Highly Sensitive and Selective Detection of Arsenic Using Electrogenerated Nanotextured Gold Assemblage.

Arsenic is considered as a toxic heavy metal which is highly detrimental to ecological systems, and long-term exposure to it is highly dangerous to life as it can cause serious health effects. Timely detection of traces of active arsenic (As3+) is very crucial, and the development of simple, cost-effective methods is imperative to address the presence of arsenic in water and food chain. Herein, we present an extensive study on chemical-free electrogenerated nanotextured gold assemblage for the detection of ultralow levels of As3+ in water up to 0.08 ppb concentration. The gold nanotextured electrode (Au/GNE) is developed on simple Au foil via electrochemical oxidation-reduction sweeps in a metal-ion-free electrolyte solution. The ultrafine nanoscale morphological attributes of Au/GNE substrate are studied by scanning electron microscopy. Square wave anodic stripping voltammetry (ASV) response for different concentrations of arsenites is determined and directly correlated with As3+ detection regarding the type of electrolyte solution, deposition potential, and deposition time. The average of three standard curves are linear from 0.1 ppb up to 9 ppb (n = 15) with a linear regression coefficient R 2 = 0.9932. Under optimized conditions, a superior sensitivity of 39.54 μA ppb-1 cm-2 is observed with a lower detection limit of 0.1 ppb (1.3 nM) (based on the visual analysis of calibration curve) and 0.08 ppb (1.06 nM) (based on the standard deviation of linear regression). Furthermore, the electrochemical Au/GNE is also applicable for arsenic detection in a complex system containing Cu2+, Ni2+, Fe2+, Pb2+, Hg2+, and other ions for the selective and sensitive analysis. Au/GNE substrate also possesses remarkable reproducibility and high stability for arsenic detection during repeated analysis and thus can be employed for prolonged applications and reiterating analyses. This electrochemically generated nanotextured electrode is also applicable for As3+ detection and analysis in a real water sample under optimized conditions. Therefore, fabrication conditions and analytical and electroanalytical performances justify that because of its low cost, easy preparation method and assembly, high reproducibility, and robustness, nanosensor Au/GNE can be scaled up for further applications.

Introduction

Contamination of water with heavy metals poses severe threat to ecological systems and human health because of their extreme toxicity which remains a great challenge for global sustainability.[1] Arsenic (As) is considered to be one of the most carcinogenic and toxic elements that exist in earth’s crust and groundwater. Active arsenic in the form of arsenic cations (As) is tremendously fatal and has long been an Environmental Protection Agency (EPA) priority pollutant.[1,2] Because of its extreme toxicity, it has been considered as a seriously hazardous element even at the parts per billion (ppb) level to a broad spectrum of living organisms.[2a] Long-term exposure to contaminated water with arsenic can lead to serious health problems and different forms of skin cancers.[2b] Therefore, timely and effective sensing of the trace levels of arsenic in contaminated water has gained considerable attention among scientific communities and is considered as a great threat to environmental sustainability.[3] Generally, arsenic exists in water as organic and inorganic arsenic. Inorganic arsenic exists in four oxidation states, where As3+ is about 60–70 times more toxic than As5+, and organoarsenic compounds have the capability to react with enzymes in both humans and animals.[4] Despondently, arsenic adulteration in drinking water causes several diseases such as skin lesions, keratosis, lung cancer, and bladder cancer and are evidently associated with adverse dermal effects such as hyperkeratosis, depigmentation, and so forth.[5]

All these harmful effects compelled the US EPA to establish a lower endurable standard for arsenic from 50 to 5 ppb.[6] Thus, the World Health Organization (WHO) and the US EPA recommends a maximum contamination limit of below 0.1 μg L–1 (10 ppb) of arsenic in water.[7] Inappropriately, the adverse climatic changes and numerous industrial processes such as metal smelting, semiconductor industries, and power generation houses are continuously causing increased accumulation of arsenic in the environment.[8] Consequently, the groundwater in many countries such as America, Thailand, England, Oregon, Bangladesh, India, Inner Mongolia, Taiwan, Vietnam, Pakistan, Canada, and so forth has been reported with a certain level of arsenic that is well above the guideline value level (10 ppb/0.1 μg L–1) specified by WHO.[9] About 44 districts in Bangladesh are affected with a high arsenic concentration, and it is reported that about 35–80 million people in Bangladesh are at a risk of arsenic toxicity.[10] According to a survey, about 50–60 million people in Pakistan are using contaminated water. Thus, it is estimated that about 150 million people around the globe are at risk because of arsenic contamination.[11]

Although the presence of arsenic in drinking water and food above a certain level is a serious threat to life and subsequently a plausible solution for its on-site steadfast detection, the development of highly sensitive, time-effective, and reliable analytical or electroanalytical methods is mandatory.[12] Various analytical techniques such as hydride generation atomic fluorescence spectrometry, inductively coupled plasma atomic emission spectroscopy (ICP-AES), ICP mass spectrometry (ICP-MS), hydride generation atomic absorption spectroscopy (HGAAS), graphite furnace AAS, and fluorescence spectrometry have been used for the detection of metals including arsenic in various samples.[13] However, all these techniques can detect low-levels of arsenic but require expensive, ponderous, and sophisticated instrumentation and a well-established laboratory set up. They are also time-intensive, vastly expensive, not easily accessible, and are not suitable for on-the-spot analysis.[14a] Moreover, with these methods, the cost of analyses can be as high as $8–10 per sample.[14b] Researchers are striving hard to develop simple, portable, and cost-effective sensing approaches for a rapid and reliable As3+ detection/analysis in the environmental samples, especially in developing countries and in areas with insufficient infrastructure and technical facilities.[15a]

Electrochemical methods, on the other hand, are advantageous as they are cost-effective, time-effective, and easy to implement. Various electroanalytical tools such as cyclic voltammetry (CV), linear sweep voltammetry (LSV), and more particularly stripping voltammetry including differential pulse ASV and square wave ASV (SWASV) can be employed to detect low concentration of arsenic with good sensitivity and reliability.[15b] Furthermore, stripping voltammetry techniques are adaptable for successful and rapid field screening, high accuracy, and enhanced sensitivity and demonstrate a very low detection limit.[16] Employing the highly sensitive stripping voltammetry technique, researchers have detected arsenic to a low detection limit and significant sensitivity using various electrode materials under optimized experimental conditions.[17−19]

