Self-Referenced Evanescent Wave Sensor for Low Limit of Detection Applications

We have designed and implemented a low noise evanescent wave sensor (EWS) based on lock-in amplifier and sensitized glass optical fibre for detection of waterborne and airborne analytes. We stripped the buffer and cladding from a 2 cm section in the middle of a 20 cm long 400 μm core multimode glass optical fibre and coated it with a film of a sensitive molecule. We then ran the coated optical fibre through a sample vessel such that the film was in contact with a sample that may contain analyte, and probed with modulated light, coupled into the fibre from an LED. We matched the LED peak emission wavelength to the absorption peak of the sensitizer-analyte complex formed and used the reference voltage output of a digital lock-in amplifier for modulation. To account for possible LED drift, we propagated the modulated light through a 50:50 beam splitter, where one beam was coupled to the coated fibre (sample beam) and the other acts as a reference beam. We then projected the two beams onto two photodiodes connected to a ‘light balance’ circuit that delivers a differential current ∆i, which is proportional to the absorbance along the sample beam and converts it into a voltage. Finally, this voltage was fed into the digital lock-in amplifier for low- noise measurement. As a proof of concept, we spray-coated the stripped section of the optical fibre with 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine (EHO), which is sensitive to acetic acid and ran the fibre through a gas exposure chamber. Then we exposed acetic acid vapour, which is being detected, into the chamber and found a limit of detection (LoD), (defined as the lowest concentration that a sensor can detect) equal to 1.491 ppt (61.48 µM). To compare the performance of our EWS in air and in aqueous media, we coated the same sensitizer (EHO) to the stripped section of the fibre and run through deionised water in a sample vessel. Titrating acetic acid solution into the vessel, we obtained a LoD equal to 18.43 µM, which is 3.3 times better than in air. We further tested the performance of our EWS by spray-coating the stripped section of the fibre with 1-(2-pyridylazo)-2-naphthol (PAN) to detect aqueous Zn2+ and obtained 31.07 nM as a LoD. To improve the analytical performance of our EWS, we roughened the stripped section of the sensing fibre with a home-made roughening tool based on a Dremel tool. Then we spray-coated optical fibres with a macrocyclic sensitizer, zinc 5-(4-carboxyphenyl),10,15,20-triphenyl porphyrin (Zn(P-CO2H-TPP)) and used them to detect waterborne octylamine realizing a LoD equal to 2.17 μM. In conclusion, we find EWS sensors are particularly useful for sensing in the aqueous medium, as the higher refractive index (1.333) of water, (compared to air (1)) allows deeper evanescence of the light propagating inside the fibre, and hence better coupling to the sensitizer coating, than in gas sensing.