Dual-Emission Carbon Dots for Ratiometric Fluorescent Water Sensing, Relative Humidity Sensing, and Anticounterfeiting Applications

We synthesized blue fluorescent carbon quantum dots (CDs) by the solvothermal strategy using 2,5-dihydroxyterephthalic acid as the precursor. The as-prepared CD surfaces contain abundant carboxyl and hydroxyl groups and show excitation-independent emission and excellent chemical stability. Interestingly, with the increment of water contents in organic solvents, the CDs underwent a visual fluorescence color transition from blue to green, which could be attributed to the excited-state intramolecular proton transfer effect via the intermolecular hydrogen bonding between CDs and water. Based on this, we designed an effective ratiometric fluorescence water sensor in organic solvents, which featured self-calibration, visualization, rapid response (<20 s), and high sensitivity (limit of detection = 0.052%, v/v, in ethanol). The smartphone-based quantitative assay was also realized using CD-loaded paper strips through a color processing application and successfully applied to monitor the water content in spirit samples. Humidity sensing was further evaluated through the as-prepared CDs/polymer films. Remarkably, the CD-based sensors were well recyclable both with paper strips and polymer films. The as-prepared CDs also hold great potential for anticounterfeiting.