Detection of DNA hybridization on indium tin oxide surfaces

Sensors and Actuators B 125 (2007) 574–580

Detection of DNA hybridization on indium tin oxide surfaces Selina Moses, Scott H. Brewer, Stephan Kraemer, Ryan R. Fuierer, Lisa B. Lowe, Chiamaka Agbasi, Marc Sauthier, Stefan Franzen ∗ Department of Chemistry, North Carolina State University, Raleigh, NC 27695, United States Received 14 January 2007; received in revised form 22 February 2007; accepted 5 March 2007 Available online 14 March 2007

Abstract Indium tin oxide (ITO) surfaces were modified with ssDNA by coupling oligonucleotides to a monolayer of 12-phosphonododecanoic acid (12-PDA) on ITO surfaces. This coupling involved the formation of an amide bond between the carboxylic acid moiety of 12-PDA to the amine group of a 5 -aminopropyl-labeled single strand of DNA. The self-assembled monolayer of 12-PDA and surface-attached oligonucleotides were characterized by X-ray photoelectron and reflectance FTIR spectroscopy. Detection of selective surface DNA hybridization was achieved by labeling the target ssDNA with gold nanoparticles. The presence of gold nanoparticles was probed using X-ray photoelectron spectroscopy, stripping voltammetry, atomic force microscopy, thermography, photoelectrochemistry (chronoamperometry) and cyclic voltammetry (CV). CV was used to successfully detect DNA hybridization for nanoparticle concentrations as low as 10 pM when using the gold nanoparticles bound to an ITO electrode as catalysts for the electrochemical oxidation of FeCl2 . The studies described here provided the basis for surface attachment methodology for various electrochemical and thermographic sensing methods that use ITO thin films as a substrate. © 2007 Elsevier B.V. All rights reserved. Keywords: Thermography; Electrochemistry; Fourier-transform infrared; Nanoparticle detection; Stripping voltammetry

1. Introduction Indium tin oxide (ITO) is a versatile material that has shown promise in novel detection strategies for biomolecules. ITO has been shown to have favorable properties for electrochemical [1], photothermal electrochemical [2], thermographic [3], and surface plasmon resonance [4–6] detection methods. ITO electrodes can support a wide range of potentials. ITO is an infrared reflective material [4,5,7–10], which is advantageous for thermographic [3] and infrared sensing technologies [11]. Recently, it has been shown that surface plasmons can be excited on ITO [5]. Surface plasmon resonance is a method widely used on gold surfaces that gained widespread acceptance during the last decade [12–14]. However, the application to conducting metal oxides provides opportunities for new surface chemistry [5,11,15,16], a wide range of simultaneous electrochemistry

∗

Corresponding author. Tel.: +1 919 515 8915; fax: +1 919 515 8909. E-mail address: Stefan [email protected] (S. Franzen).

0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.03.009

[2,17–23] applications and novel effects because of the fact that the plasmon itself is in the near infrared [4,6,8,24]. The application of ITO for any of these detection strategies requires a robust attachment strategy. ITO is much less studied than substrates such as gold [25] and glass [26,27], which have a demonstrated ability to support self-assembled monolayers on these surfaces with thiol or chloro- or alkoxysilane containing molecules [28], respectively. These systems have shown the detection of selective DNA hybridization through a variety of detection techniques including fluorescence [29–32], surface plasmon resonance (SPR) [33,34], electrochemistry [35–37], FTIR spectroscopy [38], X-ray photoelectron spectroscopy (XPS) [25] and through labeling with nanoparticles [39–41]. In this study, indium tin oxide thin films were investigated to develop a surface DNA modification procedure to be used for the subsequent detection of DNA hybridization. The electronic and optical properties of ITO have spurred interest in the formation of adlayers on ITO [11,15,42–45] and electrochemical methods have been developed for the detection of DNA or PNA on ITO [1,22,23]. The detection strategies employed

S. Moses et al. / Sensors and Actuators B 125 (2007) 574–580

in this study involve the detection of gold-nanoparticle-labeled target single-stranded DNA (ssDNA) [2]. The formation of self-assembled monolayers and the coupling of ssDNA to those monolayers on ITO thin films were confirmed by XPS, reflectance FTIR spectroscopy and chronocoulometry while the gold nanoparticle/target ssDNA conjugates were detected on the ITO surface by XPS, stripping voltammetry, atomic force microscopy (AFM), thermography, photoelectrochemistry (chronoamperometry) and cyclic voltammetry (CV).

