Eutectic Media Open a Synthetic Route to Oligocitrazinic Acid Fluorophores of Purple Hue

Under isochoric and solvent-free conditions, the reaction between ammonium formate and citric acid results in a deeply purple reaction product with fluorescent properties. This brings this reaction in the realm of bio-based fluorophores and bottom-up carbon nanodots from citric acid. The reaction conditions are optimized in terms of UV-vis spectroscopic properties and, subsequently, the main reaction product is separated. While the structural analysis does not give any indication for carbon nanodots in a general sense, it points towards the formation of molecular fluorophores that consist of oligomerized citrazinic acid derivatives. Furthermore, EPR spectroscopy reveals the presence of stable free radicals in the product. We hypothesize that such open-shell structures may play a general role in molecular fluorophores from citric acid and are not yet sufficiently explored. Therefore, we believe that analysis of these newly discovered fluorophores may contribute to a better understanding of the properties of fluorophores and CND from citric acid in general.

Polyethylenimine-functionalized nitrogen and sulfur co-doped carbon dots as effective fluorescent probes for detection of Hg2+ ions

Surface modification of nitrogen and sulfur co-doped carbon quantum dots (N, S-CDs) were performed using cysteine and polyethylenimine as raw materials. The prepared N, S-CDs exhibited excitation-independent in the range of 300-380 nm. Furthermore, mercury(II) ions (Hg²⁺) can effectively quench the fluorescence intensity of the N, S-CDs. Based on this, we developed a fluorescence sensor with high sensitivity and selectivity to detect Hg²⁺. Under optimized conditions, the sensor showed good linearity in the range of 0-500 nM, and the limit of detection is 9.2 nM. Further, the sensor showed high sensitivity to Hg²⁺ in lake water and rice samples. The recovery of the Hg²⁺ in lake water and rice samples ranged between 98.2 % and 109.5 % with a relative standard deviation below 5.8 %. With outstanding sensitivity and selectivity, the fluorescence sensor provides a promising platform for monitoring Hg²⁺ in real samples.

Advanced Carbon-Based Polymeric Nanocomposites for Forensic Analysis

Nanotechnology is a powerful tool and fast-growing research area in many novel arenas, ranging from biomedicine to engineering and energy storage. Nanotechnology has great potential to make a significant positive contribution in forensic science, which deals with the identification and investigation of crimes, finding relationships between pieces of evidence and perpetrators. Nano-forensics is related to the development of nanosensors for crime investigations and inspection of terrorist activity by analyzing the presence of illicit drugs, explosives, toxic gases, biological agents, and so forth. In this regard, carbon nanomaterials have huge potential for next-generation nanosensors due to their outstanding properties, including strength combined with flexibility, large specific surface area, high electrical conductivity, and little noise. Moreover, their combination with polymers can provide nanocomposites with novel and enhanced performance owed to synergy between the composite components. This review concisely recapitulates up-to-date advances in the development of polymer composites incorporating carbon-based nanomaterials for forensic science. The properties of the different carbon nanomaterials, several methods used to analyze functional polymeric nanocomposites, and their applications in forensic investigation are discussed. Furthermore, present challenges and forthcoming outlooks on the design of new polymer/carbon nanomaterial composites for crime prevention are highlighted.

Nitrogen-rich silicon quantum dots: facile synthesis and application as a fluorescent "on-off-on" probe for sensitive detection of Hg2+ and cyanide ions

The sensitive and reliable detection of Hg²⁺ and CN^- as harsh environmental contaminants are of great importance. In view of this, a novel "on-off-on" fluorescent probe based on nitrogen-rich silicon quantum dots (NR-SiQDs) has been designed for sensitive detecting Hg²⁺ and CN^- ions in aqueous media. NR-SiQDs were synthesized by a facile, one-step, and environment friendly procedure in the presence of 3-aminopropyl trimethoxysilane (APTMS) and ascorbic acid (AA) as precursors, with L-asparagine as a nitrogen source for surface modification. The NR-SiQDs exhibited strong fluorescence emission at 450 nm with 42.34% quantum yield, satisfactory salt tolerance, and superior photo- and pH-stability. The fluorescence emission was effectively quenched by Hg²⁺ (turn off) due to the formation of a non-fluorescent stable NR-SiQDs/Hg²⁺ complex while after the addition of cyanide ions (CN^- ), Hg²⁺ ions can be leached from the surface of the NR-SiQDs and the fluorescence emission intensity of the quenched NR-SiQDs fully recovered (turn on) due to the formation of highly stable [Hg (CN)₄ ]^2- species. After optimizing the response conditions, the obtained limits of detection were found to be 53 nM and 0.46 μM for Hg²⁺ and CN^- , respectively. Finally, the NR-SiQDs based fluorescence probe was utilized to detect Hg²⁺ and CN^- ions in water samples and satisfactory results were obtained, suggesting its potential application for environmental monitoring.

