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Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

71874
10.1038/s41598-024-71874-6
Article
A metal-insulator-metal waveguide-based plasmonic refractive index sensor for the detection of nanoplastics in water
Guchhait Shyamal 12
Chatterjee Subhasri c.subhasri@tcs.com

2
Chakravarty Tapas 2
Ghosh Nirmalya 1
1 https://ror.org/00djv2c17 grid.417960.d 0000 0004 0614 7855 Indian Institute of Science Education and Research Kolkata, Mohanpur, India
2 TCS Research, Kolkata, India
14 9 2024
14 9 2024
2024
14 2149511 6 2024
2 9 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
A metal-insulator-metal waveguide-based square-ring resonator plasmonic refractive index sensor is designed and optimized for achieving high sensitivity. The sensitivity of the sensor critically depends on the physical dimension and the geometrical parameters of the resonator. Systematic studies on varying geometrical parameters of the resonator reveal that the sensitivity increases with the number of concentric square-rings. Moreover, the full-width-half-maxima of the resonance line is found to increase with the number of square rings. Importantly, variations in the coupling length affect the transmitted intensity as well as the full-width-half-maxima of the resonance spectra in a characteristic fashion. An initial exploration of the optimized sensor for nanoplastic detection for a range of volume fractions 0.15625–0.625% shows a systematic linear increase in the resonance wavelength with changing refractive index of the surrounding medium. This offers the possibility of ultrasensitive detection of extremely small change (∼0.00025) in the local refractive index as the signature of a minute level of plastic contamination. This was achieved by using an optimized sensor design with a sensitivity of 2700 nm/RIU and a full-width-half-maxima of 333 nm. Results presented in the paper demonstrate the considerable promise of the proposed plasmonic refractive index sensor towards nanoplastic detection.

Keywords

Plasmon
Plasmonic sensor
Sensitivity
Figure of merit
Refractive index
Nanoplastic
Subject terms

Optical sensors
Optics and photonics
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pmcIntroduction

The optical properties of metal nano-particles and nano-structures have gained substantial interest in recent times for their potential applications in various domains, and the fundamental aspects related to the light-matter interactions. These unique optical properties are governed by the collective oscillations of the free electrons present on the metal surface in resonance with the incident electromagnetic fields, known as the surface plasmon resonance (SPR). As a result of this, strongly enhanced and highly localized electromagnetic fields are generated. The surface plasmon can be either propagating at planar metal-dielectric interfaces or localized (LSPR) in the case of metal nano-particles/structures. In either case, it leads to sharp resonance lines in their reflectance or transmittance spectra.1.

The surface plasmon resonance is well-researched and well-known for its wavelength-dependent distinct spectral characteristics as well as its inherent responsiveness towards the local dielectric environment. Current research interest has explored SPR for numerous practical applications such as biomedical and chemical sensing, bio-molecular manipulation, energy harvesting, optical information processing, surface-enhanced spectroscopy, development of nano-optical devices for control and manipulation of light at the nanometer scale, contrast enhancement in optical imaging, plasmonic metasurfaces and data storage, switching, lasing, filters and robust colour display, nonlinear and slow-light devices, invisibility cloaking and so forth2–7. Besides the potential applications, a number of intriguing and exotic effects associated with the interaction of light with spatially tailored plasmonic nanostructures have also been observed recently. Spin orbit interaction (SOI) and Spin Hall (SH) effect of light, quantum spin hall effect, spin (polarization) controlled plasmonics, nonreciprocity in reflection from spatial Kramers-Kronig medium, coupled plasmons and plasmonic Fano resonances, coherent perfect absorption of light, electromagnetically induced transparency (EIT) and absorption (EIA), bound state in continuum (BIC), are some of the recently discovered intricate plasmonic effects8–14. In the domain of sensing, SPR-based platforms are utilized to detect changes in the refractive index or dielectric properties of the medium surrounding the metal surface, enabling the detection of analytes such as gases, liquids, and bio-molecules15. By functionalizing the metal surface with specific recognition elements, SPR sensors can selectively capture and quantify target analytes, making them invaluable tools in environmental monitoring, food safety, and pharmaceutical quality control16–18. Furthermore, SPR can also be employed for the investigation of physical properties such as film thickness, viscosity, and density19. By analyzing the SPR response to changes in these properties, researchers can gain insights into material properties and processes, facilitating advancements in diverse fields ranging from thin film deposition, and polymer science, to surface chemistry.

