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. Using a green physical method, we prepared GNPs comprising 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, facilitating further functionalization. Thus, a cell permeable peptide (CPP) poly-arginine-11 (R11) was grafted onto the GNPs to obtain peptide-functionalized GNPs (R11@GNP). Moreover, the preparation of a non-covalent complex with 1-pyrene carboxylic acid (PyCA) demonstrated that the GNP surface can be loaded with small molecules for drug delivery purposes. The structure of R11@GNP and PyCA@GNP supramolecular systems and the presence of covalent and non-covalent bonds, respectively, was investigated. A thorough analysis of the functionalized GNPs through UV-vis, FTIR and Raman spectroscopy techniques as well as dynamic light scattering and Z-potential measurements well characterized their structures at the molecular level. Fluorescence spectroscopy allowed collecting further evidence of the formation of stable π-π complexes between GNPs and PyCA and provided a first test of the thermally induced release of absorbed molecules.