). Ultra-Low Noise Fibre Optic Sensor for Low Limit of Detection in Aqueous Medium.

Abstract

Water from some sources contains waterborne pollutants (e.g lead or cadmium cations) toxic to both human beings and animals even at low concentrations. This calls for sensitive, low cost and portable instruments that can be applied for on-site detection of these pollutants, e.g. ‘optrodes’ (optical sensors). The limit of detection of existing optical sensors is still higher than the recommended safety standards, which is attributed to the detector noise superimposed to the low level signal. In our quest 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. We coated a thin film of 1-(2-pyridylazo)- 2-naphthol as a Zn2+ sensitizer on a 2 cm stripped section of a 20 cm long sensing fibre optic and run through a sample vessel. 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 to very low levels (50 nM) of dissolved Zn2+. With suitable sensitizer, our transducer has a potential application to field measurements of low concentrations of waterborne pollutants.