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Dendritic Bimetallic Nanostructures Supported on Self-Assembled Titanate Films for Sensor Application Shengfu Tong,†,§ Yaohua Xu,† Zhixin Zhang,‡ and Wenbo Song*,† College of Chemistry, Jilin UniVersity, Changchun 130012, People’s Republic of China, and The First Hospital of Jilin UniVersity, Changchun 130021, People’s Republic of China ReceiVed: April 20, 2010; ReVised Manuscript ReceiVed: September 29, 2010
The electrochemical properties of electrodes modified with metal/metal oxides depend not only on the nature of the materials but also on the composition and substrate as well. In this study, dendritic CuNi nanostructured materials with a distinguishable bimetal phase were achieved by electrodeposition in 0.05 M Na2SO4 solution containing 0.05 M CuSO4 and 0.05 M NiCl2 at -1.0 V on the surface of titanate thin films, which were self-assembled from the titanate nanaosheets exfoliated by n-propylamine. The structures, morphologies, and elemental molar ratio of the titanate-supported CuNi were analyzed by XRD, SEM, and ICP-AES, respectively. The electrochemical activities of the CuNi nanostructured electrodes toward glucose oxidation were evaluated, and factors that affect the electrocatalytic activities of the electrodes were examined and optimized. The potential applications of the CuNi nanostructured films for fabrication of enzymeless glucose sensors were also investigated. The assay performances of the sensor evaluated by conventional electrochemical techniques revealed a quick response, good reproducibility, and enhanced sensitivity in glucose determination compared with that of pure Cu or Ni electrodeposited on the self-assembled titanate template. 1. Introduction It has been recently demonstrated that the structures and geometries of binary nanostructured materials are quite different from those of their corresponding pure ones,1,2 and their physical and chemical properties can be tuned by varying the composition and atomic ordering and the size as well.1 Mixed 3d transitionmetal/metal oxides were reported to be more active in catalysis than the single ones due to the existence of synergistic effects among alloy components relating to the electronic effect and/ or some other influence else.3-6 During the past years, such biand/or multimetallic nanostructured materials were extensively developed and reported by many articles and reviews.1,2,7-12 For instance, CuNi bimetallic materials for application in corrosion protection, catalysis, electronics, and batteries were largely investigated. For electroanalytical application, addition of a small amount of Ni into Cu-based material resulting in dramatic changes in the properties of the electrode surface and leading to sensitive and stable electrochemical detection of carbohydrates was also reported in a previous work.10 Exploring an economic and easy route for preparing size and morphology controllable CuNi bimetallic nanostructured materials still remains at the forefront of research. Compared with other methods, electrochemical routes exhibit superiority in synthesis, that is, avoiding the separation between the products and the solutions. Additionally, they are usually controllable, low in cost, easy to operate, etc.13 As one of the powerful electrochemical methods, electrodepositing a thin layer of metal or oxides on the surfaces of a foreign metal or a semiconductor is of importance in both electrocatalysis and surface science.14,15 * To whom correspondence should be addressed. Tel: +86-(0)43185168352. Fax: +86-(0)431-85167420. E-mail:
[email protected]. † College of Chemistry, Jilin University. ‡ The First Hospital of Jilin University. § Current address: Physical Chemistry Laboratory, Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0818, Japan.
