• • The Fe1-xS@CNT nanozyme-based colorimetric method achieves a limit of detection (LOD) of 0.04 μg·L−1 for Hg2+, which is below the WHO guideline for drinking water (1 μg·L−1), enabling trace-level monitoring.
• • The method exhibits a wide linear range of 0.1–500 μg·L−1, covering typical environmental contamination levels, and demonstrates high accuracy with recoveries of 94.4%–111.1% in real water samples.
• • The relative standard deviations (RSD) for replicate measurements (n=3) are consistently below 3.0% across spiked concentrations (1, 200, 450 μg·L−1), indicating excellent precision and reproducibility for field applications.
• • The colorimetric response is visible to the naked eye, allowing rapid semi-quantitative screening without sophisticated instrumentation, which is critical for resource-limited settings and on-site environmental monitoring.