 19788278
124. S. M. Z. Hossain et al. , “Development of a bioactive paper sensor for detection of neurotoxins using piezoelectric inkjet printing of sol-gel-derived bioinks,” Anal. Chem. 81 (13 ), 5474–5483 (2009). 10.1021/ac900660p 19492815
125. J. Lessing et al. , “Inkjet printing of conductive inks with high lateral resolution on omniphobic “R-F paper” for paper-based electronics and MEMS,” Adv. Mater. 26 (27 ), 4677 (2014). 10.1002/adma.201401053 24889538
126. J. Olkkonen, K. Lehtinen, and T. Erho, “Flexographically printed fluidic structures in paper,” Anal. Chem. 82 (24 ), 10246–10250 (2010). 10.1021/ac1027066 21090744
127. A. Määttänen et al. , “Paper-based planar reaction arrays for printed diagnostics,” Sens. Actuators, B 160 (1 ), 1404–1412 (2011). 10.1016/j.snb.2011.09.086
128. J. Hiltunen et al. , “Roll-to-roll fabrication of integrated PDMS-paper microfluidics for nucleic acid amplification,” Lab Chip 18 (11 ), 1552–1559 (2018). 10.1039/C8LC00269J 29708259
129. R. Ghosh et al. , “Fabrication of laser printed microfluidic paper-based analytical devices (LP-mu PADs) for point-of-care applications,” Sci. Rep. 9 (2019). 10.1038/s41598-019-44455-1
130. J. S. Ng and M. Hashimoto, “Fabrication of paper microfluidic devices using a toner laser printer,” RSC Adv. 10 (50 ), 29797–29807 (2020). 10.1039/D0RA04301J 35518222
131. C. L. Sones et al. , “Laser-induced photo-polymerisation for creation of paper-based fluidic devices,” Lab Chip 14 (23 ), 4567–4574 (2014). 10.1039/C4LC00850B 25286149
132. P. Spicar-Mihalic et al. , “CO2 laser cutting and ablative etching for the fabrication of paper-based devices,” J. Micromech. Microeng. 23 (6 ), 067003 (2013). 10.1088/0960-1317/23/6/067003
133. M. A. Mahmud et al. , “Creating compact and microscale features in paper-based devices by laser cutting,” Analyst 141 (23 ), 6449–6454 (2016). 10.1039/C6AN02208A 27792224
134. S. M. Z. Hossain and J. D. Brennan, “β-galactosidase-based colorimetric paper sensor for determination of heavy metals,” Anal. Chem. 83 (22 ), 8772–8778 (2011). 10.1021/ac202290d 22029903
135. N. Dossi et al. , “Pencil-drawn paper supported electrodes as simple electrochemical detectors for paper-based fluidic devices,” Electrophoresis 34 (14 ), 2085–2091 (2013). 10.1002/elps.201200425 23161669
136. Q. H. He et al. , “Method for fabrication of paper-based microfluidic devices by alkylsilane self-assembling and UV/O patterning,” Anal. Chem. 85 (3 ), 1327–1331 (2013). 10.1021/ac303138x 23244032
137. N. Lopez-Ruiz et al. , “Smartphone-Based simultaneous pH and nitrite colorimetric determination for paper microfluidic devices,” Anal. Chem. 86 (19 ), 9554–9562 (2014). 10.1021/ac5019205 25158126
138. C. Sicard et al. , “Tools for water quality monitoring and mapping using paper-based sensors and cell phones,” Water Res. 70 , 360–369 (2015). 10.1016/j.watres.2014.12.005 25546358
139. N. M. Myers, E. N. Kernisan, and M. Lieberman, “Lab on paper: Iodometric titration on a printed card,” Anal. Chem. 87 (7 ), 3764–3770 (2015). 10.1021/ac504269q 25711696
140. X. Q. Hu et al. , “Determination of apparent amylose content in rice by using paper-based microfluidic chips,” J. Agric. Food Chem. 63 (44 ), 9863–9868 (2015). 10.1021/acs.jafc.5b04530 26495809
141. S. Q. Jin et al. , “A cost-effective Z-folding controlled liquid handling microfluidic paper analysis device for pathogen detection via ATP quantification,” Biosens. Bioelectron. 63 , 379–383 (2015). 10.1016/j.bios.2014.07.070 25127472