Arsenic detection by electrochemical methods is investigated using mercury electrodes[20a] such as hanging mercury drop electrodes (HMDE) because they provide a wider potential window for redox reaction of various metals, and regeneration of a clean surface is quite easier via simply creating a new mercury drop. However, because of obvious toxicity considerations, it is very difficult to discard used mercury and clean the whole electrochemical setup after each measurement.[20b] Furthermore, various metals such as Au, Ag, and Hg could not be potentially detected on HMDE.[20a,20b] Therefore, they have been significantly replaced by a number of solid electrodes such as platinum,[21] silver, boron-doped diamond electrode,[22] gold microwire electrode,[23] gold microdisk,[24] and iridium oxide-modified boron-doped diamond electrodes.[25] Researchers investigated the platinum NP-modified glassy carbon electrode for electrochemical sensing of arsenic in 1 M aqueous HClO4 electrolyte solution. After optimization, a lower detection limit (LOD) of 2.1 ppb was obtained.[26] Further, an iridium-modified boron-doped diamond electrode is generated by ion implantation method and exploited directly for electrochemical detection of arsenic to an LOD of 1.5 ppb. The aforementioned sensor was favorably able to selectively sense spiked arsenic in tap water containing a substantial amount of various other elements as well.[27] However, these materials are expensive, thus making As3+ detection not feasible for long-term applications.

The successful large-scale implication of the electrochemical sensor merely depends on careful designing and fabrication of desired materials, which can facilitate adequate accumulation and subsequent oxidation of the analyte from its surface under optimized electrochemical conditions. Among many other solid electrode-based sensors, gold-centered materials have demonstrated enhanced electrochemical activity because of their advanced characteristic features in the field of catalysis, electroanalysis, and nanoscale devices.[28] They have excellent electronic, optical, and electrical characteristics which merely depend on the surface morphology and size of gold particles.[29] Therefore, microstructured/nanostructured gold electrodes have been exploited largely for application in the detection of arsenic in water. They are superior to the commercially available metal electrodes because of the presence of more electroactive sites, much enhanced electron-transfer rate, and favorable electrochemical kinetics.[30] Gold-based nanoscale materials can be fabricated by chemical synthesis, UV light or electron beam irradiation, and electrochemical methods.[31] Electrochemical methods are much facile and easy to use relative to other methods.[32]

Gold nanoparticles can be electrodeposited on the electrode from 0.1 mM HAuCl4 solution, which show good sensing for arsenic at few parts per billion levels.[33] Gold nanoelectrodes also enable simultaneous detection of arsenic, mercury, and copper.[34] Furthermore, gold NP-modified indium tin oxide (ITO)-coated glass electrodes were prepared by direct electrodeposition method from 0.5 M H2SO4 solution containing HAuCl4 solution. The Au-based films were nanostructured and detected arsenic to an LOD of 5.0 ppb using the LSV technique.[35] Another study shows the electrodeposited gold NP-modified glassy carbon electrode for sensing of arsenic in 0.1 M HNO3 electrolyte solution. After optimizing various electrochemical parameters, they were able to detect arsenic to an LOD of 0.25 ppb using the flow analysis electrochemical system via the ASV technique.[36] In short, electrodeposition approach has been widely exploited for in situ fabrication of nanoscale Au-based electrodes for As3+ detection. However, it is a tedious method, and during deposition, subsequent accumulation of Au ions on cathodic sites also poses a challenge. To invade this issue, separators are employed, which make the system more complex and thereby increasing the cost for analyses.[37]

Hence, apart from the advancement in the field of Au-based nanoscale electrochemical sensors for As3+ detection, there is a high need to fabricate an electrochemical nanotextured sensor using much simpler and facile electrochemical methods with enhanced activity and stability. It is noteworthy that sensitivity, linearity, selectivity, and limit of detection are some of the crucial parameters governing the successful implication of an electrochemical sensor which can be modified by carefully engineering the nanoscale-dimensional features of electrode materials.[38,39] We show here a unique and facile, electrochemically made, highly efficient, selective, and sensitive electrogenerated gold nanoelectrode for As3+ detection at ultralow levels comprising nanotextured Au assemblage on a simple gold foil (gold nanotextured electrode, Au/GNE). The gold nanoelectrodes are electrogenerated by oxidation–reduction-controlled potential anodization of the gold foil via multiscan CV (MSCV) in a simple near-neutral 0.1 M KCl solution containing no metal ions. The characteristic nanotexturing on gold surface enhances the electrochemical surface area (ECSA) and generates more surface-active sites, consequently facilitating the adsorptivity of arsenic on Au/GNE for highly sensitive detection. The proposed method also allows much reliable, selective, and sensitive detection of arsenic using SWASV as illustrated in Scheme . Advantageously, the method is successfully employed in a real water sample for efficient arsenic analysis of trace levels.

Scheme 1

Electro-Assisted Nanotextured Gold Assemblage on a Simple Gold Foil for Arsenic Detection

Materials and Methods

Chemical Reagents

All the chemicals and reagents were of analytical grade, obtained from Aldrich, and used as received without following any purification step, unless otherwise noted. All the solutions and subsequent dilutions are made using Milli-Q water. The arsenic solution is prepared by gently dissolving the compound in a minimum volume of 0.1 M nitric acid solution and diluting with ultrapure water up to a certain level.

Analytical Instruments

Scanning electron microscopy images for the surface morphology of Au/GNE were evaluated by a field emission scanning electron microscope (Lyra3, Tescan, Czech Republic) at an accelerating voltage of 20 kV. The elemental stoichiometry and composition of film electrodes were investigated by energy-dispersive X-ray (EDX, INCA Energy 200, Oxford Inst.) spectroscopy. Qualitative and quantitative analysis of various metal constituents in real water samples is conducted via AAS and HGAAS using a PerkinElmer PinAAcle 900T atomic absorption spectrophotometer.

Electrochemical Depictions

All the electrochemical measurements such as CV, electrochemical impedance spectroscopy (EIS), and SWASV were carried out in a standard three-electrode glass cell covered with an airtight Teflon cap on a computer-controlled potentiostat (Autolab PG-Stat 10). A spiral-shaped platinum wire was used as a counter electrode, also known as the auxiliary electrode. Before engaging into experimentations, the platinum wire was annealed and cleaned in 20% of HNO3 solution and washed several times with deionized water as described previously.[40] Electrogenerated gold nanoelectrodes comprising gold nanoparticles (NPs) assembled on the solid gold surface were used as the working electrode, also known as an indicator electrode and a saturated silver/silver chloride (sat. Ag/AgCl) reference electrode completing the assembly. The reference was calibrated against a standard before placing into the cell. CV was conducted in 0.1 M nitric acid electrolyte solution with pH ≈ 1. SWASV was employed as a sensitive technique for detection of arsenic3+ using Au/GNE. All the measurements were carried out at room temperature.

