Selective Determination of Dopamine on a Boron-Doped Diamond Electrode Modified with Gold Nanoparticle/Polyelectrolyte-coated Polystyrene Colloids

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DOI: 10.1002/adfm.200701099

Selective Determination of Dopamine on a Boron-DopedDiamond Electrode Modified with Gold Nanoparticle/Polyelectrolyte-coated Polystyrene Colloids**

By Min Wei, Li-Guo Sun, Zhuo-Ying Xie, Jin-Fang Zhii, Akira Fujishima, Yasuaki Einaga,De-Gang Fu, Xue-Mei Wang, and Zhong-Ze Gu*

egatively charged gold nanoparticles (AuNPs) and a polyelectrolyte (PE) have been assembled alternately on a polystyrene

PS) colloid by a layer-by-layer (LBL) self-assembly technique to form three-dimensional (Au/PAH)4/(PSS/PAH)4 multilayer-

oated PS spheres (Au/PE/PS multilayer spheres). The Au/PE/PS multilayer spheres have been used to modify a boron-doped

iamond (BDD) electrode. Cyclic voltammetry is utilized to investigate the properties of the modified electrode in a 1.0 M KCl

olution that contains 5.0� 10�3M K3Fe(CN)6, and the result shows a dramatically decreased redox activity compared with the

are BDD electrode. The electrochemical behaviors of dopamine (DA) and ascorbic acid (AA) on the bare and modified BDD

lectrode are studied. The cyclic voltammetric studies indicate that the negatively charged, three-dimensional Au/PE/PS

ultilayer sphere-modified electrodes show high electrocatalytic activity and promote the oxidation of DA, whereas

hey inhibit the electrochemical reaction of AA, and can effectively be used to determine DA in the presence of AA

ith good selectivity. The detection limit of DA is 0.8� 10�6M in a linear range from 5� 10�6 to 100� 10�6

M in the presence

f 1� 10�3M AA.

1. Introduction

Dopamine (DA) is an important neurotransmitter and plays

a significant role in the function of the central nervous, renal,

and hormonal systems. The loss of DA may result in serious

diseases such as Parkinson’s disease.[1–3] Hence, research on

the determination of DA is significant in many fields such as

biochemistry and medicine. Among various methods reported

for its determination, electrochemical techniques are prefer-

[�] Prof. Z.-Z. Gu, Dr. M. Wei, L.-G. Sun, Z.-Y. Xie, Prof. D.-G. Fu,X.-M. WangState Key Laboratory of BioelectronicsSoutheast UniversityNanjing 210096 (P.R. China)E-mail: [email protected]

Prof. J.-F. ZhiiTechnical Institute of Physics and ChemistryChinese Academy of SciencesBeijing 100080 (P.R. China)

Prof. A. FujishimaKanagawa Academy of Science and TedchnologyKSP, 3-2-1 Sakado, Kawasaki 213-0012 (Japan)

Prof. Y. EinagaDepartment of ChemistryFaculty of Science and TechnologyKeio University, 3-14-1 HiyoshiYokohama 223-8522 (Japan)

[��] This work was supported by theMinistry of Education of China (GranNo. 20040286024) and the National Natural Science Foundation oChina (Grant No. 60121101).

� 2008 WILEY-VCH Verlag Gm

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bH &

ential because of their advantages, which include high

selectivity, low cost, and rapid detection. However, a major

problem with the electrochemical detection of DA on bare

electrodes is the overlap of the voltammetric response of

interfering compounds such as ascorbic acid (AA).[4–9] In order

to selectively detect DA in the presence of AA, or simul-

taneously separate the oxidation potentials of DA and AA,

many studies have focused on the modification of the electrode

surface by utilizing various materials such as carbon mate-

rials,[10–14] polymers,[15–18] enzymes [19,20] and others.[21–25]

Boron-doped diamond (BDD) films have attracted a great

deal of attention as new electrode materials because of their

superior electrochemical properties, which include high

electronic conductivity, wide potential window, low back-

ground current, high sensitivity, and long-term stability.[26]

Several biological substances[27–30] have been detected with

BDD electrodes, and in most of these studies, the BDD

electrode has been demonstrated to be superior to the glass

carbon (GC) electrode and other electrodes in terms of high

signal-to-noise ratio, long-term stability, high sensitivity, and

good reproducibility. Roy et al.[31] also reported that the

sensitivity of a poly(N,N-dimethylacrylamide) (PDMA) film-

coated BDD electrode for DA determination in the presence

of a high concentration of AA is greater than that of the

PDMA film-coated GC electrode.

