Seeking value from biomass materials: preparation of coffee bean shell-derived fluorescent carbon dots via molecular aggregation for antioxidation and bioimaging applications

Bifunctional carbon dots have shown a large amount of potential in bioimaging and antioxidation applications. However, the hydrothermal method for the preparation of bifunctional carbon dots requires a high energy input and an expensive setup. Moreover, this method breaks down sensitive compounds in the raw materials and could decrease the antioxidation ability of the resulting carbon dots. Here, phenolic extracts of coffee bean shells were used to prepare carbon dots via a cheap, energy-saving, mild molecular aggregation method. The as-prepared carbon dots were characterized by TEM, HPLC, XPS and Raman spectroscopy. The carbon dots had a diameter ranging from 1 to 5 nm and mainly contained three kinds of phenolic compounds including 3,4,5-trihydroxybenzoic acid, 3,4-dihydroxybenzaldehyde and 3,4-dihydroxybenzoic acid. The carbon dots demonstrated a strong antioxidation capacity, which was comparable to the commercially available butylated hydroxytoluene. The EC50 of the carbon dots was 110 mu g mL(-1). The carbon dots had a pH-/excitation-dependent fluorescence. The as-prepared carbon dots also showed anti-bleaching fluorescence, which was better than that of the commercially available 40,6-diamidino-2-phenylindole. Based on this finding, the excellent biocompatibility of carbon dots enabled them to be successfully used for banana storage and imaging both cancer cell nuclei and tumors in vivo.