Silver-Doped UiO-66 Metal–Organic Framework as U-Bent Optical Fiber Nanocoating for Trace Cadmium Ions Sensing in Wastewater

Abstract Cadmium (Cd(II)) contamination in water poses a severe threat to public health and aquatic ecosystems, requiring continuous monitoring below the 3 parts per billion (ppb) safety limit established by the World Health Organization. Traditional detection relies on conventional techniques such as inductively coupled plasma mass spectrometry and atomic absorption spectroscopy, which are expensive laboratory-bound equipment, creating an urgent need for portable and accessible alternatives. To address this environmental challenge, we developed a simple, highly sensitive “dip and detect” optical fiber sensor designed for the continuous surveillance of trace Cd(II). This portable platform utilizes a silver-doped metal–organic framework (Ag/UiO-66) coated in situ as a nanoscale layer over a U-bent fiberoptic sensor (U-FOS). The interaction of Cd(II) ions with this functional coating triggers a structural reorganization, enabling direct and measurable optical transduction via an evanescent wave through the U-FOS. This platform achieved an experimental detection limit of 1 ppb, operating below international regulatory thresholds. The sensor demonstrated excellent selectivity against potential competing heavy metal ions. Furthermore, it maintained optimal performance at a neutral pH and retained 80% of its initial sensitivity over an eight-week storage period. To validate its real-world applicability, the sensor was tested on raw sewage treatment effluents collected across Chennai, India, yielding cadmium concentrations with remarkable recovery rates of 93–107% in the ppb range, with reference to the gold-standard mass spectrometry measurements, highlighting the potential of Ag/UiO-66 coated U-FOS probes as a scalable and deployable solution for Cd(II) monitoring in complex wastewater matrices.