Au nanoparticles (AuNPs) or clusters have received

considerable attention in recent years and are very promising

for practical applications by virtue of their superior peculia-

rities, which include size-dependent unique chemical, elec-

Co. KGaA, Weinheim Adv. Funct. Mater. 2008, 18, 1414–1421

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M. Wei et al. / Selective Determination of Dopamine

Figure 1. Wide scanning XPS spectra (A) and N 1s spectra (B) of theABDD surface (black solid line) and HBDD surface (gray dash line).

trical, and catalytic properties, and good stability and

biocompatibility. Much research has been reported on the

use of AuNP-modified electrodes to determine DA and

AA.[32–37] Although these methods have been successful in

introducing the AuNPs onto the electrode surface by

electrochemical deposition, or a self-assembled monolayer

by immobilization with –SH or –NH2 groups, in most cases,

the obtained AuNPs layers are one-dimensional or two-

dimensional arrays with a low adsorption capacity and low

specific surface area on the electrode surface. This blocks the

positive interaction between the AuNPs and DA or AA and

makes it difficult to effectively apply the AuNPs in catalysis or

biosensors. In contrast, three-dimensional negatively charged

AuNPs have unique characteristics that are different from the

one-dimensional or two-dimensional AuNPs with neutral or

positive charge assembled on the bare electrode.

The layer-by-layer (LBL) self-assembly technique based on

electrostatic interactions has attracted extensive interest owing

to its advantages such as simple operation of the procedure,

wide choice of the usable materials, and precise control of the

composition and the layer thickness on a molecular level.[38]

This technique provides a useful tool for the construction of

nanometer-scale assemblies of novel material systems.[39–41]

Several multilayer films based on LBL self-assembly have been

used to modify electrodes for the determination of DA and

AA,[10,37] but to our knowledge, Au/polyelectrolyte (PE)

nanocomposite multilayer shell–spherical colloid core particles

have not yet been developed using this technique to detect DA

and AA.

In the present work, negatively charged AuNPs and PEs

(poly(allyamine hydrochloride) (PAH) and poly(sodium

4-styrene-sulfonate) (PSS)) have been assembled alternately

on polystyrene (PS) colloid templates using the LBL assembly

to form three-dimensional negatively charged (Au/PAH)4/

(PSS/PAH)4 multilayer-coated PS spheres (Au/PE/PS multi-

layer spheres). We utilized these three-dimensional negatively

charged Au/PE/PS multilayer spheres to modify a BDD

electrode and studied its electrochemical catalytic activity

toward the oxidation of DA and AA, and further explored

possible applications for the selective determination of

DA in the presence of AA, which is different from the

one-dimensional or two-dimensional PE and AuNPs with

neutral or positive charge assembled on bare electrode.

2. Results and Discussion

2.1. Characterization of the Amine-terminated BDD

(ABDD) Surface

It is known that the surface of an as-grown diamond film is

normally hydrogen-terminated (HBDD), which is quite inert

chemically and is difficult to link with other molecules.[43]

Here, in order to obtain a nanofunctional macroscopic

interface on the BDD surface, an amine-terminated BDD

(ABDD) surface was obtained by plasma treatment.[44] X-ray

Adv. Funct. Mater. 2008, 18, 1414–1421 � 2008 WILEY-VCH Verl

photoelectron spectroscopy (XPS) was used to evaluate the

BDD surfaces before and after plasma treatment, and the

results are presented in Figure 1A and 1B. From the results of

the XPS spectra, an increase in peak intensity of the N 1s signal

centered at 399.9 eV compared with the untreated BDD

surface was observed after plasma treatment, which indicates

that the HBDD surface was successfully converted into an

ABDD surface. In order to further verify the results, the

contact angle was also tested before and after plasma treat-

ment. A water contact angle changed from 96 8 on a HBDD

surface to 68 8 on an ABDD surface, which demonstrates a

change in surface from hydrophobic to hydrophilic.

