Highly Sensitive Determination of Copper Ions as MnO₂ Etching Inhibitor in Single-Particle Nanoplasmonic Imaging

Dark-field microscopy (DFM) imaging based on plasmonic metal nanoparticles has garnered significant attention. Here, we exploit the susceptibility of MnO₂ to reduction to modulate the local dielectric environment of an Au nanoparticle core through the etching/antietching effects of specific targets on the encapsulated MnO₂ shell. The presence of d-penicillamine promotes MnO₂ etching, while the chelation of d-penicillamine with Cu²⁺ effectively inhibits this etching. By recording the Cu²⁺-induced color shift of scattered light from orange to bright green at the single-particle level and performing the statistical analysis of the green-to-red (G/R) values in DFM images, we achieved quantitative determination of Cu²⁺ with a wide linear range (0.1-10 μM) and a low limit of detection (4.55 nM). With the facile and reliable Cu²⁺ assay in real-world samples exemplifying the practicality of the single-particle nanoplasmonic imaging method, this work may inspire future DFM-based investigations of nanoshell etching inhibition processes.