A

fMfaMtp©

K

1

taapbosgei

ratt

0d

Available online at www.sciencedirect.com

Biosensors and Bioelectronics 23 (2007) 695–700

Assembly of quantum dots-mesoporous silicate hybrid materialfor protein immobilization and direct electrochemistry

Qian Zhang a,b, Ling Zhang b, Bin Liu b, Xianbo Lu b, Jinghong Li b,∗a Department of Chemistry, University of Science and Technology of China, Hefei 230026, Chinab Department of Chemistry, Key Lab of Bioorganic Phosphorus Chemistry & Chemical Biology,

Tsinghua University, Beijing 100084, China

Received 2 April 2007; received in revised form 5 August 2007; accepted 7 August 2007Available online 15 August 2007

bstract

An organized multi-components hybrid material, constructed by mesopores cellular foam silicate (MCFs) and quantum dots (QDs), was designedor the immobilization and biosensing of protein. The negative CdTe QDs were assembled on the surface of mesopores in amino group functionalized

CFs through electrostatic interaction to form QDs-MCFs hybrid material, which was used as the matrix to immobilize myoglobin (Mb) andabricate modified protein electrode (Mb-QDs-MCFs/GC). FT-IR, UV–vis and PL spectroscopies were used to monitor the assembly process

nd also demonstrated that Mb was immobilized into the hybrid matrix without denaturation. Compared with the Mb-MCFs/GC electrode, theb-QDs-MCFs/GC electrode could not only realize enhanced direct electrochemistry but also display better sensitivity and wider linear range to

he detection of hydrogen peroxide. The experiment results demonstrate that the hybrid matrix provides a biocompatible microenvironment forrotein and supplies a necessary pathway for its direct electron transfer.

2007 Elsevier B.V. All rights reserved.

ntum

aTnotphitrsgpat

eywords: Direct electrochemistry; Electrocatalysis; Mesoporous silicate; Qua

. Introduction

Direct electrochemistry of redox protein is of great impor-ance both for studying the intrinsic redox behaviors of proteinsnd fabricating biosensor without addition of mediators (Chennd Li, 2006). But unfortunately, it is difficult for the redoxroteins to directly exchange electron with electrode surfaceecause the denaturation and loss of electrochemical activitiesccurred when the proteins adsorbed directly on the electrodeurface (Lu et al., 2006). Therefore, finding new matrix withood biocompatibility for redox proteins immobilization onlectrode surface is important to obtain their direct electrochem-cal reaction and keep their bioactivities.

Nanoparticles have been intensively applied in various fieldsecently because of their intriguing properties that cannot be

chieved by their bulk counterparts (Zhao et al., 2005a). Due toheir distinctive properties, nanoparticles could greatly improvehe performance of biosensor and have attracted considerable

∗ Corresponding author. Tel.: +86 10 6279 5290; fax: +86 10 6279 5290.E-mail address: jhli@mail.tsinghua.edu.cn (J. Li).

wtQe2

i

956-5663/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.bios.2007.08.008

dots; Assembly

ttention in the field of bioelectroanalysis (Xua et al., 2006).he immobilization of redox proteins on the biocompatibleanoparticles could not only help the proteins to obtain favoredrientation but also facilitate the direct electron transfer betweenhem and the electrode (Shan et al., 2007). As one of theromising nanoparticles, semiconductor quantum dots (QDs)ave drawn intense research interest because of their interest-ng optical and electronic properties (Yu et al., 2003). Sincehe first demonstration of water soluble QDs as bioassay fluo-ophores, the number of their biological applications have grownteadily (Gao and Nie, 2004; Tana et al., 2007). Upon bioconju-ation, QDs have found great potential in fluorescence biologicalrobes (Li and Ruckenstein, 2004) and bio-imaging (Mulder etl., 2006) etc. Furthermore, its ability to promote the direct elec-ron transfer between the redox protein and electrode surfacesas also explored (Lu et al., 2005). Based on the above men-

ioned, the fabrication of hybrid nanostructures composed ofDs and proteins have become a subject of considerable inter-

st for developing novel sensors and biomaterials (Liu et al.,007).

