Role of Ionic Liquid and Phytochemical Capping on the Structure and Biological Applications of ZnO Nanoparticles

Nanoparticles (NPs) have transformed modern biomedical science by enabling targeted drug delivery, enhancing antimicrobial, antioxidant, protein-interaction capabilities, improving diagnostic imaging. Among metal oxide nanomaterials, zinc oxide (ZnO) NPs are widely investigated due to their biocompatibility and functional versatility. This study presents a comparative analysis of ZnO NPs synthesized through two distinct green chemistry approaches, utilizing the ionic liquid (IL) 1-Decyl-3-methylimidazolium tetrafluoroborate [DMIM][BF₄] and the aqueous leaf extract of Nyctanthes arbor-tristis (NAT) as capping and stabilizing agents. Comprehensive physicochemical characterizations revealed notable differences in morphology and stability. Powder X-ray diffraction (PXRD) confirmed phase-pure wurtzite ZnO in both samples. Microscopic analysis revealed that IL-ZnO NPs were smaller and more uniformly dispersed, whereas NAT-ZnO NPs exhibited comparatively larger and irregular morphologies. Zeta potential measurements indicated moderate colloidal stability for IL-ZnO (+ 15 ± 0.3 mV) than NAT-ZnO NPs (+ 8 ± 0.5 mV). FTIR spectra validated Zn–O bonding and surface functionalization derived from the respective capping agents. Biological assays demonstrated that IL-ZnO showed comparatively stronger antibacterial and antifungal activity, while NAT-ZnO exhibited selective inhibition against Enterococcus faecalis (E.faecalis) and relatively higher antioxidant activity at elevated concentrations (conc.). Circular dichroism (CD) spectroscopy of bovine serum albumin (BSA) interactions indicated minimal conformational disruption, suggesting NPs-induced stabilization rather than denaturation.

These findings demonstrate that the choice of capping agent influences NPs morphology, stability and biological response, offering valuable insights for targeted applications.