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Catalytic Deposition of Pb on Regenerated Gold Nanofilm Surface and Its Application in Selective Determination of Pb2+ Wei Zhao, Pei-Yu Ge, Jing-Juan Xu,* and Hong-Yuan Chen The Key Lab of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China ReceiVed February 6, 2007. In Final Form: April 28, 2007 We report a simple method of catalytic deposition of Pb on a gold nanofilm substrate, which was in situ prepared and used as nanocrystal seeds. Due to the unique properties of gold nanocrystal seeds, Pb could be catalytically deposited on the surface of the gold nanofilm. Compared with the deposition of Pb on bare gold electrode, a larger amount of Pb was deposited on the gold nanofilm and the electrical response was amplified significantly. The catalytic deposition of Pb on the gold nanofilm was characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and electrochemical methods. A stable and quasi-reversible redox couple was obtained in neutral solution and studied in detail. The surface of the gold nanofilm could be easily regenerated in 0.1 mol L-1 nitric acid solution. Since the redox peaks of Pb could be effectively separated from those of other metals such as Cu, Cd, and Zn, a selective determination of Pb2+ was achieved. Linear sweep voltammetry (LSV) was used for the determination of Pb2+. The peak currents of Pb varies linearly with the concentration of Pb2+ in aqueous solution ranging from 1.0 to 10.0 µmol L-1 (R ) 0.999), with a detection limit of 0.1 µmol L-1. It is expected that the gold nanofilm will facilitate the appearance of heavy metal ion sensors with good performance.
1. Introduction Recently, the synthesis of nanoscale materials and the organization of functional metal nanoparticles (e.g., Au, Ag, Cu) on surfaces has made rapid progress.1-3 The utilization of organized metal nanoparticle systems has attracted specific research efforts. The solution growth method of metal nanoparticles (NPs), concentrating metal atoms onto seed nanocrystals, has been well established.4,5 In the chemical reduction of a metal salt, the nucleation event may be localized uniformly in space and time by the presence of nanometer-size cavities. The nanocrystal seeds provide unique catalytic properties to stimulate the enlargement of seeds by the same metal or another metal. Wilcoxon et al.6-8 demonstrated a systematic growth of nanoparticles starting from a monodisperse seed population. Both Au and Ag were used as the nanocrystal seeds and shells. On the basis of this growth technique, a wide range of monodisperse metal nanoclusters could be synthesized in solutions and can serve as seed populations for further heterogenerous growth. Recently, the catalytic deposition of the metals on metal nanoparticle seeds has been successfully used for the construction of biosensors. Willner and co-workers reported the coupling of enzymes to the biocatalytic growth of Au nanoparticles (Au NPs), which was used to analyze glucose,9 to detect NAD(P)+dependent biocatalyzed transformations,10 or to follow the tyrosinase activity.11 They also introduced the growth of metal NPs on solid surfaces instead of in solution. Copper NPs were * Corresponding author: e-mail
[email protected]; fax +86 25-83594862; tel +86 25-83597294. (1) Sih, B. C.; Teichert, A.; Wolf, M. O. Chem. Mater. 2004, 16, 2712. (2) Yu, A. M.; Liang, Z. J.; Cho, J. H.; Caruso, F. Nano Lett. 2003, 3, 1203. (3) Raveendran, P.; Fu, J.; Wallen, S. L. J. Am. Chem. Soc. 2003, 125, 13940. (4) Schmid, G. Angew. Chem. 1978, 90, 417. (5) Watzky, M. A.; Finke, R. G. Chem. Mater. 1997, 9, 3083. (6) Wilcoxon, J. P.; Provencio, P. P. J. Am. Chem. Soc. 2004, 126, 6402. (7) Wilcoxon, J. P.; Martin, J. E.; Provencio, P. Langmuir 2000, 16, 9912. (8) Wilcoxon, J. P.; Martin, J. E.; Provencio, P. J. Chem. Phys. 2001, 115, 998. (9) Zayats, M.; Baron, R.; Popov, I.; Willner, I. Nano Lett. 2005, 5, 21. (10) Xiao, Y.; Pavlov, V.; Levine, S.; Niazov, T.; Markovitch, G.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 4519. (11) Baron, R.; Zayats, M.; Willner, I. Anal. Chem. 2005, 77, 1566.
