The Nanosilver Imprinted Cross‐Channel Film used in Microfluidic Chips Based on Microcavity Resonator for Mercury Assessment

In this study, a highly sensitive optical probe is designed, fabricated, and evaluated that is composed of a microfluidic device integrated with a fiber optic sensor for environmental monitoring. The resonant cavity is coated with a metal ion‐responsive molecularly imprinted nanocomposite film and monitored by a single‐mode fiber (SMF) optic sensor. The resulting Fabry–Pérot cavity‐based optical probe can accurately measure Hg levels in water with high sensitivity and stability. The nanocomposite film is synthesized using a one‐pot reaction process involving sodium alginate (SA)‐reduced nanosilver particles (nanoAg) dispersed in polyvinyl alcohol (VA). When an aqueous solution containing mercury is injected into the microfluidic device, interactions between Ag and Hg at the nanoscale form a solid Ag‐Hg solid amalgam in the resonant cavity, and surface plasmon resonance (SPR) and the Fabry–Pérot effect can be exploited to detect the presence of Hg. The synthesis, fabrication, and measurement system of this integrated device is simple and cost effective. The developed sensing heavy metals in wastewater treatment, catalysis enhancement, and nanoscale toxicity assessment, and contributes practically to achieving the sustainable development goal (SDG) of access to safe drinking water.


Introduction
Mercury is a very toxic heavy metal contaminant that persists in the environment and causes long-term health risks for humans.Hg vapor is easily inhaled and can enter the bloodstream through the lungs before spreading throughout the body.Inside cells, it is oxidized to highly reactive Hg 2+, which binds to the cysteine in proteins.The toxicity of monomethyl Hg (MeHg + ) or dimethylmercury (Me 2 Hg) is due to their ability to penetrate DOI: 10.1002/adsr.202300133cell membranes within seconds and cross the blood-brain barrier.Symptoms of Hg poisoning mainly manifest as neurological damage and affect the cardiovascular system, kidneys, and bones. [1]iber optic sensor technology is increasingly maturing with evident advantages.First, fiber optics have a low transmission loss, enabling long-distance signal transmission with minimal attenuation.Second, fiber optic sensors exhibit high sensitivity to external physical stimuli such as temperature, [2,3] strain, [4,5] acoustic vibrations, [6,7] current, [8,9] and pressure. [10,11]For sensors, the sensing layer plays a crucial role.Wetsensitive materials [12] and gas-sensitive materials [13] are commonly used for humidity and gas sensing, respectively.Optical low-coherent interferometry (LCI) is an emerging biomedical imaging technique that maintains the intensity of light transmitted through the fiber optic sensor. [14,15]Fabry-Pérot interferometer optical fiber sensors (FPIOFS) are efficient optical sensors based on LCI principles. [16,17][20][21] FPIOFS come in two types: closed and open configurations.[24][25] It is relatively easy to fabricate and demonstrates high measurement sensitivity.However, the probe is susceptible to damage, limiting its practical application.On the other hand, FPIOFS in open configuration involves injecting the test medium into the cavity, and the refractive index is determined by measuring the resonance wavelength shift. [26][29][30] Among them, Ag coatings exhibit high reflectivity in infrared light, [31,32] making them advantageous for Hg detection, biological sensing, and chemical sensing.
The detection of Hg ions in water and wastewater is crucial.There has been an increasing interest in using adsorption as a Hg detection method, particularly with the use of natural polymers, such as chitin and its derivative Chitosan, cellulose, and amino polysaccharides.Designed materials with molecularly imprinted functional groups have been used to prepare Hg-ion-sensitive materials, including dithizone, [33,34] Chitosan, [35,36] and sodium alginate (SA). [37,38]SA can form hydrogels through crosslinking, but these hydrogels have generally low stability and weak mechanical strength.On the other hand, nanomaterials and polymers exhibit larger specific surface areas, good stability, high mechanical strength, and abundant surface functional groups.Based on these properties, researchers have tried to synthesize SA composite materials with enhanced adsorption of heavy metal ions. [39,40]Moreover, when SA reacts with silver nitrate, it reduces Ag ions to nano-sized Ag particles.In 2012, Katok et al. [41] discovered the phenomenon of ultra-stoichiometric interaction between Ag and Hg.They found that the interaction between aqueous Hg(II) and Ag(0) results in a chemical ratio of 1:2, producing zero-valent Hg.As the diameter of Ag nanoparticles (NP) decreases below 32 nm, Hg(II) is reduced from aqueous solution onto the surface of AgNPs.For 11 nm AgNPs, the Hg-Ag ratio reaches 1.125:1.Additionally, Ag does not oxidize in solution but rapidly forms a Ag-Hg solid amalgam that can be fixed on inert silica substrates.This discovery has promising implications for nanoscale chemistry.In addition, the interaction between Ag particles and Hg ions generates SPR, further enhancing sensitivity. [42]Saitta et al. [43] developed an innovative SPR sensor using inkjet 3D printing.It features a microfluidic chip with sputtered Au and Ag on two channels to trigger SPR.Plastic optical fibers are used for light emission and collection, enabling simultaneous detection of refractive index and temperature changes.
Microfluidic chips utilize microscale channels and chambers on tiny chips to manipulate the flow of fluids.Microfluidics is a powerful system enabling high sensitivity, rapid analysis, and cost-effective analysis, thus, real-time monitoring of complex chemical and biological processes can be made.Guler et al. [44] introduced a method for creating microchannels on polydimethylsiloxane (PDMS) using a polymethyl methacrylate (PMMA) mold.The process involved CO 2 laser processing of PMMA in a grating pattern to produce molds for PDMS casting.The PDMS mold was then plasma-bonded to a substrate.The method allowed the production of clean and usable molds in under 20 min, resulting in microchannels with high aspect ratios (up to 2.5), narrow widths (as low as 60 μm), and low heights (as low as 23 μm).
In this research, we employ a one-pot chemical synthesis method using biocompatible and low-toxicity materials such as SA and polyvinyl alcohol (PVA) to prepare molecularly imprinted responsive films for heavy metal ion detection.The microfluidic measurement chamber was coated with Ag followed by the nanocomposite film to create a Fabry-Pérot cavity (FPC) which can be used to determine the concentration of heavy metal ions, specifically Hg, by measuring changes in the refractive index of the aqueous solution.This technique can be further developed and optimized to fabricate the next generation of microfluidic devices and optical sensors.

