Steroid-grafted silk fibroin conjugates for drug and agrochemical delivery

Steroid-grafted silk fibroin (SF) conjugates with a degree of substitution of 1.1-4.3 mol% were efficiently synthesised for controlled release of anticancer drug diosgenin and agrochemicals DI31 and S7. The esterification of tyrosine residues in SF conjugates was confirmed using elemental analysis, ATR-FTIR and H-1 NMR spectroscopies. The thermal properties of SF conjugates analysed using DSC and TGA confirmed the ATR-FTIR findings on SF conjugates that beta-sheet conformation was the predominant one in solid state. DLS revealed the impact of hydrophobic modification of SF on its hydrodynamic parameters, which resulted in more than doubled hydrodynamic sizes and zeta potential values. SF conjugates were observed as 250-600 nm aggregates formed by nanoparticles of 25-41 nm, as depicted using TEM and AFM. Controlled release of 40-50% grafted steroids from SF conjugates at pH 6.0 was achieved after 5 days. Agrochemical-grafted SF conjugates showed better plant growth enhancing effect than parent DI31 and S7 when evaluated on radish plants. SF conjugates showed low cytotoxicity at concentrations below 0.025 mg/mL when applied on MCF-7 and HEK-293 cells. Due to these results, we envisage that the synthesised DI31- and S7-grafted SF conjugates are good candidates for agrochemical use. This work also contributes to progress the use of silk fibroin in sustainable agriculture.