formed on Au seeds on the surface of quartz slides. However, although this is a convenient method to deposit metals, as we knew, this strategy has not been used for the direct detection of metal ions. In addition, since the metal nanoparticle seeds need to be synthesized first and then assembled on the solid surface, the complicated process might influence the reproducibility of the sensors. In the pioneering work, heavy metal ions in aqueous solutions were often detected via anodic stripping voltammetry (ASV), which shows a low detection limit for the trace metal analysis and is very suitable for on-site and in situ analysis.13,14 The first step of preconcentrating metals on the electrode surfaces is important for enhancing the performance of sensors and is often carried out by electrochemical reduction. Due to the easily regenerated surface, hanging mercury drop electrodes (HMDE) are mainly used as substrate for the accumulation of metals.15,16 However, mercury would bring serious environmental pollution; therefore, other, less toxic electrode materials have been investigated. Different electrodes from bare gold, iridium, bismuth-film coated, and boron-doped diamond have been used instead of mercury.17-20 Recently, chemically modified electrodes capable of accumulating target analytes from dilute aqueous solution have also been designed.21-25 This approach can deposit (12) Shlyahovsky, B.; Katz, E.; Xiao, Y.; Pavlov, V.; Willner, I. Small 2005, 1, 213. (13) Wu, H. P. Anal. Chem. 1996, 68, 1639. (14) Sanna, G. S.; Pilo, M. I.; Piu, P. C.; Tapparo, A.; Seeber, R. Anal. Chim. Acta 2000, 415, 165. (15) Achterberg, E. P.; Braungardt, C. Anal. Chim. Acta 1999, 400, 381. (16) Wang, F. Y.; Chen, J. S. EnViron. Sci. Technol. 1997, 31, 448. (17) Wang, J.; Foster, N.; Armalis, S.; Larson, D.; Zirino, A.; Olsen, K. Anal. Chim. Acta 1995, 310, 223. (18) Wang, J.; Lu, J.; Luo, D.; Hocevar, B.; Farias, P. M. Anal. Chem. 2000, 72, 3218. (19) Nolan, M. A.; Kounaves, S. P. Anal. Chem. 1999, 71, 3567. (20) Tsai, Y. C.; Coles, B. A.; Holt, K.; Foord, J. S.; Marken, F.; Compton, R. G. Electroanalysis 2001, 13, 831. (21) Arrigan, D. W. M. Analyst 1994, 119, 1953. (22) Wang, J. In Electroanalytical Chemistry; Bard, M. A., Ed.; Marcel Dekker: New York, 1998; Vol. 16, p 1. (23) Yang, W.; Gooding, J. J.; Hibbert, D. B. Analyst 2001, 126, 1573. (24) Etienne, M.; Bessiere, J.; Walcarius, A. Sens. Actuators, B 2001, 76, 531.
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metals at open circuit and the preconcentration can be extended to some metal ions, which cannot be electroreductively deposited. Rivas et al.26 reported a functionalized polymer-odified electrode that could detect Pb2+ selectively, in the concentration range from 10-5 to 5 × 10-3 mol L-1. Recently, we developed a straightforward and nonpolluting method to obtain gold nanofilm.27 With oxidation under high positive potential and reduction in β-D-glucose solution, a porous gold nanofilm with a large surface area and abundant adatomstate Au* formed on the top of the gold electrode. Herein, we report a simple method for the catalytic deposition of Pb on the gold nanofilm, which was prepared to provide nanocrystal seeds for the catalytic deposition of Pb. The catalytic properties of gold nanocrystal seeds increased the amount of Pb deposited on the surface of the electrode, which facilitated the electrical sensing of Pb2+ in aqueous solution. The surface of the gold nanofilm could be easily regenerated in nitric acid solution. In addition, as the gold nanofilm was formed in situ on the gold electrode, additional immobilization process was needless, which simplified the procedure of preparation. Compared with functionalized polymer electrode materials,26,28 the sensor based on the gold nanofilm could be constructed within 1 h.