142. W. Liu et al. , “Paper-based chromatographic chemiluminescence chip for the detection of dichlorvos in vegetables,” Biosens. Bioelectron. 52 , 76–81 (2014). 10.1016/j.bios.2013.08.024 24021659
143. Y. L. Zhang, P. Zuo, and B. C. Ye, “A low-cost and simple paper-based microfluidic device for simultaneous multiplex determination of different types of chemical contaminants in food,” Biosens. Bioelectron. 68 , 14–19 (2015). 10.1016/j.bios.2014.12.042 25558869
144. M. S. Li et al. , “‘Periodic-table-style’ paper device for monitoring heavy metals in water,” Anal. Chem. 87 (5 ), 2555–2559 (2015). 10.1021/acs.analchem.5b00040 25645265
145. S. Chaiyo et al. , “Highly selective and sensitive paper-based colorimetric sensor using thiosulfate catalytic etching of silver nanoplates for trace determination of copper ions,” Anal. Chim. Acta 866 , 75–83 (2015). 10.1016/j.aca.2015.01.042 25732695
146. S. Nouanthavong et al. , “Pesticide analysis using nanoceria-coated paper-based devices as a detection platform,” Analyst 141 (5 ), 1837–1846 (2016). 10.1039/C5AN02403J 26842266
147. W. Alahmad et al. , “A miniaturized chemiluminescence detection system for a microfluidic paper-based analytical device and its application to the determination of chromium(III),” Anal. Methods 8 (27 ), 5414–5420 (2016). 10.1039/C6AY00954A
148. M. M. Ali et al. , “A printed multicomponent paper sensor for bacterial detection,” Sci. Rep. 7 (2017). 10.1038/s41598-017-12549-3
149. T. M. G. Cardoso et al. , “A paper-based colorimetric spot test for the identification of adulterated whiskeys,” Chem. Commun. 53 (56 ), 7957–7960 (2017). 10.1039/C7CC02271A
150. Q. K. Kong et al. , “A novel microfluidic paper-based colorimetric sensor based on molecularly imprinted polymer membranes for highly selective and sensitive detection of bisphenol A,” Sens. Actuators, B 243 , 130–136 (2017). 10.1016/j.snb.2016.11.146
151. B. W. Li et al. , “Quantum dot-based molecularly imprinted polymers on three-dimensional origami paper microfluidic chip for fluorescence detection of phycocyanin,” ACS Sens. 2 (2 ), 243–250 (2017). 10.1021/acssensors.6b00664 28723147
152. Y. Wu et al. , “Sensitive inkjet printing paper-based colormetric strips for acetylcholinesterase inhibitors with indoxyl acetate substrate,” Talanta 162 , 174–179 (2017). 10.1016/j.talanta.2016.10.011 27837814
153. J. Narang et al. , “Point of care with micro fluidic paper based device integrated with nano zeolite-graphene oxide nanoflakes for electrochemical sensing of ketamine,” Biosens. Bioelectron. 88 , 249–257 (2017). 10.1016/j.bios.2016.08.043 27570055
154. S. Lee, J. Park, and J. K. Park, “Foldable paper-based analytical device for the detection of an acetylcholinesterase inhibitor using an angle-based readout,” Sens. Actuators, B 273 , 322–327 (2018). 10.1016/j.snb.2018.06.059
155. Y. D. Ma et al. , “Rapid and sensitive on-site detection of pesticide residues in fruits and vegetables using screen-printed paper-based SERS swabs,” Anal. Methods 10 (38 (2018). 10.1039/C8AY01698D
156. N. Gao, P. C. Huang, and F. Y. Wu, “Colorimetric detection of melamine in milk based on triton X-100 modified gold nanoparticles and its paper-based application,” Spectrochim. Acta, Part A 192 , 174–180 (2018). 10.1016/j.saa.2017.11.022
157. L. Li et al. , “Paper-based immune-affinity arrays for detection of multiple mycotoxins in cereals,” Anal. Bioanal. Chem. 410 (8 ), 2253–2262 (2018). 10.1007/s00216-018-0895-z 29411083
