Electrochemical Properties of a Boron-Doped Diamond Electrode Modified with Gold/Polyelectrolyte Hollow Spheres

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Electrochemical Properties of a Boron-Doped Diamond ElectrodeModified with Gold/Polyelectrolyte Hollow Spheres

Min Wei,a Zhuoying Xie,a Liguo Sun,a Zhong-Ze Gua, b*a State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Nanjing

210096, P. R. Chinab Suzhou Key Laboratory of Environment and Biosafety, Research Institute of Southeast University in Suzhou, Dushu Lake Higher

Education Town, Suzhou 215123, P. R. China*e-mail: [email protected]

Received: September 15, 2008Accepted: October 29, 2008

AbstractOppositely charged polyelectrolyte (poly(allyamine hydrochloride) (PAH) and poly(sodium 4-styrene-sulfonate)(PSS)), and negatively charged gold nanoparticles (Au) were assembled alternately on polystyrene (PS) spheres vialayer-by-layer technique, and the different PAH/(PSS/PAH)n/(Au/PAH)m/Au composite hollow spheres were derivedby dissolving PS core. These hollow spheres were used to modify boron-doped diamond (BDD) electrodes forelectrochemical sensors. The cyclic voltammetric results for dopamine (DA) detection demonstrated that hollow-sphere-modified BDD exhibited better electrocatalytic activity than did bare BDD. Influence of the wall thicknessand composition of hollow spheres on electrochemical properties were investigated. The results showed that theoxidative peak potential of DA and the peak current varied with different PSS/PAH and Au/PAH layers. Theoptimized wall structure of hollows spheres was PAH/(PSS/PAH)7/(Au/PAH)5/Au.

Keywords: Boron-doped diamond, Layer-by-layer technique, PAH/(PSS/PAH)n/(Au/PAH)m/Au hollow spheres,Electrochemical sensor

DOI: 10.1002/elan.200804411

1. Introduction

In recent years, there is considerable interest in the synthesisof organic/inorganic nanocomposite hollow spheres due totheir specific structure, unique properties, and increasingapplications in drug delivery, catalysis, chemical storage andoptoelectronics [1 – 3]. Gold nanoparticles (Au) were widelyused to fabricate core-shell and hollow spheres owing totheir favorable properties such as high biocompatibility,good conductivity, and distinctive chemical and opticalbehaviors. There have been several reports on preparing Au/organic composite hollow spheres using various methods[4 – 9]. Among a number of methods, layer-by-layer (LBL)assembly is a good approach for fabricating hollow spheredue to its ability for precise control of the thickness andcomposition. These advantages make it attractive in broadapplications such as biotechnology, medicine and otherfields [10 – 14]. Au/organic composite core-shell and hollowspheres have been fabricated by LBL technique for variousapplications [15 – 18], but to our knowledge, Au/polyelec-trolyte (PE) hollow spheres have not yet been used tomodify electrode for electrochemical sensor.

In the past few decades, boron-doped diamond (BDD)electrode attracted much attention due to its superiorproperties such as wide potential window, low backgroundcurrent, high sensitivity and long-term stability [19 – 23].However, the hydrogen-terminated surface of as-grown

BDD is chemically inert, which may limit the utility in someways. Therefore, chemical modification of BDD surface isnecessary to improve its behavior for sensor application.

Surface modification can favor the electrode with addi-tional functions such as reducing the overpotential, enhanc-ing the electrocatalytic activity, and improving the selectiv-ity. For example, we recently used Au/PE coated polystyr-ene spheres to modify BDD electrode for the separation ofdopamine (DA) from the interfering ascorbic acid (AA) inthe electrochemical detection [24]. According to literatures,properties of hollow nanostructure are always superior totheir solid counterparts. For example, Kumar et al. [25]found that gold hollow microspheres could improve theextent of ligand binding and detection sensitivity, ascompared with the solid gold nanoparticles and the gold-coated AgCl nanoparticles. Liang et al. [26] demonstratedthat the hollow Pd nanostructure is superior to solid Pdnanoparticles, Pd microparticles, and planar Pd for proton/hydrogen sensing. Wen et al. [27] indicated that the Pt/hollow carbon spheres electrode showed better electro-catalytic activity and higher stability than did Pt/carbonmicrospheres and Pt/commercial carbon electrode formethanol oxidation.