2. Materials and methods 2.1.1. ssDNA Modification of ITO Scheme 1 outlines the strategy employed in the modification of indium tin oxide (Delta Technologies, Ltd.) surfaces with ssDNA. The ITO thin films were comprised of 90% indium oxide and 10% tin oxide, had a nominal thickness of ˚ and a sheet resistance of 8–12 . Initially, a mono1500 A layer of 12-phosphonododecanoic acid (12-PDA) (Xanthon, Inc.) (10 mM in 50/50 DMSO/18 M cm H2 O for 16 h) was formed on the ITO surface (cleaned 20 min with UV/O3 (UVOcleaner (UVO-60), model number 42, Jelight Company, Inc.)). The carboxylic acid functional group of 12-PDA was activated by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) (Sigma–Aldrich) and reacted with the primary amine of a 5 modified H2 N (CH2 )3 ssDNA (5 H2 N (CH2 )3 modified 5 -GTGAGCGGATAATCCTGGTT-3 ) (Applied Biosystems, Inc.) to form an amide bond between the 12-PDA and the 5 modified H2 N (CH2 )3 ssDNA. The coupling conditions were 1 ␮M 5 modified H2 N (CH2 )3 ssDNA and 200 mM EDC for 4 h in a 0.1 M MES (2-(N-morpholino)ethane sulfonic acid) buffer at pH 5 with 0.25 M NaCl. The surface was then rinsed with 18 M cm H2 O (BARNSTEAD E-PURE) and dried with N2 gas. The characterization of ssDNA modified ITO substrates is discussed in Supporting Information.

575

2.1.2. Preparation of the gold nanoparticle/oligonucleotide conjugates The gold nanoparticle conjugates were synthesized by the attachment of 5 thiol modified ssDNA. The complementary ssDNA target strand used in the hybridization experiments was 5 -HS(CH2 )6 modified 5 -AACCAGGATTATCCGCTCAC-3 while the non-complementary ssDNA target strand was 5 -HS(CH2 )6 modified 5 -GTGAGCGGATAATCCTGGTT-3 . Gold nanoparticles (10 nm diameter) coated with bis(psulfonatophenyl)phenylphosphine dihydrate dipotassium salt (BSPP) were synthesized by a literature procedure [46]. Briefly, 10 nm citrate coated Au colloids (Ted Pella) were stirred with an excess of BSPP at room temperature overnight. The particles were aggregated slightly by progressive addition of a 1 M NaCl solution until the NaCl concentration was >100 mM. As the NaCl concentration increased, the color of the colloidal suspension changed from burgundy to a slightly purplish color. The sample was centrifuged to pellet the particles and the supernatant containing citrate solution was removed. Afterwards, the pellet was re-suspended in a 250 mg/L solution of BSPP. Methanol was added dropwise until the color of the particles turned from a burgundy red to a purplish color again. The colloidal solution was centrifuged and the supernatant was discarded. The gold nanoparticles were finally re-dissolved in the BSPP solution and stored at 4 ◦ C. For the preparation of the gold nanoparticle/ssDNA conjugates, a 1:100 ratio of gold nanoparticles to ssDNA was stirred at room temperature overnight. In order to remove non-reacted ssDNA, the solution was centrifuged at 14,000 rpm for 30 min and the precipitate was re-suspended into 1× SSC buffer, defined as 150 mM NaCl and 15 mM sodium citrate at a pH of 7. This procedure was performed twice and resulted in approximately 5–6 ssDNA strands per gold nanoparticle determined by fluorescence (data not shown). The final concentration conjugate solution had a final concentration of 3.7 nM gold nanoparticles as determined from the absorbance of the plasmon band

Scheme 1. Modification of ITO with single-stranded DNA through the formation of an amide bond between the carboxylic acid functional group of a monolayer of 12-phosphonododecanoic acid with the primary amine of 5 modified H2 N(CH2 )3 ssDNA.

