1.1.2 Tunable Long-Lifetime Emission
All this considered, the different sharp emission peaks, together with their respective
long lifetimes, make UCNPs doped with different ions of interest for multicolour
imaging and time-resolved photoluminescence imaging [22, 23]. In fact, UCNP
emissions possess long lifetime, usually in the order of several hundreds of μs, as
compared to the ns scale of common organic fluorophores. As a result, the
autofluorescence of the biomolecules can be completely removed just by selecting
the appropriate detection window in a time-gated imaging strategy [22, 24].
The sharp long-lived emission of the lanthanides embedded in the UCNP is
attributed to the shielded partially allowed intra f-f electronic transitions of lanthanide dopants. The upconversion emission is the result of the combined effect of the
crystal field, the staggered nature of lanthanides electronic configuration and the
characteristic long-lived lifetime of their excited state. In the ET upconversion
mechanism, the Yb
3+ plays an important role because of its high absorption cross
section, its long-lived excited state and its energy gap, which is similar to the energy
gap between higher excited states of the activators. The feeding of superior excited
states produces multiple sharp power-dependent emissions from the activator(s) [6, 7,
16, 25, 26].
Even more, by controlling several parameters, such as the doping ratio, the
inorganic matrix, the presence of other dopants, the size, the temperature or the
presence of shells (active or inactive), it is possible to tune the UCNP emission
features [6, 13, 15, 27]. Moreover, although the upconversion emission is the special
feature of the UCNPs, they also display the typical downconversion emission of the
doping lanthanides, which usually happens in the second biological window
(NIR-II) [10, 28, 29]. Often UV-Vis emission region has been applied in
theragnostics and sensing [26, 30, 31], while the downconversion emission has
been especially useful in bioimaging [28, 29].
2 Pre-requirements When Designing Photoactive
Nanohybrids by Combining UCNPs and Chromophores
The absorbing species (dye or nanoparticle) selected to make upconversion
nanohybrids (UCNHs) must absorb at the activator’s emissions present in the
UCNP, if the purpose is to enable NIR-induced lanthanide ET processes. A huge
variety of dyes can then be used: cationic dyes (e.g. methylene blue) [32–35],
BODIPYs [36–38] and phthalocyanines [32, 39–41]. See Chart 1 for more
examples.
Then, according to the photophysical properties of the absorbing species present
in the nanohybrid, one would select the application of the UCNH. Several possibilities can be outlined for the absorbing species:
378
J. Ferrera-González et al.
All this considered, the different sharp emission peaks, together with their respective
long lifetimes, make UCNPs doped with different ions of interest for multicolour
imaging and time-resolved photoluminescence imaging [22, 23]. In fact, UCNP
emissions possess long lifetime, usually in the order of several hundreds of μs, as
compared to the ns scale of common organic fluorophores. As a result, the
autofluorescence of the biomolecules can be completely removed just by selecting
the appropriate detection window in a time-gated imaging strategy [22, 24].
The sharp long-lived emission of the lanthanides embedded in the UCNP is
attributed to the shielded partially allowed intra f-f electronic transitions of lanthanide dopants. The upconversion emission is the result of the combined effect of the
crystal field, the staggered nature of lanthanides electronic configuration and the
characteristic long-lived lifetime of their excited state. In the ET upconversion
mechanism, the Yb
3+ plays an important role because of its high absorption cross
section, its long-lived excited state and its energy gap, which is similar to the energy
gap between higher excited states of the activators. The feeding of superior excited
states produces multiple sharp power-dependent emissions from the activator(s) [6, 7,
16, 25, 26].
Even more, by controlling several parameters, such as the doping ratio, the
inorganic matrix, the presence of other dopants, the size, the temperature or the
presence of shells (active or inactive), it is possible to tune the UCNP emission
features [6, 13, 15, 27]. Moreover, although the upconversion emission is the special
feature of the UCNPs, they also display the typical downconversion emission of the
doping lanthanides, which usually happens in the second biological window
(NIR-II) [10, 28, 29]. Often UV-Vis emission region has been applied in
theragnostics and sensing [26, 30, 31], while the downconversion emission has
been especially useful in bioimaging [28, 29].
2 Pre-requirements When Designing Photoactive
Nanohybrids by Combining UCNPs and Chromophores
The absorbing species (dye or nanoparticle) selected to make upconversion
nanohybrids (UCNHs) must absorb at the activator’s emissions present in the
UCNP, if the purpose is to enable NIR-induced lanthanide ET processes. A huge
variety of dyes can then be used: cationic dyes (e.g. methylene blue) [32–35],
BODIPYs [36–38] and phthalocyanines [32, 39–41]. See Chart 1 for more
examples.
Then, according to the photophysical properties of the absorbing species present
in the nanohybrid, one would select the application of the UCNH. Several possibilities can be outlined for the absorbing species:
378
J. Ferrera-González et al.