Electrochemical synthesis of 2D-silver nanodendrites functionalized with cyclodextrin for SERS-based detection of herbicide MCPA

Surface enhanced Raman spectroscopy (SERS) is a powerful analytical technique that has found application in the trace detection of a wide range of contaminants. In this paper, we report on the fabrication of 2D silver nanodendrites, on silicon chips, synthesized by electrochemical reduction of AgNO3 at microelectrodes. The formation of nanodendrites is tentatively explained in terms of electromigration and diffusion of silver ions. Electrochemical characterization suggests that the nanodendrites do not stay electrically connected to the microelectrode. The substrates show SERS activity with an enhancement factor on the order of 106. Density functional theory simulations were carried out to investigate the suitability of the fabricated substrate for pesticide monitoring. These substrates can be functionalized with cyclodextrin macro molecules to help with the detection of molecules with low affinity with silver surfaces. A proof of concept is demonstrated with the detection of the herbicide 2-methyl-4-chlorophenoxyacetic acid (MCPA).


Introduction
Photonic sensors are being applied to a wide variety of sensing applications.[1] These sensors can utilize various techniques such as fluorescence spectroscopy [2], infrared spectroscopy [3], plasmonic-based sensors [4], Raman spectroscopy [5], or surface enhanced Raman spectroscopy (SERS) [6].Of these, surface enhanced Raman scattering (SERS) is an emerging optical-based technique that has shown tremendous potential for trace analysis of contaminants [7] relevant to the food or environmental sectors [8,9].In SERS, the Raman signal is significantly enhanced at the surface of metallic nanostructures.Enhancement is generally divided into chemical and electromagnetic enhancements [10].Chemical enhancement relates to the increased polarizability of the target molecule upon adsorption onto the SERS surface while electromagnetic enhancement occurs due Nanotechnology Nanotechnology 35 (2024) 285704 (9pp) https://doi.org/10.1088/1361-6528/ad373c* Author to whom any correspondence should be addressed.
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to the plasmonic nanogaps or 'hotspots' located in between the metallic nanostructures.In the last couple of decades, a number of approaches have been investigated for the fabrication of SERS substrates [11].Typically, fabrication techniques employed include UV photolithography [12], electron beam lithography [13], nanoimprint lithography [14], and templating [15].The morphology of the nanostructures includes particles [16], triangles [17], or spheres [18].Recently, SERS based on nano dendritic structures has also been reported.Techniques used to obtain the dendrites include galvanic displacement [19,20] and chemical methods [21][22][23].Direct electrochemical methods are also used and generally require the addition of surfactants or additional chemicals to stabilize the growth [24].In those reports, the SERS nanodendrites were grown in 3D [25].While this provides a higher density of hot spots and therefore higher Raman enhancement, these structures are more prone to be washed away during cleaning steps and when carrying out the analysis in aqueous conditions.
While SERS is a sensitive technique, it in essence relies on the interaction of the metal surface with the target analyte.When using a 'dip and dry' approach (where the SERS substrates is dipped in liquid and allowed to dry before undertaking the Raman measurement), the target molecules eventually interact with the sensor surface as the solvent evaporates.When undertaking measurements in liquid however, the target molecules can diffuse in the solvent if they do not have a strong affinity with the metal surface (thiolated molecules typically form strong bonds with metal surfaces).One possible way to overcome this is to functionalise the surface of the sensor with layers that can capture the target analyte.This approach has recently been reported for SERS based sensing using capture layers such as antigen for biomolecular detection (SARS-CoV-2 virus, biomarkers) [26], aptamer for detection of specific DNA strands or bacteria [27], calixarene for detection of quinacridones [28] and cyclodextrin for detection of fluoroquinolone antibiotics [29].
In the present paper, we report on the rapid and costeffective synthesis of 2D silver nanodendrites on Si substrates.The synthesis is based on the electrochemical reduction of silver salts at microelectrodes.This approach requires only low volumes (10-15 μl) of chemicals of relatively low toxicity and is straightforward to be scaled up.The fabrication method presented here occurs on the order of seconds whereas other electrochemical methods can take up to an hour [24].The SERS substrate fabrication was visualized under a microscope, and it was observed that the nanodendrites grew horizontally along the substrates.The formation can be explained by the interplay between electromigration and the mass transport of silver ions.Electrochemical characterization of the fabricated substrates suggests that the nanodendrites are not fully electrically connected to the microelectrode after deposition.These samples show SERS activity with an enhancement factor (EF) of ~10 6 obtained.To demonstrate the efficacy of the sensors, 2-methyl-4-chlorophenoxyacetic acid (MCPA) was selected as it is a widely used phenoxy herbicide used to control broad leaf weeds in pasture and cereal crops.Atomistic simulations were undertaken and revealed that the interaction between MCPA and the Ag surface was unfavorable.To overcome this limitation, substrates were functionalized with γ-cyclodextrin.Cyclodextrins are cyclic oligosaccharides made up of repeating glucopyranose units [30] and have been shown to form inclusion complexes with organic contaminants owing to the hydrophobic moieties in their core [31].Using this approach and chosen experimental conditions, detection of 1 mM MCPA in water was obtained.

