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

Abstract

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 (CCDs), (3) glucose (G-CDs), and (4) spermine/glucose (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, CCDs improved cell growth with cell viability >100% after 24 h 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 photoluminescence and biocompatible properties.