SPR and LSPR sensors encompass a diverse range of configurations tailored to specific sensing applications, each offering unique advantages and capabilities. Traditional prism-based SPR sensors utilize a thin metal film deposited on a glass prism to excite surface plasmons at the metal-dielectric interface, enabling label-free detection of biomolecular interactions and chemical binding events20. Alternatively, fibre optic SPR sensors leverage the evanescent field generated by light propagating through an optical fibre, offering flexibility for remote sensing and integration into compact device21. Grating-coupled SPR sensors exploit periodic nanostructures to couple incident light to surface plasmons, enabling high-throughput and multiplexed sensing in applications such as drug discovery and proteomics22. Microfluidic-based SPR sensors integrate microfluidic channels with SPR sensing elements, enabling precise control of sample delivery and mixing for real-time analysis of biological and chemical samples23. On the other hand, LSPR typically results in the confinement of the surface plasmon within a nanoparticle, producing a high responsivity to the dielectric properties of the environment. The diversity of the SPR and LSPR sensor platforms provides researchers with the desirable versatile toolkit for addressing a wide range of analytical challenges enabling new opportunities for scientific discovery and technological innovations.

Metal–Insulator–Metal (MIM) waveguide sensors24 represent a prominent application of SPR technology, offering enhanced sensitivity and specificity for various sensing modalities25. By confining surface plasmon polaritons within a nanoscale gap between metal layers separated by insulating material, MIM waveguides create a highly localized electromagnetic field sensitive to changes in the surrounding medium. This configuration enables label-free detection of analytes with exceptional efficiency, making MIM waveguide sensors well-suited for refractive index sensing applications. Moreover, MIM waveguides offer additional advantages, including compatibility with integrated photonic circuits and the potential for on-chip integration, facilitating miniaturization and multiplexed sensing platforms26. The performance efficiency of the MIM wavegide-based SPR sensor is evaluated through the parameters Sensitivity and the Figure of Merit (FoM). Sensitivity refers to the ability of the sensor to detect changes in the refractive index or concentration of analytes in the surrounding medium. In MIM waveguide SPR sensors, sensitivity is typically quantified as the shift in the resonance wavelength or angle in response to changes in the refractive index of the medium. Higher sensitivity indicates a greater ability to detect small changes in analyte concentration, making the sensor more suitable for applications requiring high detection sensitivity. The FoM, on the other hand, is a metric used to evaluate the overall performance of an SPR sensor, taking into account both sensitivity and spectral resolution. It is defined as the ratio of sensitivity to the Full Width at Half Maximum (FWHM) of the resonance peak. A higher FoM indicates a sensor with better performance in terms of sensitivity and spectral resolution, enabling more accurate and precise detection of analytes. In MIM waveguide SPR sensors, optimizing the FoM is essential for achieving high detection sensitivity and selectivity, making it a key parameter in sensor design and optimization.

Efforts have been made to enhance the sensitivity and FoM through various design strategies and material optimizations. Studies by Kazanskiy et al.26 demonstrated improved sensitivity in MIM waveguide sensors by optimizing the waveguide geometry and dielectric materials, leading to a significant enhancement in FoM. Studies by Mohammad Reza Rakhshani proposed concentric triple racetrack resonators side-coupled to metal-insulator-metal waveguide for chemical sensing27. Jing Chen et al. reported leaky surface plasmon modes in finite, planar, metal-insulator-metal waveguides28. All investigations focus on optimizing the sensitivity and FoM of the sensors to attain a viable platform for sensing a specific analyte.