Recent investigations demonstrated that the catalytic activities of electrodes not only correlated closely with the nature of the materials but also depended on their structures, surface morphologies, and even underlying substrates.16,17 We are motivated in the search for novel supported electrocatalysts for potential applications in electrocatalysis and electroanalysis.18 Our previous studies and others2,17,19-22 demonstrated that the electrocatalytic activities of the nanostructured metal loaded on suitable substrates could be largely enhanced. Exploiting appropriate carrier materials is vital to the properties of the supported catalysts, and the properties of carriers should first meet the natural requirements, involving high surface area and good mechanical and thermal resistance. Titanate, one of the typical inorganic transition-metal oxides with layered structures, is an outstanding candidate of carriers and attracted much research attention during the past years due to its unique properties, such as stability, high surface area, interesting ion exchange, exfoliation properties, etc. Sasaki et al. contributed much to the synthesis, characterization, and functionalization of titanate.11,23-28 They also demonstrated the successful formation of titanate self-assembled thin films based on the exfoliated titanate nanosheets.27,28 On the other hand, only few reports emerged very recently concerning the application of functionalized titanate in electrocatalysis or electroanalysis; for instance, Li et al. investigated the direct electrochemistry of the intercalated myoglobin between the titanate layers and its analytical application.29 In our previous studies,13,30 the feasibility of the preparation of Cu-titanate intercalation electrode materials by electrochemical reduction of the inserted metal ions among the titanate layers has been demonstrated, and its potential application in glucose sensor fabrication has also been investigated. In this paper, titanate self-assembled thin films were constructed on the indium tin oxide (ITO) surfaces by sequential adsorption of the exfoliated titanate nanosheets and poly(diallyldimethylammonium) (PDDA) based on the electrostatic principle and
10.1021/jp1035772 2010 American Chemical Society Published on Web 11/17/2010
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were used as a support for loading bimetallic CuNi nanostructures through electrodeposition in a solution of 0.05 M Na2SO4 containing 0.05 M CuSO4 and 0.05 M NiCl2 at -1.0 V. The structures, morphologies, components, and electrochemical properties of the resulting CuNi nanostructured films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), inductive coupled plasma-atomic emission spectroscopy (ICP-AES), and traditional electrochemical methods. The potential application of the supported CuNi bimetal nanostructures films in glucose determination was also investigated in detail. 2. Experimental Section 2.1. Reagents. All chemicals in the present experiments were analytical grade and used without any further purification. The carbohydrate solutions used in the current study were prepared freshly before the experiments. The water used was redistilled water. The layered lithium potassium titanate (K0.81Li0.27Ti1.73O4, KLTO) utilized here was synthesized according Sasaki’s method.23 2.2. Apparatus. All the electrochemical experiments were carried out on a CHI660B (CH Instruments, Chenhua) at room temperature under an ultrapure nitrogen atmosphere in a conventional three-electrode cell. A Pt wire and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The working electrodes were indium tin oxide (ITO) electrodes before/after the film modification. The bare ITO electrodes were cleaned by ultrasonication in acetone, ethanol, and water alternatively prior to all experiments. XRD patterns were recorded on an XRD-6000 (Shimadzu, Japan) powder diffractometer equipped with Cu-KR radiation (λ ) 1.5418 Å). The morphologies of the resulting electrodes were characterized by SEM (SSX-550, Shimadzu, Japan), and the elemental compositions of the products were measured by ICP-AES on an ICP-1000 (PerkinElmer, U.S.A.). 2.3. Modification of Electrodes. The process of ITO substrates modified with KLTO thin films was similar as Sasaki’s work.28 In detail, 50 mg of KLTO was mixed with 2.5 mL of n-propylamine (C3N) and 2.5 mL of redistilled water. After ultrasonication of the mixture for 25 min, a homogeneous and stable suspension was obtained, and KLTO was exfoliated into nanosheets (referred to as KTO). The primed ITO electrodes (0.25 cm2 exposed for modification) were first dipped into 1% poly(diallyldimethylammonium) (PDDA) chloride for 8 min, and the substrates were coated with positively charged PDDA (PDDA/ITO). Subsequently, the PDDA/ITO electrodes were transferred into the KTO suspension for another 8 min to adsorb titanate nanosheets based on the electrostatic interaction between the positively charged PDDA and negatively charged KTO nanosheets. Multilayer films of KTO and PDDA, (KTO/ PDDA)n/ITO, were prepared by repeating the previous steps for n times. The electrodes were always rinsed thoroughly with redistilled water to get rid of excessive reagents before immersing into another solution. The final (KTO/PDDA)n/ITO films were ready for use. Electrodeposition of pure Cu, Ni, and CuNi bimetallic nanostructured materials on (KTO/PDDA)n/ITO films was carried out in 0.05 M CuSO4, 0.05 M NiCl2, and 0.05 M CuSO4 + 0.05 M NiCl2 in separate 0.05 M Na2SO4 solutions. The reduction potentials applied were -0.8, -1.0, and -1.2 V (vs SCE). The electrodeposition process was performed in a stationary electrolyte solution without stirring. The final electrodes were referred to as M/(KTO/PDDA)n/ITO (M ) Cu, Ni, or CuNi).
