Multifunctional Au nanoclusters for targeted bioimaging and enhanced photodynamic inactivation of Staphylococcus aureus

Fluorescent Au nanoclusters (NCs) have been the subject of intense study owing to their increasing applications in imaging, sensing, and nanomedicine. Herein, highly fluorescent bovine serum albumin (BSA)-directed Au-BSA NCs are synthesized as a platform for further conjugation with human antistaphylococcal immunoglobulin (antiSAIgG) and the photodynamic (PD) synthesizer Photosens (TM) (PS) to fabricate Au-BSA-antiSAIgG-PS complexes. The complexes possess three theranostic modalities: biospecific detection of Staphylococcus aureus bacteria, intense red fluorescence due to Au-BSA NCs (quantum yield, similar to 14%), and synergistic PD inactivation due to the PS dye. Owing to biospecific targeting and intense red fluorescence, the Au-BSA-antiSAIgG probe can detect pathogenic bacteria in bacterial mixtures through fluorescence microscopy or even through naked eye inspection of sediments under UV illumination. The developed complexes can be used at a physiological pH of similar to 7, in contrast to nonspecific electrostatic binding of Au-human serum albumin NCs to S. aureus at pH < 5-6. PD treatment of methicillin-sensitive and methicillin-resistant S. aureus with Au-BSA-antiSAIgG-PS complexes and 660 nm light irradiation significantly inactivates both types of bacteria. By using the developed strategy, Au-BSA-PS complexes can be further modified with other targeting and chemotherapeutic molecules, thus ensuring new theranostic applications in current nanomedicine.