Voltammetric Detection of Cr(VI) with Disposable Screen-Printed

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Environ. Sci. Technol. 2007, 41, 8129–8134

Voltammetric Detection of Cr(VI) with Disposable Screen-Printed Electrode Modified with Gold Nanoparticles GUODONG LIU,# YING-YING LIN, HONG WU, AND YUEHE LIN* Pacific Northwest National Laboratory, Richland, Washington 99352

Received July 12, 2007. Revised manuscript received September 13, 2007. Accepted September 17, 2007.

This paper was retracted March 18, 2008 (Environ. Sci. Technol. 2008, 42, 3117).

Gold nanoparticle (Au-NP) enhanced voltammetric detection of Cr(VI) is developed for determination of trace amounts of Cr(VI) in an acetate buffer media (pH 4.6). The Au-NPs were electrodeposited onto a disposable screen printed electrode (SPE) via an electrodeposition step. It was found that the electrodeposited Au-NP has strong adsorption on Cr(VI) species, which results in an enhanced reduction current of Cr(VI). Compared with the bulk gold electrode, the reduction current of Cr(VI) was enhanced 10 times with the Au-NP-modified SPE electrode. Square wave voltammetric (SWV) measurement with the disposable Au-NP-modified SPE provides a fast, simple and sensitive detection of trace amounts of Cr(VI). The adsorption of Cr(VI) on Au-NP was characterized with voltammetry, X-ray photoelectron spectroscopy and ultraviolet spectra. The different parameters including the electrodepositing time, supporting electrolyte, and pH that govern the analytical performance of the electrode have been studied in detail and optimized. The detection limit of 5 µg L-1 Cr (VI) was obtained under optimum experimental conditions. The performance of the sensor was successfully evaluated with river water samples spiked with Cr(VI), indicating this convenient and sensitive technique offers great promise for onsite environmental monitoring and biomonitoring of Cr(VI).

Introduction Analysis of Cr(VI) in environmental samples is routinely carried out using analytical techniques, such as ultraviolet and visible spectrophotometry (1), atomic emission spectrometry (2), atomic absorption spectrometry (3), high pressure liquid chromatography (4), X-ray fluorescence (5), and mass spectrometry (6). Such analysis is generally performed at centralized laboratories, requiring extensive labor and analytical resources, and often results in a lengthy turnaround time. These analytic methods have a number of disadvantages that limit their applications primarily to laboratory settings and prohibit their use for rapid analyses under field conditions. To meet the requirements of rapid * Corresponding author phone: 1-509-3760529; fax: 1-509-3765106; e-mail: [email protected]. # Current address: Department of Chemistry and Molecular Biology, North Dakota State University, Fargo, ND, 58105. 10.1021/es071726z CCC: $37.00