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Figure 1. SEM images of the gold nanofilm modified gold slides (A) prior to deposition of metal and (B) after catalytic deposition of 8.0 mmol L-1 Pb2+ in the presence of 0.1 mmol L-1 AA for 30 min.
2. Experimental Section 2.1. Reagents. β-D-Glucose, ascorbic acid, trisodium citrate, lead nitrate, copper nitrate, zinc nitrate, and cadmium nitrate were purchased from Shanghai Chemical Reagent Co. (Shanghai, China); NaBH4 was purchased from Tianjin Chemical Reagent Institution (Tianjin, China); NADH was purchased from Sigma. All the chemicals were of analytical grade. Twice-distilled water was used throughout. 2.2. Preparation of Gold Nanofilm. A bulk gold disk electrode (diameter 2.0 mm) was abraded with fine SiC paper, polished carefully with 0.3 and 0.05 µm alumina slurry, and then sonicated in water and absolute ethanol, respectively. Gold nanofilm was prepared as previously reported.27 The cleaned gold electrode was first anodized under a high potential of 5 V in 0.10 mol L-1 phosphate-buffered saline (PBS) solutions, pH 7.4, for 3 min. The color of the electrode surface turned salmon pink. Afterward, as a nontoxic and inexpensive reducing agent, β-D-glucose is used to reduce the gold oxide. The anodized gold electrode was dipped into 1.0 mol L-1 β-D-glucose aqueous solution at room temperature. The color of the gold electrode surface turned black immediately and a porous gold nanofilm was obtained on the top of the gold electrode after 5 min. Both the anodized potential and the time of polarization and reduction were optimized. 2.3. Catalytic Deposition of Metals on the Gold Nanofilm. Gold nanofilm modified electrodes were dipped into 0.10 mmol L-1 ascorbic acid (AA) aqueous solution with metal nitrate for 30 min. In this process, metal ions such as Pb2+ and Cu2+ were reduced on gold nanocrystal seeds. Both reducing agents and time were optimized. The regeneration of electrodes was carried out by immersing the electrodes in 0.10 mol L-1 nitric acid solution, followed by a cyclical scan in a potential range of -0.6 to 0.6 V at 0.1 V s-1 for 10 cycles. 2.4. Scanning Electron Microscopic Characterization. Morphologies of gold slides after in situ preparation of gold nanofilm and deposition of Pb on the gold nanofilm were studied on a JEOL JSM-6700F field emission scanning electron microscope. 2.5. X-ray Photoelectron Spectroscopic Characterization. Valences of metals reduced on the surface of gold nanofilm were (25) Turyan, I.; Atiya, M.; Mandler, D. Electroanalysis 2001, 13, 653. (26) Rivas, B. L.; Pooley, S. A.; Brovelli, F.; Pereira, E.; Basaez, L.; Puentes, J.; Moutet, J. C.; Saint-Aman, E. J. Appl. Polym. Sci. 2006, 100, 2380. (27) Zhao, W.; Xu, J. J.; Shi, C. G.; Chen, H. Y. Electrochem. Commun. 2006, 8, 773. (28) Rivas, B. L.; Pooley, S. A.; Brovelli, F.; Pereira, E.; Basaez, L.; Osorio, F.; Moutet, J.-C.; Aman, E. S. J. Appl. Polym. Sci. 2005, 98, 1192.