Among many potential applications, MIM-waveguide SPR sensors offer a promising approach to the detection of nanoplastic particles due to their high sensitivity and compatibility with nanoscale analytes. Presumably, when nanoplastic particles interact with the sensor surface, they induce changes in the refractive index within the waveguide gap in the sensor, leading to shifts in the SPR resonance wavelength or angle. These changes can be monitored in real time, providing quantitative information about the presence and concentration of nanoplastic particles in the sample. Furthermore, the nanoscale dimensions of plastic particles are well-suited to the sensitivity and spatial resolution capabilities of MIM waveguide SPR sensors, enabling the detection of individual nanoparticles or small aggregates. By optimizing the design and fabrication of MIM waveguide sensors, it is likely to develop highly sensitive platforms for the detection of nanoplastic pollution in environmental samples. This could contribute to efforts aimed at understanding and mitigating the environmental impact of nanoplastic contamination in aquatic ecosystems.

In this paper, we present the results of our systematic investigations on the design and optimization of a square-ring structured MIM waveguide-based SPR sensor. The influence of the various geometrical factors such as the number of concentric square-rings, the distance between the waveguide and the resonator, resonator dimensions etc. on the resulting sensitivity and the FoM of the SPR sensor are studied in detail. The thorough analyses addressed in this paper are aimed at conceptualizing the sensor design and evaluating its performance in terms of sensitivity and FoM. Finally, we have carried out preliminary investigations to explore the potential of this optimized sensor towards detecting a minute level of nanoplastics contamination in water.

Material and method

The proposed sensor configuration is depicted schematically in Fig. 1, which comprises a square resonator and a single waveguide; the structure being composed of the layers silver-air-silver. The chosen geometry consisted of concentric square resonators (with varying numbers of square rings for optimization) with large lateral interaction lengths along the whole flat resonator sidewall. One important feature of such a geometry is that it significantly reduces the stringent limitation on the gap separation between the side-coupled waveguide and the resonator. One side of the waveguide served as the input channel for illuminating light from a broad-band spectral light source, while the opposite side functioned as the output channel for light detection. The material to be sensed would fill the resonator, and variations in its local refractive index would prompt shifts in the surface plasmon resonance wavelength (or change in spectral line shape) recorded at the output channel. The designed sensor is capable of sensing both gaseous and liquid samples. In the respective cases, gaseous samples would fill in the resonator according to the gas diffusion forces within a vacuum environment, whereas liquid samples may be introduced through a nanofilling technique leveraging capillary attraction. In Fig. 1, the resonator length is denoted as d, and the waveguide’s width is represented by w, with the distance between the waveguide and resonator, termed as the coupling length, as g. For square rings, the ring width was denoted as w0, and in the case of concentric square rings, the gap between the rings corresponded to the square ring’s width.

As seen in the schematic, the proposed sensor is simulated in 2D. It is a general consideration to extrude 3D structure from the 2D configuration for simulation purposes in order to save the computation time. It has been previously reported that nearly all MIM sensors are simulated in 2D29.Fig. 1 Schematic illustration of the sensor geometry: (a) From the top view, the silver (Ag) film in the diagram, characterized by a square morphology, is shown by the colour cyan, and the air column acting as the waveguide (with width w=45nm) is shown by magenta colour. The black region denotes various resonator types having the maximum length of the resonator of d. One side of the waveguide is used for the input channel for incident light from a broadband source. The other side of the waveguide is used for the output channel to detect light. (b) The square dielectric resonator is presented. (c) The square-ring dielectric resonator is presented. (d–e) Examples of concentric square rings are illustrated comprising different numbers of silver and dielectric rings. In (d) three dielectric rings and two silver rings, and in (e) three dielectric & three silver rings are shown, respectively.

For the simulation of the optical response of the structure, we used COMSOL, a commercially available software based on the finite element method (FEM). Within the Perfect Matched Layer (PML), our simulation setup incorporated a scattering boundary condition to absorb outgoing waves and mitigate undesired reflections, thereby ensuring the reliability of the simulated outcomes and the generated data. Incident light was introduced via a port, while the transmitted light was detected through another port. To enhance the resolution and precision of the simulation, we adopted an approach employing extra-fine and extremely fine triangular meshing. A minimum size of the ultra-fine mesh was considered as 10 nm. This strategy effectively dissected the simulation into smaller, more finely detailed components. By leveraging the FEM within this framework, we could numerically solve the complex electromagnetic wave equations incorporating material interfaces. By decomposing the solution domain into discrete elemental components, FEM enabled the resolution of partial differential equations (PDEs) mimicking wave behaviour as they interacted with various materials, thereby facilitating comprehensive analysis and in-depth examination of complex structures.

