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This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Ternary nanocomposite electrode of polyoxometalate/carbon nanotubes/gold nanoparticles for electrochemical detection of hydrogen peroxide Shuyue Guo, Lin Xu,* Bingbing Xu, Zhixia Sun and Lihao Wang 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x In this work, a nanocomposite film electrode containing polyoxometalates (POMs) clusters K6P2W18O62 (P2W18), carbon nanotubes (CNTs) and Au nanoparticles (AuNPs) was fabricated by a smart combination of layer-by-layer (LbL) with the self-assembly technique. The synergistical effect of POM, CNTs and AuNPs on the electrocatalysis of H2O2 was investigated to improve the sensitivity of H2O2 detection. The response of (P2W18/CNTs/P2W18/AuNPs)4 electrode to H2O2 was remarkably enhanced due to large active sites and good electron conducting ability. The sensor had quick response (less than 1 second) to H2O2 with a high sensitivity (596.1 µAmM-1cm-2), and low detection limit (9.3 nM). Basing on the respective advantages of POMs, CNTs and AuNPs, the nanocomposite multilayer POMs/CNTs/POMs/AuNPs will bring some special properties and high potential of applications.
Introduction
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The rapid and accurate determination of hydrogen peroxide (H2O2) is practically important in food, pharmaceutical, clinical, industrial, and environmental analyses.1-3 Many techniques have been developed for the determination of H2O2, such as titrimetry,4 spectrophotometry,5 chemiluminescence,6 fluorometric7 and electrochemistry.8 Among these techniques, electrochemical methods are superior for their relatively low cost, high efficiency, high sensitivity and the ease of operation.9,10 In order to prepare excellent electrochemical sensors, many materials were used to decrease the overpotential and increase the electron transfer kinetics. To date, a great amount of enzyme-based and enzymefree electrochemical sensors have been developed to detect H2O2.11-14 However, the enzymatic biosensors are limited by the poor stability, high cost, complicated immobilization procedure and critical operational conditions due to the inherent nature of enzymes. Thus, major efforts have been made to use various replacement alternatives for enzyme, such as Prussian blue,15 noble metal nanoparticles,16 and nanohybrids.17,18 Polyoxometalates (POMs) are discrete nanoscale oxo-clusters of the early transition metals, which display remarkably rich redox and photoelectrochemical properties.19 The metal-oxygen framework can undergo reversible and stepwise, multielectrontransfer reactions, which make POMs an attractive candidate for the electrochemical sensor.20 Many strategies have been developed to H2O2 detection using a variety of POMs-based electrodes.21-25 Although these modified electrodes have shown interesting ability toward H2O2 detection, they also display many problems related to the immobilization of the mediator, low sensitivity and stability, and slow response. Usually, limit of This journal is © The Royal Society of Chemistry [year]
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detection of these modified electrodes is limited to micromolar and response time is around 5 seconds. Hence, fabrication of novel H2O2 sensors with low detection limit and fast response time by using new electron transfer mediators is still highly desirable in this field. Since their discovery by Iijima in 1991, carbon nanotubes (CNTs) have been widely used to immobilize POMs, due to their high surface area, high electrical conductivity, strong adsorptive ability, and good chemical stability.26 By combination of POMs and CNTs, a hybrid nanocomposite with enhanced electrochemical properties and improved stability could be obtained.27-28 In addition, gold nanoparticles (AuNPs) have attracted increasing interest in bioassays for their unique physical and chemical properties, such as easily controllable size distribution, comparative stability, and friendly biocompatibility with biomolecules.29 Recent studies indicate that the substrate material for the loading of AuNPs is very important to the fabrication of sensors with higher performance.30-32,54 The use of CNTs as the support materials for AuNPs has attracted great attention. It is proven that CNTs/AuNPs can act as both the lowpotential redox mediator and electron transfer facilitator.33-35 A large variety of conditions and protocols were followed for synthesis of nanomaterial of CNTs/AuNPs, including electroless plating, electrodeposition, chemical deposition, plasma method and the direct assembly of nanoparticles.36-40 Layer-by-layer (LbL) technique is one of the most promising methods for designing nanocomposite film because of its simplicity, versatility and wide range of materials that can be used for film assembly.41,42 In the last few years, our group fabricated several modified electrodes based on POMs/semiconductor and POMs/metal nanoparticles/semiconductor multilayers by LbL method.43-49 [journal], [year], [vol], 00–00 | 1
Analyst Accepted Manuscript
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These nanocomposite modified electrodes showed improved photovoltaic and photoelectrocatalysis performance, which was attributed to the synergistic effects of the components in the multilayers. These above-mentioned results inspired us to prepare a POMs/CNTs/AuNPs tricomponent nanocomposite to create a synergistic effect on the electrocatalytic activity. In this work, a new type of POMs/CNTs/AuNPs modified electrode was fabricated by LbL method, which was employed for a non-enzymatic electrochemical sensor of H2O2. Dawsontype heteropolytungstate anion of [P2W18O62]6− was selected as an electrochemically active component to fabricate a composite electrode of heteropolytungstate/CNTs. The electrochemical results demonstrate that the sensor exhibits quick response, high sensitivity, a low detection limit, and long-term stability. Basing on the respective advantages of POMs, CNTs and AuNPs, the nanocomposite multilayer POMs/CNTs/POMs/AuNPs will bring some special properties and high potential of applications.
