Photoluminescence of Gold Nanoparticles and Their Applications to Sensing and Cell Imaging - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 197

the decreased NP size and increased contribution of the alkanethiol-
passivated surfaces, thereby decreasing the energy gap between the
quantized levels as a result of a smaller core and a higher coverage
of thiolate. As the particle size decreases, the surface-to-volume
ratio increases and a larger fraction of the NP core atoms becomes
involved in forming bonds with the core-capping ligands.
Based on the well-established spectral assignment for the
Au(I)-thiolate complexes, the absorption of alkanethiol-Au NPs
at 375 nm presumably originates from metal-centered and/or
ligand-metal charge transfer (LMCT; S → Au) transitions. 28-30 In
addition, the luminescent alkanethiol-Au NPs have large Stokes-
shifted luminescence (1.03 eV) with long lifetimes (>250 ns) —
typical characteristics of thiol-Au(I) complexes that display ligand-
metal charge transfer and metal(I)-metal(I) interactions. 62-64
Thus, the luminescence of the alkanethiol-Au NPs presumably
originates mainly from Au NP/polynuclear Au(I)-thiol (core/shell)
complexes. 24-27 The presence of a shell of polynuclear Au(I)-thiol
units suggests that the emission arises from a low-lying triplet state
(phosphorescence), populated via intersystem crossing from the
lowest singlet state. 62-64 The emission intensities of the luminescent
RS-Au NPs are comparable with those of reported monolayer-
protected Au NCs. 50-62 The emission peak of the RS-Au NPs
( λ max

em = 501-618 nm) is located at an energy higher than those of
the Au 28 (GSH) 16 NCs and the dodecanethiolate-protected Au 38 NCs
(<1.5 nm). 50-56 The blue shift in the emission maxima upon reducing
the NC size results from the increase in the energy gaps between
the quantized levels (interband or intraband transitions) ascribed
to the smaller core sizes as well as the higher coverage of thiolates
or other capping molecules (e.g., phosphine). 83 In contrast, the value
of λ max

em for the RS-Au NPs (ca. 2 nm) is inconsistent with those of
the NCs, presumably due to the existence of the outermost layer of
Au(I)-thiolate complexes. 24-27
The luminescence properties of Au and Au/Ag NPs can be
controlled by varying the molar ratios of THPC to Au ions (for Au
NPs) and of Ag ions to Au ions (for Au/Ag alloy NPs). 26 Through
variation of the initial molar ratio of THPC to HAuCl 4 , Au NPs can be
prepared having diameters ranging from 2.2 to 3.7 nm. Introduction
of 11-mercaptoundecanoic acid (11-MUA) ligands onto the surfaces
of the as-prepared Au NPs results in luminescent 11-MUA-Au