Carbon Dots with Continuously Tunable Full-Color Emission and Their Application in Ratiometric pH Sensing

Two types of carbon dots (C dots) exhibiting respective excitation-independent blue emission and excitation-dependent full-color emissions have been synthesized via a mild one-pot process from chloroform and diethylamine. This new bottom-up synthetic strategy leads to highly stable crystalline C dots with tunable surface functionalities in high reproducibility. By detailed characterization and comparison of the two types of C dots, it is proved concretely that the surface functional groups, such as C═O and C═N, can efficiently introduce new energy levels for electron transitions and result in the continuously adjustable full-color emissions. A simplified energy level and electron transition diagram has been proposed to help understand how surface functional groups affect the emission properties. By taking advantage of the unique excitation-dependent full-color emissions, various new applications can be anticipated. Here, as an example, a ratiometric pH sensor using two emission wavelengths of the C dots as independent references has been constructed to improve the reliability and accuracy, and the pH sensor is applied to the measurement of intracellular pH values and cancer diagnosis.