Carbon quantum dots (CQDs) are a novel family of fluorescent materials that could be employed as nontoxic alternatives to molecular fluorescent dyes in biological research and also in medicine. Four different preparation approaches, including microwave assisted heating and solvent refluxing, were explored. In addition to the widely used microwave assisted methods, a simple convenient new procedure is presented here for the particle synthesis. A detailed X-ray photoelectron spectroscopic (XPS) analysis was employed to characterize the composition, and more importantly, the chemical structure of the CQD samples and the interrelation of the characteristic surface chemical groups with the fluorescence properties and with surface polarity was unambiguously established. In vitro cellular internalization experiments documented their applicability as fluorescence labels while non-toxic properties were also approved. It was demonstrated that the adequate water-dispersibility of the particles plays a crucial role in their biological application. The synthetized CQD samples turned to be promising for cellular imaging applications both in laser illuminated flow cytometric measurements and in fluorescence microscopy. (C) 2019 Elsevier Inc. All rights reserved.