Gene therapy is the treatment of genetic diseases by the delivery of nucleic genes into a patient's cells to replace the ‘impaired’ genes and regain their biological function, as well as to trigger them some additional functions. Successful cellular uptake of genetic material requires efficient biocompatible carriers. Viral gene delivery is highly effective, but safety concern limits its clinical applications. For non-viral vectors, genetic material is delivered to the nucleus by passive diffusion through the nuclear pores.
Polyethyleneimine (PEI) is a cationic polymer with high affinity for DNA and RNA. However, PEI is poorly internalized by cells. Introducing nanodiamonds within PEI-gene complexes highly improves their transfection efficiency ¹.
Biocompatible nanodiamonds with average size of 4-5 nm provide the effective gene delivery, - they are rapidly swallowed up by cells and are small enough to penetrate the nucleus.
The most studied gene-nanodiamond therapeutic complexes are based on the functionalized nanodiamonds with a cationic surface enabling to adsorb and desorb negatively charged DNA and RNA ².
In addition, nanodiamonds as a carrier can be linked to cell-targeting ligands for targeted gene delivery ³. Complexes of genes together with fluorescent nanodiamonds enable to track them in real time.

Reference:

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2. Alhaddad, et al., Nanodiamond as a vector for siRNA delivery to Ewing sarcoma cells, Small 7 (21) (2011) 3087–3095.
3. C. Cui, et al., RGDS covalently surfaced nanodiamond as a tumor targeting carrier of VEGF-siRNA: synthesis, characterization and bioassay, J. Mater. Chem, (11), 70-80.