2.2. Characterization of Prepared PS and Au/PE/PS

Multilayer Spheres

The processes that include the fabrication of three-

dimensional negatively charged Au/PE/PS multilayer spheres

andmodification of the BDD electrode are shown in Scheme 1.

Figure 2 shows the representative scanning electron micro-

scopy (SEM) (Fig. 2a,b) and transmission electron spectro-

scopy (TEM) (Fig. 2c,d) images of the uncoated PS spheres

(Fig. 2a,c) and Au/PE/PS multilayer spheres (Fig. 2b,d). The

obtained PS spheres were basically uniform in size and mor-

phology, and had smooth surfaces with an average diameter of

ag GmbH & Co. KGaA, Weinheim www.afm-journal.de 1415

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Scheme 1. Schematic illustration of the formation of (Au/PAH)4/(PSS/PAH)4/PS multilayer spheres and modification of the BDD electrode.

1416

250 nm. The LBL deposition process gave uniformly coated

particles, which maintained the spherical shape of the neat PS

spheres. Both the uniformity of the multilayer coating and the

increase in surface roughness could clearly be seen on the

surface of the PS spheres, which proved the presence of theAu/

PE multilayer on the surface. Furthermore, deposition of the

(Au/PAH)4/(PSS/PAH)4 multilayer resulted in an increase in

the overall diameter of the particles.

2.3. Characterization of Bare and

Modified BDD Electrodes

Figure 3 shows the cyclic voltammograms (CVs) for HBDD

(black solid line), ABDD (gray solid line), Au/PE/PS

Figure 2. SEM (a,b) and TEM (c,d) images of PS spheres (a,c) and (Au/PAmultilayer spheres (b,d).

www.afm-journal.de � 2008 WILEY-VCH Verlag GmbH

multilayer-sphere-modified HBDD (black dash dot line),

and Au/PE/PS multilayer-sphere-modified ABDD (gray dash

line) electrodes in 1.0 M KCl aqueous solution that contained

5.0� 10�3M K3Fe(CN)6. On the HBDD and ABDD electro-

des, well-defined CVs of K3Fe(CN)6 were obtained, which

indicates nearly reversible or quasi-reversible electron transfer

kinetics for these electrode interfaces. The peak-to-peak

separations (DEp) on the HBDD and ABDD electrodes were

157 and 248mV, respectively. A decrease of current response

and an increase of the DEp on the ABDD electrode were

attributed to slow electron transfer rate as a result of the

presence of an amine layer on the ABDD surface, which serves

as a barrier layer and imparts a resistance to the electron

transfer. However, absolutely different phenomena were

H)4/(PSS/PAH)4/PS

& Co. KGaA, Weinheim

revealed on the modified HBDD and

modified ABDD electrodes. As shown in

Figure 3, the shapes of the CVs changed

from peak shaped to plateau shaped, and

the significantly decreased currents and

dramatically increased DEp were observed.

The results suggest that the Au/PE/PS

multilayer spheres on the electrodes pro-

vide a barrier to electron transfer and block

the electrochemical reactions on the elec-

trode surface. The remarkable decrease of

peak currents was ascribed to the abundant

AuNPs with negative charge, which can

repulse and interfere with the diffusion of

the negatively charged Fe(CN)63� towards

the electrode surface.[45,46] The multiple

structure of the modified materials was

composed of electric AuNPs and insulating

PE formed by a LBL assembly. Herein, the

construction of the assembly is important

because a degree of randomness inherent in

a self-assembly process would result in

defects, disorder, and percolation effects,

which may strongly influence the final

properties of the materials.[47] As such

the charge transfer rates were accordingly

Adv. Funct. Mater. 2008, 18, 1414–1421

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Figure 3. Cyclic voltammograms (CVs) for 5.0� 10�3M K3Fe(CN)6 in a

