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Single-walled carbon nanotubes (SWNTs) became soluble in water and formed a stable solution when l-phenylalanine (Phe) was adsorbed. The adsorption selectivity of Phe zwitterions for larger diameter SWNTs was confirmed by analysis of Fourier transform infrared spectra and by differential thermogravimetric analysis. Enhanced adsorption of Phe on the oxidized single-walled carbon nanotubes (OSWNT) was observed in comparison with that of the purified single-walled carbon nanotubes (PSWNT). The Phe zwitterions are thought to adsorb on the surface of OSWNT by joint interaction of the π−π stacking, hydrogen bond, and part of the covalent bond. The π−π stacking is the dominant interaction in the sidewall of OSWNT without defects. The hydrogen bond and covalent bond formed with oxygen-containing groups becomes dominant on the end of OSWNT. For the PSWNT system, π−π stacking is an important factor to realize the adsorption of Phe zwitterions on the sidewall of PSWNT. The intermolecular hydrogen bond between Phe zwitterions is also formed when Phe zwitterions are adsorbed on the PSWNT.

Coal tar pitch (CTP), a by-product of coking industry, has a unique molecule structure comprising an aromatic nucleus and several side chains bonding on this graphene-like nucleus, which is very similar to the structure of graphene quantum dots (GQDs). Based on this perception, we develop a facile approach to convert CTP to GQDs only by oxidation with hydrogen peroxide under mild conditions. One to three graphene layers, monodisperse GQDs with a narrow size distribution of 1.7 ± 0.4 nm, are obtained at high yield (more than 80 wt. %) from CTP. The as-produced GQDs are highly soluble and strongly fluorescent in aqueous solution. This simple strategy provides a feasible route towards the commercial synthesis of GQDs for its cheap material source, green reagent, mild condition, and high yield.