Published on Web 10/26/2007

 2007 American Chemical Society

detection and field deployment, more-compact low-cost instruments, coupled to smaller sensing probes, it is highly desirable for facilitating the task of on-site monitoring of Cr(VI) compounds. Electrochemical techniques are very attractive for Cr(VI) analysis, owing to their high sensitivity, inherent simplicity, miniaturization, and low cost. A number of electrochemical methods and techniques have been developed for Cr(VI) detection. Early work of electrochemical detection of Cr(VI) started with mercury electrodes (7). Adsorptive stripping voltammetric measurements at mercury electrodes in the presence of complexing agents (diethylenetriamene pentaacetate (8, 9), triethylenetetranitrilohexaacetic acid (10, 11), bipyridyne (11, 12) pyrocatechol violet (13, 14), and Cupferron (15, 16) or masking agent (Nitrilotriacetic acid (17) have been reported and used for Cr(VI) detection in different samples. Although mercury electrodes exhibit the highest sensitivity, the potential toxicity and its operation limitations prevent its application in analytical practice. Solid electrodes, such as glassy carbon (18), carbon paste (19), platinum (20) and gold (21, 22), as well as different chemical and physical modifications of these electrodes have been widely used for the Cr(VI) detection or reaction mechanism studies in connection with various voltammetric techniques, such as anodic stripping voltammetry, adsorptive stripping voltammetry, square wave voltammetry (SWV), and cyclic voltammetry (CV). Recently, Welch et al. (21)studied the reduction mechanism of Cr(VI) at solid electrodes in acidic media and its analytical application. Most of these investigations are carried out in the laboratory and not used for in-field analysis. In this work, the aim is to develop in-field electrochemical sensor based on gold nanoparticle modified screen printed electrode (Au-NP-modified SPE) for a fast, simple, and convenient way to detect Cr(VI). The screen-printing (thickfilm) technique is widely used for large-scale fabrication of disposable electrochemical sensors and biosensors (e.g., glucose test strip) with several advantages including low cost, versatility, and miniaturization (22). SPEs have enabled the production of modern sensors which can be incorporated in portable systems, an important requirement of analytical methods for direct analysis of a sample in its “environment” without alteration of the “natural environmental conditions” (22). With the development of nanoscience and nanotechnologies, the use of nanoparticles in electroanalysis has been extensively explored (23). Particularly, gold nanoparticles have been widely used as electrocatalysts for oxygen reduction (24–26), the oxidation of nitric oxide (27), and nitrite reduction (28). Au-NP modified glassy carbon electrodes have been used for electrochemical detection of Cr(III) and arsenic (29, 30). In this paper, we developed an Au-NP modified SPE for Cr(VI) determination in the first time. The reduction of Cr(VI) was evaluated using CV and SWV analysis. The adsorption performance of Cr(VI) on Au-NPs was investigated by CV, XPS, and UV. The performance of the sensor was evaluated with river water samples spiked with Cr(VI), and the results were validated with conventional inductively coupled plasma mass spectrometry (ICP-MS). The promising stripping voltammetric performances open new opportunities for fast, simple, sensitive, and onsite analysis of Cr(VI).

Experimental Section Reagents. Hydrogen tetrachoroaurate (III) trihydrate (HAuCl4), K2Cr2O7, gold nanoparticle (diameter: 10 nm) solutions, 3 M acetate buffer were bought from Sigma (St. VOL. 41, NO. 23, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Typical SEM images of a SPE (A) and electrodeposited Au-NP-modified SPE (B). Au-NP-modified SPE was prepared using a 60 s deposition time. Louis, MO). All solutions were prepared using Milli-Q (Millipore, Bedlford, MA) A1- gradient (18 MΩ.cm) deionized water. Instrumentation. Square-wave voltammetric and cyclic voltammetric measurements were performed with an electrochemical analyzer CHI 660A (CH Instruments, Austin, TX) connected to a personal computer. Disposable electrochemical screen-printed electrode (SPE) consisted of carbon working electrode, carbon counter electrode, and Ag/AgCl reference electrode was bought from Alderon Biosciences, Inc. (cat. 0101, NC) for electrochemical measurements. Sensor connector (Alderon Biosciences, Inc., cat. 0012) allows for connecting disposable SPE to electrochemical analyzer. ICPMS measurements were performed using a Hewlett-Packard 4500 Series ICP-MS instrument. X-ray photoelectron spectroscopy (XPS) measurements were performed using a physical electronics quantum 2000 scanning ESCA microprobe. Scanning electron microscopy (SEM) was carried out using a JEOL JSM-5900 LV machine. Preparation of Au-NP-Modified SPE. The Au-NP-modified SPE was prepared by electrodeposition. Before modification, the SPE was washed with distilled water and dried under air stream. In the case of the preparation of physical adsorption of Au-NPs on the SPE surface, briefly, a 2 µL of Au-NP solution was dropped to the SPE surface and dried in the air. Before use, the Au-NP- modified SPE was washed with distilled water once to remove the unabsorbed Au-NPs. The procedure for the electrodeposition of Au-NPs at SPE was adapted from previously published reports (24). Fifty µL of AuCl4- solution in 0.5 M H2SO4 was dropped to the cleaned SPE (see above) surface, and a potential step from +1.1 V to a potential 0 V was applied for a fixed time (for example, 60 s). All solutions were degassed with a N2 stream prior to measurement for at least 10 min The Au-NP deposited electrode was washed with distilled water once and dried under a N2 stream. Figure 1 shows the representative SEM images of bare screen printed carbon electrode (A) and AuNP-modified screen printed carbon electrode (B). The distinct Au-NPs (∼50 nm) were formed on the carbon surface, and no Au-NP was observed on the bare screen printed carbon electrode under the same preparation conditions in the absence of HAuCl4. The density of Au-NPs on the electrode surface is much less than that of reported Au-NP-modified disk glassy carbon electrodes and gold electrodes (24, 28). It may be attributed to the rougher surface of the screen printed carbon electrode. Electrochemical Measurements. All electrochemical measurements were performed with an unmodified SPE or AuNP-modified SPE. A 50 µL of sample solution was dropped to the Au-NP-modified SPE surface to cover the three electrodes. The electrode was incubated 1 min with the sample solution before the electrochemical measurement. SWV measurements were performed from 0.5 to -0.5 V with a step potential of 4 mV, amplitude of 20 mV, and a frequency of 25 Hz (unless otherwise stated). Baseline correction of the resulting voltammogram was performed using the “linear baseline correction” mode of the CHI 660 software. The cyclic voltammogram was recorded between -0.5 and +0.5 V at a B

