A coumarin based Schiff base probe for selective fluorescence detection of Al3+ and its application in live cell imaging

A zwitterionic probe for ratiometric fluorescent detection of aluminium(III) ion in aqueous medium and its application in bioimaging

In the present study, we have synthesized an aminobenzoic acid containing Schiff base (compound 1) and its structure was confirmed through single crystal X-ray study. Importantly, the compound 1 crystallizes in the zwitterionic form, with an anionic carboxylate group (-COO-) and a cationic iminium group (-C = NH+-). The compound 1 is highly soluble in water due to its zwitterionic feature in the solid state. Interestingly, compound 1 acts as a ratiometric fluorescent probe for the selective detection of Al3+ ion in aqueous solution without organic cosolvent. It can also detect Al3+ ion by visual colour change to bluish-green fluorescence under 365 nm UV light. The association constant between compound 1 with Al3+ ion was estimated to be 1.67 × 104 M-1. The lowest detection limit for Al3+ ion was calculated to be 7.05 × 10-8 M in water. Compound 1 in combination with Al3+ ion demonstrated fluorescent imaging potential of the nucleus of in RAW 264.7 murine macrophage cell line. In addition, the sensing model is developed as paper based sensor ''Test Kit' 'for its practical applicability.

A ratiometric fluorescent probe with a large Stokes shift for the detection of Hg2+ and its applications in environmental sample and living system analysis

Toxic mercury ions (Hg2+) can cause serious environmental pollution and accumulate in living organisms via the food chain. Therefore, monitoring Hg2+ is crucial in ensuring the safety of ecosystems and organisms. In this work, a novel ratiometric fluorescent probe CMT (5-(4-(diphenylamino)phenyl)-1-(7-hydroxy-coumarin-3-yl)-4-pentene-1,3-dione) based on coumarin was developed for detecting Hg2+, which displayed obvious fluorescence changes, a low detection limit (2.24 × 10-7 M), good selectivity, and a large Stokes shift (255 nm). The CMT probe could detect Hg2+ in real environmental soil and water samples. Furthermore, the CMT probe enabled the naked-eye detection of Hg2+ using test paper experiments. CMT was also applied for fluorescence imaging in living zebrafish and plants. This work provides a highly efficient tool for monitoring Hg2+ in environmental samples and biological systems.

Synthesis of a super-low detection limit fluorescent probe for Al3+ and its application in fluorescence imaging of zebrafish and cells

A novel fluorescent probe N'-(2-hydroxybenzylidene)-indole-3-formylhydrazine (JHK) was designed and synthesized based on the condensation reaction of indole-3-formylhydrazine and salicylaldehyde. The probe JHK solution could highly selectively recognize Al3+ by the obvious fluorescence enhancement (288-fold) after adding Al3+. And the probe solution with Al3+ had a very high fluorescence quantum yield (89.29 %). The detection limit was calculated to be 1.135 nM, which was significantly lower than many reported detection limits, indicating that the probe JHK had pretty good sensitivity. The ratio of JHK to Al3+ (1:1) and the sensing mechanism were determined by Job's plot, 1H NMR spectra, FTIR spectra, ESI-MS and Gaussian calculation. The probe solution and medium-speed filter paper were successfully used to make test papers for more convenient detection of Al3+. Furthermore, the probe JHK had been successfully applied to the detection of Al3+ in real water, zebrafish and living cells.

A highly specific chalcone derivative grafted ethylcellulose fluorescent probe for rapid and sensitive detection of Al3+ in actual environmental and food samples

Al3+ is commonly utilized in daily life, however, the excessive accumulation of Al3+ within organisms can result in severe health problems. Herein, a highly efficient fluorescent probe EC-HTC for Al3+ was synthesized through chemical modification of ethyl cellulose. This probe exhibited a significant fluorescence enhancement response to Al3+, and it interestingly also possessed an obvious aggregation-induced emission (AIE) effect. The detection limit of probe EC-HTC for Al3+ was as low as 0.23 μM, and its pH usage range was as wide as 5-10. The complexation ratio of EC-HTC with Al3+ was determined to be 1:1 based on Job's plot, which was further confirmed by 1H NMR titration and HRMS analysis. Moreover, the probe EC-HTC was successfully employed for the determination of Al3+ in environmental and food samples. In addition, the probe EC-HTC compositing PS (polystyrene) electrostatic spun fiber membranes EHP with high specific surface area were prepared to achieve the rapid and portable detection of Al3+.