Fabrication of a Gold Nanoelectrode (Au/GNE) by Controlled Potential Anodization: Electrochemical Sensor

Prior to the electrochemical surface enhancement, gold strips were gently polished on velvet cloth with alumina powder (50 nm) and then sequentially cleaned by immersing in methanol and acetone for 10 min each and rinsed several times with Milli-Q water. Finally, the pretreated gold strips were electrochemically cleaned by cycling the potential between −0.3 and 1.8 VAg/AgCl in 0.5 M H2SO4 electrolyte solution for about 15 min. The electrochemically cleaned electrode is rinsed many times with ultrapure water and placed in an electrochemical cell for further experimentation. In order to make electrochemically enhanced Au/GNE, surface anodization is performed via MSCV under the controlled potential window in 0.1 M KCl electrolyte solution. The gold surface was roughened by intentionally keeping the conductor under controlled potential from −0.25 to 1.25 VAg/AgCl for 25 consecutive oxidation–reduction sweeps at a scan rate of 20 mV s–1. After electroanodization, the gold electrode was rinsed softly by ultrapure water and subsequently dried by nitrogen flow as reported previously.[41a]Before electrochemical testing, Au/GNE was activated in 0.2 M H2SO4 electrolyte solution using CV by cycling from −1.0 to 1.0 VAg/AgCl at a scan rate of 50 mV s–1 until a stable curve is obtained. Surface characterization was carried out using CV mode (0.1 M HNO3 and 0.1 M H2SO4). The stability of the electrode is investigated via accelerated degradation experiments, measuring about 500 concurrent CV cycles. The ECSA or microscopic surface area is calculated from the integrated reduction charge of gold oxide and assuming a conversion factor of about 400 μC cm–2. The roughness factor (Rf) is calculated as the ratio of the microscopic surface area to the geometrical area of the electrode. EIS is undertaken to comparatively examine the charge-transfer resistance at various applied potentials of bare gold surface and electrogenerated Au/GNE sample.

Electrochemical Sensing of Arsenic

SWASV was employed for the ultrasensitive detection of arsenic under the following optimized conditions: frequency, 25 Hz; amplitude, 25 mV; potential increment, 4 mV; deposition potential, 0 VAg/AgCl; and deposition time (300 s for lower concentration of arsenic and 120 s for higher concentration of arsenic). At the end of each electrochemical test, the electrode is maintained at a constant potential of 0.3 VAg/AgCl for 120 s to remove the memory effect and to ascertain the reusability of the electrode. For all measurements, freshly prepared 0.1 mM arsenic solution was used. Optimization studies were performed under the following square wave parameters: frequency (5–25 Hz), reduction potential (−0.7Ag/AgCl to 0.0 VAg/AgCl), and deposition time (60–300 s). Aq nitric acid (0.1 M), 0.1 M aq hydrochloric acid, and 0.1 M aq sulfuric acid were used as electrolyte solutions to see which electrolyte gives best results. In order to find the detection limit, the calibration plot was established. Following the abovementioned experimental conditions, arsenic detection in a real water sample is also carried out using Au/GNE.

Results and Discussion

Preparation of GNE (Au/GNE)

The Au nanotextured electrode for arsenic sensing is fabricated via in situ MSCV on the simple gold substrate under near-neutral conditions in 0.1 M KCl (Figure S1a). The chloride electrolyte has some reactive features that aid in the dissolution–redeposition process of Au and facilitating roughness on gold surface (Scheme S1).[41b] MSCV results in the microscale to the nanoscale controlled dissolution of the top surface of gold substrate with a few micron thickness with onset occurring at 0.88 VAg/AgCl (Figure S1b). This cycling initiates the formation of AuCl4– and subsequent redeposition commencing at 0.3Ag/AgCl to 0.66 VAg/AgCl of nanotextured gold on the substrate, consequently generating an enhanced gold surface demonstrating large electroactive sites.[41b] It is also recognized that during MSCV with an increase in the cycle number, the amplitude of irreversible reduction peak continuously increases, and a linear increase in peak current is also observed sequentially and can be ascribed to the formation of more electroactive nanoscale gold attributes on a simple gold substrate with cumulative CV scans (Figure S1b). However, it was observed that after consecutive 25 CV cycles, the magnitude of the reduction peak starts to stabilize with increasing cycle number and current saturation occurs (Figure S1c). The voltammograms recorded initially and after 17 and 24 successive positive–negative excursions show that all the voltammograms are quite similar, the chief difference is that the latter two demonstrated 20–30% higher peak current and shifting of onset potential toward the negative potential vertex (Figure S1d). This behavior is consistent with the moderate changes in the microscopic surface area of the gold surface because of the formation of nanoparticulate-type structures as shown by structural and electrochemical characterizations.[41a,41b]

Morphological Characterization of Au/GNE

For morphological observation, the scanning electron microscopy images for bare gold electrode and electrogenerated Au/GNE are taken and presented in Figure . It is obvious that the Au/GNE obtained by controlled electrochemical etching reveals crackled and highly rough surface features comprising a nanotextured pattern and nanoparticulate-type erections grown relatively smoothly on the simple gold surface (Figure a). The enlarged view represents the spongy texture of the top film having porous, rough structure and enhanced surface features (Figure b). The cross-sectional view shows the thickness of the nanotextured film to be 7.17 μm (Figure c) comprising gold nanoparticulate assemblage on the simple gold surface (thickness = 84.74 μm, overall thickness = 92.26 μm) (Figures S2 and S3). Conclusively, the results demonstrate that the MSCV employed here is an appropriate method to electrochemically enhance the surface feature of a simple gold substrate to fabricate the nanoscale Au/GNE (Figure S4) for electrochemical and electroanalytical investigations. Nevertheless, these types of structural nanolandscapes are not observed for the bare gold surface (Figure d).

Figure 1

Scanning electron micrography images for surface view of Au/GNE: (a) overview image; (b) enlarged view; (c,d) cross-sectional view for Au/GNE; and (d) surface view for bare gold electrode.

The elemental constituent analysis of the simple gold substrate and Au/GNE clearly suggests the presence of Au as a dominating metal in the bare gold substrate along with some carbon background signatures. However, the electrogenerated gold electrode shows some minor contribution from oxygen along with the Au matrix, which might have originated from adsorbed water on the Au surface and some leftover AuO structures during redeposition (Figures S5 and S6).

