Ligand-mediated formation of amphiphilic silver nanoclusters: synthesis, redox properties, and activity against clinically relevant bacteria

Abstract

Ultrasmall silver nanoclusters have emerged as promising tools for their sensing, optical, and biological properties, but their translation toward practical applications remains limited due to the synthetic complexity and insufficient control over colloidal stability. Here we report a straightforward one-step strategy for the direct conversion of Ag⁺ ions into blue-emitting silver nanoclusters (FcCAR@Ag NCs, average diameter 2.6 nm) using amphiphilic ferrocene carnosine (FcCAR) as both reducing and capping agents. Under mild basic conditions and in the presence of ascorbic acid, the FcCAR ligand drives the kinetic-controlled reduction of silver ions and simultaneously stabilizes the resulting nanoclusters through interfacial interactions mediated by ferrocene and carnosine functional groups. The formation of well-dispersed nanoclusters is confirmed by comprehensive optical, structural, and colloidal characterization, revealing blue photoluminescence and high colloidal stability. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) analyses performed on screen-printed electrodes (SPEs) showed a higher electrochemical response of FcCAR@Ag NCs with respect to the native FcCAR ligand, suggesting the potential application of FcCAR@Ag NCs in electrochemical sensing. Moreover, the use of ligands based on peptides functionalized with ferrocene units introducing lipophilicity confers an amphiphilic character to NCs, which became crucial for the effective interaction with bacterial envelopes. Therefore, MIC and MBC values of FcCAR@Ag NCs against Staphylococcus aureus and Escherichia coli demonstrated the superior antimicrobial efficacy of silver in nanocluster form compared with conventional silver ions (AgNO₃). Specifically, a two-fold reduction of MIC (from 11.7 to 5.8 µg mL⁻¹ for S. aureus and from 5.8 to 2.9 µg mL⁻¹ for E. coli) along with a four-fold reduction in MBC (from 46.7 to 11.7 µg mL⁻¹ for S. aureus and from 11.7 to 2.9 µg mL⁻¹ for E. coli) was observed. In addition, low MIC values (5.8 µg mL⁻¹) were found against clinically relevant bacteria, including methicillin-resistant S. aureus (MRSA), vancomycin-resistant Enterococcus faecium (VREfm), Pseudomonas aeruginosa, and ESBLs producing Escherichia coli. Moreover, FcCAR@Ag NCs were also effective in reducing biomass and metabolic activity of 24 h-established biofilms formed by S. aureus, E. coli, and P. aeruginosa strains. Overall, our findings highlight the strong antimicrobial potential of FcCAR@Ag NCs against both Gram-positive and Gram-negative bacteria, including antibiotic-resistant and biofilm-producing strains.