1.0 M KCl aqueous solution obtained on HBDD (black solid line), ABDD(gray solid line), (Au/PAH)4/(PSS/PAH)4/PS multilayer-sphere-modifiedHBDD (black dash dot line), and (Au/PAH)4/(PSS/PAH)4/PS multilayer-sphere-modified ABDD (gray dash line) electrodes, respectively. Scan ratewas 50mV s�1.

slower between the outer layers and the electrode surface as a

result of intervention at each stage of the self-assembly process

and obstruction of potentially insulting layers and complicated

architectures, which results in the increasingly separated and

wider CV peaks. These results combined with the above CV

Figure 4. CVs for 5� 10�4M DA (black solid lines) and 1� 10�3

M AA (gray daPAH)4/(PSS/PAH)4/PSmultilayer-sphere-modified HBDD, and D) (Au/PAH)4Scan rate was 50mV s�1.


data were consistently observed and have been reported in

other studies of multilayer films that contain AuNP/redox-

active linker units,[47–49] which highlights the significant effect

of nanostructured building blocks and complicated architec-

ture on influencing charge transfer. Similar results have been

reported for electrodes coated with oppositely charged PE

films that are alternately assembled onto the electrode surface,

in which the response of the Fe(CN)64�/3� redox couple was

severely suppressed when the surface was covered with a

negatively charged PE layer.[50,51] Here, the PS spheres used as

templates could change the three-dimensional structure on the

electrode surface, enhance the high surface area, and improve

the effect of Au and PE on the electrode surface. The loss in

redox activity indicated that the Au/PE/PS multilayer spheres

inhibit Fe(CN)63� from reaching the electrode surface as a

result of electrostatic repulsion, intricate nanostructure, steric

constraints and weakened double-layer fields.

2.4. Cyclic Voltammogram Behavior of DA and AA

Figure 4 compares the CVs obtained for oxidation of DA

and AA on the HBDD electrode (Fig. 4A), the ABDD

electrode (Fig. 4B), the Au/PE/PS multilayer-sphere-modified

shed lines) in 0.07 M PBS (pH 7.2) obtained on A) HBDD, B) ABDD, C) (Au//(PSS/PAH)4/PSmultilayer-sphere-modified ABDD electrodes, respectively.


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HBDD electrode (Fig. 4C), and the Au/PE/PS multilayer-

sphere-modified ABDD electrode (Fig. 4D) in 0.07M phos-

phate buffered solution (PBS) that contained 5� 10�4M DA

(black solid lines) and 1� 10�3M AA (gray dashed lines). On

bare HBDD and ABDD electrodes, the CVs of DA present

poor and irreversible waves with an oxidation peak potential at

0.524 and 0.548V and currents of 13.96 and 12.32mA,

respectively, which indicates the sluggish electrocatalytic

process. The observed slightly positive shift of the oxidation

potential and the decrease in the oxidation current of DA on

the ABDD electrode demonstrates that the amine-termination

serves as an effective and compact barrier layer. However,

on the Au/PE/PS multilayer-sphere-modified BDD electrodes,

the CVs exhibited favorable electrocatalytic behaviors for the

oxidation of DA. This could be a result of the three-

dimensional Au/PE/PS multilayer spheres on the electrodes,

which provide abundant negative charge, thus promote the

enrichment of cationic DA, improve the electron transfer, and

enhance the electrode sensitivity. Moreover, the enhancement

of catalytic activity towards DA is also attributable to the

three-dimensional surface structure, which might effectively

prevent poisoning of the electrode surface by the oxidation

products. As shown in Figure 4, the oxidation peak potential of

DA shifted from 0.524V on the bare HBDD electrode to

0.38V on the modified ABDD electrode, i.e., the oxidation

potential of DA shifted by 144mV in the negative direction,

which is evidence for the electrocatalytic oxidation of DA. In

comparison with the bare HBDD electrode, the oxidation

currents increased by 15% and 55% on the modified HBDD

and modified ABDD electrode, respectively, which indicates

that the modified ABDD electrode is superior to the modified

HBDD electrode. This conclusion might be ascribed to

the hydrophilicity and amine groups on the ABDD electrode

surface, which could preferably immobilize the nanocomposite

spheres.