Mesoporous silicates (MPSs), have shown to be more excit-ng candidates for the immobilization of proteins because of their

6 d Bio

bvsecvTocsmnietctfHupusoca

fiMiesMdpr

2

2

cfPb(cw

2Q

ua1

dfiv

daattowwT53

2

Ccaf

2m

MTd

tTaMtaQ

2m

pplstcap

96 Q. Zhang et al. / Biosensors an

iocompatibility, adjustable pore size, large surface area, poreolume and opened pore structure (Hartmann, 2005). Recently, aeries of MPSs such as SBA-15 (Xian et al., 2007) and HMS (Dait al., 2004) have been used to physically absorb proteins to fabri-ate biosensor. Additionally, MPSs can also act as host matrix forarious nanoparticles (Bedford et al., 2005; Yang et al., 2003).he successful immobilization of nanoparticles on the surfacef functionalized mesopores can well maintain the physical andhemical properties of nanoparticles and further improve theirtability. The obtained nanoparticles-mesoporous silicate hybridaterials are still porous with large surface area and well-defined

anopores, which can be used as multi-functional materials withmproved properties (Zhao et al., 2005b). Accordingly, consid-ring the ability of QDs nanoparticles that can promote electronransfer (Liu et al., 2007) and the advantage of MPSs whichan act as excellent matrix host for both protein and nanopar-icles, we attempted to fabricate QDs-MPSs hybrid materialor redox protein immobilization and direct electrochemistry.owever, to achieve the multi-steps assembly in MPSs, thetilization of MPSs with large mesopores is important. Meso-ores cellular foams silicate (MCFs) represent a new class ofltralarge-pore MPSs with well controlled, uniformly sized andhaped pores (Schmidt-Winkel et al., 2000). Compared withther mesoporous materials, the ultralarge mesopores of MCFsould provide sufficient volume for the multi-steps modificationnd develop advanced multi-functional materials.

In this work, thioglycolic acid-stabilized CdTe QDs wererstly assembled in the mesopores of amino-functionalizedCFs to fabricate QDs-MCFs hybrid material, which exhib-

ted satisfying biocompatibility and large surface area, therebynabling the immobilization and biosensing of redox proteins,uch as myoglobin (Mb). Compared with the unmodified MCFs,

b immobilized in QDs-MCFs could not only realize enhancedirect electron-transfer but also exhibit better electrocatalyticerformance to H2O2, such as good sensitivity and wide linearange.

. Experiment

.1. Materials

1,3,5-Trimethylbenzene (TMB), tetraethoxysilane (TEOS),admium chloride and sodium borohydride were obtainedrom Aldrich. Tellurium powder, thioglycolic acid (TGA),123 (poly (ethylene oxide)-block-poly (propylene oxide)-lock-poly (ethylene oxide), 3-aminopropyltrimethoxysilaneAPTMS) was obtained from ACROS. Myoglobin was pur-hased from BioBasic Incorporation. Deionized double-distilledater was used for making all the solutions (18 M� cm−1).

.2. Preparation of amino-functionalized MCFs and CdTeDs

Mesopores cellular foam silicate (MCFs) was synthesizedsing microemulsion templating in acidic solutions (Zhang etl., 2005). For post-synthesis of amino-functionalized MCFs,.0 g pure calcined MCFs was reacted with APTMS in 30 ml

T1ss

electronics 23 (2007) 695–700

ry toluene under reflux for 24 h. The resultant white solid wasltered off, washed with acetone three times and dried underacuum.

The CdTe QDs were prepared following the methodescribed in our previous papers (Wang et al., 2005; Yu etl., 2003). Briefly, N2-saturated cadmium chloride solution wasdded to NaHTe solution, which was prepared by the reac-ion between sodium borohydride and tellurium powder inhe presence of thioglycolic acid (TGA). The concentrationf Cd2+ was 2 mM, and the molar ratio of Cd2+:Te2−:TGAas fixed at 1:0.5:2.5. After mixing, the solution was heatedith microwaves for 10 min. Then CdTe QDs were obtained.he Photoluminescence peak of the obtained CdTe QDs was at60 nm and the particle diameters of the CdTe QDs were aroundnm according to the TEM image.