Tong et al.
Figure 1. EIS of (KTO/PDDA)n/ITO (n ) 0-15) in 0.1 M KCl containing 1 mM [Fe(CN)6]3-/4-. Inset: the suitable equivalent circuit of the films.
2.4. Electrochemical Measurements. Electrochemical impedance spectroscopy (EIS) measurements were performed in 0.1 M KCl solution with the presence of 1 mM [Fe(CN)6]3-/4(1:1) as a redox probe. In the EIS measurement, the initial applied potential was held at an open-circuit potential (+0.23 V), the alternating voltage was 5 mV, and the frequency range was from 100 mHz to 100 kHz. The EIS data were simulated using the software Zview. NaOH (0.1 M) was used as the supporting electrolyte in the measurements of M/(KTO/PDDA)n/ITO film electrodes for electrocatalysis toward small organic molecules (including glucose, ethanol, and other carbohydrates). 3. Results and Discussion 3.1. Electrochemical Characterization of the Self-Assembled KTO Films. 3.1.1. Electrochemical Impedance Spectroscopy (EIS) Analysis. EIS is a well-known effective means of probing the features of surface-modified electrodes, and the respective semicircle diameter corresponds to the electrontransfer resistance at the electrode surface.31,32 EIS was applied in this study to investigate the self-assembled processes and electron transfer of the KTO films. Significant differences in the impedance spectra were observed during stepwise modification of the self-assembled KTO films, as depicted in Figure 1. Obviously, both the bare ITO and the (KTO/PDDA)n/ITO electrodes exhibited semicircle portions at higher frequencies and linear parts at lower frequencies, suggesting the kinetically controlled electron-transport processes of the redox probe at the electrode interfaces at high frequency and diffusion-limited electron-transfer processes at low frequency.32 On immobilization of the self-assembled KTO films on ITO, the diameter of the Nyquist plot increased sharply when the self-assembled KTO layers (n) changed from 1 to 4, whereas when n increased from 4 to 8, the diameter of the Nyquist plot kept almost constant. With the thickness of KTO films further increased, the plot diameter increased slightly. The EIS results of KTO selfassembled films were similar to those of La2/ 33 3Li0.15Ti0.85Al0.15O3, suggesting the successful construction of self-assembled KTO films on the electrode surface28 The suitable equivalent circuit, which models the above results, is also shown in Figure 1 (inset). From the simulation with the software Zview, the self-assembled KTO films exhibited a stable resistance at 4 < n < 8 (Figure S1 in the Supporting Information), which is in good agreement with the results of Nyquist plots described above. The resistance of the titanate films will obviously affect the electrochemical behaviors of the electrodes, which will be discussed later. 3.1.2. Cyclic Voltammetric BehaWior in 0.1 M KCl Solution Containing 1 mM [Fe(CN)6]3-/4-. The cyclic voltammetric behavior of KTO self-assembled films in 0.1 M KCl solution
Dendritic Bimetallic Nanostructures on Titanate Films containing 1 mM [Fe(CN)6]3-/4- was investigated (Figure S2, Supporting Information). Well-defined redox peaks, the oxidative peak at about +0.3 V and the reductive peak at around +0.15 V, attributed to the [Fe(CN)6]3-/4- probe, were observed at the bare ITO electrode. With the increase of KTO layers assembled on the ITO surface, the oxidative peak was distinctly decayed and the reductive peak was negatively shifted, exhibiting a diode-like phenomenon.34,35 This can be explained by the electrostatic repulsion between [Fe(CN)6]3-/4- molecules and the outermost negatively charged KTO nanosheets, as well as the influence of positively charged PDDA that affected the double-layer capacitance and the interfacial electron-transfer process. 3.2. Optimization of Metal Electrodeposition and Preparation of CuNi on Self-Assembled KTO Films. 