It is well known that AA is negatively charged under

physiological conditions, so negatively charged substances

coated on the electrode surface will repel AA from the surface

by electrostatic repulsion, and then block the oxidative

reaction of AA. Some reports[21,36,52–54] on the blocking of

AA and selective differentiation of DA have investigated

utilizing various self-assembled monolayers (SAMs) with

negatively charged modified electrodes. Herein, novel materi-

als of three-dimensional, negatively charged Au/PE/PS multi-

layer spheres have been used to block AA from the electrode

surface. Figure 4 shows the CVs of oxidation for AA on

different electrodes. On a bare HBDD electrode, the CV

showed a prominent oxidation peak at 0.641V and the peak

current was 16.44mA, which was attributed to the electro-

chemical reaction for oxidation of AA. An obvious oxidation

peak also appeared at 1.145V on the bare ABDD electrode

and the peak current was 12.52mA. Compared with theHBDD

electrode, the obvious positive shift of the oxidation potential

and decrease in the oxidation current of AA on the ABDD

electrode is ascribed to the amine-terminated BDD surface,

which imparts resistance to the electron transfer. However, for


Au/PE/PS multilayer-sphere-modified electrodes, the electro-

chemical responses were significantly different. In comparison

to the CVs obtained on the bare BDDelectrodes, the oxidation

processes were obviously inhibited and the current decreased

dramatically with no or negligible peak for AA oxidation on

the modified electrodes. For the electrochemical oxidation of

AA, although one-dimensional Au exhibits good electrocata-

lytic activity,[35,36] three-dimensional negatively charged Au/

PE/PS multilayer spheres show an obvious blocking effect.

This may be a result of the high density of the negatively

charged AuNPs and the intricate three-dimensional structure

of Au/PE/PS multilayer spheres on the electrode, which blocks

access of the anionic AA to the electrode surface, thus has a

remarkably repulsive effect on the oxidation of AA. It was

also seen that the background current on the modified

ABDD electrode was lower than that on the modified HBDD

electrode, which demonstrates that the modified ABDD

electrode is preferable to the modified HBDD electrode for

determination of AA.

According to the results from Figure 4A–D, compared with

bare BDD electrodes, the observed negative shift of potential

and increase of current response for oxidation of DA on the

modified electrodes could be attributed to both the favorable

electrostatic attraction between the anionic Au/PE/PS multi-

layer spheres and cationic DA and the three-dimensional

structure of the modifiedmaterials that effectively prevents the

poisoning of the electrode surface by the oxidation products.

Whereas, for oxidation of AA on the modified electrodes, the

electrostatic repulsion between the negatively charged Au/PE/

PS multilayer spheres and anionic AA and the intricate

three-dimensional nanostructured building blocks of the

modified materials on the electrode surface result in no or a

negligible current peak.

2.5. Cyclic Voltammogram Behavior of DA and

AA Mixture

In the present study, we further investigated the CVs of a

mixture that contained DA and AA on bare and modified

BDD electrodes. Figure 5 shows the CVs obtained for

coexisting 5� 10�4M DA and 1� 10�3

M AA on bare HBDD

(black solid line), bare ABDD (gray solid line), Au/PE/PS

multilayer-sphere-modified HBDD (gray dash dot line), and

Au/PE/PS multilayer-sphere-modified ABDD (black dash

line) electrodes. As shown in Figure 5, the bare HBDD

electrode could not separate the voltammetric signals of AA

and DA, and only one broad current peak was obtained for the

mixture at about 0.6V due to overlap of the individual peaks.

On the ABDD electrode, two broad and ill-defined oxidative

current peaks were shown at about 0.54 and 1.14V, which

correspond to the oxidation of DA and AA, respectively, and

this behavior was attributed to the effect of amine groups on

the electrode surface. Although a large peak separation is

observed, it can be seen that the oxidation peak of AA is not

obvious because of interference from DA in the solution.

& Co. KGaA, Weinheim Adv. Funct. Mater. 2008, 18, 1414–1421

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Figure 5. CVs for a mixture that contains 5� 10�4M DA and 1� 10�3

M

AA in 0.07 M PBS (pH 7.2) on HBDD (black solid line), ABDD (gray solidline), (Au/PAH)4/(PSS/PAH)4/PS multilayer-sphere-modified HBDD (graydash dot line), and (Au/PAH)4/(PSS/PAH)4/PS multilayer-sphere-modifiedABDD (black dash line) electrodes, respectively. Scan rate was 50mV s�1.

