A novel DNA-modi ed indium tin oxide electrode

Transcription

1 Electrochemistry Communications 2001) 5±9 A novel DNA-modi ed indium tin oxide electrode Jinzhong Xu, Jun-Jie Zhu *, Qing Huang, Hong-Yuan Chen Department of Chemistry, The State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 21009, People's Republic of China Received September 2001; accepted 10 September 2001 Abstract By means of adsorption, ss-dna and ds-dna were modi ed onto the surface of two silanized indium tin oxide ITO) electrodes, respectively, and were found to have gained high concentration after being electrochemically characterized. The electrode, modi ed by ss-dna, recognized the complementary ss-dna in solution with a good repeatability. The hybridization±regeneration was cycled three times, with reproducible results. Ó 2001 Published by Elsevier Science B.V. Keywords: DNA-modi ed electrode; Indium tin oxide electrode; Gene detection; Adsorption of DNA; Monolayer 1. Introduction The development of DNA-sensor devices has continued to attract considerable attention in connection with gene analysis, detection of genetic disorders, tissue matching and forensic application [1±4]. The DNAmodi ed electrode, mainly used as an electrochemical DNA biosensor for gene detection [5,] over the last decade, has more advantages than conventional labeling probe techniques. Due to its potential application to molecular recognition, it will be of practical value in both molecular biology and modern biomedical engineering. However, many fundamental problems about this electrode are still unclear. Much work has been done on the optimal immobilization of DNA hybridization on electrode surfaces and a few methods for immobilizing DNA onto electrode surfaces have appeared [7±9]. Controlled potential adsorption method was adopted by Wang et al. [10] to anchor DNA to carbon paste electrodes. Herme and Tarlov [11] synthesized DNA fragments modi ed with a mercapto group by the use of a DNA synthesizer and then immobilized DNA onto gold electrode surfaces through self-assembly of the mercapto-modi ed DNA. According to the reference, covalent coupling is a good way to immobilize the DNA onto electrodes with perfect function of hybridization and bio-a nity. However, * Corresponding author. Fax: address: gxyz@nju.edu.cn J.-J. Zhu). these methods are of shortages. Due to the low surface concentration of the covalent coupling, the application of this method is a ected. It is inconvenient to prepare DNA-modi ed electrodes by self-assembly of mercapto-modi ed DNA because of the low yield of mercapto-containing DNA, so this method is tedious, time-consuming and costly. Compared with the methods above, adsorption is found to be a simple, yet e cient way to gain high concentration of DNA on electrode surface, using only small amounts of samples. The approach with simplicity, reproducibility and precision, was used to adsorb and anchor DNA molecule on the silanized surface of mica or glass [12,1]. However, it is reported the ss-dna adsorbed on electrode cannot hybridize with ss-dna in solution [14]. If the DNA-modi ed electrode can be prepared with adsorption and it can be used for DNA biosensor, it is still very signi cant. Impedance spectroscopy is an e ective method to probe the interfacial properties capacitance, electron-transfer ability) of modi ed electrodes and can be used to con rm and amplify the preparation and hybridization of DNA sensor [15,1]. In this work, the DNA molecules were adsorbed onto the silanized ITO surface with high concentration, and the ss-dna-modi ed electrode hybridized with ss-dna in solution. The preparation and the hybridization of the DNA-modi ed electrodes are characterized by di erent electrochemical methods and two popularity electrochemical probes in detail /01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S )

