Ylidenemalononitrile Enamines as Fluorescent “Turn-On” Indicators for Primary Amines

Detection of biogenic amines using a ylidenemalononitrile enamine-based fluorescence probe: Applications in food quality control

Biogenic amines (BAs) are a class of nitrogen-containing natural organic compounds. Elevated levels of BAs are a reliable indicator of food spoilage and pose a significant risk to human health. Thus, the development of real-time sensors for BAs monitoring is crucial. In this study, we present a novel fluorescence probe, TP-A, based on ylidenemalononitrile enamine, which can detect various BAs including putrescine, cadaverine, and spermine. TP-A exhibits a significant fluorescence intensity increase at 600 nm in the presence of these BAs in an aqueous system, which can effectively monitor BAs in representative vegetable onion tissues. Notably, TP-A-loaded filter paper has been developed into test strips for real-time monitoring of BAs released from shrimp under different temperatures. This work offers a precise method for probing the release of BAs in real food samples, which is promising for application in food quality control.

Programmed Heterocycle Synthesis Using Halomucononitriles as Pyridinimine Precursors

Herein we report a method to convert primary amines, ubiquitous motifs found in pharmaceutical libraries, to either imidazo[1,2-a]pyridines or 7-alkyl azaindoles in two steps from known compounds. Using halomucononitrile reagents, we can directly access 5-bromo-6-imino-1-alkyl-1,6-dihydropyridine-2-carbonitriles (pyridinimines) in a single step from primary amines (25-93% yield) through the cyclization of transient aminomucononitrile intermediates. We then demonstrate that these compounds can be readily converted to 7-alkylazaindoles using Sonogashira cross-coupling conditions (13 examples, up to 91% yield). Under oxidative conditions, the pyridinimines serve as directing groups for C-H functionalization reactions to afford imidazo[1,2-a]pyridines. We also studied the mechanism of the cyclization event using DFT calculations and propose that this takes place via sequential base-mediated E/Z isomerization and cyclization steps.

Concentration-tuned diverse response to selective biogenic amines using a reusable fluorophore: monitoring protein-rich food spoilage

Maintaining the freshness of food is essential for a healthy and quality life. Nevertheless, it remains a global challenge. Hence, an easy detection and monitoring protocol would be highly desirable. A cyanoacrylic acid (CAA)-based fluorophore is manifested as a reusable platform that responds diversely against different concentrations of selective aliphatic biogenic amines (BAs) in both solution and vapor phases. Slow spoilage of the protein-rich food is progressively monitored through emission shifts visible to the naked eye. This fluorophore provides easy and naked-eye detection of the BA vapor through a change in emission, i.e., red → orange → orange-yellow → cyan → green and quantum yield enhancement, which occur in stepwise increments of vapor concentrations. The probe design includes π-conjugated functionalized fluorescent molecules linked to multiple twisting sites, resulting in both solid and solution-state emission. The attached carboxylic acid responds quickly with selective BAs, mainly putrescine (PUT), cadaverine (CAD), and spermidine (SPM), where the concentration-based emission variation has appeared to be distinct and prominent against PUT [sensitivity (μM): 2 (solution); 3.3 (vapour)]. The selectivity towards diamine can be clarified by the formation of carboxylic acid salts and the consequent proton exchanges between free and protonated amines. In addition, -CN···H interaction is likely to develop within this ammonium carboxylate system, providing extra stability. Such ammonium carboxylate salt formation and gradual change in the molecular arrangement, resulting in symmetry development, are validated by FT-IR and wide-angle X-ray diffraction studies. Besides, this fact is supported by DFT studies that validate intramolecular H-atom exchange between free amine and ammonium salt units. A fluorophore-coated coverslip, filter paper, or silica gel-coated Al-plate is fruitfully utilized to detect the freshness of fish and chicken, which reveals the potential of this probe to prevent food waste and control food safety.

