We report on the fabrication of electrical biosensors based on functionalized ZnO nanorod surfaces with biotin for highly sensitive detection of biological molecules. Due to the clean interface and easy surface modification, the ZnO nanorod sensors can easily detect streptavidin binding down to a concentration of 25 nM, which is more sensitive than previously reported one-dimensional (1D) nanostructure electrical biosensors. In addition, the unique device structure with a micrometer-scale hole at the center of the ZnO nanorod's conducting channel reduces the leakage current from the aqueous solution, hence enhancing device sensitivity. Moreover, ZnO nanorod field-effect-transistor (FET) sensors may open up opportunities to create many other oxide nanorod electrical sensors for highly sensitive and selective real-time detection of a wide variety of biomolecules.The authors thank Prof. Hee Cheul Choi at POSTECH for helpful discussion. This work was supported by the Korea Research Foundation Grant (KRF-2003-041-C00132)

Kim, Jin Suk

Lee, Chul-Ho

Park, Won Il

Yi, Gyu-Chul

Journal of the Korean Physical Society

SNU Open Repository and Archive

Journal of the Korean Physical Society, Vol. 49, No. 4, October 2006, pp. 00ZnO Nanorod Biosensor for Highly Sensitive Detection ofSpecic Protein BindingJin Suk Kim, Won Il Park, Chul-Ho Lee and Gyu-Chul YiNational CRI Center for Semiconductor Nanorods and Department of Materials Science and Engineering,Pohang University of Science and Technology, Pohang 790-784(Received 25 July 2006)We report on the fabrication of electrical biosensors based on functionalized ZnO nanorod surfaceswith biotin for highly sensitive detection of biological molecules. Due to the clean interface andeasy surface modication, the ZnO nanorod sensors can easily detect streptavidin binding down toa concentration of 25 nM, which is more sensitive than previously reported one-dimensional (1D)nanostructure electrical biosensors. In addition, the unique device structure with a micrometer-scalehole at the center of the ZnO nanorod's conducting channel reduces the leakage current from theaqueous solution, hence enhancing device sensitivity. Moreover, ZnO nanorod eld-eect-transistor(FET) sensors may open up opportunities to create many other oxide nanorod electrical sensors forhighly sensitive and selective real-time detection of a wide variety of biomolecules.PACS numbers: 85Keywords: ZnO, Nanorod, Biosensor, Field eect transistor (FET)I. INTRODUCTIONNanosensors based on semiconductor nanostructures,such as carbon nanotubes (CNTs), nanowires, andnanorods, have recently attracted considerable attentionfor detecting biological molecules [1{5]. Among the va-riety of nanosensor systems, nanometer-scale electronicsensors based upon one-dimensional (1D) semiconduc-tors oer high sensitivity and real-time detection [3,6,7].For example, thanks to the high surface-to-volume ratioof the 1D nanostructures, the detection sensitivity of 1Deld eect transistor (FET) biosensors may be increasedto a single-molecular detection level by monitoring thevery small change in conductance caused by binding ofbiomolecular species on a long conduction channel. 1Dsemiconductor electronic biosensors, in particular, haveactive surfaces that can easily be modied for immobi-lization of numerous biomolecules. However, this advan-tage may not apply to many non-oxide semiconductornanomaterials because their surfaces are not stable inan air environment, which leads to formation of an in-sulating native oxide layer and may degrade device re-liability and sensitivity [8]. FETs based on 1D oxidesemiconductors may solve this problem [9{11]. Never-theless, detection of biological molecules using 1D oxidesemiconductor FETs has rarely been studied.As one