Photoluminescence of Gold Nanoparticles and Their Applications to Sensing and Cell Imaging - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 192

but feature monolayers of different thiols. 23 The core size-
independence of the emission energy relects the participation of
localized core surface states that have a size-independent energy;
this hypothesis is supported by ligand place exchanges and galvanic
exchange of Ag core atoms with Au core atoms (Fig. 9.3B).
Figure 9.3B(a) reveals that the luminescence intensity increases
linearly (with a near-zero intercept) with the average number of
polar N , N , N -trimethyl(mercaptoundecyl)ammonium (TMA) ligands
exchanged onto Au 140 (C6) 53 (nonpolar hexanethiolate-protected Au
MPCs). Nuclear magnetic resonance (NMR) spectroscopy is a
powerful tool for determining the ratio of TMA (3-4 ppm) and
hexanethiolate (0.8-1.0 ppm) units in the mixed monolayer; UV
absorbance and transmission electron microscopy (TEM)
measurements are effective for conirming that no difference exists
in the metal core size before and after the ligand place exchange
reaction. The luminescence appears to be localized at speciic surface
sites occupied by the TMA ligands. A similar quantitative dependence
(Fig. 9.3B(b)) can be found in core metal galvanic exchange reactions
of tiopronin-coated Ag NCs with Au(I)[ p -SCH 2 (C 6 H 4 )C(CH 3 ) 3 ], at
various molar ratios. 61 During the reaction, the emission spectrum
changes from that of Ag NCs (ca. 500 nm) to that of Au NCs. The inal
Au NC-like emission increases linearly upon increasing the reactant
ratio of Au(I)[ p -SCH 2 (C 6 H 4 )C(CH 3 ) 3 ] to Ag NCs, mainly because of
increases in the number of Au atoms per NC core. Considering that
one Au atom replaces one Ag surface atom, the luorescence intensity
reveals that the surface Au atoms act as discrete surface state
emitters. This result parallels those from the ligand exchange
experiment in Fig. 9.3B(a), suggesting that the NIR emission is an
electronic surface state phenomenon. The emission intensities
depend on the nature of both the NP core metal and the ligands
attached to it. Similarly, the NIR luminescence of monolayer-
protected Au 38 and Au 140 NCs is intensiied after exchanging nonpolar
ligands (phenylethanethiol) by more polar thiolate ligands, including
alkanethiolates ( ω -terminated with alcohol, acid, or quaternary
ammonium groups) and thio-amino acids. 62 Remarkably, place
exchanges of (i) the initial phenylethanethiolates on Au 38 NCs with
p -substituted thiophenolates or thio-amino acids and (ii)
hexanethiolates on Au 140 NCs with ω -quaternary ammonium-
terminated undecylthiolates result in changes in the NIR
luminescence intensities that increase linearly with respect to the
number of exchanged polar ligands (Fig. 9.4). 62 The intensity