Design of Lanthanide-Doped Colloidal Nanocrystals: Applications as Phosphors, Sensors, and Photocatalysts

The unique optical characteristics of lanthanides (Ln³⁺) such as high color purity, long excited-state lifetimes, less perturbation of excited states by the crystal field environment, and the easy spectral conversion of wavelengths through upconversion and downconversion processes have caught the 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 synthesis strategies for making Ln³⁺-doped nanoparticles in colloidal forms, particularly ways of functionalizing hydrophobic surfaces to hydrophilic surfaces to enhance their dispersibility and luminescence in aqueous media. We have enumerated the various strategies and sensitizers utilized to increase the luminescence of the nanoparticles. Furthermore, the use of these colloidal nanoparticle systems in sensing application by the 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 the 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.

Castor oil: a suitable green source of capping agent for nanoparticle syntheses and facile surface functionalization

Castor oil (CO) is an inedible vegetable oil (VO) that has been employed extensively as a bioresource material for the synthesis of biodegradable polymers, cosmetics, lubricants, biofuels, coatings and adhesives. It is used in medicine, pharmaceuticals and biorefineries, due to its versatile chemistry. However, there has been less focus on CO as an alternative to toxic and expensive solvents, and capping/stabilizing agents routinely used in nanoparticle syntheses. It provides a richer chemistry than edible VOs as a solvent for green syntheses of nanoparticles. CO, being the only rich source of ricinoleic acid (RA), has been used as a solvent, co-solvent, stabilizing agent and polyol for the formation of polymer-nanoparticle composites. RA is a suitable alternative to oleic acid used as a capping and/or stabilizing agent. Unlike oleic acid, it provides a facile route to the functionalization of surfaces of nanoparticles and the coating of nanoparticles with polymers. For applications requiring more polar organic solvents, RA is more preferred than oleic acid. In this review, we discuss the production, chemical and physical properties, triglyceride and fatty acid (FA) compositions and applications of CO, focusing on the use of CO and RA as well as other VOs and FAs in syntheses of nanoparticles and surface functionalization.

Double bond terminated Ln3+-doped LiYF4 nanocrystals with strong single band NIR emission: simple click chemistry route to make water dispersible nanocrystals with various functional groups

A simple thiol–ene click chemistry strategy to develop upconverting nanocrystals with different functional groups.

Methyl oleate-capped upconverting nanocrystals: a simple and general ligand exchange strategy to render nanocrystals dispersible in aqueous and organic medium

We report a simple and general ligand exchange strategy to functionalize the nanocrystals with both hydrophobic and hydrophilic ligands. This is achieved by first capping the Er/Yb-doped NaYF4 nanocrystals with a weak ligand such as methyl oleate and subsequently ligand exchanged with various organic ligands which can strongly coordinate to the surface of the nanocrystals. The method involves only a simple stirring or sonication of the nanocrystals dispersion with the ligands of interest. Dicarboxylic acids such as sebacic acid, adipic acid, succinic acid, and malonic acid-functionalized nanocrystals which are difficult to achieve via thermal decomposition method were easily prepared by this ligand exchange strategy. In addition, low boiling point ligands like hexanoic acid can easily be coated over the surface of the Er/Yb-doped NaYF4 nanocrystals. Both size and shape of the nanocrystals were preserved after the ligand exchange process. The methyl oleate-capped Er/Yb-doped NaYF4 nanocrystals display strong upconversion emission after ligand exchanged with hydrophobic and hydrophilic molecules. The high stability of the nanocrystals after ligand exchange process is verified by performing time-dependent luminescent measurements at different pH, buffers, etc.

Probing the nature of upconversion nanocrystals: instrumentation matters

Understanding upconversion nanocrystals: this review intends to summarize instrumental matters related to the characterization of upconversion nanocrystals from surface structures to intrinsic properties to ultimate challenges in nanocrystal analysis at single-particle levels.