Anomalous doping effect on Ag-doped DNA conductor H. Mayama a, * ,1 , T. Hiroya b , K. Inagaki b , S. Tanda b , K. Yoshikawa a a Department of Physics, Graduate School of Science, Kyoto University and CREST (Core Research for Evolutional and Scientific Technology) of JST (Japan Science and Technology Corporation), Kyoto 606-8502, Japan b Department of Applied Physics, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan Received 28 June 2004; in final form 11 August 2004 Available online 11 September 2004 Abstract We report novel experimental results of chemical doping effect on Ag-doped DNA conductor. The prepared Ag-doped DNA samples were confirmed by EXAFS that Ag + was doped into p-way. In I–V measurements at room temperature, nonlinear I–V curves with hysteresis emerged and systematic change in electrical conductivity r was observed from 10 10 to 10 6 X 1 cm 1 under doping condition [Ag + ]/[Base pair] = 10 6 10. The relation between the conductivity and dopant concentration showed a weak dependence r µ [Ag + ] 0.5 . This Ôweakly doping effectÕ may be caused by the long-range correlated randomness of sequence in DNA, in contrast with usual doped organic conductors. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction The uniqueness of DNA originates in its structure, in which history of life and its functions are coded in the stacking of base pairs [1]. Because of the p-way arising from the stacking, DNA is considered to be a one- dimensional electron system [2]. Recently, electrical property of DNA has been investigated intensively for possible use in molecular devices [3–11]. There is a wide range of spectra in the results of the previous studies from Anderson insulator to superconductor. To investi- gate the electrical property of DNA, other approaches may be needed. Chemical doping is a prominent strategy for control- ling the electrical properties of materials, as demon- strated in semiconductors [12], electrically conductive polyacetylene [13] and high-T c superconductors [14].A suitable dopant to the p-way may change the electrical property of DNA. There have been a few previous stud- ies on the electrical property of chemically doped DNA [8–10]. However, no systematic investigation on the chemical doping effect of electrical property has yet been made. One of the main reasons is that it is very difficult to evaluate how much of the dopant binds to DNA on the level of a single molecule. We are convinced that bundles of orientated DNA molecular chains are really suitable for a study of chemical doping effect, because precise evaluation of the amount of the dopant bound to DNA will thereby be feasible. In this Letter, we report novel experimental results on chemical doping effect on Ag-doped DNA conductor. We adopted Ag + as a dopant, which is expected to occupy easily the space between guanine (G) and cytosine (C) and forms two rigid bonds, as shown in Fig. 1 [15]. Ag + is substituted for H + which was previously bound to a nitrogen in guanine. Then the Ag takes an electron out of a double bond in cytosine and becomes 4d 9 5s 1 5p 1 structure, which corresponds to hole doping. Under such experimental design, we have prepared Ag-doped DNA bundles at different Ag + concentrations and per- formed their I–V measurements. On the basis of the 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.08.061 * Corresponding author Fax: +81 11 706 9357. E-mail address: mayama@es.hokudai.ac.jp (H. Mayama). 1 Present address: Nanotechnology Research Center, Research Institute for Electronic Science, Hokkaido University, Sapporo 001- 0021, Japan. www.elsevier.com/locate/cplett Chemical Physics Letters 397 (2004) 101–105