Electrodeposited Silver Dendrites on Laser‐Induced Graphene for Electrochemical Detection of Nitrate with Tunable Sensor Properties

Laser‐induced graphene (LIG) is a promising technology enabling cost‐effective, scalable, and high surface area 3D‐porous graphene electrodes for electrochemical applications. Nitrate in water bodies is a harmful contaminant to humans and the ecosystems. Its detection by electrochemical sensors is challenging due to the interference from nitrite. Herein, for the first time, a LIG‐based electrochemical sensor modified with electrodeposited silver dendrites (EdAg/LIG) without using surfactants is proposed for the detection of nitrate with tunable selectivity and sensitivity. The modified electrode surface is extensively characterized by spectroscopic and electrochemical methods and the underlying mechanism for the formation of dendrites is substantiated. The developed EdAg dendrites/LIG electrode shows excellent sensing properties for the detection of nitrate at pH 2. The interference with nitrite in acidic media is eliminated by implementing a novel strategy to shift the working pH of the electrode to 7. The achieved sensor properties at both pH values surpass other LIG‐based sensors with limit of detection of 0.46 at pH 2 and 5.53 µm at pH 7. The developed sensor also shows good recovery characteristics in mineral, tap, and groundwater across a wide range of concentrations and also demonstrates good stability under temperature fluctuations and deformations.


Introduction
Highly sensitive and selective detection of various analytes such as heavy metals, organic and inorganic molecules, bacteria, ambient conditions without using any further post-processing steps and chemicals.This allows for the production of costeffective and flexible graphene electrodes with high efficiency.Specifically, the interaction of CO 2 laser with polyimide sheet such as Kapton leads to the conversion of sp 3 carbon into a conductive sp 2 bonded graphene-like material. [22]Compared to chemical vapor deposition (CVD), which can produce singlelayer graphene, the superiority of laser-induced graphene (LIG) lies in its ability to create highly porous, conductive surfaces with a substantial surface area, that is apt for electrochemical sensing applications.The typical values of conductivities and surface area of the LIG are around 5-25 S cm −1 and 340 m 2 g −1 , respectively. [23]Further, it has been previously shown that the presence of defects on the surface enhances the electrochemical performance of the electrodes by increasing the electron transfer rate and reducing the overpotential for the reduction/ oxidation of the target species. [24]In this regard, owing to the underlying photothermal phenomena involved in the formation of LIG, the surfaces are typically defective with the presence of several functional groups.Apart from the properties, the fabrication method of LIG offers several advantages such as cost-effective and easy realization of LIG without the necessity of inks that leads to high production efficiency, and reproducibility.In addition, scalable synthesis and patterning of flexible graphene electrodes in a single step could be achieved and thus replacing the conventional mask-based approaches such as screen-printing methods for the electrochemical sensors.
In this regard, due to the above-mentioned advantages of LIG, they have been used for several applications that includes electrochemical sensing, [1] gas sensing, [25] wearable applications as sensors, [26] alternative power sources by biofuel cells, [27] electrodes for nanogenerators, [28] antenna based strain sensor for human motion monitoring [29] to name a few.The combination of silver (Ag) and LIG has been implemented in diverse fields such as the detection of NO x gas by coating the LIG electrode surface with Ag ink. [30]Direct laser writing on the pre-deposited Ag inks on polyimide substrate enabled the realization of conductive microstrip lines with low loss in the transmission of RF signals. [31]part from inks, Ag nanoparticles modified on LIG were used for the detection of contaminants [32] and also as SERS substrates. [33]oncerning the target analytes, nitrate, a byproduct of the nitrification process in the nitrogen cycle, has also been extensively employed as a synthetic fertilizer to enhance crop yield.Overconsumption of nitrate through drinking water is recognized to induce methemoglobinemia.Additionally, the transformation of nitrate into nitrosamines has been documented as a carcinogenic risk to both humans and animals.Furthermore, excessive nitrate levels contribute to algae blooms, resulting in damage to the aquatic ecosystem. [34,35]In this context, numerous electrochemical sensors have been previously reported, with a significant proportion utilizing copper (Cu) or Ag nanoparticles to detect nitrate through its reduction.However, the interference with nitrite in acidic media has been one of the problems with Cu nanoparticlemodified electrodes.[39][40] On the other hand, Ag nanoparticles are known to have superior properties as electrocatalysts for the reduction of nitrate at neutral conditions.For example, Ag nanoparticles modified on glassy carbon electrode could detect nitrate in neutral conditions with a good limit of detection (LoD) of 4.1 μm by rotating disc voltammetry couple to chronoamperometry.However, the presence of nitrite and chloride ions interfered with the sensing of nitrate. [41]Other works on Ag nanoparticle-modified electrodes [42][43][44][45][46][47] also have either ignored the investigation of nitrite as an interfering compound or have shown its influence in the detection of nitrate.To conclude, although the interference with nitrite during the detection of nitrate in acidic and neutral media has been well documented, there are no strategies developed to mitigate the problem until now.
The paper aims to elaborate on a sensitive and selective electrochemical sensor based on LIG modified with electrodeposited silver dendrites (EdAg dendrites) to detect nitrate.Ag dendrites also termed fractal structures are promising for electrochemical sensor applications as they provide high surface area, [48] hierarchical morphology, and good electrical conductivities. [49]Typical methods such as sonochemistry, [50] photochemistry, [51] electroless deposition, [52] and electrochemical methods [53] are known to use either high temperatures, or harmful reductants for the fabrication of Ag dendrites.Further, the surfactants may adsorb onto the surface which could hinder the sensing capability.Electroless deposition methods on the other hand, require long times to achieve the desired morphology. [52]Electrodeposition methods, offers the advantage of one-step synthesis of Ag dendrites with controllable morphologies under ambient conditions in a rapid way.
Initially, the paper elaborates on the fabrication, characterization, and mechanism of the formation of Ag dendrites on LIG.To the best of our knowledge, this is the first report presenting the findings on the electrodeposition of Ag dendrites on LIG surfaces (EdAg dendrites / LIG).Further, herein, the electrodeposition of Ag dendrites was achieved in an aqueous solution without any surfactants or reductants.The electrochemical and morphological characterization of EdAg dendrites/LIG is thoroughly investigated.Concerning selective and sensitive detection of nitrate, a novel strategy is implemented by leveraging on the pH dependent behavior of nitrite ions.The EdAg dendrites /LIG showcased highly sensitive detection of nitrate at pH 2 where the electrocatalytic activity of the electrode toward nitrate was maximum.However, as the interference with nitrite was inevitable at this pH, by tuning the pH value, weak interfering capability of nitrite at neutral pH on the developed electrode was identified.Hence, the detection of nitrate was continued at neutral pH with linear range and LoD well below the allowable levels even at non-optimal pH.Thus, a highly sensitive and selective electrochemical sensor is proposed for detecting nitrate in acidic and neutral media.

