Despite the various synthesis methods to obtain carbon dots (CDs), the bottom-up methods are still the most widely administrated route to afford large-scale and low-cost synthesis. However, as CDs are developed with increasing reports involved in producing many CDs, the structure and property features have changed enormously compared with the first generation of CDs, raising classification concerns. To this end, a new classification of CDs, named carbonized polymer dots (CPDs), is summarized according to the analysis of structure and property features. Here, CPDs are revealed as an emerging class of CDs with distinctive polymer/carbon hybrid structures and properties. Furthermore, deep insights into the effects of synthesis on the structure/property features of CDs are provided. Herein, the synthesis methods of CDs are also summarized in detail, and the effects of synthesis conditions of the bottom-up methods in terms of the structures and properties of CPDs are discussed and analyzed comprehensively. Insights into formation process and nucleation mechanism of CPDs are also offered. Finally, a perspective of the future development of CDs is proposed with critical insights into facilitating their potential in various application fields.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201901316

Evolution and Synthesis of Carbon Dots: From Carbon Dots to Carbonized Polymer Dots

Carbon and graphene quantum dots (CQDs and GQDs), known as zero-dimensional (0D) nanomaterials, have been attracting increasing attention in sensing and bioimaging. Their unique electronic, fluores...

Review of Carbon and Graphene Quantum Dots for Sensing

Carbon dots: large-scale synthesis, sensing and bioimaging

Fluorescent carbon dots (CDs) are a new class of carbon nanomaterials and have demonstrated excellent optical properties, good biocompatibility, great aqueous solubility, low cost, simple synthesis, etc. Since their discovery, various synthesis methods using different precursors have been developed, and are mainly classified as top-down and bottom-up approaches. For the mechanistic origin of CDs photoluminescence, three mechanisms have been proposed such as quantum confinement effect, surface state and molecule state. CDs have presented many applications, and this review article mainly focuses on the development of CDs based fluorescent sensors in recent 5 years. The sensing mechanisms, senor design and sensing properties to various targets are summarized. A broad range of analytes including cations, anions, small molecules, macromolecules, cells and bacteria have been discussed. In addition, the challenges and future directions for CDs as sensing materials are also presented.

Fluorescent carbon dots and their sensing applications

Nitrogen and sulfur codoped carbon dots (CDs) were prepared from garlic by a hydrothermal method. The as-prepared CDs possess good water dispersibility, strong blue fluorescence emission with a fluorescent quantum yield of 17.5%, and excellent photo and pH stabilities. It is also demonstrated that the fluorescence of CDs are resistant to the interference of metal ions, biomolecules, and high ionic strength environments. Combining with low cytotoxicity properties, CDs could be used as an excellent fluorescent probe for cellular multicolor imaging. Moreover, the CDs were also demonstrated to exhibit favorable radical scavenging activity.

Green Synthesis of Bifunctional Fluorescent Carbon Dots from Garlic for Cellular Imaging and Free Radical Scavenging.

Sulfur-doped carbon dots (S-doped C-dots)were synthesized using a simple and straightforward hydrothermal method. The as-prepared S-doped C-dots exhibit significant fluorescence quantum yield (67%) and unique emission behavior. The spherical S-doped C-dots have an average diameter of 4.6 nm and the fluorescence of S-doped C-dots can be effectively and selectively quenched by Fe3+ ions. Thus, S-doped C-dots were applied as probes toward Fe3+ detection, exhibiting a limit of detection of 0.1 μM.

Preparation of highly photoluminescent sulfur-doped carbon dots for Fe(III) detection

The Institute of Chemistry of Great Britain and Ireland. Journal and Proceedings. 1933. Part II