For the real application of hollow-sphere-modified elec-trode, it is important to investigate the effect of thicknessand composition of the hollow sphere on their electro-chemical properties for the optimization of electrode. In this

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Electroanalysis 2009, 21, No. 2, 138 – 143 � 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

work, LBL method was adopted to control the wall thick-ness and composition of Au/PE hollow spheres. Theirinfluences on the electrochemical properties of hollow-sphere-modified BDD were investigated by discussing theelectrochemical behaviors of DA.

2. Experimental

2.1. Chemicals and Apparatus

Dopamine hydrochloride was purchased from Sigma.Poly(allyamine hydrochloride) (PAH) and poly(sodium 4-styrene-sulfonate) (PSS) were purchased from Aldrich. Allother chemicals were of analytical reagent grade and Milli-Q water was used throughout the experiments. The sup-porting electrolyte was 0.07 M phosphate buffer solution(PBS) with pH 7.0.

Electrochemical measurements were performed onIM6ex instrument (Zahner Elektrick, Germany) with athree electrode electrochemical cell. BDD films weredeposited on Si(111) wafers in a microwave plasma chemicalvapor deposition system (Astex Corp.) using a mixture ofacetone and methanol (9 : 1, v/v) as the carbon source and B2

O3 as a boron source. The details of the preparation aredescribed elsewhere [28]. BDD electrodes were sonicatedsuccessively in 2-propanol and Milli-Q water (>18 MW cm)before use. The geometric area of BDD electrode was0.07 cm2. A saturated calomel reference electrode (SCE)and a Pt wire counter electrode were used. All measure-ments were performed at room temperature. The solutionsused were deoxygenated with N2 for 10 min and maintainedunder nitrogen atmosphere during measurement.

The prepared materials were characterized by transitionelectron microscope (TEM, Philips, JEM-100CX), SEM (S-3000N, HITACHI, Japan) and Fourier transform infraredspectra (FTIR, Bruker company, Germany. IFS66/s).

2.2. Preparation of Au/PE Nanocomposite HollowSpheres and Modification of BDD Electrode

Monodisperse polystyrene spheres (PS, 250 nm) as templatewere synthesized by the soap-free emulsion polymerization[29]. Gold nanoparticle (Au, 2 – 5 nm) were prepared bysodium borohydride reduction. The PAH/(PSS/PAH)n/(Au/PAH)m/Au multilayer coating PS spheres were obtained byLBL assembly following our reported approach [24] and thecorrelative literatures mentioned there, which is as follows.First, PAH/(PSS/PAH)n multilayer were alternately ad-sorbed onto the negatively charged PS template. Theconcentration of PAH solution and PSS solution used forabsorption was 1 mg/mL in ultrapure water containing0.5 M NaCl. Then, the spheres were deposited alternatelyusing Au nanoparticles and PAH to form (Au/PAH)m/Aumultilayer. Each adsorption time was 30 min and the excesssubstances were removed by four repeated centrifugation /washing/redispersion cycles. The PS spheres coated by PAH/

(PSS/PAH)n/(Au/PAH)m/Au multilayer were obtained fol-lowing the above steps. Au/PE nanocomposite hollowspheres were finally obtained by removing PS core usingN, N-Dimethylformamide (DMF) solution.

Modification of BDD surfaces was performed by immers-ing BDD electrode into the solution of Au/PE hollowspheres for 12 h. Then BDD electrode was taken out anddried naturally. Following the above-mentioned methods,we obtained 5 types of modified BDD electrodes:– PAH/(PSS/PAH)5/(Au/PAH)3/Au hollow spheres modi-

fied BDD (BDD1)– PAH/(PSS/PAH)7/(Au/PAH)3/Au hollow spheres modi-




fied BDD (BDD5).

2.3. Preparation of the (PAH/PSS)n/(PAH/Au)m

Multilayer Film-Coated BDD Electrode

The as-grown BDD electrode was treated electrochemicallyin 0.1 M NaOH solution by applying a potential of þ2.6 V(vs. SCE) for 75 min to form negative charge on the BDDsurface. Then the BDD electrode with negatively chargedsurface was dipped alternately in PAH and PSS or Au colloidsolutions for 30 min and then rinsed with water for 1 min.The (PAH/PSS)3/(PAH/Au)4 multilayer film-coated BDDand the (PAH/PSS)7/(PAH/Au)6 multilayer film-coatedBDD were prepared by repeating the above procedure.

