Physicochemical Characterization and Antibacterial Properties of Carbon Dots from Two Mediterranean Olive Solid Waste Cultivars

Carbon nanomaterials have shown great potential in several fields, including biosensing, bioimaging, drug delivery, energy, catalysis, diagnostics, and nanomedicine. Recently, a new class of carbon nanomaterials, carbon dots (CDs), have attracted much attention due to their easy and inexpensive synthesis from a wide range of precursors and fascinating physical, chemical, and biological properties. In this work we have developed CDs derived from olive solid wastes of two Mediterranean regions, Puglia (CDs_P) and Calabria (CDs_C) and evaluated them in terms of their physicochemical properties and antibacterial activity against Staphylococcus aureus (S. aureus) and Pseudomonas aeruginosa (P. aeruginosa). Results show the nanosystems have a quasi-spherical shape of 12-18 nm in size for CDs_P and 15-20 nm in size for CDs_C. UV-Vis characterization indicates a broad absorption band with two main peaks at about 270 nm and 300 nm, respectively, attributed to the pi-pi* and n-pi* transitions of the CDs, respectively. Both samples show photoluminescence (PL) spectra excitation-dependent with a maximum at lambda(em) = 420 nm (lambda(exc) = 300 nm) for CDs_P and a red-shifted at lambda(em) = 445 nm (lambda(exc) = 300 nm) for CDs_C. Band gaps values of approximate to 1.48 eV for CDs_P and approximate to 1.53 eV for CDs_C are in agreement with semiconductor behaviour. zeta potential measures show very negative values for CDs_C compared to CDs_P (three times higher, -38 mV vs. -18 mV at pH = 7). The evaluation of the antibacterial properties highlights that both CDs have higher antibacterial activity towards Gram-positive than to Gram-negative bacteria. In addition, CDs_C exhibit bactericidal behaviour at concentrations of 360, 240, and 120 mu g/mL, while lesser activity was found for CDs_P (bacterial cell reduction of only 30% at the highest concentration of 360 mu g/mL). This finding was correlated to the higher surface charge of CDs_C compared to CDs_P. Further investigations are in progress to confirm this hypothesis and to gain insight on the antibacterial mechanism of both cultivars.

Automated summary

Carbon dots (CDs), a type of carbon nanomaterials, have been synthesized from olive solid wastes and their properties and antibacterial activity have been evaluated. The study found that the CDs had a quasi-spherical shape, with different sizes for different regions. They exhibited absorption and photoluminescence characteristics, and higher antibacterial activity against Gram-positive bacteria. The surface charge of the CDs was found to influence their antibacterial properties.