Fabrication of microelectrode on chip
Silicon chip bearing Pt microelectrode was fabricated as described in [32,33], see figure S1 (supplementary information).Briefly, the sensors were fabricated on four-inch silicon wafer substrates with a 300 nm layer of thermally grown silicon dioxide.Firstly, the working electrodes were patterned using photolithography and thermal evaporation (50 nm of Pt, with 10 nm of Ti adhesion layer) followed by lift-off.A second optical lithographic and metal deposition process (Ti 10 nm/Au 100 nm) was undertaken to define HDMI pin-out, interconnection tracks, as well as the on-chip counter electrode (500 μm wide × 6 mm long).A third similar step was used to define the Pt (Ti 10 nm/ Pt 100 nm) on-chip reference electrode (500 μm wide × 6 mm long).Finally, 500 nm of PECVD SiN was blanket deposited on the whole wafer, and openings over the working/counter/reference electrodes and electrical contacts defined by lithography and dry etching.

Electrochemical characterization
Cyclic voltammetry procedures were undertaken using a portable CH Instrument 1220C bi-potentiostat (CH Instruments, Inc. Austin, Tx).A three-electrode configuration was used for electrochemical deposition of Ag nanodendrites with Pt microband arrays as working electrodes, gold on-chip counter electrode and, platinum on-chip pseudo reference electrode.Electrochemical impedance spectroscopy (EIS) measurements were carried out using Autolab Bipotentiostat/ Galvanostat (Metrohm, Netherlands) undertaken using a three-electrode silicon sensor chip with Pt microelectrode arrays as working electrodes, gold on-chip counter electrode and platinum on-chip pseudo reference electrode (frequency range: 0.01 Hz-10 kHz; amplitude: 5 mV).

Fabrication of Ag nanodendrites
SERS substrates were prepared using 10 mM AgNO 3 aqueous solutions.For electrochemical deposition, a small aliquot (ca 10 μl) of the silver solution was deposited on a chip surface covering all three electrodes (working, on chip pseudo reference and, counter electrodes).Chronoamperometry was applied to fabricate the SERS substrates by electrodeposition followed by rinsing in DI water and drying under a nitrogen flow for several seconds.

Optical, AFM and SEM characterization
An electrochemical setup could be placed under the microscope in order to visualize and record the deposition process.A Leica DMRB microscope equipped with a 20× objective (Olympus LMPLFLN, 0.40 NA) and a Thorlabs C1284R13C camera was used.SEM characterization was undertaken with a FEI QUANTA 650 HRSEM with energy dispersive x-ray spectroscopy (EDX Oxford Instruments INCA energy system).AFM images were undertaken in tapping mode with a Bruker Nanoscope dimension icon atomic force microscope.Electrochemical characterization of the substrates was undertaken using cyclic voltammetry as described above.

