Key Takeaways & Executive Findings
- •• • 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.
Abstract
The escalating environmental contamination by mercury ions (Hg2+) poses severe risks to ecosystems and human health, necessitating the development of rapid, sensitive, and cost-effective detection methods. In this study, Fe1-xS@CNT composite nanozymes were synthesized via a straightforward solvothermal approach. The nanozymes exhibit uniform morphology, structural stability, and significant peroxidase (POD)-like activity. The incorporation of carbon nanotubes (CNT) facilitates electron transfer, enhancing the Fenton reaction between Fe2+/Fe3+ to generate abundant reactive oxygen species (ROS), primarily hydroxyl radicals (·OH) and superoxide anions (·O2−). The synergistic action of these ROS and photogenerated holes (h+) promotes the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to a blue-colored product (oxTMB), establishing a colorimetric system of Fe1-xS@CNT + H2O2 + TMB. The specific binding of S2− on the nanozyme surface to Hg2+ inhibits POD activity, reducing the absorbance of the system. This principle was harnessed to develop a colorimetric method for Hg2+ quantification in environmental water samples. The method demonstrates a linear range of 0.1–500 μg·L−1 and a limit of detection (LOD) of 0.04 μg·L−1. Validation in real water samples (campus and tap water) showed recoveries between 94.4% and 111.1% with relative standard deviations (RSD) below 3.0%, comparable to atomic fluorescence spectrometry. The method offers advantages of simplicity, rapid analysis, and naked-eye visibility, providing a novel approach for on-site monitoring of heavy metal pollutants.
1. Introduction
Mercury pollution remains a critical global concern due to its severe neurotoxicity and bioaccumulation in food chains. Conventional detection techniques, such as atomic fluorescence spectrometry (AFS) and inductively coupled plasma mass spectrometry (ICP-MS), offer high sensitivity but are hindered by high operational costs, complex sample preparation, and the need for skilled personnel, limiting their deployment for routine environmental surveillance. This bottleneck has driven the search for simple, rapid, and cost-effective alternatives that maintain analytical performance while enabling on-site application.
Nanozymes, nanomaterials with enzyme-like activities, have emerged as promising candidates for colorimetric sensing due to their high stability, tunable catalytic properties, and low production costs. However, single-component nanozymes often suffer from insufficient catalytic activity and poor selectivity in complex matrices. The Fe1-xS@CNT composite nanozyme addresses these limitations by integrating iron sulfide nanoparticles with carbon nanotubes, which enhances electron transfer and promotes the generation of reactive oxygen species. This design not only amplifies the peroxidase-like activity but also provides specific binding sites for Hg2+ via surface sulfide ions, enabling a selective and sensitive colorimetric detection platform. The developed method demonstrates a low detection limit and robust performance in real water samples, offering a practical solution for heavy metal monitoring.
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WANG Hongbo, PENG Xinyu, SUN Haiyang, ZHENG Chijie, GU Zhenzhen, WANG Xuedong, LIU Tingting (2026). Preparation of Fe1-xS@CNT Composite Nanozyme and Its Application in Colorimetric Detection of Hg2+. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025022302
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Frequently Asked Questions
What is the underlying mechanism for the selective detection of Hg2+ using the Fe1-xS@CNT nanozyme?
The selectivity arises from the strong affinity between Hg2+ and sulfide ions (S2−) on the nanozyme surface. This specific binding inhibits the peroxidase-like activity of the nanozyme, likely by blocking active sites or altering the redox properties of iron species, thereby reducing the oxidation of TMB and decreasing the absorbance signal. The method shows no significant interference from other metal ions, as demonstrated in the study.
How does the Fe1-xS@CNT nanozyme compare to traditional detection methods like atomic fluorescence spectrometry (AFS) in terms of accuracy and precision?
In validation experiments with spiked water samples, the colorimetric method achieved recoveries between 94.4% and 111.1%, with relative standard deviations (RSD) below 3.0% for triplicate measurements. These values are comparable to those obtained by AFS, which showed recoveries of 94.8%–111.2% and RSDs up to 4.95%. The colorimetric method offers the advantages of lower cost, simpler operation, and faster analysis time, making it suitable for high-throughput screening.
What is the limit of detection (LOD) of the developed method and how does it compare to regulatory limits for mercury in water?
The LOD is 0.04 μg·L−1, which is significantly lower than the WHO guideline of 1 μg·L−1 for mercury in drinking water. This high sensitivity allows for the detection of trace levels of Hg2+ in environmental samples, ensuring compliance with safety standards.
What are the potential limitations or interferences in the colorimetric detection of Hg2+ in real water samples?
The study evaluated the method's performance in campus and tap water samples, showing good accuracy and precision. However, potential interferences from other heavy metal ions or organic matter could affect the assay. The authors did not report detailed interference studies, but the high specificity of S2− for Hg2+ suggests minimal cross-reactivity. Future work should investigate the effect of common coexisting ions and organic matter to ensure robustness in diverse environmental matrices.
Can the Fe1-xS@CNT nanozyme be scaled up for commercial production and what are the cost implications?
The synthesis involves a simple solvothermal method using ethylene glycol as solvent, which is scalable and cost-effective. The raw materials (iron salts, sulfur sources, carbon nanotubes) are relatively inexpensive. The method's simplicity and low cost make it attractive for commercial development, though further optimization of synthesis conditions and quality control would be necessary for large-scale production.
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