Development of a New Antimicrobial Photosensitizer from Brominated Carbon Dots: Metal-Enhanced Phosphorescence and Singlet Oxygen Generation

Abstract

The ever-growing wave of antibiotic resistance in bacterial strains has placed pressure on the scientific community to develop new, diverse methods for fighting deadly microbials. One such method investigated is antimicrobial photodynamic inactivation (aPDI), whereby photosensitizing compounds or particles that commonly occupy a triplet excited state (T1) may generate toxic species, such as reactive oxygen, when irradiated. Subsequently, these short-lived reactive agents inflict critical damage on nearby microbials. Although numerous photosensitizers have been developed and implemented for this purpose, the need remains to develop more photostable and dynamic photosensitizers. Carbon nanodots are well known to exhibit strong photostability and biocompatibility, are easily synthesized, and may undergo simple structural modification. We report a new singlet oxygen photosensitizer from a carbon nanodot scaffold, termed “Heavy Carbon Nanodots.” These luminescent, quasi-spherical nanoparticles are composed of brominated sheets of oxidized graphene that exhibit phosphorescence (T1→S0) when oxygen diffusion is slowed; however, in oxygenated, low viscosity solvents we show singlet oxygen is readily generated by these particles. Further we explore the potential of silver-coated 96-well plates “Quanta Plates™” to plasmon amplify both the phosphorescent signal and singlet oxygen generation, in processes known as metal-enhanced phosphorescence (MEP) and metal-enhanced generation of singlet oxygen (ME1O2) respectively. We report increased detection of triplet emission and increased generation of singlet oxygen by these metal-coated wells. These phenomena are also observed to be pH-dependent. The data reported here provides the foundation for future development of brominated carbon nanodots as novel agents for antimicrobial photodynamic inactivation. ∗all correspondence: [email protected]