158. L. Y. Ma et al. , “Rapid detection of clenbuterol in milk using microfluidic paper-based ELISA,” Food Chem. 246 , 437–441 (2018). 10.1016/j.foodchem.2017.12.022 29291870
159. Y. Q. Fan et al. , “Milk carton with integrated paper-based microfluidics for milk quality rapid test,” J. Food Saf. 38 (6 (2018). 10.1111/jfs.12548
160. X. Weng and S. Neethirajan, “Paper-based microfluidic aptasensor for food safety,” J. Food Saf. 38 (1 (2018). 10.1111/jfs.12412
161. A. A. Dias et al. , “Detection of analgesics and sedation drugs in whiskey using electrochemical paper-based analytical devices,” Electroanalysis 30 (10 ), 2250–2257 (2018). 10.1002/elan.201800308
162. J. Hassanzadeh, H. A. J. Al Lawati, and I. Al Lawati, “Metal-organic framework loaded by rhodamine B As a novel chemiluminescence system for the paper-based analytical devices and its application for total phenolic content determination in food samples,” Anal. Chem. 91 (16 ), 10631–10639 (2019). 10.1021/acs.analchem.9b01862 31311265
163. H. Shibata, Y. Hiruta, and D. Citterio, “Fully inkjet-printed distance-based paper microfluidic devices for colorimetric calcium determination using ion-selective optodes,” Analyst 144 (4 ), 1178–1186 (2019). 10.1039/C8AN02146E 30560965
164. M. Younas et al. , “Parametric analysis of wax printing technique for fabricating microfluidic paper-based analytic devices (μPAD) for milk adulteration analysis,” Microfluid. Nanofluid. 23 (3 (2019). 10.1007/s10404-019-2208-z
165. C. Chen et al. , “Novel wax valves to improve distance-based analyte detection in paper microfluidics,” Anal. Chem. 91 (8 ), 5169–5175 (2019). 10.1021/acs.analchem.8b05764 30869869
166. A. K. Govindarajalu et al. , “A cellulosic paper-based sensor for detection of starch contamination in milk,” Bull. Mater. Sci. 42 (6 (2019). 10.1007/s12034-019-1958-2
167. S. Teepoo, S. Arsawiset, and P. Chanayota, “One-step polylactic acid screen-printing microfluidic paper-based analytical device: Application for simultaneous detection of nitrite and nitrate in food samples,” Chemosensors 7 (3 ), 44 (2019). 10.3390/chemosensors7030044
168. A. Kumar and V. Santhanam, “Paper swab based SERS detection of non-permitted colourants from dals and vegetables using a portable spectrometer,” Anal. Chim. Acta 1090 , 106–113 (2019). 10.1016/j.aca.2019.08.073 31655635
169. S. Shariati and G. Khayatian, “Microfluidic paper-based analytical device using gold nanoparticles modified with N,N′-bis(2-hydroxyethyl)dithiooxamide for detection of Hg(II) in air, fish and water samples,” New J. Chem. 44 (43 ), 18662–18667 (2020). 10.1039/D0NJ03986A
170. T. Thongkam and K. Hemavibool, “An environmentally friendly microfluidic paper-based analytical device for simultaneous colorimetric detection of nitrite and nitrate in food products,” Microchem. J. 159 (2020). 10.1016/j.microc.2020.105412
171. N. Ratnarathorn and W. Dungchai, “Paper-based analytical device (PAD) for the determination of borax, salicylic acid, nitrite, and nitrate by colorimetric methods,” J. Anal. Chem. 75 (4 ), 487–494 (2020). 10.1134/S1061934820040127
172. A. M. Yehia, M. A. Farag, and M. A. Tantawy, “A novel trimodal system on a paper-based microfluidic device for onsite detection of the date rape drug “ketamine,”” Anal. Chim. Acta 1104 , 95–104 (2020). 10.1016/j.aca.2020.01.002 32106962
173. F. M. Gharaghani, M. Akhond, and B. Hemmateenejad, “A three-dimensional origami microfluidic device for paper chromatography: Application to quantification of tartrazine and indigo carmine in food samples,” J. Chromatogr. A 1621 , 461049 (2020). 10.1016/j.chroma.2020.461049 32252986