.3. Preparation of QDs-MCFs hybrid material

Amino-functionalized MCFs (0.5 g) was dispersed in 50 mldTe colloid solution (pH 7.0) and then stirred for 24 h. Afterentrifugation, the orange QDs-MCFs composites were obtainednd the supernatant liquor was colorless. The composites wereurther washed with distilled water three times and dried.

.4. Preparation of Mb-QDs-MCFs and Mb-MCFs hybridaterial

QDs-MCFs or MCFs (10 mg) was dispersed in a solution ofb (2 mg/ml, pH 6.5) and shaken for 1 h for protein absorption.

he resulting materials were then centrifuged and washed withistilled water three times.

The UV–vis spectrum was used to monitor the concentra-ions of Mb solutions before and after the assembly process.he decreases in absorbance at 407 nm of the solutions waspplied to quantify the loading amount of Mb entrapped intoCFs or QDs-MCFs. Calculating the difference in the concen-

ration of the protein before and after adsorption, we found thatbout 62 �g and 30 �g of Mb was adsorbed into 1 mg MCFs andDs-MCFs matrix, respectively.

.5. Preparation of Mb-QDs-MCFs/GC and Mb-MCFs/GCodified electrodes

Prior to use, GC electrode with a diameter of 3 mm wasolished on a polishing cloth with 1.0, 0.3, 0.05 �m aluminaowder, respectively, and rinsed with deionized water fol-owed by sonicating in acetone, ethanol and deionized wateruccessively. Then the electrode was allowed to dry at roomemperature. The modified electrode was prepared by a simpleasting method. 3 mg Mb-QDs-MCFs or Mb-MCFs obtainedbove were redispersed in 1 ml of water, and 5 �l of these sus-ensions were deposited on the surface of the pretreated GCE.

hey were then left to dry at room temperature. PVA sol (3%,0 �l) was then added for encapsulation. The electrodes weretored for at least 24 h at 4 ◦C. The modified electrodes weretored under the same condition when not in use.

d Bioe

2

tppHaBPcwsweathsm

3

3M

dat2bfMgbeMt

stwocmofetfusciaNgated (Fig. S2). Compared with the BET results of MCFs-QDs,the BET surface area and the total pore volume of Mb-MCFs-QDs decreased to 278.1 m2/g and 1.01 cm3/g, indicating that Mbintercalated into the mesopores of QDs-MCFs.

Q. Zhang et al. / Biosensors an

.6. Instruments

UV–vis experiments were performed with UV-2100S spec-rophotometer (Shimadzu). The FTIR spectra of samples in KBrellets were recorded on a Perkin-Elmer instrument. The mor-hologies of the as-prepared samples were observed utilizing aitachi model H-800 transmission electron microscope opened

t an accelerating voltage of 100 kV. The pore characters ofMS were measured by ASAP-2010C adsorptionmeter. TheL spectra data were monitored by a PE steady-state fluores-ence spectrometer (LS-550). Electrochemical measurementsere performed at room temperature using a CHI 630B work-

tation (CH Instruments, Inc., Austin, USA). The measurementsere based on a three-electrode system with the as-prepared

nzyme electrodes as the working electrodes, a platinum wires the counter electrode, and a saturated Ag/AgCl electrode ashe reference electrode. The buffer solution was purged withighly purified nitrogen for at least 30 min and a nitrogen atmo-phere environment was kept during the whole electrochemicaleasurements.

. Result and discussion

.1. Preparation and characterization of theb-QDs-MCFs hybrid material

MCFs is one of mesoporous materials that possesses a disor-ered array of silica struts, whose structures are reminiscent oferogels (Schmidt-Winkel et al., 2000). As shown in Fig. S1,he well defined ultralarge mesopores with diameters about5–30 nm are observed and the wall thickness is estimated toe from 2.5 to 4 nm. Scheme 1 depicts the idealized schemeor the construction of Mb-QDs-MCFs hybrid material. Firstly,

CFs material was functionalized with a high density of aminoroups by a post-synthesis method, which was important to sta-

ilize the QDs in the next step. Then, based on the stronglylectrostatic interactions between the amino groups of modifiedCFs and thioglycolic acid ligands on CdTe QDs, the quan-

um dots could successfully incorporated into the mesoporous

Scheme 1. Process of fabricating Mb-QDs-MCFs hybrid material.