Schiff Bases: A Versatile Fluorescence Probe in Sensing Cations

Metal cations such as Zn²⁺, Al³⁺, Hg²⁺, Cd²⁺, Sn²⁺, Fe²⁺, Fe³⁺ and Cu²⁺ play important roles in biology, medicine, and the environment. However, when these are not maintained in proper concentration, they can be lethal to life. Therefore, selective sensing of metal cations is of great importance in understanding various metabolic processes, disease diagnosis, checking the purity of environmental samples, and detecting toxic analytes. Schiff base probes have been largely used in designing fluorescent sensors for sensing metal ions because of their easy processing, availability, fast response time, and low detection limit. Herein, an in-depth report on metal ions recognition by some Schiff base fluorescent sensors, their sensing mechanism, their practical applicability in cell imaging, building logic gates, and analysis of real-life samples has been presented. The metal ions having biological, industrial, and environmental significance are targeted. The compiled information is expected to prove beneficial in designing and synthesis of the related Schiff base fluorescent sensors.

Tris(bipyridine)ruthenium(II)-functionalized metal-organic frameworks for the ratiometric fluorescence determination of aluminum ions

A novel ratiometric fluorescence probe was designed for the determination of Al³⁺ by self-assembling of NH₂-MIL-101(Fe) and [Ru(bpy)₃]²⁺. Under the excitation wavelength of 360 nm, the NH₂-MIL-101(Fe)@[Ru(bpy)₃]²⁺ presented a dual-emitting luminescent property at 440 and 605 nm, respectively. In the presence of Al³⁺, the blue fluorescence of NH₂-MIL-101(Fe)@[Ru(bpy)₃]²⁺ at 440 nm was enhanced remarkably, while the red emission at 605 nm was almost not influenced. Therefore, taking the fluorescence at 440 nm as the report signal and 605 nm as the reference signal, quantitative determination was achieved for Al³⁺ concentration in the ranges 0.2-25 μM and 25-250 μM. The limit of detection (LOD) and limit of quantification (LOQ) were calculated to be 73 nM and 244 nM, respectively. The sensing mechanisms were studied by theoretical calculation and optical spectra. The analysis of real food samples confirmed the suitability of the proposed method. More importantly, portable fluorescent test papers were successfully manufactured to provide a strategy for visual, rapid, and on-site detection of Al³⁺.

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.

A simple hydrazone probe for recognition of Al3+ and PPi and its applicability in lysosomal imaging

A simple hydrazone probe (1) was designed and synthesized for the successive detection of Al³⁺ and pyrophosphate (PPi) in almost 100% buffer environment. The probe provided O₂N donor set for chelation with Al³⁺, leading to a distinct fluorescence boost at 510 nm. The in-situ formed 1-Al³⁺ complex detected PPi with an "on-off" behavior. The detection limits for Al³⁺ and PPi were 35.7 nM and 76 nM, respectively. Benefiting from the existence of morpholine as lysosome-targeting group, probe 1 was successfully applied to the detection of Al³⁺ and PPi in lysosomes.

A dual-channel 'turn-on' fluorescent chemosensor for high selectivity and sensitivity detection of CN¯ based on a coumarin-Schiff base derivative in an aqueous system

Novel strategies still need to be proposed that can be used to identify and detect toxic environmental pollutants. In this paper, two channels of colorimetry and fluorescence 'turn-on' fluorescent probe 1 (7-hydroxy-8-[(2-hydroxy-phenylimino)- methyl]-4-methylbenzopyran-2-one) for the simple yet highly selective detection of CN¯ have been successfully designed and synthesized. Crystal features of probe 1 were defined using X-ray single crystal diffractometry. Probe 1 showed a strongly colorimetric and fluorescence response to CN¯ that induced obvious naked-eye colour changes in aqueous solution (DMSO/H₂ O, 3:1 v:v). In addition, probe 1 for CN¯ detection displayed low detection limits of 3.91 × 10^-8  M, which were significantly lower than the 1.9 × 10^-6  M maximum level specified by the World Health Organization (WHO) for potable water. The sensing mechanism for probe 1 was attributed to the deprotonation process as shown by ¹ H NMR titration. Moreover, based on the visible colorimetry and fluorescence change for probe 1 to CN¯, measurement was performed for simulated water samples containing CN¯. This study provides a broad prospect for solving other pollution problems and promoting the design of new fluorescent materials.