Theory
Optical fibers guide light waves for signal transmission.Optical fibers consist of a core and a cladding, with the core having a higher refractive index than the cladding, allowing total internal reflection at the core-cladding interface and enabling light waves to propagate within the fiber.The core is the primary region for light wave propagation while the cladding protects and encases the core.The Fabry-Pérot resonance cavity equations quantify and predict the performance of optical instruments utilizing resonant cavities.The position and width of the resonance peak can also be calculated.The refractive index and length of the cavity determine the strength and accumulation of interference effects, while the high reflectivity of the mirrors used enhances the precision and stability of the FPC.An FPC interferometer can output extremely fine interference fringes, [45] and this enhances the measurement accuracies for the incident wavelength and the detailed structure of the spectrum.The FPC sensor consists of a cavity between two partially reflecting surfaces or one partially reflecting surface and one totally reflecting surface.Therefore, total reflection is the result of two reflection powers (R 1 and R 2 ), represented by the following Equation [46] : where P i and P r are the incident and reflected optical powers, respectively, R 1 and R 2 are the reflectances, and ϕ is the phase difference.After the two beams of light pass through, the phase difference resulting from the difference in their paths can be calculated by Equation ( 2): where  is the refractive index of the cavity material, L is the cavity length,  is the wavelength of the incident light, and ′ is the angle of incidence.
When the optical path difference between two adjacent beams of light is an integer multiple of the wavelength, the transmission function has a maximum value of 1.In the case of a medium with no absorption, the reflectance of the FPC satisfies R = 1 − T where sin 2  2 = 1, which means that when the optical path difference is a half-integer multiple of the wavelength, the transmission function has a minimum value R max corresponding to the maximum value of reflectance.This can be written as Equation (3): For the transmission function, the wavelength interval between two adjacent transmission peaks is the free spectral range (FSR) of the FPC.
SPR under resonance conditions can be described through the interference superposition of reflected light.When the frequency of incident light matches the resonant frequency of electrons in the metal, maximum interference superposition occurs, resulting in a resonance phenomenon.Therefore, the resonance condition can be expressed using the following formula [47] : In the SPR resonance condition, n eff () is the effective refractive index, n metal () is the refractive index of the metal, Δn() signifies is the refractive index difference between the metal and the dielectric, and n medium () indicates the refractive index of the dielectric.Under the SPR resonance condition, the incident light is absorbed, and a localized enhancement of the surface electric field occurs at the metal-dielectric interface.This makes SPR a sensitive optical detection technique that can be used to monitor changes in substances, such as variations in refractive index in the liquid phase or surface adsorption events.