3.2.1. The Influence of Self-Assembled KTO Layers on Electrodeposition. As described above, ITO surfaces could be successfully modified with self-assembled KTO films, which are expected as promising catalyst supports. To investigate the effect of the thickness of KTO films on metal electrodeposition, the electrodeposition of Cu in 0.05 M Na2SO4 solution containing 0.05 M CuSO4 onto ITO surfaces precoated with various KTO layers was first attempted as the model system. In all cases, a constant amount of Cu (controlled by the coulombs consumed during electrodeposition process) was deposited onto the separate KTO films with different KTO self-assembled layers. Generally, two main parameters, potential and net currents during glucose electrochemical oxidation in a basic solution, are used to evaluate the electrochemical properties of the electrodes. Note that the net current toward glucose oxidation on each electrode with the same amount of deposition metal instead of the current density was used for comparison due to either the uncertainty of evaluating the current density by utilizing the geometric area of the substrate or the difficulty in obtaining the real surface area and/or the electroactive area of the coelectrodeposited bimetallic phases that are normally believed to be inhomogeneous and nonstoichiometric. Comparing the oxidative peak potential or the corresponding current of glucose oxidation at the supported Cu (constant deposition amount) electrodes prepared on the separate carrier films with KTO layers of 1-10 (Table S1, Supporting Information), we found that the electrodeposited Cu on a carrier film with KTO layers of 6 exhibited the higher current toward the electrochemical oxidation of glucose. On consideration of the stable resistance of the KTO films with six self-assembled layers (Figure S1, Supporting Information) and above higher electrochemical oxidation current of the supported Cu, a carrier film with six KTO layers was adopted as support for electrodeposition in the following study. Hereafter, M/(KTO/PDDA)6/ITO is shortened to M/KTO/ITO, where M ) Cu, Ni, or CuNi. 3.2.2. The Dependence of Electrochemical Properties on Electrodeposition Time. A comparison of the glucose electrochemical oxidation properties of the nanosized bimetallic films prepared at different electrodeposition times was carried out (Figure S3, Supporting Information). Obviously, the oxidative current of glucose at the electrode obtained at 80 s was almost 5 times of that at 10 s, while the deposition time increased from 80 to 180 s; the oxidative currents were almost constant. When the deposition time was prolonged to 500 s, the oxidative current increased only 1.07 times compared with that at 80 s. That is, a minor deposition time influence on the electrochemical property of the deposited electrode was found when the deposition time was longer than 80 s. It was reasonable that the substrate was less affected with more Cu deposited onto
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Figure 2. Steady-state current-time response of CuNi/KTO/ITO (a), Cu/KTO/ITO (b), and Ni/KTO/ITO (c) in 0.1 M NaOH with consecutive injection of 1.0 × 10-4 M glucose at +0.55 V.
the KTO films, and when Cu was overdeposited onto the KTO films; the resulting metal films were almost continuous rather than micro/nanostructures. In this study, 80 s was selected as the optimal deposition time for metal loading on the selfassembled KTO films. 3.2.3. Preparation of Bimetallic CuNi Nanostructures on KTO Films. As mentioned above, mixed 3d transition-metal oxides are more active in catalysis than the single ones,5 as demonstrated by the enhanced electrocatalytic activities toward small organic molecules of Cu-based electrodes doped with other transition-metal/metal oxides.10,36 The pure Cu and Ni metals have the same face-centered cubic structure with similar lattice parameters, suggesting a possible wide range of compositions for CuNi bimetal depositions. Researchers investigated the carbohydrate oxidation on a Ni-modified electrode containing a high percentage of Cu and reported its promising stability for glucose oxidation. In the current study, motivation is derived primarily from the anticipation of a synergistic electrochemical benefit from the combined properties of the two components. The effect of doping Ni in Cu-rich electrodes to fabricate bimetallic CuNi electrodes on their electrochemical oxidation properties toward glucose and other carbohydrates was investigated. It is well-known that the reduction potential of nickel is more negative than that of copper,19,37 for obtaining bimetallic CuNi nanostructured materials by coelectrodeposition; the electrodeposition processes were carried out in 0.05 M Na2SO4 solution containing CuSO4 and NiCl2 at separate reduction potentials of -0.8, -1.0, and -1.2 V (vs SCE). Two main parameters, that is, reduction potential and molar ratio of Cu2+ and Ni2+ in the electrolyte, that influence the components and electrochemical properties of the resulting materials were optimized (Table S2, Supporting Information). In experiments, we found that CuNi films coelectrodeposited under the condition of (CCu2+:CNi2+ ) 1, Ered ) -1.0 V, deposition time ) 80s) exhibited the highest electrochemical response toward glucose oxidation. The net oxidative current of glucose on this bimetallic CuNi electrode was 1.43 and 3.98 times those on the corresponding pure Cu and Ni film electrodes with a similar amount of metal loading on separate self-assembled KTO supports (Figure 2). It was 2.39 times that on the supported CuNi on a bare ITO surface without the KTO carrier films (not shown in Figure 2) and 4.63 times that on the Cu/KTO-intercalation electrode in our previous report.30 The high electrochemical response of CuNi nanostructures supported on the self-assembled KTO films toward glucose oxidation was possibly attributed to the electrochemical combination of Cu and Ni bimetal components and the related electronic effect.3-6
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Figure 3. XRD pattern of CuNi/KTO/ITO obtained at -1.0 V.
3.3. Components, Structures, and Morphologies of CuNi Materials Supported on the KTO Films. The molar ratio of composed elements of CuNi supported on the KTO films was analyzed by ICP-AES. The contents of Cu and Ni in the bimetal materials were largely dependent on the reduction potential applied for electrodeposition. For instance, the atomic molar ratios of Cu and Ni in the bimetallic materials obtained at -0.8, -1.0, and -1.2 V were 7.255, 5.927, and 2.922, respectively (Table S3, Supporting Information), indicating that the more negative the potential was, the higher ratios of Ni in the bimetal materials could be obtained. XRD measurements were applied to characterize the structure of the CuNi materials supported on the self-assembled KTO films. The CuNi phase similar to Kang’s result11 was distinguishable from Figure 3, though there also existed the peaks of oxide and hydroxide of copper due to the oxidation of copper. The morphologies of self-assembled KTO films and supported CuNi on KTO films were characterized by SEM (Figure 4). The rough self-assembled KTO films were attached onto the ITO surface (Figure 4a), implying a promising support for dispersing catalysts. Codeposition of CuNi on the self-assembled KTO films resulted in a compact bimetallic coating with different grain sizes cross-linked over the surface. In addition, some of them were combined and formed leaflike and/or dendritic nanostructures (Figure 4b and inset), which were quite different from the supported pure Cu on KTO films, as shown in Figure 4c. The physical property changes of the Cu-based materials by doping of Ni have already been reported.10 In the meantime, we found that the oxidation current of glucose at CuNi/KTO/ITO was much higher than that of either CuNi/Cu11 (Table S4, Supporting Information) or CuNi/ITO (mentioned above), suggesting an important role of the self-assembled KTO films for loading a bimetal. 3.4. Electrochemistry and Electrocatalysis of the Supported CuNi on Self-Assembled KTO Films. 3.4.1. Cyclic Voltammetric BehaWior of CuNi/KTO Films. The electrochemical behavior of the CuNi/KTO/ITO electrode was characterized by cyclic voltammetry. Continuous cyclic voltammograms (CVs) of the CuNi/KTO/ITO electrode in 0.1 M NaOH are shown in Figure S4 (Supporting Information). As indicated in our studies and other publications, pure Cu has no obvious redox peaks in 0.1 M NaOH in the potential range (inset in Figure S4, Supporting Information). At the KTO-supported CuNi bimetallic electrode surface, a Ni(OH)2 layer was rapidly formed at low oxidation potential during the positive scan, leading to a Cu-rich metal surface that was subsequently oxidized to Cu2O and further to Cu(OH)2. The surface layer may later be transformed to a mixture of NiOOH and Cu(OH)2, and some Cu(OH)2 and CuO could be further oxidized to the Cu(III) oxide, as indicated by Druska and Jafarian.38,39 Therefore, the redox
Figure 4. Morphologies of KTO/ITO (a), CuNi/KTO/ITO (b), and Cu/KTO/ITO (c) obtained at -1.0 V. The insets are local SEM images of corresponding films.
