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

Present trends towards technologies and processes that increase the use of residues make starchy vegetal biomass an important alternative material in various applications due to starch’s versatility, low cost and ease of use when its physicochemical properties are altered. Starch is increasingly used in many industrial applications and as a renewable energy resource. Starch can be modified to enhance its positive attributes and eliminate deficiencies in its native characteristics. In this article, the state of knowledge on conventional and unconventional starches and their properties, characteristics, modifications and applications are reviewed.

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Physicochemical properties, modifications and applications of starches from different botanical sources

Carbon dots (CDs) are one of the advancing fluorescent materials, which draw increasing attention in both theoretical research and practical applications. However, the clear chemical structure and photoluminescence (PL) mechanism of CDs is still an open debate, which limits the development of CDs. Because of the diversity of CDs, it is highly important to clarify this issue for specific CDs models. Herein, a type of CDs, made from citric acid with extremely high quantum yield, is investigated. Through the separation of the CDs, a type of bright blue fluorophore (IPCA) was discovered. IPCA and its derivatives were investigated and they were proven to contribute to the molecular state PL. Other components in the CDs were related to the carbon core state PL, which included polymer clusters and nanosized carbon cores. We conclude that this type of CDs contained complex components and multiple PL centers and that an independent fluorophore strongly affects the PL properties of the CDs. These two conclusions can potentially be true for similar CDs.

Investigation from chemical structure to photoluminescent mechanism: a type of carbon dots from the pyrolysis of citric acid and an amine

Nitrogen-doped carbon dots synthesized from citric acid as a carbon precursor have recently been considered to contain fluorescent derivatives of citrazinic acid, which contribute to their emission in the blue spectral range. To study the impact of such molecular fluorescent species on the optical properties of carbon dots, we synthesized three samples employing citric acid and three different nitrogen sources: ethylenediamine, hexamethylenetetramine, and triethanolamine. On the basis of the analysis of the nitrogen content and its coordination by X-ray photoelectron spectroscopy, FTIR spectra, and systematically comparing absorption, steady-state emission, and photoluminescence decays of each kind of carbon dot, we derive the influence of the molecular precursors and gain further understanding of the complex structure of carbon dots highlighting the strong impact of molecular fluorescence in the samples produced with ethylenediamine and hexamethylenetetramine.

Molecular Fluorescence in Citric Acid-Based Carbon Dots

https://rua.ua.es/dspace/bitstream/10045/62171/5/2016_Alonso_etal_EurJOC_rev.pdf

Deep Eutectic Solvents: The Organic Reaction Medium of the Century

In this report, we present the results of our investigations into the elucidation of the chemical structure of moieties responsible for the blue and green luminescence of CDs derived from the microwave-assisted pyrolysis of citric acid in the presence of urea. The molecular fluorophore that forms during the synthesis of green fluorescing CDs is 4-hydroxy-1H-pyrrolo[3,4-c]pyridine-1,3,6(2H,5H)-trione (HPPT).

Luminescence phenomena of carbon dots derived from citric acid and urea – a molecular insight

A series of novel fluorescent compounds was isolated from condensation mixtures of citric acid and specific β-amines. Their chemical structure was determined using 1H, 13C and HSQC experiments, elemental analysis data, high resolution and fragmentation spectra ESI-MS analyses. These compounds consisted of five-membered ring fused 2-pyridones. The fluorophores exhibited quantum yields as high as 79%.

Novel efficient fluorophores synthesized from citric acid

Luminescence phenomena of biodegradable photoluminescent poly(diol citrates)

Free-radical polymerization of itaconic acid in the presence of choline salts: Mechanism of persulfate decomposition

A “green” method for oxidized starch preparation was presented. Hydrogen peroxide (H2O2) was used as the starch oxidant. Two main types of experiments were investigated: catalytic (by means of sodium tungstate – Na2WO4, the hydrogen peroxide activator) and non-catalytic processes. The research covers low microwave level synthesis. Ninety and one hundred and ninety watts of microwave power were applied for the samples. All the microwave-assisted reactions were compared to conventional ones carried out at 80 and 90°C, respectively. Reaction was monitored using manganometric determination of H2O2 consumption during the reaction progress. Obtained reaction products were tested by means of carbonyl and carboxyl group content. Additionally, molecular mass changes were investigated and a preliminary rheological study was done. The observed significant differences in the degree of oxidation (monitored by means of carboxyl and carbonyl group content) as well as the changes in molecular mass of polysaccharide comparing to those found at conventional conditions were showed and discussed.

Szczepan Bednarz

Papers

Luminescence phenomena of carbon dots derived from citric acid and urea – a molecular insight

Novel efficient fluorophores synthesized from citric acid

Luminescence phenomena of biodegradable photoluminescent poly(diol citrates)

Free-radical polymerization of itaconic acid in the presence of choline salts: Mechanism of persulfate decomposition

Environmental friendly polysaccharide modification – microwave-assisted oxidation of starch