Fibre optic sensor for the detection of potassium using fluorescence energy transfer

Abstract

A fluorescence fibre optic sensor has been developed for measurement of the potassium concentration in aqueous solution based on the change in optical absorbance of the hydrophobic indicator 7-decyl-2-methyl-4-(3′,5′-dichlorophen-4′-one)indonaphth-1-ol (MEDPIN). The sensor was constructed by dipping the distal end of a single optical fibre in a poly(vinyl chloride)(PVC) coating solution containing MEDPIN, a plasticiser, the ionophore valinomycin and the fluorescent dye 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindodicarbocyanine perchlorate [DilC₁₈(5)]. The change in the absorbance of MEDPIN induced by potassium was detected from the quenching of the fluorescence of DilC₁₈(5) due to energy transfer. Glass slides dipped in the PVC coating solution were used to establish the optical properties of the sensor. The potassium concentration was detected by the absorbance of MEDPIN at 650 nm and by the quenching of the DilC₁₈(5) fluorescence by MEDPIN. The fluorescence intensity and lifetime of DilC₁₈(5) decreased by ca. 40% with the addition of 33 mM KCl, indicating a resonance energy transfer mechanism. Experiments with the fibre optic sensor showed a decrease in the fluorescence of 57% with increasing potassium concentration (0–5 mM) at pH 7.45. The potassium concentration giving a 50% decrease in the fluorescence (K_d) for the most sensitive probe was 0.05 mM KCl. The value of K_d was increased to 1.3 mM when the plasticiser was changed from 2-nitrophenyl octyl ether to tris(2-ethylhexyl) phosphate; however, the 90% response time increased from 10 s to 4.2 min. Sodium and other physiological ions, and pH (6–8), had little or no effect on the fluorescence of the probe; the reproducibility for the determination of the potassium concentration was better than 1% in both blood and aqueous buffers. These results provide a sensitive and selective fibre optic sensor for potassium for potential application to in vivo monitoring of the potassium concentration.