Killing G(+) or G(-) Bacteria? The Important Role of Molecular Charge in AIE-Active Photosensitizers

Bacterial infections pose a serious threat to human health. Photodynamic therapy is an effective medical treatment to solve the problems raised by antibiotic resistant bacteria. But it is not easy to have photosensitizers (PSs) that can simultaneously produce efficient fluorescence and reactive oxygen species. Traditional PSs show compromised performances due to the aggregation-caused quenching effect in aqueous media, however, luminogens with aggregation-induced emission (AIE) can inherently achieve high fluorescence and efficient ROS generation. In addition, electrostatic interaction is generally accepted to be responsible for initial targeting of bacteria. But for AIE PSs, the roles of molecular charges on antibacterial efficiency are rarely considered. Herein, two red-emissive AIE PSs with the same luminogenic core but carrying different number of positive charges are designed, and their antibacterial performance and the killing mechanism toward Gram-positive (G(+)) and Gram-negative (G(-)) bacteria are investigated. The AIE PSs with highly efficient singlet oxygen generation can clearly image and selectively kill bacteria over mammalian cells. With the increase in the positive charges of AIE PSs, the improvement in antibacterial efficiency is great against G(-) bacteria, but it is negligible against G(+) bacteria. This research will provide new insight into the rational design of new antibacterial materials.