pH-Regulated Surface Plasmon Absorption from Ultrasmall Luminescent Gold Nanoparticles

Luminescent gold nanoparticles (AuNPs) with ultrasmall sizes (2-3 nm) and few free electrons have attracted fundamental science and application research interests due to their robust emission properties, but discoveries on their fantastic plasmonic properties are limited. Herein, controllable surface plasmon resonance (SPR) absorption (>520 nm) is reported from ultrasmall luminescent AuNPs (approximate to 2.0 nm) coated with a small thiolate surface ligand, mercaptoimidazole (MIZ), through pH regulation. The 690 nm emitting MIZ-AuNPs show SPR absorption and emission with a reversibly pH-dependent response, i.e., a strong emission with no SPR band at high pH values (e.g., pH 5.0) and strong SPR absorption (570 nm) with weak emission at low pH values (e.g., pH 2.0). The MIZ surface coating, which has a low steric hindrance and high tendency to form intermolecular hydrogen bonds, is shown to play an important role in the creation of strong plasmon coupling and the emergence of plasmonic properties from ultrasmall MIZ-AuNPs with 44% Au(I) species. The discovery of a plasmonic property in semiconducting luminescent MIZ-AuNPs opens a new pathway for further understanding the optical properties of AuNPs and designing a new generation of optical AuNPs for various future applications.