Dendritic Bimetallic Nanostructures Supported on Self-Assembled Titanate Films for

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 Uni V ersity,Changchun130012,People’s Republic of China,and The First Hospital

of Jilin Uni V ersity,Changchun130021,People’s Republic of China

Recei V ed:April20,2010;Re V ised Manuscript Recei V ed:September29,2010

The electrochemical properties of electrodes modi?ed 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 in0.05M Na2SO4solution

containing0.05M CuSO4and0.05M NiCl2at-1.0V on the surface of titanate thin?lms,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?lms 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,1,2and their physical and chemical properties can be tuned by varying the composition and atomic ordering and the size as well.1Mixed3d transition-metal/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 in?uence else.3-6During the past years,such bi-and/or multimetallic nanostructured materials were extensively developed and reported by many articles and reviews.1,2,7-12For 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 carbohy-drates was also reported in a previous work.10

Exploring an economic and easy route for preparing size and morphology controllable CuNi bimetallic nanostructured materi-als still remains at the forefront of https://www.360docs.net/doc/4d6538774.html,pared 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.13As 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

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 mor-phologies,and even underlying substrates.16,17We are motivated in the search for novel supported electrocatalysts for potential applications in electrocatalysis and electroanalysis.18Our previ-ous studies and others2,17,19-22demonstrated that the electro-catalytic activities of the nanostructured metal loaded on suitable substrates could be largely enhanced.

Exploiting appropriate carrier materials is vital to the proper-ties of the supported catalysts,and the properties of carriers should?rst 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.11,23-28They also demonstrated the successful forma-tion of titanate self-assembled thin?lms based on the exfoliated titanate nanosheets.27,28On the other hand,only few reports emerged very recently concerning the application of function-alized titanate in electrocatalysis or electroanalysis;for instance, Li et al.investigated the direct electrochemistry of the interca-lated myoglobin between the titanate layers and its analytical application.29

In our previous studies,13,30the feasibility of the preparation of Cu-titanate intercalation electrode materials by electro-chemical 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?lms were constructed on the indium tin oxide(ITO)surfaces by sequential adsorption of the exfoliated titanate nanosheets and poly(diallyldimethy-lammonium)(PDDA)based on the electrostatic principle and

*To whom correspondence should be addressed.Tel:+86-(0)431-

85168352.Fax:+86-(0)431-85167420.E-mail:wbsong@https://www.360docs.net/doc/4d6538774.html,.

?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,Sapporo060-0818,Japan.

J.Phys.Chem.C2010,114,20925–2093120925

10.1021/jp1035772 2010American Chemical Society

Published on Web11/17/2010

were used as a support for loading bimetallic CuNi nanostruc-tures through electrodeposition in a solution of0.05M Na2SO4 containing0.05M CuSO4and0.05M NiCl2at-1.0V.The structures,morphologies,components,and electrochemical properties of the resulting CuNi nanostructured?lms were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),inductive coupled plasma-atomic emission spectroscopy(ICP-AES),and traditional electrochemical meth-ods.The potential application of the supported CuNi bimetal nanostructures?lms in glucose determination was also inves-tigated in detail.

2.Experimental Section

2.1.Reagents.All chemicals in the present experiments were

analytical grade and used without any further puri?cation.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?lm modi?cation. 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-K R radiation (λ)1.5418?).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.Modi?cation of Electrodes.The process of ITO sub-strates modi?ed with KLTO thin?lms was similar as Sasaki’s work.28In detail,50mg of KLTO was mixed with2.5mL of n-propylamine(C3N)and2.5mL of redistilled water.After ultrasonication of the mixture for25min,a homogeneous and stable suspension was obtained,and KLTO was exfoliated into nanosheets(referred to as KTO).The primed ITO electrodes (0.25cm2exposed for modi?cation)were?rst dipped into1% poly(diallyldimethylammonium)(PDDA)chloride for8min, and the substrates were coated with positively charged PDDA (PDDA/ITO).Subsequently,the PDDA/ITO electrodes were transferred into the KTO suspension for another8min to adsorb titanate nanosheets based on the electrostatic interaction between the positively charged PDDA and negatively charged KTO nanosheets.Multilayer?lms 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 immers-ing into another solution.The?nal(KTO/PDDA)n/ITO?lms were ready for use.