Preliminary Electrochemical Investigation of Au/GNE and Simple Au Substrate

The thus-obtained Au/GNEs are tested electrochemically to investigate their electrochemical properties via CV and impedance spectroscopy, and the results are compared with the commercially available bare gold strips electrodes. The forward potential sweep for Au/GNE sample in 0.1 M HNO3 electrolyte solution represents preoxidative features that appeared after 1.0 VAg/AgCl, following a current decline at 1.32 VAg/AgCl, ascribed to the formation of AuO-type material.[42] This is followed by initiation of catalytic peak at 1.58 VAg/AgCl, which quickly rises and reached >2000 μA current at 1.81 VAg/AgCl (Figure a). Likewise, during the reverse sweep, a broad reduction peak is also observed at 0.88 VAg/AgCl because of the surface reduction of AuO. However, the commercially available gold strips having the same geometrical area did not show any well-pronounced oxidative and catalytic behavior during steady-state CV measurements as shown in Figure a inset. This is a preindication of large surface area and efficient electron transfer at the Au/GNE material because of the formation of ultrafine nanoparticulate-type structures on the Au surface as revealed by SEM. The stability of any sensing material is another crucial factor for determining its implication for large-scale exploitations. NPs often suffer from sintering and agglomeration, which results in loss of activity. Therefore, it is highly desirable to introduce some facilely synthesized durable materials in the field of catalysis and sensing for ensuring long-term applications and stability. Here, after having good electrochemical activity of Au/GNE, the durability of electrode is also examined in aq HNO3 via repetitive CV scans.

Figure 2

Electroanalysis: (a) typical CV of bare Au strips and electrogenerated Au/GNE in 0.1 M HNO3 electrolyte solution at a scan rate of 50 mV s–1 (inset presents enlarged view of CV for bare Au electrode); (b) Nyquist plot for Au/GNE at 1.09, 1.6, 1.7, and 1.8 VAg/AgCl in the frequency range of 100 K to 0.1 Hz.

The electrogenerated Au/GNE shows much improved electrocatalytic activity during the 500 consecutive CV measurements without undergoing any loss of electrochemical activity (Figure S7). The enhanced current value obtained during 500th CV cycle is indicative of highly stable electroactive nature of the nanoscale Au/GNE confirming that more and more electroactive sites are being generated during the course of electrolysis. Moreover, the CVs recorded at different scan rates such as 5, 10, 20, 50, and 100 mV s–1 show an increase in current response and magnitude of redox peak with scan rate (Figure S8). The electrochemical activity of Au/GNE is also assessed in 0.1 M H2SO4 electrolyte solution. Forward potential sweep using electrogenerated Au/GNE and simple gold strips in 0.1 M H2SO4 is conducted by cycling the potential from −0.3 to 1.8 VAg/AgCl. The oxidative peak appeared beyond 1.3 VAg/AgCl and is followed by an increase in the catalytic current at the onset of 1.58 VAg/AgCl (Figure S9). A marked reduction peak is also observed during reverse CV scan. These redox peaks can obviously be attributed to the redox reaction of the catalytic gold layer, which was not obvious for simple gold strips in H2SO4 solution (Figure S9).

Next, EIS is commenced to support the argument regarding low charge-transfer resistance for electrogenerated Au/GNE relative to bare gold strip at the electrode–electrolyte interphase using various potentials in the faradaic region such as 1.09, 1.6, 1.7, and 1.8 VAg/AgCl. The corresponding Nyquist plots for Au/GNE and simple gold trips are also illustrated in Figure b (also see Figure S10). To obtain the precise results, the data were fitted using the simplest Randles circuit and the Rct value is presented in Table S1. The results reveal that Au/GNE favorably shows very low charge-transfer resistance evidenced by the smaller diameter of the semicircle as compared to simple gold strips, which show the considerably large value of Rct. All these features are owing to the much improved conductivity and promising kinetics of electron transfer on Au/GNE. From CVs and impedance data, it can be concluded that the electroactive sites and the charge-transfer capacity of the gold surface can be enhanced electrochemically by electro-anodization under controlled potential range by means of much simpler electrochemical setup.

The real surface area of Au/GNE electrode is determined by calculating the area under the reduction peak in the polarization curve considering optimized potential window (0.559–1.0 VAg/AgCl) as illustrated in Figure S11. Usually, the calculation of real surface area commonly known as the roughness factor is based on the assumption that a monolayer of chemisorbed oxygen with a gold atom in a ratio of 1:1 has been formed. In order to indicate the monolayer coverage, the minimum current observed at the decline of redox peak is considered. The exact value of the corresponding charge needed for the reduction of gold oxide depends on the composition and structural features of the gold electrode.[43] Trasatti and Petrii mentioned a value of 400 μC cm–2 for the polycrystalline gold electrode.[44] Although oxygen measurement procedure appears to be a slightly arbitrary procedure for calculating real surface area, it is a widely used method.

ECSA for Au/GNE is calculated asHere, the term QAu,O ascribes the charge associated with Au/GNE, whereas QAu,S describes the charge calculated for the polycrystalline gold surface. Consequently, the real surface area of 5.8 cm2 is estimated for the electrode which is higher than those reported previously.[33] An advantageously high roughness factor (AECSA/AGEO) of 11.6 is calculated for Au/GNE owing to its remarkable catalytic activity and stability. Furthermore, the number of electroactive gold atoms formed on the simple gold substrate due to electrochemical etching is calculated via considering the charge passed under reduction peak from potential versus current curve, which is 0.00235 Coulombs. Then, the number of electrons is calculated as (0.00235 C/1.602 × 10–19 C) 1.466 × 1016 electrons. The surface concentration of the active Au atom on the electrode is estimated by diving with the number of electrons involved in the redox reaction, which is 2 for Au2+/0+; therefore, the surface concentration of atoms is determined to be 7.34 × 1015 atoms. All these features provide compelling remarks presenting the superiority of Au/GNE sample for arsenic sensing prepared by a very simple and facile procedure.

Electrochemical Detection of Arsenic via CV and EIS

After observing the structural and electrochemical characteristics of Au/GNE, we directly employed the nanoelectrode for electrochemical sensing of arsenic in water. CV and impedance spectroscopy are applied for electrochemical arsenic analysis on a freshly fabricated Au/GNE and the results are compared with bare gold strips as well. These measurements are based on the arsenic accumulation/deposition (As3+ to As0) and then the subsequent stripping off (As0 to As3+) in 0.1 M HNO3 electrolyte. First, the electrode potential is held at 0.0 VAg/AgCl for 300 s to deposit arsenic from HNO3 solution containing 100 ppb As3+ under stirring condition. Then, the CVs of the above Au/GNE samples with and without arsenic deposition are measured by applying the vertex potential within a controlled potential window at a scan rate of 50 mV s–1. During the forward potential sweep, the characteristic peak of As0 oxidation to As3+ is observed at 0.25 VAg/AgCl, matching well with the literature values.[36,45] This current is then followed by a large reduction peak during reverse sweep because of As deposition (Figure a). However, no such redox behavior is observed in the arsenic-free electrolyte solution on Au/GNE. Usually, anodic stripping peak for arsenic appears in the potential range from 0.07 to 0.3 VAg/AgCl (Table S2). The appearance of the anodic peak for arsenic stripping at +0.25 VAg/AgCl is also indicative of its efficient affinity with Au/GNE relative to other materials, and hence, it is less likely to oxidize arsenic easily, therefore needs some more potential input for complete stripping off. The special nanoscale structural features grown on Au/GNE having large active sites favorably accumulate and deposit the arsenic on them, thereby presenting enhanced Au–As0 affinity (Figure a).

