Synergistic photothermal antimicrobial therapy using graphene oxide/polymer composite layer-by-layer thin films

We report an efficient and a simple synergistic antimicrobial therapy for lysing pathogenic E. coli K12 MG1655 using layer-by-layer (LbL) films of graphene oxide (GO) and poly(allylamine hydrochloride) (PAH). Two kinds of antimicrobial therapies have been tested in this study. The first therapy involves the inherent antimicrobial property of GO studied by incubating the bacteria with glass slides coated with GO/PAH films up to 16 h (varying number of layers from 20 to 80). The optical density (OD) measurements at 600 nm revealed that the antimicrobial activity of the films increased with an increase in the number of layers, which can be attributed to an increase in the roughness of the films and also the amount of GO in the films. The second therapy involves exploiting the photothermal property of GO by exposing the films to a near-infrared (NIR) laser at 1064 nm (Nd:YAG, 10 ns pulse, 10 Hz, 85 mW) for 15 min and incubation for 4 or 16 h. The photothermal therapy results revealed the enhanced antimicrobial activity compared to incubation of bacteria with films in the absence of a NIR-laser for 4 or 16 h. The enhanced antimicrobial activity of GO/PAH films is because of the synergistic effect resulting from membrane-stress induced on bacteria by GO sheets and photothermal heating in the presence of NIR-light. Further, we have also performed a fluorescence-based live/dead assay using confocal fluorescence microscopy to verify the viability of bacterial cells after incubating the bacteria and GO/PAH films with and without a NIR laser. All the results suggest that the synergistic antimicrobial effect of GO/PAH films causes increased lysis of E. coli compared to the individual effects.