Characterization of Bare LIG Surface
The LIG surfaces, formed at 4 W, 30 mm s −1 , and 1200 DPI, were characterized by Scanning electron microscopy (SEM), Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR).The corresponding findings are presented in Figure 1.The SEM images shown in Figure 1a and the magnified image in Figure 1b highlights the 3D structure of the multi-layer graphene foam with a micro/macro porous morphology of the LIG surfaces as reported previously. [54,55]The unique pentagon-heptagon rings observed in the magnified images give rise to defects in the structures of the LIG surfaces. [16]The highly porous 3D structure of graphene facilitates the subsequent deposition of nanoparticles and also allows the electrolyte to reach the surface and thereby enhancing the electron transfer. [7]In addition, the high surface area due to the 3D morphology prevents the agglomeration of metal nanoparticles and ensures that catalytic sites are exposed to the target for reduction/ oxidation. [56]Figure 1c shows the FTIR spectra obtained for the Kapton sheet and LIG surface.[59] On the other hand, differences in the spectra of LIG were observed compared to Kapton.The peaks within the band 2800-3000 cm −1 are analogous to the methyl groups on the surface. [29]The band at 1735 cm −1 exists due to the vibrations from carbonyl groups [60] and 1560 cm −1 represents the sp 2 bonded carbon atoms. [61]Further to the left, the bands from 1000 to 1260 are assigned to the C─O bond stretching vibrations. [60,62]he analysis of FTIR spectra concludes the presence of oxygen functional groups on the surface of LIG.
Raman spectroscopy, being a versatile technique for analyzing carbon materials, was measured on the LIG surface.Figure 1d shows the average of the spectra obtained at the three positions consisting of the standard D, G, and 2D band, that are indicative for graphitic surfaces.The strong D band in the spectra implies the defective nature of the engraved LIG surface. [63]G band originates from the stretching vibrations of in-plane sp 2 carbon atoms in the lattice. [64]The presence of defects play a vital role in not only functionalization but also promotes efficient charge transfer from the analyte. [7]The ratio of intensities of the D band to the G band provides an estimate of defect density and the value was quantified to be 0.772 ± 0.01.2D band, a second order overtone of the D band provides the differentiation between graphemic and graphitic carbon. [65]Herein, the presence of a sharp 2D band confirms the complete conversion of Kapton to multi-layer graphene.Further, the ratio of 2D band to G band intensity obtained was 0.65 ± 0.02 that qualitatively represents tri-layer graphene. [66]