Figure 3

Electroanalysis: (a) CV for Au/GNE with (red line) and without (black line) 100 ppb arsenic in 0.1 M HNO3 solution at a scan rate of 50 mV s–1; (b) Nyquist plots for Au/GNE with (red line) and without (black line) 100 ppb arsenic in solution at potential E/V 0.25 VAg/AgCl; (c) CV for simple gold electrode with (red line) and without (black line) 100 ppb arsenic in 0.1 M HNO3 solution at a scan rate of 50 mV s–1; (d) Nyquist plots for simple gold electrode with (red line) and without (black line) 100 ppb arsenic in solution at potential E/V 0.25 VAg/AgCl. (Arsenic was deposited at 0.0 VAg/AgCl for 5 min under continuous stirring.)

EIS commenced with and without arsenic also illustrates the low charge-transfer resistance at 0.25 VAg/AgCl, where no faradaic current relevant to the electrocatalytic behavior of Au/GNE is observed (Figure b). This possibly associated with successful deposition and subsequent electron-transfer reaction of arsenic on Au/GNE surface. The impedance measured without arsenic shows a very large value of charge-transfer resistance, demonstrating a very limiting or no electron-transfer reaction under this potential regime. This proves that Au/GNE is very suitable for accumulation, deposition, and subsequent stripping of arsenic during sensing applications. For a simple gold electrode, the CVs do not show any redox peak for arsenic in the polarization curve (Figure c) and the value of charge-transfer resistance during EIS remained the same when evaluating for a simple gold electrode with and without arsenic. This might be associated with the fact that no arsenic from the solution is accumulated/deposited on the simple gold substrate under the employed conditions because of the very small number of electroactive catalytic sites on Au (Figure d). It is notable that the As0–Au affinity of a certain strength is crucial for electrochemical interconversion of arsenic from zero oxidation state to +3 oxidation state on nanoscale gold electrodes. The remarkable affinity of Au/GNE may be attributed to high electroactive sites on the electrode with nanotextured porous structural features as shown by SEM. Therefore, after evaluating the successful implication of Au/GNE for arsenic sensing via CV and EIS analysis, the highly sensitive SWASV technique for trace level detection of arsenic is also applied.

Electrochemical Arsenic Detection via Stripping Analysis: Calibrating Various Testing Parameter

SWASV technique is investigated for ultratrace sensing of arsenic on electrogenerated Au/GNE under the optimized electrochemical parameters. In order to get maximum sensitivity for arsenic detection on Au/GNE, careful optimization of various voltammetry parameters such as choice of supporting electrolyte, accumulation/deposition potential, deposition time, and frequency range using the 3 ppb concentration of arsenic in the aqueous nitric acid electrolyte solution is undertaken. Furthermore, several samples are tested three times under similar conditions. Measuring errors that are obtained in the range of 3–15%, which are marked by error bars on points, are presented in Figure .

Figure 4

Typical SWASV response of fixed arsenic (3 ppb) and corresponding plots of analysis of stripping current (squares) and peak area (columns) as a function of (a) electrolyte; (b) preconcentration potential; (c) preconcentration time; and (d) frequency. The error bars are obtained by performing the parallel measurements three times.

Supporting Electrolyte

The voltammetry behavior of arsenic is strongly influenced by the type and pH of the supporting electrolyte.[35,36] Therefore, the electrolyte selection must be carefully considered. Figure S12 illustrates the SWASV response of arsenic on Au/GNE in a variety of electrolyte solutions such as aq KCl (pH ≈ 7), aq NaOH (pH ≈ 13), phosphate buffer solution (PBS, pH ≈ 6.2), borate buffer solution (BBS, pH ≈ 9.2), carbonate electrolyte solution (pH ≈ 8.3), and aq HNO3 (pH = 1). The cathodic accumulation and anodic stripping of arsenic are successfully observed in all the electrolyte solutions, thus demonstrating the favorable activity of the electrode for arsenic detection under a wide pH range. However, the low detection limit is strongly dependent on the type of electrolyte solution. In 0.1 M NaOH, no significant arsenic signals were obtained even up to 1000 ppb arsenic concentration. This might be due to the large number of hydroxide ions that can disrupt the electrode surface and limit the sensing of low concentration of analyte. However, Au/GNE was able to detect arsenic in 0.1 M borate buffer solution (pH ≈ 9.2), 0.1 M carbonate buffer solution (pH ≈ 8.3), 0.1 M KCl (pH ≈ 7), and 0.1 M phosphate buffer solution (pH ≈ 6.2) with LODs of 350, 100, 35, and 35 ppb, respectively (the limit of detection is based on the visual analysis of voltammetry response obtained during SWASV analysis). In 0.1 M KCl, appearance of an additional peak in the vicinity of 0.4–0.6 VAg/AgCl is ascribed to the adsorbed chloride ions on the electrode surface.[17] In contrast, highly reproducible electroanalytical response for arsenic detection is observed in 0.1 M nitric acid solution, as shown in Figure S12f. The low sensitivity in detection under high pH condition may be due to the valency change, hydrolysis of arsenic, and presence of other interfering ions of electrolytes, which may interact with the electrode surface and make the arsenic interaction with the electrode at its ultralow concentration quite difficult.[33] Further, pronounced stripping response for arsenic on Au/GNE in 0.1 M HNO3 can also be ascribed to the fact that gold-based materials offer a wide potential range for subsequent cathodic preconcentration and anodic oxidation of analytes under acidic conditions, where not any significant faradaic reaction of gold itself takes place as presented in the CVs of gold conducted in nitric acid and sulfuric acid (Figures a and S9).[28]

After having highly sensitive results in 0.1 M nitric acid solution further, experiments were performed under acidic conditions and aq HNO3, aq H2SO4, and aq HCl are also tested as supporting electrolytes for arsenic detection. The electrolyte solution of varying concentrations from 0.1 to 1 M are compared in respect of response at Au/GNE toward 3 ppb As3+ concentration using square wave stripping analysis. SWASV response for arsenic reveals that the magnitude of the current response due to the stripping of arsenic is strongly dependent on the choice of the supporting electrolyte (Figures and S13). The stripping of arsenic in HCl generates a peak in the vicinity of 0.16 VAg/AgCl, which can be ascribed to the electrooxidation of arsenic.[46] However, an additional peak was also observed at a slightly more positive potential, thereby masking the electrochemical response from arsenic stripping at an ultralow concentration (Figure S12). This behavior was also observed previously for arsenic sensing in HCl.[36]

Figure 5

SWASV response of 3 ppb arsenic in different supporting electrolytes, 0.1 M H2SO4, 0.1 M HNO3, and 0.1 M HCl, on Au/GNE.