3. Results and Discussion

3.1. Characterization of Au/PE Nanocomposite HollowSpheres

Scheme 1 outlines the fabrication procedure of Au/PEnanocomposite hollow spheres with various compositionand thickness. Figure 1 shows the representative TEMimages of the samples at different stages, which are PSspheres (a), PS/PAH/(PSS/PAH)7/(Au/PAH)3/Au spheres(b) and PAH/(PSS/PAH)7/(Au/PAH)3/Au hollow spheres(c). As shown in Figure 1a, the obtained PS spheres hadsmooth surfaces with an average diameter of 250 nm. Fromboth the uniformity of multilayer coating and the increase ofsurface roughness of Figure 1b, it can be concluded that theLBL assembly gave well coated spheres, which maintainedthe spherical shape of the neat PS spheres and resulted in anincrease in the overall diameter of the spheres. The darkspots on the surface of Figure 1b and c proved the existenceof Au. It can be seen that the PS core were successfullyremoved from opacity of Figure 1b to transparency ofFigure 1c. Figure 1d shows the SEM images of PAH/(PSS/PAH)7/(Au/PAH)3/Au hollow spheres deposited on the

139Electrochemical Properties of a Boron-Doped Diamond Electrode

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BDD electrode surface, and we can see that almost nohollow spheres collapse at the BDD surface.

FTIR spectra shown in Figure 2 demonstrated the re-moval of the PS core. From the spectra of PS (Fig. 2a), thetypical PS absorption bands at 1494 cm�1, 1452 cm�1,757 cm�1 and 698 cm�1 are clearly seen [6, 30]. Theadsorption peak at 3030 cm�1 is C�H stretching vibrationof benzenoid ring, and the characteristic peaks around3000 cm�1 – 2800 cm�1 are saturated C�H stretching vibra-tion. In the spectra of Figure 2b, the adsorption peak at3445 cm�1 is assigned to �NH2 stretching vibration,1650 cm�1– 1590 cm�1 are the characteristic peaks of N�H,and 839 cm�1 is attributed to N�H out-of-plane bendingvibration. The adsorption peak at 1040 cm�1 is attributed toS¼O symmetrical stretching vibration, 1190 cm�1 is assignedto S¼O asymmetrical stretching vibration. All these absorp-tion peaks showed that PAH and PSS were depositedsuccessfully [31 – 34]. From the spectra of Figure 2c, it can beseen that after treatment by DMF solutions, the intensity ofPS characteristic peaks decreased dramatically, whichproved that the PS core has been successfully removed.

3.2. Influence of Multilayer on the ElectrochemicalProperties of Modified BDD

In order to confirm the effect of thickness and compositionof hollow spheres on the electrochemical properties, theelectrochemical behaviors of DA on the different hollow-sphere-modified BDD were investigated. Figure 3 showsthe cyclic voltammograms (CVs) obtained for oxidation of4� 10�4 M DA on bare and different modified BDD. Onbare BDD, the oxidation of DA occurred at 0.524 Vand thecurrent was 10.43 mA, indicating the sluggish electrocata-lytic process. However, on the different hollow-sphere-modified BDD, the current response of DA increased andthe oxidative peak potential shifted negatively, demonstrat-ing that the existence of Au/PE hollow spheres couldaccelerate electron transfer and enhance the electrocata-lytic activity. From the results of Figure 3A, on BDD1, theoxidative peak potential of DA shifted negatively to 0.435 Vand the oxidation current increased to 11.50 mA, exhibitingthe enhanced electrocatalytic behaviors for DA oxidation.When increasing the number of PSS/PAH layer, on BDD2and BDD3, the peak potential of DA oxidation occurredlikewise at 0.415 V, and the peak currents were 11.99 mA and

12.37 mA, respectively. Compared with BDD1, the slightlynegative shift of oxidative peak potential and the increase of

Scheme 1. Schematic illustration of the procedure for gold/polyelectrolyte hollow spheres.

