Photoluminescence of Gold Nanoparticles and Their Applications to Sensing and Cell Imaging - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 183

absence of an energy gap. 17-19 When the dimensions of a metal
object decrease, quantum coninement effects become signiicant,
resulting in many interesting physical and chemical properties,
including luminescence. 6,20 Au NPs (<2 nm diameter) and Au NCs
capped with ligands, including citrate, poly(amidoamine) (PAMAM)
dendrimers, and thiolates, display luminescence at different
wavelengths with signiicantly different quantum yields. 6,20 Au
NCs capped with PAMAM or thiol compounds exhibit strong
luminescence characteristics, but Au NPs capped with citrate do not.
The luminescence energies of PAMAM-bound Au NCs vary strongly
with respect to their size, from 3.2 eV for Au 5 to 1.4 eV for Au 31 . 21
In contrast, for thiolate-protected Au NPs they vary more modestly
with size; for example, the energy gaps of glutathione monolayer-
protected Au NCs ranging in size from Au 10 to Au 39 lie in the range
1.4-1.7 eV. 22 Au 13 , Au 38 , and Au 140 NCs capped with monolayers of
various thiols [ N , N , N -trimethyl(mercaptoundecyl)ammonium thiol,
hexanethiol, tiopronin] possess energy gaps ranging from 1.1 to
1.3 eV, corresponding to near-infrared (NIR) emissions. 23 Variations
in the luminescence intensities of the Au NPs and NCs are, on the
other hand, more consistent with changes in their sizes; the emission
intensities of both PAMAM- and thiolate-protected NCs increase
upon decreasing their size. 21,22 Usually, Au NCs at the smallest sizes
provide impressive quantum eficiencies (>10%).
In addition to Au NCs, alkanethiol (RSH)-capped Au NPs
(<2.0 nm) — so-called luminescent Au NPs or Au nanodots — are
interesting because they display PL with large Stokes shifts and long
lifetimes. 24-27 The emissions of these luminescent Au NPs might arise
from electronic transitions of Au(I) coupling to the ligand-metal
charge transfer transition. 28-30 Unlike larger Au NPs, these small
luminescent Au NPs do not feature surface plasmon absorption.
Like Au NCs, the PL properties of these luminescent Au NPs are not
directly related to their sizes. 24-27 Not surprisingly, contributions
from surface characteristics become greater at diminished NP
sizes because, as the size decreases, the surface-to-volume ratio
increases and a larger fraction of the NP core atoms become involved
in bonds to the core-capping ligands. The properties of these new
types of luminescent Au NPs are also discussed in this chapter.
Luminescent Au NCs and Au NPs have great practicality in
several ields, including sensing and cell imaging. 24,25,31-36 Because
of their excellent physical and chemical properties, many methods