Although the reason for the appearance of two peaks in 0.1 M HCl electrolyte solution was not clearly understood previously,[36] in this study, the origin and position of stripping peaks strongly depend on the electrochemical properties of the electrode materials. Au/GNE is highly porous and demonstrates large electroactive sites as observed by SEM and surface area calculation analysis. Because of high surface area, Cl1– ions from aq HCl electrolyte solution having strong affinity for gold may be adsorbed on Au/GNE and cover the electrode surface and ultimately limits the availability of active site for arsenic adsorption.[17] The appearance of additional peak is ascribed to the adsorbed chloride ions. Furthermore, to confirm the above statement, SWASV experiments are performed in 0.1 M HCl electrolyte solution in the presence and absence of added arsenic. Without arsenic, a broad peak at 0.4–0.6 VAg/AgCl might appear from adsorbed Cl1– ions.[46b] However, in the presence of a low concentration of arsenic, this peak remained intact and its intensity decreased by increasing arsenic concentration in solution because of favorable cathodic preconcentration of arsenic on Au/GNE at 0 VAg/AgCl. The additional peak caused by adsorbed Cl1– ions can be completely masked in the presence of high arsenic content in solution as shown in Figure S14. Because of poor reproducibility and appearance of the additional peak, HCl was not well considered for arsenic detection in this study. However, the peak area and peak current while using the same concentration of arsenic (3 ppb) are enhanced in 0.1 M H2SO4 electrolyte solution, and a more sensitive response for arsenic detection with significantly Gaussian shape peak is observed in 0.1 M HNO3 electrolyte solution. In this respect, 0.1 M HNO3 solution is selected and used as an electrolyte medium for the further electrochemical studies.

Accumulation/Deposition Potential

A very important parameter in stripping analysis methods toward the electrochemical detection for the favorable accumulation and deposition of arsenic on the electrode surface is the application of preconcentration potential.[47] Arsenic was deposited by favorably choosing the deposition potential range from −0.7 to 0.0 VAg/AgCl. Previously, researchers disclosed that by applying much negative deposition potential, cathodic preconcentration of electron-insulating As0 on gold electrode could be enhanced probably because of electrogenerated H2, as studied via CVs.[48] However, on Au/GNE, the phenomenon is quite opposite for ultralow detection of arsenic using SWASV technique. Figure represents the electrochemical response by depositing arsenic at a much negative preconcentration potential from −0.6 to −0.4 VAg/AgCl. It is notable that in the absence of arsenic, very intense peak observed roughly centering at −0.6 to −0.1 VAg/AgCl. This peak can be ascribed to the anodic stripping of H atom adsorbing on highly electroactive gold surface during application of negative potential for a specified time.[48] However, in the presence of very low concentration of arsenic such as 1 ppb, the anodic stripping peak of hydrogen becomes less intense and a well Gaussian-shaped peak observed roughly at about 0.25 VAg/AgCl attributed to the oxidation of As0 to As3+. Furthermore, by increasing the analyte concentration, the current intensity of the arsenic stripping peak is increasing linearly and an ultimate decrease was observed in the intensity of hydrogen desorption peak (Figures S15–S17). This behavior provides compelling evidence about strong competitive adsorption of arsenic species on Au/GNE, having much enhanced electroactive sites. Moreover, oxidative signatures for arsenic stripping are not observed at an ultralow concentration, such as 0.1 ppb, due to complete coverage of hydrogen atoms on catalytic gold. At a further negative preconcentration potential of −0.7 VAg/AgCl, no arsenic could be detected even at 10 ppb level because of intense hydrogen evolution.

Figure 6

SWASV response of various concentrations of arsenic on Au/GNE applying an accumulation potential of (a) −0.4; (b) −0.5; and (c) −0.6 VAg/AgCl of arsenic stripping. Insets show an enlarged view of arsenic stripping region in 0.1 M HNO3.

Nevertheless, at further positive preconcentration potential such as −0.3 to 0.0 VAg/AgCl, no additional peak for hydrogen desorption appeared in the voltammogram (Figure S18). Eventually, arsenic species are less likely to compete with adsorbing hydrogen ion, and electroactive sites of gold are entirely available for arsenic accumulation/deposition, meanwhile making ultralow (0.1 ppb) detection of arsenic possible under the specified potential region. Additionally, it is notable that because of positive ongoing of potential from −0.6 to −0.3 VAg/AgCl than to 0.0 VAg/AgCl, the anodic stripping peak potential of arsenic is further shifting toward positive potential range, indicating the formation of more As0 deposits on Au/GNE accompanied with increased current response for arsenic stripping (Figure ). Therefore, the above discussed experimental results justify that the oxidation peak potential and current are strongly dependent on preconcentration potential. Because of these observations, and keeping in view the sensitivity of the arsenic detection on gold electrode at −0.6 VAg/AgCl (6.24 μA ppb–1 cm–2), −0.5 VAg/AgCl (6.85 μA ppb–1 cm–2), −0.4 VAg/AgCl (5.57 μA ppb–1 cm–2), and at 0.0 VAg/AgCl (39.54 μA ppb–1 cm–2 discussed in later section), the preconcentration potential falling in the positive region far away from hydrogen adsorption/desorption region is selected.

Results reveal that by accumulating the 3 ppb arsenic at −0.3 and −0.2 VAg/AgCl, a relatively small peak appeared during stripping at 0.25 VAg/AgCl; moreover, a very clear peak was observed by accumulating at −0.1 VAg/AgCl. The deposition/accumulation potential of 0.0 VAg/AgCl gave a clear peak of arsenic with a higher current response, as illustrated in Figure . Consequently, 0.0 VAg/AgCl is then selected as an optimal preconcentration potential for the arsenic detections. However, previously, it was shown that the arsenic electrooxidation peak is increasing when preconcentration potential became more negative from 0.1Ag/AgCl to −0.6 VAg/AgCl.[48] From the detailed discussion in the literature, it is concluded that the electrochemical response of any analyte on concerned material depends on careful optimization of various electrochemical parameters and fabrication of sensing electrode material under the employed conditions. Table S2 illustrates the optimal electrochemical parameters under stripping analysis adopted by researchers for the particular sensing of various analytes. We, however, believe that the optimal electrochemical parameters setting for the successful implication of electroanalytical methods merely depend on precise engineering of micro to nanoscale electrode materials and one should optimize them with great care to have sensitive and precise outcomes.

