(i) It filters the UCNP emission, but has no emission and does not induce reactive
oxygen species generation.
(ii) It absorbs the UCNP emission leading to an emission in a wavelength range
different to that of the UCNP, and this emission is used as a part of the
ratiometric response of the sensor instead of those of the UCNP emission.
(iii) It absorbs the UCNP emission leading to its characteristic emission; however, it
is ignored for the sensor response.
(iv) It absorbs the UCNP emission generating reactive oxygen species (e.g. singlet
oxygen).
These four possibilities regarding the photophysical pathways after NIR excitation will determine the capabilities of the UCNH to be used in different applications.
Next sections present several examples in order to illustrate each option either for
sensing (i–iii) or for PDT applications (iv).
3 Sensing with UCNH
The unique features of UCNPs have been exploited and combined with other
photoactive species (molecules and nanoparticles) to obtain new sensors. Adequate
combinations may produce significant and selective differences in the luminescence
response of the UCNH when an analyte is present in the medium. The common
configuration of UCNH sensors benefits from the photophysical properties of the
UCNPs and the sensing properties of known chromophores. NIR excitation of the
UCNP produces emissions in the UV-Vis spectral region. These emissions eventually lead to the excitation of the chromophore, which is a well-known molecular
probe sensitive to pH, temperature or analyte concentration (e.g. ions, proteins, or
DNA) [26, 31].
At the microscale cellular level, the sensors based on UCNPs offer superior
sensing properties than organic probes or other NPs. Fluorescence sensors supply
higher sensitivity and selectivity than absorption sensors, but their usual limitation is
that direct fluorescence measurements are sensible to interferences, such as
photobleaching or photoblinking, dye leaching or the instability of the lamps or
detectors. Ratiometric or time-resolved measurements suppress many of these factors and, consequently, are preferable [42]. UCNPs can be used in both ways since
they have multiple narrow emissions (enabling ratiometric measurements) and long
lifetime for time-resolved imaging [22, 26, 31].
According to the interaction between the UCNP and the photoactive species
mentioned above, we can classify UCNHs into two groups: inner-filter (i) or resonant ET (ii–iii) nanohybrids. In any case, both mechanisms require an efficient
spectral overlap between the emission of the donor (the lanthanide acting as activator) and the absorption of the acceptor (chromophore).
Functional Nanohybrids Based on Dyes and Upconversion Nanoparticles
379
oxygen species generation.
(ii) It absorbs the UCNP emission leading to an emission in a wavelength range
different to that of the UCNP, and this emission is used as a part of the
ratiometric response of the sensor instead of those of the UCNP emission.
(iii) It absorbs the UCNP emission leading to its characteristic emission; however, it
is ignored for the sensor response.
(iv) It absorbs the UCNP emission generating reactive oxygen species (e.g. singlet
oxygen).
These four possibilities regarding the photophysical pathways after NIR excitation will determine the capabilities of the UCNH to be used in different applications.
Next sections present several examples in order to illustrate each option either for
sensing (i–iii) or for PDT applications (iv).
3 Sensing with UCNH
The unique features of UCNPs have been exploited and combined with other
photoactive species (molecules and nanoparticles) to obtain new sensors. Adequate
combinations may produce significant and selective differences in the luminescence
response of the UCNH when an analyte is present in the medium. The common
configuration of UCNH sensors benefits from the photophysical properties of the
UCNPs and the sensing properties of known chromophores. NIR excitation of the
UCNP produces emissions in the UV-Vis spectral region. These emissions eventually lead to the excitation of the chromophore, which is a well-known molecular
probe sensitive to pH, temperature or analyte concentration (e.g. ions, proteins, or
DNA) [26, 31].
At the microscale cellular level, the sensors based on UCNPs offer superior
sensing properties than organic probes or other NPs. Fluorescence sensors supply
higher sensitivity and selectivity than absorption sensors, but their usual limitation is
that direct fluorescence measurements are sensible to interferences, such as
photobleaching or photoblinking, dye leaching or the instability of the lamps or
detectors. Ratiometric or time-resolved measurements suppress many of these factors and, consequently, are preferable [42]. UCNPs can be used in both ways since
they have multiple narrow emissions (enabling ratiometric measurements) and long
lifetime for time-resolved imaging [22, 26, 31].
According to the interaction between the UCNP and the photoactive species
mentioned above, we can classify UCNHs into two groups: inner-filter (i) or resonant ET (ii–iii) nanohybrids. In any case, both mechanisms require an efficient
spectral overlap between the emission of the donor (the lanthanide acting as activator) and the absorption of the acceptor (chromophore).
Functional Nanohybrids Based on Dyes and Upconversion Nanoparticles
379