The various ions present in the extra- and intracellular mediums, such as potassium, occupy an important role for many
biological phenomena. So far, biologists and electrophysiologists have been working hard to understand the role of
several ions in cellular behaviors. Our objective is to measure ionic concentration fluctuations using a tapered optical
(fiber) guide and an ionic indicator. The optical cylindrical waveguides are tapered to a final diameter of 10
micrometers. The resulting probes are first used to transmit excitation light (349 nm wavelength) into the solution, and
then, they are used to collect a potassium indicator (PBFI) fluorescence. The indicator emission depends on the
potassium concentration and, by monitoring this fluorescence, a correlation can be made with the potassium current.
Two types of optical waveguides have been studied: a multimode fiber optic (Thorlabs FG-200-UCR) and a borosilicate
capillary (generally used as an electrophysiological electrode). Results show that concentration fluctuations in the order
of 10 mM can be monitored using tapered optical guides. However the signal to noise ratio and the sensing repeatability
are requiring further improvements. Thus, tapered optical waveguides can be used as ionic sensors. It has been
demonstrated that sensors as small as 10 micrometers are sensitive to concentration fluctuations. Optical indicators are
widely used in microscopy and they offer many possibilities in terms of their specificity (for ions as well as for other
particles). Thus optical fibers, by guiding the light into deep regions, allow for the use of optical indicators in vivo.