Solvatochromic Response of Carbon Dots: Evidence of Solvent Interaction with Different Types of Emission Centers

The fluorescence emission of carbon dots (CDs) of ∼4 nm diameter was studied in a large number of solvents ranging from the weakly polar 1,4-dioxane to the strongly polar and protic water and formamide. The emission spectra in all solvents comprise of two major components, attributed to two distinct populations of emission centers, the edge-states at the periphery of the sp²-hybridized CD core and the surface fluorophores. Emission from both these centers are sensitive to solvent polarity and H-bonding capacity and undergo prominent redshift with solvent <i>E</i>_T(30) polarity parameter, which reveals a comparable polar character of these centers and their strong interaction with solvent molecules. However, between these two centers, the emission of surface fluorophores is more drastically quenched at high <i>E</i>_T(30) solvents, suggesting the opening up of additional nonradiative relaxation channels. This distinct solvatochromic response of these two emission centers also enables the CDs to perform as ratiometric probes for solvent polarity