Precursor effects on surface functionalization, photoluminescence, and cytotoxicity of carbon dots synthesized via microwave-assisted methods

• Surface functional groups and surface charge of CDs are dependent on the precursors • CDs made from spermine show positive charge as compared to other CDs • More surface oxygen-containing groups on CDs causes redshifted emission • Cell viability of CDs made from chitosan depends on the incubation time Carbon dots (CDs) produced by microwave-assisted methods have been extensively used in many applications such as theranostics. The microstructures and properties of CDs are highly associated with their precursors. However, few studies have reported on the effects of precursors on functional group-associated properties of CDs. Herein, different precursors were used to produce CDs with a microwave-assisted method, namely CDs made of (1) citric acid/urea (CA/U-CDs), (2) chitosan (C-CDs), (3) glucose (G-CDs), and (4) spermine (SG-CDs). CDs are all negatively charged except SG-CDs which show some positive charges. The photoluminescence of the CDs was measured at excitation of 480 nm, and Fourier Transform Infrared (FTIR) spectroscopy was employed to analyze the relative proportion of functional groups on CD surfaces, with results further confirmed by X-ray photoelectron spectroscopy (XPS) analysis. The more oxygen-containing groups compared to nitrogen-containing groups CDs have, the longer the emission wavelength (λ em ). In addition, the cytotoxicity of different CDs was investigated on NIH/3T3 mouse fibroblast cells. In particular, C-CDs improved cell growth with cell viability >100% after 24 hours but decreased viability at longer incubations, likely due to increased uptake and lysosomal stress. In contrast, G-CDs exhibited stable biocompatibility over 72 h. This study demonstrates the effect of functional groups on CDs made with different precursors on their surface charge, emission, and cytotoxicity, which may provide guidance to the design and development of CDs with tunable luminescent and biocompatible properties.