Raman measurements
Raman spectra were acquired with a Horiba XploRA Plus confocal Raman microscope equipped with a 532 nm laser.For measurements in liquid, the SERS substrates were placed in a dedicated holder.A 2 μl aliquot of the solution was deposited over the substrate and covered with a glass slide coverslip.All measurements were acquired using a 10× microscope (Olympus, 0.25 NA).All spectra presented herein are an average of 10 individual scans.A laser power of 0.35 mW and an integration time of 10 s were used.Smoothing and baseline correction were undertaken with Labspec 6 software.

Computational details
Density functional theory (DFT) calculations were carried out using the VASP5 code [34,35].The core electrons were described by pseudopotentials constructed with the projector augmented-wave (PAW) method [36], where Ag 5s 1 and 4d 10 , O 2s 2 and 2p 4 , Cl 3s 2 and 3p 5 , C 2s 2 and 2p 2 , and H 1s 1 are considered valence states.The exchange-correlation functional was approximated with the Perdew-Burke-Ernzerhof (PBE) functional [32].The cut-off energy for the plane waves is set to 400 eV, and the convergence criteria used for energy and atomic forces are 10 −4 eV and 0.02 eV Å −1 , respectively.Spin polarization was considered for all calculations, with a Γ-point sampling grid.The Ag surface was cleaved from a bulk fcc structure with an equilibrium DFT lattice parameter 4.17 Å.Our surface model uses an Ag (111)oriented slab with four layers, a side length of 17.67 Å and hexagonal symmetry.The supercell size was selected to minimize the adsorbed molecule's periodic interactions across the periodic boundary conditions.The adsorption was simulated by DFT relaxation of a single MCPA molecule at different orientations with respect to the surface in order to assess the most favorable configuration.The adsorption energies (E ads ) were computed with the following expression, including vdW interactions with the Grimme's D3 method [37]: where E surface and E total are the energy of the system before and after the adsorption of the respective adsorbate.The energy of the adsorbed molecule (E adsorbate ) was calculated for an isolated molecule, using the same computational setup as the SERS surface.

Fabrication of the SERS substrates
Silicon chips used for the fabrication of the SERS sensors can be seen in figure 1(a).These sensors chips were originally developed for electrochemical based sensing applications [38].The chips have a HDMI-C edge connector format for easy connectivity to a potentiostat and bear eight individually addressable electrodes.Interdigitated Pt microbands arrays (1 μm wide, 40 μm long, 50 nm height and 2 μm gap) were used for SERS substrate fabrication.Figure 2(b) shows the experimental setup used to record videos of nanodendrites formation.The sensors chips were placed in a dedicated holder and a small aliquot of an AgNO 3 solution dropped on top of the electrodes.Chips were then covered with a coverslip and imaged under the microscope, allowing real-time monitoring of the SERS sensor's fabrication.The current approach is rapid, scalable, and environmentally friendly due to the low volumes of chemicals required.Also, unlike silver nanoparticle synthesis using wet chemistry, the nanostructures do not require stabilizing agents and are pure silver, allowing easy post-synthesis functionalization via, e.g.thiol chemistry.This method also ensures that no residues, such as citric acid, would be incorporated into the metallic nanostructured surface which could potentially interfere with the recorded spectra.