Electrochemical Deposition of Ag on LIG and its Characterization
The electrodeposition of Ag was performed by chronoamperometry that includes parameters such as deposition potential and time that has to be optimized.To investigate the effect of deposition potential, Ag was deposited on LIG at −0.2, −0.4,−0.6, −0.8, and −1 V by making sure that the charge deposited on the electrode is constant.The corresponding charge versus time plots at the defined potentials can be seen in Figure S1 (Supporting Information) in the supplementary information.Figure 2a shows the SEM images of the surfaces and its magnified images at different potentials.At −0.2 V, irregular shaped polygon crystals are formed on the surface with dimensions of ≈5 μm.As the potentials are raised to −0.4 V, these polygon structures break down or are highly distorted and aggregate into more non-uniform morphologies.At −0.6 V, the distorted crystals breakdown and nanosheets and rods structures protrude out from their surface from different directions.Ag dendrites with branches arising from either side of the stem were observed at −0.8 V.These branches also included side branches protruding and can be termed as secondary branches.In addition, some were formed perpendicular to the dendrite stem as well. [67]The dimensions of the entire dendritic structure were ≈15 μm and the twigs arising from the side branches in the order of few hundreds of nanometres.However, at −1 V, surprisingly the size of the side branches does not increase but the secondary branches combine among each other and leads to a leafy or sheet morphology on the surface.
Based on the analysis, two different mechanisms depending on the applied potentials could be interpreted.At low potentials, the mechanism is based on Volmer-Weber growth model wherein the low driving force leads to less diffusion of Ag ions to the surface and leads to aggregation-controlled growth of polygon structures (−0.2 V).Nevertheless, at −0.4 V the amount of Ag ions on the surface is sufficient to induce interparticle diffusional coupling among the neighboring particles that leads to distorted and aggregated shapes. [68,69]The formation of non-equilibrium morphologies (dendritic) at high cathodic potentials can be related with the diffusion limited aggregation (DLA) model. [70,71]erein, due to high driving force, large amount of Ag ions diffuse to the surface that initially a random walk of Ag ions on surface leads to aggregates to reduce their overall energy.Subsequent Ag ions diffusing toward the surface grow upon these aggregates resulting in the dendrite structures. [68]igure 2b shows the Raman spectroscopy of the electrodeposited Ag surfaces on LIG.An enhancement in the intensity of D and G bands can be noticed for Ag modified surfaces irrespective of the type of morphology as compared with bare LIG.Further, the increase in intensity of 2D band relative to G and D band is smaller and is almost same or lower than bare LIG.Due to the introduction of surface bound species, the coupling effect necessary for the two phonon process required for 2D band is reduced that leads to low intensity. [72]he electrocatalytic activity of the Ag surfaces with different morphologies toward nitrate was investigated in order to identify the best surface for further investigations.The peak currents from square wave voltammetry (SWV) were recorded for all electrodes.As can be seen from the pie chart in Figure 2c, the electrocatalytic activity was highest for Ag dendrites formed at -0.8 V and the lowest for Ag polygons on LIG formed at −0.2 V.The reduction in peak currents at −1 V is most likely related to the planar morphologies of Ag that reduces the number of active sites on the surface.Further, the Ag surfaces at −1 V tend to be distributed in the form of clusters rather than homogenous distribution at −0.8 V seen from the unmagnified SEM images.Figure 2c shows the energy dispersive spectra of the EdAg dendrites / LIG confirming the presence of Ag on the surface.The corresponding EDS images of the mapping of the surface are presented in Figure S2 (Supporting Information).

