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A portable lateral flow distance-based paper sensor for drinking water hardness test
A paper sensor for drinking water hardness test
https://orcid.org/0000-0003-1801-9351
Liu Yulin Formal analysis Funding acquisition Investigation Visualization Writing – original draft 1 2
Dong Longzhan Formal analysis Investigation 2
Wu Wenli Formal analysis Funding acquisition Writing – review & editing 1
https://orcid.org/0000-0003-3175-6946
Ping Jiantao Formal analysis Funding acquisition Writing – review & editing 1
Chen Jingbo Formal analysis Supervision Writing – original draft 2 *
Hu Qiongzheng Conceptualization Formal analysis Funding acquisition Methodology Project administration Supervision Writing – review & editing 1 *
1 Shandong Analysis and Test Center, Qilu University of Technology (Shandong Academy of Sciences), Jinan, China
2 Department of General Surgery, The First Affiliated Hospital of Shandong First Medical University, Jinan, China
Sauli Elingarami Editor
Nelson Mandela African Institute of Science and Technology, UNITED REPUBLIC OF TANZANIA
Competing Interests: The authors have declared that no competing interests exist.

* E-mail: huqz@qlu.edu.cn (QH); qychenjingbo@163.com (JC)
6 9 2024
2024
19 9 e030842430 1 2024
23 7 2024
© 2024 Liu et al
2024
Liu et al
https://creativecommons.org/licenses/by/4.0/ This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Hardness is one of the basic parameters of water, and a high-level hardness of drinking water may be harmful to human health. Thus, it is very important to monitor drinking water hardness. In this work, a portable lateral flow distance-based paper sensor for the semi-quantitative detection of drinking water hardness is demonstrated. In the presence of Ca2+/Mg2+, the hydrogel can be formed via the chelation between sodium alginate and Ca2+/Mg2+, inducing a phase separation process. The viscosity change of the sodium alginate solution is directly related to the Ca2+/Mg2+ concentration and can be determined by the water lateral flow distance on test strips. The sensor successfully realizes the quantification of Ca2+ and Mg2+ in the range of 0–10 mmol L-1 and 4–20 mmol L-1, respectively. The recoveries are found varied from 95% to 108.9%. The water hardness is acceptable for drinking if the Cr values lies in the range of 0.259 to 0.419, and it is high with the Cr value above 0.595. Remarkably, the performance of the sensor is comparable with the commercial kit for real water samples, which avoids the subjective judgment. Overall, this method provides a portable approach for semi-quantitative detection of drinking water hardness with the merits of convenience and low cost, which shows great potential for the potential application.

http://dx.doi.org/10.13039/501100007129 Natural Science Foundation of Shandong Province ZR2021QH106 https://orcid.org/0000-0003-1801-9351
Liu Yulin http://dx.doi.org/10.13039/501100007129 Natural Science Foundation of Shandong Province ZR2020QB153 Wu Wenli http://dx.doi.org/10.13039/501100007129 Natural Science Foundation of Shandong Province ZR2022YQ123 Hu Qiongzheng http://dx.doi.org/10.13039/501100010029 Taishan Scholar Foundation of Shandong Province tsqn201812088 Hu Qiongzheng Education and Industry Integration Pilot Project of Qilu University of Technology 2023PY058 Hu Qiongzheng Education and Industry Integration Pilot Project of Qilu University of Technology 2022PY036 https://orcid.org/0000-0003-3175-6946
Ping Jiantao Natural Science Foundation of Shandong Province (ZR2021QH106, ZR2020QB153, ZR2022YQ123); the Taishan Scholars Program (tsqn201812088); the Education and Industry Integration Pilot Project of Qilu University of Technology (2023PY058, 2022PY036) The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Data AvailabilityAll relevant data are within the manuscript and its Supporting Information files.
Data Availability

All relevant data are within the manuscript and its Supporting Information files.
==== Body
pmcIntroduction

Water is an indispensable natural resource for human beings. Drinking water quality is vital to human health. Water hardness is one of the basic parameters for evaluating water quality, and a high-level drinking water hardness may cause diseases [1, 2]. Water hardness is generally referred to the sum content of calcium, magnesium, iron, aluminum, zinc and other ions contained in water, and usually calculated by Ca2+ and Mg2+ contents considering the lower concentrations of other ions, which is also called as Ca2+ hardness. Although the Ca2+ concentration in drinking water is not strictly defined, the World Health Organization recommends that calcium ion level in drinking water should be no more than 5 mmol L-1 [3, 4]. Therefore, it is very important to detect Ca2+ hardness in drinking water samples.

Until now, several methods for monitoring Ca2+ have been reported despite its difficulty to be differentiated from other interfering ions. Among them, atomic absorption spectrometry and complexometric titration are the classic methods used for calcium quantification [5, 6]. However, they often suffer from the disadvantages such as complex procedures, bulky sample requirement, complicated instrumentation, and trained operators. For instance, the commercial colorimetric kit has already been accessible, which is developed based on the titration method. However, a large amount sample of 10 mL was generally required. Meanwhile, the color change of titration terminal is subjectively judged by operators, which may inevitably lead to systematic errors. Additionally, the fluorescence method is available for the simplified detection process with high sensitivity [7–9], but it still requires the usage of large-scale instrument and trained operators. Ionophore-based ion-selective optode also have been used for the colorimetric detection of Ca2+ [10, 11]. Nevertheless, the results are greatly influenced by the pH of the samples, which hampers their broad application. Thus, it is greatly demanded to construct a portable sensor for monitoring Ca2+ in water.

