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Photoelectrochemical Stripping Analysis Yanmei Xin, and Zhonghai Zhang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04381 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on January 1, 2018
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Analytical Chemistry
Photoelectrochemical Stripping Analysis Yanmei Xin, Zhonghai Zhang* School of Chemistry and Molecular Engineering, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China. Email:
[email protected] Tel: 0086-21-54345359
ABSTRACT: The electrochemical stripping analysis (ECSA) is a promising method for metal ions detection. However, the low sensitivity and poor reproducibility limits its practical applications. The combination with other powerful detection techniques to address these concerns is highly desirable. Herein, the anodic stripping method and photoelectrochemical (PEC) technique are integrated into a new detection platform of PEC stripping analysis (PECSA) with bismuth vanadate (BiVO4) as both optoelectronic material and electrochemical enrichment candidate. The new PECSA strategy presents high sensitivity and excellent reproducibility, in addition, inherited from the ECSA, this strategy also offers new selectivity dimensions through the potential-dependent response, and thus implements reproducible, sensitive, and selective detection of silver ion (Ag+) in real biological and environmental samples. The success of PECAS strategy shed light on the rational combination of various analysis techniques for versatile applications.
The combination of effective enrichment techniques with advanced measurement methods render the electrochemical stripping analysis (ECSA) to be a powerful method for sensitive and selective detection of trace metal ions.1,2 The mercury-based electrodes, mercury-film electrode and hanging mercury drop electrode, have been widely employed in the early stage of the development of ECSA.3,4 However, these mercury-based electrodes have not been anymore accepted to operate the measurements due to their high toxicity. A series of electrode candidates of carbon, gold, iridium, antimony, and bismuth have been investigated as possible alternatives to mercury.5-10 Among them, bismuth-based electrodes distinguish themselves due to their excellent stripping performance approaching to that of mercury-based electrodes: comparable sensitivity and similar potential-dependent selectivity for metal ions detection, more importantly, the bismuth-based electrodes show very low toxicity to environmental and biological mediums. The bismuth film electrodes have been first proposed by Wang and coworkers,11 and are generally fabricated in an electrochemical reduction process with bismuth and target metal ions simultaneously depositing on the surface of glass carbon or carbon fiber electrodes, however, which procedures inevitably bring about poor reproducibility. In addition, the sensitivity of bismuth-based ECSA method needs further increase to satisfy the requirement of ultra-trace metal ions detection. To address these concerns, we hypothesize that if the bismuth film can be in-situ generated from a solid bismuth compound electrode, which would avoid potential interferences from ion diffusion in the solution, and thus effectively enhance the detection reproducibility;
furthermore, if the solid bismuth compound electrode can be directly utilized not only in ECSA method, but also in new advanced detection methods, such as photoelectrochemical (PEC) analysis,12 which would promote the ECSA to a new PEC stripping analysis (PECSA) method, and thus raise the possibility to increase the sensitivity. The PEC analysis, as a new kind of high sensitivity analytical method, have developed rapidly in recent years due to the its unique signal transducing modality of excitation energy sources of light and readout signal of electricity,13-20 which reduces the background noise, and thus increase the detection sensitivity. To realize the prior hypothesis of integrating ECSA with PEC analysis to a new PECSA method for reproducible and sensitive detection, the exploration of the solid bismuth compound with both ECSA property and PEC response activity is a prerequisite. Coincidentally, bismuth vanadate (BiVO4) can be one of the promising candidates. First, the BiVO4 is a visible light driven PEC material with narrow band gap of 2.4 eV, good optical