A three-in-one versatile sensor for concise detecting biogenic amines and beef freshness

Biogenic amines (BAs), as important indicators for evaluating food spoilage caused by fermentation processes or microbial activities, present significant risks of food safety. Consequently, the development of a simple, sensitive, and selective detection method for amines is of great importance. In this study, we proposed a three-in-one sensor 3,6-bis(dimethylamino)-9-(ethylthio)xanthylium (PSE) for high sensitivity and selectivity detecting BAs with multimodal responses, including olfactory, colorimetric, and fluorescent signals, thus facilitating convenient real-time detection of BAs. Mechanism study indicated that the nucleophilic substitution of PSE with BAs induced such rapid multi-responses with a low detection limit (LOD = 0.03 μM). We further fabricated PSE loaded paper for portable detection of BAs vapors. And the accurate determination of BAs levels is achieved through analyzing the RGB color mode. Finally, we successfully applied these test strips for non-destructive assessing meat beef freshness with the assistance of a smartphone in on-site scenarios.

Benzothiazole-based novel fluorescence probe sensing 1, 3-diaminopropane

Amines are extensively present in biological systems and are abundantly used in research, industries and agriculture. Systematic detection and quantification of certain amines can help us in food quality control and diagnosis of many diseases. A Schiff base probe HL was designed and successfully synthesized. It was proposed as a sensor for the exclusive detection of 1, 3- diaminopropane through turn-on fluorescence response in a variety of solvents including water. Micromolar limits of detection was achieved in all these solvents. Mechanism of detection was proposed by investigating mass spectrometric and NMR results. These were corroborated with DFT/TD-DFT calculations. Spiking experiments performed in various real water samples revealed the potential of the sensor to be used in day-to-day applications. Paper strip experiments demonstrated the suitability of the probe for real-life applications.

Concentration-Guided Visual Detection of Multiphase Aliphatic Biogenic Amines through Amine-Phenol Recognition Using a Dual-State Emitter

Intermolecular amine-phenol interactions are largely recognized as unique models with diverse supramolecular interactions. However, fluorescence (FL) variations originating from such interactions are rare. Herein, FL changes are well realized from amine-phenol interactions to identify an important biomarker, biogenic amines (BAs). A simple, inexpensive, and thermally stable anthracenylphosphonate is linked with 2,2'-biphenol to design a functional dual-state emitter. Among the various amines tested, this emitter displays superior sensitivity with the lowest possible limit of detection as 5.8-9.7 ppb with aliphatic polyamines such as 1,3-, 1,4-, 1,5-, and 1,6- diamines and spermidine in the solution phase. Fast, on-spot detection of the BA vapors was visually conducted through a notable high-contrast change from blue to yellow emission in the solid state. FT-IR, ¹H/³¹P NMR, and mass spectroscopic studies identify the ground-state amine-phenol interactions. The failure in BA detection with the 2,2'-dimethoxy-biphenyl-linked analog verifies the role of amine-phenol interactions. Mechanistic studies determine amine-phenol interactions in the ground and excited states. The molecular structure and packing of the doubly twisted probe are documented with a substantial void space facilitating close contact of the BAs with the strong amine-phenol interactions desired for efficient detection. Finally, this probe governs the freshness of a piece of Catla catla fish and prawn. Further, a remarkable concentration-controlled diverse emission with a red shift difference of 141 nm is detected with 1,3-diaminopropane (1,3-DAP) vapor (from 29 to 319 mg/L) for the first time. Thus, a cost-effective device is developed to detect 1,3-DAP at a precise concentration, visible through the naked eye.

Enlightening the Path to Protein Engineering: Chemoselective Turn-On Probes for High-Throughput Screening of Enzymatic Activity

Many successful stories in enzyme engineering are based on the creation of randomized diversity in large mutant libraries, containing millions to billions of enzyme variants. Methods that enabled their evaluation with high throughput are dominated by spectroscopic techniques due to their high speed and sensitivity. A large proportion of studies relies on fluorogenic substrates that mimic the chemical properties of the target or coupled enzymatic assays with an optical read-out that assesses the desired catalytic efficiency indirectly. The most reliable hits, however, are achieved by screening for conversions of the starting material to the desired product. For this purpose, functional group assays offer a general approach to achieve a fast, optical read-out. They use the chemoselectivity, differences in electronic and steric properties of various functional groups, to reduce the number of false-positive results and the analytical noise stemming from enzymatic background activities. This review summarizes the developments and use of functional group probes for chemoselective derivatizations, with a clear focus on screening for enzymatic activity in protein engineering.

