Atomic force microscope: a tool for studying ionophores

Abstract

The aim of the investigations was to show the analytical use of an atomic force microscopy (AFM) tip coated with an ion-selective membrane and to show that the ion-selective boundary potential is detectable as a force induced by ion-selective electrostatic interactions, which are more pronounced than double-layer forces. This new technique allows the area-specific ion exchange over boundaries to be displayed with a destruction-free technique by AFM in an aqueous buffer. From experiments with ISEs (ion-selective electrodes), a boundary potential for valinomycin was assumed to be established in close vicinity to a K+-releasing surface. To trace this boundary potential, an AFM tip was coated with a potassium-selective polymer film containing valinomycin as used in preparing ISEs. The K+-releasing substrate was prepared by incorporating a lipophilic potassium salt into a plasticized PVC layer. In contact with an electrolyte such as sodium chloride solution, an ion exchange takes place. Analogue experiments were performed using a sodium-selective ionophore, DD16C5, incorporated into the coating of the AFM tip, with a Na+-releasing substrate. The boundary potential was traced and investigated with the help of force vs distance curves. The resulting adhesion forces for a valinomycin-coated tip in a 150 mM NaCl solution were 9.8+/-3.275 nN using a blank PVC substrate and 330.15+/-113.0 nN using a substrate containing 10 wt % potassium tetrakis(4-chlorophenyl) borate. The selectivity of the ion-selective AFM tips was measured on sodium relative to potassium-releasing substrates and studied in different salt solutions with concentrations between 10 mmol L(-1) and 1 mol L(-1). For valinomycin, a force selectivity coefficient log Kf(K,Na) of -2.5+/-0.5 for K+ against Na+ and a selectivity coefficient log Kf(Na,K) of -4 +/- -0.5 for DD16C5 were measured. In addition, the surface of the polymer substrate was imaged by AFM in contact mode with and without lipophilic potassium salt. The modulation of the force-distance curves induced by the ion exchange was compared to the experimental change in elasticity of the blank and ion-exchanging substrate. The Young's moduli measured with strain force analysis on a potassium-containing substrate were 5 times smaller than the ones registered with nanoindentation and did not explain the modulation of the force vs distance curves.