Synergistic anti-bacterial activity of imidazole-based azo dyes with polyvinylpyrrolidone-coated silver nanoparticles

The high demand for novel antimicrobial textiles by the medical, healthcare, hygiene, sportswear, personal protective equipment and filtration sectors has promoted the growth of functional textiles. However, the efficacy of antimicrobial agents against different pathogens is a considerable challenge because of the distinctive mechanisms of action and resistance. The development of novel synergistic antimicrobial dyes may offer numerous opportunities to enhance antimicrobial effectiveness. In this work, a novel imidazole-based azo dye with a p-methoxyphenyl group in the N-1 substituent of the imidazole ring (AzoIz-a), and corresponding amidrazone precursor (AmIz-a), were combined with polyvinylpyrrolidone-coated silver nanoparticles (AgNPs). The molecules, alone and combined with the AgNPs, were characterised by ultraviolet-visible spectrophotometry and zeta potential. Their synergistic effect was assessed against Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa. The results were compared with a previously reported imidazole-based azo dye and precursor containing a methyl group in the imidazole ring (AmIz-b and AzoIz-b). The results showed interesting antimicrobial properties of the novel AzoIz-a molecules when combined with a very small concentration of AgNPs. The combination showed an additive effect for S. aureus and a synergistic effect for E. coli and P. aeruginosa. Considering the synergistic results, the effective concentration of the AzoIz-a decreased from more than 128 to 16-32 μg.mL⁻¹ by the addition of a small concentration of AgNPs (0.6-1.3 μg.mL⁻¹), which displayed comparable results with the AzoIz-b molecule. Thus, the application of these conjugates in textiles may lead to highly coloured materials with remarkable anti-bacterial properties, which warrant further exploration.