We assumed that the medium filling the resonator and waveguides was air, characterized by a refractive index of 1 (n=1). The permittivity function of the background metal (silver (Ag)) was described using the Lorentz–Drude model. This model served as a valuable theoretical framework for examining the optical properties of metals by encompassing their electronic configurations. Widely utilized in materials science and optics, it facilitates the exploration of metals’ behaviour at atomic and molecular scales. The model elucidated the movement of electrons within the metal through a series of equations that account for collisions between electrons and lattice vibrations. Using this model, the refractive index of metal was described as1 ϵ(ω)=1-ωp2ω(ω-iΓ0)+∑n=1kfnωn2ωn2-ω2+iωΓn

In this context, ωp denoted the plasma frequency of the metal. The damping constant associated with the n-th harmonic oscillator (bound electrons) of the material was designated as Γn, while the damping constant at zero frequency was termed Γ0. The oscillator strength of the n-th oscillator was represented by fn, and its natural frequency was denoted as ωn. Furthermore, the frequency of the electromagnetic radiation was denoted by ω.

Results and discussion

In this section, we first investigate how the material refractive indices of two distinct resonator types (as illustrated in Fig. 1b) impact the surface plasmon resonance wavelength. Figure 2a,d present the transmission spectra of the proposed sensor for surrounding medium refractive indices of 1.00 and 1.01, respectively, for different resonator geometries. Figure 2a showcases the spectral response derived from a square resonator (Fig. 1b 1.) with a length, d=400nm positioned on a silver substrate for the incident Transverse Magnetic (TM) polarization of light. While the dipolar resonance is observed to peak at a wavelength of 968 nm, and the quadrupolar resonance peak appears at 719 nm for n=1, the corresponding dipolar resonance peaks at a wavelength of 978 nm and the quadrupolar resonance occurs at 726 nm for n=1.01. Accompanying Fig. 2b,c portray the total magnetic field distributions for the dipolar and quadrupolar resonances, respectively.Fig. 2 Spectral response for different resonators. (a) The spectral response obtained from a square (length, d=400nm) on a silver substrate for incident transverse magnetic (TM) polarization (magnetic field perpendicular to the plane). The dipolar resonance peak appears at a wavelength of 968 nm, while the quadrupolar resonance occurs at 719 nm for n=1.00. (b) and (c) illustrate the corresponding total magnetic field distributions (H) for the dipolar and the quadrupolar resonances, respectively. (d) The spectral response observed from a square ring (outer length, d=400nm, width, w0=40nm) on a silver substrate with incident TM polarization. The dipolar resonance peak wavelength is 2884 nm, and the quadrupolar resonance occurs at 1452 nm for n=1.01. (e) and (f) illustrate the total magnetic field distributions (H) for the dipolar and the quadrupolar resonances. The solid blue line in the plots represents a refractive index of 1.00, while the magenta dotted line corresponds to a refractive index of 1.01.