Compared with the bare HBDD electrode, there was only one

voltammetric response at about 0.38V on the modified

electrodes, which was nearly the same potential as that in

single DA solutions. AA showed no voltammetric response on

themodified BDDelectrodes, which was consistent with that in

single AA solutions. The results demonstrate that the modified

electrodes could repel negativeAA in this mixed system, so the

Figure 6. A) CVs and B) calibration plots for an increasing concentrationof DA in the presence of 1� 10�3

M AA in 0.07 M PBS (pH 7.2) obtainedwith a (Au/PAH)4/(PSS/PAH)4/PS multilayer-sphere-modified ABDD elec-trode. Scan rate was 50mV s�1.


presence of the unoxidized AA would not cause any side

reactions. The oxidative peaks of DA are almost uninfluenced

by the presence of AA in the mixture solution, compared with

those in single DA solutions. The above results indicate that

electrostatic repulsion between the Au/PE/PS multilayer

spheres and AA is effective to block AA from the electrode

surface and eliminate any interfering factors. As such it is

feasible to selectively detect DA in the presence of AA using

Au/PE/PS multilayer-sphere-modified BDD electrodes.

Figure 6 shows the CVs and calibration plots for the oxidation

of different concentrations of DA in the presence of 1� 10�3M

AA at a fixed scan rate of 50mV s�1 on a (Au/PAH)4/(PSS/

PAH)4/PS multilayer-sphere-modified ABDD electrode.

Obviously, there is no voltammetric response for the oxidation

ofAA,whereas the oxidation peak currents ofDA increasewith

increasing concentration of DA while the concentration of AA

is kept constant. Even the presence of a high concentration

of AA did not interfere with the determination of a low

concentration ofDA. Linearitywas observedwithin the range of

(5–100)� 10�6M (r¼ 0.999). The detection limit of DA in the

presence of 1� 10�3M AA was �0.8� 10�6

M according to the

formula 3sb/m criteria,[55] which was lower than that obtained on

a tyrosinase-modified ABDD electrode and oligo(phenylene

ethynylene)-modified Au disk electrode,[19,21] which demon-

strates that the (Au/PAH)4/(PSS/PAH)4/PS multilayer-

sphere-modified ABDD electrode is a better choice for the

selective detection of DA in the presence of AA.

3. Conclusion

In the present work, AuNPs and PEs have been successfully

alternately assembled on a PS colloid using the LBL assembly

technique to form three-dimensional negatively charged (Au/

PAH)4/(PSS/PAH)4/PS multilayer spheres. The results

demonstrate that the prepared sphere-modified BDD electro-

des exhibit high electrocatalytic activities toward the oxidation

of DA, show almost no response for the oxidation of AA, and

display an effective determination of DA in the presence of

AA with good selectivity. Moreover, by comparing different

electrodes, it is concluded that the modified ABDD electrode

is the best choice for use in the determination of DA and AA.

The detection limit of DA was 0.8� 10�6M in a linear range

from 5� 10�6 to 100� 10�6M in the presence of 1� 10�3

MAA.

The preparation of the three-dimensional, negatively charged

Au/PE/PS multilayer spheres is not only useful in electro-

analytical chemistry but also helpful in other fields such as

understanding the optical properties of the three-dimensional

multilayer on the nanometer scale. Ongoing and further

studies will develop a variety of three-dimensional nanostruc-

ture-modified electrodes for electrochemical biosensors.

4. Experimental

Chemicals and Apparatus: DA was purchased from Sigma, andAA was obtained from Shanghai Bio Life Science & Technology Co.


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1420

Ltd, China. PAH and PSS were purchased from Aldrich. All otherchemicals were of analytical reagent grade and Milli-Q water was usedthroughout the experiments. The supporting electrolyte was 0.07M

PBS with pH 7.2.Electrochemical measurements were performed on IM6ex instru-

ment (ZAHNER elektrick, Germany) and a three electrode electro-chemical cell. The geometric area of the BDD electrode was 0.07 cm2.A saturated calomel reference electrode (SCE) and a Pt wire counterelectrode were used. All measurements were made at roomtemperature in solutions deoxygenated with N2 for 15min andmaintained under nitrogen atmosphere during measurement.