Design and Fabrication of Microfluidic Chips
To manufacture microfluidic chips with a poly(methyl methacrylate) (PMMA) substrate, a CO 2 laser engraver (Beambox Pro, FLUX) was utilized.The microfluidic chip was designed as a multi-layered structure comprising of upper and lower covers and the channel itself, as shown in Figure 1a.Each layer had a thickness of 1 mm, and two positioning holes with a diameter of 4 mm were included to facilitate accurate alignment during assembly and prevent misalignments.The channel design includes straight channels for the inlet, outlet, and detection area.To enable efficient fluid monitoring, a detection area with a diameter of 3 mm allows the fluid to dwell for measurement, as depicted in Figure 1b.A parallel beam with a diameter of ≈5 mm was produced by the laser head, which features a glass focusing lens that narrows the beam to ≈0.2 mm in diameter.Due to minimal divergence, the parallel beam could propagate through the system with negligible energy loss.The laser power is set to 27.5 W, and the cutting speed is 150 mm s −1 .The microfluidic chip was composed of three layers as shown in Figure 1c.
Reflective surfaces were required to obtain signals in the Fabry-Pérot sensor, therefore, a high-reflectivity Ag film was deposited on the chip substrate.The metal film was deposited using direct current sputtering (DC Sputtering System SPS-501, I SHIEN Vacuum Corporation).DC sputtering utilizes gas glow discharge to generate a positive ion beam in the plasma to bombard the Ag target.The Ag deposition rate could be precisely controlled to obtain an even coating of the substrate surface with the required film thickness.To enhance the production of highenergy ions for target bombardment, Ar gas was used as the sputtering gas.Since the target may be oxidized or contaminated by particles, a pre-sputtering process was carried out for half an hour before sputtering onto the substrate, ensuring the deposition of high quality Ag film.The sputtering parameters were set with a flow rate of 15 SCCM, a chamber pressure of 10 mTorr, and a power of 40 W. For Ag films with thicknesses above 200 nm, the reflectivity does not change with increasing thickness, [48][49][50] so the sputtering time was set to 1200 s.
To bond the microfluidic chip layers and achieve a leak-proof seal, a hot press was used (HT-81228, Hung Ta Instrument Co. Ltd.).Hot pressing is a simple, fast, and cost effective bonding method.Before hot-pressing, the microfluidic chip layers were cleaned with deionized (DI) water and dried in an oven at 75 °C.Next, the chip was positioned on the heat press platform, and then screwed tight to ensure proper alignment of the hotpressing pattern.Heat-resistant tape was used to bond the two sides of the chip together, preventing any displacement during the hot-pressing process.Heat-resistant paper was also used to cover both the top and bottom surfaces of the chip to avoid contamination from the iron blocks of the heat press.

Fabrication of the Crosslinked Sensing Layer Incorporated with AgNP
A one-pot method was used to fabricate the sensing layer composed of SA-reduced nanosilver particles (nanoAg) embedded in the PVA film [51] (SA/nanoAg/PVA).SA enables the facile reduction of AgNPs [52][53][54] with various functional groups. [55]The synthesis processes are shown schematically in Figure 2. SA and AgNO 3 with a purity of 99.85% were procured from Honeywell Fluka.PVA was obtained from Sigma-Aldrich.DI water was used as a solvent.The experimental set-up comprised a precision balance (Model XS-625M-SCS, Precisa), magnetic stirrer (Model HMS 102, Hongyu Instrument Co., Ltd.), and relevant laboratory glassware (UNI-ONWARD Co., Ltd.)The preparation process was as follows: First, 0.5 wt.% SA was stirred in DI water at 70 °C, releasing numerous carboxyl groups (-COOH) and hydroxyl groups (-OH) into the solution from SA. Next, 60 mm AgNO 3 was added to the reaction vessel, and the mixture was stirred at 70 °C for 30 min.Finally, 0.05 wt.% PVA, dissolved in DI water, was added to the same reaction mixture and stirred thoroughly for 30 min.The resulting solution could be dispensed on a surface to form a nanocomposite film containing nanoAg after curing (SA/nanoAg/PVA), which could be used for the selective recognition and detection of Hg ions.The surface morphology of the nanocomposite thin films was observed by field emission scanning electron microscopy (Zeiss Auriga, with advanced focused ion beam system, FIB-SEM) with energy dispersive X-ray

