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Enhanced Photostability of Fluoroquinolone Antibacterials Capped on Silver Nanoparticles

The photolabile and phototoxic fluoroquinolone (FQ) antibacterial drugs norfloxacin and rufloxacin have been used as capping agents for 25 nm Ag nanoparticles, exploiting the noncovalent interaction between the cationic piperazinyl ring of both FQs and the negatively charged metal surface. The resulting FQ‐protected silver nanoclusters (Ag@FQs), ca. 80 nm in diameter, are dispersible in phosphate buffer at physiological pH and their response to light excitation has been studied by steady‐state and time‐resolved spectroscopic and photochemical techniques. The fluorescence emission of the FQs chromophores is not extensively quenched in the Ag@FQs. In contrast, both Ag@FQs exhibit a photochemical stability more than one order of magnitude larger than that of the free drugs. This is the result of significant and differentiated quenching effects by the Ag surface on the kinetic and population of the excited triplet states of FQs, which are the key intermediate in the drugs photodecomposition. In view of the well‐known antimicrobial properties of the Ag nanoparticles, the Ag@FQs could represent intriguing nanohybrids in the perspective multifunctional nanodrugs.