Graphene OxideâSilver Nanowire Nanocomposites for Enhanced

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Graphene Oxide - Silver Nanowire Nanocomposites for Enhanced Sensing of Hg2+ Md Tawabur Rahman, Md Faisal Kabir, Ashim Gurung, Khan Mamun Reza, Rajesh Pathak, Nabin Ghimire, Aravind Baride, Zhenqiang Wang, Mahesh Kumar, and Qiquan Qiao ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00789 • Publication Date (Web): 19 Jul 2019 Downloaded from pubs.acs.org on July 20, 2019

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ACS Applied Nano Materials

Graphene Oxide - Silver Nanowire Nanocomposites for Enhanced Sensing of Hg2+ Md Tawabur Rahman†, Md Faisal Kabir†, Ashim Gurung†, Khan Mamun Reza†, Rajesh Pathak†, Nabin Ghimire†, Aravind Baride‡, Zhenqiang Wang‡, Mahesh Kumar§, and Qiquan Qiao†* †Department

of Electrical Engineering and Computer Science, South Dakota State University, Brookings, SD 57007, USA. ‡Department

of Chemistry, University of South Dakota, 414 East Clark Street, Vermillion, SD

57069, USA.

§Department of Electrical Engineering, Indian Institute of Technology Jodhpur, Jodhpur 342011, India Abstract We demonstrate a highly sensitive and selective sensing platform for the electrochemical detection of Hg2+ in aqueous media. Graphene oxide (GO) and silver nanowire (AgNW) nanocomposites modified platinum (Pt) electrode has been applied to determine Hg2+ using square wave anodic stripping voltammetry (SWASV). The synergistic effect of graphene oxide and conductive AgNW greatly facilitates faster electron-transport and sensing behavior for Hg2+. Under the optimum conditions, the sensor shows a high sensitivity of ~ 0.29 µA/nM and linear response in the range of 1 - 70 nM towards Hg2+. The detection limit of the GO-AgNW nanocomposites modified electrode towards Hg2+ is ~ 0.1 nM, which is significantly less than the safety limit defined by the World Health Organization. The sensor has excellent selective response to Hg2+ against other interfering heavy metal ions such as Pb2+, Cd2+, Cu2+, and Na+. In addition, the sensor exhibits a high repeatability and reproducibility. The sensor is employed for the detection of Hg2+ in tap water samples with an outstanding performance, suggesting it is a very promising platform for on-site monitoring of Hg2+ in water.

Keywords: Graphene oxide, nanowire, nanocomposites, Hg sensor, electrochemical.

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1. INTRODUCTION Mercury (II) is one of the harmful environmental pollutants and threats for human health due to its high toxicity even at a trace amount and non-biodegradability.1-2 High accumulation of mercury ions (Hg2+) would cause many detrimental effects such as arrhythmia, cardiomyopathy, kidney and respiratory failure, and damage in nervous system.3-5 The major source of Hg2+ ions in human body is through drinking water. The maximum acceptable limit of Hg2+ in drinking water defined by the World Health Organization (WHO) is 5 nM.6 Obviously, the development of a simple, rapid, and sensitive analytical method for the determination of a trace level of mercury has attracted great attentions. Electrochemical methods for heavy metal ion detection provide several advantages such as simplicity, low cost, high sensitivity, good selectivity and in-situ analysis.1, 712 Among all the electrochemical methods, anodic stripping voltammetry (ASV) is an effective tool for the detection of metal ions owing to its low cost, easy operation, high sensitivity and selectivity.13-15 Besides, the effective preconcentration (deposition) of target metal ions on working electrode surface followed by stripping of the deposited analytes provides a low detection limit.1617