Electrochemical Characterization of EdAg Dendrites/LIG and pH Studies
Prior to the electrochemical analysis of EdAg dendrites/LIG, the deposition time of the electrode was optimized by recording SWV curves in 500 μm nitrate solution and 10 s provided the best response as seen in Figure S3 (Supporting Information).The electrochemical characterization of the EdAg dendrite/LIG electrodes were performed by running cyclic voltammetry curves at a fixed starting potential and increasing the anodic inversion potential in 0.1 m potassium hydroxide (KOH) solution.Figure 3a shows the CV curves by gradually incrementing the inversion potential with a step size of 0.025 V. Four anodic peaks A 1 , A 2 , A 3 , and A 4 and two cathodic peaks C 1 and C 2 were identified, and their origin and meaning are discussed by comparing with previous literature.A 1 is related to the dissolution of Ag and the formation of monolayer of Ag(OH) 2 on the surface.A 2 peak is ascribed to the monolayer of Ag 2 O formed by precipitation of Ag(OH) 2 and A 3 is due to the thickening of the monolayer of Ag 2 O. [73,74] Concerning the cathodic peaks, C 1 is related to the reduction of Ag 2 O to Ag. Concerning the peak C 2 , it doesn't exist from the start of the CV and is gradually formed as the inversion potential is increased to 0.55 V.At this potential, the anodic currents begins to rise (A 4 ) that signifies the electrooxidation of Ag 2 O to AgO and C 2 is the corresponding reduction peak of AgO back to Ag 2 O. [74,75] Figure 3b shows the catalytic activity of the developed electrode toward the reduction of nitrate at different pH values.At a pH value of 2, the maximum current was obtained from SWV that is also following with previous reports [40,76] on the investigation of electrochemical reduction of nitrate.Although the reduction of nitrate is favorable in acidic media, at pH values <2, the onset of hydrogen evolution dominates the reduction process of nitrate leading to lower currents.
Figure 3c shows the high electrocatalytic behaviour of EdAg dendrites /LIG compared to bare LIG with clearly visible reduction peak in SWV that was absent for bare LIG electrode.Herein, although LIG does not offer catalytic activity toward reduction of nitrate, the high surface is essential for depositing abundant Ag dendrites on the surface and also for electrolyte to move toward the surface.To investigate the underlying reaction mechanism, the reduction of nitrate was performed by cyclic voltammetry (CV) at different scan rates from 50 to 400 mV s −1 as can be seen in Figure 3d.The linear dependency of peak current obtained at different scan rates versus the root of scan rate as seen from Figure 3e with an R 2 value of 0.995 confirms the diffusion-controlled process for the reduction of nitrate on EdAg dendrites/LIG electrode. [38]Further, shift in the peak potential as a function of scan rate as shown in Figure S4a (Supporting Information) also suggests the irreversible nature of the nitrate reduction.The plot of (E p -E p/2 ) versus scan rate in Figure S4b (Supporting Information) is almost constant irrespective of scan rate implying the independency of the transfer coefficient on the scan rate. [77]Based on the conclusions above, the number of electrons involved in the reduction process was evaluated by the following equations [44,78] where the left-hand side in Equation ( 1) is the slope obtained from the plot of peak potential versus the (log) scan rate,  is the charge transfer coefficient, n a is the number of electrons transferred in the rate determining step.In Equation ( 2), D denotes the diffusion coefficient [79] for the bulk nitrate concentration (C) and with a value ≈2 × 10 −5 cm 2 s −1 ,  is the scan rate (V s −1 ), A is the geometric area of the electrode (7 mm 2 ) and n is the actual number of electrons transferred for the first reaction.The value of n obtained by solving both the equations was 1.83 that confirms a two-electron process in the reduction of nitrate.Among the several reaction pathways, the one involving 2e − for the reduction of nitrate to nitrite is as follows [80,81] NO

