One-step synthesis of carbon dots with bacterial contact-enhanced fluorescence emission: Fast Gram-type identification and selective Gram-positive bacterial inactivation

Carbon dots (CDs) are a type of metal-free fluorescent quantum dots and can be synthesized facilely and cost-effectively. In this work, we develop a one-pot solvothermal method to prepare a novel type of quaternized CDs using two commercial reagents (glycerol and dimethyloctadecyl[3-(trimethoxysilyl)propy]ammonium chloride, abbreviated as Si-QAC). The introduction of the long alkyl chain-containing quaternary ammonium compound Si-QAC endows the as-prepared CDs with the capability to interact with the negatively-charged bacterial cells via both electrostatic and hydrophobic interactions. Owning to the different surfaces of Gram-positive and Gram-negative bacteria, the CDs can selectively interact with the Gram-positive bacteria. More interestingly, once the CDs are adsorbed onto the Gram-positive bacteria, their fluorescence emission is significantly enhanced. Furthermore, the CD adsorption can further cause the bacterial cell surface destruction since the CDs may change the charge balance of the bacterial surface and can insert into the bacterial surface via the long alkyl chains, which ultimately leads to the inactivation of Gram-positive bacteria. As compared with the conventional Gram-staining method, the CD-based Gram-type differentiation approach is accurate, fast, and easy-operating. Overall, the quaternized CDs represent a metal-free nanoparticle-based nanotheranostic for imaging-guided bacterial differentiation and inhibition.