Fluorescence turn-off sensing of TNT by polyethylenimine capped carbon quantum dots

Nitrophenyl Thiourea-Modified Polyethylenimine Colorimetric Sensor for Sulfate, Fluorine, and Acetate

A thiourea-based colorimetric sensor incorporating polyethyleneimine (PEI) and chromophoric nitrophenyl groups was synthesized and utilized for detecting various anions. Structural characterization of the sensor was accomplished using FTIR and 1H-NMR spectroscopy. The sensor's interactions and colorimetric recognition capabilities with different anions, including CI-, Br-, I-, F-, NO3-, PF6-, AcO-, H2PO4-, PO43-, and SO42-, were investigated via visual observation and UV/vis spectroscopy. Upon adding SO42-, F-, and AcO- anions, the sensor exhibited distinct color changes from colorless to yellow and yellowish, while other anions did not induce significant color alterations. UV/vis spectroscopic titration experiments conducted in a DMSO/H2O solution (9:1 volume ratio) demonstrated the sensor's selectivity toward SO42-, F-, and AcO-. The data revealed that the formation of the main compounds and anion complexes was mediated by hydrogen bonding, leading to signal changes in the nitrophenyl thiourea-modified PEI spectrum.

Chemical reduction as a facile colorimetric approach for selective TNT detection by spectrophotometry and photothermal lens spectroscopy

In this study, the simple determination of TNT is achieved through the vivid stable red color products generated after chemically reduction by NaBH₄ as a common and accessible reducing/colorimetric reagent. Some other nitroaromatics were impressed under reduction reaction and led to the colorful products. The color of these reduced nitroaromatics were unstable and approximately vanished after some few minutes which ameliorated the selectivity in TNT determination. Utilizing the time-dependent selectivity, the method was applied specifically for discriminating of TNT from other nitroaromatic compounds (NACs). UV-vis spectrophotometry and photothermal lens spectrometry were employed as detection techniques. The former was simpler and more available in various laboratories while the latter provides higher sensitivity. It was revealed that the photothermal lens responses were linear from 2.0 to 55.0 nM with a limit of detection (LOD) of about 0.8 nM. The LOD of the photothermal lens measurement were found to be 241 times lower than that of the UV-vis spectrophotometry in TNT quantification. The evolved method was successfully carried out for TNT vapor determination after trapping into the colorimetric reagent. The recoveries and relative standard deviations (RSD, n = 3) calculated for 3 gas samples were ≥91% and ≤7%, respectively.

Characterization and Application of Supramolecular Junctions

The convergence of supramolecular chemistry and single-molecule electronics offers a new perspective on supramolecular electronics, and provides a new avenue toward understanding and application of intermolecular charge transport at the molecular level. In this review, we will provide an overview of the advances in the characterization technique for the investigation of intermolecular charge transport, and summarize the experimental investigation of several non-covalent interactions, including π-π stacking interactions, hydrogen bonding, host-guest interactions and σ-σ interactions at the single-molecule level. We will also provide a perspective on supramolecular electronics and discuss the potential applications and future challenges.

Dual-emission fluorescence detection of histidine using carbon dots and calcein/Ni2+ complexes

Histidine (His) is a natural amino acid that plays very important roles in biota. However, the low concentrations of His in biological fluids and the similar structures and properties of other amino acids mean it is difficult to selectively determine His concentrations in biological fluids with a high degree of sensitivity. A novel ratiometric fluorescence probe for detecting His in aqueous solutions is described here. The method involves carbon dots (CDs) and calcein/Ni²⁺ complexes. At an excitation wavelength of 480 nm, the CD/calcein system emits green fluorescence (maximum emission from calcein at 512 nm) and red fluorescence (maximum emission from CDs at 617 nm). The presence of Ni²⁺ decreases the calcein fluorescence intensity because of static quenching caused by the formation of calcein/Ni²⁺ complexes but the CD fluorescence intensity remains almost unchanged. Fluorescence of calcein/Ni²⁺ complexes provides the response, and the presence of His binds to Ni²⁺ via cooperative chelation and produces free calcein causing fluorescence to be recovered. CDs provide a self-calibration fluorescence signal, the intensity of which remains almost unchanged in the presence of His. The ratio of the fluorescence intensities at 512 and 617 nm (I₅₁₂/I₆₁₇) was strongly related to the His concentration in the range 0.5-22 μM, and the detection limit was 0.16 μM. The specificity of Ni²⁺/His interactions allows His to be determined without interference from other species. The method was successfully used to determine His in diluted human urine. The recovery was acceptable, suggesting that the biosensor can be used to determine His in real samples.