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Polyoxometalate clusters K6P2W18O62 (P2W18) was synthesized according to the literature method and identified by UV-vis adsorption spectra and cyclic voltammetry.50 PEI-capped Au nanoparticles were prepared by HAuCl4 with polyethylenimine branched (PEI) according to the reported procedure.48 TEM image of AuNPs was shown in Fig. S1. Poly(allylamine hydrochloride) (PAH, MW 70,000), Poly(styrenesulfonate) (PSS MW 70,000), and (3-amino-propyl)trimethoxysilane (APS) were purchased from Aldrich and used as received. Multiwalled CNTs (diameter: 20-30 nm, length: ~30 µm) were purchased from Tsinghua-Nafine Nano-power Commercialization Engineering Center. The CNTs were prepared by a reported method.51 PAH was dissolved in deionized water at a concentration of 0.1 mg mL-1, containing 0.05 M NaCl. Certain amounts of CNTs were dispersed in doubly distilled water with sufficient ultrasonication for about 1 h. When they were dispersed thoroughly, PAH aqueous solution was added. After 3 h ultrasonication, black sediment was observed and separated by centrifugation. The residual PAH polymer was remove by highspeed centrifugation and the complex was rinsed with water at least three times. The collected complex was redispersed in water with mild ultrasonication to produce a stable solution of the complex. Preparation of the (P2W18/CNTs/P2W18/Au)n composite film
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Layer-by-layer films were assembled onto indium tin oxide (ITO)-coated glass, silicon wafers and quartz substrates. ITOcoated glass, silicon wafers (Si‹100›, polished on one side), and quartz substrates were cleaned according to a literature procedure.52 APS modified substrates were immersed into HCl (pH 2.0) for 30 min to get an amino cation surface, followed by washing with deionized water and drying in nitrogen. And then dipped into PSS (1 mM containing 0.5 M NaCl) and PAH (1 mM containing 0.5 M NaCl) for 20 min, respectively. For the (P2W18/CNTs/P2W18/Au)n films, the precursor films were dipped into solution in the order of P2W18, CNTs, P2W18 and AuNPs solution for 10, 20, 10 and 4 min, respectively. LbL assembly of P2W18/CNTs/P2W18/Au is schematically depicted in 2 | Journal Name, [year], [vol], 00–00
Fig. S2. To obtain (P2W18/CNTs)n thin films, the substratesupported precursor films were alternately dipped into the P2W18 (4 mM in 0.1 M H2SO4) solution and the CNTs (0.5 mg/ml) for 10 min and 20 min, respectively, then rinsed with deionized water and drying in nitrogen after each dipping. For comparison, (P2W18/PAH)n films were prepared in a similar way.
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UV-vis absorption spectra of the quartz-supported films were recorded on a Cary 500 Scan UV-Vis-NIR spectrophotometer after each layer deposition. AFM measurements were performed in air with a SPI3800N Probe Station. XPS were measured on silicon wafers using an ESCALAB MK II Surface Analysis System. All electrochemical experiments were performed on a CHI611D Electrochemical Workstation (Shanghai Chenhua Instrument Corp., China) at room temperature. A three-electrode system was used. The composite films-assembled ITO glass was used as the working electrode, and a saturated calomel electrode (SCE) and a platinum wire were used as reference and auxiliary electrode, respectively.
Results and discussions UV-vis spectra characterization of the P2W18/CNTs/P2W18/Au multilayer
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UV-vis absorption spectroscopy was used to monitor the LbL assembling procedure. In an acid solution, negatively charged P2W18, positively charged CNTs and AuNPs adsorbed with each other depends on the electrostatic interactions. Fig. 1 shows the
Fig. 1 UV-vis absorption spectra of multilayer films (P2W18/CNTs/P2W18/Au)2 on quartz substrates (from lower to upper curves). The solid line (—) represents spectra after P2W18 deposition, the dotted line (…) represents spectra after CNTs deposition, the dash line (-..-.) represents spectra after AuNPs deposition. The inset shows relationship of absorbance at 206 nm after P2W18 deposition vs. the number of P2W18 layers.