2 J. Xu et al. / Electrochemistry Communications 2001) 5±9 2. Experimental 2.1. Apparatus and chemicals Calf thymus DNA CT-DNA) was obtained from Sigma Company, and was used as received. Native double stranded DNA ds-dna) was dissolved in water prior to use. Denatured single stranded DNA ss-dna) was produced by heating native ds-dna solution in a water bath at 100 C for 5 min, followed by rapid cooling in ice bath [14]. Tris 1,10-phenanthroline)-cobalt )perchlorate Co phen ClO 4 H 2 OŠ was prepared as [17]. -Aminopropyl)trimethoxysilane APS) was obtained from Nanjing Shuguang Chemical Factory China). Other chemicals were of analytical reagent grade. Water used was twice distilled. Cyclic voltammograms CV), di erential pulse voltammetry DPV) and A.C. impedance were performed with CHI0 Instrument CH Instrument, USA). Threeelectrode system was employed with saturated calomel reference electrode SCE), an ITO working electrode and a platinum auxiliary electrode Methods Electrode ITO-coated sheets were cut into 0:5 1:0 cm 2. The ITO electrodes were cleaned after sonication in acetone for 20 min, in detergent solution for 15 min, and twice in water for 15 min each cycle. The electrodes were then treated with 5 M NaOH for 5 h; prior to rinsing with water and deriving in an aqueous solution of 0.2% -aminopropyl)trimethoxysilane for 10 min. Substrates were rinsed with water and allowed to dry in air. The electrodes were incubated in ds-dna or ss-dna solution for 5 min, extracted from the solution at a constant speed and dried in air. The DNA-modi ed electrodes were rinsed with water and 20 mm Tris ph ˆ 7.0) with 20 mm NaCl solution before using. The ss-dna-modi ed electrode was immersed in a hybridization bu er 00 mm NaCl + 0 mm sodium citrate ph ˆ 7.0), denoted as a 2 SSC buffer) containing 200 lg=ml ss-dna for 1 h at 70 C under shaking. The whole system was gradually cooled to 25 C. Thus the hybridization of ss-dna in solution took place. The ds-dna on the electrode was denatured after being heated in a 10 mm Tris-HCl + 1 mm EDTA bu er ph ˆ 8.0) at 100 C for 5 min. Due to its strong interaction with DNA at low ionic strengths, Co phen Š can be used to judge the quantity of immobilized DNA on the electrode indirectly. So it is a good indicator to characterize DNA-modi ed electrodes. In addition, the shifts of the CV peak potentials provide information on the interaction mode of DNA and the indicators. Co phen Š binded into ds- DNA by hydrophobic interaction, thus the peak potential shifts positively. Fig. 1 shows the modi cation of the ITO electrode with ds-dna. Curve a) is the CV for Co phen Š obtained at the ITO electrode surface with ±OH OH/ ITO) after treating with 5 M NaOH [18]. Then the electrode was modi ed with ±NH 2 by treatment with APS, the result is shown in curve b). The curve c) shows that ds-dna molecules were anchored on the electrode by adsorption. The current of peak increases dramatically in comparison with that of curve b), and the formal potential shifts positively. The results indicate that ds-dna immobilized on the electrode surface can enrich Co phen Š due to its strong bound interaction with Co phen Š. Co phen Š were accumulated onto the ds-dnamodi ed electrode after immersing the electrode in 0.1 mmol/l Co phen Š solution for 5 min. Then the electrode was rinsed with water, and ready for Ip±v experiments. Fig. 2 shows a linear dependence of the cathodic peak current with the scan rate in the range of 10±200 mv/s, indicating that Co phen Š is strongly bound to the ds-dna-modi ed electrode surface. A total surface concentration of C ˆ 4: mol=cm 2 Co phen Š on the ds-dna-modi ed electrode surface can be calculated, according to the equation: Ip ˆ n 2 F 2 msa=4rt and the slope of the Ip±v curve. Supposing ve base pairs bind one molecular of Co phen Š [], and the saturate monolayer adsorbance is 2: mol=cm 2, the amount of ds-dna adsorbed on the electrode is estimated to be ca.. Results and discussion.1. Electrochemical characterization of ds-dna-modi- ed ITO electrode Fig. 1. Cyclic voltammograms of 0.1 mm Co phen Š in 20 mm Tris ph ˆ 7.0) with 20 mm NaCl at 50 mv/s: a) hydroxylated/ito; b) silanized/ito; c) ds-dna/ ITO.

