NaCl nanocrystal-encapsulated carbon dots as a solution-based sensor for phosphorescent sensing of trace amounts of water in organic solvents.

The phosphorescence of solid-state carbon dots (CDs) has been demonstrated to be susceptible to water molecules. However, solution-based CDs have been rarely exploited for phosphorescence detection of trace amounts of water in organic solvents. Here, we present a straightforward method to embed the CDs into NaCl nanocrystals and show their application for phosphorescence detection of the water content in organic solvents. The phosphorescent CDs inside NaCl nanocrystals were fabricated by hydrothermal treatment of poly(diallyldimethylammonium) (PDDA) polymers and their counter chloride ions (Cl-) in the presence of NaOH. Because of the interaction with quaternary ammonium surface groups of PDDA-based CDs (PDDA-CDs), the Cl- ions serve as a nucleation site to trigger NaCl nanocrystal formation. Electron microscopy and spectroscopy techniques demonstrate the embedment of PDDA-CDs into NaCl nanocrystals (PDDA-CDs@NaCl). The PDDA-CDs@NaCl exhibited excitation-independent phosphorescence and excitation-dependent fluorescence in ethanol, methanol, dimethyl sulfoxide, and dimethylformamide. In four different organic solvents, the phosphorescence QYs and lasting times of PDDA-CDs@NaCl range from 23 to 35% and 1.2 to 1.5 s, respectively. Once trace amounts of water are present in an organic solvent, the water-induced dissolution of NaCl nanocrystals switches off the phosphorescence of PDDA-CDs@NaCl. It was found that PDDA-CDs@NaCl was capable of detecting as low as 0.25% v/v water in ethanol and 0.125% v/v water in methanol. The above-discussed results provide fundamental insights regarding the embedment of phosphorescent CDs into a solid matrix as a solution-based sensor.