Similarly, Fig. 2d presents the spectral response observed from a square ring resonator (Fig. 1b 2.) with an outer length, d=400nm, and width, w0=40nm on a silver substrate for incident TM polarization. Here the dipolar resonance peaks at 2884 nm, and the quadrupolar resonance occurs at 1452 nm for n=1.00. For n=1.01, the corresponding dipolar resonance peaks at 2913 nm, and the quadrupolar resonance occurs at 1466 nm. Figure 2e,f illustrate the total magnetic field distributions for the dipolar and the quadrupolar resonances. The solid blue line in the plots represents a refractive index of 1.00, while the magenta dotted line corresponds to a refractive index of 1.01. It is evident that as the refractive index increases, significant redshifts occur in the resonance wavelengths, with the dipolar resonance experiencing a larger shift (10 nm for square and 29 nm for the square-ring resonator) compared to the quadrupolar resonance (7 nm for square and 14 nm for the square-ring). A discernible red shift in the spectrum is observed in the square-ring configuration compared to the spectrum of the square resonator. Moreover, the spectral separation resulting from a refractive index change of 0.01 is enhanced in the square-ring structure relative to the square structure.Fig. 3 Spectral response of the square-ring resonator for different geometrical parameters. The solid line represents the spectral response of the resonator for the surrounding medium refractive index of n=1.00. The dotted line represents the spectral response of the resonator for n=1.01 (a) The spectral response of the square-ring resonator having different widths (w0= 20–60 nm), while the outer length (d=400nm) and coupling length (g=15nm) are kept constant. An increase in the width leads to a blue shift of the spectral line shape (a), a decrease in the sensitivity (∂n/∂λ) (shown in d, left axis), and a reduction of the FWHM -δλ (shown in d, right axis). (b) The spectral response is shown for different lengths (d=  300–500 nm)of the square ring configuration, while the width (w0=40nm) and coupling length (g=15nm) are kept constant. An increase in the length leads to a red shift of the spectral response (b), an increase in the sensitivity (shown in e, left axis), and an increase of FWHM (shown in e, right axis). (c) The spectral response is shown for varying coupling length (g= 15–35 nm), while the outer length (d=400nm) and width (w0=40nm) are kept constant. The gap between the square ring and the waveguide, i.e., the coupling length is varied between (g=15–35 nm). An increase in the coupling length reduces the coupling between the resonator and the waveguide affecting the transmitted intensity (c). The sensitivity remains unaffected (shown in f, left axis), but the FWHM increases (shown in f, right axis).

Fig. 4 Spectral response for varying numbers of square-rings. (a) Spectral response obtained from concentric square-rings with outer length (d=400nm), width w0=40nm, and gap width equivalent to the square-ring width, placed on a silver substrate for TM polarized light. As the number of concentric square-rings increases, the spectrum exhibits a redshift, with the spectral shift between refractive indices 1.00 and 1.01. (b) Sensitivity (∂n/∂λ) increases with the number of rings (shown in b, left axis), and the FWHM-δλ also increases with the number of rings (shown in b, right axis). (c–f) Magnetic field distributions (H) at resonance for square-ring numbers 1–4 are shown.

Fig. 5 Optimization of the spectral response of square-rings resonator for nanoplastic detection. (a) The spectral resonance of the square-ring resonator for numbers of square-rings 2 is presented. The spectral response for g=15nm coupling length is shown using the blue line, and for g=20nm coupling length is shown using the red line. The solid and the dotted lines are for the refractive index (RI) of 1.34 and 1.35, respectively. The length of the square and the width of the rings are 400 nm and 50 nm, respectively. (b) The resonance wavelength is observed to vary systematically with very small changes in RI. The RI of water is taken 1.34, and plastic is 1.5, respectively. Small changes in RI can be related to a small quantity of plastic present in water.

In Fig. 3, we summarize the spectral response, sensitivity, and full-width half maxima of the square-ring resonator for different geometrical parameters, namely, resonator length (d), width (w0), and coupling length (g). The results show distinct trends across different configurations. We first discuss the effect of the variation of width (w0) (Fig. 3a, d). For this purpose, the spectral response was analyzed for differing widths (w0 = 20–60 nm with 10 nm step) while maintaining constant outer length (d=400nm) and coupling length (g=15nm). While for the width w0=20nm, the sensitivity and the full-width half maximum were found to be 4000 and 679 nm, for width w0=60nm, these parameters were obtained to be 2400, and 153 nm, respectively. As the width increases, a blue shift in the spectrum response is noted, which is accompanied by a decrease in the sensitivity and a subsequent reduction in the FWHM. Consequently, the FoM is observed to increase, i.e., varying inversely with the sensitivity. The influence of the variation of the length (d) is summarized in (Fig. 3b,e), where the length of the square-ring is varied between (d = 300–500 nm with 50 nm step) keeping a fixed width of (w0=40nm) and coupling length (g=15nm). The results reveal a systematic red shift in the spectral response with increasing length of the square-ring resonator. While for the length d=300nm, the sensitivity and FWHM are obtained to be 2100 and 167 nm, for length d=500nm, these values are changed to 3600, and 414 nm, respectively. Additionally, an improvement in sensitivity and an accompanying increase in the FWHM are observed. Finally, the influence of coupling length (g) is studied and the results are summarized in (Fig. 3c,f). Here, the coupling length is varied between (g = 15–35 nm with 5 nm step) by keeping the outer length (d=400nm) and width (w0=40nm) constant. As can be seen, for the gap g=15nm, sensitivity and FWHM-δλ are 2900 and 273 nm, respectively. Values of these parameters change to 2800 and 165 nm, respectively, when the coupling length is changed to g=35nm. It is found that an increase in the coupling length leads to a reduction of the coupling between the resonator and the waveguide, thereby impacting the transmitted intensity profile. Importantly, the sensitivity remains unaffected by the change in the coupling length, whereas the FWHM decreases with increasing coupling length.