Paper-based detection methods have become appealing in recent years, with the merits of convenient operation, fast response, and easy modification. They are extensively applied in clinical diagnostics, food quality management, and environmental monitoring [12–15]. In particular, the distance-based paper sensor can quantify the analyte by measuring the change of water flow distance, which has shown great application prospect as point-of-care test, because of its advantages of simple portability, visualization, easy quantification, and short analysis time [16–18]. The pH indicator papers can be employed as test strips because they are cheap and can clearly show the water flow marks. In addition, analyte-responsive hydrogels are series of polymers with three dimensional network structures. Various hydrogels have been designed and developed for biosensing of metal ions [19, 20], nucleic acids, proteins [21], and microorganisms [22, 23]. With the specific experimental design, hydrogels can respond to analytes and induce physical or chemical changes that generate subsequent readable signals. Among these methods, the gel-sol transition triggered by external stimuli is a most commonly used detection principle [23–25]. Therefore, it possesses great potential for the exploration of the distance-based lateral flow sensor using stimuli-responsive polymers for the evaluation of drinking water hardness.

Herein, a portable lateral flow sensor with the distance readout signal for the semi-quantitative determination of drinking water hardness on the paper strip was developed (Scheme 1). In the presence of Ca2+/Mg2+, the sodium alginate (Alg) hydrogel network with “egg-box” structure can be formed during the phase separation process. Correspondingly, the viscosity of the Alg solution drops sharply due to reduction of the Alg concentration. Thus, the concentration of Ca2+/Mg2+ can be determined via the measurement of water lateral flow distance on paper strips of the residual Alg solution. This method offers a simple and convenient method for the water Ca2+ hardness evaluation using a small amount of samples with satisfacoty accuracy, which also avoids the subjective color endpoint judgment.

10.1371/journal.pone.0308424.g001 Scheme 1 The principle of the portable distance readout paper sensor for evaluating drinking water hardness.

Experimental

Materials

Sodium alginate (CP, viscosity 200 ± 20 mPa·s), (NH4)2S2O8 (AR, 98.1%), and NaHSO3 (AR, 99.99%) were obtained from Macklin. Hydrochloric acid (AR) for pH adjustment was obtained from Sinopharm Group. NaH2PO4 (AR, 99.0%) and Na2HPO4 (AR, 99%) were purchased by Aladdin Technology Co., Ltd., China. Water hardness test kit was bought from Lohand Biological. Co., Ltd. The pH strips with the dimension of 60 mm × 5 mm (length × width) were used in this investigation. The PVC plates were cleaned by ethanol thoroughly and dried before use. Tris-HCl buffer (pH = 7.4) was offered from Sangon Biotech Co., Ltd., China.

Optimization of the sodium alginate concentration

Tris-HCl buffer (100 mmol L-1, pH = 7.4) was used in this study. The Alg solutions with different concentrations from 0.1 wt% to 0.5 wt% were firstly prepared at 25°C. The solutions of CaCl2 with various concentrations of 0, 2, 4, 10, 20 mmol L-1 were then obtained. According to our previous study, the mixture of CaCl2 and Alg solution was incubated at 25°C after vortex for 30s [26]. Then, 30 μL of the supernatant solution obtained by centrifuging for 1 min was transferred onto the left side of the test strip. After waiting for 2 min, the images of the paper sensor were captured by smartphone and analyzed by Adobe Photoshop software. All of the experiments were conducted at least three times to obtain the standard deviations.

Determination of the water hardness using the commercial test kit

Firstly, 10 mL of the test solution was accurately pipetted into the cleaned conical flask, followed by the addition of a package of total hardness reagent I. If the solution color is pure blue after reagent I dissolved, the hardness value of the water sample is 0 mg/L. If the solution exhibits purple red, hardness reagent II aqueous solution was then vertically added with the continuous shaking of the conical flask. The dropping speed should be controlled as 1 drop every 3 s until the solution changed from purple red to pure blue. Then, the number of drops consumed (N) was recorded and hardness (mg L-1, calculated as CaCO3, 1 mg L-1 = 0.01 mmol L-1) was calculated by the following equation, the detection range of this kit is 30–600 mg L-1: Hardness=N×30

Data analysis

The pH indicator papers were photographed with a smartphone. Then, the pixel values of areas including water marked and whole test strips were obtained by Adobe Photoshop and recorded as Pmark and Ptotal, respectively. Finally, the ratio of Pmark and Ptotal was calculated as the water trace coverage ratio (Cr) as follows: Cr=Pmark/Ptotal

Results and discussion

Feasibility of the paper sensor for the evaluation of Ca2+/Mg2+

The responses of the paper sensor in different conditions were recorded. As shown in Fig 1, the flow distance of the Ca2+ solution was obviously longer than that of the 0.2 wt% Alg solution. The Cr values were 0.8 and 0.5, respectively. A similar phenomenon also occurred for the Mg2+ solution (S1 Fig). The difference was mainly ascribed to the viscosity discrepancy caused by Alg itself. Upon the Alg solution was incubation with the 10 mmol L-1 Ca2+ solution, the Cr value increased to 0.65 with the longer water flow distance as shown in Fig 1A and 1B. The solution viscosity was increased obviously in the existence of Alginate compared with that of Ca2+ and Mg2+ existing alone in Fig 1C. These results are consistent with the proposed principle above. The viscosity decrease of the Alg solution was caused by the consumption of Alg via cross-linking with Ca2+/Mg2+ during the formation of the hydrogel [26–29]. Overall, this method provides an effective means to monitor Ca2+/Mg2+, which is candidate for the drinking water hardness determination.

