Abstract

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 π-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 π-way may change the electrical 1 Present address: Nanotechnology Research Center, Research Institute for Electronic Science, Hokkaido University, Sapporo 001- 0021, Japan. 000-2614/$ - see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.08.061 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 95s 15p1 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 experimental results, we discuss the chemical doping effect on Ag-doped DNA conductor.
2. Experimental
Ag-doped DNA bundles were prepared with different dopant concentrations as follows: Small bundles (<10 mg) of dry fibrous calf thymus DNA (SIGMA) were im- mersed into small amount (a few µL) of aqueous solu- tions of AgNO 3 (Wako) for ≈1 min under [Ag +]/[Base pair] = 0  (non-doped), 10 – 5, 10 – 4, 10 – 3, 10 – 2, 10−1, 1 and 10, where [Ag +] and [Base pair] are defined as the numbers of silver ions and base pairs, respectively, obtained from weight of dry fibrous DNA. The ratio of [Ag + ]/[Base pair] was determined stoichiometrically. The DNA bundles were then stretched and fixed to cover glasses (Matsunami Glass, Ind. Ltd.). The samples were dried at ≈26 °C for at least several hours. The prepared bundles were 50–100 μm-diameter in the center and ≈10 mm-length. We confirmed that rapid evaporation occurred at both ends of the DNA bundle in the dry process and then impurities including Ag * were accumu- lated there. To determine the Ag binding site, we carried out an extended X-ray absorption fine structure (EXAFS) anal- ysis for Ag-doped DNA at [Ag +]/[Base pair] = 0.1 by a transmission method at Ag K-edge in BL01B1 of SPring-8 [16]. In this analysis, we used a software, R EX2000J (Rigaku). The I–V measurement was made by a two-probe method at room temperature under room humidity (30–80%). The bundles were put in a vacuum (≈10−6 Torr) to remove water. Then gold electrodes were deposited on both ends of the DNA bundles. A typical electrode gap was 30–50 μm. The bias voltage was swept between ±1 V. The average diameter and length of the bundles on the electrode gaps were measured by an in- verted microscope IX-70 (Olympus) to obtain electrical conductivity. We confirmed that leakage current through the glass surface was <10−13 A and can be ne- glected, and also that the electrical conductivity of the DNA bundle did not depend on the ambient humidity.
3. Results and discussion
3.1. EXAFS spectra
The binding site of Ag + in DNA was determined by an EXAFS analysis [17]. Fig. 2 indicates the Fourier transforms of the spectra at Ag K-edge of Ag-doped DNA  at  [Ag *]/[Base  pair] = 0.1. The  distances  of  all peaks are displayed shorter than the actual values be- cause of the phase shift [17]. The height of main peak at R = 1.8 A, assigned to the nearest neighbor(s) of Ag, depends strongly on the kind and number of the nearest neighbor atom(s) [17]: i.e., two nitrogen atoms in G–Ag–C, a nitrogen atom in G (a coordination bond) and an oxygen atom in phosphate group (Ag–O). The fitting peaks are fixed at R = 2.1 Å from covalent radii and reference values, but their amplitudes are different. It is clearly shown that the fitting peak on G–Ag–C (Fig. 2a) is the best agreement among the three possibil- ities. Other possible configurations, including the forma- tion of silver wire outside of DNA [18], are also ruled out from the EXAFS results. We have also found from EXAFS spectra that the amount of binding Ag* is pro- portional to [Ag + ]/[Base pair]. The details will be pub- lished separately.
3.2. Electrical properties
To evaluate the conductivity, we carried out the I–V measurements for various Ag-doped DNA bundles. Fig. 3 shows typical I–V curves of the non-doped and the Ag-doped DNA bundles at [Ag +]/[Base pair] = 0.01 and 0.1. Nonlinear I–V curves with hysteresis were ob- served for all samples. Nonlinear conductions in DNA have been reported in previous studies on DNA conduc- tion [6,8–10], but the hysteresis has not been reported to be very slight [10]. By analogy with the I–V curves in dielectrics [19] and charge–density-wave (CDW) [20,21], the clear hysteresis observed here reflects the existence of transient states in electrical conduction in the DNA bundles. Since guanine (G) is easily oxidized and then charged [11], DNA is a dielectrics consisting of an array of numerous G–C (charging site) and A–T (insulating site) pairs. Formation of bundle may en- hance the hysteresis of a single DNA molecule. We have also found other characteristics of the I-V curves: asymmetry of I–V curves and conductance fluc- tuations. The observed I–V curves are asymmetric with respect to the origin. These features have also been ob- served [6,10], but the reasons have never been discussed. Very recently, it has been shown that a symmetry/asym- metry of I–V curves of single organic molecules depend on spatially-symmetry/asymmetry of the organic mole- cule [22]. The asymmetric I-V curves observed here seems to be affected by the chirality of DNA. The con¬ ductance fluctuations, observed in junction arrays as single electron tunneling [23], have observed as small steps on the nonlinear I–V curves in Fig. 3a, similarity to that reported in [6]. The conductance fluctuations indicate that DNA also has the nature of a junction array. Asymmetric I–V curves and conductance fluctua- tions thus are important to understand the electrical property of DNA beside the chemical doping effect. Here, we emphasize that the electrical conduction arising from physical adsorption and chemisorption of Ag + can be denied from the view points of sample prep- aration (the rapid evaporation at both ends of the DNA bundles), EXAFS and the electronic structure of Ag compounds.
