We reported recently the first application of the scanning electrochemical microscope (SECM) to probe the redox activity of individual living cells. The possibilities of measuring the rate and investigating the pathway of transmembrane charge transfer (CT) were demonstrated. Significant differences were detected in the redox responses given by normal human breast epithelial cells (MCF-10A) and metastatic breast cancer cells (MDA-MB-231). Here, we demonstrate that the intracellular redox potentials and concentrations of redox components in the cell can be evaluated noninvasively by the SECM. Such measurements provide mechanistic explanation of the observed differences in cell redox activities.

In SECM, a microscopic electrochemical sensor, usually called the tip, is scanned over the surface of a sample, and topographic images and maps of chemical reactivity across the surface are obtained. The schematic diagram of such measurements is shown in Figure 1. The oxidized (or reduced) form of redox species initially present in solution reacts at the surface of the ultramicroelectrode tip.