Recent Advances in Chemodosimeters Designed for Amines

The analysis of amines has long been a very important task in science, industry, and healthcare. To date, this task has been accomplished by using expensive and time-consuming methods. Colorimetric and fluorescent chemodosimeters enable the fast, accurate, and sensitive analysis of various species with inexpensive instruments or the naked eye. Accordingly, the studies on these probes have gained great momentum in the last 20 years. In this review, amine chemodosimeters developed in the last 10 years were investigated. The investigated chemodosimeters are metal-free structures based on small organic compounds. The strategies for the detection, differentiation, and quantification of amines were discussed by considering the reaction types.

An effective long-wavelength fluorescent sensor for Cu2+ based on dibenzylidenehydrazine-bridged biphenylacrylonitrile

Although numerous fluorescence sensors for Cu²⁺ have been presented, a long-wavelength sensor in aqueous media has rarely been reported as expected due to practical application requirements. In this work, a novel AIE molecule (DHBB) containing two biphenylacrylonitrile units bridged by dibenzylidenehydrazine was prepared. It possessed the merits of long-wavelength emission, good emission in aqueous media, and multiple functional groups for binding Cu²⁺. It exhibited good sensing selectivity for Cu²⁺ among all kinds of tested metal ions. The detection limit was as low as 1.08 × 10^-7 M. The sensing mechanism was clarified as 1:1 stoichiometric ratio based on the binding cooperation of O and N functional groups of DHBB. The selective sensing ability for Cu²⁺ remained stable at pH = 5-9 and was influenced little by other metal ions. The Cu²⁺ sensing ability of DHBB was applied in real samples with 96% recovery rate. The bio-imaging experiment of living cells suggested that DHBB possessed not only good bio-imaging performance but also sensing ability for Cu²⁺ in living environments. This work suggested the good application prospect of DHBB to sense Cu²⁺ in real samples and living environment.

An effective dual sensor for Cu2+ and Zn2+ with long-wavelength fluorescence in aqueous media based on biphenylacrylonitrile Schiff-base

Although some sensors for Cu²⁺ and Zn²⁺ had been reported, the sensor with long-wavelength emission in aqueous media for in-situ detecting Cu²⁺ and Zn²⁺ was always expected. Herein, a biphenylacrylonitrile Schiff-base (OPBS) with large aromatic conjugated system was designed and synthesized in yield of 82%. OPBS possessed excellent long-wavelength fluorescence at 550-750 nm in aqueous media, which selectively response to sense Cu²⁺ with quenched fluorescence and Zn²⁺ with chromotropic fluorescence from red to yellow. The detection of Cu²⁺ and Zn²⁺ were realized without mutual interference in their coexistence system by means of the assistance of ATP. The detection limits were 2.3 × 10^-7 M for Cu²⁺ and 1.8 × 10^-6 M for Zn²⁺, respectively. The sensing mechanism was elucidated by binding MS spectra, fluorescence Job's plot and ¹H NMR spectra. Moreover, OPBS exhibited good bioimaging performance and the in-situ sensing abilities for Cu²⁺ and Zn²⁺ in living cells, suggesting the application potential for detecting Cu²⁺ and Zn²⁺ in both vitro assay and vivo environment.

Smartphone-based portable fluorescence sensor with gold nanoparticle mediation for selective detection of nitrite ions

A simple, portable device for the detection of NO₂^- via a fluorescence method was developed. The proposed device consisted of a dark box containing a blue LED as a low-power excitation light source and a smartphone with a mobile application for RGB analysis as a light detector. Detection was mediated by using synthesized cetyltrimethylammonium bromide-stabilized gold nanoparticles (CTAB-AuNPs). The CTAB-AuNPs were etched with NO₂^- to yield Au³⁺, which catalyzes the oxidation of o-phenylenediamine (OPD) in the presence of H₂O₂ to generate 2,3-diaminophenazine (DAP). Triton X-100 (TX-100) micelles were introduced to improve the DAP fluorescence emission. The fluorescence intensity of DAP was recorded by the smartphone in terms of RGB intensity, which was correlated with the NO₂^- concentration. This method provided a wide linear working concentration range (0.5-100 μM), a limit of detection of 0.17 μM and excellent selectivity for NO₂^- over other anions.