Graphene, a novel two-dimensional single layer of sp2 hybridized carbon,18-21 has attracted broad attention for developing electrochemical sensors due to its high electrical conductivity,22-23 large surface area, chemical stability, and high electro catalytic activities.24-29 Graphene oxide (GO) is considered as a widely used precursor to synthesize graphene by a variety of chemical or thermal routes.30 GO has many oxygen containing functional groups such as hydroxyl and epoxy on the basal plane along with carboxyl and carbonyl at the edges,31-33 which are useful to immobilize various heavy metal species.15, 34-35 However, they suffered from the aggregation and π-π stacking interactions of graphene layers.36-39 In order to prevent such aggregation, various materials including metal nanoparticles,40 conducting polymer,41-42 silver nanowires (AgNWs),43-45 and multiwall carbon nanotubes (MWCNTs)46 were incorporated into graphene suspensions. Recently, the use of graphene based nanocomposites enhanced the electrochemical sensing performance. For instance, gold nanoparticles (AuNPs),40 ionic liquid functionalized graphene oxide,47 reduced graphene oxide (rGO),48 and graphene quantum dots49 were incorporated into graphene to fabricate ultrasensitive Hg2+ sensors. Besides the electrochemical method, other types of techniques have also been implemented for Hg2+ detection.50-56 For instance, Kamaruddin et al. developed a surface plasmon resonance (SPR) sensor based on gold/silver/gold/chitosan–graphene oxide (Au/Ag/Au/CS–GO) for the detection of Pb2+ and Hg2+, respectively. They found the linear detection range of 0.5-25 µM, which is significantly higher than the safety limit defined by the WHO. Real field testing on normal drinking water was not performed. Golsheikh et al. employed rGO-silver nanoparticles (AgNPs) composites for the detection of Hg2+ ions using UV-Visible absorption spectroscopy. They found the linear detection range of 0.1 µM-100 µM and detection limit of 20 nM, which is higher than the safety limit defined by the WHO. No data regarding the real field testing on normal drinking water were presented.


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Surface modification of the electrode can support a faster electron transfer between electroactive analytes and the electrode in electrochemical sensors.57-59 Owing to unique electrical, catalytic, and chemical properties, one-dimensional (1-D) nanomaterials have drawn attention in electrochemical sensors.60 The 1-D conductive nanowires can provide faster electron transport between electrode and electroactive analytes and hence, can improve the sensitivity of electrochemical sensors.61-62 The presence of various functional groups in GO makes it suitable for Hg2+ detection, however, with its intrinsic poor conductivity,30 optimum sensor performance is difficult to be realized. With this into consideration, this work presents the incorporation of AgNWs to form nanocomposites with GO, in order to enhance the conductivity of GO by providing faster electron transport pathway. The use of GO-AgNW nanocomposites utilizes the advantages of the graphene oxide (large surface area, binding affinity towards Hg2+) together with AgNWs (high conductivity). This is the first report on use of this GO-AgNW nanocomposites for Hg2+ detection. A low cost, easily operated and highly sensitive electrochemical technique (SWASV) has been employed. The achieved linear response towards Hg2+ detection was in the range of 1 - 70 nM. Further, the detection limit of the GO-AgNW nanocomposites modified Pt electrode towards Hg2+ was ~ 0.1 nM. Therefore, our sensor can detect 5 nM Hg2+ which is the safety limit defined by the WHO and even less. The sensor was also employed for the detection of Hg2+ in real field testing including tap water and satisfactory recovery was realized, suggesting that the developed sensor is a promising platform for on-site monitoring of Hg2+ in water. 2. EXPERIMENTAL 2.1 Chemicals Graphene oxide (GO) powder, and potassium ferricyanide (III) were purchased from Sigma Aldrich (St. Louis, MO, USA). Silver Nanowire (AgNW) suspension was obtained from Blue Nano Inc. (NC, USA). Lead chloride (PbCl2), cupric chloride (CuCl2), cadmium nitrate (CdNO3), sodium chloride (NaCl), and potassium chloride (KCl) were purchased from Fisher Scientific (NH, USA). Mercury (II) chloride was purchased from STREM chemicals (MA, USA). 0.1 M KCl was served as the supporting electrolyte. Deionized water was used throughout this project. 2.2 Instruments The surface morphology was characterized by the field emission scanning electron microscopy (FESEM, Sigma, Zeiss) and transmission electron microscopy (TEM, Tecnai G2, FEI). 2D band of graphene was recorded by Raman spectroscopy (HORIBA Scientific). Electrochemical measurements were performed on a VersaSTAT 3 potentiostat (AMETEK Scientific Instruments, USA) with a conventional three-electrode system consisting a platinum wire as the counter electrode, Ag/AgCl (3 M KCl) as reference electrode and platinum (Pt) (1.6 mm diameter) as working electrode, respectively. The platinum working electrode (Bioanalytical Systems Inc. West Lafayette, IN, USA) was modified with GO-AgNW nanocomposites. All electrochemical measurements were carried out in a 20 mL cell. 2.3 Preparation of the modified electrode