576

S. Moses et al. / Sensors and Actuators B 125 (2007) 574–580

of the gold nanoparticles using an extinction coefficient of 8.9 × 107 M−1 cm−1 obtained from UV–Vis measurements and based on the concentration reported by the manufacturer (Ted Pella, Inc.). 2.1.3. DNA hybridization conditions The DNA hybridization was carried out either at 37 ◦ C for 19 h in a shaking incubator or by keeping the hybridization solution for 1 h at 45 ◦ C and then allowing the solution to gradually cool to room temperature overnight. The hybridization conditions were varied to study its effect on the efficiency of hybridization and the limit of detection. The concentrations of the gold nanoparticle/ssDNA conjugate solutions used in this study were in the range of 1 nM to 1 pM of the gold nanoparticle.

2.1.7. Atomic force microscopy (AFM) The ITO surface modified with ssDNA and hybridized with 10 nm gold nanoparticle labeled complimentary target ssDNA was imaged by Digital Instruments Nanoscope IIIa Scanning Probe Microscope in tapping mode. The AFM images were 1 ␮m2 with a height scale of 50 nm. 3. Results and discussion The focus of this study is the surface attachment of ssDNA probes and the detection of hybridization of DNA using nanoparticle labels on ITO surfaces. Fig. 1 shows three methods for monitoring dsDNA on ITO surfaces using 10 nm gold-colloidlabeled target strands hybridized to surface-attached probe strands. Fig. 1A shows XPS spectra of Au 4f7/2,5/2 (83.3,

2.1.4. X-ray photoelectron spectroscopy (XPS) XPS spectra were recorded on a Riber LAS 2000 Surface Analysis System equipped with a cylindrical mirror analyzer (CMA) and a MAC2 analyzer with Mg K␣ X-rays (model CX 700 (Riber source) (hν = 1253.6 eV). The elemental scans had a resolution of 1.0 eV and were the result of five scans. XPS spectra were smoothed using a 9-point (second order) Savitzky–Golay algorithm, baseline corrected and the peaks were fitted using Gaussian line shapes. 2.1.5. Electrochemistry The photoelectrochemical measurements were performed in a custom electrochemical cell allowing for the direct illumination of the electrode surface. The electrolyte was 0.1 M potassium phosphate buffer (pH 7) with 0.05 M EDTA. The change in electrode potential was monitored with chronoamperometry. Therefore, the potential was stepped to 0.7 V versus Ag(s)/AgCl while the current was monitored as a function of time. Electrochemical oxidation (stripping) of the gold nanoparticles on the ITO surface was performed in 0.5 M KCl at a scan rate of 100 mV/s. The cyclic voltammograms (CV) of the oxidation of FeCl2 (100 mM), catalyzed by the gold nanoparticles were performed at a scan rate of 100 mV/s. 2.1.6. Thermographic measurements The thermographic detection method involves irradiation of a gold nanoparticle label with visible light so that incident light energy is absorbed while detecting the local heating due to blackbody radiation in the mid-infrared. For all thermographic measurements 10 nm gold nanoparticle labels were used on the target strand (in a two strand assay). The modified ITO surfaces were illuminated with 532 nm light from a frequency doubled quasi-CW Nd:YAG laser (Coherent Antares 76-Nd:YAG laser) and the change in surface temperature was monitored by infrared (IR) thermography (Inframetrics 740). The laser beam was focused to approximately 1 mm diameter and the light intensity was adjusted to 1 W.

Fig. 1. (A) XPS spectra of Au 4f7/2,5/2 are shown. (B) Stripping voltammograms (in a 0.5 M KCl solution) of ITO modified with ssDNA (long dash) exposed to the complementary (solid) or non-complementary (short dash) ssDNA labeled with 10 nm gold nanoparticles (1 nM). (C) Tapping mode AFM image (1 ␮m2 with a height scale of 50 nm) of ITO modified with ssDNA exposed to the complementary ssDNA labeled with 10 nm gold nanoparticles (1 nM).