stability and environmentally benign properties, and have been widely investigated for photocatalytic and PEC applications,21-23 which predicts its superior performance in PEC analysis with good sensitivity. Second, the bismuth film can be in-situ generated on the top surface of solid BiVO4 in an electrochemical reduction process, which enables to the operation of anodic stripping applications. Inspired from the advantages of BiVO4 material, herein, we first propose a new PECSA strategy with BiVO4 as both PEC material and precursor of bismuth film for sensitive detection of ultra-trace metal ions. Silver ion (Ag+) is selected as target ion in the PECSA because it is one of the most poisonous heavy metal ions in many
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environmental and biological systems,24 and, more meaningfully, it has never been successfully detected through conventional bismuth film based ECSA process. The new PECSA strategy presents much better sensitivity and reproducibility than BiVO4-based and bismuth film-based ECSA methods. Furthermore, most of the conventional PEC sensors need to couple with recognition units, such as aptamers, antibodies, and enzymes, to achieve selective detection,25-27 however, which limit their practical applications due to the harsh conditions for complex design process, instability, and excessive cost, on the contrary, the PECAS strategy offers new selectivity dimensions through the potential-dependent PEC response. Finally, the rationally designed PECAS strategy successfully implements reproducible, sensitive, and selective detection of Ag+ in real biological and environmental samples. The BiVO4 was synthesized as previously reported meth21 od, and the detailed processes can be found in Experimental Section in Supporting Information. Briefly, the BiOI film was first electrodeposited on fluorine doped tin oxide (FTO) glass, then the BiOI/FTO electrode was rinsed in dimethyl sulfoxide (DMSO) solution containing vanadyl acetylacetonate (VO(acac)2), and followed annealing in air. After cooling to room temperature, the electrodes were soaked in sodium hydroxide (NaOH) solution for 30 min, and washed with deionized water and dried in ambient air.
ure S1) demonstrated the atomic ratios of Bi: V: O to be close to 1: 1: 4, clearly suggested the stoichiometric chemical composition of BiVO4. The BiVO4 film was then utilized to in situ generate bismuth film for electrochemical enrichment of silver (Ag/Bi/BiVO4). After this process, the nanoporous structure of BiVO4 was unchanged, and no clear bismuth film and Ag nanoparticles can be observed from the SEM image (Figure S2) or have been detected from the XRD measurement (Figure S3), which implied that the small amount of generated bismuth and enriched silver may be beyond the detection limit of SEM and XRD. To better elucidate the electrochemical enrichment process and to reveal the formation of bismuth film and silver nanoparticles, transmission electron microscopy (TEM) was performed and is presented in Figure 1c, where a nanoparticle can be observed on the surface of BiVO4. Closer observation from high-resolution transmission electron microscopy (HRTEM) in Figure 1d and Figure 1e clearly presented distinctive layers of (i) bulk BiVO4 with lattice spacing of 0.306 nm, indexed to (112) planes of scheelite BiVO4, (ii) in situ generated bismuth film with lattice spacing of 0.328 nm, ascribed to (012) plane of bismuth metal, (iii) amorphous bismuth-silver alloy zone, and (iv) silver nanoparticle with lattice spacing of 0.236 nm, corresponded to (111) plane of silver metal. The formation of these sequential multi-layer structures not only revealed that the bismuth film was generated on the BiVO4 surface followed the simple electrochemical reduction mechanism, but + also depicted that the Ag was enriched and converted to silver nanoparticle on the top surface of BiOV4 in the same electrochemical enrichment processes, all of which helped to explain the formation mechanism of Ag/Bi/BiVO4 electrodes.
Figure 1. (a) XRD pattern of BiVO4/FTO glass electrode, (b) top-view SEM image of BiVO4, (c,d) TEM image of Ag/Bi/BiVO4, and (e) HRTEM image of Ag/Bi/BiVO4 with different zones of (i) bulk BiVO4, (ii) bismuth film, (iii) amorphous bismuth-silver alloy, and (iv) silver nanoparticle.
Figure 2. Core-level XPS of (a) Bi 4f, (b) V 2p, (c) O 1s and (d) Ag 3d of BiVO4 and Ag/Bi/BiVO4.