FQ(

lectronics 23 (2007) 695–700 697

tructure under stirring together for 24 h. It was observed thathe resulting hybrid material, after washing thrice with distilledater, attained a deep orange color, evidently due to the presencef CdTe QDs in the cavities of the silicate matrix. The typi-al nitrogen adsorption–desorption isotherm of the QDs-MCFsaterials is presented in Fig. S2. The isotherm is a type IV curve

f mesoporous materials. A steep hysteretic loop is observedrom this curve, which is typical for mesoporous materials thatxhibit capillary condensation and evaporation, indicating thathe mesoporous structure was still preserved after the two stepsunctionalization. The BET surface area and the total pore vol-me are 334.8 m2/g and 1.38 cm3/g respectively, thus made ituitable for the further protein loading. Therefore, positivelyharged Mb (pI = 7.0) at pH 6.5 could assemble easily into cav-ty of QDs-MCFs by electrostatic interaction. To clarify thedsorption of Mb into the hybrid mesoporous materials, the2 adsorption isotherms after Mb loading were also investi-

ig. 1. (A) Uv–vis absorption spectra of CdTe QDs (a), QDs-MCFs (b), Mb-Ds-MCFs (c) and Mb (d) in water. (B) PL spectra of CdTe QDs (a), QDs-MCFs

b) and Mb-QDs-MCFs (c).

6 d Bio

tpssQQaMt(ctSilwitm

gAwsopbtsMes

3M

M

FQ2

e0wgAaaeIwppatrsteotwMaaebifheMf

as

98 Q. Zhang et al. / Biosensors an

The self-assemble process was monitored by UV–vis absorp-ion and PL spectroscopies. As shown in Fig. 1A, an adsorptioneak at 530 nm exhibits the character of pure CdTe QDs. Corre-pondingly, the reflectance UV–vis spectrum of the QDs-MCFshows an obvious peak at the same region, suggesting that CdTeDs has been successfully incorporated into MCFs matrix. AfterDs-MCFs treated with Mb, it is evident that a new peak further

ppeared at 407 nm, which is due to the alternative absorption ofb into the hybrid matrix. Since Mb is a heme-containing pro-

ein which is an efficient quencher for molecular fluorescenceLarsen et al., 1997), PL spectra were also used to monitor theonjugation of Mb and QDs-MCFs. Fig. 1B shows the PL spec-ra of CdTe QDs, QDs-MCFs and Mb-QDs-MCFs respectively.imilar to that of the Uv–vis spectra, the PL peak of QDs-MCFs

s essentially the same as that of CdTe QDs. However, a greatoss in the intensity of the PL peak could be found after the Mbas further assembled with QDs-MCFs, indicating the quench-

ng of CdTe QDs in MCFs, which further demonstrated thathe conjugation of the Mb with CdTe QDs functionalized in the

esopores.UV–vis absorption experiment was also carried out to investi-

ate conformational changes of Mb entrapped into QDs-MCFs.s shown in Fig. 1A, the position of the Soret band at 407 nmas unaffected after Mb immobilized into the hybrid material,

uggesting that the protein was associated with the matrix with-ut denaturation. FT-IR absorption was often used to monitorossible structural changes of proteins because the vibrationalands of the amide group in proteins provide sensitive signa-ures of protein secondary structure. The FT-IR studies (Fig. S3)how the shapes of the amide I and amide II bands of Mb in

b-QDs-MCFs located at 1658 cm−1and 1546 cm−1, which aressentially the same as those of the native Mb, indicating that theecondary structures of the bound proteins were undisturbed.