174. L. L. Jin et al. , “A facile microfluidic paper-based analytical device for acetylcholinesterase inhibition assay utilizing organic solvent extraction in rapid detection of pesticide residues in food,” Anal. Chim. Acta 1100 , 215–224 (2020). 10.1016/j.aca.2019.11.067 31987143
175. E. Trofimchuk et al. , “Determination of norfloxacin residues in foods by exploiting the coffee-ring effect and paper-based microfluidics device coupling with smartphone-based detection,” J. Food Sci. 85 (3 ), 736–743 (2020). 10.1111/1750-3841.15039 32017096
176. E. Trofimchuk et al. , “Development of paper-based microfluidic device for the determination of nitrite in meat,” Food Chem. 316 (2020). 10.1016/j.foodchem.2020.126396
177. W. Li et al. , “A novel microfluidic paper-based analytical device based on chemiluminescence for the determination of β-agonists in swine hair,” Anal. Methods 12 (18 ), 2317–2322 (2020). 10.1039/C9AY02754H 32930256
178. K. Phoonsawat and W. Dungchai, “Highly sensitive, selective and naked-eye detection of bromide and bromate using distance based paper analytical device,” Talanta 221 , 121590 (2021). 10.1016/j.talanta.2020.121590 33076125
179. J. Mettakoonpitak et al. , “Simple biodegradable plastic screen-printing for microfluidic paper-based analytical devices,” Sens. Actuators, B 331 , 129463 (2021). 10.1016/j.snb.2021.129463
180. J. L. Han et al. , “A ZnFe O-catalyzed segment imprinted polymer on a three-dimensional origami paper-based microfluidic chip for the detection of microcystin,” Analyst 147 (6 ), 1060–1065 (2022). 10.1039/D2AN00032F 35191458
181. H. Martínez-Pérez-Cejuela et al. , “Combining microfluidic paper-based platform and metal-organic frameworks in a single device for phenolic content assessment in fruits,” Microchim. Acta 190 (4 (2023). 10.1007/s00604-023-05702-5
182. S. Masoomi, H. Sharifi, and B. Hemmateenejad, “A paper-based optical tongue for characterization of Iranian honey: Identification of geographical/botanical origins and adulteration detection,” Food Control 155 (2024). 10.1016/j.foodcont.2023.110052
183. Y. Wu et al. , “A surface molecularly imprinted microfluidic paper based device with smartphone assisted colorimetric detection for butachlor in mung bean,” Food Chem. 435 , 137659 (2024). 10.1016/j.foodchem.2023.137659 37816277
184. R. Lucas, “Ueber das zeitgesetz des kapillaren aufstiegs von flüssigkeiten,” Kolloid-Z. 23 (1 ), 15–22 (1918). 10.1007/BF01461107
185. E. W. Washburn, “The dynamics of capillary flow,” Phys. Rev. 17 (3 ), 273–283 (1921). 10.1103/PhysRev.17.273
186. E. Fu et al. , “Transport in two-dimensional paper networks,” Microfluid. Nanofluid. 10 (1 ), 29–35 (2011). 10.1007/s10404-010-0643-y 22140373
187. Z. Liu et al. , “Experimental and numerical studies on liquid wicking into filter papers for paper-based diagnostics,” Appl. Therm. Eng. 88 , 280–287 (2015). 10.1016/j.applthermaleng.2014.09.057
188. S. Jahanshahi-Anbuhi et al. , “Creating fast flow channels in paper fluidic devices to control timing of sequential reactions,” Lab Chip 12 (23 ), 5079–5085 (2012). 10.1039/c2lc41005b 23079674
189. E. A. Phillips et al. , “Thermally actuated wax valves for paper-fluidic diagnostics,” Lab Chip 16 (21 ), 4230–4236 (2016). 10.1039/C6LC00945J 27722697
190. T. Kong et al. , “A fast, reconfigurable flow switch for paper microfluidics based on selective wetting of folded paper actuator strips,” Lab Chip 17 (21 ), 3621–3633 (2017). 10.1039/C7LC00620A 28945259