peaks at this supported CuNi electrode surface revealed in Figure S4 (Supporting Information), the oxidative peak at around +0.58 V and the reductive peak at around +0.32 V, could be assigned to the nickel hydroxide/nickel oxyhydroxide redox couple, that is, Ni(OH)2/NiOOH, in the bimetal composite.10,40 The elevated baseline current in the potential range of +0.20 to +0.55 V should be associated with the co-oxidation of Cu and Ni metal
Dendritic Bimetallic Nanostructures on Titanate Films surfaces at low oxidation potential to their corresponding lower oxidative states, such as Ni(OH)2 and Cu(OH)2, etc. By careful comparison, we found that the oxidation current at a low oxidation state of the CuNi surface was much higher than those on the KTO-supported pure Cu and Ni electrodes, suggesting a very efficient strategy of enriching the electrochemical active surface area of the KTO-supported bimetallic composite electrode by combination of a small amount of nickel with copper species. The progressive enrichment of the accessible electroactive species, Ni(II) and Ni(III), on or near this CuNi nanocomposite electrode was not found, as indicated by the almost invariable peak potentials and peak currents associated with the redox of the nickel hydroxide and nickel oxyhydroxide couple during continuous potential scanning. These phenomena demonstrated that the changes in the crystal structures of the nickel hydroxide and oxyhydroxide of the electrochemically formed surface film were achieved very fast in this KTO-supported CuNi electrode, unlike that of the electrodeposited pure Ni electrode, potentially due to the expected synergistic interaction. The KTO-supported CuNi electrode was rather electrochemically stable in alkaline solution; after 50 continuous potential scannings, the redox current was found without any decay. Therefore, the KTOsupported CuNi nanostructures are expected to be a better electroactive material for high electrochemical performance in alkaline solution. 3.4.2. Glucose Oxidation on CuNi/KTO Films. As mentioned previously, glucose and other small organic molecules were utilized as the targets for evaluation of the electrochemical activities of the resulting electrodes. The electrooxidation of glucose at KTO/ITO and CuNi/KTO/ITO electrodes was compared. No obvious redox currents were observed at the KTO/ITO electrode in the presence of glucose in the potential range of -0.1 to +1.0 V, suggesting that the direct oxidation of glucose at KTO/ITO was disabled. On the other hand, on addition of a low concentration glucose solution into the electrolyte, dramatic enhancement of the oxidation currents in a potential range of +0.25 to +0.8 V was observed at the CuNi/ KTO/ITO electrode with two oxidation peaks around +0.50 and +0.65 V (Figure S5, Supporting Information). The basis of the oxidation mechanism of glucose at these electrode surfaces is suggested to involve the electron-transfer mediation of the multivalent metal redox couple of the anodized electrodes.36 As revealed by Figure S5 (Supporting Information), the oxidized potential of glucose at the CuNi/KTO/ITO electrode in alkaline solution started from +0.25 V and the major oxidative process fell into the potential range of +0.25 to +0.80 V, which was the potential range involved with the formation of Cu(III) and Ni(III), as discussed above. Therefore, the second oxidation peak around +0.65 V was characterized by the mechanism involving electron-transfer mediation by a Ni(OH)2/NiOOH redox couple in the bimetallic films at the anodized electrode surface.39 The first oxidation peak around +0.50 V might be associated mainly with the oxidation of glucose at the surface of the anodized CuNi electrode mediated by the Cu(II)/Cu(III) redox couple, and doping Ni into Cu metal in the KTO-supported electrodeposition processes should also be beneficial, as revealed by the pronounced enrichment of the electroactive surface area of the supported CuNi electrode by addition of a small amount of Ni, as discussed above. The comparison of the electrochemical behavior of the pure electrodeposited nickel and Cu on KTO supports with CuNi toward glucose oxidation was also carried out. We found that the oxidation current of the same concentration of glucose at