Electrodeposition process
Figure 2(a) shows a cyclic voltammogram acquired at a scan rate of 100 mV s −1 at a single Pt microband.When the CV is swept cathodically it is observed that an under deposition of silver is initially observed at −0.2 V.By increasing the applied potential further, below −0.6 V, a rapid dendritic silver growth is observed.Dendrites continue to grow during the reverse (anodic) sweep until the applied potential is more positive than −0.6 V.As the anodic sweep is swept more positive a sharp peak can be seen at a potential of ~0.1 V.This sharp peak corresponds to the stripping of electrodeposited silver.Following CV characterization, SERS substrates were subsequently fabricated using an amperometric approach where a potential of −0.65 V applied to the working electrode for 10 s.A typical current versus time plot is presented in figure 2(b).It can be seen that in this deposition process, the current does not plateau but instead keeps increasing exponentially as the potential is applied with time.This suggests that the overall deposition process is not diffusion limited since the growth continues laterally across the electrode surface.
Figures 2(c) and (d) are optical micrographs of an interdigitated electrode recorded after 2 and 7 s, respectively (a video is available in the SI).As expected, the silver deposition starts initially on the working electrode.Interestingly, under the experimental conditions chosen here, the silver deposition is observed to grow laterally away from the working electrode.The lateral growth rate was measured to be about 10 μm s −1 .

Electrodeposited electrode topography
When inspected under a 100× magnification, the electrodeposited material adopts a dendritic like formation, with a number of stalks originating from the working electrode.As the deposition process continues, these stalks are decorated with lateral branches, see videos S2 in SI and figure 3(a), and the dendrites grows along the substrate, away from their original locations.No overlap between the branches, or growth in a vertical direction was observed.Figure 3(b) shows a typical SEM of the electrodeposited material, where the dendritic structure can be observed in more details.The dendrites were composed of nanoparticle clusters in close proximity to each other.A high number of nanosized gaps (that can support electromagnetic hot spots) can be observed between adjacent nanostructures.Figure 3(c) shows a typical AFM 2D image and shows the silver nanodendrite structure on a large scale.The average height measured in the middle of the structure was measure to be 20 nm (see figure S3).The average roughness across 1 μm 2 was calculated to be 125 nm. Figure 3(d) shows the 3D-AFM image of nanostructure above the electrode, the average roughness of 426 nm across 1 μm 2 was calculated indicating the denser nanostructure above the microelectrodes.This is consistent with the formation process (see section 5 below).

Electrochemical characterization
The nanocluster dendrite formation observed under the microscope (video in SI figure 2) as well as the SEM and AFM images in figure 4, suggest there is always a continuous path from any point on the nanodendrites back to the working microelectrode.To test whether the nanodendrites were electrically connected to a microband, CV characterization of a modified electrode using a 10 mM ruthenium in 10 mM PBS solution was undertaken.Ruthenium hexafluoride was chosen as its redox reaction occurs at negative potential-and therefore away from the silver stripping potential observed at   0.1 V. Figures 4((a)-(c)) shows silver nanodendrites formed following electrodeposition for 1, 5 and 12 s, respectively, with the corresponding CVs presented in figure 4(d).As can be seen, the current measured at −0.23 V (corresponding to the reduction of Ru) is 12.5 nA a bare electrode and 12.5 nA, 13.9 nA and 13.3 nA for samples with 1, 5 and 12 s silver depositions, respectively.As the nanodendrites grow, it is expected that the measured faradaic current should grow accordingly.However, the measured signals are very similar from the ones measured at the bare microelectrode.In addition, the signal measured at 12 s electrode is smaller than the one measured at 5 s despite the spread of the silver increasing the footprint of the electrode from 60 to 150 microns.The lack of correlation between signal intensity and electrode surface area suggests that the silver nanodendrites do not participate in the electrochemical reaction and that they are thus not fully electrically connected to the original microelectrode.However, this is good for SERS substrates as it indicates the presence of significant number of nano gaps or hotspots.The small increase observed may be attributed to an increase in surface area at the microelectrode itself.One possible explanation for the observed behavior is that after application of the deposition potential, the open potential of the electrode is close to the silver stripping potential and, as a result, silver dissolves slightly back into solution, see video S2, but does not dissolve completely.
To further study this hypothesis, electrochemical deposition using cyclic voltammetry (three full scans) was undertaken using different voltage windows.In the first set of experiments, the potential range was set to be −1.0 to 0.0 V, i.e. with the most positive potential maintained below the potential for Ag stripping.Figure 5(a) shows an optical micrograph of the corresponding electrode.It can be seen that the nanodendrites grew in three waves as evidenced by the concentric rings.This suggests that under these conditions, the nanodendrites are still electrically connected to the original microelectrode.A second CV-based deposition was undertaken using a wider potential window of −1.0 to 0.2 V, i.e. above the Ag stripping voltage.In this case, the first cycle showed the typical slow controlled 2D growth of nanodendrites as observed previously.However, during the second deposition, the silver growth was observed to switch to a more vertical direction from the surface of microelectrode occurred more rapidly.The resulting silver deposition appeared to be larger agglomerates which were not dendritic in nature see figure 5(b).We hypothesize that the increased concentration of silver in the vicinity of the working electrode, following the stripping process, resulted in a rapid silver deposition with a different morphology.