UV-vis absorption spectra of the (P2W18/CNTs/P2W18/Au)2 multilayers assembled on a precursor-coated quartz substrate (on both sides). The characteristic bands of P2W18 at 206 and 278 nm increased linearly with the number of layers, and the inset of Fig. 1 shows the plots of the absorbance at 206 nm. The broad and weak peak at 278 nm is ascribed to an overlap of characteristic absorption of P2W18 and CNTs. The absorption band of AuNPs was not seen at 520 nm for the low adsorption quantity. A similar This journal is © The Royal Society of Chemistry [year]
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behavior was observed for (P2W18/PAH)2 and (P2W18/CNTs)2 multilayer films. X-ray photoelectron spectrum (XPS)
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XPS spectra were used to identify the chemical composition and the binding energy of the (P2W18/CNTs/P2W18/Au)4 film on silicon substrates. Fig. 2 shows the signal peaks of P, W, C, and Au in the film which confirmed the presence of these elements. The peaks of W 4f and P 2p were detected at 35.1, 37.2 and 132.7 eV, which also gave an approximate ratio of phosphorous to tungsten in 1 : 9 for P2W18. The C 1s peak was observed at 284.7 eV, corresponding to C-C chemical binding states.52 There is a Au 4f7/2 and 4f5/2 doublet with the binding energies of 83.3 and 87.1 eV, respectively. These values suggest that gold is present in the LbL film in the metallic form.53 These results indicate that P2W18, CNTs, and Au nanoparticles are incorporated into the composite film indeed.
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Fig. 2 XPS spectra of the (P2W18/CNTs/P2W18/Au)4 film (a) P; (b) W; (c) C; (d) Au 20
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Morphology of the multilayer AFM can supply detailed information about the surface morphology and homogeneity of the deposited film. Fig. 3 shows AFM images of the (P2W18/CNTs)4 film (A) , and the (P2W18/CNTs/P2W18/Au)4 film (B). The surface roughness of the (P2W18/CNTs)4 film is 29.12 nm, whereas the (P2W18/CNTs/P2W18/Au)4 film is 23.32 nm. The interface roughness decreases when AuNPs are assembled into the composite material. In addition, a vertical grain structure of the multilayer surface can be observed from three-dimensional AFM images which show that the distribution of aggregated clusters is almost uniform.
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Fig. 3 AFM images of (A) the (P2W18/CNTs)4 film, (B) the (P2W18/CNTs/P2W18/Au)4 film on ITO-coated glass.
and (P2W18/CNTs/P2W18/Au)4 also exhibit four redox waves. After modified with CNTs, the redox peak (peaks I, II and III) currents of the (P2W18/CNTs)8 film is a slightly larger than those of the (P2W18/PAH)8 film, which indicates that CNTs could improve the active surface area of the electrode.27 Comparation (P2W18/CNTs/P2W18/Au)4 to (P2W18/CNTs)8, the peak currents (peaks I, II and III) further increases with AuNPs deposition onto the electrode surface. Actually, AuNPs in the (P2W18/CNTs/P2W18/Au)4 film act as an electron conductor for promoting the electron in the modified material transfer into the electrode.48 Since the CNTs and the Au NPs are both excellent electron conductors for improving the charge transfer between the modifying layer and substrate ITO-coated glass, we deduced that synergistic effect of Au NPs and CNTs exists. Instead of a well
Electrochemical behavior of composite film electrode
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The comparative cyclic voltammograms (CV) of the multilayer of (P2W18/PAH)8, (P2W18/CNTs)8 and (P2W18/CNTs/P2W18/Au)4 in the same buffer solution were shown in Fig. 4. Redox peaks I, II, III and IV of the (P2W18/PAH)8 correspond to four consecutive electron transport processes involving P2W18.20 In the same scan range, the CV curves of other two thin films of (P2W18/CNTs)8
This journal is © The Royal Society of Chemistry [year]
Fig. 4 Cycle voltammogram obtained with the (P2W18/PAH)8 film, the (P2W18/pCNTs)8 film, the (P2W18/pCNTs/P2W18/Au)4 film in 0.2 Na2SO4H2SO4 buffer solution (pH 2.0). Scan rate: 50 mV s-1.
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Analyst Accepted Manuscript
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DOI: 10.1039/C4AN01734J
Electrochemical response of H2O2 (P2W18/CNTs/P2W18/Au)4 modified electrode
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H2O2 +e- → OHad + OHOHad + e- → OH2OH- +2H+ → 2H2O 35
The relevant electrode reactions for P2W18 are as follows:
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P2W18O626- + e ↔ P2W18O627P2W18O627- + e ↔ P2W18O628P2W18O628- + 2e + 2H+ ↔ H2P2W18O628H2P2W18O628- + 2e + 2H+ ↔ H4P2W18O628-
the
The comparison investigations of the electrochemical response of H2O2 at different electrodes were demonstrated in Fig.5. It can be observed that an obvious increase in the reduction current of the (P2W18/CNTs/P2W18/Au)4 nanocomposite as compares with that of the (P2W18/PAH)8 film, and the (P2W18/CNTs)8 film. In the
Fig. 5 Cycle voltammogram obtained with the (P2W18/PAH)8 film, the (P2W18/pCNTs)8 film, the (P2W18/pCNTs/P2W18/Au)4 film in the presence of 0.5 mM H2O2 in 0.2 M Na2SO4-H2SO4 buffer solution (pH 2.0). Scan rate: 50 mV s-1.
case that the surface coverage of P2W18 in these three films is close, the better performance of the nanocomposite is attributed to the following two factors: i) the highly conductive CNTs and AuNPs improved the charge transfer between the modifying layer and the substrate ITO electrode; ii) a synergistic effect of P2W18, CNTs and AuNPs is beneficial to the excellent electrocatalytic activity.
