Dual surface functionalised curcumin-shellac nano-delivery system with enhanced antimicrobial action

We report a strong boost of the antimicrobial action of curcumin (CUR) upon encapsulation in sterically stabilized shellac-based nanoparticles (NPs) with cationic surface functionalisation. The CUR-loaded shellac NPs were fabricated by solvent attrition and co-precipitation by mixing an aqueous solution of ammonium shellac and an ethanolic solution of curcumin followed by a pH drop from 8 to 5 in the presence of the sterically stabilising polymer Poloxamer 407 (P407). The surface functionalisation of the produced curcumin nanocarrier was done by subsequent doping with the water-insoluble cationic surfactant octadecylthrimethylammonium bromide (ODTAB). Optimal nanocarrier stability was obtained at a fixed ratio of (0.25 : 0.2) wt% of shellac : poloxamer 407 concentrations. Shellac NP formulations containing 0.01-0.07 wt% concentration range of encapsulated CUR with 0.25 wt% shellac at pH 5 were successfully produced and examined for the efficiency of CUR encapsulation and its release from these nanocarriers. We studied the antibacterial action of CUR-NPs before and after the cationic surface functionalisation to evaluate the encapsulation efficiency, the role of the nanocarrier components and surface properties on its antibacterial, antiyeast and antialgal action. The antimicrobial effect of the surface functionalised CUR loaded-shellac NPs was evaluated on different proxy microorganisms, including <i>E. coli</i>, <i>C. reinhardtii</i> and <i>S. cerevisiae</i>. The cationic coating of the shellac NPs strongly enhanced the antimicrobial effect of the encapsulated CUR for all the examined microorganisms. We envisage that the enhanced effect is due to the strong electrostatic attraction of the coated CUR-loaded shellac NPs and the anionic surface of the cell walls which promote the nanocarrier accumulation directly on the microbial cell membrane and the local delivery of CUR which increased its bioavailability. This nanotechnology-aided amplification of the antimicrobial effect of CUR may potentially offer new antialgal, anti-yeast and antibacterial formulations based on natural ingredients as shellac and curcumin without the use of conventional antibiotics.