10.1371/journal.pone.0308424.g002 Fig 1 Feasibility of the paper sensor for the evaluation of Ca2+/Mg2+.

(A) The photopgraphs and (B) the Cr values of the paper sensor in the Ca2+ solution, the Alg solution, and the mixture of Alg and Ca2+, respectively. The concentrations of Ca2+ and Alg are 10 mmol L-1 and 0.2 wt%, respectively. (C) The viscosities of Ca2+, Mg2+, Alg/ Ca2+ and Alg/Mg2+ solution, respectively.

Optimization of condition for the paper sensor

The performance of Alg solutions at different concentrations on pH test strips was studied. As shown in Fig 2A and S2 Fig, with the concentration of Alg solution elevating from 0.1 wt% to 0.5 wt%, the water flow distance descended obviously due to the increasing viscosity. The concentration of Ca2+ from 0 to 20 mmol L-1 was evaluated at a fixed Alg concentration of 0.1 wt%. Surprisingly, the water flow distance becomes shorter with the concentration increase of Alg from 0–4 mmol L-1, while further distance increases with the Alg concentration from 4 to 20 mmol L-1 (Fig 2B and S3 Fig). Moreover, the similar variation tendency was found for the solution and the mixture of Alg and Mg2+ (S1 Fig), respectively. The concentrations of Ca2+ and Alg were 10 mmol L-1 and 0.2 wt%, respectively. Moreover, the similar variation tendency was observed with different concentrations of Alg (Fig 2C–2F). The results are reasonable considering the crosslinking process. At the initial stage, the addition of Ca2+ into Alg solution can cause the rapid formation of the hydrogel based on the chelation between them, leading to the increased viscosity. While the continued increase of the Ca2+ concentration contributed to increase of the cross-linking degree of the hydrogel, which was separated from the aqueous phase. As only the remainly Alg was in the aqueous phase, the viscosity is decreased with further increase of the Ca2+ concentration. Thus, the viscosity change can be coupled to the concentration change of Ca2+. As concluded from Fig 2, the fixed 0.2 wt% Alg solution was chosen for following experiments, because it can clearly distinguish different concentrations of Ca2+.

10.1371/journal.pone.0308424.g003 Fig 2 Optimization of conditions for the paper sensor.

(A) The Cr values of the paper sensor responses towards the Alg solutions with different concentrations. (B)-(F) The responses of the paper sensor towards different concentrations of Alg with different concentrations of Ca2+ (0, 2, 4, 10 and 20 mmol L-1).

Detection of Ca2+ and Mg2+

Inspired by the excellent performance of the paper sensor above, the study investigating determination of the concentration of Ca2+ and Mg2+ was also conducted. Based on the results in Fig 2, the Cr value droped at the Ca2+ concentration from 0–4 mmol L-1 and increased at the Ca2+ concentration from 4–20 mmol L-1, which makes it challenging for the direct quantitative detection of Ca2+. Thus, 4 mmol L-1 Ca2+ was initially added directly into the test sample to address this issue. In this way, the concentration of Ca2+ can be quantitatively determined. As shown in Fig 3A, the water flow distance increased complying with the progressive concentration increase in the range of 4–14 mmol L-1. Thus, the function between Cr value and the Ca2+ concentration from 4–14 mmol L-1 was plotted as Fig 3B. A satisfactory linear relationship was obtained with R2 as 0.990. With the initially added 4 mmol L-1 Ca2+ case, the function exhibited the same trendency in the range of 0–10 mmol L-1 (Fig 3C, 3D). Besides, the responses of the paper sensor to Mg2+ were also studied and the pictures were showed in S3 Fig. The Cr value exhibited decreasing trendy accompanied by the Mg2+ concentration rising from 0 to 4 mmol L-1. Following, the Cr values increased from 4–16 mmol L-1 and reached a plateau until the concentration of Mg2+ increased to 20 mmol L-1 (Fig 3E and 3F). A linear relationship was also plotted with R2 calculated to be 0.993. The results showed the potential of the paper sensor for Ca2+ and Mg2+ hardness detection in water.

10.1371/journal.pone.0308424.g004 Fig 3 Detection of Ca2+ and Mg2+.

(A) The images of the sensors and (B) the linear plot between the Cr and Ca2+ concentration from 4–14 mmol L-1. (C) The images of the sensors and (D) the linear plot between the Cr and Ca2+ concentration from 0–10 mmol L-1. (E) The images of the sensors and (F) the linear plot between the Cr and Mg2+ concentration from 4–20 mmol L-1.