Platinum (Pt) electrode was polished carefully with 1.0 µm, 0.3 µm and 0.05 µm alumina slurries on micro-cloth pads until a mirror shiny surface was appeared, followed by rinsing with deionized (DI) water, ethanol and DI water, respectively. Afterward, the electrode was dried in air at room temperature. Ultrasonic agitation (for 1h) was used to disperse the GO (1 mg/mL) into ethanol. AgNW (1 mg/mL) suspension was stirred with a magnetic bar for 30 minutes. Then a composite of GO-AgNW (1:1) was vigorously stirred for another 30 minutes. A 5 µL of GO-AgNW nanocomposites was drop coated on the surface of the clean Pt and dried in the air. The prepared electrode was kept in a refrigerator at 4 oC before use. 2.4 Electrochemical measurement procedures The square wave anodic stripping voltammetry (SWASV) measurements were carried out in presence of 0.1 M KCl and different concentrations of Hg2+. The preconcentration (deposition) of mercury was conducted at -0.4 V for 500 s under stirring. The stirring stopped at the end of the deposition time. The square wave voltammetry was recorded between 0 V to +1.0 V with a frequency of 25 Hz, step increment of 5 mV and amplitude of 25 mV. Prior to the next cycle, the electrode was cleaned at +1.0 V for 200 s to remove the residual Hg under stirring condition. In addition, a series of SWV scanning was also performed to confirm the complete removal of any residual mercury until the stripping peak current disappeared.40 All experiments were carried out in air at room temperature. 3. RESULTS AND DISCUSSION 3.1 Sensing Mechanism and Morphological Characteristics Figure 1 illustrates the steps in fabricating the electrochemical sensor and sensing mechanism for Hg2+ detection. The carboxylic group (–COOH) in GO can selectively bind Hg2+ ions due to its strong affinity towards Hg2+ by forming a stable R-COO-Hg2+-COO-R linkage.63 The Hg2+ ions were deposited (preconcentrated) onto the GO-AgNW nanocomposites modified Pt electrode by an initially applied negative voltage (- 0.4 V for 500 s). Square wave voltammetry was carried out subsequently to oxidize (anodic stripping) the absorbed mercury. It is essential to get faster electron transport between electrode and target analyte (Hg2+) to improve the sensitivity of electrochemical sensors. Most of the electrochemical sensors using different GO composites including 5-methyl-2-thiouracil (MTU)-AuNPs-GO,64 cysteamine-GO,65 L-CysteinerGO,66 SnO2-rGO2 suffer from lower sensor performance. One possible reason is the poor conductivity of GO.30 In our work, AgNWs have been employed to increase the conductivity of GO by making conduction pathways between GO sheets and Pt electrode. This phenomenon has been illustrated in Figure S1. Initially, the Hg2+ ions were deposited on the surface of GO by an initially applied negative voltage (-0.4V for 500s) and the negatively charged carboxylic group (– COOH) on GO. The AgNWs accelerate the conduction of necessary electrons for the deposition (reduction reaction) of Hg2+ ion. After that, the stripping of the adsorbed mercury are performed by square wave voltammetry where the AgNWs provide faster transport of released electrons from GO to Pt electrode (shown by red arrows). The rapid charge (electron) transfer during the 4 ACS Paragon Plus Environment