Electrochemical Detection of Nitrate and Selectivity Studies
The sensing performance of the EdAg dendrite/LIG sensor was assessed by recording SWV curves at various nitrate concentrations in a pH 2 solution with concentration of 0.1 m PBS.where SD (I blank ) is the standard deviation of current recorded in the blank solution and m signifies the slope of the regression line.The quantified LoD was 0.46 μM with a linear range from 10 to 10000 μm.The selectivity of the EdAg dendrite/LIG electrode for nitrate was assessed by recording reduction currents in the presence of common cationic and anionic interferants at ten-fold concentrations in the buffer solution.Figure 4c shows the peak current data obtained for the reduction of 100 μm concentration of nitrate in the presence of NaHCO 3 , K 2 CO 3 , KNO 3 , MgCl 2 , CaSO 4 , CH 3 CO 2 K, and 4-nitrophenol.The electrode showed almost negligible interference from other ions, with a maximum relative error percentage of 11.05% observed with NaHCO 3 as interferant.
In the case of nitrite as an interferant, at pH 2, as shown in symbol curves in Figure 4d the electrode had significant interference from nitrite during the detection of nitrate.This can be explained by the fact that nitrite disproportionate itself to NO and nitrate ions in acidic media by the following reaction [82][83][84] 3HNO 2 → HNO 3 Nevertheless by taking advantage of the fact that the pKa value of HNO 2 is ≈3.3 [85,86] and its existence diminishes in alkaline media, the detection of nitrate in the presence of nitrite was carried out at pH 7 and negligible interference was recorded as seen from the solid line in Figure 4d.To conclude, by modulating the pH values to 7 from 2 the challenge of interference with nitrite in several papers reported previously was eliminated.Further, one important aspect to highlight in order to understand the negligible interference from nitrite is the requirement of very high potentials for the reduction of nitrite ions. [82]The electrochemical reduction of nitrite at different pH values on EdAg dendrites/LIG electrode was investigated and Figure 4e confirms the low catalytic activity of the developed electrode toward nitrite at neutral pH values.
Further, the sensing properties of the EdAg dendrites/LIG electrode toward the nitrate at neutral pH were investigated by recording SWV curves from 50 to 1000 μm as seen in Figure 4f.A rise in the peak currents was observed with a rise in the concentration and the calibration curve shown in the inset a revealed a reasonably good linear fit with an R 2 value of 0.99 and linear range from 50 to 1000 μm.The LoD value was calculated from Equation (4) and was found to be 5.53 μm that is well below the permissible level.The properties obtained also indicate a high electrocatalytic activity of the electrode in nitrate reduction, which can also be detected at non-optimal pH values.The selectivity with the other ions at pH 7 was investigated similar to Figure 4c.A minimal interference with all the other ions was obtained and the corresponding bar plot is shown in Figure S5 (Supporting Information).Table 1 shows the comparison of sensor properties of the developed sensor with respect to other laser-based electrodes from the state of the art for the detection of nitrate.As can be seen, most of the electrodes were based on ion-selective membranes and the detection was done by the potentiometry method.The sensor developed in this work outperformed other electrodes in terms of LoD, even though it worked at less-than-optimal pH values.Further, the issue of interference with nitrite was mitigated by reporting on an innovative strategy of working at different pH values.The other reported research in the table either had an interference with nitrite or was not evaluated at all.
Despite the developed sensor offers superior properties, its selectivity and sensitivity are influenced by nitrite and indirectly by the pH of the matrix.Therefore, accurate assessment of real unknown matrices necessitates a system capable of measuring pH and nitrite alongside nitrate.Previous research have attempted to address these challenges by developing multi modal systems, aiming to obtain diverse information for accurate quantification of target. [90]Several reviews have been published for using multimodal detection by optical methods. [91,92]In the case of electrochemical sensors, Electronic tongue (E-tongue) concept has been widely implemented wherein a multisensory system with low selectivity is utilized to obtain huge information, followed by advanced signal processing tools to decouple the information, analyze and extract meaning from the complex data. [93]For instance, E-tongue based on potentiometric detection was developed, employing non-selective sensors for the detection of nitrate, nitrite and ammonia by using artificial neural networks for signal analysis and quantification. [94]Similarly, another E-tongue for the detection of ammonium nitrate was developed that features eight different electrodes and the diverse data obtained from these sensors facilitated selective detection of the target. [95]A combination of five noble metal and three non-noble metal electrodes was used for the detection of nitrate, nitrite and chloride.Nevertheless, the system had accuracy issues for the detection in real samples due to the effect of the complex matrix.In the present work, the developed sensor was selective toward nitrate in the presence of wide range of contaminants except for nitrite at pH 2 and at pH 7 the selectivity with nitrite was also mitigated.Accordingly, leveraging from the appreciable selectivity of EdAg/LIG toward nitrate, only two other sensors for monitoring pH and nitrite concentrations are required that is substantially lower than the number of non-selective of sensors used for E-tongue previously.This would reduce the system bulkiness, the simplifies complexity in electronics and does not require large amounts of training data for the machine learning and signal processing.