Carbon dots versus nano-carbon/organic hybrids - dramatically different behaviors in fluorescence sensing of metal cations with structural and mechanistic implications

Carbon dots (CDots) are defined as surface-passivated small carbon nanoparticles, with the effective passivation generally achieved by organic functionalization. Photoexcited CDots are both potent electron donors and acceptors, and their characteristic bright and colorful fluorescence emissions make them excellent fluorescence sensors for organic analytes and metal ions. For the latter extraordinarily low detection limits based on extremely efficient quenching of fluorescence intensities by the targeted metal cations have been observed and reported in the literature. However, all of the dot samples in those reported studies were made from "one-pot" carbonization of organic precursors mostly under rather mild processing conditions, unlikely to be sufficient for the required level of carbonization. Those dot samples should therefore be more appropriately considered as "nano-carbon/organic hybrids", characterized structurally as being highly porous and spongy, which must be playing a dominating role in the reported sensing results. In this study, we compared the dot samples from carbonization syntheses under similarly mild and also more aggressive processing conditions with the classically defined and structured CDots for the fluorescence sensing of copper(ii) cations in aqueous solutions. The observed dramatic decoupling between quenching results for fluorescence intensities and lifetimes of the carbonization samples, with the former being extraordinary and the latter within the diffusion controlled limit, suggested that the quenching of fluorescence intensities was greatly affected by the higher local quencher concentrations than the bulk associated with the porous and spongy sample structures, especially for the sample from carbonization under too mild processing conditions. The major differences between the classical CDots and the nano-carbon/organic hybrids are highlighted, and the tradeoffs between sensitivity and accuracy or reproducibility in the use of the latter for fluorescence sensing are discussed.

Carbon Nanomaterials: Synthesis, Functionalization and Sensing Applications

Recent advances in nanomaterial design and synthesis has resulted in robust sensing systems that display superior analytical performance. The use of nanomaterials within sensors has accelerated new routes and opportunities for the detection of analytes or target molecules. Among others, carbon-based sensors have reported biocompatibility, better sensitivity, better selectivity and lower limits of detection to reveal a wide range of organic and inorganic molecules. Carbon nanomaterials are among the most extensively studied materials because of their unique properties spanning from the high specific surface area, high carrier mobility, high electrical conductivity, flexibility, and optical transparency fostering their use in sensing applications. In this paper, a comprehensive review has been made to cover recent developments in the field of carbon-based nanomaterials for sensing applications. The review describes nanomaterials like fullerenes, carbon onions, carbon quantum dots, nanodiamonds, carbon nanotubes, and graphene. Synthesis of these nanostructures has been discussed along with their functionalization methods. The recent application of all these nanomaterials in sensing applications has been highlighted for the principal applicative field and the future prospects and possibilities have been outlined.

Modified Facile Synthesis for Quantitatively Fluorescent Carbon Dots

A simple yet consequential modification was made to the popular carbonization processing of citric acid - polyethylenimine precursor mixtures to produce carbon dots (CDots). The modification was primarily on pushing the carbonization processing a little harder at a higher temperature, such as the hydrothermal processing condition of around 330 °C for 6 hours. The CDots thus produced are comparable in spectroscopic and other properties to those obtained in other more controlled syntheses including the deliberate chemical functionalization of preprocessed and selected small carbon nanoparticles, demonstrating the consistency in CDots and reaffirming their general definition as carbon nanoparticles with surface passivation by organic or other species. Equally significant is the finding that the modified processing of citric acid - polyethylenimine precursor mixtures could yield CDots of record-setting fluorescence performance, approaching the upper limit of being quantitatively fluorescent. Thus, the reported work serves as a demonstration on not only the need in selecting the right processing conditions and its associated opportunities in one-pot syntheses of CDots, but also the feasibility in pursuing the preparation of quantitatively fluorescent CDots, which represents an important milestone in the development and understanding of these fluorescent carbon nanomaterials.

Carbon dots: surface engineering and applications

Carbon dots have attracted a great deal of attention because of their high performance, cheap and facile preparation, and potential applications in a wide area. In order to broaden their applications, especially to meet specific requirements, surface engineering, including tailoring surface functional group coating and subsequent chemical modification as required, is an effective strategy for further functionalization of carbon dots. In this article, representative approaches to coating the surface with various functional groups, and strategies for conjugating specific materials onto the surface of carbon dots for functional modification via covalent bonds, electrostatic interactions and hydrogen bonds are highlighted, as well as the results from explorations of their various applications in target modulated sensing, accurate drug delivery and bioimaging at high resolution.

Carbon dots based turn-on fluorescent probes for the sensitive determination of glyphosate in environmental water samples

Schematic illustration of Cu²⁺ and glyphosate detection using the CDs.

Design of Fe₃O₄@SiO₂@Carbon Quantum Dot Based Nanostructure for Fluorescence Sensing, Magnetic Separation, and Live Cell Imaging of Fluoride Ion

A robust reusable fluoride sensor comprised of a receptor in charge of the chemical recognition and a fluorophore responsible for signal recognition has been designed. Highly fluorescent carbon quantum dot (CD) and magnetically separable nickel ethylenediaminetetraacetic acid (EDTA) complex bound-silica coated magnetite nanoparticle (Fe3O4@SiO2-EDTA-Ni) have been used as fluorophore and fluoride ion receptor, respectively. The assay is based on the exchange reaction between the CD and F(-), which persuades the binding of fluoride to magnetic receptor. This method is highly sensitive, fast, and selective for fluoride ion in aqueous solution. The linear response range of fluoride (R(2) = 0.992) was found to be 1-20 μM with a minimum detection limit of 0.06 μM. Excellent magnetic property and superparamagnetic nature of the receptor are advantageous for the removal and well quantification of fluoride ion. The practical utility of the method is well tested with tap water. Because of high sensitivity, reusability, effectivity, and biocompatibility, it exhibits great promise as a fluorescent probe for intracellular detection of fluoride.