J. Phys. Chem. C, Vol. 114, No. 49, 2010 20929 the CuNi/KTO/ITO electrode was much higher than that of electrodepositing pure Cu or Ni on the KTO support, suggesting the expected synergistic electrochemical effect of bimetal loading on the self-assembled KTO films. Additionally, in all concentration levels, the two oxidation peak currents increased with the glucose concentration, and the oxidative peak potentials gently shifted toward the anodic direction. A similar phenomenon was also found at the surfaces of Cu, Ni, and their bimetallic electrodes,22,30 ascribing to the strong interaction of glucose with the surface already covered by low-valence metallic species. As one kind of outstanding candidate for oxidation of carbohydrates, the advantages to employing Cu, Ni, CuNi, and other transition-metal-based electrode materials instead of noble metals are that they can oxidize carbohydrates at constant potential and, therefore, simplify the instrumentation and operation.30 The virtual foundation of the glucose sensor is the relation between the concentration of glucose and the analytical signals that correspond to the steady-state currents’ flow due to the Faradaic oxidation of glucose at the electrode surfaces. The stability of the KTO-supported CuNi electrode during glucose oxidation was also investigated. In alkaline solution containing a certain amount of glucose, the oxidation current was found with negligible decay after 10 continuous potential scannings, implying a good stability of this CuNi electrode in glucose oxidation processes. Promising stability for carbohydrate oxidation on Ni-modified electrodes containing a high percentage of Cu has been demonstrated by others. From the results of the linear current response to glucose concentration depicted in Figure S5 (Supporting Information), the good electrochemical stability in alkaline solution, and the good electrooxidation reproducibility toward glucose, the CuNi/KTO/ITO electrode might be a good candidate for enzymeless glucose sensors. 3.5. Amperometric Performance of the CuNi/KTO/ITO Electrode to Glucose. 3.5.1. Optimal Potential Selection. On the basis of results described above, it appeared that amperometric detection of glucose at the CuNi/KTO/ITO electrode might be applicable. For obtaining a high sensitivity in the determination, hydrodynamic amperometry was carried out to determine the optimum potential for the sensor operation. Curve a in Figure S6 (Supporting Information) shows the hydrodynamic voltammogram of the CuNi/KTO/ITO electrode in the presence of 1.0 mM glucose in the stirring electrolyte solution; the current response was measured at a constant interval of 0.05 V in the potential range of +0.25 to +0.70 V. With the oxidation potential positively shifted, the oxidation current of 1.0 mM glucose at the CuNi/KTO/ITO electrode increased, while the signal-to-background (S/B) ratio (curve b in Figure S6, Supporting Information) initially kept on rising, reached the topmost value at +0.55 V, and then sharply decayed when the potential was more positive than +0.55 V due to the dramatic increase in the baseline current at high oxidation potential. For a high sensitivity determination of glucose, a constant potential of +0.55 V was selected and employed in all the subsequent amperometric detections. 3.5.2. Amperometric Analysis. Figure S7 (Supporting Information) shows the typical steady-state current-time response of the CuNi/KTO/ITO electrode in 0.1 M NaOH with consecutive injection of 1.0 × 10-5 M glucose at +0.55 V. The response time (from glucose injecting to reaching 95% of the steadystate current) was within 5 s, suggesting a facile electron-transfer process through the CuNi/KTO films. The CuNi/KTO/ITO electrode displayed a well-defined concentration dependence. The relationship between the electrocatalytic current and glucose
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Tong et al. indicated that the AA, UA, ethanol, and SC did not cause any observable interference in the designated concentration of glucose. Additionally, other carbohydrates that can be electrocatalytically oxidized by copper-based materials at positive potentials were also investigated (see Table S5, Supporting Information). Compared with other carbohydrates, the response generated by glucose was much higher. The CuNi/KTO/ITO electrode was selective to glucose. 3.5.4. Standard Glucose Sample Analysis. Standard glucose samples were detected by the standard addition method to verify the reliability of the electrode. All the concentrations of the standard glucose were in the linear range, and the detections were carried out in a separate process. The determinations were performed in 5.0 mL of 0.1 M NaOH at +0.55 V. The current responses were directly interpolated in the linear regression. The values determined were satisfactory with a good recovery (see Table S6, Supporting Information), attributed to the superiority of the CuNi dispersed on the self-assembled KTO support. 4. Conclusions
Figure 5. Panels A-C display the relationship between electrocatalytic current and concentration of glucose in different concentration decades at the CuNi/KTO/ITO electrode.