191. B. Li et al. , “Controlling capillary-driven fluid transport in paper-based microfluidic devices using a movable valve,” Anal. Chem. 89 (11 ), 5707–5712 (2017). 10.1021/acs.analchem.7b00726 28474516
192. J. Han et al. , “Simple way To fabricate novel paper-based valves using plastic comb binding spines,” ACS Sens. 3 (9 ), 1789–1794 (2018). 10.1021/acssensors.8b00518 30080022
193. J. Qi et al. , “Rotational paper-based microfluidic-chip device for multiplexed and simultaneous fluorescence detection of phenolic pollutants based on a molecular-imprinting technique,” Anal. Chem. 90 (20 ), 11827–11834 (2018). 10.1021/acs.analchem.8b01291 30136577
194. B. J. Toley et al. , “Tunable-delay shunts for paper microfluidic devices,” Anal. Chem. 85 (23 ), 11545–11552 (2013). 10.1021/ac4030939 24245747
195. B. J. Toley et al. , “A versatile valving toolkit for automating fluidic operations in paper microfluidic devices,” Lab Chip 15 (6 ), 1432–1444 (2015). 10.1039/C4LC01155D 25606810
196. M. Fratzl et al. , “Magnetic two-way valves for paper-based capillary-driven microfluidic devices,” ACS Omega 3 (2 ), 2049–2057 (2018). 10.1021/acsomega.7b01839 31458514
197. J. Houghtaling et al. , “Dissolvable bridges for manipulating fluid volumes in paper networks,” Anal. Chem. 85 (23 ), 11201–11204 (2013). 10.1021/ac4022677 24228812
198. K. M. Schilling et al. , “Fully enclosed microfluidic paper-based analytical devices,” Anal. Chem. 84 (3 ), 1579–1585 (2012). 10.1021/ac202837s 22229653
199. H. Noh and S. T. Phillips, “Fluidic timers for time-dependent, point-of-care assays on paper,” Anal. Chem. 82 (19 ), 8071–8078 (2010). 10.1021/ac1005537 20809563
200. H. Chen et al. , “A fluidic diode, valves, and a sequential-loading circuit fabricated on layered paper,” Lab Chip 12 (16 ), 2909–2913 (2012). 10.1039/c2lc20970e 22699228
201. E. B. Strong et al. , “Wax-printed fluidic time delays for automating multi-step assays in paper-based microfluidic devices (MicroPADs),” Inventions 4 , 20 (2019). 10.3390/inventions4010020
202. E. Fu et al. , “Controlled reagent transport in disposable 2D paper networks,” Lab Chip 10 (7 ), 918–920 (2010). 10.1039/b919614e 20300678
203. A. Apilux et al. , “Development of automated paper-based devices for sequential multistep sandwich enzyme-linked immunosorbent assays using inkjet printing,” Lab Chip 13 (1 ), 126–135 (2013). 10.1039/C2LC40690J 23165591
204. J. H. Shin et al. , “Programmed sample delivery on a pressurized paper,” Biomicrofluidics 8 (5 ), 054121 (2014). 10.1063/1.4899773 25584116
205. G. Dai et al. , “A colorimetric paper sensor for lactate assay using a cellulose-binding recombinant enzyme,” Sens. Actuators, B 238 , 138–144 (2017). 10.1016/j.snb.2016.07.008
206. K. Yang et al. , “Novel developments in mobile sensing based on the integration of microfluidic devices and smartphones,” Lab Chip 16 (6 ), 943–958 (2016). 10.1039/C5LC01524C 26899264
207. S. Chaiyo et al. , “High sensitivity and specificity simultaneous determination of lead, cadmium and copper using μPAD with dual electrochemical and colorimetric detection,” Sens. Actuators, B 233 , 540–549 (2016). 10.1016/j.snb.2016.04.109
208. G. G. Morbioli et al. , “Technical aspects and challenges of colorimetric detection with microfluidic paper-based analytical devices (μPADs)—A review,” Anal. Chim. Acta 970 , 1–22 (2017). 10.1016/j.aca.2017.03.037 28433054
209. S. Burnham et al. , “Towards rapid on-site phage-mediated detection of generic Escherichia coli in water using luminescent and visual readout,” Anal. Bioanal. Chem. 406 (23 ), 5685–5693 (2014). 10.1007/s00216-014-7985-3 24969469