.2. Direct electrochemical properties of the

b-QDs-MCFs/GC electrode

Fig. 2 displays the typical cyclic voltammograms (CVs) forCFs/GC (a), Mb-MCFs/GC (b) and Mb-QDs-MCFs/GC (c)

ig. 2. Cyclic voltammograms of MCFs/GC (a), Mb-MCFs/GC (b) and Mb-Ds-MCFs/GC (c) electrodes in 0.1 M PBS solution (pH 7.0). Scan rate,00 mV s−1.

pimdMbttTmaapo

3e

e(e

electronics 23 (2007) 695–700

lectrodes in 0.1 M PBS (pH 7.0) over the potential range fromto +0.6 V at scan rate of 200 mV s−1. No obvious redox peaksere observed at the MCFs/GC electrode (curve a), which sug-ested that MCFs was not electroactive in the potential range.fter Mb assembled into the MCFs matrix, a couple of rel-

tively small, but quasi-reversible redox peaks were observedt Mb-MCFs/GC electrode (curve b), indicating the weak directlectron transfer between the immobilized Mb and the electrode.n contrast, the redox peaks observed on the Mb-QDs-MCFs/GCas remarkable larger (curve c). The cathodic and anodic peakotentials were −0.306 and −0.346 V, respectively. The formalotential (Ep), calculated from the average value of the cathodicnd anodic peak potentials, was −0.326 V, which was charac-eristic of the reversible electrode process of the heme FeIII/FeII

edox couple in the immobilized Mb (Zhang et al., 2006). Themall potential difference of cathodic and anodic peak poten-ials of 40 mV at the scan rate of 200 mV s−1 indicated a fastlectron transfer process obtained for the electroactive centref the immobilized Mb. So by comparison, it was obviouslyhat the direct electron transfer between the Mb and electrodeas greatly enhanced after the protein immobilized in QDs-CFs matrix. It revealed that QDs-MCFs, whose components

re both friendly materials for protein immobilization, providedbiocompatible microenviorment for the entrapped Mb. Consid-ring of the striking distinction of these voltammetric responsesetween the Mb-QDs-MCFs/GC and Mb-MCFs/GC electrodes,t was also obviously that CdTe QDs played an important role inacilitating the direct electron transfer of Mb in mesoporous. Theigh surface to volume ratio and the corresponding high surfacenergy of CdTe QDs could result in strong interaction betweenb and QDs, which probably allow the protein to obtain a more

avorable orientation.Fig. S4 is the CVs of the Mb-QDs-MCFs/GC electrode

t various scan rate and the plots of peak currents versuscan rate. As shown in Fig. S4, the reduction and oxidationeak currents increased linearly with scan rate, demonstrat-ng that the redox process of Mb immobilized in the hybrid

esopores was surface-confined process. According to Fara-ay’s law, Q = nFAΓ *, the surface concentration of electroactiveb (Γ *) at Mb-QDs-MCFs/GC electrode was calculated to

e 9.5 × 10−11 mol cm−2, which was about 25 times higherhan that at the Mb-MCFs/GC electrode and was also largerhan the theoretical monolayer value (3.4 × 10−11 mol cm−2).his shows that multi-layers of Mb entrapped in the hybridatrix participated in the electron-transfer process. The remark-

bly improved portion of electroactive protein was probablyttributed to the high loading of Mb in the mesoporous com-osites with three-dimensional structure, the nanoscale effectsf QDs, and the favorable orientation of Mb.

.3. Electrocatalytic properties of Mb-QDs-MCFs/GClectrode

Mb immobilized on the electrode surface could usuallyxhibit electrocatalytic activity for oxygen, H2O2 or NO2

−Zhang et al., 2006). Taking H2O2 as models, we studied thelectrocatalytic properties of Mb-QDs-MCFs/GC electrode. As

Q. Zhang et al. / Biosensors and Bioe

Fig. 3. (A) Cyclic voltammograms of Mb-QDs-MCFs/GC electrode in 0.1 MPBS solution (pH 7.0) with 0 �M (a), 2.5 �M (b) and 5.0 �M (c) and 7.5 �M(vt

st−aTMqMitaoM

tesaMo

cdTtotwtsHa(tFM6pbMsetM

3

ieutet(dllvb

4

gQtcMwidi

d) H2O2 scan rate, 200 mV s−1. (B) Plots of the electrocatalytic current (Icat)s. H2O2 concentration for Mb-MCFs/GC (a) and Mb-QDs-MCFs/GC (b) elec-rodes in 0.1 M PBS (pH 7.0) solution.