3 J. Xu et al. / Electrochemistry Communications 2001) 5±9 7 Fig. 2. Plot of Ip vs v of the ds-dna-modi ed electrode. 2: mol=cm 2, which is equivalent to a coverage of ca. 84.5%..2. Electrochemical characteristics of the ss-dna-modi ed ITO Scheme 1 shows the structure of ss-dna-modi ed electrode and the hybridization with ss-dna. The impedance spectra include a semicircle portion and a linear portion, the semicircle portion at higher frequencies corresponds to the electron-transfer limited process, and the linear part at lower frequencies corresponds to the di usion process. The semicircle diameters correspond Fig.. Nyquist-diagram Z im vs Z re for the A.C. impedance measurements of an ITO electrode in the present of 10 mm Fe CN =4 Š 1:1-mixture) in 0.1 M KCl: a) hydroxylated/ito; b) silanized/ito; c) ss-dna/ito; d) ds-dna/ito. The impedance spectra were recorded wthin the frequency range 0.21 Hz±100 khz at the formal potential of the Fe CN =4 Š redox couple. The amplitude of the alternate voltage was 5 mv. to the electron-transfer resistance R et, R et at the electrode surface respective. The changes of the impedance spectra were observed in the course of stepwise formation of the DNA sensor as shown in Fig.. Curve a) shows the impedance features of the ITO electrode after the ITO electrode was treated with 5 M NaOH, the R et is ca X. Then, the electrode was modi ed with ±NH 2 by treatment with APS, the result is shown in curve b), (A) (B) Scheme 1. Preparation A) and recognization B).

4 8 J. Xu et al. / Electrochemistry Communications 2001) 5±9 and the R et is ca. 400 X. The curve c) shows that ss- DNA were anchored on the electrode surface, the R et is ca X. This is consistent with the fact that the interface negatively charged with ±OH repels the negatively charged redox probe Fe CN =4 Š. When the surface was modi ed with ±NH 2, the interface was charged positively. Fe CN =4 Š can reach the surface easily, which leads to the decrease of R et. When the ss- DNA molecules were immobilized on the surface, the negative charge of the DNA leads to the increase of the R et. The results are similar to those obtained on gold electrode according to [18]. A highly sensitive electrochemical technique, DPV is usually used to detect the trace composition in sample. Here, we use DPV to observe the changes of the peak current and potential of the positive probe Co phen Š. The results are displayed in Fig. 4. Curve a) is the DPV feature of the negatively charged interface with ±OH: curve b), the positively charged interface with ±NH 2 ; curve c), the interface which ss-dna are immobilized on. Interaction between positive probe Co phen Š and charged surface of the electrode gave rise to the highest peak current and the most positive potential in curve a), the small peak current in curve b), the increase of peak current and the shift of the peak potential in curve c), comparing with curve b). In addition, the interaction between ss-dna and Co phen Š also accounts for the di erence of curve b) and c) in peak current and peak potential. The result shows that ss-dna have been immobilized on the electrode surface, and the coverage is estimated to be ca. 77%, according to reference method... Characterization of hybridization of the ss-dnamodi ed ITO As is well known, an electrochemical DNA sensor is prepared by immobilizing ss-dna with the special sequence onto the electrode surface, and the electrochemical method is used to detect whether the ss- DNA on the surface can hybridize and recognize the target DNA e ciently. The quality of the sensor relies on the hybridization of ss-dna with its target DNA. A.C. impedance is a sensitive method to detect the quality of the sensor. The curve d) in Fig. is the impedance spectrum after the sensor was hybridized with the target DNA. The R et ca. 500 X) in curve d) is much larger than the R et 1400 X) in curve c), which con rms the good quality of this sensor and its e ciency in hybridizing and recognizing the target DNA. After the sensor was hybridized with ss-dna in solution, some of the ss-dna on the surface of the sensor turned to ds-dna. Since the Co phen Š can be embedded into the DNA groove. The interaction between Co phen Š and DNA molecule is more than an electrostatic interaction. As shown in Fig. 4, the peak current of the curve d) increases and the peak potential shifts positively in comparison with the curve c). The ds-dna-modi ed electrode is used to repeat the experiment in the same conditions, but the peak current does not increase. The results are in agreement with those at glass carbon electrode reported in the reference, further showing that the ss- DNA on the surface of ITO electrode can hybridize with the DNA in solution. According to the reference, ss-dna adsorbed on the electrode surface cannot hybridize with ss-dna in solution, but it is realized in our experiment. The ss-dna adsorbed on the silanized ITO electrode is di erent from those adsorbed on the bare electrode. We think that the APS layer between the electrode and the DNA molecules plays an important role. The in uence of concentration of the APS solution is invested, and 0.2% is chosen as the optimum concentration with the silanized time of 10 min. Fig. 4. The DPV of 0.1 mm Co phen Š in 20 mm Tris ph ˆ 7.0) with 20 mm NaCl: a) hydroxylated/ito; b) silanized/ito; c) ss- DNA/ ITO; d) ds-dna/ ITO. Fig. 5. The DPV of 0.1 mm Co phen Š in 20 mm Tris ph ˆ 7.0) with 20 mm NaCl: a) hybridization with ss-dna in solution; b) regeneration in TE bu er at 100 C; c) rehybridization with ss-dna in solution.