Since the cathodal current of peak (III) and (IV) was clearly increased with the addition of H2O2, the redox peaks (III) and (IV) should be responsible for the H2O2 reduction.
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Fig. 6 Comparative current-time curves for the (a) the (P2W18/PAH)8 film, (b) the (P2W18/pCNTs)8 film, and (c) the (P2W18/pCNTs/P2W18/Au)4 film with successive addition of 0.5 mM H2O2 per 50s.
By using the composite electrode, the mechanism electrocatalytic reduction for H2O2 is as follows: 4 | Journal Name, [year], [vol], 00–00
(I) (II) (III) (IV)
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Fig. 7 The CVs of the (P2W18/pCNTs/P2W18/Au)4 film in 0.2 M Na2SO4H2SO4 buffer solution containing (a) 0 mM, (b) 0.5 mM, (c) 1 mM, (d) 2 mM, (e) 4 mM, (f) 6 mM, (g) 8 mM, (h) 10 mM H2O2. Scan rate: 50 mV s-1. The inset shows the relationship between the third reduction peak current with the concentration of H2O2.
Fig. 7 shows CVs of the (P2W18/CNTs/P2W18/Au)4 film in 0.2 M Na2SO4-H2SO4 buffer solution containing H2O2 in various concentrations. The strong catalytic waves appear on the third and the fourth reduction waves of P2W18, and the corresponding oxidation current decrease, which demonstrates that the (P2W18/CNTs/P2W18/Au)4 film present electrocatalytic activity toward the reduction of H2O2. The catalytic current taken at the third reduction peak was plotted against the concentration of H2O2. As shown in the inset of Fig. 6, the reduction peak current increases linearly along with the increase of the concentration of H2O2. Therefore the composited film could be used in detecting the concentration of H2O2. The amperometric response to H2O2 of the (P2W18/CNTs/P2W18/Au)4 film was studied to make sure that the as-prepared nanocomposite possess promise for application as H2O2 sensors. Fig. 7 shows the typical amperometric i–t curve obtained for (P2W18/CNTs/P2W18/Au)4 nanocomposite modified electrode upon successive addition of different concentrations of H2O2 in 0.2 M Na2SO4-H2SO4 buffer solution. The electrode potential was held at -0.65 V. Upon addition of H2O2, a good and well-defined amperometric response is observed for the reduction of H2O2. The electrode reached 95% of the steady-state current within 1 s, demonstrating the rapid electrocatalytic reduction of H2O2 by P2W18 present in the nanocomposite modified electrode. Moreover, the obvious increase of the current could be observed This journal is © The Royal Society of Chemistry [year]
Analyst Accepted Manuscript
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defined redox peak, there is a sharp increase in cathodic current of peak IV in (P2W18/CNTs)8 and (P2W18/CNTs/P2W18/Au)4modified electrodes, which is ascribed to the overlap with the hydrogen reduction reaction. This result suggests the availibility of a electrocatalyzing the hydrogen involved reactions.20
Current/µA
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when the concentration of H2O2 as low as 1 µM.
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The CNTs improve the active surface area of the electrode, which mainly enhanced the catalytic current density, and then improved the catalytic velocity; it is the reason that the electrode reached 95% of the steady-state current within 1 s. On the other hand, the diffusion rate of H2O2 molecules from the solution to the modified electrode is mainly depending on the stirring speed of the buffer solution. The AuNPs acts as a tunnel of electron in the composite between the redox active center (WV/WVI) of the P2W18 and the electrode surface that leads to the sharp amperometric response and fast electrocatalytic reduction of H2O2 at the modified electrode. As shown in the inert of Fig. 7, the reduction peak current increases linearly upon increasing H2O2 concentration and the linear response range is from 1 to 98 µM. The sensitivity is calculated from the fitted linear regression equation as 596.1 µAmM-1cm-2 (slope value/effective surface area of modified electrode). The detection limit of as-prepared
Fig. 8 Current-time responses for the (P2W18/pCNTs/P2W18/Au)4 film at -0.65 V with the addition of 1 to 98µM H2O2.