Validation in Real Samples
Testing the developed EdAg dendrite/LIG electrode in real matrices is crucial in order to evaluate its practical applicability.In this regard, the credibility of the developed sensor was tested in tap, mineral, and groundwater after adjusting the pH by the standard addition method directly without any further purification or filtration of the matrix.For the tap and mineral water, traces of nitrate were not identified before spiking with known concentrations from 50 to 500 μm.In ground water from Saxony, Germany, 890 μm concentration of nitrate was already existing prior to spiking with known concentrations in the steps of 500 μm. Figure 5a-c shows the SWV curves for the above-mentioned samples.Triplicate measurements were obtained for a single concentration and their standard deviations were included in the calibration curves.The electrode showed a linear response for all three real samples with R 2 values of 0.998, 0.995, and 0.987 for mineral, tap, and groundwater, respectively, as seen from the calibration curves in Figures 5d-f.Based on the peak currents recorded for these concentrations, the sensor showed a good recovery, with values not exceeding 15% for all the concentrations.The recovery evaluation for all the concentrations is presented in Table S1 (Supporting Information).Further to confirm the analytical capability of EdAg dendrites/LIG electrode, the effect of the matrix was evaluated by the following equation.

M.E.% = S real samples
S buffer samples × 100 − 100 (6)   where in M.E% stands for matrix effect, S real samples is the slope of the calibration curves of real samples, and S buffer samples is the slope of the regression line in the buffer solution.The slope of the calibration curve from the buffer solution was extracted by considering only the concentrations used for real sample analysis.The corresponding calibration plots are shown in Figure S6a,b (Supporting Information).The obtained values are 8.39% for mineral water, 8.11% for tap water, and 6.36% for groundwater.The low values obtained for M.E.% combined with good recoveries in the real matrices indicate an excellent analytical capability of the developed sensor.In addition, the validation of obtained results from the developed sensor was performed by commercial colorimetry test strips based on Griess reaction.In the Table S1 (Supporting Information), the not in range term implies that the strips to test at those concentrations were not available and were excluded from the study.Nevertheless, the quantified concentrations from the sensors for all the three real samples fall within the concentration range derived from the colorimetrybased evaluation and thus suggests the accuracy of the developed sensor.

Repeatability, Reproducibility, Shelf Life, and Stability of the Sensor
Another important aspect to evaluate the sensor's applicability in practical scenarios are its reproducibility, repeatability, and stability, apart from the sensing properties and capability of detection in real samples.These aspects of the EdAg dendrites/LIG sensor were evaluated at 500 μm concentration of nitrate by recording the peak currents from SWV. Regarding repeatability, 15 consecutive measurements were obtained on the same electrode and its peak currents across the measurements are shown in Figure 6a.The obtained RSD value from all the measurements was 4.07% with changes in the peak current <10 μA that indicates good repeatability of the sensor.The minor deviations in the data could be related to the time allocated as rest time before measurements.
In the case of reproducibility, seven electrodes prepared under the same conditions were evaluated by recording their peak currents measured in triplicate.As can be seen from the bar plot in Figure 6b, a minimal change in the peak currents was observed with an RSD of 1.85%, which supports the reproducible fabrication procedure of our electrodes and their functionalization by silver dendrites.However, the small variations observed in reproducibility analysis is owed to the semi manual procedure implemented for fabrication of electrodes such as applying silver and the passivation layer.Shelf life, a crucial aspect to investigate in electrochemical sensors was evaluated by recording the peak currents over a period of 30 days as seen from Figure 6c.The sensor was capable of retaining 94.1% and 84.2% of the current intensity after 20 and 30 days, respectively, which demonstrates the good shelf life of the electrochemical sensor.A nonmonotonic rise and fall in the recorded peak current values was observed especially in the initial five days.The possible explanation could be that the as fabricated sensor was directly exposed to the solution containing 500 μm nitrate solution without any preconditioning step using buffer solution.Further, the sensors were stored directly after the measurement without cleaning its surface with buffer.These steps were deliberately made to mimic the practical scenario and would have probably contributed to such a non-monotonous response of the sensor with sudden rise and fall in the recorded peak current values.Nevertheless, a monotonous fall in the peak current was observed after the first 5 days.The experiments from Figure 6a-c also reveal an important aspect that the time interval between each measurement, automation of the fabrication procedure and development of preconditioning protocol is essential to achieve very precise response from the sensor with negligible deviations.
To explore the influence of temperature on the electrochemical performance, the freshly prepared EdAg/LIG electrode was subjected to temperatures from 10 to 50 °C inside a climate chamber, mimicking practical scenarios.After equilibrating the sensor at each temperature for 30 min, the response was measured, and peak current was evaluated.As depicted in Figure 6d, the variation in temperature from 10 to 50 °C showed minimal effect on sensor response indicating the suitability of the sensor for accurate measurement in real applications under temperature fluctuations.
Besides temperature, given the flexible nature of LIG based electrochemical sensor, it is worthwhile to investigate the influence of deformation or strain on the electrochemical response.Accordingly, two different experimental approaches were investigated.In the first case, the sensor was affixed onto glass bottles exhibiting a certain curvature.The response of the sensor was assessed after 15 min, followed by its placement on a bottle with differing curvature.The images of the sensors placed on the glass bottles are shown in the inset of Figure 6e.Only a marginal reduction in the peak current, ≈2.5% relative to the initial value obtained at large curvatures was observed.Subsequently, the same sensor underwent manual deformation, as illustrated in the inset of Figure 6e.The process involved subjecting the sensor to 20 deformation cycles prior to the measurement.The peak current, as shown in Figure 6e, was largely constant even after 60 cycles of deformation.However, further deformation up to 80 cycles damaged the sensor contacts, resulting in the loss of signal.The comprehensive analysis on the influence of strain by two distinct methods underscores the sensors capability to withstand any significant strain in real applications, affirming its flexible nature.

