Abstract: Recently, many reversible optical chemical sensors based on ion-exchange schemes for sensing metal ions with comparable selectivities are developed. Most sensors of this kind reported so far are based on absorbance measurements. However, luminescence technique is an especially suitable optical chemical sensing method for designing these sen sors. This is due in part to its inherent sensitivity, its almost complete inertness to turbidity effects, and its flexibility with respect to geometric arrangements. In this paper, a fibre-optic sensor of fluorescence enhancement for sodium ions has been developed on the basis of ion-exchange sensing scheme. Asuitable fluorescer is incorporated in the sensing membrane containing a lipophilic pH-indicator and sodium-selective neutral ionophore. Fluorescence varies as a function of the varying absorption of the pH-indicator which in turn is modulated by the sodium concentration through the inner filter effect of fluorescence. The sensor shows a linear reversible response to sodium ion over the range of 1× 10^-2─2×10^-5mol/Lwith a detection limit of 8 × 10^-6mo/L. The response time of the sensor is about 2-3min. The concentrations of sodium ions in serum and mineral water samples are determined by this sensor with satisfactory results.

Key words: Inner filter effect of fluorescence, Sodium ion, Optical chemical sensor, Optode