A coumarin-based portable fluorescent probe for rapid turn-on detection of amine vapors

Amines are widely used in many fields including agriculture, dyes, medicine and food processing. However, volatile amine vapors could initiate acute and serious damage to human bodies. Thus, highly efficient detection of volatile amine vapors has great importance for academic research as well as practical application. In this work, a turn-on type fluorescent sensor BZCO has been developed, which could be used to detect volatile amine vapors. The portable BZCO sensor can be easily prepared through immersing filter paper into its CH₂Cl₂ solution and then evaporating it to dryness. This paper-based amine vapor sensor exhibits high sensitivity with a relatively low detection limit at 3.82 ppm. It also has good selectivity for discriminating amine vapors from volatile organic solvents. The detection mechanism has been confirmed by UV-vis spectral analysis. The practical applications of this paper-based BZCO sensor, such as detection of food spoilage and fluorescent security ink, have been investigated. This work has developed a new fluorescent sensor BZCO, which has broad applications in various fields, including amine gas detection, security and anti-counterfeiting materials.

Development of a novel chromophore reaction-based fluorescent probe for biogenic amines detection

Biogenic amines (BAs) are important biomarkers to monitor meat spoilage. However, the design of efficient BA fluorescent probes with distinct colorimetric and ratiometric fluorescent dual-channels is still a critical challenge because of similar chemical properties and basicity between BAs and other amines. Herein, pyrrolopyrrole cyanine (PPCy-1) is reported to display distinctly high reactivity toward BAs through an ultrasensitive irreversible chromophore reaction for the first time. The reaction mechanism is ascribed to synergistic aza-Michael addition and B-N detachment, followed by hydrolysis to produce low-conjugated diketopyrrolopyrrole and heteroaromatic acetonitrile compounds. As a result, colorimetric and ratiometric fluorescent dual-channel (Δλ_ab = 188 nm and Δλ_em = 151 nm) signals and a limit of detection up to 62.1 nM level for BA solution are acquired. In addition, the colorimetric detection of volatile amine vapor using the PPCy-1-loaded filter paper, showing a color change from green to yellow, is feasible. A simple and cost-effective fluorescence "turn on" method using the filter paper or the CAD-40 resin loaded with PPCy-1 to detect TVB (total volatile bases) originating from shrimp spoilage is further demonstrated.

Fluorescent chameleon labels for bioconjugation and imaging of proteins, nucleic acids, biogenic amines and surface amino groups. a review

Chameleon labels (ChLs) possess the unique property of changing (visible) color and fluorescence on binding to amino groups of biomolecules. MostChLs react with primary aliphatic amino groups such as those in lysine or with amino groups artificially introduced into polynucleic acids or saccharides, but someothers also react with secondary amino groups. Under controlled circumstances, the reactions are fairly specific. The review is subdivided into the following sections: (1) An introduction and classification of fluorescent labels; (2) pyrylium labels that undergo shortwave color changes upon labelling, typically from blue to red; (3) polymethine type of labels (that also undergo shortwave color changes, typically from green to blue; (4) various other (less common) chromogenic and fluorogenic systems; (5) hemicyanine labels that undergolongwavecolor changes, typically from yellow to purple; (6) the application of ChLs to labeling of proteins and oligonucleotides; (7) applications to fluorometric assays and sensing; (8) applications to fluorescence imaging of biomolecules; (9) applications in studies on affinity interactions (receptor-ligand binding); (10) applications in surface and interface chemistry; and (11) applications in chromatography, electrophoresis and isotachophoresis of biomolecules.

