Optical sensors for sodium, potassium and ammonium ions based on lipophilic fluorescein anionic dye and neutral carriers

Thin plasticized polymer membranes doped with a neutral pH chromophore together with selective ionophores have been employed for developing cation optical sensors. The response is based on cation exchange which results in deprotonation of the pH dye, and therefore changes in the membrane absorbance. In this study, optical sensors incorporated with appropriate lipophilic ionophores and a fluorescein anionic chromophore are investigated for the sensing of several small cations. The valinomycin, nonactin and sodium ionophore IV doped membrane films yield optical responses to potassium, ammonium and sodium respectively with detection limits on the order of 10−5.3 M, and improved selectivities compared with ion-selective electrodes based on the same ionophore. Interestingly, the sodium ionophore III doped membrane yields very little response to sodium and other monovalent cations, yet it exhibits significant response to calcium and magnesium. The response principle and application of these optodes are discussed.