Fiber Optic Sensor and Microfluidic Chip System
The detection area of the microfluidic chip was coated with SA/nanoAg/PVA (Figure 3).First, N 2 gas was used to blow away contaminants and dust particles from the Ag film-coated microfluidic chip as impurities negatively impact sensor performance.Next, a dropper was used to dispense SA/nanoAg/PVA  solution onto the detection area of the microfluidic chip.This step requires precise handling to ensure even coverage of the entire detection area, which in turn enables efficient interaction with the target analyte.After coating, the solution-coated chips were placed in an electric oven (Model RUD-302, Hongjun Instruments Co., Ltd.) and the chips were cured at a temperature of 50 °C for 24 h to allow for drying.This step was essential in forming a robust SA/nanoAg/PVA film on the chip surface, ensuring the stability and reliability of the sensor.Therefore, a highly integrated sensor structure was fabricated.
The influence of the cavity length on the resonance wavelength was evaluated by distance between the tip of fiber optic wire and resonance cavity.To determine the optimal resonance wavelength for monitoring fluid refractive index changes, an encapsulated single-mode fiber (SMF 28) was connected to an optical spectrum analyzer (OSA) (Model MS9740A, Anritsu) and a superluminescent diode (SLD) (Dense Light) was used as the light source.The light was transmitted through a 2 × 1 coupler.The height was adjusted using a height gauge, and the resulting spectrum was recorded at each setting.Once the optimal height was determined, the encapsulated optical fiber was securely fixed onto the microfluidic chip using UV adhesive.The schematic diagram of the experimental setup is shown in Figure 4.

Detection of Hg Ions by the Fabry-Pérot Resonator Microfluidic Sensing System
In this experiment, the liquid was introduced into the microfluidic chip using an infusion pump to control the fluid velocity and pressure.The experiment maintained a constant flow rate of 10 μL min −1 in the microfluidic channel. [56,57]Different concentrations of heavy metal aqueous solutions were supplied through the microfluidic channel, and the spectral changes were recorded and observed using the spectrometer.As described in Ref., [58] when the pH is acidic, the amino and carboxyl groups on the sensitive membrane are protonated, resulting in the release of chelated Hg ions.Therefore, after each cycle, the microfluidic chip was rinsed using DI water and a pH 5.0 phosphate-buffered saline (PBS) solution (UNI-ONWARD Corp.).The experimental set-up is illustrated in Figure 5.A cavity height of 100 μm was selected.This height offers a larger FSR for resonance and ensures good stability of the resonance characteristics.