3.3. Chemical doping effect
To study the chemical doping effect on conductivity, we evaluated the electrical conductivity of non-doped and Ag-doped DNAs from the I–V curves. The conduc- tivity σ was determined from the slopes of I–V curves at 0 V and sample size and then the dependence of σ on [Ag + ] was evaluated. Fig. 4 illustrates the Ag + concen- tration dependence of σ , which systematically changes from 10- 10 to 10- 6 Ω- 1 cm- 1 within the range of [Ag*]/[Base pair] from 10~6 to 10. The dependence of σ on [Ag +] is determined by a least-squares method within experimental error as
(1) The scattering of data points in Fig. 4 primarily arises from the measurement of thickness of DNA bundles. The σ of non-doped DNA is (8.24 ± 2.06) × 10−10 Ω− 1 cm− 1 , which agrees with that in a recent study [24] (not plotted in Fig. 4).
3.4. Conduction mechanism
To understand the conduction mechanism described by Eq. (1), we discuss the possibilities based on the dop- ing effect in semiconductor including a critical behavior in metal-insulator transition, ionic conductions and or- ganic conductors (π-electron system) as typical exam- ples, as shown below. First, we discuss the possibility of doped semiconduc- tor. The doping effect in semiconductor is described by σ ∞ n [12], where n is the number of dopant. This indi- cates that DNA differs from a conventional semiconduc- tor. Moreover, we compared with a critical behavior in metal-insulator transition of doped semiconductor [25], but Eq. (1) is quite different from that because the range in which Eq. (1) is satisfied is too wide as the critical behavior. Next, we examine the possibility of an ionic conduc- tions: whether Ag + moves along DNA chain. In this case, the mechanism is explained by ionic conductions in a strong electrolyte solution, where σ ∞ n [26], or solid state ionics, where σ ∞ [Ag +] x , x = +1.93 and 1.64 for Ag–Ge–S and Ag–Ge–Sb–Se at 298 K, respectively [27]. In either case, the dependence is much stronger than in Eq. (1). Therefore, the ionic conductions can be disregarded in our problem. Finally, we compare the doping effects between DNA and common π-electron system. The latter sys- tem such as polyacetylene, poly( p -phenylene) and poly( p -phenylene sulphide) exhibits a steep metal-insu- lator transition [28–30]. For example, in [CH(AF 5 ) x]n the σ increases over eight orders from 10 –5 to 10 2 Ω− 1 cm− 1 at x  < 0.05 [28], whereas the doping effect for DNA is much weaker. Therefore, the Ag-doped DNA is different from usual π-electron system. What is the origin of the difference between usual π-electron systems and DNA? The essential difference is regular- ity of their structures, because the π-system has a peri¬ odic structure, whereas DNA has a long-range correlated disordered sequence [31]. Very recently, Stanley and co-workers [32] have predicted that one- dimensional long-range correlated disorder electron system such as DNA shows a metal-insulator transi- tion. Such a study would be necessary not only to understand the 'weakly doping effect' obtained here, but could also lead to a better understanding of DNA itself.
4. Conclusion
The novel experimental results of the chemical dop- ing effect on Ag-doped DNA conductor are reported. Nonlinear I–V curves with clear hysteresis and conduct- ance fluctuations are observed and the doping effect is determined to be σ ∞ [Ag +] 0.5 , which is discussed in com- parison with that in semiconductor, ionic conductions in a strong electrolyte solution and solid state ionics, and π-electron system. The origin of the 'weakly doping ef- fect' may be discussed in terms of long-range correlated disordered sequence reflecting history of life.
Acknowledgements
This work has been partially supported by a Grant- in-Aid for the 21st Century Centre of Excellence (COE) Program, entitled "Topological Science and Technology", from the Ministry of Education, Culture, Sport, Science and Technology of Japan (MECSST).