Electrochemical formation process
Concerning 2D growth, the 2D morphology of the nanostructure can be explained in terms of mass transport and interplay between diffusion and electromigration.During the initial electrochemical deposition, silver ions located in the diffusion layer adsorb on the surface of the microelectrode where they are reduced, forming seed nanoparticles that effectively increase the surface roughness of the electrode.During this initial process, the volume just above the electrode becomes depleted in Ag ions limiting the electrode formation in the vertical direction.However, there are still Ag ions in the horizontal plane, i.e. along the chip surface, allowing the electrode to grow laterally.As the process continues, the electrode spreads along the substrate, effectively 'chasing' available Ag ions.The dendritic pattern of the electrode can be explained in terms of electric fields.As the electrode spreads, the newly deposited silver supports the electric field underpinning the electrodeposition process.The electric fields at adjacent branches are equal thereby preventing the bridges shorting and instead favoring the electrode extension away from the original electrode.As time evolves, silver ions migrate from the bulk to the top of the electrode and further deposition can occur then, resulting in thicker deposition near the electrode.

Silver nanodendrites as surface enhanced Raman scattering substrates
The electrodeposited silver nanodendrites consisted of a high density of metallic nanostructures with associated nanogaps.It was found that the 2D nanodendrites were strongly attached to the silicon surface of the sensor chip and did not lift off when washed or used in aqueous test solutions.By contrast, the 3D structures were found to detach easily from the surface of the chip and were not studied further.Initial Raman experiments showed that the as fabricated silver nanodendrites could support plasmonic hot spots and therefore could be attractive for SERS applications.In order to test their SERS capabilities, a monolayer of Raman reporter 4-Aminobenzothiol (4-ABT) was deposited on the nanodendrites surface, and its Raman response recorded and compared to the that obtained from bulk 4-ABT, under the same experimental conditions.The Raman signal was found to be significantly higher for the monolayer than it was for the solid confirming a SERS response, see figure S3.The enhancement factor of 4-ABT on the silver nanodendrites was calculated to be 7.9 × 10 6 (see calculation details and peak assignment in supporting information).This is consistent with the EF observed for other nanodendrites structures [39].
5. Consecutive depositions undertaken using cyclic voltammetry when the highest potential is (a) lower and (b) higher than the stripping potential of the silver.