of the protein-receptor interactions, we se-lected a high-anity biotin-streptavidin binding systemE-mail: gcyi@postech.ac.krbecause this system is widely utilized in clinical diag-nostic applications and has been under evaluation asa molecular component in tumor-targeted cancer ther-apeutics [12, 13]. For the detection of protein-receptorinteractions, we functionalized ZnO nanorod surfaces byusing polyethylene glycol (PEG), similar to the case ofcarbon nanotube FETs [14]. Here, we report highly sen-sitive biological nanosensors based on air-stable, single-crystal ZnO nanorod FETs for electronic detection ofspecic protein binding.II. EXPERIMENTZnO nanorod FETs with n-channel depletion modes(normally ON) were fabricated for real-time detectionof biological species monitoring protein-receptor interac-tions. Prior to the fabrication of the ZnO nanorod FETs,high-quality, single-crystalline ZnO nanorods were pre-pared using catalyst-free metal-organic chemical vapordeposition [15,16], and then dispersed on silicon waferswith a 250-nm-thick thermal oxide. Source and drainelectrode contacts were made by evaporating 80-nm-thick Ti and 100-nm-thick Au metal layers on ZnOnanorods, respectively. The details of the FET fabri-cation method are reported elsewhere [14]. Althoughhigh performance ZnO nanorod FETs were previouslyfabricated as reported elsewhere [17], they exhibited con-siderable leakage current passing through aqueous solu-tion. Thus we further modied the biosensor structure-1--2- Journal of the Korean Physical Society, Vol. 49, No. 4, October 2006Fig. 1. (a) Schematic of single ZnO nanorod biosensorsystem and (b) optical microscopic and (c) AFM images. Thebiosensor has a micrometer-scale hole with a width of 2.5 mand a length of 6 m at the center of the ZnO nanorod'sconducting channel.by using a conventional e-beam lithography techniqueto make a micrometer-scale hole using a conventional e-beam lithography technique at the center of the ZnOnanorods conducting channel (Fig. 1(a)) in order tomonitor the conductance of the nanorod itself in aqueoussolution. Figs. 1(b) and (c) show optical microscopy andAFM images of the nanorod sensor, respectively, exhibit-ing a hole with a width of 2.5 m and a length of 6 m.In this nanosensor structure, the source and the draincontact electrodes are buried in copolymer/poly methylmethacrylate (PMMA) with a thickness of 2 m, signi-cantly reducing the leakage current through the aqueoussolution and enhancing the sensitivity and the reliabilityof the nanorod biosensor.ZnO nanorod FETs detect biological or chemicalmolecules by measuring changes in conductance. ZnOnanorod surfaces have been modied by coating themwith PEG or applying one drop of biotin solution (0.17M solution of 10 mg biotin-PEG wax in de-ionized wa-ter) onto a nanodevice, followed by the transport mea-surements. The biotin-modied ZnO nanorod deviceswere then exposed to 0.025-, 0.25-, and 2.5-M solu-tions of streptavidin in 0.01-M phosphate buered saline(PH = 7.2) in sequence by monitoring the real time con-ductance change. After each exposure, the source-draincurrent-voltage (Ids-Vds) characteristics were measuredafter the conductance had been stabilized. For the gatebias dependent current-voltage measurements, Vg wasswept from a negative bias to a positive bias and thenback to a negative bias with a sweep rate of 0.2 V/s. Allelectrical measurements were performed at room temper-Fig. 2. AFM topographic images of a biotin-modied ZnOnanorod (a) before and (b) after exposure to streptavidin la-beled with gold nanoparticles. After the exposure, brightdots with diameters of 10 nm are clearly observed, and thenanorod diameter shown as the height in the line proles in-creased from 120 { 130 nm to 270 nm, indicating that strep-tavidin