Highly Sensitive Determination of Copper Ions as MnO<sub>2</sub> Etching Inhibitor in Single-Particle Nanoplasmonic Imaging

Dark-field microscopy (DFM) imaging\nbased on plasmonic metal nanoparticles\nhas garnered significant attention. Here, we exploit the susceptibility\nof MnO<sub>2</sub> to reduction to modulate the local dielectric environment\nof an Au nanoparticle core through the etching/antietching effects\nof specific targets on the encapsulated MnO<sub>2</sub> shell. The\npresence of d-penicillamine promotes MnO<sub>2</sub> etching,\nwhile the chelation of d-penicillamine with Cu<sup>2+</sup> effectively inhibits this etching. By recording the Cu<sup>2+</sup>-induced color shift of scattered light from orange to bright green\nat the single-particle level and performing the statistical analysis\nof the green-to-red (G/R) values in DFM images, we achieved quantitative\ndetermination of Cu<sup>2+</sup> with a wide linear range (0.1–10\nμM) and a low limit of detection (4.55 nM). With the facile\nand reliable Cu<sup>2+</sup> assay in real-world samples exemplifying\nthe practicality of the single-particle nanoplasmonic imaging method,\nthis work may inspire future DFM-based investigations of nanoshell\netching inhibition processes.