Interaction between target pesticides and SERS substrates
MCPA is a widely used herbicide used to control weed growth in agricultural fields that has been found at exceeded level (limit of 0.1 ppb or 0.1 μg l −1 in EU' WFD) in bodies in Ireland and Europe.At present, MCPA is typically detected by liquid chromatography with tandem mass spectroscopy (LC/MS/MS).Clearly, this method is laboratory based, requires extensive sample preparation and bulky equipment, and therefore cannot be used for in situ or online monitoring.Having the ability to monitor levels of MCPA onsite would be a great resource for water management in order to have a better assessment of the status of the water bodies and allow mitigation measures to be put in place in case exceedance are detected.
The measured bulk Raman spectrum of MCPA is presented in SI (figure S5).However, initial SERS experiments failed to detect MCPA even at elevated concentrations ~100 mM.To understand why this was the case, a DFT simulation study was undertaken to explore the adsorption of MCPA at the silver surfaces.Figures 6(a In the most energetically favored configuration figure 6(a), with an adsorption energy of −0.15 eV, the molecule-surface interactions seem to be mediated by the acid and methyl groups in MCPA.We simulated an O-Ag distance (between the terminating O and the closest Ag atom at the surface) of 2.55 Å, and H-Ag distances (between H in methyl and their closest surface Ag) in a range between 2.73 and 3.44 Å.In figure 6(b) the methyl group stays far from the surface, and the acid group mainly mediates the adsorption.In this case, the computed adsorption energy is weaker than in (a), with a computed adsorption energy of −0.07 eV and bond distances between MCPA and the surface of 3.14 Å for O and 2.61 Å for the terminating H.
In the remaining possible interaction modes, the carbonyl group is directed away from the surface and the weak interaction is mediated by the methyl, H and Cl terminations in the aromatic ring.The distances between the molecule and their closest Ag atoms on the surface are 3.08 Å for Cl, 2.67 Å for H in the ring and 3.16 Å for H in the methyl group.Rotating the MCPA molecule so that the terminating H interacts with the surface at distances between 2.76 and 3.30 Å, and the O from the -OH of the acid sits 2.89 Å from the surface gives the weakest interaction mode.
These results suggest that the interaction of the target pesticide with the SERS substrate is very weak, making detection of MCPA very challenging in agreement with experiments carried out on pristine dendrite substrates where SERS spectra of MCPA were not obtained.

Functionalization of SERS substrate with γ-cyclodextrin
To address this limitation, functionalization of the SERS substrate was undertaken using cyclodextrin.Cyclodextrins are cup shaped macromolecules with a hydrophobic core and a hydrophilic outer rim.For this reason, they have been used extensively to increase dissolution of drugs or other compounds having hydrophobic properties [40].They were also shown to form host-guest complexes with pesticides such as MCPA [41].In the present work, γ-cyclodextrin with internal dimensions of 0.78 nm (diameter) by 0.79 nm (height) was used to modify the electrodes.By coating the sensors with this molecule, it should be possible to bring the target analyte to the surface and allow its detection using Raman based techniques.Functionalization of the developed SERS substrates was undertaken by incubation for 10 min in thiol γcyclodextrin solution in DMSO followed by thorough rinsing with pure DMSO.
Electrochemical impedance spectroscopy (EIS) can effectively determine the electron charge transfer resistance  in the electron transfer resistance of the Ag nanodendrite/Pt is due to higher surface area.However, the Rct value increases significantly as γ-cyclodextrin is attached on the surface, showing its electrical isolating properties.Similar insulating properties were reported for cyclodextrins [42].This further suggested the microelectrode modification for MCPA detection was successful.To confirm this, SERS spectra were recorded in DI water in both the presence and absence of MCPA.A typical spectrum of the functionalized substrates in DI water is shown in figure 8 bottom panel.The spectrum shows peaks at 446, 487/498, 673, 720, 933, 1026 and 1264 cm −1 onwards.The two peaks at 636 and 680 cm −1 can be attributed to CS bond of the thiolated cyclodextrin.The rest of the spectrum shows a series of broad peaks between 1100 and 1700 cm −1 that correspond to the various CC and CH vibration of γ-cyclodextrin scaffold.
Figure 8 top panel shows the Raman spectrum of the γ-cyclodextrin functionalized substrate incubated in a 1 mM solution of MCPA.Some features like the peaks at 487/498, 932 and 1026 cm −1 seen in DI water only are still present.However, new peaks emerge at 357, 804, 881, 1081/1095 and 1237 cm −1 and are attributed to the MCPA/ γ -CD complex.Also, some peaks present in the original sample are shifted or split (640/657 or 743 cm −1 ).The change observed in the Raman spectrum suggest that the MCPA molecule interacts strongly with γ-cyclodextrin core.The strong coupling leads to a restriction of the number of vibrations from both the host and the guest and results in new vibrations for the host/guest system.Similar observations have been reported for FTIR analysis [43].These results confirm the DFT simulations, and that modification of the surface with γ -CD is sufficient to capture MCPA allowing subsequent SERS detection.This approach may be extended to other molecules where SERS detection is difficult.