Recent advances in the development of fluorescent chemosensors for Al3

Detection of Al3+ has become important as it is related to several health issues and other problems. Different fluorophoric platforms, such as naphthalene, benzene, rhodamine, etc., have been explored to sense Al3+ and a good number of research articles are being published. This article focuses on the synthesis of recently reported aluminum sensors constructed from 2-hydroxy-1-naphthaldehyde, salicylaldehyde, rhodamine, coumarin and different metal based-MOFs.

A highly efficient yet stable salamo-type fluorescent chemosensor with multiple responses to Cu2+ and S2−

A new salamo-type compound H3L has been synthesized, and exploited as a chemosensor with multiple responses to Cu2+ and S2−. The crystal structure of the Cu(ii)  complex has been determined by X-ray crystallographic analysis.

A lysosome-targetable fluorescent probe for real-time imaging cysteine under oxidative stress in living cells

As an effective lysosomal biomarker for oxidative stress status, cysteine (Cys) plays an important role in lysosomal proteolysis. Herein, we report the first lysosome-targetable fluorescence probe (MCAB) for Cys-selective detection based on nucleophilic addition reaction of sulfhydryl toward a α, β-unsaturated ketone and demonstrate its application to lysosomal-targetable imaging. MCAB is designed based on a α, β-unsaturated ethanoylcarbazole as the fluorophore and the thiols reaction site, and a methylcarbitol unit as a lysosome-targetable group. Upon reacting with Cys, this probe turns on highly specific fluorescence signals linearly proportional to Cys concentrations over the range of 0-300 μM. MCAB detects Cys with a rapid response time (within 12 min) and low limit of detection (0.38 μM). MCAB is highly selective to Cys over other similar biothiols including homocysteine (Hcy) and glutathione (GSH). Moreover, it also exhibits significant lysosomal-targetable ability, which is ideal for lysosomal Cys-selective imaging. Using MCAB, we have successfully visualized the fluctuation endogenous Cys in lysosomes under oxidative stress status in real-time.

Coumarin Derived "Turn on" Fluorescent Sensor for Selective Detection of Cadmium (II) Ion: Spectroscopic Studies and Validation of Sensing Mechanism by DFT Calculations

A novel coumarin based Schiff base sensor probe 1, was synthesized and structural elucidation was carried out by FTIR, UV-vis, ¹H and ¹³C NMR and MS spectroscopy. The optical properties of the sensor probe were investigated by employing absorption and fluorescence titrations which showed specific recognition behaviour being highly selective towards Cd²⁺ over the other 3d transition metal ions. The strong fluorometric response of probe 1 towards Cd²⁺ ion is attributed to inhibition of C=N isomerization effect upon coordination of the metal ion. The binding stoichiometry was determined by Job's plot and the probable sensing mechanism of the probe towards Cd²⁺ was investigated by employing FTIR spectra analysis and ¹H NMR titration experiments. Computational validation of the sensing mechanism in various modes towards Cd²⁺ was also performed by carrying out the DFT studies which were found to be in good concordance with the experimental results. The reversible nature of the probe was studied by EDTA titration indicating that it can be reused. Interaction studies of the sensor probe with the BSA showed the practical applicability for the quantitative determination of Cd²⁺ concentration in the blood plasma. The lower detection limit of the probe upto 0.114 μM further proves its practical application in the sensing phenomenon.

Coumarin-Based Small-Molecule Fluorescent Chemosensors

Coumarins are a very large family of compounds containing the unique 2H-chromen-2-one motif, as it is known according to IUPAC nomenclature. Coumarin derivatives are widely found in nature, especially in plants and are constituents of several essential oils. Up to now, thousands of coumarin derivatives have been isolated from nature or produced by chemists. More recently, the coumarin platform has been widely adopted in the design of small-molecule fluorescent chemosensors because of its excellent biocompatibility, strong and stable fluorescence emission, and good structural flexibility. This scaffold has found wide applications in the development of fluorescent chemosensors in the fields of molecular recognition, molecular imaging, bioorganic chemistry, analytical chemistry, materials chemistry, as well as in the biology and medical science communities. This review focuses on the important progress of coumarin-based small-molecule fluorescent chemosensors during the period of 2012-2018. This comprehensive and critical review may facilitate the development of more powerful fluorescent chemosensors for broad and exciting applications in the future.

