Single sensor for multiple analytes in different optical channel: Applying for multi-ion response modulation

A coumarin-modified graphene quantum dot-based luminogen for the detection of cysteine in aqueous media

A biocompatible fluorescence sensor for cysteine detection receives wide appreciation recently, because of its importance in the medical field. Functionalized graphene quantum dots (GQDs) are recently emerging biocompatible quantum dots, which are considered as suitable candidates for biomolecule detection. Motivated by this concept, here we have developed a versatile fluorescent probe based on 3-aminocoumarin (AMC) functionalized GQDs for the detection of cysteine (Cys). Modification on GQD with AMC resulted in a stable fluorescent probe with an enhancement in quantum yield of about 84% and 40 nm redshift in emission peak compared with bare GQD. The modified GQD is then used for the sensitive and selective detection of cysteine in aqueous media. The detection of Cys within the linear range of 50 nM to 1.5 μM was achieved with a detection limit (LOD) of 0.86 nM. Here, the AMC-GQD exhibit a turn-off fluorescence sensing behavior. The quenching mechanism was also explored. The sensing process follows dynamic quenching mechanism, which is attributed to the photoinduced charge transfer from AMC-GQD to Cys. The Stern-Volmer plot, energy-level alignment obtained from cyclic voltammetry measurements and density functional theory predictions give a valid proof for this. Furthermore, the sensor was applied efficiently to the determination of Cys in real water samples.

A Significant Fluorescence Turn-On Probe for the Recognition of Al3+ and Its Application

An easy prepared probe, BHMMP, was designed and synthesized, which displayed a significant fluorescence enhancement (over 38-fold) and obvious color change in the recognition of Al³⁺. The binding ratio of probe BHMMP to Al³⁺ was determined as 1:1, according to Job plot. The binding mechanism was fully clarified by the experiments, such as FT-IR spectrum, ESI-MS analysis, and ¹H NMR titration. A DFT study further confirmed the binding mode of BHMMP to Al³⁺. The limit of detection (LOD) for Al³⁺ was determined as low as 0.70 µM, based on the fluorescence titration of BHMMP. Moreover, the results from real sample experiments, including real water samples, test papers, and cell images, well-demonstrated that BHMMP was capable of sensing Al³⁺ in environmental and biological systems.

Organic polymorphs based on an AEE-active tetraphenylethene salicylaldehyde Schiff-base derivative: the effect of molecular conformation on luminescence properties

An aggregation-enhanced emission (AEE)-active tetraphenylethene salicylaldehyde Schiff-base derivative, TPE-Nap, was prepared using a facile synthesis. The AEE property of TPE-Nap was studied by luminescence and absorption spectra, and was attributed to the C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N isomerization restriction and the excited-state intramolecular proton transfer (ESIPT) process. Polymorphs TPE-Nap-Y and TPE-Nap-O were prepared from TPE-Nap, and their emission color and intensity were compared. TPE-Nap-Y is a yellow block crystal with a very weak yellow emission, with its main peak at 565 nm, while TPE-Nap-O is an orange plate crystal that gave a stronger orange emission, with its main peak at 583 nm. Single crystal diffraction data were used to demonstrate the structure–property relationship. The most unique feature was that the torsion angle of TPE-Nap-Y between the benzene ring of the TPE unit and the Nap unit was 54.08°, while that of TPE-Nap-O was 14.19°. Interestingly, the TPE unit assumed propeller-like nonplanar conformations that likely led to different intermolecular interactions, such as C–H⋯O interactions (2.529 Å and 2.617 Å) in TPE-Nap-O and C–H⋯π interactions (3.224 Å and 3.791 Å) in TPE-Nap-Y. These were influenced by the torsion angle, although the molecules in both crystals were arranged in a similar end-to-end slip-stacking mode. These results inferred that the molecular conformation was evidently affected by luminescent properties. Crystals possessing a slightly twisted molecular conformation exhibited stronger emission than those possessing a heavily twisted molecular conformation. These investigations will expand the research on the relationship between the molecular conformation and the emission properties of organic solids, and might provide a new development strategy for organic polymorphs.

