Golden Guardians: Advanced pH Sensors with Built-in Antimicrobial Defense

Gold nanoclusters (Au NCs) capped with a small biomolecule, namely (+)-norephedrine, (Au@Nore) display exceptional performance as fluorescent pH sensors. They display high emission quantum yield (Φ_PL of ≈33% in strongly basic media); absence of nanocluster (NC) aggregation and/or induced rigidification of the organic shell; a linear dependency of their emission on the pH in the 8.2–9.5 range; and reversible pH sensing. In addition, excitation at 310 nm, where both the NC and the ligand absorb, enables ratiometric sensing. Studies on the fungus Candida albicans and bacterium Bacillus subtilis demonstrate that while Au@Nore do not cause any impact on the growth of the fungus, it inhibited the growth of the Bacillus, showing the lowest cell viability of 18% at the highest pH values via a mechanism of action involving the disruption of the bacterial wall.