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deposition and stripping of Hg2+ through AgNWs helps to achieve high sensitivity and lower limit of detection. The crystal structure of the AgNWs was investigated by X-ray diffraction (XRD) analysis. As shown in Figure S2, AgNWs show typical diffraction peaks at 38.2 and 44.4 corresponding to the (111) and (200) planes of face-centered cubic silver, which confirms the formation of AgNWs 6768. The SEM image in Figure 2a shows that the AgNWs have diameter of 50-100 nm and length of 8-15 µm. It can be seen from Figure 2b that graphene has aggregated due to π- π interaction between individual layers.36 AgNWs helps to form a hybrid network with thin graphene sheet, which can be clearly illustrated in Figure 2c. The groups of GO sheets are indicated in the SEM image (Figure S3a). Further, elemental mapping was carried out using energy dispersive X-ray spectroscopy (EDS) as shown in Figure S3b-d. The elemental mapping shows that C and O are homogeneously distributed and Ag is distributed as nanowires. The EDS spectrum (Figure S3e) shows that weight percentage of C, O and Ag in the GO-AgNW nanocomposites are 80.4%, 9.1%, and 10.5%, respectively. Dispersed GO sheets become negatively charged due to the ionization of the carboxyl (-COOH) and hydroxyl (-OH) groups. The positively charged AgNWs (Figure S4a) can be absorbed on these negatively charged GO sheets (Figure S4b) due to electrostatic interaction.69 TEM images of the GO-AgNW nanocomposites (Figure 2d-e) show that there is overlapping of AgNWs which can make electron conduction pathways through the graphene sheets. It is also observed that single layer or few layer graphene sheets have been exfoliated from GO. The lightcolored and homogenous areas are attributed to the regions of single layer graphene. The less transparent regions are the accumulation of multiple graphene sheets.70-71 Raman spectra of graphene oxide (GO), AgNW and GO-AgNW nanocomposites are shown in Figure 2f. The Raman spectra of GO exhibited the presence of D and G bands. The D band at 1343.36 cm-1 arises from sp3 hybridized carbon and the graphitic lattice (G) peak at 1567.26 cm-1 arises due to the first order scattering of the E2g phonon of the sp2 carbon lattice.66, 72-73 AgNWs showed a characteristic peak at 987.63 cm-1. The individual peaks of AgNW and GO have been observed in GO-AgNW nanocomposites, which confirms the presence of both materials.



Figure 1. A schematic of GO-AgNW nanocomposites modified electrochemical sensor for Hg2+ detection.

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GO-AgNW

(e) Intensity (a.u.)