The selectivity of the paper sensor

The Ca2+ of 10 mmol L-1 was chosen in this section. The water flow distance remained almost same with different pH values from 5–9 (Fig 4A), which indicated the pH stability of the paper sensor. Subsequently, the influence of ionic strength of the solution was also considered (Fig 4B). The actual drinking water samples rarely contains heavy metal ions. Therefore, common anions were selected for selectivity detection. The results show that ignorable difference was observed at a fixed Ca2+ concentration of 10 mmol L-1 in the presence of NaCl solution (0–200 mmol L-1). The corresponding photograph is shown in S4 Fig. Finally, the potential interfering ions in real samples including NaCl, KCl, NaH2PO4, NaH2PO4, (NH4)2S2O8, Na2S2O3, NaHSO3 and NaHCO3 were mixed with Ca2+. As shown in Fig 4C and 4D, the water flow distances exhibited no obvious alternation. All of these results suggest the satisfactory selectivity of the paper sensor.

10.1371/journal.pone.0308424.g005 Fig 4 The selectivity of the paper sensor.

(A) The Cr values responses for the mixture solution of Alg and Ca2+ with different pH values, (B) for aqueous solutions with NaCl solutions (0 to 200 mmol L-1) and Ca2+, (C) for aqueous solutions with other ions and Ca2+, and (D) the corresponding photohraphs, respectively. The concentration of each ion was 10 mmol L-1.

The semi-quantitative detection of water hardness using the paper sensor

The World Health Organization recommends that the Ca2+ level in drinkable water should be less than 5 mmol L-1. Thus, the corresponding Cr value of drinkable water sample detected by the paper sensor should be in the range of 0.259 to 0.595 in the absence of Mg2+ according to Fig 3B. If only Mg2+ is existing in the tested sample, the concentration of Mg2+ should be below than 12.5 mmol L-1 Mg2+ for drinkable water, which means the same water hardness and calculated by the equation: Waterhardness=Ca2+(gL‐1)×2.5+Mg2+(gL‐1)×4.1.

According to Fig 5, the Cr values should be below 0.419. In short, the water hardness is suitable for drinking if the Cr values lie in the range of 0.259 to 0.419, and unqualified with Cr value above 0.595. Thus, the proposed sensor has been successfully applied for the semi-quantitative detection of water hardness.

10.1371/journal.pone.0308424.g006 Fig 5 The illustration of the semi-quantitative detection of water hardness using the paper sensor.

(A) The Cr values and (B) corresponding images of the paper sensor under different conditions: in the presence of 4 mmol L-1 Ca2+ + 12.5 mmol L-1 Mg2+, 4 mmol L-1 Ca2+ + 5 mmol L-1 Ca2+, and 4mmol L-1 Ca2+. (C) The Cr values ranges illustration of the sensor for the water hardness detection for the drinking water.

Comparison of the paper sensor with the commercial kit

The applicability of the paper sensor was also evaluated in water. The standard addition method was used herein. After 4 mmol L-1 of Ca2+ was initially added into the tested purified water, the additional different concentrations of Ca2+ with 0, 2, 4 and 6 mmol L-1 were spiked respectively. The concentrations of Ca2+ were determined by the paper sensor (S5 Fig), the water hardness kit and inductively coupled plasma-mass spectrometry (ICP-MS), respectively. The recoveries were found varied from 95% to 108.9% and from 105% to 145% for the paper-based sensor and the commercial kit, respectively (Table 1). The results also exhibited the comparable accuracy of the proposed paper sensor with the commercial kit. Test-t analysis was performed between the proposed paper-based method and the ICP-MS method which is the gold standard for metal content measurement. The p values were calculated as 0.317, 0.353, and 0.596 respectively, corresponding to the marked samples with Ca2+ concentrations of 2, 4, 6 mM. No significant difference were observed, which demonstrated the reliability of the paper method. More importantly, only 30 μL of the test sample is required for the paper sensor, which is much less than the amount of the test sample (10 mL) used for the commercial kit. In addition, the comparison of this paper-based sensor with different methods for the detection of Ca2+ and Mg2+ were performed (S1 and S2 Tables), which clearly demonstrated the merits of low-cost, convince, and easily-read for the paper-based sensor. Thus, this method works as an effective means for the quantification of Ca2+ in water.

10.1371/journal.pone.0308424.t001 Table 1 Hardness detection in purified water by the paper-based sensor, commercial kit and ICP-MS.

Method	Additional added Ca2+ (mM)	Found (mM)	Recovery (%)	RSD (%)	
Paper-based sensor	2.0	2.1	108.9	4.4	
4.0	3.8	95.0	7.3	
6.0	5.7	95.0	7.4	
Water hardness kit	2.0	2.9	145.0	12.9	
4.0	5.0	125.0	7.5	
6.0	6.3	105.0	7.8	
ICP-MS	2.0	2.0	100.0	1.5	
4.0	4.1	102.5	8.8	
6.0	5.9	98.3	8.9	
mM, mmol/L; RSD, relative standard deviation; ICP-MS, inductively coupled plasma-mass spectrometry.

Conclusions

In summary, this work reported a portable distance-based lateral flow paper-based sensor for drinking water hardness semi-quantitative detection based on the phase separation induced by the gel-sol transition. Briefly, the chelation between sodium alginate and Ca2+/Mg2+ triggers the formation of hydrogel to cause the viscosity changes of sodium alginate solution. The viscosity change can be quantified by the water flow distance on test strips. This method effectively avoids the usage of a large amount of samples, complicated operation, and high cost, which also exhibits high accuracy performance. Though the paper sensor could not exclude thoroughly the interference produced by other metal ions in some specific water samples, this approach can eliminate the error caused by the subjective color endpoint judgment in tested commercial kit, which shows the great potential of the paper sensor for further application.