An effective fluorescent sensor for ClO- in aqueous media based on thiophene-cyanostilbene Schiff-base

Although some reports on sensing ClO^- had been presented, the ClO^- sensor with high selectivity and sensitivity in aqueous media was still expected. Herein, an effective fluorescent sensor for ClO^- in aqueous media was designed and synthesized by simple procedure based on cyanostilbene derivative (TCS). TCS exhibited strong fluorescence in aqueous media, which could be selectively quenched by ClO^- among various species. The detection limit was as low as 3.2 × 10^-8 M. The sensing mechanism of the oxidation of sulfur in thiophene unit was confirmed by the FT-IR spectrum, fluorescence Job's plot, ¹H NMR spectrum and MS spectrum. This sensor was successfully applied on detecting ClO^- in real sample and living-cell imaging, suggesting its potential application for sensing ClO^- in both vitro assay and vivo environment.

Synthesis of multi-substituted pyridines from ylidenemalononitriles and their emission properties

Numerous methodologies to obtain pyridines from ylidenemalononitriles are described in the literature. Nevertheless, they are limited to the use of microwave or conventional heat and few lead to 2,3,4 or 2,3,4,5-substituted pyridines as multi-proposal molecular scaffolds or even universal pyridines. Herein, we present a mild and facile solvent-free methodology to obtain a scope of multi-substituted pyridines at room temperature. We also report an example where one of the resulting amino-nicotinonitriles exhibits a preliminary evidence of aggregation-induced emission (AIE).

"Turn-on" fluorescent sensor for Th4+ in aqueous media based on a combination of PET-AIE effect

Previously reported fluorescent sensors for Th⁴⁺ experienced emission quenching or generated false positive signal upon aggregate formation in aqueous media. Herein, a simple and novel thorium sensor (CDB-BA) based on cyanodistyrene structure was designed and synthesized, which integrated the highly emitting characteristic of AIE effect and off-on response of PET modulation for the first time to construct the "turn-on" fluorescent probe for Th⁴⁺. Besides excellent selectivity, CDB-BA exhibited remarkable fluorescent enhancement which was linearly related to the concentration of Th⁴⁺ in the range of 0.25-8 μM. The detection limit was attained 0.074 μM, which was lower than that of most previously reported sensors. The mechanism of tris-chelate complex of CDB-BA with Th⁴⁺ was confirmed by mass spectra, IR spectra and DFT calculation. The excellent Th⁴⁺ sensing ability of CDB-BA was successfully applied to detecting Th⁴⁺ on TLC plates, in real water samples and living-cell imaging. This work suggested that the combination of AIE and PET photophysical mechanism could offer the merits of minimized background and enhanced signal fidelity to develop novel "turn-on" fluorescent probe in complicated aqueous environment and biological research.

Cu-MOF Material Constructed with a Triazine Polycarboxylate Skeleton: Multifunctional Identify and Microdetecting of the Aromatic Diamine Family (o,m,p-Phenylenediamine) Based on the Luminescent Response

Organic aromatic amines are widely used in various fields such as pharmaceuticals, pesticides, dyes, and tobacco smoke. The pollution of organic amines has become a problem that cannot be ignored, due to the extensive harmful effects on the environment and public health, which has become one of the most concerned frontier fields in the world. Identifying and microdetecting o-phenylenediamine (OPD), m-phenylenediamine (MPD), and p-phenylenediamine (PPD) using MOFs have rarely been reported. On the basis of the blue emission properties of Cu-TBDA constructed with 5,5'-((6-chloro-1,3,5-triazine-2,4-diyl)bis(azanediyl))diisophthalic acid (H₄TBDA) ligand, Cu-TBDA was studied primarily to identify and detect aromatic diamine family as a multifunctional chemical sensor. Interestingly, Cu-TBDA has a very high selectivity and sensitivity to OPD and MPD with a low limit of detection (5.00 μM for OPD and 1.77 μM for MPD). Especially for OPD, Cu-TBDA has a unique switching function for it. When the concentration of OPD is less than 9.1 × 10^-4 M, the fluorescence response of Cu-TBDA suspension exhibit enhanced. However, when the concentration of OPD is more than 9.1 × 10^-4 M, the emission intensity displays quenching phenomenon. Therefore, Cu-TBDA as a chemical sensor not only has recognition and detection functions for organic aromatic amines but also first exhibits turn-on and -off sensing behavior toward OPD.