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Figure 2. SEM images of (a) AgNWs (b) GO (c) GO-AgNW nanocomposites (d) and (e) TEM images of GO-AgNW nanocomposites (f) Raman spectra of the GO, AgNW and GO-AgNW nanocomposites. 3.2 Electrochemical Characterization and Optimization The electron transfer properties of GO, AgNW, and GO-AgNW nanocomposites modified Pt electrodes were studied by electrochemical impedance spectra (EIS) shown in Figure 3a. It can be seen that Nyquist plots contained a semicircle at higher frequencies corresponding to the electrontransfer-limited process and a straight line at lower frequency indicating the diffusion-limited process.74 The diameter of the semicircle represents the charge transfer resistance (Rct) of electrode/electrolyte interface. The GO-AgNW nanocomposites modified Pt electrode (curve iii) exhibited a Rct value of 460.3 Ω, which is much lower than the GO electrode only with ~ 812.6 Ω (curve i).This implies that the presence of AgNW in GO-AgNW nanocomposites enable the enhancement of electron transfer kinetics suitable for achieving superior sensor response. Meanwhile, the AgNW modified Pt electrode (curve ii) had a Rct value of 396.3 Ω. The EIS results were well supported by the CV results (Figure S5). As seen in Figure S5, the anodic and cathodic peak current for the GO-AgNW nanocomposites modified Pt electrode were higher than GO only modified electrode and less than AgNW only modified electrode. Figure 3b shows the square wave voltammetry response for GO, AgNW, and GO-AgNW nanocomposites modified Pt electrodes. There was no stripping peak current in the AgNW modified Pt electrode. A small stripping peak at around + 0.16 V was observed for the GO modified Pt electrode because the carboxylic group (-COOH) in GO can selectively bind Hg2+ ions as R-COO-Hg2+-COO-R. The stripping peak current of the GO-AgNW nanocomposites modified Pt electrode was enhanced nearly 3.5-fold of that of the GO modified Pt electrode, indicating the synergistic effect of AgNW and GO. In order to get a high performance stripping analysis of Hg2+, the experimental parameters including deposition potential and time were optimized. Figure 3c showed the influence of deposition potential on the stripping peak current responses for Hg2+. Starting from - 0.1V vs. Ag/AgCl, the stripping peak current showed a significant increase as the deposition potential decreased to - 0.40 V. The maximum stripping peak current was observed at - 0.40 V, indicating the most reduction of Hg2+. At deposition potential less than -0.4 V, less Hg2+ ions were reduced into solid Hg due to the competitive generation of H2.40 Consequently, the stripping peak current decreased at deposition potential lower than -0.40 V. Therefore, a deposition potential of - 0.40 V was chosen as an optimum potential for Hg2+. The effect of deposition time in the range of 50-700 s on the stripping peak current responses for Hg2+ was studied. Figure 3d depicts the stripping peak currents of Hg2+ as a function of deposition time at - 0.40 V for the GO-AgNW nanocomposites modified Pt electrode. The stripping peak current increased as the deposition time increased. At deposition time longer than 500 s, the stripping peak current became saturated, which was probably due to saturation of available active sites for the Hg2+ deposition on the GO-AgNW nanocomposites modified Pt electrode surface. Therefore, the optimized deposition time of 500 s was chosen for all the experiments. 7 ACS Paragon Plus Environment


The effect of different weight ratio of GO to Ag NW in the nanocomposites on sensing performance of detecting Hg2+ is shown in Figure S6. The peak current towards 50 nM Hg2+ was maximum (~59 µA) for the optimized weight ratio of 1 (GO:AgNW=1:1). It can be attributed to the formation of hybrid network between AgNW and graphene sheets, which is also supported by SEM and TEM. On the other hand, the peak current significantly decreased to ~45 µA for the weight ratio of 0.5 (GO:AgNW=1:2). The associated reason may be the increase of inter-nanowire junction resistance between AgNWs due to the aggregation. When the weight ratio was increased to 2 (GO:AgNW=1:0.5), the peak current of ~52 µA was obtained which is attributed to the lack of sufficient electron conduction pathways. Therefore, the weight ratio of 1 was found to be the optimum.

Figure 3e illustrates the SWASV responses of the GO-AgNW nanocomposites modified Pt electrode at various Hg2+ concentrations in 0.1M KCl under the optimized deposition potential of -0.4 V and time at 500 s. The stripping peak currents increased with the increase of Hg2+ concentration. The peak current vs Hg2+ concentration followed a linear relationship over the range 1.0 - 70 nM with a correlation coefficient of 0.9947 (Figure 3f). The sensitivity of the sensor was ~ 0.29 µA/nM according to the slope of the linear curve. The detection limit (3σ/s, where σ and s are standard deviation and sensitivity, respectively) of the GO-AgNW nanocomposites modified Pt toward Hg2+ was calculated to be ~ 0.1 nM, which is very well below the World Health Organization defined limit in drinking water.

