Photoluminescence of Gold Nanoparticles and Their Applications to Sensing and Cell Imaging - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 195

luminescent, large Au NPs were obtained through irradiation of
a mixed aqueous solution of HAuCl 4 and GSH with room-light for
3 days. BRIGHT-yellow and red luminescence Au NPs having sizes
of 2.2 ± 0.3 and 1.9 ± 0.3 nm are usually obtained from mixtures
containing Au 3+ -to-GSH molar ratios of 0.5 and 1.0, respectively.
The absorption spectra of the luminescent Au NPs are distinct
from those of fully reduced, ca. 2 nm Au NPs, which feature a
weak, but distinguishable, plasmon absorption at ca. 520 nm. The
luminescent Au NPs have no plasmon absorption, but feature a
strong and broad absorption in the UV range (centered at 375 nm).
The UV absorption likely arises from strong Au(I)-S charge
transfer and/or Au(I)-Au(I) transitions. The strong emission
bands of GSH-encapsulated luminescent Au NPs having diameters
of 2.2 ± 0.3 and 1.9 ± 0.3 nm are centered at ca. 568 and 600 nm,
respectively, with excitation wavelength maxima at ca. 398 nm and
265 nm, respectively. The strong UV excitation proiles and long
lifetimes (ca. 5 μs) are reminiscent of the photophysical properties
of the luminescent Au NPs at low temperature, suggesting that
emission might result from analogous electronic transitions, such
as interband transitions (dp-sp) mixed with ligand-metal charger
transfer.
RS-Au NPs of various sizes luminesce at wavelengths within
the range 501-613 nm, with quantum yields ranging from 0.0062
to 3.1%. 24 The luminescence wavelength of these RS-Au NPs can
be controlled by varying the size of the alkanethiol. First, the non-
luminescent Au NPs (ca. 2.9 nm) are prepared through reduction of
HAuCl 4 · 3H 2 O with tetrakis(hydroxymethyl)phosphonium chloride
(THPC), which acts as both a reducing and capping agent. The Au
NPs are self-assembled with alkanethiol ligands of various lengths
(C 2 -C 11 ) by introducing alkanethiol stock solutions into the as-
prepared Au NPs. The positioning of these alkanethiol ligands on the
surfaces of the as-prepared Au NPs results in luorescent alkanethiol-
Au NPs. TEM imaging revealed that the as-prepared alkanethiol-Au
NPs are spherical and have smaller diameters than those of the
original spherical particles. Presumably, this phenomenon is due to
the formation of very strongly covalent, distinctly directional, Au-S
bonds, and to the preference for dissociation into very small Au and
Au-thiolate NCs (on the basis of fragmentation energies), as opposed
to the breaking of the Au-S bonds and detaching the Au NCs from the
thiolate radical. 69,70