Supporting information

S1 Fig The photographs of the paper-based sensors in response to different concentrations of Mg2+ in the presence of 0.2 wt% Alg.

(TIF)

S2 Fig The condition optimization of the sensor.

(A) The performance of the sensor towards different concentrations of sodium alginate alone (0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt% and 0.5 wt%), (B-F) the images of the sensors towards the different concentrations of sodium alginate with different concentrations Ca2+ (0, 2, 4, 10 and 20 mmol L-1).

(TIF)

S3 Fig PThe images of the paper-based sensor in response to different concentrations of Ca2+.

(TIF)

S4 Fig The selectivity evaluation of the paper-based sensor.

(A) The images of the paper-based sensor towards the aqueous solution with different pH value; (B) The images of the paper-based sensor with the coexistence of different concentrations NaCl (0, 50, 100 and 200 mmol L-1) with Ca2+ (10 mmol L-1), respectively.

(TIF)

S5 Fig The images of the paper-based sensors in response to different spiked concentration of Ca2+.

(TIF)

S1 Table Comparison of this work with different methods for the detection of Ca2+.

(DOCX)

S2 Table Comparison of this work with different methods for the detection of Mg2+.

(DOCX)

We thank Yanhui Bi and Binglu Zhao for expert technical assistance.

10.1371/journal.pone.0308424.r001
Decision Letter 0
Sauli Elingarami Academic Editor
© 2024 Elingarami Sauli
2024
Elingarami Sauli
https://creativecommons.org/licenses/by/4.0/ This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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PONE-D-24-04161A portable lateral flow distance-based paper sensor for drinking water hardness testPLOS ONE

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4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: No

Reviewer #2: Yes

Reviewer #3: Yes

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: This manuscript reported a portable lateral flow distance-based paper sensor for drinking water hardness test. However, the manuscript is not well-written. Here gives some suggestions for improvement of the manuscript:

1. The abstract section needs to be revised, more information should be mentioned, such as the recoveries of the tested purified water.

2. The third paragraph of Introduction needs to be revised. The paper-based lateral flow sensor and its applications should be introduced first, followed by the distance signal readout method and its applications, and finally the analyte-responsive hydrogels.

3. “Condition optimization of the paper sensor” section, the author declared that “Moreover, the similar variation tendency was found for the solution and the mixture of Alg and Ca2+, respectively.” The data should be provided.

4. The author declared that “The concentrations of Ca2+ were determined by the paper sensor, the water hardness kit and inductively coupled plasma-mass spectrometry (ICP-MS) (Fig.S5).” However, Fig.S5 is just a graph of the paper-based sensor in response to different added concentration of Ca2+.

5. Table 1 is missing.

6. In Fig. S2 captions, the concentrations of Ca2+ (0, 1, 2, 5 and 10 mM) is wrong.

7. The discussion of Fig.S4 in the main text is missing. Moreover, other metal ions in the water sample can also affect the sensor's results, not just chloride ions.

Reviewer #2: In the present manuscript, the authors have made an interesting design of A portable lateral flow distance-based paper sensor for testing hardness of drinking water. The study is simple therefore, novelty should be highlighted. A thorough literature survey must be included. The characterizations especially for the paper based sensors should be improved. The study related to repeatability and stability must be presented. Paper can be accepted after these minor revision.

Reviewer #3: The authors developed a simple and portable lateral flow sensor with distance reading signal for the semiquantitative detection of drinking water hardness. Where the Ca2+/Mg2+ concentration can be determined by the water lateral flow distance on paper strips of the upper aqueous solution. Several studies have been carried out, including 1) Optimization of sodium alginate concentration, 2) Determination of water hardness using commercial test kit, 3) Feasibility of Ca2+/Mg2+ detection of the paper sensor, 4) Condition optimization of the paper sensor, 5) Detection of Ca2+ and Mg2+, 6) The semi-quantitative detection of water hardness using the paper sensor and 7) Comparison of the paper sensor with commercial kit for Ca2+ determination. Sample analyze was also performed. The article shows important results for the area of knowledge. However, the manuscript presents few points that should be improved to final decision.

Required comments:

1) Review the units. Preferably use mol L-1, g L-1, Temperature in K, etc. According to the international system of units.

2) Does study involving pH and temperature variation is necessary for the optimization of sodium alginate concentration?

3) Did the authors measure sensor stability?

4) If possible, include a table comparing the values obtained with literature data from other sensors.

**********

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: Yes: Dr Khairunnisa Amreen

Reviewer #3: Yes: Emerson Schwingel Ribeiro

**********

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

10.1371/journal.pone.0308424.r002
Author response to Decision Letter 0
Submission Version1
11 Apr 2024

We would first like to appreciate the editors and the reviewers for your positive and constructive comments and suggestions, and give us a chance to improve our manuscript.

Responses to Editor:

Question 1: Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming.

Answer: Thanks for your careful comments. We have modified our manuscript as PLOS ONE's style requirements.

Question 2: We note that the grant information you provided in the‘Funding Information’ and ‘Financial Disclosure’ sections do not match.

Answer: Thanks for your friendly suggestion. We have provided the correct grant numbers for the awards for our study in the ‘Funding Information’ section.

Question 3: Please state what role the funders took in the study.

Answer: Thanks a lot for your kind suggestion. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. We have included Role of Funder statement in our cover letter.