210. S.-Q. Jin et al. , “A cost-effective Z-folding controlled liquid handling microfluidic paper analysis device for pathogen detection via ATP quantification,” Biosens Bioelectron. 63 , 379–383 (2015). 10.1016/j.bios.2014.07.070 25127472
211. Q. He et al. , “Method for fabrication of paper-based microfluidic devices by alkylsilane self-assembling and UV/O3-patterning,” Anal. Chem. 85 (3 ), 1327–1331 (2013). 10.1021/ac303138x 23244032
212. D. Tsikas, “Analysis of nitrite and nitrate in biological fluids by assays based on the Griess reaction: Appraisal of the Griess reaction in the l-arginine/nitric oxide area of research,” J. Chromatogr. B 851 (1 ), 51–70 (2007). 10.1016/j.jchromb.2006.07.054
213. M. E. I. Badawy and A. F. El-Aswad, “Bioactive paper sensor based on the acetylcholinesterase for the rapid detection of organophosphate and carbamate pesticides,” Int. J. Anal. Chem. 2014 , 536823 (2014). 10.1155/2014/536823 25484901
214. M. Salve et al. , “Quantifying colorimetric assays in μPAD for milk adulterants detection using colorimetric android application,” Micro Nano Lett. 13 (11 ), 1520–1524 (2018). 10.1049/mnl.2018.5334
215. P. Nath, R. K. Arun, and N. Chanda, “Smart gold nanosensor for easy sensing of lead and copper ions in solution and using paper strips,” RSC Adv. 5 (84 ), 69024–69031 (2015). 10.1039/C5RA14886C
216. A. Apilux et al. , “Simple and rapid colorimetric detection of Hg(II) by a paper-based device using silver nanoplates,” Talanta 97 , 388–394 (2012). 10.1016/j.talanta.2012.04.050 22841097
217. D. A. Bruzewicz, M. Reches, and G. M. Whitesides, “Low-cost printing of poly(dimethylsiloxane) barriers to define microchannels in paper,” Anal. Chem. 80 (9 ), 3387–3392 (2008). 10.1021/ac702605a 18333627
218. W. Dungchai, O. Chailapakul, and C. S. Henry, “Electrochemical detection for paper-based microfluidics,” Anal. Chem. 81 (14 ), 5821–5826 (2009). 10.1021/ac9007573 19485415
219. J. A. Adkins and C. S. Henry, “Electrochemical detection in paper-based analytical devices using microwire electrodes,” Anal. Chim. Acta 891 , 247–254 (2015). 10.1016/j.aca.2015.07.019 26388383
220. Z. Li et al. , “Direct writing electrodes using a ball pen for paper-based point-of-care testing,” Analyst 140 (16 ), 5526–5535 (2015). 10.1039/C5AN00620A 26079757
221. J. Narang et al. , “Point of care with micro fluidic paper based device integrated with nano zeolite–graphene oxide nanoflakes for electrochemical sensing of ketamine,” Biosens. Bioelectron. 88 , 249–257 (2017). 10.1016/j.bios.2016.08.043 27570055
222. A. K. Yetisen, M. S. Akram, and C. R. Lowe, “Paper-based microfluidic point-of-care diagnostic devices,” Lab Chip 13 (12 ), 2210–2251 (2013). 10.1039/c3lc50169h 23652632
223. M. Luo et al. , “A paper-based plasma-assisted cataluminescence sensor for ethylene detection,” Sens. Actuators, B 240 , 132–141 (2017). 10.1016/j.snb.2016.08.156
224. J. Yu et al. , “Microfluidic paper-based chemiluminescence biosensor for simultaneous determination of glucose and uric acid,” Lab Chip 11 (7 ), 1286–1291 (2011). 10.1039/c0lc00524j 21243159
225. W. Zheng et al. , “Strategies for the detection of target analytes using microfluidic paper-based analytical devices,” Anal. Bioanal. Chem. 413 (9 ), 2429–2445 (2021). 10.1007/s00216-021-03213-x 33712916
226. F. Li et al. , “High-resolution temporally resolved chemiluminescence based on double-layered 3D microfluidic paper-based device for multiplexed analysis,” Biosens. Bioelectron. 141 , 111472 (2019). 10.1016/j.bios.2019.111472 31272061
