Papaya Peel Waste Carbon Dots/Reduced Graphene Oxide Nanocomposite: from Photocatalytic Decomposition of Methylene Blue to Antimicrobial Activity

Carbon dots (CDs) have gained unprecedented attention as a novel luminescent zero-dimensional carbon nanomaterial owing to their diverse industrial applications. Herein, we reported the sustainable synthesis of fluorescent CDs from papaya peel waste, acting as a natural carbon originator. As-prepared CDs and reduced graphene oxide (RGO) were fabricated in the composites through a facile one-step hydrothermal method. Synthesized RGO/CDs (RC) nanocomposites were characterized using spectroscopic, diffraction, and electron-microscopic techniques. Nanocomposites with variable RGO to CD mass ratios were tested for photodegradation of textile dye methylene blue (MB). The highest photocatalytic activity (degradation efficiency of 87% in 135 min) was obtained in the nanocomposite containing a 2.1 mass ratio (RC2). The RGO sheets in the nanocomposite acted as media for electron acceptors, promoting the fast transfer and separation of photoinduced electrons during CDs excitation, thus preventing the recombination of the electron and holes. Based on the agar well diffusion assay, the nanocomposites exhibited excellent antibacterial activity than other tested materials against Bacillus subtilis (Gram-positive) and Pseudomonas aeruginosa (Gram-negative) bacterium. The largest inhibition zone area (22 mm), i.e., the highest antimicrobial activity, was obtained in the nanocomposite tested against Gram-positive strains. Taken together, the synergistic effect of RGO and CDs enhanced the photocatalytic and antibacterial performance of synthesized nanocomposite material.