Figure 2. XPS of (A) Pb 4f spectra and (B) Cu 2p spectra after the deposition of Pb and Cu in the presence of 0.1 mmol L-1 AA with 8.0 mmol L-1 Pb2+ and 8.0 mmol L-1 Cu2+ for 30 min. studied by an Escalab MKII X-ray photoelectron spectrometer, using nanomonochromatized Mg KR X-ray as the excitation source and choosing C1s (284.6 eV) as the reference line. 2.6. Electrochemical Experiments. All electrochemical experiments were performed on a CHI 660B electrochemical workstation (Shanghai Chenhua Apparatus Corp., China). A conventional threeelectrode system with a gold disk electrode (2.0 mm diameter) as a working electrode, a platinum foil as an auxiliary one, and a saturated calomel electrode (SCE) as a reference electrode is employed.
3. Results and Discussion 3.1. Scanning Electron Microscopic Characterization of Pb Deposited on Gold Nanofilm. Support for the catalytic deposition of lead on the gold nanofilm with nanocrystal seeds is obtained from SEM imaging of the surface (Figure 1). The surface micrograph of gold nanofilm reveals a porous film with small gold nanocrystal seeds (Figure 1A). After the deposition process in the presence of Pb2+ and AA, which results in the accumulation of Pb on the gold nanocrystal seeds, a thicker and rougher film with nanoparticles combined on the surface of gold nanocrystal seeds is observed (Figure 1B). 3.2. XPS Characterization of Metals Deposited on the Gold Nanofilm. The deposition of a second metal (lead or copper) onto the gold nanoseeds enables X-ray photoelectron spectroscopy (XPS) investigation for the valence of metal after reduction. Figure 2A shows the Pb 4f X-ray photoelectron spectra. The binding energies are calculated on the basis of the binding energy
Catalytic Deposition of Pb on Gold Nanofilm
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Figure 3. Cyclic voltammograms of gold nanofilm modified electrodes before (A, curve a) and after the deposition of metals in the presence of 0.1 mmol L-1 AA with 8.0 mmol L-1 Cu2+ (A, curve b), 8.0 mmol L-1 Pb2+ (B), 8.0 mmol L-1 Cd2+ (C), and 8.0 mmol L-1 Zn2+ (D) for 30 min, in 0.1 mol L-1 PBS, pH 7.0, at a scan rate of 50 mV s-1.
of carbon (284.6 eV). The binding energy locates at 138.6 eV, which is approaching the binding energy of Pb(II) (138.0 eV) but is much higher than that of Pb(0) (136.6 eV).29 Since the elemental Pb(0) could be easily oxidized in the air,30 it is assumed that Pb(0) formed on the gold nanofilm due to the reduction of Pb2+ and then was oxidized to be Pb(II). For support of this expectation, Cu was catalytically deposited on the gold nanofilm. Figure 2B shows the Cu 2p X-ray photoelectron spectra. The binding energy locates at 931.9 eV, approaching the binding energy of Cu(0), 932.6 eV.31 This would imply mostly elemental Cu(0) on the surface of gold nanofilm, thus supporting the conclusion that the formation of metal(0) on the gold nanofilm is due to the reduction of metal ions. 3.3. Electrochemical Characterization of Metals Deposited on Gold Nanofilm. Further support for the deposition of metals on the gold nanofilm is investigated by the electrochemical experiments. After the preparation of gold nanofilm, a pair of quasi-reversible redox peaks is obtained at 0.191 and 0.119 V in PBS, pH 7.0 (Figure 3A, curve a). It is caused by the formation of gold adatom sites Au* and the adsorption of OHads at these sites, which is indicated by32
Au* + H2O f AuOH(1-n)- + H+ + ne-
(1)
After the deposition of Pb, the peak currents of Au*/AuOHads decrease, which may be caused by the covering of Au* on the surface of gold nanofilm by the metal nanoparticles, and a pair of redox peaks appears at -0.169 and -0.276 V (Figure 3B). We also investigated Cu2+, Cd2+, and Zn2+, the common elements (29) Abdel-Samad, H.; Watson, P. R. Appl. Surf. Sci. 1998, 136, 46. (30) Cai, S. H. Element Inorg. Chem. 1998, 118. (31) Bernede, J. C.; Hamdadou, N.; Khelil, A. J. Electron Spectrosc. Relat. Phenom. 2004, 141, 61. (32) Aoun, S. B.; Bang, G. S.; Koga, T.; Yasuhiro, N.; Sotomura, T.; Taniguchi, I. Electrochem. Commun. 2003, 5, 317.