227. S. Wang et al. , “Molecularly imprinted polymer grafted paper-based multi-disk micro-disk plate for chemiluminescence detection of pesticide,” Biosens. Bioelectron. 50 , 262–268 (2013). 10.1016/j.bios.2013.07.003 23871875
228. J. Hassanzadeh, H. A. J. Al Lawati, and I. Al Lawati, “Metal–organic framework loaded by rhodamine B As a novel chemiluminescence system for the paper-based analytical devices and Its application for total phenolic content determination in food samples,” Anal. Chem. 91 (16 ), 10631–10639 (2019). 10.1021/acs.analchem.9b01862 31311265
229. Y. Zhang, P. Zuo, and B.-C. Ye, “A low-cost and simple paper-based microfluidic device for simultaneous multiplex determination of different types of chemical contaminants in food,” Biosens. Bioelectron. 68 , 14–19 (2015). 10.1016/j.bios.2014.12.042 25558869
230. L. Zong et al. , “A new red fluorescent probe for Hg2+ based on naphthalene diimide and its application in living cells, reversibility on strip papers,” Sens. Actuators, B 238 , 735–743 (2017). 10.1016/j.snb.2016.07.052
231. J. Prabpal, T. Vilaivan, and T. Praneenararat, “Paper-based heavy metal sensors from the concise synthesis of an anionic porphyrin: A practical application of organic synthesis to environmental chemistry,” J. Chem. Educ. 94 (8 ), 1137–1142 (2017). 10.1021/acs.jchemed.6b00943
232. J. Qi et al. , “Three-dimensional paper-based microfluidic chip device for multiplexed fluorescence detection of Cu2+ and Hg2+ ions based on ion imprinting technology,” Sens. Actuators, B 251 , 224–233 (2017). 10.1016/j.snb.2017.05.052
233. X. Weng and S. Neethirajan, “Paper-based microfluidic aptasensor for food safety,” J. Food Saf. 38 (1 ), e12412 (2018). 10.1111/jfs.12412
234. M. M. Ali et al. , “A printed multicomponent paper sensor for bacterial detection,” Sci. Rep. 7 (1 ), 12335 (2017). 10.1038/s41598-017-12549-3 28951563
235. P. Zuo et al. , “A PDMS/paper/glass hybrid microfluidic biochip integrated with aptamer-functionalized graphene oxide nano-biosensors for one-step multiplexed pathogen detection,” Lab Chip 13 (19 ), 3921–3928 (2013). 10.1039/c3lc50654a 23929394
236. R. Pelton, “Bioactive paper provides a low-cost platform for diagnostics,” TrAC Trends Anal. Chem. 28 (8 ), 925–942 (2009). 10.1016/j.trac.2009.05.005
237. P. Mulvaney, “Surface plasmon spectroscopy of nanosized metal particles,” Langmuir 12 (3 ), 788–800 (1996). 10.1021/la9502711
238. H. Kudo et al. , “Paper-Based analytical device for zinc Ion quantification in water samples with power-free analyte concentration,” Micromachines 8 , 127 (2017). 10.3390/mi8040127
239. W. Chen et al. , “A simple paper-based colorimetric device for rapid mercury(II) assay,” Sci. Rep. 6 (1 ), 31948 (2016). 10.1038/srep31948 27554633
240. L. Yao et al. , “Paper matrix based array for rapid and sensitive optical detection of mercury ions using silver enhancement,” Microchim. Acta 184 (2 ), 569–576 (2017). 10.1007/s00604-016-2052-8
241. K. Chaiendoo, T. Tuntulani, and W. Ngeontae, “A paper-based ferrous ion sensor fabricated from an ion exchange polymeric membrane coated on a silver nanocluster-impregnated filter paper,” Mater. Chem. Phys. 199 , 272–279 (2017). 10.1016/j.matchemphys.2017.07.013
242. F. Figueredo et al. , “Enhanced analytical performance of paper microfluidic devices by using Fe3O4 nanoparticles, MWCNT, and graphene oxide,” ACS Appl. Mater. Interfaces 8 (1 ), 11–15 (2016). 10.1021/acsami.5b10027 26693736