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Figure 3. (a) Nyquist plots for different electrodes in 5 mM K3Fe(CN)6 aqueous solution containing 0.1 M KCl (i) GO modified Pt, (ii) AgNW modified Pt, (iii) GO-AgNW nanocomposites modified Pt electrode. The inset is the equivalent circuit used to model impedance data in the form of charge transfer resistance (Rct), warburg diffusion resistance (Ws), and constant phase element (CPE), respectively. (b) SWASV responses for Hg2+ determination at three different modified Pt electrodes. The effect of (c) deposition potential and (d) deposition time on the stripping peak current for 1µM Hg2+ at GO-AgNW nanocomposites modified Pt electrode. (e) SWASV response of the GO-AgNW nanocomposites modified Pt electrode for Hg2+ with different concentrations. (f) The plot of the stripping peak current vs. Hg2+ concentration. The error bars represent the standard deviation for the mean of three replicate tests. 3.3 Selectivity, Repeatability, Reproducibility, and Stability The possible interferences arising from various heavy metal ions including Pb2+, Cd2+, Cu2+, Na+, and Ag+ were studied under the optimum deposition potential and time to evaluate the selectivity of the GO-AgNW nanocomposites modified Pt sensor. Figure 4a displays the stripping response of the Hg2+ sensor in presence of 10-fold concentration of each interfering agent in the solution with respect to the target analyte. It was observed that the stripping peak current for 50 nM Hg2+ was almost the same, indicating that these metal ions had no significant effect on the detection of Hg2+. Since the standard reduction potential of Hg2+ (+0.85 V) is close to that of Ag+ (+0.80 V), a 9 ACS Paragon Plus Environment


very small interference from Ag+ ion on the response of Hg2++Ag+ was observed. SWASV response of the GO-AgNW nanocomposites modified Pt electrode for Hg2+, Ag+ and Hg2+/Ag+ mixture are shown in Figure S7. The interference effects of different metal ions to GO-AgNW nanocomposites modified Pt electrode with and without Hg2+ are shown in Figure S8. As observed, the sensor did not show obvious responses to the interfering species studied. These results indicate the high selectivity of the sensor to Hg2+. It is well known that many metal ions can complex with carboxylic group (-COOH). When two metal ions, both having affinity to -COOH group, the metal ion having higher reduction potential will get preferably reduced on the surface of GO by the COOH group.75 Thus, Hg2+ with its higher standard reduction potential (+0.85 V) shows more tendency to reduce over interfering Pb2+ (-0.13 V), Cd2+ (-0.40 V), Cu2+ (+0.34V), Na+ (-2.71 V), and Ag+ (+0.80 V). 75 Hence, the interfering ions will find it more difficult to bond with -COOH group and less number of these ions will be adsorbed. This implies that Hg2+ will be easily adsorbed and reduced by -COOH of GO in the presence of interfering ions. Repeatability of the sensor was studied after ten replicate tests for 10nM Hg2+ under the optimum deposition potential and time with a single GO-AgNW nanocomposites modified Pt sensor. As seen in Figure 4b, the obtained stripping peak current towards Hg2+ for every test was almost identical. The low relative standard deviation (RSD) of peak current (3.01%) demonstrates a good repeatability of the sensor. The reproducibility of the GO-AgNW nanocomposites modified Pt sensor was also tested with three different sensors prepared independently by the same procedure. Under the optimum deposition potential and time, the sensors were employed to detect 10nM Hg2+ using SWASV. Figure 4c represents the SWASV response of the sensors, where the inset shows a histogram plot for peak current with respect to different sensors tested. Three replicate tests were performed for each sensor. The low RSD of peak current (2.0%) demonstrates a good reproducibility of the sensor. The good repeatability and reproducibility of the GO-AgNW nanocomposites modified Pt sensor make it promising for electrochemical detection of Hg2+. To study the short-term stability of the GO-AgNW nanocomposites modified Pt electrode, cyclic voltammetry (CV) was performed over 10 cycles. Figure S9 shows the CV curves of the 1st, 5th, and 10th cycle. The overlapping of the three CV curves indicates that the GO-AgNW nanocomposites modified Pt electrode shows good short-term stability. Long-term stability will be investigated in the future. No depletion or falling-off the GO-AgNW nanocomposites modified electrode was observed as is evident in Figure S10, which shows the photographs of the electrode after Hg2+ detection for 10 mins as well as 20 mins. This confirms that the GO-AgNW nanocomposites are very stable with excellent adhesion property during the electrochemical measurement for Hg2+ detection.