in the soil. After the deposition of Cu, a pair of redox peaks is obtained with peak potentials of 0.206 and 0.098 V, which are near the peak potentials of Au*/AuOHads (Figure 3A, curve b). Between -0.6 and 0.6 V, reducing the Cd2+ (Figure 3C) and Zn2+ (Figure 3D) does not induce obvious redox peaks but results only in the decrease of the peak currents of Au*/AuOHads. The oxidation peaksof Cu and Pb approach those reported by Rahman and co-workers. The redox peak potentials of Cd and Zn were also proved to be more negative than -0.6 V.33 Figure 4A shows the CVs of gold nanofilm modified gold electrode before (curve a) and after (curve b) reduction of Pb2+, Cu2+, Cd2+, and Zn2+ at the same time. Compared with depositing the four metals separately, the redox peaks of Pb and Cu deposited from mixed solution do not change obviously. The result indicated that Cu2+, Cd2+, and Zn2+ would not influence the reduction of Pb2+. It facilitates the selective detection of Pb2+ in aqueous solution. For comparison, we also investigated the CVs of the bare gold electrode before and after reduction of these four metal ions (Figure 4B). Both the redox peaks of reactions of Pb and Cu are obtained, with potential differences of the peak-to-peak (DEp) of 0.187 and 0.123 V, respectively, which are wider than those of Pb and Cu on gold nanofilm, 0.091 and 0.098 V, respectively. In addition, the peak currents of Cu and Pb preconcentrated on gold nanofilm are 100 and 200 times higher than those on bare gold electrode. Here, the real surface areas of the bare gold electrode and gold nanofilm modified gold electrode were calculated by cyclic voltammetry in 1.0 mol L-1 H2SO4 in the potential range of 0-1.5 V (vs SCE).34 By integration of the reduction peak, the real surface area of the gold nanofilm was determined to be 1.21 cm-2 by assuming that the reduction of (33) Rahman, M. A.; Park, D. S.; Won, M.-S.; Park, S.-M.; Shim, Y.-B. Electroanalysis 2004, 16, 1366. (34) Woods, R. Electroanal. Chem. 1976, 9, 1.
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Figure 5. Cyclic voltammograms of gold nanofilm modified electrode after the deposition of Pb in the presence of 0.1 mmol L-1 AA with 10 µmol L-1 Pb2+ for 30 min, in 0.1 mol L-1 PBS, pH 7.0, at 50, 100, 200, 300, 400, and 500 mV s-1 (from inner to outer). Insets: Plots of (a) peak currents vs scan rate and (b) Ep vs log V. Figure 4. Cyclic voltammograms of (A) gold nanofilm modified electrode and (B) bare gold electrode, (a) before and (b) after the deposition of metals in the presence of 0.1 mmol L-1 AA with 8.0 mmol L-1 Pb2+, 8.0 mmol L-1 Cu2+, 8.0 mmol L-1 Cd2+ and 8.0 mmol L-1 Zn2+ for 30 min in 0.1 mol L-1 PBS, pH 7.0, at a scan rate of 50 mV s-1.