A “turn-on” fluorescent sensor for cytosine in aqueous media based on diamino-bridged biphenyl acrylonitrile

A “turn-on” fluorescent sensor for cytosine in aqueous media was prepared.

Increasing Scope of Clickable Fluorophores: Electrophilic Substitution of Ylidenemalononitriles

Recently, we demonstrated that ylidenemalononitriles (YMs) react with amines to form cyclic amidines and that the starting linear YMs are nonemissive in solution and the cyclic amidines are fluorescent. These turn-on systems were of interest to us because of their potential as biosensors and synthons for accessing functionalized pyridines. While our original method was promising, several limitations persisted, including access to more functionalized and polar-solvent-soluble structures as well as increased control over the rate of cyclization. Herein, we report a new approach that allows the electrophilic substitution of YMs. These substituted YMs exhibit faster turn-on rates, color tunability, access to polar-solvent-soluble species, and increased control over cyclization rate. This allowed us to significantly expand the fluorophore's chemical space.

First fluorescent sensor for curcumin in aqueous media based on acylhydrazone-bridged bis-tetraphenylethylene

This work designed and synthesized the first organic fluorescent sensor for curcumin in aqueous media based on red-to-green fluorescence change of acylhydrazone-bridged bis-tetraphenylethylene (Bis-TPE). Bis-TPE was prepared by condensation of formyltetraphenylethylene with dihydrazide oxalate in 86% yield. It has the large conjugated electron effect with strong red AIE fluorescence in aqueous solution. It displayed high selective sensing ability for curcumin with red-to-green fluorescence change in THF-H₂O (5:95). The detection limit was as low as 1.15 × 10^-7 M. The sensing mechanism was confirmed as 1:1 stoichiometric ratio based on quadruple hydrogen bonds. Bis-TPE was successfully applied for analyzing curcumin of ginger and living cell imaging, supplying a new detecting strategy for curcumin in real sample and living environment.

Efficient synthesis of cyclic amidine-based fluorophores via 6π-electrocyclic ring closure

Novel 10π-electron cyclic amidines with excellent fluorescence properties were synthesized by a general and efficient 6π-electrocyclic ring closure of ketenimine and imine starting from N-sulfonyl triazoles and arylamines. The photophysical properties of cyclic amidine fluorophores have been studied in detail and have shown good properties of a large Stokes shift, pH insensitivity, low cytotoxicity and higher photostability, which have great potential for biological imaging. Furthermore, this novel fluorophore was successfully applied to the localization of the NK-1 receptor in living systems.

Novel 10π-electron cyclic amidines with excellent fluorescence properties were synthesized by a general and efficient 6π-electrocyclic ring closure of ketenimine and imine starting from N-sulfonyl triazoles and arylamines.

Propargylamine-selective dual fluorescence turn-on method for post-synthetic labeling of DNA

We have developed a propargylamine-selective dual fluorescence turn-on system, using ylidenemalononitrile enamines, for post-synthetic DNA labeling, allowing the direct monitoring of DNA using dual emission in living cells.

Discrimination of Various Amine Vapors by a Triemissive Metal-Organic Framework Composite via the Combination of a Three-Dimensional Ratiometric Approach and a Confinement-Induced Enhancement Effect

Multiemissive sensors are being actively pursued, because of their ratiometric luminescent detection capabilities, which demonstrates better sensitivity and selectivity than conventional single-emission sensors. Herein, we present a trichromatic white-light-emitting metal-organic framework (MOF) composite (Z3) by simultaneously incorporating red/green-emitting Pt/Ru complex cations into porous blue-emitting bio-MOF-1 through post-synthetic modification. With the help of a three-dimensional (3-D) dual-ratiometric luminescence recognition method, and unique turn-on responses of the red emission toward amine compounds (ACs), including NH₃ and aliphatic amines, via confinement-induced luminescence enhancement effect, Z3 can work as a dual-ratiometric luminescent sensor for discrimination of 7 out of 11 AC vapors. This work not only provides a new AC sensing mechanism (confinement effect) that can induce a "turn-on" response but also proves that the accuracy and selectivity of composite sensor can be greatly improved through the combination of 3-D recognition method and the confinement effect. Thus, it open up fresh opportunities to develop composite sensors with excellent sensing and differentiating ability.

