serum albumin (BSA)-protected Au 25 : $6% QY). In particular, Jin and coworkers
reported that surface ligands with many electron-rich atoms (e.g., N, O) or groups
(e.g., –COOH, NH 2 ) can promote the fluorescence of gold clusters either by charge
transfer through the Au-S bonds or by direct donation of delocalized electrons to the
metal core [72].
Besides the aforementioned route (“Au atoms to Au clusters”), there is alternative
route to prepare gold nanoclusters by etching the surface atoms of gold nanoparticles
by appropriate ligands. For example, gold nanoclusters capped with dihydrolipoic
acid (AuNC@DHLA) with a quantum yield of around 1–3% were synthesized.
Upon etching and ligand exchanging with DHLA, the original gold nanoparticles
stabilized with didodecyldimethylammonium bromide (AuNP@DDAB) (~5.6 nm)
becoming smaller. These water-soluble gold clusters (<2 nm) have red
photoluminescence under UV excitation (Fig. 7) [65]. As for the etchant, hyperbranched polymers were also applied to induce the gold nanoclusters with a quantum
yield of 10–20% [73].
The photoluminescence quantum yield of gold clusters is still lower than the
organic fluorophore or QDs. However, they are ultra-small and exceptionally biocompatible nanoparticles with reduced photo-blinking behavior. Therefore, in a
recent study by Hung-I Yeh and coworkers, gold nanoclusters have been exploited
as a fluorescent biomarker for live cell tracking in vivo using hind-limb ischemic
mice. Here, the Au clusters showed nonspecific incorporation into living endothelial
progenitor cells (EPC) with no acute cytotoxicity. Thus, after intramuscular injection
of Au-labelled human EPC, the cells preserved angiogenic potentials and exhibited
detectable fluorescent signals for up to 21 days [74].
In another example to demonstrate utility in the early diagnosis for cancers, mice
with MDA-MB-45 and HeLa tumor xenografts were treated with ultra-small
NIR-emitting Au nanoclusters (BSA-capped gold cluster). These clusters accumulated in the tumors and showed tumor-to-background signals of ~15 for 6 h
postinjection [75]. Most importantly, gold nanocluster can also have good clearance
after administration through the kidneys due to its renal cutoff size (<5.5 nm) (this is
below the kidney filtration threshold (7–8 nm). Zheng et al. recently demonstrated
renal-clearable, NIR-emitting, gold nanoclusters (zwitterionic thiolated gold clusters) enabling the real-time fluorescence visualization of kidney clearance kinetics
with a 50-fold increase of contrast to conventional organic dyes. This is a useful and
sensitive tool for early staging of kidney dysfunctions [76].
5 Nano-diamond and Persistent Luminescent Nanoparticles
Nano-diamonds (ND) mainly consist of sp
3 carbon and are optically transparent,
biologically inert, and chemically modifiable. When these materials are irradiated by
high-energy ion beam, followed by thermal annealing, they can be immobilized with
a high concentration of point defects (e.g., nitrogen-vacancy (NV
À ) complexes) in
the sp
3 carbon lattice (Fig. 8a) [77]. These point defects can form a photoluminescent
66
T. Kim and J. V. Jokerst
reported that surface ligands with many electron-rich atoms (e.g., N, O) or groups
(e.g., –COOH, NH 2 ) can promote the fluorescence of gold clusters either by charge
transfer through the Au-S bonds or by direct donation of delocalized electrons to the
metal core [72].
Besides the aforementioned route (“Au atoms to Au clusters”), there is alternative
route to prepare gold nanoclusters by etching the surface atoms of gold nanoparticles
by appropriate ligands. For example, gold nanoclusters capped with dihydrolipoic
acid (AuNC@DHLA) with a quantum yield of around 1–3% were synthesized.
Upon etching and ligand exchanging with DHLA, the original gold nanoparticles
stabilized with didodecyldimethylammonium bromide (AuNP@DDAB) (~5.6 nm)
becoming smaller. These water-soluble gold clusters (<2 nm) have red
photoluminescence under UV excitation (Fig. 7) [65]. As for the etchant, hyperbranched polymers were also applied to induce the gold nanoclusters with a quantum
yield of 10–20% [73].
The photoluminescence quantum yield of gold clusters is still lower than the
organic fluorophore or QDs. However, they are ultra-small and exceptionally biocompatible nanoparticles with reduced photo-blinking behavior. Therefore, in a
recent study by Hung-I Yeh and coworkers, gold nanoclusters have been exploited
as a fluorescent biomarker for live cell tracking in vivo using hind-limb ischemic
mice. Here, the Au clusters showed nonspecific incorporation into living endothelial
progenitor cells (EPC) with no acute cytotoxicity. Thus, after intramuscular injection
of Au-labelled human EPC, the cells preserved angiogenic potentials and exhibited
detectable fluorescent signals for up to 21 days [74].
In another example to demonstrate utility in the early diagnosis for cancers, mice
with MDA-MB-45 and HeLa tumor xenografts were treated with ultra-small
NIR-emitting Au nanoclusters (BSA-capped gold cluster). These clusters accumulated in the tumors and showed tumor-to-background signals of ~15 for 6 h
postinjection [75]. Most importantly, gold nanocluster can also have good clearance
after administration through the kidneys due to its renal cutoff size (<5.5 nm) (this is
below the kidney filtration threshold (7–8 nm). Zheng et al. recently demonstrated
renal-clearable, NIR-emitting, gold nanoclusters (zwitterionic thiolated gold clusters) enabling the real-time fluorescence visualization of kidney clearance kinetics
with a 50-fold increase of contrast to conventional organic dyes. This is a useful and
sensitive tool for early staging of kidney dysfunctions [76].
5 Nano-diamond and Persistent Luminescent Nanoparticles
Nano-diamonds (ND) mainly consist of sp
3 carbon and are optically transparent,
biologically inert, and chemically modifiable. When these materials are irradiated by
high-energy ion beam, followed by thermal annealing, they can be immobilized with
a high concentration of point defects (e.g., nitrogen-vacancy (NV
À ) complexes) in
the sp
3 carbon lattice (Fig. 8a) [77]. These point defects can form a photoluminescent
66
T. Kim and J. V. Jokerst