Organic polymorphs displaying different emission colors and intensities were obtained from aggregation-enhanced emission (AEE)-active tetraphenylethene derivatives, and their luminescent properties were affected mainly by molecular conformation.

TDDFT study on aluminum and fluoride dual-sensing mechanism of a Schiff-Base sensor

The aluminum and fluoride dual-sensing mechanism of a previously reported sensor with a Schiff-base moiety (Spectrochim. Acta A, 2017, 183, 267-274) has been investigated by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. The present calculations reproduce the photoproperties of the sensor as well as its aluminum and fluoride complexes, which illustrates that DFT and TDDFT constitute a reliable tool for uncovering detailed fluorescence-based sensing mechanisms in diverse electronic states. Theoretical results indicate that there are two OH⋯N hydrogen bonds in the sensor and two OH⋯F hydrogen bonds in its F¯ complex. Different degrees of coplanarity caused by these hydrogen bonds are responsible for their distinct absorption wavelengths. However, excited-state geometry optimization and a scan of the potential-energy surface show that there is twisted intramolecular charge transfer about the CN bond in the sensor molecule and an excited-state proton-transfer process from the fluoride anion to the neighboring N atom in the fluoride-sensor complex, whereby the fluorescence is quenched. A chelation-enhanced fluorescence effect associated with the aluminum-sensor complex shows a different excited-state process. The local excitation and emission occur exclusively within the planar fluorophore, and negligible structural change upon excitation of the aluminum-sensor complex leads to its strong fluorescence. Therefore, it is theoretically explained why the sensor may be successfully used to analyze the fluoride anion by absorption spectroscopy and the aluminum cation by emission spectroscopy.

Fluorescence light-up detection of cyanide in water based on cyclization reaction followed by ESIPT and AIEE

Schiff base 1 (2,4-di-tert-butyl-6-((2-hydroxyphenyl-imino)-methyl)phenol) containing two hydroxyl groups could undergo an oxidative cyclization reaction and then generate hydroxyphenylbenzoxazole (HBO) 2 when CN^- was present as a catalyst. The multistep cyclization reaction was proved by spectroscopy, ¹H NMR, ¹³C NMR and mass spectra. C[double bond, length as m-dash]N isomerization is the predominant decay process of the excited states, so sensor 1 is weakly emissive in solution at ambient temperature. When 1 reacts with CN^-, the emission is remarkably enhanced, where 1 is converted to 2. The cyclization product HBO 2 displays bright green luminescence in micellar due to the ESIPT (excited-state intramolecular proton transfer) as well as AIEE (aggregation-induced emission enhancement) effect. The detection limit is 5.92 × 10^-7 M, lower than the WHO guideline of CN^- in drinking water (1.9 μM). The selective and competitive experiments reveal that sensor 1 shows high sensing selectivity and sensitivity for CN^- over other anions. Test papers containing absorbed 1 were prepared and applied for practical application of cyanide detection.

Highly selective and sensitive turn-on fluorescent sensor for detection of Al3+ based on quinoline-base Schiff base

A new aluminum ion fluorescent probe (4-(diethylamino)-2-hydroxybenzylidene)isoquinoline-1-carbohydrazide (HL¹) has been conveniently synthesized and characterized. HL¹ exhibited a highly selective and pronounced enhancement for Al³⁺ in the fluorescence emission over other common cations by forming a 2:1 complex, with a recognition mechanism based on excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT). The strong fluorescent emission can be observed even at ppm level concentration of the probe in the presence of Al³⁺ with 41 fold intensity enhancement at 545 nm. HL¹ displays good linear relationship with Al³⁺ in the low concentration and the limit of detection is 8.08 × 10^-8 mol/L. Similar molecules with different substituents on salicylaldehyde phenyl ring were synthesized for studying the structure-activity relationship. Density-functional theory (DFT) calculations are in agreement with the proposed mechanism. It is confirmed that HL¹ could be used to detect Al³⁺ ions in real sample by fluorescence spectrometry and Al³⁺ ions in cells by bioimaging.