However, one limitation for the conventional Yb
3+ -doped UCNPs [42] is the
heating effect that is generated by water molecules under 980 nm laser excitation.
Therefore, in recent years, considerable effort has been made to control the excitation wavelength because this is where water has lower absorption. In one study,
Andersson-Engels and coworkers utilized NaYbF 4 :Tm;Er and NaYbF 4 :Tm;Ho
nanoparticles under 915 nm excitation [43]. Nd
3+ was introduced to the conventional
Yb
3+ -doped UCNPs as a new sensitizer to be excited at 800 nm [44]. These
nanoparticles showed a larger absorption cross section and deeper image penetration
depth than those with 980 nm excitation. It is also very desirable to construct UCNPs
with both excitation and emission in the NIR range (NIR in -NIR out UCNPs) to further
improve the imaging contrast. Therefore, Prasad and coworkers reported core/shell
(α-NaYbF 4 :Tm
3+ )/CaF 2 nanoparticles with excitation at $980 nm and PL emission
at 800 nm. In this study, the authors found that these nanoparticles allowed tenfold
higher signal-to-background ratio (SBR) than previously reported UCNPs for
in vivo imaging enabling deep-penetration imaging through 3.2 cm pork tissue [45].
4 Carbon Dots, Porous Silicon Nanoparticles, and Au
Nanoclusters
Quantum-sized carbon- and silicon-based optical imaging probes have recently been
developed as benign alternatives to conventional semiconductor QDs. Although
these nanomaterials lack a classical bandgap structures of QDs, they can achieve
fluorescence emission from the surface passivation-created defects (surface energy
traps [46]). Here, surface passivation stabilize the surface defects and facilitate more
effective radiative recombination of surface-confined excitons [47, 48].
In a pioneering study, Sun et al. [49] prepared 5 nm carbon dots (C-Dots) via laser
ablation of graphite powder and cement. The surface of the C-Dots was then
effectively passivated with organic moieties (diamine-terminated oligomeric PEG;
PEG1500N) resulting in strong photoluminescence with no blinking as well as
tunable emissions from visible to NIR under the argon ion laser excitation
(458 nm) [49]. They also found that these C-Dots capped with poly(propionylethylenimine-co-ethylenimine) (PEI-EI) were two-photon active with
pulsed laser excitation in the NIR region (800 nm). The two-photon absorption
cross sections of the C-Dots were comparable with the best-performing semiconductor QDs. Next, upon incubation to the human breast cancer cells, the authors
demonstrated the potential of C-Dots for cell imaging with two-photon luminescence
microscopy [50]. Indeed, recent studies have included the careful selection of carbon
source as well as surface modifier for C-dots with enhanced photoluminescence [51]
resulting in C-Dots with a quantum yield 2–2.5-fold that of CdSe/ZnS QDs
[52, 53]. In particular, C-dots can offer significant advantages in terms of potential
translatability and applicability because they exhibit very low toxicity and great
availability in scale-up production through various inexpensive renewable resources
[54–56].
Inorganic Fluorescent Nanomaterials
63
3+ -doped UCNPs [42] is the
heating effect that is generated by water molecules under 980 nm laser excitation.
Therefore, in recent years, considerable effort has been made to control the excitation wavelength because this is where water has lower absorption. In one study,
Andersson-Engels and coworkers utilized NaYbF 4 :Tm;Er and NaYbF 4 :Tm;Ho
nanoparticles under 915 nm excitation [43]. Nd
3+ was introduced to the conventional
Yb
3+ -doped UCNPs as a new sensitizer to be excited at 800 nm [44]. These
nanoparticles showed a larger absorption cross section and deeper image penetration
depth than those with 980 nm excitation. It is also very desirable to construct UCNPs
with both excitation and emission in the NIR range (NIR in -NIR out UCNPs) to further
improve the imaging contrast. Therefore, Prasad and coworkers reported core/shell
(α-NaYbF 4 :Tm
3+ )/CaF 2 nanoparticles with excitation at $980 nm and PL emission
at 800 nm. In this study, the authors found that these nanoparticles allowed tenfold
higher signal-to-background ratio (SBR) than previously reported UCNPs for
in vivo imaging enabling deep-penetration imaging through 3.2 cm pork tissue [45].
4 Carbon Dots, Porous Silicon Nanoparticles, and Au
Nanoclusters
Quantum-sized carbon- and silicon-based optical imaging probes have recently been
developed as benign alternatives to conventional semiconductor QDs. Although
these nanomaterials lack a classical bandgap structures of QDs, they can achieve
fluorescence emission from the surface passivation-created defects (surface energy
traps [46]). Here, surface passivation stabilize the surface defects and facilitate more
effective radiative recombination of surface-confined excitons [47, 48].
In a pioneering study, Sun et al. [49] prepared 5 nm carbon dots (C-Dots) via laser
ablation of graphite powder and cement. The surface of the C-Dots was then
effectively passivated with organic moieties (diamine-terminated oligomeric PEG;
PEG1500N) resulting in strong photoluminescence with no blinking as well as
tunable emissions from visible to NIR under the argon ion laser excitation
(458 nm) [49]. They also found that these C-Dots capped with poly(propionylethylenimine-co-ethylenimine) (PEI-EI) were two-photon active with
pulsed laser excitation in the NIR region (800 nm). The two-photon absorption
cross sections of the C-Dots were comparable with the best-performing semiconductor QDs. Next, upon incubation to the human breast cancer cells, the authors
demonstrated the potential of C-Dots for cell imaging with two-photon luminescence
microscopy [50]. Indeed, recent studies have included the careful selection of carbon
source as well as surface modifier for C-dots with enhanced photoluminescence [51]
resulting in C-Dots with a quantum yield 2–2.5-fold that of CdSe/ZnS QDs
[52, 53]. In particular, C-dots can offer significant advantages in terms of potential
translatability and applicability because they exhibit very low toxicity and great
availability in scale-up production through various inexpensive renewable resources
[54–56].
Inorganic Fluorescent Nanomaterials
63