Revealing the nature of optical activity in carbon dots produced from different chiral precursor molecules

Abstract

Carbon dots (CDs) are light-emitting nanoparticles that show great promise for applications in biology and medicine due to the ease of fabrication, biocompatibility, and attractive optical properties. Optical chirality, on the other hand, is an intrinsic feature inherent in many objects in nature, and it can play an important role in the formation of artificial complexes based on CDs that are implemented for enantiomer recognition, site-specific bonding, etc. We employed a one-step hydrothermal synthesis to produce chiral CDs from the commonly used precursors citric acid and ethylenediamine together with a set of different chiral precursors, namely, L-isomers of cysteine, glutathione, phenylglycine, and tryptophan. The resulting CDs consisted of O,N-doped (and also S-doped, in some cases) carbonized cores with surfaces rich in amide and hydroxyl groups; they exhibited high photoluminescence quantum yields reaching 57%, chiral optical signals in the UV and visible spectral regions, and two-photon absorption. Chiral signals of CDs were rather complex and originated from a combination of the chiral precursors attached to the CD surface, hybridization of lower-energy levels of chiral chromophores formed within CDs, and intrinsic chirality of the CD cores. Using DFT analysis, we showed how incorporation of the chiral precursors at the optical centers induced a strong response in their circular dichroism spectra. The optical characteristics of these CDs, which can easily be dispersed in solvents of different polarities, remained stable during pH changes in the environment and after UV exposure for more than 400 min, which opens a wide range of bio-applications.

Hydrothermal synthesis of chiral carbon dots

“…XPS of CDots was compared with that of Cys (Figure 4A–C). From the XPS spectra of CDots synthesized for 4 h at 150°C (Figure 4B), the C 1s peak consisted of peaks attributed to 284.80, 286.10, 287.44, and 289.97 eV peaks that corresponded to CC, CO, OCO, and OCO bonds, respectively, 22,34 which aggress very well with the conclusions from 1 H NMR study. The intensity and integrated area of the CC bond peak increased with the increasing synthesis duration from 4 to 20 h (Figure 4B,C), indicating the increasing dominance of CC bonds and a greater degree of carbonization.…”

Carbon Dots from d-Cysteine and Citric Acid Increase the Activity of the Enzyme Laccase

“…As shown in Figure f, two symmetrical CD signals at 248 and 354 nm are newly generated. The chiral signal at 248 nm is attributed to the interaction of the chiral supramolecular network in the aggregated state, and the chiral signal at 354 nm is attributed to the coupling of the energy states of the chiral CDs with those of the chiral molecules when they are close enough to form a chiral conformation, which indicates that a new chiral center is generated. − To sum up, the l - or d -CDots are consistent in morphology, functional groups, and fluorescence properties, with only difference in chirality.…”

Carbon Nanoparticles Extracted from Date Palm Fronds for Fluorescence Bioimaging: In Vitro Study

“…The CNPs obtained in this report were highly graphitic and crystalline, with diffraction peaks at 25.77° and 28.70° (002) and the presence of excess oxygen as the only surface functional group. Excess oxygen plays a vital role in creating defects within CNPs and increasing PL properties [ 37 ]. The absorption spectra reported in this study agreed with those in other reports [ 38 , 39 , 40 ].…”