Carbon Quantum Dots from Amino Acids Revisited: Survey of Renewable Precursors toward High Quantum-Yield Blue and Green Fluorescence

Abstract

Carbon quantum dots (CQDs) were synthesized via a green, one-step hydrothermal method. As CQD precursors, nine amino acids of different structural descriptors (negatively/positively charged in water, polar, hydrophobic, sulfur-containing, and other/complex ones) were surveyed: Asp, Cys, Gly, His, Leu, Lys, Phe, Pro, and Ser. The reactions were performed in an autoclave in the presence of citric acid at 180 °C for 24 h and yielded core–shell CQDs. CQDs were comprehensively characterized by transmission electron microscopy, dynamic light scattering, Raman, UV/Vis, infrared, X-ray photoelectron spectroscopy, and fluorescence spectroscopy. At the excitation wavelength of λex = 350 nm, Cys-, Phe-, Leu-, and Lys-based CQDs displayed the highest quantum yield blue fluorescence90 ± 5, 90 ± 4, 87 ± 5, and 67 ± 3%, respectivelysuperior to the conventional fluorescent dyes. Strikingly, for Lys- and Phe-CQDs, dissimilar trends in the excitation–emission wavelength relationships were identified, that is, constantly strong red shifts versus excitation wavelength-independent emission. Cys- and Lys-CQDs were water-dispersible toward the narrow unimodal distribution of hydrodynamic diameters0.6 and 2.5 nm, respectively. Additionally, Lys- and Cys-CQDs, with high absolute zeta potential values, formed stable aqueous colloids in a broad range of pH (2, 7, and 12). The results constitute important premises for water-based applications of CQDs, such as bioimaging or photocatalysis.

Carbon Dots: Opportunities and Challenges in Cancer Therapy

“…Currently, the most widespread theory is that surface groups at the edge of the CDs, rather than the same size as in QDs, are responsible for the PL emission [ 39 ]. After the transition of aromatic sp2 C-C bond, the absorption of CDs in the UV-vis spectrum reveals a peak at 230–280 nm [ 40 ]. According to Zheng and his group [ 41 ], a shoulder between 300 and 550 nm is attributed to the n-* transition of C=O/C=N and C-O/C-N bonds or other related groups, where GQDs with altered size are linked to shifts at the maximum of the emission spectrum.…”

Development of biomass waste-based carbon quantum dots and their potential application as non-toxic bioimaging agents

“…The elements of C, H, O, and N are the primary elements found in CQDs, and the surface of CQDs can be functionalized with polar functional groups such as carboxyl, amine, and hydroxyl, which is benecial for surface passivation, which will improve the solubility of the CQDs. [26][27][28] CQDs can be doped with heteroatoms or graed with other substances to boost the quantum yield or affinity to certain biomolecules. 29,30 By graing NH 2 groups onto biological structures, amines such as ethylenediamine, poly(ethylenediamine), and trimethylamine can improve affinity to the structures.…”

From dots to tubes – the reversed scenario of bottom-up external-catalyst-free synthesis of N-doped carbon nanotubes

“…15 One of the topdown methods of CD synthesis is a controlled, fragmentary decomposition of CNTs, 16 or graphite/graphene, 17 whereas bottom-up synthetic methods are based on a progressive polyaromatization and graphitization of low-molecular-weight precursors. 18 And although the synthesis of CNTs and N-CNTs has been studied for more than three decades, the catalyst-free methods were elaborated only for CNTs (with a single report on N-CNTs) (Table S1, ESI †). [19][20][21][22][23] But more importantly, there are no reports on the synthesis of N-CNTs from CDs as a carbon source while, as mentioned above, only the reverse scenario was realized.…”

“…S1, ESI †). 18 TGA synthesis transformed the yellow-orange Phe-CDs into black Phe-CD-N-CNTs of a well-defined, aerogel-like morphology at the macroscale (density 6.7 g dm À3 ). As shown in the SEM image (Fig.…”

“…Firstly, CDs were synthesized from six structurally different natural l -amino acids according to the previously reported protocol (ESI). 18 Next, CDs (10 mg) were placed in an alumina crucible, heated at the 20 °C min −1 rate to 800 °C, kept thereat for 3 min, and left to cool down to room temperature – with the total synthesis time of ∼1 h in a TGA furnace (TGA 8000, PerkinElmer) (Scheme 1). The whole process was run under a nitrogen flow (20 mL min −1 ).…”