Synergistic Effect Improves the Response of Active Sites to Target Variations for Picomolar Detection of Silver Ions

Heavy metal ions seriously threaten human health; even a trace of them can damage the renal, nervous, and immune systems irreversibly. Although established nanozyme-based colorimetric assays have been designed for the rapid detection of heavy metal ions, the general contained surface organic ligands of nanocatalysts and low absorptivity of metal ions on solid substrates might result in a weak effect on active sites and prevent the realization of their full detection potential. Here, we developed a nanozyme-based colorimetric sensor (CPM-Pt) made by pyrolysis of peat moss with preabsorbed traces of Pt ions to ultrasensitively detect Ag⁺. The calcination removes organic components and produces bare nanozymes that expose rich active sites. The strong protective effect from the porous carbon support enables the embedded Pt nanoparticles (Pt NPs) with a partially stable positive charge after pyrolysis (∼28% Pt²⁺ species). By the d⁸–d¹⁰ metal–metal interactions between Pt²⁺ (4f¹⁴5d⁸) and Ag⁺ (4d¹⁰), the high proportion of Pt²⁺ species on the surface of Pt NPs can readily capture/absorb Ag⁺. Subsequently, Ag⁺ accepts electrons from the support to form Ag atoms, which rapidly cover the peroxidase-like active sites of bare Pt NPs, weakening the activation of H₂O₂ to realize the response of Ag⁺. The colorimetric detection limit of Ag⁺ reached an unprecedented 1.1 pM, and the corresponding naked-eye color recognition is ultrasensitive to extremely low levels (100 pM).