hown in Fig. 3A, when H2O2 was added to a pH 7.0 PBS solu-ion, an obvious increase of the reduction peak current at about

0.346 V was observed on the Mb-QDs-MCFs/GC electrode,ccompanied by the decrease of the oxidation peak current.he increase of reduction current in the reaction is becauseb–FeII can be oxidized with H2O2 and transferred to Mb–FeIII

uickly when Mb–FeIII is directly electrochemical reduced tob–FeII (Liu and Hu, 2005). The reduction peak current also

ncreased with the increasing H2O2 concentration, indicatinghat Mb immobilized in QDs-MCFs can retain its bioelectrocat-lytic activity. In contrast, the relative weaker peak current wasbserved when the cyclic voltammetric scan was performed atb-MCF/GC electrode under the same conditions.Calibration curves of steady-state currents versus concen-

rations of H2O2 for Mb-QDs-MCFs/GC and Mb-MCFs/GClectrodes were shown in Fig. 3B. As shown in Fig. 3B, under the

ame concentration of H2O2, electrocatalytic current obtainedt Mb-QDs-MCFs/GC is obviously larger than that at the Mb-CFs/GC. It is known that the enhanced Faradic responses are

f great importance in voltammetric investigations of interfa-

rvas

lectronics 23 (2007) 695–700 699

ial electron transfer for redox proteins and it is also highlyesired for development of the biosensor (Yan et al., 2005).he currents of Mb-QDs-MCFs/GC and Mb-MCFs/GC elec-

rodes all had good linear relationship with the concentrationf H2O2 in the range of 2.5–60 �M and 5.0–35 �M, respec-ively. The detection limit for Mb-QDs-MCFs/GC electrodeas 0.7 �M based on S/N = 3 and its linear regression equa-

ion is y = 0.096x + 0.273 (R = 0.9981, n = 14), where y and xtand for the peak current (�A) and the concentration (�M) of2O2, respectively. The prepared Mb-QDs-MCFs/GC electrode

lso had good reproducibility. The relative standard deviationR.S.D.) of the prepared Mb-QDs-MCFs/GC electrode responseo 25 �M H2O2 was 3.75% for six successive measurements.urthermore, the sensitivity of the Mb-QDs-MCFs/GC andb-MCFs/GC were calculated to be 9.0 mA cm−2 M−1 and

.9 mA cm−2 M−1 based on the slope of the curves. By com-arison, it was obvious that the Mb-QDs-MCFs/GC electrodeehaved better electrocatalytic performance to H2O2 than Mb-CFs/GC electrode for its wider linear detection range, higher

ensitivity, which probably due to the unique electronic prop-rties of QDs resulting from quantum size confinement andhe biocompability of both components in the hybrid QDs-

CFs.

.4. Stability of Mb-QDs-MCFs/GC electrode

The stability of the Mb-QDs-MCFs/GC electrode was alsonvestigated. Firstly the modified electrode was evaluated byxamining the cyclic voltammetric peak currents after contin-ous scanning for 50 cycles. There was nearly no decrease ofhe voltammetric response, indicating that Mb-QDs-MCFs/GClectrode was stable in buffer solution. Then the protein elec-rode was stored in PBS at 4 ◦C and measured intermittentlytwice every week), the current response to 20 �M H2O2ecreased about 4% over a 30-day period, indicating the goodong-term stability of Mb-QDs-MCFs/GC electrode. The goodong-term stability can be attributed to the favorable microen-ironment for the immobilized Mb, which was provided by theiocompatible MCFs together with QDs.

. Conclusion

In summary, QDs was successfully assembled into aminoroups functionalized MCFs material to form biocompatibleDs-MCFs material. Since QDs-MCFs possessed mesostruc-

ure, good biocompatibility and large surface area, Mb wasonsequently immobilized into the matrix to construct Mb-QDs-CFs. FT-IR and UV–vis spectroscopies demonstrated that Mbas entrapped into the hybrid matrix without denaturation. The

mmobilized Mb in Mb-QDs-MCFs/GC electrode realized fastirect electron transfer process and behaved good electrochem-cal performance to H2O2 with good sensitivity, wide linear

ange. Therefore, such multi-components hybrid material pro-ided a novel matrix for protein immobilization and could findpplications in the fabrication of the third generation of biosen-or.

