Abstract: Optical chemical sensors are becoming increasingly popular and a multitude of designs have been reported recently.All these devices rely on a light source and a detector.Optical fibres are often employed to bring the incident light to the detector.Spectral selectivity is achieved conventionally using monochromators.Here we report on a compact probe that consists of a photodiode coated with a poly (vinyl chloride) (PVC) sensitive membrane and a light emitting diode having a relatively narrow spectral output.The two diodes are embedded in a plastic barrel opposite each other and the photodiode can be dipped into solutions similar to an ion-selective electrode.ETH1810 and ETH 2439 were chosen as the lithium ion carrier and pHindicator, respectively.The ETH2439 shows an absorption band at 514 nm in the unprotonated form which changes to about 650 nm when protoned.The competitive binding of lithium ion and hydrogen ion will change the color of the PVCmembrane.The red light emitting diodes have a maximum output at 660 nm, so that the change can be adequately monitored.The sensor shows a linear response to lithium ion over the range of 1×10^-4-7.9 × 10^-2 mol/Lwith the same selectivity as in potentiometric system.By use of the standard-addition method, different serum samples containing Li⁺ in the clinical range were assayed with this photodiode sensor.

Key words: Photodicxle sensing device, Lithium ion, pH indicator