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Development of an Amperometric Sensor for Chiral Recognition in Organic Solvent Using the Gate Effect of Molecularly Imprinted Polymer

Shin-ichi Sekine1, Yuta Watanabe1, Yasuo Yoshimi1, Koji Hattori2, and Kiyotaka Sakai2. (1) Applied Chemistry, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo, 135-8548, Japan, (2) Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan

Abstract: The purpose of this work is development of a sensor with chiral selectivity in organic solvent, which is necessary for monitoring many chemical processes. We grafted poly(methacrylic acid-co-ethyleneglycol dimethacrylate) imprinted by D- or L-phenylalanine anilide (PAA) on indium-tin oxide (ITO) electrodes. Cyclic voltammetry of ferrocene with the grafted ITO was performed in the organic solvents. The effect of D- or L-PAA on current was estimated. The chiral-selective change in current was observed by the cyclic voltammetry in nonpolar solvent.

Introduction: Biosensors using enzymes or antibodies as biochemical recognitive materials have been used for the detection of optically active compounds (e.g., amino acid) but are unusable in organic solvents. High-performance liquid chromatographic (HPLC) methods with chiral stationary phases have been used for chiral recognition and resolution. However, HPLC methods cannot be utilized for real-time monitoring inherently. Recently, molecularly imprinted polymers (MIPs) have been developed as substitutes for biochemical recognitive materials. MIPs have specific binding sites suitable for target molecules (templates) [1, 2]. MIPs have some advantages compared to biochemical recognitive materials (e.g., robustness and inexpensive preparation). In addition, MIPs can function in organic solvent. However, no established methodology is available for transducing the specific binding events of MIPs toward the electric signal. We focus attention on "gate effect", which is a change in solute permeability of the MIP membrane by specific interaction with its template [3]. We reported previously that the diffusive permeability of the redox molecule in a thin layer of MIP immobilized on an electrode changed in the presence of the template [4]. The gate effect was observed as the change in the current of the redox molecule. Results indicated that MIP with the gate effect functions as a transducer and as a recognitive material of a chemical sensor. In this study, we estimated the chiral recognition in organic solvent electrochemically using the gate effect of MIP.

Experimental: Surface of indium-tin (ITO) electrode was methacrylated by 3-methacryloxypropyltrimethoxysilane. The methacrylated ITO was soaked in the toluene solution of D- or L-phenylalanine anilide (PAA; template), ethylene glycol dimethacrylate, methacrylic acid, and a polymerization initiator. The solution was polymerized by heating to 60 °C for 12 h to graft the D- or L-PAA imprinted polymer onto the surface of the ITO. The ITO was ultrasonicated in distilled water to remove the weakly-adsorbed copolymer, and that was submerged in a methanolic solution of acetic acid followed by methanol to remove the template. The electrode imprinted with D- and L- PAA is referred to as DIP and LIP-ITO, respectively. Conventional cyclic voltammetry of ferrocene in organic solvent [chloroform, dichloromethane (DCM), pyridine, N,N-dimethylformamide (DMF), or acetonitrile (AN)] was conducted using DIP-ITO or LIP-ITO as a working electrode. Effects of the guest (D- or L-PAA) on the anodic current of ferrocene were estimated.

Results: Relative changes in maximum anodic current by 5 mM guest and chiral selectivity defined by (1) were shown in Table 1 for the each solvent of the test solution. The anodic currents in nonpolar solvent (chloroform or DCM) changed significantly in the presence of the template, but were insensitive to the enantiomer of the template. However, anodic currents in polar solvent (pyridine, DMF, or AN) were minimally sensitive to the template and enantiomer. Results indicate that the MIP on ITO expressed the gate effect with chiral recognition in nonpolar solvents. The template interact with the carboxyl group of the binding site in MIP mainly through the hydrogen bonding [6]. However, the interactions are weakened in polar solvents, which interact strongly with the polar groups of the template and MIP. Consequently, the gate effect with chiral recognition only occurs with nonpolar solvents. This gate effect could be applied to simple amperometric sensors to provide chiral selectivity in nonpolar solvent where biosensors usually do not function.

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