Local pH Measurement with Scanning Ion Conductance Microscopy

Abstract

We report microscale pH probes and their application as a scanning probe in Scanning Ion Conductance Microscopy (SICM). Experiments are performed with electrodes that consist of a thin gold layer deposited on one side of a nanopipet. Subsequent insulation of the electrode with parylene and electropolymerization of aniline to form a polyaniline (PANi) thin film on the gold electrode surface creates a microscale pH probe with a dynamic range of pH 2.5–12 and sensitivity of 0.02 ΔpH. Porous membranes were examined with prepared electrodes. Ion current measured at the nanopipet opening served to control the position of the electrode with respect to the membrane surface and to report local conductance in the vicinity of the nanopipet, while local pH was measured by recording the potential difference between the PANi electrode and a reference electrode. Application of a transmembrane potential difference was utilized for additional control over migration across the membrane. Lateral resolution of the EMF response was studied with micrometer diameter pores. Additionally, the vertical resolution of the EMF response was studied as the pipet was lowered into the pores where a ΔpH of 0.13 was measured.