a monolayer of gold oxide requires 386 µC cm-2, which was 18 times larger than that of bare gold electrode (0.065 cm-2). Thus the 100 and 200 times increase of the peak currents for deposited Cu and Pb on the gold nanofilm modified electrode are attributed not only to the bigger surface area of the gold nanofilm but also to the large amount of gold nanocrystal seeds on the film, which provide more sites and shows catalytic properties for the deposition of metals. In addition, reduced on the bare gold electrode, the peak currents of Cu are bigger than those of Pb; this is because Cu2+ could be reduced more easily than Pb2+, since the standard electrode potential (φ vs SHE) of Cu2+/Cu is 0.337 and φ of Pb2+/Pb is -0.126. However, the peak currents of Pb are slightly larger than those of Cu on gold nanofilm modified electrode, which indicates that Pb could be more easily deposited on the gold nanofilm than Cu (Figure 4A). It might be induced by the easily deposition of Pb atoms on the gold nanocrystal seeds. Thus, it is advantageous for the detection of Pb2+ to use gold nanofilm for the preconcentration of Pb. Since the peak potentials could be distinguished, the redox process of the reaction of Pb on the surface of gold nanofilm is investigated effectively. At pH 7.0, the redox peak currents are proportional to the scan rates in the range less than 0.5 V s-1 (Figure 5), indicating a typical surface-controlled quasi-reversible process. On the basis of the model of Laviron,35 the chargetransfer coefficient R and electron number n were calculated to be 0.44 and 2.01, respectively. According to the equation Γ ) Q/nFA, the amount of Pb reduced on the gold nanofilm in the presence of 0.1 mmol L-1 AA with 10 µmol L-1 Pb2+ can be deduced to Γ ) 1.4 × 10-7 mol cm-2 (geometric area). 3.4. Influence of Solution pH. It was reported that Pb(II) is extensively hydrolyzed, forming both mononuclear and polynuclear species in solutions, except at low pH.36 Thus the pH value of the background solution played an important role in the (35) Laviron, E. J. Electroanal. Chem. 1979, 101, 19. (36) Perera, W. N.; Hefter, G.; Sipos, P. M. Inorg. Chem. 2001, 40, 3974.
Figure 6. Cyclic voltammograms of gold nanofilm modified electrode after the deposition of Pb in the presence of 0.1 mmol L-1 AA with 8.0 mmol L-1 Pb2+ for 30 min, in 0.1 mol L-1 PBS, pH 5.0, 6.0, 7.0, 8.0, 9.0, or 10.0, at a scan rate of 50 mV s-1. Insets: Plots of (a) peak currents vs pH and (b) peak potentials vs pH.
electrochemical reaction of Pb. Figure 6 shows the influence of the solution pH on the redox processes. In the pH range of 5.010.0, an increase in solution pH caused a negative shift in both cathodic and anodic peak potentials. Plot of the formal potential versus pH (from 5.0 to 10.0) produced a line with slope of -29.6 mV/pH (R ) 0.997) (inset a in Figure 6), which was close to half the value of -59.0 mV/pH, indicating a one-proton and two-electron reaction process, and the mononuclear species Pb(OH)+ existed mainly on the surface of the deposited Pb-modified electrode. Thus the main reaction on the surface of gold electrode is assumed to be
Pb(OH)+ + H+ + 2e- a Pb + H2O
(2)
In general, all changes in CV peak potentials and currents with pH were reversible in the pH range of 7.0-10.0; that is, the same CV could be obtained if the electrode was transferred from a solution with different pH value to its original solution. However, in weak acidic solutions, the peak currents irreversibly decreased during cyclic scan, indicating part of the Pb was oxidized as
Catalytic Deposition of Pb on Gold Nanofilm
Figure 7. Linear sweep voltammograms of gold nanofilm modified electrode after deposition of Pb in the presence of 0.1 mmol L-1 AA, 5.0 mmol L-1 trisodium citrate, 1.0 mmol L-1 glucose, 5.0 mmol L-1 NaBH4, or 1.0 mmol L-1 NADH, respectively, with 10 µmol L-1 Pb2+ for 30 min, by use of electrochemical deposition method in 10.0 µmol L-1 Pb2+ aqueous solution (the deposition potential is -0.5 V and the deposition time is 40 s), in 0.1 mol L-1 PBS solution, pH 7.0, at a scan rate of 50 mV s-1.