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Figure 4. (a) Interference effects of different metal ions on the stripping signals of Hg2+ at GOAgNW nanocomposites modified Pt electrode (50.0 nM Hg2+ and 500 nM each for Pb2+, Cd2+, Cu2+, and Na+). The error bars represent the standard deviation for the mean of three replicate tests. (b) Repeatability study of GO-AgNW nanocomposites modifed Pt sensor in 0.1 M KCl containing 10 nM Hg2+ under optimum deposition potential and time. Data are obtained from every SWASV response. (c) Reproducibility test carried out at 10 nM Hg2+ for three sensors. Inset shows a histogram plot for peak current with respect to the three sensors. Error bars are obtained from three replicate tests. 3.4 Analysis of Real Sample The feasibility of the GO-AgNW nanocomposites modified Pt sensor was evaluated by determining Hg2+ in tap water. Tap water was collected from Daktronics Engineering Hall, South Dakota State University , SD, USA. Tap water was diluted with 0.1M KCl in a ratio of 1:9, without any further sample treatment.76 The tap water samples were spiked with different concentrations of Hg2+ and then analyzed with SWASV. Without spiking, the mercury ion content in tap water was zero. From Table 1, the high recovery percentage suggests that the GO-AgNW nanocomposites modified Pt sensor has an excellent capability for accurate detection of Hg2+ in tap water. 11 ACS Paragon Plus Environment


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Table 1. Determination of Hg2+ in real water samples using GO-AgNW nanocomposites modified Pt sensor (n = 3). Concentration of Hg2+ Sample

Recovery (%) Add (nM)

Found (nM)

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0.9

90.0

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87.0

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69.0

98.6

Comparison of sensing performance among different electrochemical Hg2+ sensors in terms of limit of detection (LOD) and linear detection range (LDR) are summarized in Table 2, demonstrating that the GO-AgNW nanocomposites modified Pt sensor is a promising candidate for ultrasensitive detection of Hg2+. Based on different methods including electrochemical, fluorometric, luminescent, etc., the GO-AgNW nanocomposites modified Pt sensor showed lower LOD compared to previously reported works,53-54, 66, 76-79 where the lowest reported LOD was 0.000001 nM.75 Besides, the GO-AgNW nanocomposites modified Pt sensor demonstrated higher LDR than these previous works.77,48-49, 55, 80 The GO-AgNW nanocomposites modified Pt sensor exhibited better LOD and LDR than other reports on graphene nanocomposites,81,82 and nanoporus AuNPs.77 The reported LODs in some previous works76,54, 79 are above than the safety limit of Hg2+ in drinking water (5 nM) defined by the WHO. While, the GO-AgNW nanocomposites modified Pt sensor met this safety limit. Some reports including DNA-MoS2/AuNPs,83 DNA-rGO,84 Oligonucleotide-WS2 nanosheet,55 GQDAuNPs/GCE,49 AgNPs/GCE,80 AuNPs-rGO48 had lower and comparable LODs but lower LDRs. The following works L-Cys-rGO/GCE,66 DNA-WS2 nanosheet,53 Graphene Aerogel-MOF78 met the safety limit with high LDRs, however, had higher LODs than our work. In addition, only few reports including MoS2/GCE,75 N-doped rGO/MnO2/GCE85 had better LODs and LDRs than our work. Further, the MoS2/GCE75 showed the LODs of 0.000001 nM, however, these LOD value is far away from their achieved LDRs. The reported LDR for Au/Ag/Au/CS–GO56 is above than the safety limit of Hg2+ in drinking water. Table 2. Comparison of sensing performance among different electrochemical Hg2+ sensors. Modifications Nanoporus AuNPs/ITO