Results and Discussion
Hg ions were detected using a Fabry-Pérot resonator microfluidic sensing system.The detection area of the microfluidic chip was coated with a SA/nanoAG/PVA thin film which was synthesized using a one-pot reaction method (Figure 6a).Hg 2+ (aq) can be detected by the SA/nanoAG/PVA via SPR phenomena.The surface morphology and element distribution of the coated chip was examined using a scanning electron microscope (SEM) equipped with a focused ion beam (FIB) and EDS.The integrated sensing system was connected to the OSA and SLD via a 2 × 1 coupler.Optical spectra for different concentrations of Hg ions in solution were measured to optimize the resonance effect and determine the effect of cavity length on the resonance wavelength.
Figure 6b shows an SEM image of the sputtered Ag coating on the detection area, revealing a uniform deposition of Ag with grain sizes ≈100 nm.The XRD spectrum of the deposited Ag, presented in Figure 6b, clearly indicates a polycrystalline structure with prominent peaks at 2 angles of 38.11°, 44.30°, 64.45°, and 77.40°.These peaks are attributed to the (111), ( 200), (220), and (311) reflections of sputtered Ag, which were confirmed through literature and matched with the JCPDS (Joint Committee on Powder Diffraction Standard) data for Ag. [59,60]EDS analysis of the Ag surface shows a clear spectrum, demonstrating the even distribution of Ag in the detection area.The Ag content was found to be approximately 96.42 wt.%.
Figure 7 displays SEM and EDS images.It can be observed that Ag is uniformly deposited on the surface, along with evident pits, possibly due to the matrix properties of the cured nanocomposite film (Figure 7a).EDS analysis of the SA/nanoAg/PVA thin film reveals that it is composed of C, O, Na, and Ag with approximate contents of 11.40 wt.%, 31.80 wt.%, 5.15 wt.%, and 51.65 wt.%, respectively.To further investigate element distribution, EDS mapping was conducted.Figure 7b reveals the even distribution of Ag and Na on the surface of the thin film, indicating the homogeneous dispersion of SA and silver nitrate in the aqueous solution.Additionally, owing to the properties of PVA, a compact network structure was formed.PVA exhibits excellent adhesion and film-forming properties, enabling the formation of a continuous matrix that provides a robust network structure to support and encapsulate SA (Figure 7c) and nanoAg particles (Figure 7d), thereby achieving their uniform distribution.Therefore, PVA plays a crucial role in the preparation process.
UV-vis spectroscopy is a standard tool for characterizing the formation and reactivity of AgNPs.Specifically, the UV-vis spectra of the synthesized AgNPs exhibit a distinct and broad SPR band ranging from 430 to 460 nm. Figure 8a shows different absorption peaks at different reaction temperatures (50 °C, 70 °C, 80 °C, and 90 °C) within the wavelength range of 300 to 700 nm.For AgNP formation in SA aqueous solution, the weight percentage of SA and the reaction time were kept constant.At 90 °C, the ability to convert Ag ions into Ag particles is more critical, resulting in a relatively lower SPR peak, as most of the ions have already been converted into particle form.At 50°C, the reaction temperature is insufficient to produce a significant SPR peak.However, at 70°C, the conversion temperature is ideal, exhibiting a high SPR peak, making it the perfect reaction temperature for the metallic sensing layer.These findings illustrate that precise control over the AgNP formation process and their optical prop-erties can be achieved by manipulating the reaction temperature, resulting in the distinct SPR shown in Figure 8a.Consequently, a reaction temperature of 70°C was determined as optimal for obtaining SA/nanoAg/PVA nanocomposite thin films with outstanding optical characteristics that can be used as the sensing layer for microfluidic chips.
Functional groups on a surface can initiate chemical reactions or adsorb specific molecules or ions.In this study, the prepared SA/nanoAg/PVA film contains carboxylate ion function groups, -CH, and -CO.These functional groups undergo specific reactions with Hg ions, providing the film with the capability to detect Hg ions.From the FTIR spectrum, the following characteristic features were observed: -OH stretching vibrations at 3749 cm −1 and 3661 cm −1 , -CH stretching vibrations at 2977 cm −1 and 2341 cm −1 , carboxylate ion group (-COO) stretching vibration at 1571 cm −1 , -CH stretching vibration at 1391 cm −1 , and -CO stretching vibrations at 1252 cm −1 and 1058 cm −1 .Additionally, the bending oscillation of -COH was observed at 911 cm −1 , as shown in Figure 8b.These observations indicate that the SA/nanoAg/PVA film contains multiple functional groups, providing favorable conditions for the detection of Hg ions.
The microfluidic chip with a built-in Fabry-Pérot resonator was designed for Hg ion sensing, with the Ag metal film as the reflecting layer and the SA/nanoAg/PVA coating as the sensing layer.From the measured spectra, it is evident that as the concentration of Hg ions in the aqueous solution increases from 0 to 40 ppm, the resonance wavelength is redshifted, and the transmission losses fluctuate.During the experiment, the system was washed with DI water and PBS.The redshifting phenomenon is due to the SPR effect.When Hg ions come into contact with AgNPs, it induces surface electric field oscillations and electron oscillations, leading to changes in the refractive index of the composite SA/nanoAg/PVA film which redshifts the resonance wavelength, in accordance with Equation ( 4) and as shown in Figure 9a.
The resonance peaks in the 1525 nm to 1535 nm wavelength were measured and the results for three sensing cycles are summarized in Table 1 and presented in Figure 9b.Analysis of the results reveal that the adsorption of Hg ions on the film surface follows a multi-step process, and at low concentrations, the reaction is more pronounced.Saturation is reached at 20 ppm Hg concentration, where the resonance wavelength almost remains unchanged, resulting in a maximum total wavelength variation of 8.016 nm.At concentrations ranging from 0.001 to 10 ppm, the sensitivity and linearity are found to be 0.450 nm ppm −1 and above 0.75, respectively.Remarkably, at concentrations from 0.01 to 10 ppm (as shown in Figure 9c), the sensitivity only decreases by 0.007 nm, while the linearity remains above 0.92.Statistical analysis of the dispersion in the three cycles, as depicted in the standard deviation analysis graph, shows that the standard deviation of the resonance wavelength in all three cycles is below 0.2 nm.For concentrations of 0.01 to 10 ppm (Figure 9d), the average wavelength sensitivity and linearity are calculated as 0.391 nm ppm −1 and 0.930, respectively.
For the microfluidic chip, a cavity height of 100 μm was applied, and a remarkable linearity exceeding 0.85 for both transmission loss and resonance wavelength was achieved.For comparison, the optical spectrum of a FPC without SA/nanoAg/PVA nanocomposite thin film coating was measured (See Supporting Information).This device yielded a narrow reflection spectrum transmission dip of 1 to 2 nm (Figure S1, Supporting Information, blue solid line).In contrast, the optical spectrum of the FPC with SA/nanoAg/PVA nanocomposite thin film coating has a broad reflection spectrum transmission dip of 4 to 5 nm (Figure 9a).This broad dip is due to the induced SPR in Hg ions by the SA/nanoAg/PVA/Ag composite film.
For Hg ion concentrations ranging from 0 to 40 ppm, as the concentration increases, the resonance wavelength is redshifted.For Hg ion concentrations ranging from 0.001 to 10 ppm, sensitivity linearity remains at 0.92 or higher.The standard deviation of the resonance wavelength for three measurement cycles remains below 0.2 nm.When the sputtered Ag and SA/nanoAg/PVA multilayer composite interacts with Hg ions at the nanoscale, compounds with a sub-stoichiometric ratio are formed.This interaction was verified through SEM imaging, as shown in Figure 10a.EDS characterization of the multilayer composite coating exposed to Hg reveals the presence of C, O, Na, Ag, Hg, with approximate weight percentages of 9.56%, 13.32%, 0.18%, 34.39%, and 42.55%, respectively.The interaction between aqueous Hg(II) and Ag(0) occurs in a 1:1.5 stoichiometric ratio.This process leads to the reduc-tion of Hg(II) from aqueous solution onto AgNPs, resulting in the formation of elemental Hg.
EDS elemental mapping results indicate a uniform distribution of Ag (Figure 10b), Na (Figure 10c), and Hg (Figure 10d) on the surface of the film.Significantly, the distribution of Hg is concentrated on the surface layer.This observation suggests that the functional groups released by SA can effectively bind to Ag nanoparticles, creating molecular imprints for Hg in Figure 10e.As a result, the adsorption of Hg onto the film surface is  confirmed.The composite nanofilm coating of the microfluidic chip can be prepared with a simple one-pot synthesis method, and the Fabry-Pérot effect can be achieved by first coating the microfluidic channel with a thin layer of Ag by sputtering.Therefore, an economically viable and sensitive sensor combining microfluidics and fiber-optics that can be utilized for sensing heavy metal ions was successfully fabricated and evaluated in this study.

