Challenges in Using Time-resolved Fluorescence Anisotropy for the Size Determination of Carbon Dots

In this work, we exploited time-resolved fluorescence depolarization imaging (r-FLIM) to determine the averages size of carbon dots (CDs). The characteristic rotational correlation time connected to the size of the particle through classical Stokes–Einstein–Debye equation. The r-FLIM technique applied in this study achieves picoseconds time resolution, which approximately corresponds to the particle size determination at sub-nanometer precision. The depolarization time constants of the heteroatom-doped CDs in aqueous solution are found ca. 315 ps. The calculated diameter of the carbon dots is about 1.4 nm, which is smaller than the actual size of the carbon dots (5 nm). It has been recently reported that the proton transfer between water molecules and CDs might contribute to the faster depolarization process of CDs in aqueous solution. The discrepancy between the measured and calculated size of the CDs, therefore, can be attributed to the interaction between CDs and polar solvent molecules. Further studies required to ensure the reliability of r-FLIM as nanoparticle metrology standards of CDs size determination.