The spectral response obtained from varying numbers of square-rings in the resonator is summarized in Fig. 4. Simulations were conducted for 1–5 concentric rings, each with an outer length d=400nm, width w0=40nm, and a gap between two square-rings is equivalent to the square-ring width. These rings were positioned on a silver substrate and results were obtained for incident TM polarized light. As shown in Fig. 4a, the resonance wavelengths for square-rings 1 to 5 are found to be 2884 nm, 3100 nm, 3258 nm, 3280 nm, and 3286 nm, respectively. With an increase in the number of concentric square-rings, the spectrum exhibits a redshift, and the spectral shift between refractive indices of 1.00 and 1.01 are shown. Additionally, the sensitivity increases systematically with the number of rings (while the sensitivity for 1 ring is 2900 and that for 5 rings is 3400, shown in left axis of b). Furthermore, the FWHM is also observed to increase with the number of rings (increases from 273 to ∼500nm for 1–5 rings, shown in right axis of b). The magnetic field distribution at resonance for square-ring numbers 1 to 4 shows a delocalization of the field distribution (Figs. 4c–f), expanding the spectral width of the resonance and characteristically influencing the FWHM.

Results presented in Figs. 1, 2, 3, 4 provided optimization of the geometric parameters of the proposed refractive index sensor. Further exploration was aimed to investigate the sensor performance in the detection of tiny plastic particles suspended in water. Fig. 5 summarizes the results aimed at detecting small changes in the refractive index of water due to minute quantities of plastic contamination. A sensor design with two concentric square rings was chosen based on the optimized sensitivity and FWHM values. The square length (L) and square-ring width (w0) were set to 400 nm and 50 nm, respectively. The spectral resonance of the resonator is plotted for different coupling lengths (g) (shown in Fig. 5a). The spectral response for a coupling length (g) of 15 nm is depicted by the blue line and that for 20 nm is shown by the red line. The solid and the dotted lines correspond to refractive indices (RI) of 1.34 and 1.35, respectively. The resonator’s sensitivity for the base RI of 1.34 (considered as the RI of water) is found to be 2700. Increasing the coupling length from 15 to 20 nm reduced the FWHM from 392 to 333 nm. Thus, a coupling length of 20 nm was selected for further simulations. In Fig. 5b, we simulated shifts in the resonance wavelength ranging from 3500 to 3750 nm with 0.25 nm steps. The corresponding RI of nanoplastic varied from 1.3 to 1.7, and an average RI of 1.5 was used for the simulation. Considering the RI of water as 1.34 and that of plastic as 1.5, small changes in RI can be the presence of tiny plastic particles in water. Thus, a change of RI of 0.00025 represents 0.15625% plastic in water whereas that of 0.001 represents 0.6250% plastic in water. As shown in Fig. 5, the resonance wavelength for RI 1.34 is found to be 3606.5 nm, which shifts to 3607.25 nm with a slight RI change of 0.00025. Further simulations for additional RI changes reveal a resonance at 3609.25 nm for RI 1.341. The change in the wavelength of resonance was found to be quite systematic and varied linearly with change in RI, which can be exploited for the detection of minute levels of contamination of nanoplastic in water.

