Label-Free, Single-Particle, Colorimetric Detection of Permanganate by GNPs@Ag Core-Shell Nanoparticles with Dark-Field Optical Microscopy.

Rapid quantification of permanganate (MnO4-) in aqueous solution with a convenient and sensitive single-particle-detection (SPD) method is demonstrated by dark-field optical microscopy. The design is based on the selective etching of the Ag shell of a glucose-protected GNPs@Ag nanoparticle by MnO4-. In the presence of MnO4-, a noticeable red-shift of localized surface-plasmon resonance (LSPR, from blue to green) together with a tremendous decrease in the extinction coefficient from individual GNPs@Ag nanoparticles is observed. MnO4- can then be quantified by calculating the ratio between the number of green and blue particles on the cover glass surface after the etching process. A linear dynamic range of 0-6 µM and a limit of detection (LOD) as low as 46 nM were readily achieved, which are much lower than those of spectroscopic measurements in bulk solution. In tap water, a comparable LOD (50 nM) and satisfactory recovery efficiency are demonstrated. As a consequence of these merits, the method demonstrated herein will find promising applications for the ultrasensitive detection of MnO4- under complex milieu in the future.