Fluorescent carbon dots with tunable negative charges for bio-imaging in bacterial viability assessment

Applying bio-imaging in bacterial viability assessment is vital to human health. Nitrogen, phosphorus co-doped carbon dots (NPCDs) with high visible fluorescence and tunable negative charges were synthesized by a feasible hydrothermal reaction. The as-prepared NPCDs possess abundant surface functional groups, excellent photostability and bio-compatibility. Especially, the surface negative charges of NPCDs can be adjusted by changing the kind of the precursors. The obtained NPCDs as a fluorescent dye could selectively stain dead bacteria instead of live ones due to electrostatic repulsion since the cell walls of bacteria possess negative charges. The NPCDs with less negative charges provides a low accurate mortality rate due to a weaker repulsion, while NPCDs with the most negative charges give a comparable mortality rate with traditional plate counting method but consumes shorter time. Our study suggests that NPCDs with highly negative charges could serve as a precise and efficient fluorescent probe to determine the living state of bacteria, widening the path for bio-imaging application in bacterial viability assessment. (C) 2017 Elsevier Ltd. All rights reserved.