Question 4: Please remove any funding-related text from the manuscript and let us know how you would like to update your Funding Statement.

Answer: Thank you for your kind reminder. We have removed any funding-related text from the manuscript. We will no longer update our Funding Statement.

Question 5: Please ensure that you have an ORCID iD.

Answer: Thanks a lot for your kind suggestion. I have an ORCID iD which is 0000-0003-1801-9351.

Question 6: Please include a copy of Table 1 which you refer to in your text on page 17 (in PDF format).

Answer: Thanks for your careful and kind suggestion. We apologize for our carelessness. We have added Table 1 to the latest version.

Question 7: Please include captions for your Supporting Information files at the end of your manuscript, and update any in-text citations to match accordingly

Answer: Thanks for your critical comments. We have attached the captions of Supporting Information files at the end of our manuscript, updated any in-text citations to match accordingly.

Question 8: Please review your reference list to ensure that it is complete and correct.

Answer: Thanks a lot for your kind suggestion. We have carefully reviewed our references list. We ensure that it is complete and correct.

Responses to Reviewer #1:

Question 1: The abstract section needs to be revised, more information should be mentioned, such as the recoveries of the tested purified water.

Answer: Thanks for your professional comments. We have added the recoveries of paper-based sensor into abstract section.

Question 2: The third paragraph of Introduction needs to be revised.

Answer: Thanks for your professional suggestion which will significantly improve the logic of our article. We have adjusted the order of introduction for paper-based lateral flow sensor , distance signal readout method and analyte-responsive hydrogels.

Question 3: “Condition optimization of the paper sensor” section, the author declared that “Moreover, the similar variation tendency was found for the solution and the mixture of Alg and Ca2+, respectively.” The data should be provided.

Answer: Thank you for your critical comments. We apologize for our oversight. Originally, we wanted to declared that “the similar variation tendency was found for the solution and the mixture of Alg and Mg2+”, as shown in Fig S1.

Question 4: The author declared that “The concentrations of Ca2+ were determined by the paper sensor, the water hardness kit and inductively coupled plasma-mass spectrometry (ICP-MS) (Fig.S5).” However, Fig.S5 is just a graph of the paper-based sensor in response to different added concentration of Ca2+.

Answer: Thanks for your careful and kind suggestion. We apologize for our carelessness. Fig S5 is a graph of the paper-based sensor in response to different added concentration of Ca2+. We have modified it to “The concentrations of Ca2+ were determined by the paper sensor (Fig S5), the water hardness kit and inductively coupled plasma-mass spectrometry (ICP-MS).”

Question 5: Table 1 is missing.

Answer: Thanks a lot for your careful comment. We have added Table 1 to the latest version.

Question 6: In Fig. S2 captions, the concentrations of Ca2+ (0, 1, 2, 5 and 10 mM) is wrong.

Answer: Thanks for your careful review. We apologize for our oversight. We have modified the concentration in Supporting Information.

Question 7: The discussion of Fig.S4 in the main text is missing. Moreover, other metal ions in the water sample can also affect the sensor's results, not just chloride ions.

Answer: Thanks for your professional review. In the main text, we added a brief discussion about the anti-interference capacity of the paper sensor for Ca2+ detection, as shown in Figs 1-4 and Fig S4.

Responses to Reviewer #2:

Question 1:A thorough literature survey must be included. The characterizations especially for the paper based sensors should be improved. The study related to repeatability and stability must be presented.

Answer: Thanks for your professional suggestions.

We made comparison of this work with different methods for the detection of Ca2+ and Mg2+ by a thorough literature survey, which were presented as Tables S1 and S2 in Supporting Information.

In addition, we apologize for missing Table 1 in uploaded manuscript. In Table 1, the RSD of Paper-based sensor ranges from 4.4 to 7.4, which indicates good stability of this method. We have added Table 1 to the latest version.

Responses to Reviewer #3:

Question 1:Review the units. Preferably use mol L-1, g L-1, Temperature in K, etc. According to the international system of units.

Answer: Thanks a lot for your kind suggestion. We have reviewed all the units in the manuscript. According to the international system of units, we have converted the units in the manuscript.

Question 2: Does study involving pH and temperature variation is necessary for the optimization of sodium alginate concentration?

Answer: Thanks for your valuable suggestion. We have tested the anti-interference capacity of the paper sensor for Ca2+ detection including pH and ionic strength. The ionic strength in actual drinking water samples exhibits simple, which rarely contains heavy metal cations. Therefore, we selected common anions for anti-interference detection.

The effect of different temperatures on the formation of calcium oleate has been observed in our previous study. With the increase in temperature, the amount of calcium oleate particles on the wall of the centrifugal tube decreased signiffcantly due to the increase in its solubility. However, the reaction time was largely prolonged if the temperature was too low. Therefore, the temperature of calcium oleate formation was selected as 298K[1].

The corresponding results were represented in Results and discussion.

Reference

[1] Xia S, Yin F, Xu L, Zhao B, Wu W, Ma Y, et al. Paper-Based Distance Sensor for the Detection of Lipase via a Phase Separation-Induced Viscosity Change. Anal Chem . 2022; 94(49): 17055-17062.

Question 3: Did the authors measure sensor stability?

Answer: Thanks for your valuable suggestion. We apologize for missing Table 1 in uploaded manuscript. In Table 1, the RSD of Paper-based sensor ranges from 4.4 to 7.4, which indicates good stability of this method. We have added Table 1 to the latest version.

