Atomic Force Microscope: A Tool for Studying Ionophores

摘要

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 boundaiy potential is detectable as a force induced by ion-selective electrostatic interactions, which are i*ore pronounced than double-layer forces. This new technique allows the area-specific ion exchange over boundaries to be displayed wi*h a destruction-free tech-nique 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 Km-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 plasti-cized 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 coat-ing 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 NaCI 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 Ltand 1 mol L’. For valinomycin, a force selectivity coefficient log 4Na of—2.5 ± 0.5 for K~ against Na~ and a selectivity coefficient log K~aK 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.