S. Moses et al. / Sensors and Actuators B 125 (2007) 574–580

87.0 eV) that demonstrate the presence of gold nanoparticle labels by a comparison of ITO modified with ssDNA coupled through a monolayer of 12-PDA (long dash) exposed to the complementary (solid) or non-complementary (short dash) ssDNA labeled with a 10 nm gold nanoparticle (1 nM). Fig. 1B shows the stripping voltammograms in a 0.5 M KCl solution of ITO modified with ssDNA coupled through a monolayer of 12-PDA exposed to the complementary (solid) or non-complementary (short dash) ssDNA labeled with a 10 nm gold nanoparticle (1 nM). The stripping voltammogram shows an oxidative (864 mV) and reductive (465 mV) peak of gold on the surface. The stripping voltammograms showed in Fig. 1B illustrate that non-specific binding of the non-complementary ssDNA to the ssDNA modified ITO surface based on the integrated area of the reductive peak of the gold was at most ∼13% for the 1 nM solution of Au nanoparticles used in these experiments. In fact, if the analysis were based on the oxidative wave alone one would predict significantly less non-specific binding. The corresponding AFM image (1 ␮m2 image with a 50 nm height scale) captured in tapping mode for the hybridization of the complementary ssDNA/gold nanoparticle conjugate to the ssDNA modified ITO surface is shown in Fig. 1C. AFM was used to estimate the coverage of gold nanoparticles by counting the number in areas of 1 ␮m2 , as shown in Fig. 1C. The average number found was 8.1 ± 1.7 × 1010 /cm2 based on these counts. Chronocoulometry was used to obtain an upper bound for the surface coverage of probe ssDNA on the ITO surface (see Supporting Information). Given that the 12-PDA monolayer is negatively charged, the measured negative charge on the surface by chronocoulometry involves some amount of binding to the 12-PDA layer itself. The measurement is based on the binding of [Ru(NH3 )6 ]3+ to negatively charged molecules on the surface. The control experiment with 12-PDA gave signals one order of magnitude lower than the surfaces with DNA conjugated. The uncorrected value for ssDNA coverage based on chronocoulometry is 1.9 ± 0.4 × 1013 ssDNA strands/cm2 . This value is a factor of 5 higher than that observed on gold surfaces [41]. There is no objective reason to discount this value and it is the best quantitative measure of ssDNA coverage on ITO. Moreover, this value was roughly linear in ssDNA coverage on the surface. Nonetheless, we use the value based on chronocoulometry an upper bound of value of the surface coverage given the possibility that 12-PDA also plays some role in the observed signal. Based on this value the hybridization efficiency can be estimated to have a lower bound value of 0.4 ± 0.1%. This value is near one order of magnitude lower than the hybridization efficiency observed on Au surfaces [41]. The presence of gold nanoparticles attached to the DNA modified ITO surface through DNA hybridization was also detected by thermography [3], photoelectrochemistry [2] and stripping voltammetry. The thermographic and photoelectrical experiments are based on laser induced temperature jumps where the plasmon band of 10 nm gold nanoparticles bound to the DNA modified ITO surface through DNA hybridization is excited by 532 nm laser radiation [3]. In the case of thermographic detection the temperature increase was monitored on a dry surface (ex situ) using a thermographic (InSb CCD) camera. In the case of

577

photoelectrochemical detection the temperature increased was detected in solution using a home-built electrochemical cell that allowed the sample to be irradiated while monitoring the resulting current from the temperature increase [2]. The presence of the gold nanoparticles was also detected by stripping voltammetry where the oxidation and reduction of the gold nanoparticles was detected. After irradiating the hybridized sample for 30 s with 532 nm radiation (1 W/cm2 ), the gold nanoparticles attached to the ITO substrate through DNA hybridization (1 nM gold nanoparticle/ssDNA conjugate) yielded a temperature increase of ∼4 ◦ C over background (ssDNA modified ITO). The change in current due to this temperature increase was monitored with chronoamperometry where the potential was stepped to 0.7 V versus Ag(s)/AgCl and the current was monitored over time (Fig. 2). When the sample was irradiated with 532 nm light, the current decreased rapidly at first and then slowly leveled off over a period of 120 s. At this point the laser was switched off and it was observed that the current returned to its initial value. Similar transients have been observed by us [2] and in other studies [47–50]. DNA hybridization on ITO surfaces was also detected by the oxidation of a specific redox species catalyzed by gold nanoparticles. The identity of the redox species is crucial because the electron transfer kinetics of this species must be slow on bare ITO but fast in the presence of gold nanoparticles. The change in electron transfer rate can easily be detected using cyclic voltammetry. The best redox species for this type of electrochemical detection strategy was determined to be FeCl2 by comparison of a number of sacrificial donors such as triethanolamine and ethylene diamine tetraacetic acid. The presence of FeCl2 in solution generated oxidation currents that are much larger than those recorded by electrochemical stripping. The limit of detection of gold nanoparticles was determined to be 10 pM. The results of this technique are shown in Fig. 3A where a range of concentrations of target gold nanoparticle/ssDNA conjugates was used to hybridize to ssDNA modified ITO surfaces. The concentrations of the gold