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FIGURE 2. Voltammograms of bare SPE (A) and Au-NP-modified SPE (B) in the absence (black curve, a) and presence (red curve, b) of 10 mg L-1 chromate. Potential scanning rate: 100 mV/s; supporting electrolyte: 0.2 M acetate buffer (pH 4.6). Also shown is the corresponding square wave voltammograms of Au-NP-modified SPE electrode in the absence (blue curve) and presence (red curve) of 10 mg L-1 Cr(VI)(C). SWV scan with a step potential of 4 mV; amplitude 20 mV; frequency, 25 Hz. scan rate of 100 mV/s. All measurements were performed at room temperature. River Water Analysis and Recovery. River water samples were collected from two locations (I–II) on the Columbia River in Washington state. Sample solutions were prepared by adding 10 µL untreated river water into 40 µL of 0.2 M acetate buffer (pH 4.6). Recovery experiments were performed by spiking standard Cr (VI) stock solution to the sample solution. For comparison, Cr(VI) recovery experiments were also performed with ICP-MS.

Results and Discussion Electrochemical Response of Cr(VI) on Au-NP-Modified SPE. The electrochemical characteristics of Cr(VI) are well documented at different electrodes (bulk Hg electrode (9), gold electrode, glassy carbon, platinum electrode 20–23) under different supporting electrolytes. To examine the electroanalytical performance of Au-NP-modified SPE, CV was employed first. Figure 2 shows the cyclic voltammograms of SPE(A) and Au-NP-modified SPE (B) in 0.2 M acetate buffer(pH 4.6) in the absence(a, black curve) and presence(b, red curve) of 10 mg L-1 Cr(VI). No redox activity was observed at SPE both in the absence and in the presence of chromate at the interested potential range. (Figure 2A). However, a well-defined reduction peak was observed at -0.3 V with Au-NP-modified SPE and can be attributed to the threeelectron reduction of Cr (VI) to Cr(III) (Figure 2B). On the reverse anodic scan, an oxidation wave at +0.12 V (vs Ag/ AgCl) was observed. This process is ascribed to the subsequent reoxdiation of Cr(III) to the parent Cr(VI) species. SWV analysis has a higher sensitivity than other electrochemical technologies, such as CV and differential pulse voltammetry. Figure 2C shows corresponding SWV voltammograms of 10 mg L-1 Cr (VI) at SPE electrode (a) and Au-NP-modified SPE (b) in 0.2 M acetate buffer (pH 4.6). There is no reduction peak observed at the SPE (Figure 2 C, curve a). The gold NPs modified SPE exhibits a very sharp and well-defined reduction peak at the potential range from 0.5 to -0.5 V (Figure 2C, curve b). The peak potential of the reduction peak (-0.16 V) shifts 140 mV to a positive potential direction compared with that in the cyclic voltammogram. Similar voltammetric

