Purification and Fractionation of Carbon Dots using pHcontrolled Cloud Point Extraction Technique

In the present work a simple, fast, and green procedure is described for the purification and fractionation of carbon dots (CDs) coated by oxygencontaining groups. The CDs were synthesized by thermal pyrolysis of citric acid. The synthesis product was purified by separating them from the synthesis precursors. They also could be fractionated into two distinct kinds of CDs (f4 and f1) using pH-controlled cloud-point extraction (CPE) technique. The fractions f4 and f1 were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-Vis absorption, and FT-IR spectroscopy techniques. The results obtained were also used for studying and explaining the mechanism of the extraction procedure. It was revealed that the fractionation relies on the different surface chemistry of the CDs (i.e. the number or surface-density of oxygen-containing functional groups) which plays the main role in their different behavior at different pH values. Comparing fluorescence spectra of the separated fractions indicated that the surface-chemistry had also a marked effect on photoluminescence behavior of the CDs. The developed procedure can be used for the preparation of pure CDs or particular pure fractions of them for research laboratory purposes, or potentially for their industrial production, as well.