Electrodeposition of pure Cu,Ni,and CuNi bimetallic nanostructured materials on(KTO/PDDA)n/ITO?lms was carried out in0.05M CuSO4,0.05M NiCl2,and0.05M CuSO4 +0.05M NiCl

2

in separate0.05M Na2SO4solutions.The reduction potentials applied were-0.8,-1.0,and-1.2V(vs SCE).The electrodeposition process was performed in a stationary electrolyte solution without stirring.The?nal elec-trodes were referred to as M/(KTO/PDDA)n/ITO(M)Cu,Ni, or CuNi).

2.4.Electrochemical Measurements.Electrochemical im-pedance spectroscopy(EIS)measurements were performed in 0.1M KCl solution with the presence of1mM[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 was5mV,and the frequency range was from100mHz to100kHz.The EIS data were simulated using the software Zview.

NaOH(0.1M)was used as the supporting electrolyte in the measurements of M/(KTO/PDDA)n/ITO?lm electrodes for electrocatalysis toward small organic molecules(including glucose,ethanol,and other carbohydrates).

3.Results and Discussion

3.1.Electrochemical Characterization of the Self-As-sembled KTO Films.3.1.1.Electrochemical Impedance Spec-troscopy(EIS)Analysis.EIS is a well-known effective means of probing the features of surface-modi?ed electrodes,and the respective semicircle diameter corresponds to the electron-transfer resistance at the electrode surface.31,32EIS was applied in this study to investigate the self-assembled processes and electron transfer of the KTO?lms.Signi?cant differences in the impedance spectra were observed during stepwise modi?ca-tion of the self-assembled KTO?lms,as depicted in Figure1. 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.32On immobiliza-tion of the self-assembled KTO?lms on ITO,the diameter of the Nyquist plot increased sharply when the self-assembled KTO layers(n)changed from1to4,whereas when n increased from 4to8,the diameter of the Nyquist plot kept almost constant. With the thickness of KTO?lms further increased,the plot diameter increased slightly.The EIS results of KTO self-assembled?lms were similar to those of La2/ 3Li0.15Ti0.85Al0.15O3,33suggesting the successful construction of self-assembled KTO?lms on the electrode surface28

The suitable equivalent circuit,which models the above results,is also shown in Figure1(inset).From the simulation with the software Zview,the self-assembled KTO?lms exhibited a stable resistance at4

3.1.2.Cyclic Voltammetric Beha W ior in0.1M KCl Solution Containing1mM[Fe(CN)6]3-/4-.The cyclic voltammetric behavior of KTO self-assembled?lms in0.1M KCl

solution Figure1.EIS of(KTO/PDDA)n/ITO(n)0-15)in0.1M KCl containing1mM[Fe(CN)6]3-/4-.Inset:the suitable equivalent circuit of the?lms.

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containing1mM[Fe(CN)6]3-/4-was investigated(Figure S2, Supporting Information).Well-de?ned redox peaks,the oxida-tive peak at about+0.3V and the reductive peak at around +0.15V,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,exhibit-ing a diode-like phenomenon.34,35This 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 in?uence of positively charged PDDA that affected the double-layer capacitance and the interfacial electron-transfer process.

3.2.Optimization of Metal Electrodeposition and Prepa-ration of CuNi on Self-Assembled KTO Films.3.2.1.The In?uence of Self-Assembled KTO Layers on Electrodeposition.