soluble Pb2+. With the decrease of solution pH, the proportion of Pb2+ increased, resulting in a decrease of the peak currents (inset b in Figure 6). Since Pb could dissolve in acid solution with cyclical scanning, we choose 0.1 mol L-1 nitric acid solution to regenerate the surface of gold nanofilm. After 10 cycles of cyclical scanning in a potential range of -0.6 to 0.6 V at 0.1 V s-1, Pb deposited on the gold nanofilm could be completely cleared from the surface. The gold nanofilm was used repeatedly with changeless performance. 3.5. Optimization of Deposition Conditions. Optimal deposition conditions were determined by reducing Pb2+ with different reducing agents. Since the electrochemical reduction is fast and widely used,37 we also deposited Pb via the underpotential deposition (UPD) for 40s, at -0.5 V. Linear sweep voltammetry (LSV) is utilized to detect the deposition quantity of Pb on electrode surface. By comparisin of the voltammograms (Figure 7), the oxidation peak current of Pb reaches the highest level via reducing Pb2+ in 0.10 mmol L-1 AA aqueous solution and is twice as high as that reduced by UPD. Therefore, as a nontoxic and inexpensive reducing agent, AA was used to reduce Pb2+. Concentration of 0.10 mmol L-1, reducing time of 30 min, and temperature of 4 °C were the optimal parameters. 3.6. Selective Detection of Pb2+ in Aqueous Solution. In comparison with the conventional anodic stripping analysis, the metals would not be stripped during a linear scan in neutral solution. The amount of Pb deposited on the electrode surface is proportional to the peak currents of the redox couple. Therefore, measurement of the peak heights was preferred for the determination of Pb2+ in aqueous solution. An advantage of this strategy is stability, and thus the electrode preconcentrated by Pb can be scanned repeatedly during the detection process. Figure 8A shows LSV of gold nanofilm modified electrodes after deposition of Pb in the presence of 0.10 mmol L-1AA with different concentrations of Pb2+. The oxidation peak currents of (37) Herzog, G.; Arrigan, D. W. M. Anal. Chem. 2003, 75, 319.
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Figure 8. Linear sweep voltammograms of gold nanofilm modified electrodes after deposition of Pb (A) without interferential ions and (B) with 100.0 µmol L-1 Cu2+, Cd2+, and Zn2+ in the presence of 0.1 mmol L-1AA with (a) 0.0, (b) 1.0, (c) 3.0, (d) 5.0, (e) 8.0, (f) 10.0, (g) 25.0, or (h) 50.0 µmol L-1 Pb2+ for 30 min, in 0.1 mol L-1 PBS solution, pH 7.0, at a scan rate of 50 mV s-1. Inset: Calibration curves of the peak currents associated with deposition of different concentrations of Pb2+.
Pb exhibit a linear dependence on Pb2+ concentration in deposition solution in the range of 1.0-10.0 µmol L-1 (R ) 0.999), with a detection limit of 0.1 µmol L-1. For comparison, we also investigated the detection of Pb2+ in the solution with interferential ions. Figure 8B shows LSV of gold nanofilm modified electrodes after deposition of Pb in 0.10 mmol L-1AA with 0-50 µmol L-1 Pb2+ and 100 µmol L-1 Cu2+, Cd2+, and Zn2+. Compared with curves in Figure 8A, the oxidation peak potentials and currents of Pb barely changed and the linear range and sensitivity of the sensor were not influenced. Thus, a quantitative and selective determination of Pb2+ is possible with such electrodes.
4. Conclusion In conclusion, we developed a simple method for the catalytic deposition of metals on gold nanofilm, which was prepared to provide nanocrystal seeds with catalytic properties. The redox peaks of Pb could be effectively distinguished from those of Cu, Cd, and Zn, and the surface of the gold nanofilm can be easily regenerated in acid solution. All these properties make for the development of a metal sensor, which achieves simple, repeatable, sensitive, and selective detection of Pb2+ in aqueous solution. LSV is utilized for the detection of Pb2+ in aqueous solution. The good sensitivity and selectivity of the sensor facilitate its use in Pb2+ analysis in the environment. Acknowledgment. We gratefully thank the National Natural Science Foundation of China for financial support of this research (20675037, 20575029, 20435010, 90206037) and the National Natural Science Funds for Creative Research Groups (20521503). LA7003276
