Water-Soluble Upconversion Nanoparticles by Micellar Route

Upconversion nanoparticles (UCNs) have been of interest in applications such as biological imaging and sensing because of properties like low autofluorescence and negligible photobleaching. In this paper, a micellar encapsulation route was worked upon to make hydrophilic UCNs that are compatible with biological systems and enable conjugation of biomolecules. Phospholipid micelles cannot be easily synthesized with chemical recognition groups of biological interest so synthetic amphiphiles with suitable functional head groups were used to encapsulate and solubilize the hydrophobic UCNs. The encapsulated nanoparticles were characterized using transmission electron microscopy, fluorescence spectroscopy, and dynamic light scattering. The micelle-encapsulated UCNs had an average diameter of 125 nm and the fluorescence emission at 540/660 nm of the bare UCNs did not show any shift after encapsulation. The cytotoxicity of the micelle-encapsulated UCNs was tested using lactose dehydrogenase/MTS (tetrazole salt) assays. The cell viability was estimated to be 80 % at the working concentration of the micelle-encapsulated UCNs. Finally, the micelle-encapsulated UCNs bearing Arg-Gly-Asp tripeptidic RGD surface functionalization were tested on cancer cells expressing integrins for specificity. The micelle-encapsulated UCNs were found to be suitable for cellular targeting and imaging.