Conclusions
The properties and performance of a fabricated Hg sensor composed of a PMMA microfluidic device coated with highly reflective Ag followed by SA/nanoAg/PVA film were investigated and the following conclusions were drawn: First, the spectral results show a significant wavelength redshift of the resonance peak with an increase in Hg ion concentration in aqueous solution.
Therefore, the change in the resonance wavelength is strongly correlated with Hg ion concentration.This phenomenon can be attributed to induced SPR.When aqueous Hg(II) interacts with Ag(0), forming a Hg-Ag solid amalgam with a stoichiometric ratio of 1:1.5, Hg(II) is reduced from solution, generating elemental Hg that induces oscillation of the surface electric field of AgNPs.This further alters the refractive index of the nanofilm, resulting in a shift in the resonance wavelength.Second, by statistical analysis of the spectral graphs, we observe varying slopes of resonance wavelength changes at different concentration ranges.In the low concentration range (0.01 to 10 ppm), the resonance wavelength is highly sensitive to concentration changes, exhibiting a high sensitivity and linearity of 0.391 nm ppm −1 and 0.930, respectively.As the concentration exceeds 20 ppm, the shift in resonance wavelength becomes more gradual, possibly due to the saturation of adsorption.Furthermore, we conducted  SEM observations and elemental analysis of the sensing layer.We found that after the sensing experiments, the morphology of the film remained relatively unchanged, and the surface coating contained elements including C, O, Na, Ag, Hg, with approximate weight percentages of 9.56%, 13.32%, 0.18%, 34.39%, and 42.55%, respectively.EDS mapping revealed a uniform distribution of Ag, Na, and Hg in the film, with a particularly dense distribution of Hg elements on the surface layer.This indicates that functional groups released by SA can bind to AgNPs, creating molecular imprints for Hg.This further confirms the adsorption of Hg ions, demonstrating the stability and reliability of the sensing layer.In conclusion, this study successfully achieved the detection of Hg in aqueous solutions using an integrated sensor composed of a microfluidic device with a built-in Fabry-Pérot resonator coated with a SA/nanoAg/PVA film and monitored using a fiber optic sensor.The integrated sensor demonstrated excellent performance with high sensitivity and remarkable linearity.This study provides a new fabrication and optical sensing strategy for developing efficient and reliable sensors for heavy metal ions.