We selected an optimal sensor design for nanoplastic detection in this case. As observed, with an increase in the number of concentric square-rings, the spectrum exhibits a redshift, and the spectral shift between refractive indices of 1.00 and 1.01 are shown. Additionally, the sensitivity increases systematically with the number of rings (while the sensitivity for 1 ring is 2900 and that for 5 rings is 3400). On the other hand, the FWHM also increases with the number of rings (increases from 273 to 500 nm for 1 ring to 5 ring). Though the sensor with a high ring number would provide a high sensitivity, the spectral width would also become large which is not required for the sensor. Therefore, we optimized for the square-ring number 2 for which the sensitivity and FOM both are reliable to use for practical sensing purposes.

Results presented in this paper reveal the potential of the proposed sensor to detect nanoplastics from water which is manifested in the spectral signature due to changes in the refractive index of the medium. In a realistic scenario, however, the refractive index change in the sample might occur due to other reasons as well, for example, variations in temperature or salinity, contamination of other particles etc. In such a situation, in order to enhance the specificity of the sensor, a binding agent specific to the nanoplastic may be used. A poly-peptide named Histidine-tryptophan-glycine-methionine-tryptopahn-serine-tyrosine (HWGMWSY) has been reportedly used as a binding agent for microplastics which may be explored for enhancing the specificity of the proposed sensor in our future effort30.

It is noteworthy that several efforts have been made by different research teams in order to develop MIM-based sensors comprising various resonators. Investigations led by Kum Song Ho and Song Jin Im31 presented a sensor comprising a rectangular nanocavity coupled to a MIM nanowaveguide that produces a sensitivity of 1400 nm/RIU. Khonina et al.32 reported a plasmonic sensor comprising MIM waveguide side-coupled to a square ring cavity. This was designed for the application of gas sensing that showed a high sensitivity towards carbon dioxide, i.e., 135.95 pm/ppm. Studies by Zhang et al.33 proposed a plasmonic refractive index sensor based on a MIM waveguide coupled with concentric double rings resonator that offers a sensitivity of 1060 nm/RIU. Guo et al.34 presented a plasmonic sensor based on a MIM waveguide coupled with a concentric ring and disk resonator offering a sensitivity of 1039 nm/RIU. Studies by Yuan-Fong Chou Chau et al.35showed a multi-mode plasmonic sensor based on the square ring-shaped resonators, exhibiting a sensitivity of 2473 nm/RIU. The proposed sensor in this paper presents a detailed design to tune the sensitivity through a range of 2000–4000 nm/RIU, with an optimal design (with 2 concentric rings) offering a sensitivity of 2700 nm/RIU; also depending on the application the proposed sensor works through a wide range of RI.

Conclusion

In conclusion, we have systematically investigated the dependency of the sensitivity and FWHM of the MIM waveguide sensor on the various geometrical parameters, such as resonator length, waveguide width, gap between resonator and waveguide, number of rings, coupling length etc. The results show a prominent red shift in the dipolar resonance wavelength as compared to the shift in the quadrupolar resonance. The coupling length is seen to significantly impact the transmitted intensity but not the sensitivity. The sensitivity of the sensor is found to increase with the increasing ring number and resonator length but decreases with the waveguide width. Finally, the feasibility of this resonator sensor for detecting small changes in the local dielectric environment (refractive index of the surrounding medium) affected by nanoplastic contamination in water has been explored, with initial results showing considerable promise and warranting further exploration.

Author contributions

S.G. has carried out the simulations and contributed to the manuscript preparation. S.C. participated in the simulations and contributed to the manuscript preparation. TC has overseen the collaborative project and guided through the project progression. NG shared his expertise and guidance towards planning, executing the project and preparing the manuscript.

Data availibility

All data generated during the current study are available from the corresponding authors upon request.

Code availability

All code used in this study is available from the corresponding authors upon request.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

These authors contributed equally: Shyamal Guchhait and Subhasri Chatterjee.
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