Fig. 2. Anodic current vs. time for an ITO electrode modified with ssDNA and hybridized with the complementary ssDNA gold–nanoparticle conjugate (1 nM). The measurement was done in 0.1 M potassium phosphate buffer with 0.05 M EDTA. At 30 s the sample was illuminated with 532 nm laser light with a power density of approximately 1 W/cm2 . After 90 s the laser light was switched off. The potential was held at 0.7 V vs. a Ag(s)/AgCl reference electrode which was not fixed isothermally.

578

S. Moses et al. / Sensors and Actuators B 125 (2007) 574–580

Fig. 4. Electrochemical response for 100 mM FeCl2 (scan rate 100 mV/s) as a function of the amount of target ssDNA in the hybridization solution with improved hybridization conditions. The three target amounts were 100 pM (—), 10 pM (· · ·) and 1 pM (- - -) of the gold nanoparticles (10 nm diameter) modified with the target single-stranded DNA. The hybridization was allowed to proceed at 37 ◦ C for 19 h in a shaking incubator.

4. Conclusions

Fig. 3. (A) Electrochemical response for 100 mM FeCl2 (scan rate 100 mV/s) as a function of the amount of target ssDNA/gold nanoparticle conjugate in the hybridization solution. The four target concentrations were 1 nM, 500 pM, 100 pM and 10 pM of the gold nanoparticles (modified with the target ssDNA). The hybridization was allowed to proceed for 1 h at 45 ◦ C and the solution was then gradually cooled to room temperature overnight. (B) Electrochemical response as a function of the amount of the gold nanoparticle/target ssDNA in the hybridization solution at 887 mV (squares) for the oxidation current and 109 mV (triangles) for the reduction current fit globally to a straight line (solid). The error bars are smaller than the symbols in the plot.

nanoparticle/ssDNA conjugate solutions used in these experiments were 1 nM, 500 pM, 100 pM and 10 pM. Each of these solutions were hybridized to the ssDNA modified ITO surfaces by maintaining the solution at 45 ◦ C for 1 h and then allowing it to cool to room temperature overnight. The presence of the gold nanoparticle–ssDNA conjugates were then detected electrochemically. The currents measured at the maximum of the oxidation (887 mV) and reductive peak (109 mV) recorded by CV were found to increase linearly with the target concentration in solution (Fig. 3B). Fig. 4 shows the electrochemical results for the detection of gold nanoparticles attached to the ITO surface through DNA hybridization using FeCl2 as the redox species in solution. Three gold nanoparticle/ssDNA target concentrations were used: 100 pM, 10 pM and 1 pM. The figure shows that Fe2+ is oxidized at +0.85 V, which is the same potential as Au metal. Given that Fe2+ is not oxidized in the absence of gold nanoparticles we call this mode of detection catalytic electrochemistry with the meaning that the presence of the gold nanoparticle acts as a catalyst for electron transfer from Fe2+ to the electrode surface. The limit of detection of gold-nanoparticle-modified ssDNA targets bound to the ITO substrate was determined to be 10 pM.

The hybridization of DNA to surface-attached ssDNA probes strands on indium tin oxide thin films was monitored with a variety of electrochemical and optical techniques. The coupling of probe strands to the surface was achieved by means of a reaction of the amine group of 5 -H2 N(CH2 )3 -labeled ssDNA probes to the carboxylic acid moiety of a monolayer of 12phosphonodododecanoic acid (12-PDA) on the ITO surface by EDC coupling to create an amide linkage. The method for surface attachment of DNA using 12-PDA acid self-assembled monolayers has utility in three different detection methods that use the properties of ITO transparent electrodes. These methods are electrochemistry [2], thermography [3] and surface plasmon resonance [4–6]. Herein, we focused mainly on an electrochemical method involving the catalytic oxidation of FeCl2 by gold nanoparticles on ITO electrodes. The experimental methods pointed out two possible issues that will affect the electrochemical signal. One issue is the negative charge of the12-PDA monolayer and its effect on negatively charged probe strands and nanoparticles. A second issue is the texture of ITO thin films. In spite of these issues, three methods were demonstrated here as elsewhere with limits of detection for goldnanoparticle-modified target ssDNA in the range of ∼10 pM, ∼100 fM and ∼1 fM for catalytic electrochemistry, photothermal electrochemistry [2] and thermography [3], respectively. Although, catalytic electrochemistry in its present implementation is less sensitive than the other methods, it is a novel method described here that adds to the repertoire of methods possible using ITO electrodes. Acknowledgments Selina Moses, Marc Sauthier, and Lisa Lowe were supported by Applied Biosystems, Inc. Scott H. Brewer was supported by NIH Biotechnology Training Grant T32-GM08776. Support and computational facilities were provided by the North Carolina State University High Performance Computing Resource. The