7 d Bio

A

ea2

A

i

R

B

CDGHL

LLL

LLM

S

S

TW

X

X

Y

Y

YZZ

33, 43–50.

00 Q. Zhang et al. / Biosensors an

cknowledgements

This work was supported by the National Natural Sci-nce Foundation of China (No. 20435010, No. 20675044)nd National Basic Research Program of China (No.007CB310500).

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at doi:10.1016/j.bios.2007.08.008.

eferences

edford, R.B., Singh, U.G., Walton, R.I., Williams, R.T., Davis, S.A., 2005.Chem. Mater. 17, 701–707.

hen, D., Li, J., 2006. Surf. Sci. Rep. 61, 445–463.ai, Z., Liu, S., Ju, H., Chen, H., 2004. Biosens. Bioelectron. 19, 861–867.ao, X., Nie, S., 2004. Anal. Chem. 76, 2406–2410.artmann, M., 2005. Chem. Mater. 17, 4577–4593.arsen, R.W., Omdal, D.H., Jasuja, R., Niu, S.L., Jameson, D.M., 1997. J. Phys.

Chem. B 101, 8012–8020.i, Z.F., Ruckenstein, E., 2004. Nano Lett. 4, 1463–1467.iu, H., Hu, N., 2005. J. Phys. Chem. B 109, 10464–10473.iu, Q., Lu, X., Li, J., Yao, X., Li, J., 2007. Biosens. Bioelectron. 22, 3203–

3209.

Z

Z

electronics 23 (2007) 695–700

u, Q., Hu, S., Panga, D., Hea, Z., 2005. Chem. Commun., 2584–2585.u, X., Wen, Z., Li, J., 2006. Biomaterials 27, 5740–5747.ulder, W.J.M., Koole, R., Brandwijk, R.J., Gert Storm, P.T.K.C., Strijkers,

G.J., Donega’, C.d.M., Nicolay, K., Griffioen, A.W., 2006. Nano Lett. 6,1–6.

chmidt-Winkel, P., Wayne, W., Lukens, J., Yang, P., Margolese, D.I., Lettow,J.S., Ying, J.Y., Stucky, G.D., 2000. Chem. Mater. 12, 686–696.

han, D., Wang, S., Xue, H., Cosnier, S., 2007. Electrochem. Commun. 9,529–534.

ana, W.B., Jiang, S., Zhang, Y., 2007. Biomaterials 28, 1565–1571.ang, Z., Jun Li, B.L., Hu, J., Yao, X., Li, J., 2005. J. Phys. Chem. B 109,

23304–23311.ian, Y., Xian, Y., Zhou, L., Wu, F., Ling, Y., Jin, L., 2007. Electrochem.

Commun. 9, 142–148.ua, Q., Mao, C., Liu, N.N., Zhu, J.J., Sheng, J., 2006. Biosens. Bioelectron.

22, 768–773.an, Y., Zheng, W., Zhang, M., Wang, L., Su, L., Mao, L., 2005. Langmuir 21,

6560–6566.ang, C.M., Liu, P.H., Ho, Y.F., Chiu, C.Y., Chao, K.J., 2003. Chem. Mater. 15,

275–280.u, W.W., Qu, L., Guo, W., Peng, X., 2003. Chem. Mater. 15, 2854–2860.hang, H., Lu, H.Y., Hu, N.F., 2006. J. Phys. Chem. B 110, 2171–2179.hang, X., Guan, R.F., Wu, D.Q., Chan, K.Y., 2005. J. Mol. Catal. B-Enzym.

hao, G., Feng, J.J., Zhang, Q.L., Li, S.P., Chen, H.Y., 2005a. Chem. Mater. 17,3154–3159.

hao, L., Zhu, G., Zhang, D., Chen, Y., Qiu, S., 2005b. J. Solid State Chem.178, 2980–2986.