A highly efficient, visible wavelength (λMAX = 700 to 800 nm) fluorescence is reported for four water-soluble monolayer-protected gold clusters (MPCs). For 451 nm excitation, the quantum yield of the luminescence of MPCs with 1.8 nm diameter cores and protected by monolayers of tiopronin thiolate is estimated as 0.003 ± 0.001. The efficiency and wavelength of the luminescence vary with the ligands of the monolayer around the gold core. The mechanism for the luminescence is hypothesized to be associated with interband transitions between the filled 5d10 band and 6(sp) conduction band.

Visible-near-IR luminescence spectra of gold MPCs that are similar, irrespective of the number of core atoms (all <2 nm diameter) and different monolayers, are reported. The luminescence can be quantitatively invoked by introducing polar ligands into nonpolar MPC monolayers and by galvanic exchange of metal atoms on the MPC core surface with different metals. The observed emissions are believed to result from surface-localized states that depend on both the core metal of the nanoparticle and the ligands attached to the metal surface.

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