Table1. Comparison of various H2O2 sensors
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Analyst Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: S. Guo, L. Xu, B. Xu, Z. Sun and L. Wang, Analyst, 2014, DOI: 10.1039/C4AN01734J.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
www.rsc.org/analyst
Page 1 of 6
Journal Name
Analyst
Dynamic Article Links ► View Article Online
DOI: 10.1039/C4AN01734J
Cite this: DOI: 10.1039/c0xx00000x
ARTICLE TYPE
www.rsc.org/xxxxxx
Ternary nanocomposite electrode of polyoxometalate/carbon nanotubes/gold nanoparticles for electrochemical detection of hydrogen peroxide Shuyue Guo, Lin Xu,* Bingbing Xu, Zhixia Sun and Lihao Wang 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x In this work, a nanocomposite film electrode containing polyoxometalates (POMs) clusters K6P2W18O62 (P2W18), carbon nanotubes (CNTs) and Au nanoparticles (AuNPs) was fabricated by a smart combination of layer-by-layer (LbL) with the self-assembly technique. The synergistical effect of POM, CNTs and AuNPs on the electrocatalysis of H2O2 was investigated to improve the sensitivity of H2O2 detection. The response of (P2W18/CNTs/P2W18/AuNPs)4 electrode to H2O2 was remarkably enhanced due to large active sites and good electron conducting ability. The sensor had quick response (less than 1 second) to H2O2 with a high sensitivity (596.1 µAmM-1cm-2), and low detection limit (9.3 nM). Basing on the respective advantages of POMs, CNTs and AuNPs, the nanocomposite multilayer POMs/CNTs/POMs/AuNPs will bring some special properties and high potential of applications.
Introduction
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The rapid and accurate determination of hydrogen peroxide (H2O2) is practically important in food, pharmaceutical, clinical, industrial, and environmental analyses.1-3 Many techniques have been developed for the determination of H2O2, such as titrimetry,4 spectrophotometry,5 chemiluminescence,6 fluorometric7 and electrochemistry.8 Among these techniques, electrochemical methods are superior for their relatively low cost, high efficiency, high sensitivity and the ease of operation.9,10 In order to prepare excellent electrochemical sensors, many materials were used to decrease the overpotential and increase the electron transfer kinetics. To date, a great amount of enzyme-based and enzymefree electrochemical sensors have been developed to detect H2O2.11-14 However, the enzymatic biosensors are limited by the poor stability, high cost, complicated immobilization procedure and critical operational conditions due to the inherent nature of enzymes. Thus, major efforts have been made to use various replacement alternatives for enzyme, such as Prussian blue,15 noble metal nanoparticles,16 and nanohybrids.17,18 Polyoxometalates (POMs) are discrete nanoscale oxo-clusters of the early transition metals, which display remarkably rich redox and photoelectrochemical properties.19 The metal-oxygen framework can undergo reversible and stepwise, multielectrontransfer reactions, which make POMs an attractive candidate for the electrochemical sensor.20 Many strategies have been developed to H2O2 detection using a variety of POMs-based electrodes.21-25 Although these modified electrodes have shown interesting ability toward H2O2 detection, they also display many problems related to the immobilization of the mediator, low sensitivity and stability, and slow response. Usually, limit of This journal is © The Royal Society of Chemistry [year]
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detection of these modified electrodes is limited to micromolar and response time is around 5 seconds. Hence, fabrication of novel H2O2 sensors with low detection limit and fast response time by using new electron transfer mediators is still highly desirable in this field. Since their discovery by Iijima in 1991, carbon nanotubes (CNTs) have been widely used to immobilize POMs, due to their high surface area, high electrical conductivity, strong adsorptive ability, and good chemical stability.26 By combination of POMs and CNTs, a hybrid nanocomposite with enhanced electrochemical properties and improved stability could be obtained.27-28 In addition, gold nanoparticles (AuNPs) have attracted increasing interest in bioassays for their unique physical and chemical properties, such as easily controllable size distribution, comparative stability, and friendly biocompatibility with biomolecules.29 Recent studies indicate that the substrate material for the loading of AuNPs is very important to the fabrication of sensors with higher performance.30-32,54 The use of CNTs as the support materials for AuNPs has attracted great attention. It is proven that CNTs/AuNPs can act as both the lowpotential redox mediator and electron transfer facilitator.33-35 A large variety of conditions and protocols were followed for synthesis of nanomaterial of CNTs/AuNPs, including electroless plating, electrodeposition, chemical deposition, plasma method and the direct assembly of nanoparticles.36-40 Layer-by-layer (LbL) technique is one of the most promising methods for designing nanocomposite film because of its simplicity, versatility and wide range of materials that can be used for film assembly.41,42 In the last few years, our group fabricated several modified electrodes based on POMs/semiconductor and POMs/metal nanoparticles/semiconductor multilayers by LbL method.43-49 [journal], [year], [vol], 00–00 | 1
Analyst Accepted Manuscript
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Analyst
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DOI: 10.1039/C4AN01734J
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These nanocomposite modified electrodes showed improved photovoltaic and photoelectrocatalysis performance, which was attributed to the synergistic effects of the components in the multilayers. These above-mentioned results inspired us to prepare a POMs/CNTs/AuNPs tricomponent nanocomposite to create a synergistic effect on the electrocatalytic activity. In this work, a new type of POMs/CNTs/AuNPs modified electrode was fabricated by LbL method, which was employed for a non-enzymatic electrochemical sensor of H2O2. Dawsontype heteropolytungstate anion of [P2W18O62]6− was selected as an electrochemically active component to fabricate a composite electrode of heteropolytungstate/CNTs. The electrochemical results demonstrate that the sensor exhibits quick response, high sensitivity, a low detection limit, and long-term stability. Basing on the respective advantages of POMs, CNTs and AuNPs, the nanocomposite multilayer POMs/CNTs/POMs/AuNPs will bring some special properties and high potential of applications.
