The present application relates to functionalized nanodiamonds, to complexes comprising a functionalized nanodiamond reversibly bound to a nucleic acid and to compositions comprising such functionalized nanodiamonds and complexes. In particular, the functionalized nanodiamonds comprise at least one naturally occurring basic amino acid, or analogs or derivatives thereof, covalently linked to a nanodiamond. The present application also includes methods and uses of the complexes and compositions, for example for delivering a nucleic acid to a cell.