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1. Recent development of carbon quantum dots regarding their optical properties, photoluminescence mechanism, and core structure

   https://doi.org/10.1039/c8nr10059d  

   Mintz, Keenan J.; Zhou, Yiqun; Leblanc, Roger M. (March 2019, Nanoscale)

   Carbon quantum dots (CDs) are a relatively new class of carbon nanomaterials which have been studied very much in the last fifteen years to improve their already favorable properties. The optical properties of CDs have drawn particular interest as they display the unusual trait of excitation-dependent emission, as well as high fluorescence quantum yields (QY), long photoluminescence (PL) decay lifetimes, and photostability. These qualities naturally lead researchers to apply CDs in the field of imaging (particularly bio-imaging) and sensing. Since the amount of publications regarding CDs has been growing nearly exponentially in the last ten years, many improvements have been made in the optical properties of CDs such as QY and PL lifetime. However, a great deal of confusion remains regarding the PL mechanism of CDs as well as their structural properties. Therefore, presented in this review is a summary and discussion of the QYs and PL lifetimes reported in recent years. The effect of method as well as precursor has been evaluated and discussed appropriately. The current theories regarding the PL mechanism of CDs are discussed, with special attention to the concept of surface state-controlled PL. With this knowledge, the improvement of preparation and applications of CDs related to their optical properties will be easily accomplished. Further improvements can be made to CDs through the understanding of their structural and optical properties. 

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2. Investigation into Red Emission and Its Applications: Solvatochromic N-Doped Red Emissive Carbon Dots with Solvent Polarity Sensing and Solid-State Fluorescent Nanocomposite Thin Films

   https://doi.org/10.3390/molecules28041755  

   Domena, Justin B.; Celebic, Ermin; Ferreira, Braulio C.; Zhou, Yiqun; Zhang, Wei; Chen, Jiuyan; Bartoli, M.; Tagliaferro, A.; Johnson, Qiaxian; Chauhan, Bhanu P.; et al (February 2023, Molecules)

   In this work, a NIR emitting dye, p-toluenesulfonate (IR-813) was explored as a model precursor to develop red emissive carbon dots (813-CD) with solvatochromic behavior with a red-shift observed with increasing solvent polarity. The 813-CDs produced had emission peaks at 610 and 698 nm, respectively, in water with blue shifts of emission as solvent polarity decreased. Subsequently, 813-CD was synthesized with increasing nitrogen content with polyethyleneimine (PEI) to elucidate the change in band gap energy. With increased nitrogen content, the CDs produced emissions as far as 776 nm. Additionally, a CD nanocomposite polyvinylpyrrolidone (PVP) film was synthesized to assess the phenomenon of solid-state fluorescence. Furthermore, the CDs were found to have electrochemical properties to be used as an additive doping agent for PVP film coatings. 

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3. Road Map of Semiconductor Metal-Oxide-Based Sensors: A Review

   https://doi.org/10.3390/s23156849  

   Dutta, Taposhree; Noushin, Tanzila; Tabassum, Shawana; Mishra, Satyendra K. (August 2023, Sensors)

   Identifying disease biomarkers and detecting hazardous, explosive, flammable, and polluting gases and chemicals with extremely sensitive and selective sensor devices remains a challenging and time-consuming research challenge. Due to their exceptional characteristics, semiconducting metal oxides (SMOxs) have received a lot of attention in terms of the development of various types of sensors in recent years. The key performance indicators of SMOx-based sensors are their sensitivity, selectivity, recovery time, and steady response over time. SMOx-based sensors are discussed in this review based on their different properties. Surface properties of the functional material, such as its (nano)structure, morphology, and crystallinity, greatly influence sensor performance. A few examples of the complicated and poorly understood processes involved in SMOx sensing systems are adsorption and chemisorption, charge transfers, and oxygen migration. The future prospects of SMOx-based gas sensors, chemical sensors, and biological sensors are also discussed. 

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4. Properties of Carbon Dots versus Small Molecules from “Bottom-up” Synthesis

   https://doi.org/10.1021/acsnano.3c07486  

   Bian, Zhengyi; Wallum, Alison; Mehmood, Arshad; Gomez, Eric; Wang, Ziwen; Pandit, Subhendu; Nie, Shuming; Link, Stephan; Levine, Benjamin G; Gruebele, Martin (November 2023, ACS Nano)

   A major challenge in the “bottom-up” solvothermal synthesis of carbon dots (CDs) is the removal of small-molecule byproducts, noncarbonized polyamides, or other impurities that confound the optical properties. In previously reported benzene diamine-based CDs, the observed fluorescence signal already has been shown to arise from free small molecules, not from nanosized carbonized dots. Here we have unambiguously identified the small-molecule species in the synthesis of CDs starting with several isomers of benzene diamine by directly matching their NMR, mass spectrometry, and optical data with commercially available small organic molecules. By combining dialysis and chromatography, we have sufficiently purified the CD reaction mixtures to measure the CD size by TEM and STM, elemental composition, optical absorption and emission, and single-particle blinking dynamics. The results can be rationalized by electronic structure calculations on small model CDs. Our results conclusively show that the purified benzene diamine-based CDs do not emit red fluorescence, so the quest for full-spectrum fluorescence from isomers of a single precursor molecule remains open. 

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5. Unconventional aliphatic fluorophores discovered as the luminescence origin in citric acid–urea carbon dots

   https://doi.org/10.1039/D2NR02361J  

   Yao, Xiaoxiao; Wang, Yinhan; Li, Fangjia; Dalluge, Joseph J.; Orr, Galya; Hernandez, Rigoberto; Cui, Qiang; Haynes, Christy L. (July 2022, Nanoscale)

   Carbon dots (CDs) are emerging as the material of choice in a range of applications due to their excellent photoluminescence properties, ease of preparation from inexpensive precursors, and low toxicity. However, the precise nature of the mechanism for the fluorescence is still under debate, and several molecular fluorophores have been reported. In this work, a new blue fluorophore, 5-oxopyrrolidine-3-carboxylic acid, was discovered in carbon dots synthesized from the most commonly used precursors: citric acid and urea. The molecular product alone has demonstrated interesting aggregation-enhanced emission (AEE), making it unique compared to other fluorophores known to be generated in CDs. We propose that this molecular fluorophore is associated with a polymer backbone within the CDs, and its fluorescence behavior is largely dependent on intermolecular interactions with the polymers or other fluorophores. Thus, a new class of non-traditional fluorophores is now relevant to the consideration of the CD fluorescence mechanism, providing both an additional challenge to the community in resolving the mechanism and an opportunity for a greater range of CD design schemes and applications. 

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