Exchange of Oligonucleotide (dC15) on Single-walled Carbon Nanotubes
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Exchange of Oligonucleotide (dC15) on Single-walled Carbon Nanotubes Yuichi Kato1, Ayaka Inoue1, Naotoshi Nakashima1,2,3 and Yasuro Niidome1,2 1 Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan 2 World Premier International (WPI) Research Center, International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan 3 JST-CREST, 5 Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan ABSTRACT Exchange of solubilizers adsorbed on single-walled carbon nanotubes (CNTs) is probed by analysis of the peak shifts of the absorption bands of CNTs in the near-infrared region. Equilibrium constants and thermodynamic parameters of the exchange of sodium cholate for DNA (15-mers of oligo-DNAs, cytosine) on CNTs of different chirality are determined. INTRODUCTION Single-walled carbon nanotubes (CNTs) are rolled-up graphene sheets with rigid onedimensional structures. The diameter of CNTs is uniquely related to their chiral indices (n,m), which represent how the graphene sheets are rolled up. Evaluation of the interactions between molecules and CNTs has been a hot research target. The quenching of fluorescence, Raman shifts, and chromatography-based analysis using CNTs as the stationary phase were used to monitor the adsorption/desorption of molecules on CNTs. However, quantitative evaluation of the interaction between molecules and CNTs has been very limited [1, 2]. Exchange of CNT solubilizers has been used to prepare individually solubilized CNTs [3-5]. The benefit of this method is that it does not require sonication, so destruction or denaturation of the CNTs is avoided. Furthermore, the exchange of the solubilizers on the CNTs is important to understanding the interaction between the solubilizers and CNTs. Here we describe the thermodynamics of the exchange of solubilizers adsorbed on the CNTs based on the analysis of the shifts in the near-IR (NIR) absorption spectra of the CNTs. The equilibrium constants (Ka) and thermodynamic parameters, such as enthalpy changes (ΔH) and entropy changes (ΔS) of the solubilizer exchange of sodium cholate (SC) with oligo-DNA (15mers of cytosine) on the CNTs are determined. The dependence of tube diameter on Ka and changes in ΔH and ΔS are also described. In order to prove our presented thermodynamic treatment, we used SC and oligo-DNA, which are well-known CNT solubilizers. EXPERIMENT As CNTs, HiPco-CNTs (Unidym) were used in this study. The CNTs were dissolved in aqueous micelles of SC (denoted SC-CNTs). Experimental conditions to obtain individually solubilized CNTs were the same as those reported previously [6]. The concentration of SC ([SC]) was adjusted to 2 mM, while the concentration of the DNA ([DNA]) ranged from 0-15.6
μM. All of the solutions contained phosphate buffer (pH 11.5, 20 mM). The solutions were stored for one day at a temperature of 15–40 °C, then absorption spectra were measured using a spectrophotometer. RESULTS and DISCUSSION Figu
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