The unique optical characteristics of lanthanides (Ln3+) like high colour purity, long excited state lifetimes, less perturbation of excited states by the crystal field environment and ability towards easy spectral conversion of wavelengths through upconversion and downconversion processes have caught the research attention of many scientists in the recent past. To broaden the scope of using these properties, it is important to make suitable Ln3+-doped materials, particularly in colloidal forms. In this feature article, we discuss the different synthetic strategies to make Ln3+-doped nanoparticles in colloidal forms, particularly on ways of functionalizing hydrophobic surfaces to hydrophilic ones for enhancing their dispersibility and luminescence in aqueous medium. We have enumerated the various strategies and sensitizers utilized to increase the luminescence of the nanoparticles. Further, the use of these colloidal nanoparticle systems in sensing application by appropriate selection of capping ligands has been discussed. In addition, we have shown how the energy transfer efficiency from Ce3+ to Ln3+ ions can be utilized for detection of toxic metal ions and small molecules. Finally, we discuss examples where the spectral conversion ability of these materials has been used in photocatalysis and solar cell applications.
The
property of upconverting nanoparticles to convert the low-energy
near-infrared (NIR) light into high-energy visible light has made
them a potential candidate for various biomedical applications including
photodynamic therapy (PDT). In this work, we show how a surface functionalization
approach on the nanoparticle can be used to develop a nanocomposite
hydrogel which can be of potential use for the PDT application. The
upconverting hydrogel nanocomposite was synthesized by reacting 10-undecenoic
acid-capped Yb
3+
/Er
3+
-doped NaYF
4
nanoparticles with the thermosensitive
N
-isopropylacrylamide
monomer. The formation of hydrogel was completed within 15 min and
hydrogel nanocomposites showed strong enhancement in the visible light
emission compared to the emission obtained from 10-undecenoic acid-capped
Yb
3+
/Er
3+
-doped NaYF
4
nanoparticles
via the upconversion process (under 980 nm laser excitation). The
upconverting hydrogel nanocomposites displayed high swelling behavior
in water because of their porous nature. The porous structure ensured
a higher loading of methylene blue dye (∼78% in 1 h) into the
upconverting hydrogel, which was achieved via the swelling diffusion
phenomenon. Upon excitation with the NIR light, the visible light
emitted from the hydrogel activated the photosensitizer methylene
blue which generated reactive oxygen species. Our results were able
to show that the methylene blue-loaded composite hydrogel can be a
potential platform for the future of NIR-triggered PDT in skin cancer
treatment.
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