Submicron sensors for ion detection based on measurement of luminescence decay time

Submicron optochemical sensors for pH and chloride were developed by coating silanised optical fibre tips of ∼300 nm diameter and aluminium-coated SNOM fibres of ∼50 nm aperture with polymeric membranes containing luminescent indicators. Luminescence decay time was measured using a phase-modulation technique. Changes in decay time are induced by resonance energy transfer from a ruthenium complex (the donor) to the pH indicator bromothymol blue (the acceptor). The donor–acceptor ion pair was immobilised in a hydrogel membrane and undergoes a change in decay time with pH. The chloride sensor was made by combining the ion pair for optical transduction with the chloride-carrier tridodecylmethylammonium chloride in a plasticised PVC membrane. Chloride ions present in the solution are carried into the membrane. In order to maintain electroneutrality, an equivalent molar quantity of protons is coextracted into the membrane where the dye is protonated. Both sensors are suitable for measurements in physiological fluids.