Silver Nanoparticle Conjugation with Thiopyridine Exhibited Potent Antibacterial Activity Against Escherichia coli and Further Enhanced by Copper Capping

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Background: Nanomaterials-based antibacterial agents are anticipated as future generation antibiotics. Silver nanoparticles are promising candidates to enhance the antibacterial effects of antibiotic drugs and lead compounds. Pyridine compounds and thiol moieties are important classes of pharmacophores that are part of many drugs used against numerous diseases; therefore, we conjugated synthetic thiopyridine (ThPy) with silver nanoparticles. Objectives: The study was designed to evaluate the antibacterial potential of ThPy and the effects of silver nanoparticles conjugation with it and to explore the synergistic effects of other metal ions. Methods: Using formyl pyridine reaction with dibromopropane, N-alkylated product was obtained. Bromo group was substituted by thioacetate nucleophile resulting in the formation of thiopyridine (ThPy). Thiopyridine was used to stabilize silver nanoparticles synthesized by one-phase reduction. Silver nanoparticle-conjugated thiopyridine (ThPy-AgNPs) showed typical surface plasmon resonance band, while atomic force microscopy (AFM) showed size and morphology of spherical polydispersed nanoconjugates of 60 nm. Results: Antibacterial properties of synthesized cationic compound thiopyridine was enhanced by conjugation with silver nanoparticles. Moreover, we presented a new strategy in which thiopyridine-AgNP nanoconjugates’ affinity towards copper ions is utilized to further enhance antibacterial activity of nanoconjugates. ThPy-AgNPs exhibited more inhibitory effects against Escherichia coli (MIC of 100 µg/mL compared to 200 µg/mL with ThPy). Nanoconjugates showed selective affinity for Cu(I) ions to cross the E. coli membrane. Conclusions: The addition of Cu(I) ions with ThPy-AgNPs has a synergistic effect on the activity of nanoconjugates against E. coli. Atomic force microscopy is proved to be an excellent choice to study the morphological changes occurring during the antibacterial process.