Conclusion
LIG electrodes fabricated by a CO 2 laser, modified with electrodeposited silver dendrites were demonstrated as sensitive, selective and cost-effective alternatives for the electrochemical detection of nitrate.Initially, the electrodeposition potentials (−0.2 to −1 V) for modification of LIG by Ag were investigated and based on the obtained surface morphologies two different mechanisms depending on the applied potential were presented.Amongst the different morphologies ranging from polygon to dendrites and leafy structures, Ag dendrites had the best response toward the reduction of nitrate and were selected for subsequent investigations.
The EdAg dendrites/LIG electrode had a good sensitivity toward nitrate with a LoD value of 0.46 m and a large linear range from 10-10000 μm at a pH 2. Selectivity toward nitrate the presence of nitrite one of the major challenges as reported in the previous investigations.The challenge was overcome in this work by shifting the working pH of the solution from 2 to neutral pH values.A negligible interference with nitrite at pH 7 was recorded and the underlying mechanism was elaborated.The Ag dendrites modified electrode, although is selective to nitrate at pH 2 with different contaminants, due to the unstable nature of nitrite at acidic pH values by its disproportionation to nitrate the currents increased significantly.On the other hand, the high selectivity of the electrode toward nitrate was highlighted in case of pH 7 where the nitrite is stable and present in the solution, and the electrode has minimal changes in response in its presence.Moving on, the developed sensor revealed a reasonable reproducibility and repeatability with very small RSD and also demonstrated with good shelf life with the capability to retain almost 84% of the signal after 30 days.In addition, the sensor had minimal changes in the response with varying temperature from 10 and 50 °C.The sensor performance was also unaffected while deforming the sensor to maximum extent possible by different methods.The sensors showed overall an excellent recovery characteristic and a very low matrix effect percentage in mineral, tap and ground water.
To conclude, from a broader perspective, LIG could be a cost effective and scalable option for realizing porous graphene-based electrodes and thereby avoiding not only the complex synthesis procedures for graphene but also as an alternative for the commercial screen-printed electrodes.The realization of electrodeposited Ag dendrites without using any surfactants on LIG has been reported for the first time and these surfaces have the potential for other applications as well.Finally, coming to the electrochemical detection of nitrate, the novel methodology proposed in this work to eliminate the interference with nitrite by modulating the pH of the solution would open up a broad range of possibilities in the area of electrochemical sensors by achieving flexible sensitivity and selectivity depending on the target to monitor and intended application.In future, a multi modal sensor system featuring electrochemical sensor for nitrate, nitrite and pH would allow the developed sensor to be field deployable.By implementing machine learning with the training data comprising of the sensor response towardf nitrate and different pH values in the presence and absence of nitrite would enable accurate quantification of nitrate.
Fabrication of LIG Electrodes: The LIG electrodes were fabricated by a computer-controlled CO2 laser system (Epilog Mini 24) with a wavelength of 10.6 μm from Epilog.The three-electrode pattern was generated in the computer and laser power of 4 W, scan rate of 30 mm s −1 and 1200 DPI was selected in raster mode for engraving the LIG.A Kapton film covered with a mask was cut at prescribed locations in vector mode by the laser in the shape of the contacts and silver was applied for the contacts and reference electrodes.Once the silver was dried, the mask was removed, and LIG-based working and counter electrodes were engraved by ensuring the alignment with the realized contacts.Finally, an insulation tape was used as a passivation layer for the three-electrode system Figure 7.
Instrumentation and Experimental Methods: All the electrochemical measurements were performed by commercial potentiostat, PalmSens4 from EKTechnologies, Germany.The electrodeposition was carried out by immersing the LIG electrode in 0.1 m AgNO 3 solution and running chronoamperometry for specified times.The electrochemical detection was performed by SWV from −0.6 to −1-4 V at pH 2 and −0.9 to −1.4 V at pH 7 with a frequency of 10 Hz.FTIR analysis of the surfaces was performed by INVENIO S spectrometer from Bruker, Germany, in attenuated total reflectance mode, and the software with inbuilt functionality to compensate the influence of the environment.Raman measurements