is attached to biotin-modied ZnO nanorods.ature under an ambient air environment.III. RESULTS AND DISCUSSIONAtomic force microscopy (AFM) was employed priorto electrical measurements of the nanorod biosensorsin order to investigate changes in the nanorod sur-face morphology after functionalization with biotin andsubsequent exposure to streptavidin labeled with goldnanoparticles. Bare ZnO nanorods displayed a cleanZnO surface, but the AFM image of ZnO nanorod afterbiotin-modication in Fig. 2(a) exhibits a bumpy surfacedue to biotin molecules coated on the ZnO nanorod sur-face. In the image of the ZnO nanorods exposed to strep-tavidin labeled with gold nanoparticles with a 10 nmdiameter, however, bright dots with diameters of 10 nmare clearly observed, indicating that streptavidin is eec-tively attached to biotin-modied ZnO nanorods. Thebiotin-streptavidin binding on the ZnO surface is alsoconrmed by the increase in nanorod diameter (heightin the AFM images). As shown in the line proles cross-ing a ZnO nanorod (Fig. 2), the ZnO nanorod diameterincreased from 120  130 nm to 270 nm after strepta-vidin attachment on the biotin-modied nanorod.ZnO exhibits strong adsorption of molecules on thesurface, which aects the electrical characteristics ofZnO-based devices, dependent on surface-mediated phe-nomena. For example, the conductance of ZnO nanorodFETs drastically changes when molecules adsorb on theZnO surface. Fig. 3(a) shows the source-drain current-ZnO Nanorod Biosensor for Highly Sensitive Detection   { Jin Suk Kim et al. -3-Fig. 3. (a) Ids-Vds curves of bare, biotin-modied, andstreptavidin-exposed ZnO nanorod FET. No gate bias wasapplied during these measurements. (b) Current versus timefor the biotin-modied ZnO nanorod device following addi-tion of 0.025, 0.25, and 2.5 M streptavidin, respectively. Thegate and the source-drain voltages were 0 and 1 V, respec-tively.voltage (Ids-Vds) characteristic curves of a ZnO nanorodFET before and after functionalization of ZnO nanorodsurfaces with biotin, and binding of streptavidin with thebiotin-functionalized surface. The ZnO nanorod FETexhibited only a small increase from 0.77 to 1.0 S inconductance even after the biotin-functionalization, ata source-drain voltage (Vds) of 1 V and a gate volt-age (Vg) of 0 V. However, after the biotin-modied ZnOnanorod FET had been exposed to a 250 nM streptavidinsolution, the conductance at the same voltages drasti-cally increased to 17 S, presumably due to the biotin-streptavidin binding. The order of magnitude change inthe conductance of the ZnO nanorod FET upon expo-sure to streptavidin solution is signicantly higher thanthe conductance change of 3 % from 1600 to 1650 nS af-ter the exposure of Si nanowire FET sensors to a 250-nMstreptavidin solution [3].Additional control experiments were performed in or-der to conrm that the conductance change resulted fromthe specic binding of streptavidin to the biotin ligandon the ZnO nanorod's surface. Most signicantly, whenbare ZnO nanorod devices (without biotin modication)were exposed a 250-nM streptavidin solution, the con-ductance change was less than a few percent, represent-ing a weak electrical interaction or charge transfer be-tween the bare ZnO nanorod surface and streptavidin.As in Fig. 3 shown, additionally, the ZnO nanorodbiosensors with biotin-modied surfaces did not showany signicant changes in conductance due to exposureto a buer solution without any streptavidin. However,the conductance of the ZnO nanorod FET with a biotin-modied surface responded signicantly to a streptavidinsolution with even a small concentration of 25 nM. Theseresults strongly suggest that the conductance change inthe biosensor results from the biotin-streptavidin bind-ing.Fig. 3(b) shows plots of current