5 J. Xu et al. / Electrochemistry Communications 2001) 5± The hybridization±regeneration The hybridization±regeneration was cycled three times, with reproducible results. The DPV of one hybridization±regeneration cycle are shown in Fig. 5. Little change has been observed in peak currents and peak potential of di erent circle. The ds-dna on the electrode was denatured after being heated in a 10 mm Tris- HCl + 1 mm EDTA bu er ph ˆ 8.0) at 100 C for 5 min. 4. Conclusions The DNA molecules have been adsorbed onto the silanized ITO surface to develop a novel DNA-modi ed electrode, and the electrodes have high surface concentration. The ss-dna-modi ed electrode can be applied to recognize the complementary ss-dna in solution. This method is simple and convenient, with a perfect repeatability. The sensor is stable for at least several days upon storage at 4 C in a dry state. Acknowledgements We greatly the nancial support from the National Natural Science Foundation of China and the Ministry of Education of China. References [1] M. Yang, M.E. Mcgovern, M. Thompson, Anal. Chim. Acta ) 259. [2] P.G. Righetti, C. Gel, Electrophoresis ) [] K.M. Millan, S.R. Mikkeisen, Anal. Chem ) 217. [4] E. Palecek, Electroanalysis 8 199) 7. [5] K. Hashimoto, I. Keiko, Y. Ishimori, Anal. Chem. 1994) 80. [] K. Hashimoto, I. Keiko, Y. Ishimori, Anal. Chim. Acta ) 219. [7] X. Xu, A.J. Bard, J. Am. Chem. Soc ) 227. [8] Y. Zhao, D. Pang, Z. Wang, J. Cheng, Y. Qi, J. Electroanal. Chem ) 20. [9] S. Liu, J. Ye, P. He, Y. Fang, Anal. Chim. Acta 5 199) 29. [10] J. Wang, M. Chicherro, G. Rivas, X. Cai, N. Dontha, P. Farias, H. Shiraishi, Anal. Chem ) [11] T.M. Herme, M.J. Tarlov, J. Am. Chem. Soc ) 891. [12] X. Michalet, R. Ekong, F. Fougerousse, S. Rousseaux, C. Schurra, N. Hornigold, M.V. Slegtenhorst, J. Wolfe, S. Povey, J.S. Beckmann, A. Bensimon, Science ) [1] A. Bensimon, A. Simon, A. Chi audel, V. Croquette, F. Heslot, D. Bensimon, Science ) 209. [14] D. Pang, Z. Wang, Y. Qi, J. Cheng, Z. Liu, J. Electroanal. Chem ) 18. [15] C. Gabrielli, Use and Application of Electrochemical Impedance Techniques, Famborough, [1] A. Bardea, F. Patosky, A. Dagan, I. Willer, Chem. Commun. 1999) 21. [17] L.S. Dollimere, R.D. Gillard, J. Chem. Soc. Dalton Trans. 197) 9. [18] R.B. Kenneth, P.F. Audrey, J.N. Michael, J. Am. Chem. Soc ) 1154.