Antimicrobial Peptide-Reduced Gold Nanoclusters with Charge-Reversal Moieties for Bacterial Targeting and Imaging

To combat the increasing risk of infection by pathogenic bacteria, the new generation of antimicrobial agents is expected to exhibit nonmetabolic killing mechanisms, high potency and biocompatibility. In this work, cysteine-terminated antimicrobial peptide (AMP) was employed directly as a reducing ligand to synthesize AMP-coated gold nanoclusters (Au NCs), bypassing the use of other reagents which might interfere with the efficacy of the resulting NCs. In addition to the use of a biocompatible Au core, the primary amines of AMP coating were functionalized with anionic citraconyl moieties to further reduce cytotoxicity. The citraconyl amides could autocleave to re-expose the cationic amines at low pH. As a result, the AMP-coated Au NCs with citraconyl protection were stable and cytocompatible under physiological conditions as determined by fluorescamine, hemolytic, cytotoxicity, and in vivo toxicology studies, but would switch into a cationic bactericidal mode in an acidic environment commonly encountered at bacterial infection sites. Furthermore, the AMP-coated Au NCs system exhibited bacterial binding and photoluminescence features as determined by flow cytometry and confocal microscopy, which were useful for the detection and imaging of bacterial contamination. The AMP-coated Au NCs with citraconyl moieties therefore represent a "smart" design of pH-responsive antimicrobial agents that can serve multiple functions of bacterial detection, bacterial imaging, and anti-infection therapy.