Double-detecting fluorescent sensor for ATP based on Cu2+ and Zn2+ response of hydrazono-bis-tetraphenylethylene

Although all kinds of sensors with unique detecting ability for one guest were reported, the fluorescence sensor with multiple detecting abilities was seldom presented. This work designed and synthesized a novel AIE fluorescence probe bearing double detecting for ATP based on Cu²⁺ and Zn²⁺ response of hydrazono-bis-tetraphenylethylene (Bis-TPE). Bis-TPE was prepared in 82% yield with simple procedure. It exhibited strong red AIE fluorescence based on the large conjugated electron effect in aqueous media. It showed outstanding selective sensing abilities for Cu²⁺ by strong fluorescence quenching and for Zn²⁺ by red-orange fluorescence change. The sensing mechanism of 1:1 stoichiometric ratios was confirmed by ¹H NMR and MS study. The strong red fluorescence of Bis-TPE + Cu²⁺ system could be recovered by adding ATP. The orange fluorescence of Bis-TPE + Zn²⁺ system could be quenched by adding Cu²⁺ and then was recovered by adding ATP. These double detecting abilities for ATP with the "off-on" red fluorescence in Bis-TPE + Cu²⁺ system and "allochroic-off-on" orange fluorescence in Bis-TPE + Zn²⁺+Cu²⁺ system were successfully applied to test Cu²⁺, Zn²⁺ and ATP in test paper and living cell imaging, displaying the good application prospects for sensing Cu²⁺, Zn²⁺ and double detecting ATP in the complicated environment.

Development of a 1,3a,6a-triazapentalene derivative as a compact and thiol-specific fluorescent labeling reagent

For the fluorescence imaging of biologically active small compounds, the development of compact fluorophores that do not perturb bioactivity is required. Here we report a compact derivative of fluorescent 1,3a,6a-triazapentalenes, 2-isobutenylcarbonyl-1,3a,6a-triazapentalene (TAP-VK1), as a fluorescent labeling reagent. The reaction of TAP-VK1 with various aliphatic thiols proceeds smoothly to afford the corresponding 1,4-adducts in high yields, and nucleophiles other than thiols do not react. After the addition of thiol groups in dichloromethane, the emission maximum of TAP-VK1 shifts to a shorter wavelength and the fluorescence intensity is substantially increased. The utility of TAP-VK1 as a compact fluorescent labeling reagent is clearly demonstrated by the labeling of Captopril, which is a small molecular drug for hypertension. The successful imaging of Captopril, one of the most compact drugs, in this study demonstrates the usefulness of compact fluorophores for mechanistic studies.

(Tetrahydrodibenzo[a,i]phenanthridin-5-yl)phenol as a Fluorescent Probe for the Detection of Aniline

Two (tetrahydrodibenzo[a,i]phenanthridin-5-yl)phenols that differ in their substituents at the para position (P1, R = H and P2, R = NEt₂) were designed and synthesized. The presence of a -NEt₂ group in probe P2 facilitates the twisted intramolecular charge transfer (TICT) process, making P2 emissive, which distinctly coordinated with boron trifluoride in the presence of different amines with different electronic properties. A substantial increase in emission intensity with increasing viscosity of the surrounding environment and smooth formation of a planar complex with boron and Zn²⁺ ions concluded the presence of a TICT process. The selective reactivity of P2 toward a tetracoordinated boron complex has been explored as a potential tool for colorimetric and fluorescent discrimination of aromatic primary amines, i.e., anilines. Selective detection of aniline with probe P2 can be viewed through the naked eye, and the corresponding fluorescence turn-on detection limit was found to be 12.65 nM. In addition, the detection of aniline on precoated aluminum-backed thin-layer chromatography plates and Whatman filter paper strips was found to be in good agreement with the color change of P2 in solution and in vapor phase.

