Biomedical Engineering Reference
In-Depth Information
9.4
Preparation and Optical Properties of
Luminescent Au NPs
Unlike large Au NPs (>2 nm), luminescent Au NPs capped with
thiols exhibit luminescence. The luminescence of luminescent Au
NPs is not simply dependent on the number of free electrons in
the NCs, as is the situation for PAMAM-protected Au NCs. 6 Au(I)-
thiol complexes are located on the surface of each small Au NP
(<2 nm). Because Au(I)-thiol complexes exhibit luminescence, 29,30
the luminescence of luminescent Au NPs arises from both the
complexes and the Au cores. In luminescent Au NPs, Au ions provide
available states and hybridize with organic ligands to form charge
transfer bands. 24-27 Luminescence is roughly attributed to metal-
metal electronic transitions, coupled with strong ligand-metal/
metal-metal unit charge transfer transitions. 28-30,62-64 These Au(I)
complexes generally exhibit large Stokes-shifted luminescence with
lifetimes in the microsecond regime. 62-64 Charge transfer between
ligands and Au or Au-Au units is responsible for these large Stokes
shifts and long-lifetime emissions. The emission center is usually
assigned to metal-metal electronic transitions. In addition, at low
temperature, the emission energy is inversely proportional to the
Au-Au distance. 65 Moreover, lowering the temperature can result in
a decrease in the Au(I)-Au(I) bond length, corresponding to a blue
shift of the emission energy. 66
A general strategy for synthesizing luminescent small Au NPs
is using multidentate ligands featuring soft electron donor groups
to coordinate with Au 3+ ions. 67,68 With assistance of closed-shell
interactions among Au(I) ions (5d 10 ) and bridging structures of the
ligand groups, small luminescent Au(I) NCs can be prepared. 28-30,62-
64 In these luminescent Au(I) NCs, closed-shell interactions drive
the aggregation of Au atoms to form dimers and oligomers. The
multidentate ligands are necessary to stabilize the Au(I) complexes/
NCs because the bond energies of Au(I)-Au(I) bonds (20-50 kJ/mol)
are much smaller than those (ca. 225 kJ/mol) of Au(0)-Au(0) bonds
in fully reduced Au NPs or NCs. 68 The average Au(I)-Au(I) distance
(2.7-3.4 Å) in such Au(I) NCs is also shorter than the sum (3.7 Å) of
the two van der Waals radii of Au(I) ions, but it is longer than the
average Au(0)-Au(0) distance (2.2-2.5 Å). 68
GSH has been used to prepare larger (ca. 2 nm) GSH-encapsulated
luminescent Au NPs. 27 These water-soluble, biocompatible,
 
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