Sonication power-induced enhancement of porosity and optical properties in transfer-etched rGO films for optical sensing

Reduced graphene oxide (rGO) is a promising material for optical biosensors due to its tunable electronic and optical properties. In this study, rGO films were synthesized through the chemical reduction of graphene oxide (GO) using ascorbic acid under various sonication powers (30, 45, 60, and 75 W), followed by fabrication using a transfer-etching method. Particle size analysis showed that 45 W produced the smallest and most uniform particles, yielding better dispersion stability and thinner films, whereas higher sonication powers caused partial re-agglomeration. Nevertheless, SEM-EDX analysis revealed that carbon content increased with higher sonication power, reaching 79.31% at 75 W, accompanied by an increasingly porous morphology. Although Raman spectroscopy showed an increased I D / I G ratio, indicating increased structural defects, FTIR spectra confirmed successful GO reduction, while four-probe measurements revealed the lowest electrical resistance at 75 W, suggesting improved restoration of the π-conjugated carbon network. Furthermore, surface plasmon resonance (SPR) simulations at 633 nm (He–Ne laser) showed that film thickness critically affects plasmon coupling. The fabricated rGO films (∼200 nm) were too thick for efficient SPR excitation, whereas thinner simulated films (<50 nm) generated sharper responses. Notably, the simulated 75 W sample exhibited the strongest plasmon coupling, characterized by a sharper dip and a distinct rightward shift in the SPR curve. These results demonstrate an environmentally friendly approach for rGO production and highlight the importance of controlling sonication power to tailor its structural and optical properties, with further insights indicating that managing film thickness is also crucial for maximizing SPR sensing performance.