Lysine-functionalized nanodiamonds: synthesis, characterization and potential as gene delivery agents

Detonation nanodiamonds (NDs), due to their 4-5 nm primary particle size, stable inert core, reactive surface, ability to form hydrogel, are emerging as intracellular delivery vehicle for small and large molecules. Despite several favorable characteristics, the use of NDs in biological systems is impeded by their high aggregation propensity in polar liquid medium. To develop NDs as potential gene delivery vectors, pristine carboxylated NDs (pNDs) were functionalized with lysine through covalent conjugation. Raman and FTIR spectroscopic determinations confirmed the functionalization of NDs with lysine molecules, while thermogravimetric analysis estimated a surface loading of 1.7 mmol/g. Through lysine-functionalization, the dispersion stability of NDs in water increased considerably, showing a zeta potential of +49 mV. The average particle size of pNDs as measured by dynamic light scattering was substantially reduced from 1281 to 21 nm after lysine functionalization. Atomic force microscopy further substantiated the disaggregation of pNDs achieved through lysine functionalization. The lysine-functionalized NDs (fNDs) were able to electrostatically bind and block the migration of the nucleic acids at a weight ratio of 5:1 and 20:1 of fNDs:pDNA and fNDs:siRNA, respectively, with a shift in zeta potential from negative to positive value. The particle size of the complexes stabilized around 110 nm for fNDs-pDNA and less than 280 nm for fNDs-siRNA at the weight ratios of 50:1 fNDs:nucleic acid. While the Raman-fluorescence maps were equivocal with regards to the cellular association of NDs, backscattering maps clearly indicated the interaction of the fNDs with the cells. Cellular internalization of a few fNDs was suggested by laser confocal scanning microscopy. MTT assay demonstrated no significant in vitro cytotoxicity of pNDs and fNDs in the concentration range from 4 to 250 µg/mL. Flow cytometeric assessment of the gene expression (GFP intensity measurements) suggested that a strong binding of siRNA with fNDs might have prevented the release of nucleic acid into the cytoplasm of the cells. Overall, in this study, stable aqueous dispersion of NDs was generated using a mechanochemical approach feasible at a small laboratory scale, and early evidence was presented that the fNDs can be optimized for safe delivery of nucleic acids into mammalian cells