The crystal structure and micro-morphology of BiVO4 were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) respectively. As shown in Figure 1a, all XRD peaks, except from SnO2 of FTO glass, can be ascribed to scheelite BiVO4 structure (JCPDS No. 78-1534). The SEM image of BiVO4 in Figure 1b displayed a worm-like nanoporous network structure with large specific surface area, which would be beneficial for enlarging solid-liquid contact surface and accelerating electrolyte diffusion. The corresponding energy dispersive X-ray (EDX) spectrum (Fig-
To further characterize the formation of Ag/Bi/BiVO4 electrode, the surface analysis technique of X-ray photoelectron spectroscopy (XPS) has been employed to reveal their chemical compositions and valence states. The XPS survey spectra of BiVO4 and Ag/Bi/BiVO4 were recorded in Figure S4, and indicated the existence of Bi, V and O elements in BiVO4 and extra Ag element in Ag/Bi/BiVO4. The core-level XPS of Bi 4f were showed in Figure 2a, and the double peaks with binding energy of 159.4 eV and 164.7 eV in BiVO4 were consistent with the characteristic Bi 4f7/2 and Bi 4f5/2 respectively, which
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Analytical Chemistry indicated the existence of trivalent oxidation state of bis28 muth. After electrochemical enrichment, the Ag/Bi/BiVO4 also presented double peaks with negative shifted binding energy of 159.0 eV and 164.3 eV, which implied the reduction of valence state of bismuth due to the formation of bismuth film. The core-level XPS of V 2p and O 1s were also recorded in Figure 2b and Figure 2c respectively. For BiVO4 sample, the binding energy located at 516.5 eV and 524.0 eV can be 5+ 29 ascribed to V 2p3/2 and V 2p1/2 of V , and binding energy of 529.6 eV and 531.1 eV can be indexed to O in lattice of BiVO4 30 and in hydroxyl oxygen or adsorbed oxygen. After electrochemical enrichment, the binding energy in both the corelevel XPS of V 2p and O 1s presented positive shift in Ag/Bi/BiVO4 electrode, which implied the possible charge transfer due to the change of local coordination environments of V and O ions after Ag enrichment. Finally, the corelevel XPS of Ag 3d was also detected in Ag/Bi/BiVO4 electrode and presented in Figure 2d, where two peaks with binding energies of 367.8 eV and 373.7 eV can be attributed to Ag 3d3/2 and 3d5/2 respectively, which values suggested the 0 31 formation of metallic silver (Ag ). Both the HRTEM and core-level XPS characterizations helped to reveal the successful generation of bismuth film and simultaneous enrichment of silver, thus the sensitive and selective detection of silver ion on the Ag/Bi/BiVO4 can be expected with the PECSA strategy.
+
Figure 3. (a) ECSA and (b) PECSA detection of Ag on Bi+ VO4, (c) current density vs Ag concentration calibration curves, (d) anti-interference property with the initial addi+ 2+ + 2+ 3+ 2+ 2+ tion of 400 nM Ag , 1 µM Mg , K , Ca , Ce , Cr , Cd ， 2+ 2+ 3+ Zn , Hg , Ni , and followed by the addition of the above metal ions mixture, (e) reproducibility of six BiVO4 based + PECSA sensors for detection of 400 nM Ag , and (f) PECSA + sensing of Ag in real serum sample. +
The ECSA for Ag detection was first performed on BiVO4 electrode and presented in Figure 3a. After electrochem-
ical enrichment, obvious anodic stripping peaks were observed at peak potential of 0.27 V, and the peak current den+ sity increased with the increase of concentrations of Ag , which displayed a linear relationship in the ranges of 30-500 nM with limit of detection of 10 nM (Figure 3c). In addition, the conventional Bi-film electrodes were also synthesized for comparison (Figure S5a). The BiVO4-based electrodes presented better sensitivity (Figure 3c) than Bi-film electrodes (Figure S5b), which can be ascribed to the in-situ formation + of alloy of Bi-Ag (Figure 1e) for efficient accumulation of Ag in the electrochemical enrichment process, and thus indicated that the BiVO4 could be a good candidate for Bi-based ECSA applications. For further increasing the sensitivity, with the same electrochemical enrichment conditions and + anodic stripping potential, the PECSA for Ag detection on BiVO4 was performed with chronoamperometry under illumination of visible light. As presented in Figure 3b, with in+ creasing the concentrations of Ag , continually increased photocurrent densities were recorded and a wider linear range of 1-1200 nM with much lower limit of detection of 0.3 nM in the PECSA than that in ECSA (Figure 3c) was achieved. The amount of BiVO4 has been optimized through controlling the electrodeposition time of BiOI as