A Highly Selective Fluorescence "Turn on" and Absorbance-Ratiometric Detection of Al3+ in Totally H2O and its Application in Test Paper

A novel naphthalene based fluorescence probe NBDH was designed and synthesized. Probe NBDH exhibited highly selective and sensitive responses towards Al³⁺ in HEPES-NaOH buffer solution (pH = 7.4). In addition, the detection of NBDH to Al³⁺ could be achieved through dual channels embodied in significant fluorescent turn-on signal and ratiometric absorbance response. The stoichiometry ratio of NBDH-Al³⁺ was 1:1 by fluorescence job' plot and binging mechanism was further varified by the FT-IR, NMR titration and HRMS. Furthermore, NBDH was achieved in real sample detection, and a series of color test paper were developed for visual detecting Al³⁺ ions.

A Highly Selective Turn-on and Reversible Fluorescent Chemosensor for Al3+ Detection Based on Novel Salicylidene Schiff Base-Terminated PEG in Pure Aqueous Solution

The development of highly selective and sensitive chemosensors for Al³⁺ detection in pure aqueous solution is still a significant challenge. In this work, a novel water-soluble polymer PEGBAB based on salicylidene Schiff base has been designed and synthesized as a turn-on fluorescent chemosensor for the detection of Al³⁺ in 100% aqueous solution. PEGBAB exhibited high sensitivity and selectivity to Al³⁺ over other competitive metal ions with the detection limit as low as 4.05 × 10⁻⁹ M. PEGBAB displayed high selectivity to Al³⁺ in the pH range of 5–10. The fluorescence response of PEGBAB to Al³⁺ was reversible in the presence of ethylenediaminetetraacetic acid (EDTA). Based on the fluorescence response, an INHIBIT logic gate was constructed with Al³⁺ and EDTA as two inputs. Moreover, test strips based on PEGBAB were fabricated facilely for convenient on-site detection of Al³⁺.

A phenolphthalein-based fluorescent probe for the sequential sensing of Al3+ and F- ions in aqueous medium and live cells

A novel fluorescent probe, phenolphthalein‑dialdehyde‑(2‑pyridyl) hydrazone (L), for sequentially detecting Al³⁺ and F^- in almost 100% aqueous medium was successfully designed and synthesized. The probe offers two binding pockets for Al³⁺ to form a 1: 2 ligand/metal complex, leading to a significant fluorescence enhancement at 465 nm. Further, the in-situ formed L-Al complex acts as a secondary fluorescent chemosensor for F^- by quenching the fluorescence of the complex with high selectivity. The detection limit for Al³⁺ and F^- sensing is 2.28 nM and 0.13 μM, respectively, which are far below the World Health Organization (WHO) acceptable limits (7.41 μM for Al³⁺ ion and 79 μM for F^-) in drinking water. The probe L was successfully applied to the detection of Al³⁺ and F^- in cells using fluorescence microscopy.

Polydopamine Dots-Based Fluorescent Nanoswitch Assay for Reversible Recognition of Glutamic Acid and Al3+ in Human Serum and Living Cell

We developed a facile and feasible fluorescent nanoswitch assay for reversible recognition of glutamate (Glu) and Al³⁺ in human serum and living cell. The proposed nanoswitch assay is based on our recently developed method for controlled synthesis of fluorescent polydopamine dots (PDADs) at room temperature with dopamine as the sole precursor. The fluorescence of nanoswitch assay could be quickly and efficiently quenched by Glu (turn-Off), and the addition of Al³⁺ could recover the fluorescence of the PDADs-Glu system (turn-On). Meanwhile, the reversible recognition of Glu and Al³⁺ in this nanoswitch system was stable after three cycles. Additionally, the system displayed excellent performance for Glu and Al³⁺ determination with a low detection limit of 0.12 and 0.2 μM, respectively. Moreover, PDADs are successfully applied to determine Glu and monitor Al³⁺ in human serum. Noteworthy, the nanoswitch assay is transported into HepG2 cells and realized "Off" detection of Glu and "On" sensing Al³⁺ in the living cells. Therefore, this PDADs-based nanoswitch assay provides a strategy to develop reversible recognition biosensors for intracellular and external molecular analysis.