Heteroatom doped carbon dots (CDs) with consummate photoluminescence quantum yield (PLQY) are of great interest in various applications such as trace element detection, biomolecule markers, and chemical sensing. However, due to the low doping efficiency of the reaction, a high precursor ratio is routinely used to obtain CDs with considerable photoluminescence quantum yield (PLQY). In this contribution, we report a single-step hydrothermal method having the highest doping efficiency to synthesize sulfur and nitrogen co-doped semi-crystalline carbon dots (S,N-CDs) with superior quantum yield (QY). Here, the unprecedented doping efficiency of the reaction enables an order of magnitude reduction in the starting precursor ratio, in comparison with previous reports. Moreover, for the first time, complementary theoretical and comprehensive spectroscopic techniques were employed to derive deep insight into the photoluminescence mechanism and the shifting of specific energy levels in doped CDs was identified as the reason behind the enhanced photoluminescence of doped CDs. The PLQY and luminescent characteristics of the S,N-CDs can be tuned by controlling the precursor molar ratio, the extent of surface oxidation, and chemical status of S in the CDs. While most methods report high PLQY for amorphous CDs, our technique produces semi-crystalline CDs with more than 55% QY. Another unique attribute of the S,N-CDs is the high monodispersity and defined surface chemistry and the resultant highly robust excitation-independent luminescence that is stable over a broad range of pH values and in an extremely reactive environment. The detailed structural and chemical investigations using spectroscopic and microscopic techniques combining molecular simulation revealed that the superior PLQY and luminescence of S,N-CDs are due to the heteroatom directed, oxidized carbon-based surface passivation. The remarkable, robust fluorescence properties of S,N-CDs were applied for the ultra-trace detection of Hg2+ with a detection limit of 100 pM. The novel insights into the photoluminescence of doped CDs, the robust luminescence, and enhanced doping reaction efficiency reported here are envisaged to drive transformative changes in the design of CDs with peerless properties for futuristic applications.

Synthesis, mechanistic investigation, and application of photoluminescent sulfur and nitrogen co-doped carbon dots

Heteroatom doped carbon dots, due to their excellent fluorescent properties and related applications, have attracted great attention from different fields. Herein, we reported for the first time a facile and economic approach to synthesize copper doped carbon dots (Cu-CDs) via a one-step hydrothermal approach. The chemical and fluorescent properties of these Cu-CDs as well as the mechanism involved for the enhancement of the fluorescent were investigated through various microscopic and spectroscopic analyses, such as transmission electron microscopy (TEM), fluorescence spectra, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). It is observed that the Cu-CDs with the average size of 3.76 nm are spherical and monodisperse. Additionally, the as-prepared Cu-CDs exhibited a strong emission at 440 nm when excited at 350 nm (λex) and the photoluminescence quantum yields (PLQY) reached 9.81% after the optimization of synthesis conditions. The excellent fluorescent properties of Cu-CDs make them great candidates for the selective and fast detection of Fe3+ in range of 0.001–200 μM, while the limit of detection (LOD) is 1 nM, which can be further applied to detect Fe3+ in human blood serum and other biomedical applications.

Facile synthesis of copper doped carbon dots and their application as a “turn-off” fluorescent probe in the detection of Fe3+ ions

Carbon dots (CDs) attract great interests from scientists for their low toxicity and biocompatibility properties and their important applications in the fields of photocatalysts, batteries, bio-images and supercapacitors. Most strategies of making CDs contain several steps, which can be a time-consuming and costly procedure. In this study, nitrogen-doped CDs have been prepared by a one-step hydrothermal strategy with sodium citrate and ethylenediamine as precursors. It is found that fluorescence intensity of CDs is enhanced with the increased content of doped nitrogen, which implies that nitrogen-doped element plays an important role to improve the fluorescence intensity of CDs. Most importantly, those CDs yielded high selectivity of Fe3+, with good linearity, precision and accuracy. Hence, the as-prepared nitrogen-doped CDs could be used as probes for quantitative analysis of Fe3+ in environmental applications.

Chun Gao

Papers

Preparation of highly photoluminescent sulfur-doped carbon dots for Fe(III) detection

The Institute of Chemistry of Great Britain and Ireland. Journal and Proceedings. 1933. Part II

Synthesis, mechanistic investigation, and application of photoluminescent sulfur and nitrogen co-doped carbon dots

Facile synthesis of copper doped carbon dots and their application as a “turn-off” fluorescent probe in the detection of Fe3+ ions

Enhancing the luminescence of carbon dots by doping nitrogen element and its application in the detection of Fe(III)