An amine-terminated BDD surface was obtained by plasmatreatment using a PLASMA CLEANER/STERILIZER (HARRICKPlasma, PDC-002, Harrick Scientific Corp., New York). XPS analysiswas carried out on a ESCALAB MK II X-ray photoelectronspectrometer. Spheres were observed by SEM (S-3000N, HITACHI,Japan) and TEM (Philips, JEM-100CX).

Preparation of the BDD Film: A microwave-assisted plasma

chemical vapor deposition (CVD) technique was employed to prepareBDD films on silicon (100) wafers using a commercial microwaveplasma reactor (ASTeX Corp., Woburn, MA) at 5 kW with high purityhydrogen as the carrier gas. First, the silicon substrates werehand-polished with diamond powder (0.5mm) for nucleation, theywere then rinsed with 2-propanol. The carbon source was a mixture ofacetone and methanol (9: 1, v/v). The boron source was B2O3, whichwas dissolved in the above-mentionedmixture at a B/Cmolar ratio of 1:100. After a 10 h deposition process, a BDD film thickness of �40mmwas achieved.

Preparation of PS Particles and AuNPs: Monodisperse PS spheres

(250 nm) were synthesized by a soap-free emulsion polymerization[42]. AuNPs (2–5 nm) were prepared by the reduction of chloroauricacid with sodium borohydride. First, 6mL of 1% HAuCl4 was addedinto 400mL of ultrapure water stored at 4 8C with vigorous stirring,followed by the addition of 2mL of an aqueous solution of K2CO3

(0.2 M). Under vigorous stirring, 4mL of a freshly prepared aqueoussolution of NaBH4 (0.5mg mL�1) was then quickly added to themixture 3–5 times until the solution turned wine-red in color.

Preparation of Au/PE/PS Multilayer Spheres: An LBL assembly

technique was used to prepare Au/PE multilayer shell–sphericalcolloid core particles. First, a precursor four-layer polymer film wasassembled onto the negatively charged PS particles, in order to providea higher charge density and a smooth PE surface to aid subsequentadsorption of AuNPs. The actual steps included the following: 200mLof the above PS spheres (3.87wt%, 250 nm) were dispersed in 5mL ofpositively charged PAH solution (1mg mL�1, which contained 0.5 M

NaCl). After 30min of adsorption, the excess PAH was removed bythree repeated centrifugation (13 000 rpm, 20min) and washing cycles.Negatively charged PSS solution (1mg mL�1, which contained 0.5 M

NaCl) was then deposited onto the coated PS particles using the sameconditions. NaCl was used to modify the ionic strength of the solutions.

The Au/PAH multilayer was deposited on the PS spheres by thefollowing processes: The PAH/(PSS/PAH)4-coated PS particles weredispersed in 5mL of a AuNP colloid solution. Forty minutes wasallowed for AuNP adsorption, and then excess AuNPs were removedby four repeated centrifugation (13 000 rpm, 20min), water washing,and redispersion cycles. Subsequently, PAH was absorbed in anidentical fashion. The desired number of Au/PAH multilayers weredeposited as described above. The negatively charged (Au/PAH)4/(PSS/PAH)4/PS multilayer spheres were obtained by the above-mentioned approaches.

Preparation of ABDD and Modification of the BDD Electro-

de: The as-grown BDD (hydrogen-terminated, HBDD) electrodes

were sonicated successively in 2-propanol and Milli-Q water (>18MV

cm) before use. An amine-terminated BDD (ABDD) surface wasobtained by plasma treatment according to the literature [44].Modification of the BDD surfaces was performed simply by immersingthe HBDD and ABDD electrodes into the above solutions ofthree-dimensional negatively chargedAu/PE/PSmultilayer spheres for


12 h and the electrodes were then removed and allowed to dry at roomtemperature.

Received: September 24, 2007Revised: January 02, 2008

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