One-step synthesis of hybrid copper humate-chitosan nanoparticles to control the development of human bacterial pathogens

Positively charged chitosan chains are commonly crosslinked with compounds of synthetic origin to obtain nanoparticles (NP). We hereby improved the sustainability of the production of chitosan NP by exploiting as crosslinkers compost-isolated Humic Substances (HS) (HS-C) or leonardite (HS-L) with increasing amount of chelated copper. An inverse relationship between NP z-average and humic-complexed copper added was found, whereas the zeta potential remained unaltered. A greater antibacterial activity was observed at increasing copper ion contents in NP, due to noxious effects of this metal ion against bacterial development. Furthermore, chitosan-derived positive zeta potential of NP may have favoured their adhesion onto bacterial walls, with their consequent internalization into bacterial cells. Also, the bioactivity depended on specific HS chemical features, as a higher antibacterial power was noticed for nanoparticles made with the alkyl- and aromatic-rich HS-L, whereas the hydrophilic HS-C displayed a lower antibacterial power. We furthermore noted that, regardless of the compositional differences, the humic-derived nanoparticles always performed better than the raw HS, likely due to an enhanced interaction between nanoparticles and bacterial cells. Overall, we demonstrated that copper-humates might be successfully exploited to synthesize sustainable chitosan/humic-based copper nanocarriers to efficiently control the development of bacteria responsible of human pathologies.