Analysis of cellular boundary layers: application of electrochemical microsensors

Electrochemical techniques offer considerable potential for applications in the biological sciences, particularly with regard to sampling chemical gradients in the diffusive boundary layer surrounding cells and tissues. However, their implementation is constrained by problems of discriminating relevant signals from non-specific reactions and bulk concentrations. This paper introduces an approach, referred to as self-referencing, which circumvents some of these problems opening up the application of potentiometric and amperometric sensors to probing the activity of living cells in near to real-time. New applications are described. The design and implementation of an enzyme based self-referencing microsensor capable of measuring a glucose flux of 79 pmol cm −2 s −1 and a cellular consumption of 58±7 fmol nl −1 s −1(mean ± SEM, n=5), where nl represents the biologically active volume, is described. Additionally, we introduce two novel designs, termed electro-optical, where electrochemical sensors are fused with fiber optics, such that intracellular calcium levels can be monitored while collecting collateral data with high spatial and temporal resolution. The amperometric devices are built on the fiber optic surface and are designed to measure cellular oxygen consumption. Potentiometric designs require a single mode fiber to be inserted through the liquid membrane and body of an ion selective electrode. Biological data are presented for the latter design.