S. Moses et al. / Sensors and Actuators B 125 (2007) 574–580

authors would like to thank Dr. Kimberly Briggman at NIST for her help in the acquisition of the reflectance FTIR spectra. [20]

Appendix A. Supplementary data [21]

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.snb.2007.03.009. [22]

References [1] N.D. Popovich, B.K. Yen, S. Wong, Langmuir 19 (2003) 1324–1329. [2] L.B. Lowe, S.H. Brewer, S. Kramer, R.R. Fuierer, G.G. Qian, C.O. AgbasiPorter, S. Moses, S. Franzen, D.L. Feldheim, Laser-induced temperature jump electrochemistry on gold nanoparticle-coated electrodes, J. Am. Chem. Soc. 125 (2003) 14258–14259. [3] M.G. Cerruti, M. Sauthier, D. Leonard, D. Liu, G. Duscher, D.L. Feldheim, S. Franzen, Au and silica-coated gold nanoparticles as thermographic labels for DNA detection, Anal. Chem. 78 (2006) 3282–3288. [4] S.H. Brewer, S. Franzen, Optical properties of indium tin oxide and fluorine-doped tin oxide surfaces: correlation of reflectivity, skin depth, and plasmon frequency with conductivity, J. Alloys Compd. 338 (1–2) (2002) 73–79. [5] S.H. Brewer, S. Franzen, Indium tin oxide plasma frequency dependence on sheet resistance and surface adlayers determined by reflectance FTIR spectroscopy, J. Phys. Chem. B 106 (2002) 12986–12992. [6] C. Rhodes, S. Franzen, J.-P. Maria, M. Losego, D.N. Leonard, B. Laughlin, G. Duscher, S. Weibel, Surface plasmon resonance in conducting metal oxides, J. Appl. Phys. 100 (2006) Art. No. 054905. [7] A. Ambrosini, A. Duarte, K.R. Poeppelmeier, M. Lane, C.R. Kannewurf, T.O. Mason, Electrical, optical, and structural properties of tin-doped In2 O3 –M2 O3 solid solutions (M = Y, Sc), J. Solid State Chem. 153 (1) (2000) 41–47. [8] I. Hamberg, A. Hjortsberg, C. Granqvist, High quality transparent heat reflectors of reactively evaporated indium tin oxide, Appl. Phys. Lett. 40 (5) (1982) 362–364. [9] I. Hamberg, C.G. Granqvist, K.F. Berggren, B.E. Sernelius, L. Engstrom, Band-gap widening in heavily Sn-doped In2 O3 , Phys. Rev. B 30 (6) (1984) 3240–3249. [10] I. Hamberg, C.G. Granqvist, Evaporated Sn-doped In2 O3 films – basic optical-properties and applications to energy-efficient windows, J. Appl. Phys. 60 (11) (1986) R123–R159. [11] S.H. Brewer, D.A. Brown, S. Franzen, Formation of thiolate and phosphonate adlayers on indium tin oxide: Optical and electronic characterization, Langmuir 18 (18) (2002) 6857–6865. [12] L. Jung, H.B. Lu, C.T. Campbell, Sensors based on functionalized surface plasmon resonance devices, Abs. Pap. Am. Chem. Soc. 213 (1997), 20BTEC. [13] A.G. Frutos, R.M. Corn, SPR of ultrathin organic films, Anal. Chem. 70 (13) (1998) 449A–455A. [14] A. Frostell-Karlsson, O. Jansson, R. Karlsson, S. Lofas, BIACORE studies on the characteristics of new sensor chips for biospecific interaction analysis, Abs. Pap. Am. Chem. Soc. 213 (1997) 78-BTEC. [15] C. Yan, M. Zharnikov, A. Golzhauser, M. Grunze, Preparation and characterization of self-assembled monolayers on indium tin oxide, Langmuir 16 (2000) 6208–6215. [16] A. Porch, D.V. Morgan, R.M. Perks, M.O. Jones, P.P. Edwards, Electromagnetic absorption in transparent conducting films, J. Appl. Phys. 95 (9) (2004) 4734–4737. [17] N.H. Chou, J.C. Chou, T.P. Sun, S.K. Hsiung, Study on the disposable urea biosensors based on PVC-COOH membrane ammonium ion-selective electrodes, IEEE Sensors J. 6 (2) (2006) 262–268. [18] Y. Du, H. Wei, J.Z. Kang, J.L. Yan, X.B. Yin, X.R. Yang, E.K. Wang, Microchip capillary electrophoresis with solid-state electrochemiluminescence detector, Anal. Chem. 77 (24) (2005) 7993–7997. [19] N. Araki, M. Obata, A. Ichimura, Y. Amao, K. Mitsuo, K. Asai, S. Yano, Redox and photochemical behaviour of a porphyrin monolayer