Preconcentration Time

Deposition time is an important parameter affecting the sensitivity and detection limit for analyte detection in stripping analysis. Different deposition times of 60, 120, 180, 240, and 300 s are chosen using SWASV to evaluate the deposition time effect on arsenic sensing signals (shape and response) using 3 ppb solution. It is observed that by increasing the preconcentration time from 60 to 300 s, a linear increase in current response is perceived, indicating more effective deposition with time from only a 3 ppb As3+ solution and subsequent stripping of arsenic (Figure S19). However, to generate the reaction, the effective deposition time was not further increased. At a much lower concentration of As3+, we choose 300 s as the optimal deposition time, whereas to avoid the saturation effect, at a relatively high concentration of the analyte, 120 s was used as the optimal deposition time.

Frequency

Finally, the SWASV parameter for frequency is also optimized by evaluating the response of arsenic on Au/GNE in 0.1 M aq HNO3 electrolyte using 3 ppb As3+ solution. It was observed that the frequency range does not affect the shape and potential of the peak, which remains the same by changing the frequency range from 5 to 25 Hz while keeping other parameters constant such as deposition potential (0.0 VAg/AgCl) and deposition time (120–300 s). However, a proportional increase in current response and peak area is observed by linearly increasing the frequency and highest response toward arsenic detection is obtained at 25 Hz (Figure S20). However, by further increasing the frequency range to 30 Hz, there is a drop in the value of the current response. Therefore, the frequency parameter of 25 Hz is used for the arsenic detection experiments.

Complete Test Analyses for Arsenic Detection on Au/GNE System: Calibration, Linearity, Sensitivity, and LOD

Employing the carefully augmented electrochemical parameters discussed above, the SWASV response of Au/GNE in 0.1 M aq HNO3 electrolyte containing different concentrations of arsenic is presented in Figure a. In Figure a, the analyte ion concentration is increased in sequence from 0.1 ppb (1.3 nM) to 9 ppb (120 nM), where a = 0.1 ppb, b = 0.5 ppb, c = 1 ppb, d = 1.5 ppb, e = 2 ppb, f = 2.5 ppb, g = 3 ppb, h = 3.5 ppb, i = 4 ppb, j = 4.5ppb, k = 5 ppb, l = 5.5 ppb, m = 6 ppb, n = 7.5 ppb, and o = 9 ppb (120 nM), from bottom to top. Black line presents SWASV response of GNE/Au without arsenic in solution. The peak current obtained at E/V 0.26 ± 0.05 is indicative of oxidation stripping of arsenic. It is obvious that at a much lower concentration of arsenic, the responsive current peak is observed at a slightly negative potential and the current maxima is shifted to a more positive potential by increasing the concentration of arsenic. This might have originated from increasing the concentration of the reactant and subsequently more deposition of arsenic on Au/GNE, which is the oxidizable species on the electrode. This is also related to an ultimate decrease in the activity coefficient by more deposits of metal as stated by Debye–Hückel theory, and ultimately, the required signals are observed at slightly higher applied potentials.[48]

Figure 7

(a) Typical SWASV response of Au/GNE toward the different concentrations of arsenic in the 0.1 M HNO3 electrolyte solution arsenic is deposited at 0.0 VAg/AgCl, applying an accumulation time of 300 and 120 s, amplitude of 0.004 V, frequency 25 Hz; and (b) corresponding calibration plot (initial arsenic 0.1 ppb). The error bars are obtained by performing the parallel measurements three times.

The calibration plots for a peak current of arsenic are also shown in Figure b. To obtain a calibration curve, the background current or the peak area of blank (black line) is subtracted from the peak area at each arsenic concentration. A plot was constructed between the concentration of the analyte and the current response. The same measurements are performed three times and error bars are also shown. The linearization equation is attained by fitting and stated as i (μA) = 19.77x (ppb) + 33.107. These results reveal that the average of the three standard curves was linear up to 9 ppb (where n = 15) with a correlation coefficient R2 = 0.9952. The remarkable sensitivity of electrode is determined to be 39.54 μA ppb–1 cm–2 with the theoretical limit of detection of 0.08 ppb (1.06 nM) (based on the standard deviation of linear regression).[49] The limit of detection is much lower than those reported earlier for other gold nanomaterial samples (Table ) and is well below the guideline value of arsenic level in drinking water as specified by WHO.[38,39] Therefore, the reported method in this study can be successfully employed for real water sample analysis for arsenic. It is notable that the Au/GNE generated by controlled potential anodization process from metal-ion-free electrolyte solution is the first example to be employed as an electrochemical sensing electrode for ultralow detection of arsenic and thus obtained sensing performance is favorably well adequate for practical applications.

Table 1

Comparison of Electrochemical Activity of Different Gold-Based Sensors for Arsenic Detectiona

sensor	electrolyte	method (ASV)	oxidation peak	sensitivity (μA μM–1)	LOD (ppb)	refs
Au-disk	1 M HNO3	SWV	As0–As3+	13.1	0.861	(28)
nano Au-(CRV)	PBS	DPV	As0–As3+	0.8075	14.92	(50)
AuNPs/BPPG	1 M HCl	LSV	As0–As3+	3.52	17.52	(51)
sub-BT/Au	0.1 M PBS	SWV	As0–As3+	0.548	0.281	(17)
Au/GNE	0.1M HNO3	SWV	As0–As3+	39.54*	0.083	TW

“Au-NPs, gold nanoparticles; CRV, crystal violet films; sub-BT/Au, Au(111)-like poly-Au electrode/n-butyl thiol (n-BT); AuNPs/BPPG, gold nanoparticles/basal plane pyrolytic graphite; GCE, glassy carbon electrode; ITO, indium tin oxide; Au/GNE, electro-generated gold nanoelectrode on simple gold surface; ASV, anodic stripping voltammetry; DPV, differential pulse voltammetry; LSV, linear sweep voltammetry; SWASV, square wave anodic stripping voltammetry; * = sensitivity is in μA ppb–1; LOD is based on 1S/N = 3 ratio method, 23σ method, 3standard deviation of regression line.”

