Arsenic detection in drinking water using evanescent fiber cavity ring down spectroscopy

Arsenic is a significant drinking water contaminant whose prolonged consumption can cause cancer, skin lesions, and cardiovascular diseases. The environmental protection agency (EPA) declares 10 ppb and lower as an acceptable limit for arsenic in drinking water. Several technologies, such as biomolecules, nanoparticles, nanowires, carbon nanotubes, and quantum dots have been deployed under optics to develop viable optical sensors for arsenic detection in water. Although these approaches offer decent accuracy, they require trained laboratory personnel and state-of-the-art lab facilities, impose several adaptability constraints, and take hours in results production. Therefore, there is a need for an economical and easy-to-fabricate optical sensor that can provide rapid, precise, and portable detection of arsenic in drinking water. Here, we demonstrate a novel fiber sensor that employs phase shift cavity ring down spectroscopy (PS-CRDS) to record phase shift measurements in fiber cavities. In PS-CRDS, the cavity ring down time is proportional to the phase shift between the reference modulating signal and the cavity output, a measure of sensing event (absorption due to arsenic) inside the cavity. The sensor utilizes a tapered fiber of waist ˂ 12 µm as a sensing head. We place the tapered fiber in a fluidic cell and insert the assembly inside the optical cavity. Furthermore, we chemically treat the solution with Azure B to add specificity toward arsenic detection. Azure B acts as a chromogenic reagent that enhances the absorption loss of arsenic in water samples at 633 nm. We inject 3 mL of arsenic-contaminated water and Azure B solution into the fluidic cell and record phase shift measurements. We experimentally demonstrate that the sensor has a minimum detection limit of 5 ppb and a sensitivity of 0.0133o /ppb. We anticipate our work will lead to rapid, portable, and accurate optical sensors for physical, chemical, and biological applications.