[23]

[24] [25] [26] [27]

[28] [29] [30]

[31]

[32]

[33]

[34]

[35]

[36] [37]

[38]

[39] [40]

[41]

579

on an indium-tin oxide electrode, Electrochim. Acta 51 (4) (2005) 677– 683. W.M. Yeh, K.C. Ho, Amperometric morphine sensing using a molecularly imprinted polymer-modified electrode, Anal. Chim. Acta 542 (1) (2005) 76–82. R.N. Goyal, M. Oyama, A. Sangal, S.P. Singh, Differential pulse voltammetric determination of uric acid at nano-gold modified indium tin oxide (ITO) electrode, Indian J. Chem. A: Inorg. Bio-Inorg. Phys. Theo. Anal. Chem. 44 (5) (2005) 945–949. P. Armistead, H. Thorp, Modification of indium tin oxide electrodes with nucleic acids: detection of attomole quantities of immobilized DNA by electrocatalysis, Anal. Chem. 72 (2000) 3764–3770. P. Armistead, H. Thorp, Oxidation kinetics of guanine in DNA molecules adsorbed onto indium tin oxide electrodes, Anal. Chem. 73 (3) (2001) 558–564. P.F. Robusto, R. Braunstein, Optical measurements of the surface plasmon of indium-tin oxide, Phys. Stat. Sol. A 119 (1990) 155–168. T.M. Herne, M.J. Tarlov, Characterization of DNA probes immobilized on gold surfaces, J. Am. Chem. Soc. 119 (38) (1997) 8916–8920. T.A. Taton, C.A. Mirkin, R.L. Letsinger, Scanometric DNA array detection with nanoparticle probes, Science 289 (5485) (2000) 1757–1760. L.A. Chrisey, G.U. Lee, C.E. Oferrall, Covalent attachment of synthetic DNA to self-assembled monolayer films, Nucleic Acid Res. 24 (15) (1996) 3031–3039. A. Ulman, Formation and structure of self-assembled monolayers, Chem. Rev. 96 (4) (1996) 1533–1554. D.J. Lockhart, E.A. Winzeler, Genomics, gene expression and DNA arrays, Nature 405 (6788) (2000) 827–836. M. Chee, R. Yang, E. Hubbell, A. Berno, X.C. Huang, D. Stern, J. Winkler, D.J. Lockhart, M.S. Morris, S.P.A. Fodor, Accessing genetic information with high-density DNA arrays, Science 274 (5287) (1996) 610– 614. S.P.A. Fodor, J.L. Read, M.C. Pirrung, L. Stryer, A.T. Lu, D. Solas, Lightdirected, spatially addressable parallel chemical synthesis, Science 251 (4995) (1991) 767–773. M.L. Bulyk, E. Gentalen, D.J. Lockhart, G.M. Church, Quantifying DNAprotein interactions by double-stranded DNA arrays, Nat. Biotech. 17 (6) (1999) 573–577. L. He, M.D. Musick, S.R. Nicewarner, F.G. Salinas, S.J. Benkovic, M.J. Natan, C.D. Keating, Colloidal Au-enhanced surface plasmon resonance for ultrasensitive detection of DNA hybridization, J. Am. Chem. Soc. 122 (38) (2000) 9071–9077. J.M. Brockman, A.G. Frutos, R.M. Corn, A multistep chemical modification procedure to create DNA arrays on gold surfaces for the study of protein–DNA interactions with surface plasmon resonance imaging, J. Am. Chem. Soc. 121 (35) (1999) 8044–8051. M.B. Esch, L.E. Locascio, M.J. Tarlov, R.A. Durst, Detection of viable Cryptosporidium parvum using DNA-modified liposomes in a microfluidic chip, Anal. Chem. 73 (13) (2001) 2952–2958. A.B. Steel, T.M. Herne, M.J. Tarlov, Electrochemical quantitation of DNA immobilized on gold, Anal. Chem. 70 (22) (1998) 4670–4677. A.B. Steel, T.M. Herne, M.J. Tarlov, Electrostatic interactions of redox cations with surface-immobilized and solution DNA, Bioconj. Chem. 10 (3) (1999) 419–423. S.H. Brewer, S.J. Anthireya, S.E. Lappi, D.L. Drapcho, S. Franzen, Detection of DNA hybridization on gold surfaces by polarization modulation infrared reflection absorption spectroscopy, Langmuir 18 (11) (2002) 4460–4464. Y.W. Cao, R. Jin, C.A. Mirkin, DNA-modified core-shell Ag/Au nanoparticles, J. Am. Chem. Soc. 123 (32) (2001) 7961–7962. L.M. Demers, C.A. Mirkin, R.C. Mucic, R.A. Reynolds, R.L. Letsinger, R. Elghanian, G. Viswanadham, A fluorescence-based method for determining the surface coverage and hybridization efficiency of thiol-capped oligonucleotides bound to gold thin films and nanoparticles, Anal. Chem. 72 (22) (2000) 5535–5541. M. Sauthier, R. Carroll, C. Gorman, S. Franzen, Nanoparticle layers assembled through DNA hybridization: characterization and optimization, Langmuir 18 (5) (2002) 1825–1830.

