ULTRA-LOW NOISE FIBRE OPTIC SENSOR FOR LOW LIMIT OF DETECTION IN AQUEOUS MEDIUM

Abstract

Water from some sources contains waterborne pollutants like lead or cadmium cations, which are toxic to both human beings and animals even at low concentrations. This calls for sensitive, low cost and portable instruments like ‘optrodes’ (optical sensors) that can be applied for on-site detection of these pollutants. However, the limit of detection of existing optical sensors is still higher than the recommended safety standards, due to the detector noise superimposed to the low level signals arising from lower dissolved pollutant concentrations. To solve this problem, we introduced a self-referenced evanescent wave optical fibre as a transducer, and a ‘light balance’ with lock-in read-out for minimum noise levels. From these techniques, we report low noise levels of 64 nV due to lock-in detection and a lower LoD as compared to earlier transducers. This was demonstrated with a clear signal response of ~10 μV from very low levels of dissolved Zn²⁺ (50 nM). With suitable sensitizer, our transducer has a potential application to field measurements of low concentrations of waterborne pollutants.