in different concentration decades at the CuNi/KTO/ITO electrode is shown in Figure 5A-C, demonstrating a good linear relationship between the current and glucose concentration in the range of 1.0 × 10-6 to 5.0 × 10-4 M (correlation coefficient ) 0.995). The detection limit of glucose at the electrode was 3.5 × 10-7 M (S/N ) 3). The sensitivity of the present nonenzymatic glucose sensor was 661.5 µA mM-1 (apparent electrode surface area is 0.25 cm2), which was much improved compared with those of the Cu/titanate intercalation electrode and others in our previous reports.16,22,30 Under the optimal oxidative potential of +0.55 V, seven repetitive tests of 1.0 × 10-5 M glucose were carried out in 0.1 M NaOH solution, and the relative standard deviation (RSD) in the determination was 5.12%. Three CuNi/KTO/ITO electrodes were fabricated in the same manner and utilized for detection of 0.1 mM glucose, and an RSD of 5.26% was obtained. The above results demonstrated a satisfied stability and reproducibility of the CuNi/KTO/ITO electrode in both measurements and fabrication. The good stability during glucose electrooxidation, high sensitivity in the determination, and satisfied reproducibility for sensor fabrication suggest that the CuNi/KTO/ITO electrode is a promising candidate for amperometric enzymeless glucose sensors for potential applications. 3.5.3. Interferential Analysis. The interference tests were carried out in 0.1 M NaOH solutions containing 2.0 × 10-4 M glucose at +0.55 V in the presence of 0.2 times of ascorbic acid (AA), 0.8 times of uric acid (UA), 50 times of ethanol, and 250 times of sodium chloride (SC), respectively. The results
The facile exfoliation of layered titanate with n-propylamine and successful formation of self-assembled titanate thin films were accomplished. Bimetallic CuNi nanostructures were coelectrodeposited on the self-assembled titanate films in 0.05 M Na2SO4 solution containing Cu2+ and Ni2+, and the molar ratio of Cu and Ni in the nanocomposite could be modulated by tuning the potential applied for electrodeposition. Dendritic CuNi microstructures with a distinguishable bimetal phase obtained at -1.0 V exhibited superior electrocatalytic properties toward glucose oxidation, attributed to the high surface area and excellent physical and chemical properties of the self-assembled KTO support. Sensing and assay performances of CuNi/KTO/ ITO films at +0.55 V revealed a high sensitivity and good reproducibility in glucose determination. A wide linear range of 1.0 × 10-6 to 5.0 × 10-4 M and a low detection limit of 3.5 × 10-7 M (S/N ) 3) were obtained. Results suggest that dendritic CuNi bimetallic microstructures supported on the selfassembled KTO films were promising in the fabrication of amperometric glucose sensors. Acknowledgment. This work was supported by the National Natural Science Foundation of China under Grant Nos. 21075048 and 20543003, as well as the Scientific Research Foundation for Returned Overseas Chinese Scholars, Ministry of Education of China. Supporting Information Available: The Rct and CVs of the (KTO/PDDA)n/ITO films depend on the different KTO layers; the oxidative currents depend on the deposition time. CVs of the CuNi/(KTO/PDDA)6/ITO film electrode in 0.1 M NaOH without and with glucose were tested in our current work. These materials and other figures and tables are available in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ferrando, R.; Jellinek, J.; Johnston, R. L. Chem. ReV. 2008, 108, 845–910. (2) Raimondi, F.; Scherer, G. G.; Kotz, R.; Wokaun, A. Angew. Chem., Int. Ed. 2005, 44, 2190–2209. (3) Morales, M. R.; Barbero, B. P.; Cadus, L. E. Appl. Catal., B 2007, 74, 1–10. (4) Morales, M. R.; Barbero, B. P.; Cadus, L. E. Appl. Catal., B 2006, 67, 229–236.
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