Fig. 1. TEM images of a) PS spheres, b) PS/PAH/(PSS/PAH)7/(Au/PAH)3/Au spheres, and c) PAH/(PSS/PAH)7/(Au/PAH)3/Auhollow spheres. d) SEM image of PAH/(PSS/PAH)7/(Au/PAH)3/Au hollow spheres deposited on the BDD electrode surface.

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peak current may be ascribed that the appropriate increaseof PSS/PAH layer could accordingly enhance the structurestability. As shown in Figure 3B, compared with BDD2,when increasing number of Au/PAH layer, on BDD4 andBDD5, the peak potentials for DA oxidation occurred at0.385 V and 0.395 V, and the peak currents were 13.84 mAand 13.24 mA, respectively, demonstrating that Au/PAHlayer facilitated the oxidation of DA. The slight shift ofoxidation peak potential in positive direction and thedecrease of peak current from BDD4 to BDD5 wereprobably due to an increase in the film coverage, which inturn increased the charge-transfer resistance and sup-pressed diffusion in the thicker films, resulting in a lowervalue of the diffusion coefficient [35 – 38]. By investigatingthe effect of different PSS/PAH and Au/PAH layers onelectrochemical properties, it was found that BDD4 modi-fied with PAH/(PSS/PAH)7/(Au/PAH)5/Au hollow sphereswas optimum for DA detection.

Figure 3C shows a comparison between multilayer hollowsphere-modified BDD and multilayer film-coated BDDtoward DA detection. It can be seen that the response at the(PAH/PSS)3/(PAH/Au)4 multilayer film-coated BDD wasseverely decreased (dash line), when increasing multilayerfilm (dot line), the response at the (PAH/PSS)7/(PAH/Au)6

multilayer film-coated BDD was suppressed further, whichwas ascribed that a simple multilayer film deposited on theBDD surface via layer-by-layer method introduced insulat-ing layers with poor conductivity and severe diffusionbarrier.

We further investigated the CVs of DA oxidationobtained on bare BDD and BDD4 with increasing concen-tration at a fixed scan rate of 50 mV s�1. From the results ofFigure 4A and 4B, it can be found that the oxidative peakcurrents increased with adding DA concentration on the twoelectrodes. The oxidation peak potentials shifted in morenegative direction, and the oxidation peak responses werehigher on BDD4 than those on bare BDD, demonstrating

that BDD4 showed better electrocatalytic activity andhigher sensitivity than did bare BDD for DA detection.Figure 4C showed the dependence of peak currents on DAconcentration on the two electrodes. Sensitivities corre-sponding to the linear range (5 – 200 mM) for bare BDD andBDD4 were 357.14 mA M�1 cm�2 and 514.29 mA M�1 cm�2,

Fig. 2. FTIR spectra of a) PS spheres, b) PS/PAH/(PSS/PAH)7/(Au/PAH)3/Au spheres, and c) PAH/(PSS/PAH)7/(Au/PAH)3/Auhollow spheres.

Fig. 3. Cyclic voltammograms (CVs) for 4� 10�4 M DA in0.07 M PBS (pH 7) on bare and different modified BDD(0.07 cm2). Scan rate was 50 mV s�1.




respectively. And the detection limits were 1.4 mM and0.6 mM according to the calculated formula 3sb/m criteria[39].

4. Conclusions

In this work, a series of PAH/(PSS/PAH)n/(Au/PAH)m/Aumultilayer hollow spheres were prepared using LBL assem-bly and applied to modify BDD electrodes. The hollow-sphere-modified BDD could accelerate the electron trans-fer, and enhance the electrocatalytic activity for DAdetection, as compared with bare BDD. By investigatingthe influence of the wall thickness and composition onelectrochemical properties, it was found that an appropriatedesign of PSS/PAH and Au/PAH layers of the hollowspheres is important to optimize the electrocatalytic activityof electrode.

5. Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (Grant No. 50533030, 60121101). Wethank Prof. Yasuaki Einaga from Keio University (Japan)for providing diamond electrodes.

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Electrochemical Properties of a Boron-Doped Diamond Electrode Modified with Gold/Polyelectrolyte Hollow Spheres