FIGURE 3. (A) Cyclic voltammograms of the Au-NP-modified SPE with different potential scan rate (10 ∼100 mV S1-) in acetate buffer (pH 4.6) containing 10 mg L-1 Cr (VI). Inset is the relationship plot between the reduction peak current of Cr(VI) and scan rate. (B) XPS spectra of Cr(VI) adsorbed Au-NP (a) and standard Cr (VI) (b); (C) UV spectra of Au-NP in the absence (a) and presence of 10 mg L-1 Cr(VI) (b). behavior was also observed with a physical adsorbed AuNP-modified SPE (results not shown). We compared the SWV response of Cr(VI) on bulk gold electrode and the Au-NP- modified SPE. The response current of Cr(VI) was amplified around 10 times by using the AuNP-modified SPE (results not shown). The enhanced signal may be attributed to the strong adsorption of Cr(VI) on AuNPs. It was suggested that the reduction process of Cr(VI) is related with active surface metal atoms, which is playing a significant role in Cr(VI) reduction. Electrodeposited AuNPs would have more active surface gold atoms, which results in a higher reduction current than that of the bulk gold. Characterization of Cr(VI) Adsorption on Au-NP. The reduction of Cr(VI) on the solid metal electrodes has been studied in detail (21). Welch et al. reported that the reduction process of Cr(VI) on bulk gold electrode is diffusion-controlled process in 0.1 M HCl solution (21). CV was first used to investigate the reduction process of Cr (VI) on the Au-NPmodified SPE. Figure 3A shows the recorded voltammograms of 10 mg L-1 Cr (VI) on Au-NP-modified SPE at different scan rates (10–100 mV s-1) in 0.2 M acetate buffer (pH 4.6). These reveal process with a peak potential at -0.18 V (10 mV s-1), which was found to shift up to -0.3 V (100 mV s-1) with a scan rate increase. The fact that peak potential is significantly dependent on scan rate suggests that reaction is electrochemically inreversible at the Au-NP-modified SPE. Inset of Figure 3A presents the relationship plot between the reduction peak current and scan rate (v). It was found that the reduction peak current is proportional to the potential scan rate, instead of the square root of scan rate, indicating the reduction reaction of Cr(VI) is a surface-reaction-controlled process, instead of a diffusion-controlled process. It indicates that Cr(VI) would be absorbed on the Au-NP surface. The observed reduction behavior of Cr(VI) on Au-NP-modified

