Design of polarity-dependence orange emission multifunctional carbon dots for water detection and anti-counterfeiting

In this work, polarity-dependence orange emission multifunctional carbon dots (o-CDs) were designed for water detection in organic solvents and anti-counterfeiting. The o-CDs were readily fabricated by a one-step hydrothermal method with citric acid and p-aminoazobenzene as precursors. The o-CDs possess bright orange fluorescence, good photochemical stability, and polarity-dependence characteristic. With the increase of polarity, the emission peak of o-CDs gradually red-shifted. Meanwhile, with the increase of water content in organic solvent, the fluorescence intensity of o-CDs decreased significantly and the emission peak position gradually red-shifted. This is due to the specific water-fluorophore interaction (particularly hydrogen bond effects), part of o-CDs gathered, making the carbonized cores suffer from π–π stacking and reduced the transition energy gap. Based on this, a fluorescence sensing platform with satisfactory recoveries and relative standard deviation was constructed to detect water content in real alcohol solvents. Furthermore, the o-CDs were further designed into fluorescent ink and prepared fluorescent flexible films, realizing information encryption and solid-state fluorescence. This work opens up a possibility for the development of polarity-dependence orange emission nanomaterial based on CDs for water detection, anti-counterfeiting, and solid-state fluorescence.