Asymmetric N2NiS2 Luminescent Single-Atom Catalyst for pH-Versatile Ultratrace Detection and Selective Catalytic Remediation of Carcinogenic Cr(VI) Ions

Abstract Simultaneous trace monitoring and ambient detoxification of hexavalent chromium (Cr(VI)) in complex industrial wastewater remain a critical challenge due to matrix interferences and the conventional need for auxiliary chemical reductants. Herein, an asymmetrically coordinated Ni(MBI)2 single-atom catalyst (SAC) featuring a polarized N2NiS2 electronic architecture was synthesized via a one-pot, thiol-assisted route for dual-mode ultratrace detection and ambient chemical remediation. The atomic dispersion and molecular identity of the Ni sites were established by integrating HR-ESI-MS (<3 ppm), XRD, and quantitative XPS analysis to confirm isolated Niδ+ centers. Operating across an extreme pH range (1–14), the system achieved subppb electrochemical detection limits (down to 0.290 ppb in acidic medium) and was validated in raw, alkaline leather-industry wastewater with a recovery rate of 99.58 ± 3%. Electrocatalytic testing demonstrated a maximum sensitivity of 9702.5 μA ppm–1 cm–2 and ultralow LODs of 0.530 ppb (alkaline), 0.413 ppb (neutral), and well below the WHO limit of 0.005 ppm. Concurrently, the immobilized SAC array on portable polypropylene discs enabled rapid, visual photoluminescent monitoring (92.4% quenching efficiency) with high tolerance against cocontaminants. Without external light or chemical inputs, the catalyst sustained ambient reduction of 81.2% Cr(VI) within 30 min (k ≈ 0.0505 min–1). DFT, XPS, and EIS revealed that broken coordination symmetry induces an exceptional surface dipole moment (2.82D) and hard–soft “push-pull” charge redistribution. This localized polarization promotes Cr oxyanion chemisorption (−4.25 eV) and overcomes electric double-layer transport resistance, establishing a versatile framework for self-monitoring remediation of carcinogenic chromium in water.