precursor (Figure S6). The PECSA performances on different potentials have also been evaluated (Figure S7), and the measurements on anodic stripping potential presented the highest photocurrent density value. The enhancement of photocurrent density in the PECSA process can be ascribed to the PEC + stripping of Ag to Ag with photo-generated holes, which helped for the separation of holes and electrons, thus contributed to the improvement of PEC performance. + The performances for Ag detection on Bi-film and BiVO4 with ECSA and PECSA models were all summarized in Table S1, and the PECSA on BiVO4 presented the highest sensitivity, the lowest limit of detection, and the widest linear ranges. Furthermore, the PECSA strategy also presented comparable detection performance to other reported detection methods (Table S2). In the processes of PECSA, we noted that the prior electrochemical enrichment was a very important step to increase the detection sensitivity, and the none-electrochemical enriched samples presented much lower photocurrent response (Figure S8) and poor PEC detection performance (Figure S9), thus, the electrochemical enrichment parameters of applied potentials and time have been well optimized to -0.3 V and 300 s respectively (Figure S10). The selectivity is another vital criterion to evaluate the 32,33 performance of detection strategy. Inherited from the ECSA method, the PECSA presented an excellent selectivity through controlling the stripping potential. As presented in + Figure 3d, the addition of 400 nM Ag rendered the obviously increased photocurrent density, while, the addition of other metal ions with higher concentration of (1.0 μM), no significantly photocurrents were recorded. Further addition of + metal ion mixture solution including 400 nM Ag , a very + similar photocurrent response to that for pure Ag solution was observed, which indicated that the BiVO4-based PECSA + was specifically responding to Ag at that potential. In addition, the sequence of metal ions addition has also been changed to evaluate the anti-interference property, and no significant change can be observed (Figure S11) on the BiVO4based PECSA method. The reproducibility of BiVO4 based PECSA was evaluated through comparing the sensing per-
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formance of six sensors. As depicted in Figure 3e, for parallel + detection of 400 nM Ag , these six sensors achieved a good reproducibility with a low relative standard deviation of 1.85%. In addition, the BiVO4-based electrodes also presented much better reproducibility for Ag+ detection in a ECSA model than that with Bi film-based electrodes (Figure S12). To further explore the potential practical application of + the PECSA in real samples, the concentrations of Ag in fetal bovine serum and in lake water were measured with the PECSA strategy. As presented in Figure 3f and in Figure S13, + the concentration of Ag in fetal bovine serum and in lake water were detected to be 269.4±3.2 nM and 1.2±0.4 nM, respectively, which values were in the normal ranges that defined by the United States Environmental Protection Agency (USEPA). In addition, the accuracy of the PECSA strategy was further examined by a standard addition method and good recoveries between 98% to 101% were achieved. The versatility of PECSA has been further estimated with 2+ Hg at a stripping potential of 0.36 V vs Ag/AgCl (Figure 2+ S14), which indicated that the Hg can also be effectively 2+ detected with the PECSA method. However, the Pb and 2+ Cd have as well as been detected with the ECSA method on the BiVO4 electrodes, but the more negative stripping potentials (Figure S15) than the PEC response potential of BiVO4 (Figure S16) limited their PECSA detection performance.
In summary, the new PECSA strategy based on BiVO4 electrode was proposed and successfully implemented sensitive, selective, and reproducible detection of silver ions with low limit of detection and wide linear range. This successful combination of different analysis techniques of ECSA and PEC shed light on the exploration of other versatile applications.
ASSOCIATED CONTENT Supporting Information Experimental section, EDS, SEM, XRD, XPS characterizations, and additional electrochemical and PEC measurements. The Supporting Information is available free of charge on the ACS Publications website.
AUTHOR INFORMATION Corresponding Author * Email:
[email protected] ACKNOWLEDGMENT We appreciate the support from “Yingcai” program of ECNU, and National Natural Science Foundation of China (No. 21775045, 21405046).
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