Conclusions
In this paper, we report on the fabrication of 2D silver nanodendrites on silicon chips by electrochemical deposition at platinum microelectrodes.The dendritic structuring can be explained by the interplay of electromigration and diffusion of silver ions.Characterization of the modified electrode by cyclic voltammetry revealed the deposited silver nanostructures were not electrically connected to the original microelectrode.High resolution imaging revealed the sample had a high density of nanostructures and nanogaps, necessary for SERS effect to occur.Raman reporter 4-ABT was used to calculate an enhancement factor of c.a. 10 6 .Density functional theory (DFT) simulations revealed the interaction between herbicide MCPA and silver surface was weak.To increase this interaction, the silver nanodendrites were functionalized with thiolated cyclodextrin, and SERS based detection of the pesticide reported.This concept can be extended to other contaminants with low affinity to silver.

Figure 1 .
Figure 1.(a) Photograph of a silicon chip used for the electrochemical-based fabrication of the SERS substrate; (b) electrochemical setup on the microscope stage.

Figure 2 .
Figure 2. (a) Cyclic of a 10 mM AgNO 3 solution in DI water at the fabricated electrode; (b) deposition current measured at the working electrode when −0.65 V is applied; Screenshots of the electrode after (c) 2 s and (d) 7 s deposition.

Figure 3 .
Figure 3. (a) High magnification optical, (b) SEM and (c), large area 2D-AFM micrographs of the outer nanodendrite structure and (d) 3D-AFM of the electrodeposited silver on the microelectrode.

Figure 4 .
Figure 4. (a) Optical micrographs of Ag nanodendrites corresponding to Ag deposited for 1 s, 5 s, and 12 s; (b) cyclic voltammogram of a 1 mM Ruthenium solution on a bare electrode and electrode with Ag nanodendrites for 1 s, 5 s, and 12 s.
) to (d) represent the geometries from the most to the least favorable adsorption orientations.The computed adsorption energies in a range between −0.15 (figure6(a)) to −0.01 eV (figure6(d))showing that the MCPA molecule is very weakly adsorbed in all potential configurations.
(Rct) and was used to study the different modifications of the Pt working microelectrods.The electrochemical behaviour of bare Pt electrode, Ag nanodendrite/Pt, and γ-cyclodextrin/ Ag nanodendrite/Pt are presented in figure 7(a).The Nyquist plot obtained from EIS studies was fitted with an equivalent circuit (NOVA software).The electron transfer resistance (Rct) values for bare Pt microelectrodes, Ag nanodendrite/pt, and γ-cyclodextrin /Ag nanodendrite/Pt were found to be 10.8 MΩ, 9.16 MΩ and 15.1 MΩ respectively.The decrease

Figure 6 .
Figure 6.Structure of MCPA adsorption at the silver surface in different orientations.The figure is organized from (a) the strongest (most energetically favourable) to (d) the weakest (energetically unfavourable) adsorption.

Figure 7 (
c) shows a SEM image of a typical silver deposition, while figure 7(d) shows the same SEM with elemental analysis with the purple colour representing the presence of sulphur.This confirms the deposited thiolated cyclodextrin layer on the surface of the silver.The table shown figure 7(b) shows the full elemental analysis for the sample.

Figure 7 .
Figure 7. (a) EIS Nyquist plot for each stage of deposition and functionalization; (b) table for EDX of elemental analysis of the dendrites after functionalization; (c) SEM of deposited Ag nanodendrites; (d) EDX/SEM showing presence of S on surface confirming γ-CD.

Figure 8 .
Figure 8. Raman spectra of γ-CD in DI H 2 O and 1 mM MCPA.3.9 detection of MCPA using modified SERS substrate.