Thin-Film Composite Polyamide Membranes Decorated with Photoactive Carbon Dots for Antimicrobial Applications

Abstract

In this study, we modified thin-film composite (TFC) membranes by coating them with antimicrobial and photoactive lignin-derived carbon dots (CD) using polydopamine (PDA) chemistry. The CD solution was applied to the membrane surface at different concentrations (100, 300, 500, and 1000 μg/ mL) to create CD-100, CD-300, CD-500, and CD-1000 membranes. The CD-modified membranes were tested against Escherichia coli and Bacillus subtilis cells under three light conditions: darkness, simulated sunlight, and 365 nm ultraviolet (UV). The CD-500 and CD-1000 membranes showed the highest toxicity against E. coli and B. subtilis cells under UV photoactivation. However, the exceptional antibacterial properties of the CD-1000 membrane were accompanied by significant losses in the water permeability (parameter A) and water flux. The CD-500 membrane, on the other hand, demonstrated slightly lower toxicity than CD-1000 while maintaining the transport properties of the membrane compared to the pristine TFC. Under darkness, simulated sunlight, and UV light, the CD-500 inactivated 69.9 ± 1.7, 76.4 ± 1.8, and 99.92 ± 0.03% of attached E. coli cells, respectively, compared with the pristine TFC control in darkness. Prolonged pre-exposure treatments to UV light showed that the CD-500 membrane loses its antibacterial activity significantly after 9, 15, and 24 h of continuous contact with UV radiation. The CD-500 also reduces its toxicity to E. coli cells from 99.92 ± 0.03 to 71.4 ± 0.6% when tested in the presence of simulated wastewater compared to the results collected in saline solution under UV light. Our findings demonstrate the potential of CD for controlling bacterial attachment in TFC membranes while highlighting the need to optimize the coating design to maximize stability under UV exposure and toxicity under real-life conditions.