Viologen Cross-Linked Chitosan Hydrogel as Biobased Antiseptic Surface-Coating Materials

The recent COVID-19 pandemic has triggered an avalanche of research seeking powerful antibacterial and antiviral material-based technologies. Herein, we report the cross-linking of chitosan fishery waste using three different aldehyde-terminated viologens to design iono-functional, shapeable biobased antiseptic hydrogels. Structural investigation has corroborated the presence of the cross-linked cationic 4,4′-bipyridium motif inside and confirmed its accessibility to the guests through chemical reduction. The network can be moreover cross-linked with the simultaneous entrapment of gold, silver, and copper nanoparticles, enabling the accommodation of additional synergistic functionalities inside of the framework. The multifaceted character of the resulting soft hydrogel network is illustrated through its use as a spray to coat glassy surfaces, its casting to deliver transparent and flexible micrometer-thick films, and its coagulation to provide open porous microspheres. Interesting antibacterial activity has been noticed for the cross-linking films, owing to the presence of cationic viologen, with the inhibitory effect being more significant for negative-gram bacteria compared to positive-gram bacteria. Within the two series, the trend in the biological response correlates with increasing the number of the viologen units and/or the engaged molar ratio with respect to native chitosan films. A maximum inhibitory effect of 85% was recorded for negative-gram bacteria, while 50% inhibition was the best performance reached for positive-gram bacteria. Significant amplification of the antibacterial activity has been further noticed using transparent films entrapping a tiny amount of metal nanoparticles (gold, silver, and gold), outperforming holistically those entrapping metal nanoparticles without cross-linking. Specifically, gold nanoparticles grown within the reticular viologen-containing framework enabled the highest antibacterial activity (98% inhibition for negative-gram bacteria and 75% inhibition for positive-gram bacteria), contrasting with a very negligible side effect in terms of hemolytic activity. The straightforwardness, biodegradability, and cost-effectiveness of the disclosed approach open great possibilities toward large-scale use as an antiseptic spray for surface-coating against nosocomial infections.