Control the solubility of carbon quantum dots by solvent engineering

Solubility parameters of carbon quantum dots (CQDs) are important physical properties to deploy CQDs in various applications such as organic light emitting diodes (OLEDs), light-converting materials in LEDs, and photoluminescent sensors for metal ion detection. Because of the low toxicity and tunable emission most CQDs have been designed to be water-soluble and biocompatible; oil-soluble CQDs that are essential for OLEDs are not yet explored. Herein, we used a solvothermal method to prepare CQDs and demonstrated that the solubility of CQDs could be controlled by the synthetic solvent. We used hydrophobic (toluene), ambipolar (ethanol), and polar (water) solvents to prepare three types of CQDs from citric acid and thiourea. Correlating surface chemistry, solubility, and fluorescent properties of the CQDs suggests that the solubility of CQDs is governed by the dominate surface functional groups while the more diversity in the surface functional groups shift the emission of CQDs to longer wavelengths.