versus time for abiotin-modied ZnO nanorod FET following sequentialadditions of buer solutions without streptavidin andwith 0.025-, 0.25-, and 2.5-M streptavidin, where Vgand Vds were xed at 0 and 1 V, respectively. The ar-rows indicate conductance changes observed whenever asolution with streptavidin was added. Even with a 25-nM streptavidin solution, the conductance change wasas large as 1.8 A, corresponding to an increase of 140% in conductance. In addition, device conductance in-creased almost linearly with increasing streptavidin con-centration up to 250-nM. However, further addition of2.5-M streptavidin resulted in only a small change indevice conductance. Since most biotin-binding sites arebonded with streptavidin molecules after exposure to the250-nM solution, only a small number of empty biotin-binding sites remains, which presumably results in onlya small change in the conductance at high streptavidinconcentrations above 250-nM. These electronic sensingexperiments indicate that a rapid and drastic conduc-tance change is observed even for small streptavidin con-centration. Addition of streptavidin solution, produceddrastic conductance changes in a few seconds, and thehighest conductance in was reached 25  30 seconds.Moreover, the conductance decayed exponentially afterreaching the peak. Accordingly, we strongly suggest thatZnO nanorod FET sensors can be used for highly sensi-tive and specic real-time molecular recognition.The changes in the electrical characteristics result-ing from the biotin-streptavidin interaction were furtherinvestigated by measuring the Ids-Vg characteristics ofZnO nanorod devices. Figs. 4(a) and (b) show the Ids-Vgcurves of ZnO nanorod FETs without and with biotin-streptavidin complexes, respectively. Streptavidin bind-ing resulted in an increase in the conductance response toVg or transconductance (gm = dIds/dVg), accompaniedby an increase in the maximum ON state current and adecrease in the absolute value of the threshold voltage.That is, from the Ids-Vg curve sweeps in a forward direc-tion from negative to positive bias, gm increased from 0.3to 4 S, the maximum ON state current increased from3 to 20 A, and the threshold voltage increased from 30 to  5 V. Meanwhile, a large hysteresis curve was-4- Journal of the Korean Physical Society, Vol. 49, No. 4, October 2006Fig. 4. Ids-Vg curves of the ZnO nanorod FETs (a) beforeand (b) after biotin-streptavidin binding. The source-drainvoltage (Vds) was 1 V.also observed, which was attributed to adsorbed molec-ular species working as charge traps [18]. The hysteresiseect has previously been observed in most instances ofligand-receptor interactions, as well as in simple adsorp-tion of proteins onto solid surfaces [19,20].The observed changes in the conductance and in thecurrent-voltage characteristics due to biotin-streptavidinbinding may be explained in terms of a chemical gatingeect and a charge transfer process [3,9,21]. We believethat these dierent processes coexist in ZnO nanorodbiosensors. That is, the conductance reduction for neg-ative gate voltages (Vg <   5 V) and threshold volt-age shifts presumably result from the chemical gatingeect [3, 9]. In an n-channel ZnO nanorod FET, thischemical gating eect from the binding of the negativelycharged streptavidin induces electron depletion, therebyrendering the device less n-type. For positive gate volt-ages, however, the conductance increasess presumablydue to charge transfer from streptavidin to the n-typeZnO channel via biotin [21]. The charge transfer pro-cess may also result in a drastic increase and subsequentslow decrease in the FET conductance right after deviceexposure to the strepatividin solution, as indicated bythe arrows in Fig. 3(b). That is, as soon as the strep-tavidin binds