Reusability and Stability

Stability, good shelf life, and reusability of the electrochemical sensor for heavy metal contaminant detection are the decisive features in determining its broad-spectrum and long-term applications. The stability, reproducibility, and reusability of Au/GNE are assessed by subjecting the same Au/GNE to 3 ppb arsenic-containing solution for 10 days successively. The results reveal that no observable changes in the electrochemical response of arsenic are observed after repeated analysis and relative standard deviation is 2.17%, confirming the high reproducibility and reusability of the electrogenerated Au/GNE system (Figure a). The inset shows that the position and current response of arsenic almost remain the same owing to its high stability for the long-term application (Figure a). Furthermore, the reusability of the Au/GNE system is also investigated by using the same electrode for the voltammetric response of 3 ppb arsenic repeatedly 10 times. After each measurement, the reusability is confirmed by keeping the electrode at 0.3 V for 2–5 min to remove memory effects and to strip off impurities from the electrode surface. Figure b shows the SWASV response of the electrode toward arsenic sensing with an RDS of 3.12%, and the inset shows that there is no change in the position of the characteristic peak. All these features suggest that the electrogenerated Au/GNE can be advantageously employed for direct sensing of arsenic in water under the employed electrochemical condition with remarkable stability and reusability.

Figure 8

(a) Stability measurements of Au/GNE with repeated analysis of 3 ppb arsenic consecutively using the same Au/GNE electrode for different days; (b) stability measurements of Au/GNE with repeated analysis of 3 ppb arsenic. Data were collected from an SWASV response of 3 ppb spiked arsenic in 0.1 M HNO3 electrolyte solution. The inset shows the typical SWASV responses of Au/GNE under the conditions as mentioned in Figure . The error bars are obtained by performing the parallel measurements three times.

Study of Foreign Ion Interference and Selectivity of Au/GNE for Traces of Arsenic Detection

The effect of various foreign ion interferences on the detection of arsenic using the abovementioned protocol on Au/GNE is also studied. A range of possible interfering cations such as Ni2+, Co2+, Fe2+, Pb2+, Cu2+, Cd2+, Zn2+, Hg2+, Cr2+, and Zn2+ have been deployed in the test solution containing As3+. The electroanalytical performance of Au/GNE is evaluated by SWASV analysis of arsenic in the presence of 5–50-fold concentration of the abovementioned metal cations. The effect on the peak height of arsenic with a 50-fold concentration of interfering metal ions is presented in Figure . Results are remarkable and reveal that the current response of arsenic does not change in the presence of 5–50-fold concentration of Ni2+, Co2+, Fe2+, Pb2+, Cu2+, Cd2+, Zn2+, and Hg2+ ions and other interfering cations under optimized experimental conditions. From these data, it can be deduced that no apparent interference is observed with commonly interfering ions, particularly copper ions in the acidic media under the controlled potential window with optimized parameters for arsenic detection on Au/GNE. This also shows that Au/GNE presented here is highly sensitive and selective for arsenic detection in water samples.

Figure 9

Typical SWASV response of Au/GNE toward the arsenic detection in 0.1 M HNO3 electrolyte solution in the presence of 50-fold concentration of various interfering metal cations (Ni2+, Co2+, Fe2+, Pb2+, Cu2+, Cd2+, Zn2+, Hg2+, Cr2+, and Zn2+) under the conditions as mentioned in Figure .

Application to a Real-Life Matrix

In order to see the validity of the Au/GNE electrode using a chosen method, the analysis is also performed for few real water samples collected from different areas of Lahore, Pakistan. The sample is diluted with 0.1 M aq HNO3 electrolyte solution (1:19) and tested directly for arsenic sensing. In the case of real water samples, a minute oxidation peak appeared at the vicinity of E/V = 0.27 ± 0.03 VAg/AgCl can be attributed clearly to arsenic stripping. To further demonstrate the practical use of the present electrode, the water samples were spiked with arsenic solution and subsequent linear increase in the current response is observed by increasing the arsenic amount (Figure S21a,c,e). The linear calibration curve is plotted and illustrates a linear relationship between the peak current and increasing arsenic concentration (Figure S21b,d,f). The concentration of arsenic in real water samples is calculated to be 40 ppb in sample a, 19 ppb in sample b, and 30 ppb in sample c, which is already very high and alarming. These values are in satisfactory agreements with results 38, 20.5, and 29.58 ppb, respectively, conducted on HGAAS from Pakistan Council of Research in Water Resources (PCRWR Pakistan). This indicates that the proposed method is highly accurate, precise, and reproducible and can be applied for real-life sample analysis for arsenic sensing. Furthermore, to ascertain the selectivity of the proposed method, analyses of various interfering ions (Table S3) such as Fe2+, Ni2+, Co2+, Pd2+, Cu2+, and Zn2+ are conducted in real water samples that are not shown to produce any electroanalytical signals using Au/GNE under employed electrochemical conditions.

Summary and Outlook

Highly electroactive nanotextured gold electrode (Au/GNE) assemblage is presented here with advantageously enhanced electrocatalytic features for highly sensitive and selective electrochemical detection of trace levels of arsenic in water. Au/GNE also exhibits much improved electronic transfer traits because of unique nanostructure and thereby facilitates the electrooxidation of arsenic for its ultralow sensing/detection in water up to 0.08 ppb concentration. Gold NPs immobilized on the conductor surface are made decent electrochemical sensors for metal ion detection. However, lack of durability, reproducibility, and agglomeration of Au-NPs remains the main challenge. In future, this needs more optimization in terms of preventing the abovementioned disadvantages. The proposed Au/GNE assemblage is exceedingly stable with promising reproducibility maintaining highly active nanoscale surface features by repeating the analyses several times and allows very reliable, selective, and highly sensitive detection of arsenic using CV and SWASV. This method is also successfully employed in real water samples for arsenic analysis. The GNE is developed on a simple Au foil via a straightforward electrochemical method from a metal-ion-free electrolyte solution and having an ultrafine nanoscale surface morphology. For arsenic analyses, SWASV responses under the optimized conditions show remarkable sensitivity for metal detection with the LOD of 0.1 ppb (1.3 nM) (based on the visual analysis of calibration curve) and 0.08 ppb (1.06 nM) (based on the standard deviation of linear regression). In the complex system containing Cu2+, Ni2+, Fe2+, Pb2+, Hg2+, and other ions, Au/GNE is amazingly applicable for highly selective and sensitive detection of arsenic in water. Next, the experiments are under process to scale up the Au/GNE-based electrochemical sensors for real-time applications owing to its simple fabrication and assembly, high reproducibility and robustness, and electroanalytical performance for arsenic sensing.