580

S. Moses et al. / Sensors and Actuators B 125 (2007) 574–580

[42] T. Gardner, C. Frisbie, M. Wrighton, Systems for orthogonal selfassembly of electroactive monolayers on Au and ITO: an approach to molecular electronics, J. Am. Chem. Soc. 117 (1995) 6927– 6933. [43] S. VanderKam, E. Gawalt, J. Schwartz, A. Bocarsly, Electrochemically active surface zirconium complexes on indium tin oxide, Langmuir 15 (1999) 6598–6600. [44] S. Oh, Y. Yun, D. Kim, S. Han, Formation of a self-assembled monolayer of diaminododecane and a heteropolyacid monolayer on the ITO surface, Langmuir 15 (1999) 4690–4692. [45] I. Markovich, D. Mandler, Preparation and characterization of octadecylsilane monolayers on indium-tin oxide (ITO) surfaces, J. Electroanal. Chem. 500 (2001) 453–460. [46] C.J. Loweth, W.B. Caldwell, X.G. Peng, A.P. Alivisatos, P.G. Schultz, DNA-based assembly of gold nanocrystals, Ang. Chem.-Int. Ed. 38 (12) (1999) 1808–1812.

[47] J.F. Smalley, C.V. Krishnan, M. Goldman, S.W. Feldberg, I. Ruzic, Laser-induced temperature-jump coulostatics for the investigation of heterogeneous rate-processes – theory and application, J. Electroanal. Chem. 248 (2) (1988) 255–282. [48] J.F. Smalley, L. Geng, A. Chen, S.W. Feldberg, N.S. Lewis, G. Cali, An indirect laser-induced temperature jump study of the influence of redox couple adsorption on heterogeneous electron transfer kinetics, J. Electroanal. Chem. 549 (2003) 13–24. [49] J.F. Smalley, K. Chalfant, S.W. Feldberg, T.M. Nahir, E.F. Bowden, An indirect laser-induced temperature jump determination of the surface pK(a) of 11-mercaptoundecanoic acid monolayers self-assembled on gold, J. Phys. Chem. B 103 (10) (1999) 1676–1685. [50] J.F. Smalley, L. Geng, S.W. Feldberg, L.C. Rogers, J. Leddy, Evidence for adsorption of Fe(Cn)6 3-/4- on gold using the indirect laser-induced temperature-jump method, J. Electroanal. Chem. 356 (1–2) (1993) 181–200.