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Polyoxometalate clusters K6P2W18O62 (P2W18) was synthesized according to the literature method and identified by UV-vis adsorption spectra and cyclic voltammetry.50 PEI-capped Au nanoparticles were prepared by HAuCl4 with polyethylenimine branched (PEI) according to the reported procedure.48 TEM image of AuNPs was shown in Fig. S1. Poly(allylamine hydrochloride) (PAH, MW 70,000), Poly(styrenesulfonate) (PSS MW 70,000), and (3-amino-propyl)trimethoxysilane (APS) were purchased from Aldrich and used as received. Multiwalled CNTs (diameter: 20-30 nm, length: ~30 µm) were purchased from Tsinghua-Nafine Nano-power Commercialization Engineering Center. The CNTs were prepared by a reported method.51 PAH was dissolved in deionized water at a concentration of 0.1 mg mL-1, containing 0.05 M NaCl. Certain amounts of CNTs were dispersed in doubly distilled water with sufficient ultrasonication for about 1 h. When they were dispersed thoroughly, PAH aqueous solution was added. After 3 h ultrasonication, black sediment was observed and separated by centrifugation. The residual PAH polymer was remove by highspeed centrifugation and the complex was rinsed with water at least three times. The collected complex was redispersed in water with mild ultrasonication to produce a stable solution of the complex. Preparation of the (P2W18/CNTs/P2W18/Au)n composite film
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Layer-by-layer films were assembled onto indium tin oxide (ITO)-coated glass, silicon wafers and quartz substrates. ITOcoated glass, silicon wafers (Si‹100›, polished on one side), and quartz substrates were cleaned according to a literature procedure.52 APS modified substrates were immersed into HCl (pH 2.0) for 30 min to get an amino cation surface, followed by washing with deionized water and drying in nitrogen. And then dipped into PSS (1 mM containing 0.5 M NaCl) and PAH (1 mM containing 0.5 M NaCl) for 20 min, respectively. For the (P2W18/CNTs/P2W18/Au)n films, the precursor films were dipped into solution in the order of P2W18, CNTs, P2W18 and AuNPs solution for 10, 20, 10 and 4 min, respectively. LbL assembly of P2W18/CNTs/P2W18/Au is schematically depicted in 2 | Journal Name, [year], [vol], 00–00
Fig. S2. To obtain (P2W18/CNTs)n thin films, the substratesupported precursor films were alternately dipped into the P2W18 (4 mM in 0.1 M H2SO4) solution and the CNTs (0.5 mg/ml) for 10 min and 20 min, respectively, then rinsed with deionized water and drying in nitrogen after each dipping. For comparison, (P2W18/PAH)n films were prepared in a similar way.
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UV-vis absorption spectra of the quartz-supported films were recorded on a Cary 500 Scan UV-Vis-NIR spectrophotometer after each layer deposition. AFM measurements were performed in air with a SPI3800N Probe Station. XPS were measured on silicon wafers using an ESCALAB MK II Surface Analysis System. All electrochemical experiments were performed on a CHI611D Electrochemical Workstation (Shanghai Chenhua Instrument Corp., China) at room temperature. A three-electrode system was used. The composite films-assembled ITO glass was used as the working electrode, and a saturated calomel electrode (SCE) and a platinum wire were used as reference and auxiliary electrode, respectively.
Results and discussions UV-vis spectra characterization of the P2W18/CNTs/P2W18/Au multilayer
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UV-vis absorption spectroscopy was used to monitor the LbL assembling procedure. In an acid solution, negatively charged P2W18, positively charged CNTs and AuNPs adsorbed with each other depends on the electrostatic interactions. Fig. 1 shows the
Fig. 1 UV-vis absorption spectra of multilayer films (P2W18/CNTs/P2W18/Au)2 on quartz substrates (from lower to upper curves). The solid line (—) represents spectra after P2W18 deposition, the dotted line (…) represents spectra after CNTs deposition, the dash line (-..-.) represents spectra after AuNPs deposition. The inset shows relationship of absorbance at 206 nm after P2W18 deposition vs. the number of P2W18 layers.