Question 4: If possible, include a table comparing the values obtained with literature data from other sensors.

Answer: Thanks a lot for your professional comment. We added Tables S1 and S2 for the comparison of existing sensor, which were attached to Supporting Information.

Attachment Submitted filename: Response to Reviewers.docx

10.1371/journal.pone.0308424.r003
Decision Letter 1
Sauli Elingarami Academic Editor
© 2024 Elingarami Sauli
2024
Elingarami Sauli
https://creativecommons.org/licenses/by/4.0/ This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Submission Version1
22 May 2024

PONE-D-24-04161R1A portable lateral flow distance-based paper sensor for drinking water hardness testPLOS ONE

Dear Dr. liu,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

==============================

​The authors should improve the introduction and discussion parts, including justification of the procedure used when compared with other similar methods.

==============================

Please submit your revised manuscript by Jul 06 2024 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.

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If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

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We look forward to receiving your revised manuscript.

Kind regards,

Elingarami Sauli, PhD

Academic Editor

PLOS ONE

Journal Requirements:

Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice.

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: (No Response)

Reviewer #3: All comments have been addressed

Reviewer #4: (No Response)

Reviewer #5: (No Response)

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #3: Yes

Reviewer #4: Yes

Reviewer #5: No

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #3: Yes

Reviewer #4: (No Response)

Reviewer #5: No

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #3: Yes

Reviewer #4: (No Response)

Reviewer #5: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #3: Yes

Reviewer #4: (No Response)

Reviewer #5: No

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: After the revisions, the paper improved significantly. The manuscript's structure, flow, or writing is acceptable for publication.

Reviewer #3: The authors made the necessary corrections presented by the reviewers. The article is ready to be published in Plos One.

Reviewer #4: This manuscript has been carefully revised according to the suggestions of first-round review. Followings are some suggestions for further revisions.

1, There are commercially available instrument for measuring the hardness of water, which like the pH meter. Please also introduce them in the introduction section, and include this type instrument for comparison in the measurement of hardness of water in the experimental section.

2, According to the Reviewer #1 of first-round review, there are several errors in the manuscript. Please double check the manuscript.

3, Do not use non-English terms such as "anti-interference" or "foreign ions". Make use of correct terms "Selectivity" and "potentially interfering ions", respectively.

4, Please avoid using first-person narratives for a scientific paper.

5, The shortcomings of present work can be emphasized in the conclusion section.

6, Please carefully check and unify the units. For example, both the “mM” and “mmol-1” have been used in the manuscript.

Reviewer #5: The manuscript "A portable lateral flow distance-based paper sensor for drinking water hardness test" describes a paper-based device for the determination of water hardness, expressed as Ca2+ and Mg2+ concentration, based on the formation of a hydrogel after the chelation with sodium alginate. Although the idea of the study is interesting, I do not think the quality of the manuscript support its publication. To be considered, an improvment must be performed. Some specific comments:

a) The English language throughout the manuscript must be revised. There are many typos (e.g. thrice) and strange sentences (e.g. Drinking water quality is a problem concerned by people). The peer-reviewing is not a proof reading service, so the English must be revised by someone dedicated to it.

b) A broader literature revision must be done. There are, at least, two papers dealing with the determination of water hardness using paper-based devices that were not mentioned (10.1016/j.measurement.2020.108085 and 10.3390/su14063363). It should be inserted and discussed.

c) Eriochrome T is the most usual "reagent" for the determination of water hardness. Why your procedure is better?

d) "anti-interference" sounds strange

e) The "Cr values" is not clear how to calculate. Please clarify it.

f) The images were taken with no illumination control? Is the results affected by different illumination conditions? It should be discussed.

g) Test-t should be performed to compare the results obtained by the proposed method with the reference ones.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #3: Yes: Emerson Schwingel Ribeiro

Reviewer #4: No

Reviewer #5: No

**********

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

10.1371/journal.pone.0308424.r004
Author response to Decision Letter 1
Submission Version2
6 Jul 2024

We would like to appreciate the editors and the reviewers for the positive and constructive comments and suggestions, and give us a chance to improve our manuscript.

Responses to Reviewer #4:

Question 1: There are commercially available instrument for measuring the hardness of water, which like the pH meter. Please also introduce them in the introduction section, and include this type instrument for comparison in the measurement of hardness of water in the experimental section.

Answer: Thanks for your careful and kind suggestions. We investigate carefully all the available related instrument. Several methods for measuring the hardness of water have been reported. Among them, atomic absorption spectrometry and complexometric titration are the classic methods. Additionally, the fluorescence method and ionophore-based ion-selective optode also have been used for for monitoring water hardness. The above content has been included in the introduction section. In the study, we employed the commercial kit for Ca2+ determination in control group in the experimental section.

Question 2: According to the Reviewer #1 of first-round review, there are several errors in the manuscript. Please double check the manuscript.

Answer: Thanks for your kind suggestion. We have carefully corrected the errors pointed out by the Reviewer #1. Meanwhile, we checked the manuscript carefully.

Question 3: Do not use non-English terms such as "anti-interference" or "foreign ions". Make use of correct terms "Selectivity" and "potentially interfering ions", respectively.

Answer: Thanks for your professional suggestion which will significantly improve the readability of our article. We have adjusted non-English terms according to your advice .

