Nanomaterial-enhanced sensors for trace heavy metal ion detection: a review of electrochemical and optical methods

Heavy metal ions (HMIs), such as mercury (Hg), lead (Pb), cadmium (Cd), arsenic (As), and chromium (Cr), are a serious environmental issue due to their toxicity, bioaccumulation, and long-term persistence, making it necessary to develop sensitive and selective detection technologies. Laboratory-based methods, such as atomic absorption spectroscopy, provide high accuracy, but their point-of-need deployment is limited by their reliance on large equipment and complicated sample preparation. This review highlights the critical role of nanomaterial in sensing platforms in overcoming these limitations and advancements across electrochemical and optical detection techniques. The integration of nanomaterials-including carbon-based, metallic-based, silicon-based, and quantum dots-is shown to significantly enhance sensor performance through increased surface area, electron transfer efficiency, and plasmonic effects. Despite this progress, challenges such as matrix interference, ensuring signal reproducibility, and developing scalable fabrication methods remain. Future research will focus on developing hybrid, multiplexed, and antifouling sensor architectures integrated with digital technologies like the Internet of Things (IoT) to realize next-generation, ultra-sensitive HMIs monitoring platforms.