On the nature of solvothermally synthesized carbon nanodots

Abstract

Due to their facile synthesis, biocompatibility, high apparent photoluminescence (PL) quantum yield and applications in photocatalysis, sensing and biomarkers carbon nanodots (CNDs) have been investigated intensively in recent years. Solvothermal synthesis of CNDs, however, results in complex mixtures of CNDs with molecular byproducts and some of the reported properties of CNDs are rather due to the latter. Here, we present an extensive analysis of two types of solvothermal CNDs synthesized from (a) citric acid and ethylenediamine (B-CNDs) and (b) p-phenylenediamine (R-CNDs). Study of the fractions separated by dialysis confirms the overwhelming formation of molecular fluorophores and non-conjugated small molecules. PL, Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) confirm the presence of the fluorophores in the retentate fraction even after prolonged dialysis. By means of liquid chromatography coupled to mass spectrometry (UPLC-ESI-Q-TOF-MS/MS) we unequivocally identify some of the molecules. AFM and HR-TEM reveals graphitic cores with a diameter of approximately 3 nm which are further studied by grazing incidence wide angle X-ray scattering (GI-WAXS) and surface-enhanced micro-Raman spectroscopy (SERS). In the case of B-CNDs, the formation of photoproducts was observed, which readily explains differences in the optical properties of dialysate and retentate fractions. Surprisingly, for both types of CNDs the carbon dots formed and their expected sp² domains are not detected at all by PL, PL excitation, UV-Vis spectroscopy or fluorescence correlation spectroscopy. On the other hand, a significant photophysical effect of the CNDs, as opposed to the molecular byproducts, is observed in view of singlet oxygen generation.