SPE is different than that on the bulk gold electrode. To verify this adsorption, Au-NP-modified SPE was dipped in 10 mg L-1 Cr(VI) solution for 1 min, then washed with distilled water and transferred to a 0.2 M acetate buffer for SWV measurement. A similar reduction peak of Cr(VI) was observed (result not shown). The above voltammetric experiments indicate that Cr(VI) has strong adsorption on the Au-NP surface. To confirm the adsorption of Cr(VI) on a gold naoparticle surface, XPS experiments was performed. The sample was prepared by incubating the Au-NP solution with 10 mg L-1 Cr (VI) for 2 min, then centrifuging and washing to remove the unadsorbed Cr(VI). The Cr(VI)-adsorbed Au-NP was redispersed in 0.1 M KCl. A control sample (Cr(VI) standard solution, 10 mg L-1) was also prepared for comparison. Figure 3B shows the typical XPS spectra of standard Cr(VI) and Cr(VI) adsorbed Au-NP. The binding energy of the core-electrons for the Cr 2p3/2 line at 580.2 eV and the Cr 2p1/2 line at 588.5 eV in the prepared Cr (VI) adsorbed Au-NP sample (curve a) are consistent with the standard Cr(VI) (curve b). The XPS spectra confirm our electrochemical results. The adsorption of Cr on the Au-NP is also confirmed by UV spectra. Figure 3C shows the adsorption spectra of AuNP solution in the absence (a) and presence (b) of Cr(VI). It can be seen that the adsorption peak of Au-NP at 525 nm decreases after addition of Cr(VI), a new adsorption peak appears at 350 nm, which is attributed to the adsorption of Cr(VI). The adsorption wavelength of Au-NP shifts to 523 nm. The decrease of adsorption intensity of Au-NP and shift of maximum adsorption wavelength thus support our hypothesis. Optimization of Voltammetric Parameters. The parameters that govern the analytical performance of Cr(VI) on the Au-NP-modified SPE have been optimized. Figure 4 shows the effect of electrodepositing time of Au NP (A), supporting VOL. 41, NO. 23, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Effect of electrodeposition time (A), electrolytes (B), and pH (C) on the reduction of Cr(VI) on Au-NP-modified SPE electrode. Concentration of Cr(VI): 10 mg L-1. The conditions of SWV same as in Figure 2C. electrolyte (B) and pH (C) of supporting electrolyte upon the SWV signals for 10 mg L-1 Cr(VI). The electrodeposition time during the preparation of Au NP has strong effect on the SWV signal of Cr(VI). The reduction current increased significantly in the interval from 5 to 15 s, then more slowly in the region up to 60 s (Figure 4A); at a longer electrodepositing time (>60s), the signal started to decrease. The decrease of reduction current may be attributed to the formation of gold thin film on the SPE surface with the longer depositing time, which shows the characteristics of the bulk gold electrode. So a 60 s electrodeposition time was used in further experiments. The effects of supporting electrolytes were studied to find out the optimum conditions for the analytical detection of Cr(VI). Welch et al. found the similar sensitivity in the acid electrolytes (0.1 M HCl, 0.1 M H2SO4, and 0.1 M HNO3) with a bulk gold electrode (23). However Danilov and Protsenko reported that the sensitivity in various electrolytes decreased in the following order: H2SO4> HCl> HNO3 (31). Thus, we compared the electrochemical responses of Cr(VI) with the Au-NP-modified SPE in 0.1 M HCl, 0.1 M H2SO4, 0.1 M HNO3, and 0.2 M acetate buffer (pH 4.6). Figure 4B shows the histogram of the reduction current of Cr(VI) in the above electrolytes. It can be seen that the maximum response was obtained in acetate buffer, the sensitivity in these four electrolytes decreases in the following order: acetate buffer (pH 4.6)> HNO3> HCl> H2SO4. It may be ascribed to the strong adsorption of Cr(VI) in acetate buffer (pH 4.6). To further verify the reduction response of Cr(VI) in acetate buffer, we studied the pH effect of acetate buffer on the reduction current of Cr (VI). Figure 4C shows the relationship plot between the reduction current of Cr(VI) and pH of acetate buffer. The pH of acetate buffer was adjusted to a desired value by adding 3 M acetate acid or 1 M NaOH into a 0.2 M acetate buffer (pH 4.6). It can be seen that the reduction of Cr(VI) is very sensitive for the pH of acetate buffer. The maximum reduction response was obtained at pH 4.6, which is closed to pKa value (4.75) of acetate buffer, i.e. closed to the maximum buffer capacity, smaller response was obtained at pH 5.0, and no response was obtained at pH more than five or less than 4.0. The above results show that there is some proton contribution for the reduction of Cr(VI) on the Au-NP-modified SPE. Welch et al. proposed a reaction mechanism of Cr(VI) on bulk gold electrode in hydrochloric acid and suggested that reduction of hexavalent chromium proceeds via a one-electron and one-proton reaction, folD