Question 4: Please avoid using first-person narratives for a scientific paper.

Answer: Thanks for your careful and kind suggestion. We have switched from first-person narratives to third-person narratives to present scientific facts.

Question 5: The shortcomings of present work can be emphasized in the conclusion section.

Answer: Thanks for your professional comments. We have emphasized the shortcomings of present work in the conclusion section.

Question 6: Please carefully check and unify the units. For example, both the “mM” and “mmol-1” have been used in the manuscript.

Answer: Thank you for your critical comments. We apologize for our oversight. We have carefully checked and unified the units as mmol L-1.

Responses to Reviewer #5

Question 1: The English language throughout the manuscript must be revised. There are many typos (e.g. thrice) and strange sentences (e.g. Drinking water quality is a problem concerned by people). The peer-reviewing is not a proof reading service, so the English must be revised by someone dedicated to it.

Answer: Thanks for your kind suggestion. We have carefully revised the English language throughout the manuscript.

Question 2: A broader literature revision must be done. There are, at least, two papers dealing with the determination of water hardness using paper-based devices that were not mentioned (10.1016/j.measurement.2020.108085 and 10.3390/su14063363). It should be inserted and discussed.

Answer: Thank you for your valuable suggestion. We have unified and updated the format of the literature throughly. Meanwhile, we have added the mentioned literatures (10.1016/j.measurement.2020.108085 and 10.3390/su14063363) with discussion in the manuscript.

Question 3: Eriochrome T is the most usual "reagent" for the determination of water hardness. Why your procedure is better?

Answer: Thanks for your professional comments. Eriochrome T is the most usual "reagent" for the determination of water hardness. In this study, the commercial kit for Ca2+ determination employed in the control group is made up of Eriochrome T. Both approaches have advantages and disadvantages. Our procedure provides a portable approach for semi-quantitative detection of drinking water hardness with convenience and low cost. However, the detection of water samples in complex environments needs to be further optimized.

Question 4: "anti-interference" sounds strange.

Answer: Thank you for your critical comments. We apologize for using non-English terms. We have replaced it with “Selectivity”.

Question 5: The "Cr values" is not clear how to calculate. Please clarify it.

Answer: Thanks a lot for your suggestions. The calculation of the coverage ratios of water mark was illustrated as below:

Pmark = the pixel values of the watermark area= 58765.

Ptotal= the pixel values of the whole test strip = 101797

Therefore, Cr = Pmark / Ptotal =58765/101797 = 0.577

Question 6: The images were taken with no illumination control? Is the results affected by different illumination conditions? It should be discussed.

Answer: Thanks for your professional comments. In this study, the photographs were taken at ambient light conditions. Moreover, the detection relies on measuring the water flow distance. It is not be affected by the illumination conditions. This is an advantage of the distance sensor.

Question 7: Test-t should be performed to compare the results obtained by the proposed method with the reference ones.

Answer: Thanks for your professional comments. We performed the test-t analysis between the proposed paper-based method and the ICP-MS method which is the gold standard for metal content measurement. The p values were calculated as 0.317, 0.353, and 0.596 respectively, corresponding to the marked samples with Ca2+ concentrations of 2, 4, and 6 mM, respectively. No significant difference was observed, which demonstrate the reliability of the paper-based method.

Attachment Submitted filename: Response to Reviewers.docx

10.1371/journal.pone.0308424.r005
Decision Letter 2
Sauli Elingarami Academic Editor
© 2024 Elingarami Sauli
2024
Elingarami Sauli
https://creativecommons.org/licenses/by/4.0/ This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Submission Version2
24 Jul 2024

A portable lateral flow distance-based paper sensor for drinking water hardness test

PONE-D-24-04161R2

Dear Dr. Yulin Liu

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

An invoice will be generated when your article is formally accepted. Please note, if your institution has a publishing partnership with PLOS and your article meets the relevant criteria, all or part of your publication costs will be covered. Please make sure your user information is up-to-date by logging into Editorial Manager at Editorial Manager® and clicking the ‘Update My Information' link at the top of the page. If you have any questions relating to publication charges, please contact our Author Billing department directly at authorbilling@plos.org.

If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they’ll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

Kind regards,

Elingarami Sauli, PhD

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

All reviewers' (original and secondary) comments have been addressed to my satisfaction. The submission can be accepted with minor editorial revisions as highlighted below.

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #5: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #5: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #5: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #5: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #5: No

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #5: The authors made susbtantial changes in the revised version of the manuscript. The results are now better presented and the introduction section has improved. Prior to the publication, some revisions are still needed.

a) Some expressions and sentences still needs improvment, such as: the usage of the word "Remarkably" in the abstract is unecessary. The sentence "Upon the Alg solution was incubation..." and "S2 Fig" need revision.

b) Only cations Na+ and K+ were evaluated as interferent. Why? More cations should be investigated.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #5: No

**********

10.1371/journal.pone.0308424.r006
Acceptance letter
Sauli Elingarami Academic Editor
© 2024 Elingarami Sauli
2024
Elingarami Sauli
https://creativecommons.org/licenses/by/4.0/ This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
29 Jul 2024

PONE-D-24-04161R2

PLOS ONE

Dear Dr. Liu,

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now being handed over to our production team.

At this stage, our production department will prepare your paper for publication. This includes ensuring the following:

* All references, tables, and figures are properly cited

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