to biotin on a ZnO nanorod FET, excesselectron carriers are donated from charged streptavidinto ZnO, presumably resulting from the absence of aninsulating barrier layer between the ZnO and the biotin-streptavidin complex.IV. CONCLUSIONSWe fabricated electrical biosensors based on function-alized ZnO nanorod surfaces with biotin for highly sensi-tive detection of biological molecules. The ZnO nanorodsensors can easily detect streptavidin binding down toa concentration of 25-nM, which is more sensitive thanpreviously reported 1D electrical biosensors, includingSi nanowire biosensors and carbon nanotube FET sen-sors. Presumably this may result from the clean inter-face between the ZnO nanorod surface and the biologi-cal or chemical species and from easy surface modica-tion of oxide surfaces for immobilization of the species.In addition, our unique FET device structure with amicrometer-scale hole at the center of the ZnO nanorod'sconducting channel reduces the leakage current fromthe aqueous solution, hence enhancing device sensitivity.More generally, we believe that ZnO nanorod electricalsensors may be expanded to create many other oxidenanorod electrical sensors for highly sensitive and selec-tive real-time detection of a wide variety of biomolecules.ACKNOWLEDGMENTSThe authors thank Prof. Hee Cheul Choi atPOSTECH for helpful discussion. This work was sup-ported by the Korea Research Foundation Grant (KRF-2003-041-C00132).REFERENCES[1] P. Alivisatos, Nat. Biotechnol. 22, 47 (2004).[2] a) J. Kong, N. Franklin, C. Zhou, M. Chapline, S. Peng,K. Cho and H. Dai, Science 287, 622 (2000); b) J. Kongand H. Dai, J. Phys. Chem. 105, 2890 (2001).[3] Y. Cui, Q. Wei, H. Park and C. M. Lieber, Science 293,1289 (2001).[4] K. Besteman, J. O. Lee, F. G. M. Wietz, H. A. Heeringand C. Dekker, Nano Lett. 3, 727 (2003).[5] R. J. Chen, S. Bangsaruntip, K. A. Drouvalakis, N. W.S. Kam, M. Shim, Y. Li, W. Kim, P. J. Utz and H. Dai,Proc. Natl. Acad. Sci. 100, 4984 (2003).[6] J. Hahm and C. M. Lieber, Nano Lett. 4, 51 (2004).[7] Z. Li, Y. Chen, X. Li, T. I. Kamins, K. Nauka and R. S.Williams, Nano Lett. 4, 245 (2004).[8] X. T. Zhou, J. Q. Hu, C. P. Li, D. D. D. Ma, C. S. Leeand S. T. Lee, Chem. Phys. Lett. 369, 220 (2003).ZnO Nanorod Biosensor for Highly Sensitive Detection   { Jin Suk Kim et al. -5-[9] M. S. Arnold, P. Avouris, Z. W. Pan and Z. L. Wang, J.Phys. Chem. B 107, 659 (2003).[10] C. Li, B. Lei, D. Zhang, X. Liu, S. Han, T. Tang, M.Rouhanizadeh, T. Hsiai and C. Zhou, Appl. Phys. Lett.83, 4014 (2003).[11] C. Y. Yim, D. Y. Jeon, K. H. Kim, G. T. Kim, Y. S.Woo, S. Roth, J. S. Lee and S. Kim, J. Korean Phys.Soc. 48, 1565 (2006).[12] M. Ruskowski, M. Fogarasi, B. Fritz and D. J. Hna-towich, Nucl. Med. Biol. 24, 263 (1997).[13] S. L. McCune, J. P. Gockerman and D. A. Rizzieri, J.Am. Med. Assoc. 286, 1149 (2001).[14] A. Star, J.-C. P. Gabriel, K. Bradley and G. Gruner,Nano Lett. 3, 459 (2003).[15] W. I. Park, D. H. Kim, S. W. Jung and G.-C. Yi, Appl.Phys. Lett. 80, 4232 (2002).[16] W. I. Park, J. Yoo and G.-C. Yi, J. Korean Phys. Soc.46, L0167 (2005).[17] W. I. Park, J. S. Kim, G.-C. Yi, M. H. Bae and H. J.Lee, Appl. Phys. Lett. 85, 5052 (2004).[18] K. Bradley, J. Cumings, A. Star, J.-C. P. Gabriel and G.Gruner, Nano Lett. 3, 639 (2003).[19] A. Docoslis, L. A. Rusinski, R. F. Giese and C. J. Oss,Colloid Surf. B 22, 267 (2001).[20] A. L. J. C. Jeuken and F. A. Armstrong, J. Phys. Chem.B 105, 5271 (2001).[21] K. Bradley, M. Briman, A. Star and G. Gruner, NanoLett. 4, 253 (2004).

ZnO Nanorod Biosensor for Highly Sensitive Detection of Specific Protein Binding

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ZnO Nanorod Biosensor for Highly Sensitive Detection of Specific Protein Binding

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