Functional modification of graphene nanoparticles: covalent grafting of peptides and π bonding for drug loading and delivery

Graphene nanoparticles (GNPs) can serve as a versatile platform for the development of drug-delivery systems by means of suitable functionalization strategies. By using a green physical method, we prepared GNPs formed by N < 10 stacked graphene layers with an average lateral size of about 102 nm. These GNPs are naturally endowed with carboxyl groups decorating their edges that facilitates further functionalization. Thus, the cell permeable peptide (CPP) poly-arginine-11 (R11) was grafted on the GNPs to obtain the peptide-functionalized GNPs (R11@GNP). Moreover, the preparation of a non-covalent complexes with 1-pyrene carboxylic acid (PyCA) demonstrates that the GNPs surface can be loaded with small molecules for drug delivery purposes. The structure of R11@GNP and PyCA@GNP supramolecular systems as well as the evidence of the covalent and non-covalent bond, respectively, was investigated. The thorough analysis of the functionalized GNPs by UV-vis, FTIR and Raman spectroscopies, Dynamic Light Scattering, and Z-potential measurements illustrates the potential of well-established and easily available techniques to achieve a structural characterization at the molecular level. Fluorescence spectroscopy allows collecting further evidence of the formation of stable π-π complexes between GNPs and PyCA, and provides a first test of the thermally induced release of the absorbed molecules.