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

Abstract

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 π-π* and n-π* transitions of the CDs, respectively. Both samples show photoluminescence (PL) spectra excitation-dependent with a maximum at λem = 420 nm (λexc = 300 nm) for CDs_P and a red-shifted at λem = 445 nm (λexc = 300 nm) for CDs_C. Band gaps values of ≈ 1.48 eV for CDs_P and ≈ 1.53 eV for CDs_C are in agreement with semiconductor behaviour. ζ 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 µg/mL, while lesser activity was found for CDs_P (bacterial cell reduction of only 30% at the highest concentration of 360 µg/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.

Broad-Spectrum Antibacterial Activity of Synthesized Carbon Nanodots from d-Glucose

“…The antibacterial activity and mechanism are affected by several factors including CND physiochemical properties such as their sizes, surface charges, and the nature of surface defects and functional groups . Various previous reports showed that the carboxylated carbon nanotubes and graphene oxide nanomaterials exhibit broad-spectrum antimicrobial activity through the oxidative stress which leads to bacterial cell death. − …”

“…The different response of the various bacterial strains tested to CNDs could be ascribed to their cellular membrane nature and composition …”

“…36 The different response of the various bacterial strains tested to CNDs could be ascribed to their cellular membrane nature and composition. 32 To confirm the ability of the prepared CNDs to produce ROS, we tested the percentage glutathione oxidation as shown in Figure 9.…”

“…30 CNDs synthesized from glucose by acid−base neutralization spontaneous heat have been used as antibacterials showing inhibition of both Gram-positive (Staphylococcus aureus and Bacillus subtilis) and Gram-negative (Escherichia coli) with minimum inhibitory concentration (MIC) equal to 192 μg/mL for E. coli, 384 μg/mL for S. aureus, and 768 μg/mL for B. subtilis. 31 Furthermore, other CNDs were synthesized from different resources such as olive wastes, 32 sago starch, 33 and metronidazole, 34 and they have been utilized as antibacterial agents showing different MIC and colony forming reduction percent.…”

“…coli, 384 μg/mL for S. aureus , and 768 μg/mL for B. subtilis . Furthermore, other CNDs were synthesized from different resources such as olive wastes, sago starch, and metronidazole, and they have been utilized as antibacterial agents showing different MIC and colony forming reduction percent.…”