A fluorescent optode for sodium ion based on the inner filter effect

A fluorescent optode for sodium ion has been developed based on the inner filter effect (IFE). A suitable fluorescing compound (5, 10, 15, 20-tetraphenylporphine (TPP)) is incorporated in a plasticized poly(vinyl chloride) (PVC) membrane containing the sodium-selective ionophore (calix[4]arene derivative) and the proton-selective chromoionophore (ETH5294). On account of the overlap between the fluorescent emission spectrum of TPP and the absorption spectrum of ETH5294, fluorescence intensity of such a PVC membrane varies as a function of the varying absorbance of ETH5294 induced by sodium concentration due to the IFE. The optode showed excellent selectivity and reversibility to sodium ion over the range of 5 x 10(-5)-1 mol 1(-1) with a response time of less than 2 min. The proposed method offers a general approach, which can be applied to any other cations by replacing the sodium-selective ionophore with any other suitable cation-selective one. (C)2000 Elsevier Science B.V. All rights reserved.