Silicon Doped Carbon Dots as an New Ratiometric Fluorescence Probe for Proanthocyanidins Assay Based on the Redox Reaction Between Cr(VI) and Proanthocyanidins

In the study, silicon doped carbon quantum dots (Si-CQDs) was prepared by one-pot hydrothermal method with (3-aminopropyl) triethoxysilane (APTES) and o-phenylenediamine (OPD) as raw materials. Then a new ratiometric fluorescent probe (RF-probe) was successfully established for sensitively and selectively monitoring proanthocyanidins (PAs) with a linear range of 10-500 nM and limit of detection (LOD) of 5.6 nM. that is, the fluorescence (FL) intensity of Si-CQDs at 570 nm was used as the built-in reference, while dopamine (DA) reacting with 4-hexylresorcinol (4-HR) could produce a new fluorescent substance (named as azamonardine, AZMON), and its FL intensity at 480 nm was reduced because Cr(VI) could oxidize DA to generate quinone without fluorescence. In the presence of PAs, Cr(VI) was reduced to Cr(III), which caused that the amount of DA reacting with 4-HR was increased, thus the FL intensity of AZMON was recovered. Furthermore, the RF-probe was successfully used for the determination of PAs in black goji berry from two different areas and PAs capsule with satisfactory results compared to the standard HPLC method.

Polyethyleneimine-capped copper nanoclusters for detection and discrimination of 2,4,6-trinitrotoluene and 2,4,6-trinitrophenol

The detection and discrimination of 2,4,6-trinitrotoluene (TNT) and 2,4,6-trinitrophenol (TNP) from analogues are of great importance to global security and are full of challenges in the field of trace sensing. Here, benefitting from the strong electrophilicity of TNT, a sensing strategy is established by synthesizing polyethyleneimine capped copper nanoclusters (PEI-Cu NCs) with abundant -NH₂ groups. By carefully controlling the size and structure of PEI-Cu NCs, Förster resonance energy transfer (FRET) from PEI-Cu NCs to the Meisenheimer complex occurs resulting from their spectral overlap when detecting TNT, while, due to the energy level match of TNP with PEI-Cu NCs, as well as the strong affinity between its -OH and -NH₂ in PEI-Cu NCs, photo-induced electron transfer (PET) is feasibly expected. As a result, TNT and TNP could be detected from 26 types of analogues and cations with a limit of detection (LOD) of 26.57 and 12.82 nM, respectively. Besides, owing to the brown color of the Meisenheimer complex, the discrimination of TNT and TNP could be additionally realized by colorimetric detection. We expect that the proposed methodology would not only shine light on the detection and discrimination of TNT and TNP that mitigate against public security concerns, but also pave a way for the deep understanding of FRET and PET related fluorescence quenching mechanisms from the aspect of controllable sensing material design and synthesis.

A Review on Carbon Dots: Synthesis, Characterization and Its Application in Optical Sensor for Environmental Monitoring

The development of carbon dots (CDs), either using green or chemical precursors, has inevitably led to their wide range application, from bioimaging to optoelectronic devices. The reported precursors and properties of these CDs have opened new opportunities for the future development of high-quality CDs and applications. Green precursors were classified into fruits, vegetables, flowers, leaves, seeds, stem, crop residues, fungi/bacteria species, and waste products, while the chemical precursors were classified into acid reagents and non-acid reagents. This paper quickly reviews ten years of the synthesis of CDs using green and chemical precursors. The application of CDs as sensing materials in optical sensor techniques for environmental monitoring, including the detection of heavy metal ions, phenol, pesticides, and nitroaromatic explosives, was also discussed in this review. This profound review will offer knowledge for the upcoming community of researchers interested in synthesizing high-quality CDs for various applications.

Cluster of Hexamolybdenum [Mo6Cl14]2- for Sensing Nitroaromatic Compounds

This contribution describes a novel method for the detection of trace amounts of trinitrotoluene (TNT) using a cluster of hexamolybdenum with general formula [Mo₆Cl₁₄]^2-. The molybdenum cluster was characterized by UV-visible, FT-IR, and fluorescence techniques, and the synthesis was efficient and reproducible. The evaluation of the molybdenum cluster by TNT detection was perfomed by fluoresecent measurements, and the results were interpreted by the Stern-Volmer equation, obtaining K _SV values of 2.9 × 10⁵ and 1.6 × 10⁴ M^-1 in different concentration ranges. Further, the results suggest that at TNT concentrations higher than 4 × 10^-5 mM (0.01 mg L^-1) it is possible to measure the quenching of the cluster fluorescence. The DFT calculations indicate that the contribution of the TNT in the active lowest unoccupied molecular orbitals that are involved in the higher intensity transitions in the complex cluster-TNT are significant. This situation differs from all the luminescent [M₆X₈L₆]^2- clusters (M = Mo; X = facial bridging ligand, and L = labile axial ligands), where most of the closely spaced excited states are located in the {M₆X₈} ^q+ core. Thus, the TNT switches off the cluster luminescence. The approach using a [Mo₆Cl₁₄]^2--based fluorescence sensor has the potential to be a sensing technology for the detection of nitroaromatic explosives.