As described above,ITO surfaces could be successfully modi?ed with self-assembled KTO?lms,which are expected as promising catalyst supports.To investigate the effect of the thickness of KTO?lms on metal electrodeposition,the electrodeposition of Cu in0.05M Na2SO4solution containing0.05M CuSO4onto ITO surfaces precoated with various KTO layers was?rst attempted as the model system.In all cases,a constant amount of Cu(controlled by the coulombs consumed during elec-trodeposition process)was deposited onto the separate KTO ?lms 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 dif?culty in obtaining the real surface area and/or the electroactive area of the coelectrodeposited bimetallic phases that are normally believed to be inhomogeneous and https://www.360docs.net/doc/4d6538774.html,par-ing the oxidative peak potential or the corresponding current of glucose oxidation at the supported Cu(constant deposition amount)electrodes prepared on the separate carrier?lms with KTO layers of1-10(Table S1,Supporting Information),we found that the electrodeposited Cu on a carrier?lm with KTO layers of6exhibited the higher current toward the electrochemi-cal oxidation of glucose.On consideration of the stable resistance of the KTO?lms with six self-assembled layers (Figure S1,Supporting Information)and above higher electro-chemical oxidation current of the supported Cu,a carrier?lm with six KTO layers was adopted as support for electrodepo-sition 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 electro-chemical oxidation properties of the nanosized bimetallic?lms prepared at different electrodeposition times was carried out (Figure S3,Supporting Information).Obviously,the oxidative current of glucose at the electrode obtained at80s was almost 5times of that at10s,while the deposition time increased from 80to180s;the oxidative currents were almost constant.When the deposition time was prolonged to500s,the oxidative current increased only1.07times compared with that at80s.That is, a minor deposition time in?uence on the electrochemical property of the deposited electrode was found when the deposition time was longer than80s.It was reasonable that the substrate was less affected with more Cu deposited onto the KTO?lms,and when Cu was overdeposited onto the KTO ?lms;the resulting metal?lms were almost continuous rather than micro/nanostructures.In this study,80s was selected as the optimal deposition time for metal loading on the self-assembled KTO?lms.

3.2.3.Preparation of Bimetallic CuNi Nanostructures on KTO Films.As mentioned above,mixed3d transition-metal oxides are more active in catalysis than the single ones,5as demonstrated by the enhanced electrocatalytic activities toward small organic molecules of Cu-based electrodes doped with other transition-metal/metal oxides.10,36The 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-modi?ed 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 bene?t 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,19,37for obtaining bimetallic CuNi nanostructured materials by coelectrodeposition;the elec-trodeposition processes were carried out in0.05M Na2SO4 solution containing CuSO4and NiCl2at separate reduction potentials of-0.8,-1.0,and-1.2V(vs SCE).Two main parameters,that is,reduction potential and molar ratio of Cu2+ and Ni2+in the electrolyte,that in?uence the components and electrochemical properties of the resulting materials were optimized(Table S2,Supporting Information).In experiments, we found that CuNi?lms coelectrodeposited under the condition of(C Cu2+:C Ni2+)1,E red)-1.0V,deposition time)80s) exhibited the highest electrochemical response toward glucose oxidation.The net oxidative current of glucose on this bimetallic CuNi electrode was1.43and3.98times those on the corre-sponding pure Cu and Ni?lm electrodes with a similar amount of metal loading on separate self-assembled KTO supports (Figure2).It was2.39times that on the supported CuNi on a bare ITO surface without the KTO carrier?lms(not shown in Figure2)and4.63times that on the Cu/KTO-intercalation electrode in our previous report.30The high electrochemical response of CuNi nanostructures supported on the self-assembled KTO?lms toward glucose oxidation was possibly attributed to the electrochemical combination of Cu and Ni bimetal compo-nents and the related electronic effect.3-

6

Figure2.Steady-state current-time response of CuNi/KTO/ITO(a), Cu/KTO/ITO(b),and Ni/KTO/ITO(c)in0.1M NaOH with consecu-tive injection of1.0×10-4M glucose at+0.55V.