243. S. Kumar, P. Bhushan, and S. Bhattacharya, “Facile synthesis of Au@Ag–hemin decorated reduced graphene oxide sheets: A novel peroxidase mimetic for ultrasensitive colorimetric detection of hydrogen peroxide and glucose,” RSC Adv. 7 (60 ), 37568–37577 (2017). 10.1039/C7RA06973A
244. E. Núnez-Bajo et al. , “In situ gold-nanoparticle electrogeneration on gold films deposited on paper for non-enzymatic electrochemical determination of glucose,” Talanta 178 , 160–165 (2018). 10.1016/j.talanta.2017.08.104 29136807
245. A. Nilghaz et al. , “Paper-based microfluidics for food safety and quality analysis,” Trends Food Sci. Technol. 118 , 273–284 (2021). 10.1016/j.tifs.2021.08.029
246. T. Vo-Dinh, “Surface-enhanced Raman spectroscopy using metallic nanostructures1The submitted manuscript has been authored by a contractor of the U.S government under contract No.: DE-AC05-96OR22464. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes.1,” TrAC Trends Anal. Chem. 17 (8 ), 557–582 (1998). 10.1016/S0165-9936(98)00069-7
247. M. Fan, G. F. S. Andrade, and A. G. Brolo, “A review on recent advances in the applications of surface-enhanced Raman scattering in analytical chemistry,” Anal. Chim. Acta 1097 , 1–29 (2020). 10.1016/j.aca.2019.11.049 31910948
248. Y. Zhu, L. Zhang, and L. Yang, “Designing of the functional paper-based surface-enhanced Raman spectroscopy substrates for colorants detection,” Mater. Res. Bull. 63 , 199–204 (2015). 10.1016/j.materresbull.2014.12.004
249. X. Sha et al. , “A rapid detection method for On-site screening of estazolam in beverages with Au@Ag core-shell nanoparticles paper-based SERS substrate,” Anal. Sci. 36 (6 ), 667–671 (2020). 10.2116/analsci.19P361 31787667
250. Y. Ma et al. , “Rapid and sensitive on-site detection of pesticide residues in fruits and vegetables using screen-printed paper-based SERS swabs,” Anal. Methods 10 (38 ), 4655–4664 (2018). 10.1039/C8AY01698D
251. L. Tian et al. , “Bacterial nanocellulose-based flexible surface enhanced Raman scattering substrate,” Adv. Mater. Interfaces 3 (15 ), 1600214 (2016). 10.1002/admi.201600214
252. S. Jafari et al. , “ASSURED point-of-need food safety screening: A critical assessment of portable food analyzers,” Foods 10 (6 ), 1399 (2021). 10.3390/foods10061399 34204284
253. N. Tamanna and N. Mahmood, “Food processing and Maillard reaction products: Effect on human health and nutrition,” Int. J. Food Sci. 2015 , 526762 (2015). 10.1155/2015/526762 26904661
254. M. Z. Hua et al. , “Detecting chemical hazards in foods using microfluidic paper-based analytical devices (μPADs): The real-world application,” Micromachines (Basel) 9 (1 (2018).
255. Elabsciences, Food safety kits (2024), see https://www.elabscience.com/Products-food_safety_kits-174.html.
256. R-Biopharm, R-Biopharm dedicated to food safety (2024), see https://food.r-biopharm.com/.
257. Merck, Lateral Flow Tests for Immunological Detection in Food and Environmental Samples (2024), see https://www.sigmaaldrich.com/AE/en/technical-documents/technical-article/microbiological-testing/pathogen-and-spoilage-testing/lateral-flow-tests-for-immunological-detection.
258. Amas, Australasian Medical and Scientific Ltd. (2024), see https://www.amsl.com.au/salmonella-vip-gold/.
259. PaperdopDx, Rapid diagnostic tests based on paper microfluidics (2024).
260. USDA, I-Corps: Paper-based microfluidic viral diagnostic device project (2017), see https://www.nal.usda.gov/research-tools/food-safety-research-projects/i-corps-paper-based-microfluidic-viral-diagnostic.