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FIGURE 5. (A) Typical square wave voltammograms of the increasing concentration of Cr (VI) on the Au-NP-modified SPE electrode. (B) Corresponding calibration curve. Electrodepositing time of Au-NP: 60s; supporting electrolyte: 0.2 M acetate buffer (pH 4.6). The inset of panel A shows the SWV response of 10 µg L-1 Cr(VI). Other conditions, same as Figure 2C. lowed by disproporonation. There was no reduction response at pH more than 3.0 (see details in ref 21). In the current studying, the reduction Cr(VI) may precede a similar reaction mechanism. The maximum current response at pH 4.6 indicates that Cr(VI) may be easily adsorbed on the Au nanoparticle surface under these pH conditions. The reason is not clear at this time and is under investigation in our laboratory. Therefore, a pH 4.6 acetate buffer is used as supporting electrolyte for further detection of Cr(VI). Analytical Performance. Figure 5A displays the SWV response of the Au-NP-modified SPE for increasing Cr(VI) concentrations under the optimum experimental conditions. Well-defined peaks, proportional to the concentration of the corresponding Cr(VI), were observed. A small shoulder peak at 0.1 V can be noticed. This peak was also observed in the blank measurements and may be attributed to a trace of metal contaminants in carbon ink. A linear relationship between the reduction current and Cr(VI) concentration was obtained covering the concentration range from 10 µg L-1 to 5 mg L-1. (Figure 5B). Inset of Figure 5A is the SWV response of 10 µg L-1 Cr(VI). A detection limit of 5 µg L-1 was estimated on the basis of an S/N ) 3 characteristic of the 10 µg L-1 data points. The detection limit obtained is much lower than that reported so far with the bulk gold electrode (0.22 mg L-1 and 0.25 mg L-1) (21, 30). Such improvement of the reduction signal and extremely low detection limit benefits from the electrodeposited Au-NPs. A series of measurements (six) of a solution containing 50 µg L-1 Cr (VI) with six newly prepared Au-NP-modified SPE yielded a reproducible reduction peak with relative standard deviations of 4.8%.

TABLE 1. Recovery of Cr(VI) from Spiked Samples of River Water sample origin

Cr (VI) added (mg/L)

recovery (% mean ( SDa) Au-NP-SPE

ICP-MS

I I I I II II II II

0 0.1 0.5 1.0 0 0.1 0.5 1.0

N/Ab 99 ( 4 103 ( 5 102 ( 7 N/A 103 ( 4 109 ( 8 98 ( 8

N/A 95 ( 6 98 ( 7 101 ( 5 N/A 100 ( 6 105 ( 7 100 ( 6

a

Standard deviation, n ) 3.

b

Not available.

The high sensitivity is also followed by high selectivity. The interference of other metal ions (Cr3+, Cu2+, Fe3+, Ni2+, Ca2+, Pb2+, Mg2+, MoO42-, and VO3-) on the determination of Cr(VI) was studied using a fixed concentration of Cr(VI) of 100 µg mL-1. It was found that at least 104-fold amounts of Ca2+, Mg2+; 103-fold Ni2+ and Cu2+, 100-fold Fe 3+ and Cr 3+, 50-fold MoO 2- and VO3- did not interfere. 4 Determination of Cr(VI) in River Water. The performance of the Au-NP-modified SPE was tested on river water samples collected from two locations (I–II) in the Columbia River in Washington State and compared with ICP-MS. The analysis of these samples by two methods did not find any trace of Cr(VI), which indicates values below 5 µg L-1 (Au-NPmodified SPE) or 2.5 µgL-1 (ICP-MS). To determine whether substances present in the river samples could interfere the Cr(VI) detection (false negative or false positive), 0.1, 0.5, and 1 mg/L Cr(VI) was added to the untreated river samples. The recoveries were calculated, and their values are summarized in Table 1. Recoveries ranging from 98 to 109% are not significantly different from those obtained with ICP-MS test, which suggest that the developed Au-NP-modified SPE can be used as an effective tool for the detection of Cr(VI). Comparing with the conventional ICP-MS, the current electrochemical detection approach based on the Au-NPmodified SPE is low-cost and does not require very expensive instrumentation. For example, the cost of an electrochemical detector and a commercial SPE are less than $2000 and $2 (U.S.), respectively. The results obtained from this work demonstrated that it is feasible to use a portable electrochemical detector and the Au-NP-modified SPE for in-field rapid detection of Cr(VI). Because of the pH sensitivity of current electrochemical detection of Cr(VI), one should pay attention to the pH of sample solution, for example using a buffer solution to control the pH value of the samples. The proposed method thus holds great promise for onsite environmental monitoring and biomonitoring of Cr(VI).

Acknowledgments The work performed at Pacific Northwest National Laboratory (PNNL) is supported by the U.S. Department of Energy’s (DOE’s) Environmental Remediation Science Program and the National Institute of Health (NIH)/1R01 ES010976-01A2. The research described in this paper was performed at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for DOE under Contract DEAC05-76RL01830.

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