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The rich chemical, physical, electronic, and surface properties of two-dimensional (2D) materials have witnessed an explosion of research leading to enrichment in the 2D family of materials. Cobalt sulfide (Co x S y ), a transition metal chalcogenide, has attracted significant interest as its different stoichiometric and non-stoichiometric phases offer diverse properties. To date, the Co3S4 phase has seen limited developments with recent reports suggesting good catalytic properties of this material. In the current study, we show the facile synthesis of phase pure 2D Co3S4 nanosheets using a hydrothermal approach and demonstrate the importance of the chemical reactivity of the surface atoms in achieving target selectivity during sensing. First, the catalytic activity of Co3S4 nanosheets was observed to follow Michaelis–Menten kinetics, suggesting that the nanosheets mimic the catalytic activity of the natural peroxidase enzymes. The presence of dangling sulfur atoms on the surface of the Co3S4 nanosheets allowed them to specifically interact with l-cysteine amino acid that resulted in the temporary loss of its catalytic activity. This interaction allowed us to develop a “turn-off” colorimetric sensor to detect l-cysteine even in complex mixtures of amino acids and other sulfur containing compounds. In-depth understanding of the sensor mechanism revealed a reversible competition between l-cysteine and peroxidase substrates for binding to the Co3S4 nanosheets. Due to the higher affinity of l-cysteine to the nanosheets, it first binds to the Co3S4 nanosheets blocking its nanozyme activity. Subsequently, the outstanding catalytic activity of 2D Co3S4 nanosheets assists in the oxidation of l-cysteine, and as oxidized products leave the nanozyme surface, its catalytic activity resumes. The findings of our study are likely to instigate further research into the exploration of the chemical reactivity of 2D surfaces for diverse applications.

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