A fluorescent sensor based on aggregation-induced emission: highly sensitive detection of hydrazine and its application in living cell imaging

Aggregation-induced emission (AIE) molecules eliminate the aggregation-caused quenching (ACQ) phenomenon effectively and exhibit excellent properties of a fluorescent sensor in the aggregated state. In this paper, an allochroic fluorescent sensor based on AIE molecules with a diphenylacrylonitrile structure was prepared in high yield by a simple procedure. This molecule possessed good AIE properties and exhibited a sensitive sensor ability for aliphatic amines with an obvious color change from orange to blue-green. The detailed investigation on the detection of hydrazine suggested that the detection limit for hydrazine was 3.67 × 10-6 M, and the highly sensitive sensor for hydrazine was not influenced by other species. The sensor mechanism was confirmed by using 1H NMR and MS spectra. The sensor for hydrazine was successfully applied in a test paper, exhibiting good practical application potential for detecting hydrazine. The experiment of living cell imaging suggested that this sensor showed superior bioimaging performance and presented sensitive detection for hydrazine with an obvious color change from orange to blue-green.

Crystallographic characterization of ethylammonium salts of tetracyanopyridine (TCPy) and fluorescence determination of the degree of substitution of the amino nitrogen atom thereof

Luminescence determination of the substitution degree of the amino nitrogen atom using ethylamine, diethylamine, triethylamine and tetraethylammonium has been shown.

A highly efficient, colorimetric and fluorescent probe for recognition of aliphatic primary amines based on a unique cascade chromophore reaction

Pyrrolopyrrole aza-BODIPY based nanoaggregates were reported as a highly selective and sensitive probe for recognition of aliphatic primary amines with a novel cascade chromophore reaction. Due to its distinct reaction characteristics, this probe loaded test strip can conveniently detect n-hexylamine vapor and monitor the freshness of shrimp.

AIEE Active Donor-Acceptor-Donor-Based Hexaphenylbenzene Probe for Recognition of Aliphatic and Aromatic Amines

In the present investigation, an intramolecular charge transfer (ICT) and aggregation induced emission enhancement (AIEE) active donor-acceptor-donor (D-A-D) system 5 having fumaronitrile as the acceptor and hexaphenylbenzene (HPB) as the donor moieties joined through rotatable phenyl rings has been designed and synthesized that is highly emissive in the solid state and exhibits stimuli-responsive reversible piezochromic behavior upon grinding and heating. Because of its AIEE characteristics, HPB derivative 5 undergoes aggregation to form fluorescent aggregates in mixed aqueous media that exhibit ratiometric fluorescence response toward aliphatic amines (primary/secondary/tertiary) and turn-off response toward aromatic amines and hence differentiates between them. Further, the solution-coated portable paper strips of derivative 5 showed pronounced and sensitive response toward aromatic and aliphatic amines with a detection limit in the range of picogram and nanogram level, respectively.

New 1,2-Dihydropyridine-Based Fluorophores and Their Applications as Fluorescent Probes

New 1,2-dihydropyridine (1,2-DHP)-based fluorophores 1a-1h were designed and synthesized by a one-pot four-component condensation reaction using dienaminodioate, aldehydes, and an in situ-generated hydrazone mediated by trifluoroacetic acid. The photophysical properties of 1,2-DHPs were studied in detail, and a few of them exhibited selective mitochondrial staining ability in HeLa cell lines (cervical cancer cells). A detailed photophysical investigation led to the design of 1,2-DHP 1h as an optimal fluorophore suitable for its potential application as a small molecule probe in the aqueous medium. Also, 1,2-DHP 1h exhibited sixfold enhanced emission intensity than its phosphorylated analogue 1h' in the long wavelength region (λ_em ≈ 600 nm), which makes 1,2-DHP 1h' meet the requirement as a bioprobe for protein tyrosine phosphatases, shown in L6 muscle cell lysate.

Fluorescent β-ketothiolester boron complex: substitution based “turn-off” or “ratiometric” sensor for diamine

Boron diketonate 3 provides a choice of turn-off or ratiometric detection of diamine with respect to substitution on the boron-chelating ring. Detection of the diamine involves a substitution reaction by elimination of a methylsulfanyl group, favouring selective detection of diamine.

A novel aggregation-induced-emission-active supramolecular organogel for the detection of volatile acid vapors

A novel supramolecular organogel with AIE properties was synthesized and applied for sensing volatile acid vapors with excellent performance.