<p> The successful low-temperature synthesis of Bi₂S₃ nanostructures via an in-situ solvothermal route employing the single-source precursor [Bi{S₂P(OC₃H₇)₂}₃]&#xa0;is reported. Harnessing this synthesis strategy, we engineered a pristine Bi₂S₃-based sensor platform that enables the simultaneous, multiplexed colorimetric detection of Co²⁺, Hg²⁺, and Cr³⁺ ions on unmodified hydrophobic substrates. This uniquely integrated sensor streamlines analytical workflows by eliminating extensive sample pre-treatment and specialized probes, simplifying field operation. The device exhibits remarkable detection limits,1 nM for Co²⁺, 10 nM for Hg²⁺, and 1000 nM for Cr³⁺ alongside high selectivity, operational stability, and repeatability. A regression-based machine learning algorithm was employed to process the optical response, enhancing the accuracy of heavy metal ion classification and quantification. The IoT-enabled architecture supports real-time, remote monitoring and validation studies in diverse water samples from Hyderabad, India, confirming platform robustness. This study positions single-source precursor-derived Bi₂S₃ nanostructures as a transformative solution for smart, decentralized water quality analysis, advancing progress toward universal access to safe water.</p> Graphical Abstract <p></p>