Dendritic Bimetallic Nanostructures on Titanate Films J.Phys.Chem.C,Vol.114,No.49,201020927

https://www.360docs.net/doc/4d6538774.html,ponents,Structures,and Morphologies of CuNi Materials Supported on the KTO Films.The molar ratio of composed elements of CuNi supported on the KTO ?lms 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 -1.2V 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 ?lms.The CuNi phase similar to Kang’s result 11was distin-guishable 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 ?lms and supported CuNi on KTO ?lms were characterized by SEM (Figure 4).The rough self-assembled KTO ?lms were attached onto the ITO surface (Figure 4a),implying a promising support for dispersing catalysts.Codeposition of CuNi on the self-assembled KTO ?lms resulted in a compact bimetallic coating with different grain sizes cross-linked over the surface.In addition,some of them were combined and formed lea?ike and/or dendritic nanostructures (Figure 4b and inset),which were quite different from the supported pure Cu on KTO ?lms,as shown in Figure 4c.The physical property changes of the Cu-based materials by doping of Ni have already been reported.10In the meantime,we found that the oxidation current of glucose at CuNi/KTO/ITO was much higher than that of either CuNi/Cu 11(Table S4,Supporting Information)or CuNi/ITO (mentioned above),suggesting an important role of the self-assembled KTO ?lms for loading a bimetal.

3.4.Electrochemistry and Electrocatalysis of the Sup-ported CuNi on Self-Assembled KTO Films.3.4.1.Cyclic Voltammetric Beha W ior of CuNi/KTO Films.The electrochemi-cal behavior of the CuNi/KTO/ITO electrode was characterized by cyclic voltammetry.Continuous cyclic voltammograms (CVs)of the CuNi/KTO/ITO electrode in 0.1M NaOH are shown in Figure S4(Supporting Information).As indicated in our studies and other publications,pure Cu has no obvious redox peaks in 0.1M NaOH in the potential range (inset in Figure S4,Supporting Information).At the KTO-supported CuNi bimetallic electrode surface,a Ni(OH)2layer was rapidly formed at low oxidation potential during the positive scan,leading to a Cu-rich metal surface that was subsequently oxidized to Cu 2O 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)2and CuO could be further oxidized to the Cu(III)oxide,as indicated by Druska and Jafarian.38,39Therefore,the redox

peaks at this supported CuNi electrode surface revealed in Figure S4(Supporting Information),the oxidative peak at around +0.58V and the reductive peak at around +0.32V,could be assigned to the nickel hydroxide/nickel oxyhydroxide redox couple,that is,Ni(OH)2/NiOOH,in the bimetal composite.10,40The elevated baseline current in the potential range of +0.20to +0.55V should be associated with the co-oxidation of Cu and Ni

metal

Figure 3.XRD pattern of CuNi/KTO/ITO obtained at -1.0

V.

Figure 4.Morphologies of KTO/ITO (a),CuNi/KTO/ITO (b),and Cu/KTO/ITO (c)obtained at -1.0V.The insets are local SEM images of corresponding ?lms.

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surfaces at low oxidation potential to their corresponding lower oxidative states,such as Ni(OH)2and 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 ef?cient strategy of enriching the electrochemical active surface area of the KTO-supported bimetallic composite elec-trode 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?lm 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;after50continuous potential scannings,the redox current was found without any decay.Therefore,the KTO-supported 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 men-tioned 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-0.1to+1.0V,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+0.25to+0.8V was observed at the CuNi/ KTO/ITO electrode with two oxidation peaks around+0.50and +0.65V(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.36As revealed by Figure S5(Supporting Information),the oxidized potential of glucose at the CuNi/KTO/ITO electrode in alkaline solution started from+0.25V and the major oxidative process fell into the potential range of+0.25to+0.80V,which was the potential range involved with the formation of Cu(III)and Ni(III),as discussed above.Therefore,the second oxidation peak around+0.65V was characterized by the mechanism involving electron-transfer mediation by a Ni(OH)2/NiOOH redox couple in the bimetallic?lms at the anodized electrode surface.39The ?rst oxidation peak around+0.50V 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 electrodepo-sition processes should also be bene?cial,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 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?lms.

