Humic extracts from raw or composted coffee husks as novel chitosan crosslinkers for the synthesis of sustainable nanoparticles with antioxidant properties

Coffee husks have been valorised by isolating humic materials before (HLS-Raw) or after (HS-Comp) composting them. Such substrates were reacted with chitosan at different ratios to synthesize novel nanoparticles (NP) with radical scavenging properties. Size and antioxidant activity of nanomaterials increased at higher HS/chitosan ratio, while zeta potential decreased. Our results furthermore highlighted that he molecular features of raw humic materials, as studied by solid-state NMR spectroscopy, affected the studied NP properties. Indeed, at equal reactant ratio, NP from HS-Comp had lower zeta potential due to their higher amount of carboxyl moieties contained, which counterbalanced a greater number of chitosan positive charges. Moreover, these NP showed lower antioxidant activity despite the greater phenolic content of the original HS. We hypothesized that the higher hydrophobicity of HS-Comp led to the partial partitioning of their phenolic components into the inner domains of the derived NP, lowering their antioxidant capacity. Additionally, in a short-term stability assay, NP coalescence was observed, especially at higher humic/chitosan ratio, with nanomaterial precipitation and consequent decrease in antioxidant capacity. Overall, we demonstrated that humic materials from raw or composted coffee husks could successfully crosslink chitosan chains to synthesize sustainable NP with tuneable physical-chemical and antioxidant properties.