Zeptomolar heavy metal ion detection with density of states sensing

Heavy metal contamination in water poses serious risks to human health, and several metal species such as Cr, Ni, and As are classified by the World Health Organization as Group 1 carcinogens. Here, we present an on-chip heavy metal ion sensor based on density of states (DOS) sensing, using the quantum capacitance of monolayer MoS₂ to probe adsorption-induced electronic-structure changes. As a proof of concept, Cr³⁺ was used as the target ion. After adsorption from aqueous solution, the DOS spectra reveal four additional in-gap states together with clear perturbations near the valence- and conduction-band edges, in qualitative agreement with first-principles calculations. Multiple DOS-derived parameters, including defect state intensities, band-edge slopes, and the effective gap extracted from the quantum capacitance spectra, evolve with the Cr³⁺ concentration from 10^-20 to 10^-15 M. These metrics show strong linear relationships with the logarithm of concentration, with an average R² of 0.93, enabling ultralow-concentration detection with multi-signal fingerprints. This work demonstrates the potential of DOS sensing for heavy metal ion detection based on adsorption-induced DOS fingerprints.