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,22,30ascribing 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.30The 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’?ow 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 after10continuous potential scannings, implying a good stability of this CuNi electrode in glucose oxidation processes.Promising stability for carbohydrate oxida-tion on Ni-modi?ed 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 ampero-metric 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 hydrody-namic voltammogram of the CuNi/KTO/ITO electrode in the presence of1.0mM glucose in the stirring electrolyte solution; the current response was measured at a constant interval of0.05 V in the potential range of+0.25to+0.70V.With the oxidation potential positively shifted,the oxidation current of1.0mM glucose at the CuNi/KTO/ITO electrode increased,while the signal-to-background(S/B)ratio(curve b in Figure S6,Sup-porting Information)initially kept on rising,reached the topmost value at+0.55V,and then sharply decayed when the potential was more positive than+0.55V due to the dramatic increase in the baseline current at high oxidation potential.For a high sensitivity determination of glucose,a constant potential of +0.55V was selected and employed in all the subsequent amperometric detections.

3.5.2.Amperometric Analysis.Figure S7(Supporting Infor-mation)shows the typical steady-state current-time response of the CuNi/KTO/ITO electrode in0.1M NaOH with consecu-tive injection of1.0×10-5M glucose at+0.55V.The response time(from glucose injecting to reaching95%of the steady-state current)was within5s,suggesting a facile electron-transfer process through the CuNi/KTO?lms.The CuNi/KTO/ITO electrode displayed a well-de?ned concentration dependence. The relationship between the electrocatalytic current and glucose

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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 1.0×10-6to 5.0×10-4M (correlation coef?cient )0.995).The detection limit of glucose at the electrode was 3.5×10-7M (S/N )3).The sensitivity of the present nonenzymatic glucose sensor was 661.5μA mM -1(apparent electrode surface area is 0.25cm 2),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 +0.55V,seven repetitive tests of 1.0×10-5M glucose were carried out in 0.1M NaOH solution,and the relative standard deviation (RSD)in the determination was 5.12%.Three CuNi/KTO/ITO elec-trodes were fabricated in the same manner and utilized for detection of 0.1mM glucose,and an RSD of 5.26%was obtained.The above results demonstrated a satis?ed 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 satis?ed reproducibility for sensor fabrication suggest that the CuNi/KTO/ITO electrode is a promising candidate for ampero-metric enzymeless glucose sensors for potential applications.3.5.3.Interferential Analysis.The interference tests were carried out in 0.1M NaOH solutions containing 2.0×10-4M glucose at +0.55V in the presence of 0.2times of ascorbic acid (AA),0.8times of uric acid (UA),50times of ethanol,and 250times of sodium chloride (SC),respectively.The results

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 electrocatalyti-cally 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

5.0mL of 0.1M NaOH at +0.55V.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

The facile exfoliation of layered titanate with n -propylamine and successful formation of self-assembled titanate thin ?lms were accomplished.Bimetallic CuNi nanostructures were co-electrodeposited on the self-assembled titanate ?lms in 0.05M Na 2SO 4solution containing Cu 2+and Ni 2+,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 -1.0V 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 ?lms at +0.55V revealed a high sensitivity and good reproducibility in glucose determination.A wide linear range of 1.0×10-6to 5.0×10-4M and a low detection limit of 3.5×10-7M (S/N )3)were obtained.Results suggest that dendritic CuNi bimetallic microstructures supported on the self-assembled KTO ?lms 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.21075048and 20543003,as well as the Scienti?c Research Foundation for Returned Overseas Chinese Scholars,Ministry of Education of China.

Supporting Information Available:The R ct and CVs of the (KTO/PDDA)n /ITO ?lms depend on the different KTO layers;the oxidative currents depend on the deposition time.CVs of the CuNi/(KTO/PDDA)6/ITO ?lm electrode in 0.1M NaOH without and with glucose were tested in our current work.These materials and other ?gures and tables are available in the Supporting Information.This material is available free of charge via the Internet at https://www.360docs.net/doc/4d6538774.html,.References and Notes

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Dendritic Bimetallic Nanostructures on Titanate Films J.Phys.Chem.C,Vol.114,No.49,201020931