UV-vis absorption spectra of the (P2W18/CNTs/P2W18/Au)2 multilayers assembled on a precursor-coated quartz substrate (on both sides). The characteristic bands of P2W18 at 206 and 278 nm increased linearly with the number of layers, and the inset of Fig. 1 shows the plots of the absorbance at 206 nm. The broad and weak peak at 278 nm is ascribed to an overlap of characteristic absorption of P2W18 and CNTs. The absorption band of AuNPs was not seen at 520 nm for the low adsorption quantity. A similar This journal is © The Royal Society of Chemistry [year]
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DOI: 10.1039/C4AN01734J
behavior was observed for (P2W18/PAH)2 and (P2W18/CNTs)2 multilayer films. X-ray photoelectron spectrum (XPS)
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XPS spectra were used to identify the chemical composition and the binding energy of the (P2W18/CNTs/P2W18/Au)4 film on silicon substrates. Fig. 2 shows the signal peaks of P, W, C, and Au in the film which confirmed the presence of these elements. The peaks of W 4f and P 2p were detected at 35.1, 37.2 and 132.7 eV, which also gave an approximate ratio of phosphorous to tungsten in 1 : 9 for P2W18. The C 1s peak was observed at 284.7 eV, corresponding to C-C chemical binding states.52 There is a Au 4f7/2 and 4f5/2 doublet with the binding energies of 83.3 and 87.1 eV, respectively. These values suggest that gold is present in the LbL film in the metallic form.53 These results indicate that P2W18, CNTs, and Au nanoparticles are incorporated into the composite film indeed.
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Fig. 2 XPS spectra of the (P2W18/CNTs/P2W18/Au)4 film (a) P; (b) W; (c) C; (d) Au 20
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Morphology of the multilayer AFM can supply detailed information about the surface morphology and homogeneity of the deposited film. Fig. 3 shows AFM images of the (P2W18/CNTs)4 film (A) , and the (P2W18/CNTs/P2W18/Au)4 film (B). The surface roughness of the (P2W18/CNTs)4 film is 29.12 nm, whereas the (P2W18/CNTs/P2W18/Au)4 film is 23.32 nm. The interface roughness decreases when AuNPs are assembled into the composite material. In addition, a vertical grain structure of the multilayer surface can be observed from three-dimensional AFM images which show that the distribution of aggregated clusters is almost uniform.
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Fig. 3 AFM images of (A) the (P2W18/CNTs)4 film, (B) the (P2W18/CNTs/P2W18/Au)4 film on ITO-coated glass.
and (P2W18/CNTs/P2W18/Au)4 also exhibit four redox waves. After modified with CNTs, the redox peak (peaks I, II and III) currents of the (P2W18/CNTs)8 film is a slightly larger than those of the (P2W18/PAH)8 film, which indicates that CNTs could improve the active surface area of the electrode.27 Comparation (P2W18/CNTs/P2W18/Au)4 to (P2W18/CNTs)8, the peak currents (peaks I, II and III) further increases with AuNPs deposition onto the electrode surface. Actually, AuNPs in the (P2W18/CNTs/P2W18/Au)4 film act as an electron conductor for promoting the electron in the modified material transfer into the electrode.48 Since the CNTs and the Au NPs are both excellent electron conductors for improving the charge transfer between the modifying layer and substrate ITO-coated glass, we deduced that synergistic effect of Au NPs and CNTs exists. Instead of a well
Electrochemical behavior of composite film electrode
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The comparative cyclic voltammograms (CV) of the multilayer of (P2W18/PAH)8, (P2W18/CNTs)8 and (P2W18/CNTs/P2W18/Au)4 in the same buffer solution were shown in Fig. 4. Redox peaks I, II, III and IV of the (P2W18/PAH)8 correspond to four consecutive electron transport processes involving P2W18.20 In the same scan range, the CV curves of other two thin films of (P2W18/CNTs)8
This journal is © The Royal Society of Chemistry [year]
Fig. 4 Cycle voltammogram obtained with the (P2W18/PAH)8 film, the (P2W18/pCNTs)8 film, the (P2W18/pCNTs/P2W18/Au)4 film in 0.2 Na2SO4H2SO4 buffer solution (pH 2.0). Scan rate: 50 mV s-1.
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Analyst Accepted Manuscript
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Analyst
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DOI: 10.1039/C4AN01734J
Electrochemical response of H2O2 (P2W18/CNTs/P2W18/Au)4 modified electrode
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H2O2 +e- → OHad + OHOHad + e- → OH2OH- +2H+ → 2H2O 35
The relevant electrode reactions for P2W18 are as follows:
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P2W18O626- + e ↔ P2W18O627P2W18O627- + e ↔ P2W18O628P2W18O628- + 2e + 2H+ ↔ H2P2W18O628H2P2W18O628- + 2e + 2H+ ↔ H4P2W18O628-
the
The comparison investigations of the electrochemical response of H2O2 at different electrodes were demonstrated in Fig.5. It can be observed that an obvious increase in the reduction current of the (P2W18/CNTs/P2W18/Au)4 nanocomposite as compares with that of the (P2W18/PAH)8 film, and the (P2W18/CNTs)8 film. In the
Fig. 5 Cycle voltammogram obtained with the (P2W18/PAH)8 film, the (P2W18/pCNTs)8 film, the (P2W18/pCNTs/P2W18/Au)4 film in the presence of 0.5 mM H2O2 in 0.2 M Na2SO4-H2SO4 buffer solution (pH 2.0). Scan rate: 50 mV s-1.
