Nanoscale Effects in the Room-Temperature UV–Visible Photoluminescence from Silica Particles and Its Cancer Cell Imaging

Silica nano/microparticles have generated significant interest for the past decades, emerging as a versatile material with a wide range of applications in photonic crystals, bioimaging, chemical sensors, and catalysis. This study focused on synthesizing silica nano/microparticles ranging from 20 nm to 1.2 μm using the Stöber and modified Stöber methods. The particles exhibited photoluminescence emission across a UV-visible range, specifically in the UV (∼290, ∼327, ∼339, and ∼377 nm), blue (∼450 nm), green (∼500 nm), yellow (∼576 nm), and red (∼634 nm) range of the electromagnetic spectrum. These emissions are due to radiative relaxation processes involving oxygen-deficient centers arising due to unrelaxed oxygen vacancies, strong interacting surface silanols, 2-fold coordinated silicon, self-trapped excitons, hydrogen-related species, strain-induced defects, and nonbridging oxygen hole centers excited via two-photon and single photon absorption. The increased PL intensity with a decreasing particle size was attributed to higher concentrations of defect sites in the case of smaller-sized particles. The MTT assay, AO/EB staining, and the DCFDA assay confirmed the biocompatible nature of silica particles in the HepG2 cell line. In addition, the cell viability assay in a normal cell line (HEK293) also showed no substantial cell death. Successful bioimaging of HepG2 cells was performed with silica nano/microparticles, which exhibited blue and green fluorescence, along with Hoechst33258 dye. Even though 20 nm-sized silica particles showed higher PL emission, particles sized above 20 nm showed better fluorescence in HepG2 cells, citing their potential in in vitro bioimaging applications.