APTES mediated controlled functionalization of biogenic ZnO nanoparticles: improved surface charge, crystallinity, size distribution, and optical performance

In this study, biogenic zinc oxide nanoparticles (ZnO NPs) were synthesized via a green chemistry route and subsequently functionalized with 3-aminopropyltriethoxysilane (APTES) under controlled conditions. A series of five formulations (A-E) were evaluated to investigate the impact of synthesis parameters on physicochemical, structural, and optical properties. Structural characterization by x-ray diffraction (XRD) confirmed the preservation of the wurtzite crystal structure in all samples, with no evidence of secondary phases or crystallographic distortion. Raman spectroscopy revealed variations in the E₂(high) mode intensity, indicative of differing degrees of crystallinity and defect density depending on APTES and acetic acid concentrations. Notably, the 2E₂(low) second-order mode was used as a probe of phonon confinement and interfacial stress. UV-vis spectroscopy showed band gap tunability between 1.78 and 2.27 eV, correlating with surface modification and structural integrity. Dynamic light scattering (DLS), zeta potential (Z-potential), and electrophoretic mobility analyses revealed that formulation D achieved superior colloidal stability (Z = −500.69 mV, hydrodynamic diameter = 26.5 nm), consistent with high silane crosslinking and surface charge repulsion. In contrast, the control sample exhibited marked aggregation (Z = −0.34 mV, diameter = 108 nm), underscoring the relevance of chemical functionalization in stabilizing nanomaterials. These results demonstrate that APTES functionalization effectively modulates surface and optical properties of ZnO NPs, enabling their tailored use in optoelectronics, photocatalysis, and antimicrobial applications. Formulation D emerged as the most promising, combining high crystallinity, low particle size, excellent colloidal stability, and a favorable band gap.