Purification and Fractionation of Carbon Dots using pH-controlled Cloud Point Extraction Technique

In this work, we describe a simple, green and general procedure for the purification and fractionation of carbon dots (CDs). CDs coated with oxygen-containing functional groups were synthesized by thermal pyrolysis of citric acid. The product of the reaction was first Purified and then fractionated into two distinct kinds of CDs (f4 and f1) using pH-controlled cloud point extraction (CPE) technique. Characterization of f4 and f1 fractions by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-Vis absorption, and FT-IR spectroscopy not only lead to study and explain the mechanism of the extraction procedure but also revealed that the two fractions were different from each other in terms of their different optical properties and surface chemistry (the number or surface-density of oxygen-containing functional groups) which in turn plays the main role in their fractionation at different pH values. Comparing fluorescence spectra of f4 and f1 fractions indicated that the surface-density or number of the oxygen-containing functional groups has a marked effect on photoluminescence behavior of CDs. The developed general procedure method can be used for large-scale production of different fractions of any pure CDs in the industry as well as research laboratory preparation purposes.