LOD (nM)

LDR (nM)

0.15

5-50

Method Electrochemical


References 77

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rGO-Au/CPE

2.04

5-40

Electrochemical

82

MnFe2O4Cysteine/GCE

208

14003000

Electrochemical

76

AgNPs/GCE

0.028

0.1-10

Electrochemical

80

AuNPs-rGO

0.0075

0.05-5

Electrochemical

48

L-Cys-rGO/GCE

4.958

0-1600

Electrochemical

66

3.3

6-650

Fluorometric

53

Graphene AerogelMOF

2

5-3000

Electrochemical

78

DNA-MoS2/AuNPs

0.1

0.1-10

FET

83

DNA-rGO

0.1

0.1-10

Electrochemiluminescence

84

rGO-AgNPs

20

10010,0000

UV-vis absorption spectroscopy

54

MoS2/GCE

0.000001

0.120,000

Electrochemical

75

OligonucleotideWS2 nanosheet

0.10

0.5-20

Fluorometric

55

GO-DNA

0.356

0-0.05

Fluorometric

81

GQD-AuNPs/GCE

0.02

0-1

Electrochemical

49

100-1200 Electrochemical

79

DNA-WS2 nanosheet

CNFs/AuNPs Au/Ag/Au/CS–GO N-doped rGO/MnO2/GCE GO-AgNW/Pt

30 -

50025,000

Surface plasmon resonance

56

0.0414

10-200

Electrochemical

85

0.10

1-70

Electrochemical

This work

GO: graphene oxide; AuNPs: gold nanoparticles; ITO: indium tin oxide; CPE: carbon-paste electrode; AgNPs: silver nanoparticles; GCE: glassy carbon electrode; L-Cys: L-cysteine; rGO: reduced graphene oxide; MoS2: molybdenum disulfide; WS2: tungsten disulfide; GQD: graphene quantum dot; CNFs: carbon nanofibers; CS: chitosan; AgNW: silver nanowire; MOF: metalorganic framework; FET: field effect transistor;



4. CONCLUSION In summary, GO-AgNW nanocomposites modified high performance Hg2+ sensor was successfully developed. The GO-AgNW nanaocomposites greatly facilitate faster electron-transfer kinetics and lead to an improved sensing of Hg2+. The outcomes reveal that the GO-AgNW nanocomposites modified Pt sensor is highly sensitive to Hg2+ in the range of 1 - 70 nM, and the limit of detection is ~ 0.1 nM. Due to the formation of R-COO-Hg2+-COO-R linkage, the sensor showed a strong affinity to Hg2+, while other heavy metal ions had no interference. Moreover, the sensor exhibited excellent repeatability, reproducibility, and applicability for the determination of Hg2+ in tap water. ASSOSCIATED CONTENT Supporting Information Sensing mechanism of Hg2+ (Figure S1), XRD of AgNWs (Figure S2), SEM image, EDS mapping and spectrum (Figure S3), Zeta potential (Figure S4), CV of different modified electrodes (Figure S5), Effect of weight ratio (Figure S6), Interference effect (Figure S7-S8) and stability (Figure S9S10). AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] ORCID Md Tawabur Rahman: 0000-0003-4056-1526 Md Faisal Kabir 0000-0001-5221-0593 Ashim Gurung: 0000-0002-9114-0177 Rajesh Pathak: 0000-0002-4237-4209 Nabin Ghimire: 0000-0003-4016-2744 Aravind Baride: 0000-0001-7582-1522 Zhenqiang Wang: 0000-0002-8716-482X Mahesh Kumar: 0000-0002-5357-7300 Qiquan Qiao: 0000-0002-4555-7887 Author Contributions The paper was written through contributions of all authors. All authors have given approval to the final version of the paper.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors acknowledge support from EDA University Center (ED18DEN3030025), NSF-MRI (Grant 1428992), and NSF I-Corps (1906755).

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