Figure 1 .
Figure 1.Structural characterization of bare LIG surface by a) SEM images of the porous surface and b) showing the interconnected 3D network with pentagon-heptagon ring system, c) FTIR spectra of bare LIG and Kapton, and d) showing the Raman spectra of LIG.

Figure 2 .
Figure 2. a) Morphological characterization of Ag structures formed at different potentials, b) Raman spectra of LIG modified by Ag structures, c) bar diagram depicting the peak currents recorded for the reduction of nitrate with LIG modified with different Ag morphologies, and d) shows the EDS spectra of EdAg dendrites/LIG surface.

Figure 3 .
Figure 3. a) CV curves in 0.1 m NaOH solution with increasing inversion potential from 0.125 to 0.8 V, b) shows the dependency of the peak current for nitrate reduction on pH of the solution, c) electrocatalytic activity of EdAg dendrites/LIG compared to bare electrode, d) scan rate studies of the electrode from 50 to 400 mV s −1 in the presence of 500 μm concentration of nitrate and d) shows the corresponding plot of peak current recorded at different scan rates versus the root of scan rate.

Figure 4 .
Figure 4. Electrochemical detection of nitrate in pH 2 by a) SWV curves recorded from 10 to 10000 μm and inset shows the curves from 10 to 1000 μm, b) shows the corresponding calibration curve and its linear fit with R 2 of 0.984, c) selectivity investigation in the presence of ions mentioned in the bar plot, d) shows the SWV curves of nitrate recorded in the presence of nitrite at pH 2 (symbol) and pH 7 (solid), e) the response of electrode toward nitrite at different pH values and f) shows the electrochemical detection of nitrate at pH 7 from 50 to 1000 μm and inset shows the corresponding calibration curve.

Figure 4a depicts the
Figure 4a depicts the increase in the reduction current as the concentration of nitrite increases from 10 to 10000 μm and the inset shows the curves obtained from 10 to 1000 μm concentration of nitrate.The calibration curve in Figure 4b was plotted by averaging the peak currents of three measurements at specific concentrations with their respective standard deviation.The plot demonstrates a linear correlation over a broad concentration spanning from 10 to 10000 μm with coefficient of determination of 98.4% indicating an excellent linear fit of the data.The LoD value was calculated via LoD = 3 * SD ( I blank ) m (4)

Figure 5 .
Figure 5. Electrochemical detection of nitrate in a) mineral, b) tap, and c) ground water with d-f) showing the calibration curves in the same order by calculating the peak currents from the curves.

Figure 6 .
Figure 6.a) Plot of peak current versus the number of measurement son the same electrode, b) shows the reproducibility of seven different electrodes as bar plot, and c) shows the shelf life of the developed electrode as function of number of days, d) the influence of temperature on the sensor response, e) the influence of deformation or strain on the sensor response and inset shows the images of two different methods implemented for deforming the sensor.

Figure 7 .
Figure 7. Schematic showing the fabrication procedure of LIG electrode where a) shows the application of mask, b) laser cutting of the mask, c) shows the silver applied in the cut areas of laser which are the contacts for the electrodes, and d) shows the laser engraving of working and counter electrode.

Table 1 .
Comparison of sensor properties with other laser-based electrodes for the detection of nitrate from literature.