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Contaminated drinking water is a global health issue, particularly for third-world countries. Fluoride is a widely found contaminant whose prolonged exposure in quantities greater than 1.5 mg/L causes various health hazards, including dental fluorosis and stiffness in the backbone or joints. State-of-the-art techniques for detecting fluoride in water include titrimetric, potentiometric, spectrophotometric, and chromatographic systems. Although these techniques provide fair accuracy, these systems face mass adaptability constraints due to their resource-intensive nature. This suggests a need to develop sensors that can provide rapid, portable, and accurate fluoride detection in drinking water. Here, we demonstrate a fluoride sensor by employing the principle of phase shift cavity ring down spectroscopy (PS-CRDS) in fiber cavities. In PS-CRDS, the magnitude of phase shift between output and input sinusoids is a measure of an absorption event within the cavity. We realize the sensor by building a cavity using fiber Bragg gratings. We insert a tapered fiber of < 10 μm waist within the cavity as a sensing head. We inject a 2 mL sample solution along with 2 mL of pre-stored SPADNS−zirconyl acid complex reagent for enhancing absorption at 633 nm into a customized fluidic cell. The cell holds the tapered portion of the cavity. We then quantify fluoride in the sample using PS-CRDS with the detection limit of less than 2 ppm. We anticipate that the current work with a modified chemistry protocol can be extended to a rapid and accurate biosensor for detecting a variety of diseases, including tuberculosis and pneumonia.
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