Figure 1 .
Figure 1.a) The three-layer structure of the PMMA microfluidic chip.b) Channel width, length, and FPC resonance cavity dimensions c) Photograph of the actual microfluidic chip layers.

Figure 3 .
Figure 3. Schematic of SA/nanoAg/PVA film coating.SA/nanoAg/PVA solution is uniformly dispensed onto the detection area of the microfluidic chip.The solution is dried to produce an SA/nanoAg/PVA film that facilitates effective interaction with the target analyte, specifically, Hg.

Figure 4 .
Figure 4. Schematic the height experiment setup.The single-mode fiber is positioned over the detection area of the microfluidic chip.Fabry-Pérot resonator spectra were measured at different fiber optic heights to determine the optimal resonance wavelength.

Figure 5 .
Figure 5. Schematic diagram and photograph of the actual measurement setup for Hg detection.An infusion pump introduces the liquid into the microfluidic chip.This configuration measures the changes in the refractive index the fluid and enables spectrum analysis to be performed.

Figure 6 .
Figure 6.a) Process for coating the detection area of the microfluidic chip with SA/nanoAG/PVA thin film.b) SEM image of the Ag film obtained through sputtering using a DC sputtering.c) XRD spectrum of the Ag film.

Figure 7 .
Figure 7. a) SEM image of the prepared SA/nanoAg/PVA thin film.b) EDS mapping elemental analysis stack image of the SA/nanoAg/PVA thin film.Individual EDS mapping image for c) Na and d) Ag.

Figure 8 .
Figure 8. a) UV-vis spectra and b) FTIR spectra of SA-reduced AgNPs at different temperatures.

Figure 9 .
Figure 9. a) Spectral plot of Hg ion aqueous solution using the integrated microfluidic and fiber optic sensing system.b) Standard deviation analysis of three cycles for Hg ion detection.Standard deviation analysis of three cycles for Hg ion detection in the c) 0.001 to 10 ppm and d) 0.01 to 10 ppm range.

Table 1 .
Sensitivity and linearity results for the SA/nanoAg/PVA and Ag dual-layer coated detection system with built-in Fabry-Pérot resonator for Hg ion aqueous solution for three cycles.