case that the surface coverage of P2W18 in these three films is close, the better performance of the nanocomposite is attributed to the following two factors: i) the highly conductive CNTs and AuNPs improved the charge transfer between the modifying layer and the substrate ITO electrode; ii) a synergistic effect of P2W18, CNTs and AuNPs is beneficial to the excellent electrocatalytic activity.
Since the cathodal current of peak (III) and (IV) was clearly increased with the addition of H2O2, the redox peaks (III) and (IV) should be responsible for the H2O2 reduction.
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Fig. 6 Comparative current-time curves for the (a) the (P2W18/PAH)8 film, (b) the (P2W18/pCNTs)8 film, and (c) the (P2W18/pCNTs/P2W18/Au)4 film with successive addition of 0.5 mM H2O2 per 50s.
By using the composite electrode, the mechanism electrocatalytic reduction for H2O2 is as follows: 4 | Journal Name, [year], [vol], 00–00
(I) (II) (III) (IV)
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Fig. 7 The CVs of the (P2W18/pCNTs/P2W18/Au)4 film in 0.2 M Na2SO4H2SO4 buffer solution containing (a) 0 mM, (b) 0.5 mM, (c) 1 mM, (d) 2 mM, (e) 4 mM, (f) 6 mM, (g) 8 mM, (h) 10 mM H2O2. Scan rate: 50 mV s-1. The inset shows the relationship between the third reduction peak current with the concentration of H2O2.
Fig. 7 shows CVs of the (P2W18/CNTs/P2W18/Au)4 film in 0.2 M Na2SO4-H2SO4 buffer solution containing H2O2 in various concentrations. The strong catalytic waves appear on the third and the fourth reduction waves of P2W18, and the corresponding oxidation current decrease, which demonstrates that the (P2W18/CNTs/P2W18/Au)4 film present electrocatalytic activity toward the reduction of H2O2. The catalytic current taken at the third reduction peak was plotted against the concentration of H2O2. As shown in the inset of Fig. 6, the reduction peak current increases linearly along with the increase of the concentration of H2O2. Therefore the composited film could be used in detecting the concentration of H2O2. The amperometric response to H2O2 of the (P2W18/CNTs/P2W18/Au)4 film was studied to make sure that the as-prepared nanocomposite possess promise for application as H2O2 sensors. Fig. 7 shows the typical amperometric i–t curve obtained for (P2W18/CNTs/P2W18/Au)4 nanocomposite modified electrode upon successive addition of different concentrations of H2O2 in 0.2 M Na2SO4-H2SO4 buffer solution. The electrode potential was held at -0.65 V. Upon addition of H2O2, a good and well-defined amperometric response is observed for the reduction of H2O2. The electrode reached 95% of the steady-state current within 1 s, demonstrating the rapid electrocatalytic reduction of H2O2 by P2W18 present in the nanocomposite modified electrode. Moreover, the obvious increase of the current could be observed This journal is © The Royal Society of Chemistry [year]
Analyst Accepted Manuscript
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defined redox peak, there is a sharp increase in cathodic current of peak IV in (P2W18/CNTs)8 and (P2W18/CNTs/P2W18/Au)4modified electrodes, which is ascribed to the overlap with the hydrogen reduction reaction. This result suggests the availibility of a electrocatalyzing the hydrogen involved reactions.20
Current/µA
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when the concentration of H2O2 as low as 1 µM.
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The CNTs improve the active surface area of the electrode, which mainly enhanced the catalytic current density, and then improved the catalytic velocity; it is the reason that the electrode reached 95% of the steady-state current within 1 s. On the other hand, the diffusion rate of H2O2 molecules from the solution to the modified electrode is mainly depending on the stirring speed of the buffer solution. The AuNPs acts as a tunnel of electron in the composite between the redox active center (WV/WVI) of the P2W18 and the electrode surface that leads to the sharp amperometric response and fast electrocatalytic reduction of H2O2 at the modified electrode. As shown in the inert of Fig. 7, the reduction peak current increases linearly upon increasing H2O2 concentration and the linear response range is from 1 to 98 µM. The sensitivity is calculated from the fitted linear regression equation as 596.1 µAmM-1cm-2 (slope value/effective surface area of modified electrode). The detection limit of as-prepared
Fig. 8 Current-time responses for the (P2W18/pCNTs/P2W18/Au)4 film at -0.65 V with the addition of 1 to 98µM H2O2.
Table1. Comparison of various H2O2 sensors
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MWCNTs/[C8Py]- POM
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