Novel rhodamine Schiff base type naked-eye fluorescent probe for sensing Fe3+ and the application in cell

Three rhodamine schiff-base type fluorescent sensors R1-R3 for detecting iron ion (Fe³⁺), 2-furanacrolein rhodamine hydrazone (R1), furfural rhodamine hydrazone (R2) and 2-furanacrolein rhodamine ethylenediamine (R3) have been synthesized by using rhodamine B derivatives and furan derivatives as staring materials. And their recognition abilities for Fe³⁺ were studied by fluorescence spectroscopy. The result showed that R1 is a best selective probe for Fe³⁺ over other metal ions in EtOH/H₂O (1:1, v/v) due to having 2-furanacrolein for unique space coordination structural. The recognition of Fe³⁺ and mechanism of the sensor were characterized and determined by fluorescence spectra and Fukui function. And the fluorescence intensity of the probe R1 for Fe³⁺ was proportional to its concentration with the linear correlation coefficient of 0.9965 and the binding constant of 7.66×10⁴M^-1. And the cell imaging experiment indicated a successful application of the probe R1 for Fe³⁺ in living cell.

Spectroscopic investigations on the interaction of thioacetamide with ZnO quantum dots and application for its fluorescence sensing

The purpose of the present work was to develop a method for the sensing of thioacetamide by using spectroscopic techniques. Thioacetamide is a carcinogen and it is important to detect its presence in food-stuffs. Semiconductor quantum dots are frequently employed as sensing probes since their absorption and fluorescence properties are highly sensitive to the interaction with substrates present in the solution. In the present work, the interaction between thioacetamide and ZnO quantum dots has been investigated by using UV-visible, fluorescence and infrared spectroscopy. Besides, dynamic light scattering (DLS) has also been utilized for the interaction studies. UV-visible absorption studies indicated the bonding of the lone pair of sulphur atom of thioacetamide with the surface of the semiconductor. The fluorescence band of the ZnO quantum dots was found to be quenched in the presence of micromolar concentrations of thioacetamide. The quenching was found to follow the Stern-Volmer relationship. The Stern-Volmer constant was evaluated to be 1.20×10⁵M^-1. Infrared spectroscopic measurements indicated the participation of the NH₂ group and the sulphur atom of thioacetamide in bonding with the surface of the ZnO quantum dots. DLS measurements indicated that the surface charge of the semiconductor was shielded by the thioacetamide molecules.

Facile preparation of highly sensitive and selective fluorescent paper sensor for the visual and cyclic detection of Cu2+ and Hg2+

The fluorescent paper sensor based on LAA-CQDs was prepared and applied to detect heavy metal ions Cu²⁺ and Hg²⁺. Notably, the paper sensor can be recycled for detecting at least four times, which greatly reduced resource consumption.

A novel off-on fluorescent chemosensor for Al3+derived from a 4,5-diazafluorene Schiff base derivative

The performance of a chemosensor is closely related to its structure. A new Schiff bass (DFSB) based on 4,5-diazafluorene units has been synthesized in this work. The interaction of DFSB with different metal ions has been studied using UV-vis absorption spectra and fluorescent spectra. The results show that DFSB is a highly selective and sensitive probe for Al³⁺ ions over other commonly coexisting metal ions in ethanol. A very obvious fluorescence enhancement effect was observed, and a turn-on ratio over 1312-fold was triggered with the addition of 10 equiv. of Al³⁺ ions. What is more, such fluorescent responses could be detected by the naked eye under a UV-lamp. The lowest detection limit for Al³⁺ was determined as 3.7 × 10⁻⁸ M. The complex solution (DFSB–Al³⁺) exhibited reversibility with EDTA. These results may be caused by the unique molecular structure.

A highly selective and sensitive“turn-on” fluorescent sensor for detecting Al³⁺ derivated from 4,5-diazafluorene.

A carbon dots/rutin system for colorimetric and fluorimetric dual mode detection of Al3+ in aqueous solution

Colorimetric and fluorimetric dual mode detection of Al³⁺ in aqueous solution was achieved using a carbon dots/rutin (CDs/rutin) coexisting system.