Synthesis and Functionalization of Second Harmonic Generation Nanocrystals and Their Application in Biological Imaging

Abstract

The discovery and use of fluorescent proteins has been of extreme importance in biological imaging of cells, tissues, and organs. In order to address some of the limitations of fluorescent tags, second harmonic generation can be used. Second harmonic generating nanoprobes allow nontoxic, long-term imaging that, with proper functionalization, can be utilized for biological imaging applications. As a proof of principle, commercial tetragonal barium titanate nanoparticles were functionalized to expose surface amine groups, which could be further modified for a plethora of biological applications. Barium titanate nanoparticles were functionalized for selective targeting of tissue sections, biorthogonal linkages and for nonspecific long-term imaging using biocompatible polymers enabling the study of cells as they differentiate during zebrafish development. Since the commercial barium titanate nanoparticles do not have a narrow size distribution, which limits the application of such nanoprobes, synthesis of monodisperse nanocrystals was attempted. Zinc oxide nanocrystals were synthesized by solvothermal methods involving the base hydrolysis of zinc salts in the presence of capping ligands. Different synthesis procedures were investigated based on the properties of the prepared nanoparticles. To prevent nanoparticle aggregation and to achieve good dispersion, a capping agent was chosen that provides a tightly bound shell around the nanoparticles. It was observed that the polymerization of PEG molecules with the organic ligands bound to the surface of zinc oxide provided the most adequate coating for the desired size control and dispersion of the nanocrystals. Exploration of the experimental conditions enabled production of variable size hexagonal zinc oxide nanocrystals, which can be used both as SHG probes and as quantum dots.