A turn-on Schiff base fluorescence sensor for zinc ion

Synthesis of a BINOL-Based C3 Symmetric Schiff Base and Its Fluorescence Response to Zn2

A novel C₃ symmetric 1,1'-bi-2-naphthol-based Schiff base (R,R,R)-6 has been synthesized which shows highly selective fluorescence enhancement with Zn²⁺ among 21 metal cations examined. Its sensitivity and selectivity are found to be greater than other related C₂ (1) and C₁ [(R)-9] symmetric compounds in the fluorescent recognition of Zn²⁺ . The mechanistic study reveals that the selective fluorescence enhancement of the probe can be attributed to the formation of a unimolecular multidentate 6-coordinated Zn²⁺ complex.

Comprehensive Empirical Model of Substitution—Influence on Hydrogen Bonding in Aromatic Schiff Bases

In this work, over 500 structures of tri-ring aromatic Schiff bases with different substitution patterns were investigated to develop a unified description of the substituent effect on the intramolecular hydrogen bridge. Both proximal and distal effects were examined using Density Functional Theory (DFT) in the gas phase and with solvent reaction field (Polarizable Continuum Model (PCM) and water as the solvent). In order to investigate and characterize the non-covalent interactions, a topological analysis was performed using the Quantum Theory of Atoms In Molecules (QTAIM) theory and Non-Covalent Interactions (NCI) index. The obtained results were summarized as the generalized, empirical model of the composite substituent effect, assessed using an additional group of simple ring-based Schiff bases. The composite substituent effect has been divided into separate increments describing the different interactions of the hydrogen bridge and the substituent: the classical substituent effect, involving resonance and induction mediated through the ring, steric increment based on substituent proximity to the bridge elements, and distal increment, derived from substitution on the distal ring.

Variation of the Emission Efficiency and Wavelength from Fluorescent Zinc Salen Complexes upon Systematic Structural Modifications

Understanding the photophysical properties of metal salen complexes is not straightforward because the emission efficiency is altered irregularly upon structural modifications. The present study prepared zinc salen complexes with systematic structural variations to pinpoint critical factors to determine the emission efficiency. One of the important experimental observations is the regiochemistry of a phenolate substituent affecting emission efficiency from a salicylidene fluorophore, which is nicely assigned as arising from the photoexcited electronic structure of metal salen complexes. Another significant finding is the thermal fluctuation of a salen ligand arising from the mismatched ligand-metal interaction, which has a significant impact on fluorescence lifetime. The present study sheds light on hidden factors that alter photophysical properties of a metal salen complex, which provide valuable insights into designing new photoactive salen ligands.

A Review on Organic Colorimetric and Fluorescent Chemosensors for the Detection of Zn(II) Ions

Organic compounds display several electronic and structural features which enable their application in various fields, ranging from biological to non-biological. These compounds contain heteroatoms like sulfur, nitrogen and, oxygen, which provide coordination sites to act as ligands in the field of coordination chemistry and are used as chemosensors to detect various metal ions. This review article covers different organic compounds including Schiff bases, thiourea, pyridine, rhodamine, triazole, pyrene, coumarin, imidazole, diaminomaleonitrile, naphthoxazole, pyrimidine, thiophene, thioether, and other functional groups based chemosensors that contain heteroatoms like sulfur, nitrogen and, oxygen for fluorimetric and colorimetric detection of Zn(II) ions in different environmental, agricultural, and biological samples. Further, the sensing performances of these chemosensors have been compared and discussed which could help the readers for the future design of organic fluorescent and colorimetric chemosensors for the detection of Zn(II) ions. We hope this study will support the new thoughts to design a simple, efficient, selective, and sensitive chemosensor for the detection of Zn(II) ions in different samples (environmental, agricultural, and biological).

On the Electroanalytical Detection of Zn Ions by a Novel Schiff Base Ligand-SPCE Sensor

A novel bidentate Schiff base (L) is here proposed for the detection of Zn ions in water. The structure of the synthesized Schiff base L was characterized by FT-IR, ¹H NMR and ¹³C NMR. Optical characteristics were addressed by UV-Visible spectroscopy and Photoluminescence (PL) measurements. PL demonstrated that L displays a "turn-off" type fluorescence quenching in the presence of Zn²⁺ ion in aqueous solution, indicating its ability to preferentially coordinate this ion. Based on these findings, an L-M (where M is a suitable membrane) modified screen-printed carbon electrode (SPCE) was developed to evaluate the electrochemical behavior of the Schiff base (L) with the final objective of undertaking the electroanalytical determination of Zn ions in water. Using various electrochemical techniques, the modified L-M/SPCE sensor demonstrates high sensitivity and selectivity to Zn ions over some common interferents ions, such as Ca²⁺, Mg²⁺, K⁺, Ni⁺⁺ and Cd⁺⁺. The potentiometric response of the L-M/SPCE sensor to Zn ions was found to be linear over a relatively wide concentration range from 1 μM to 100 mM.

Stimuli-Responsive Zinc (II) Coordination Polymers: A Novel Platform for Supramolecular Chromic Smart Tools

The unique role of the zinc (II) cation prompted us to cut a cross-section of the large and complex topic of the stimuli-responsive coordination polymers (CPs). Due to its flexible coordination environment and geometries, easiness of coordination-decoordination equilibria, "optically innocent" ability to "clip" the ligands in emissive architectures, non-toxicity and sustainability, the zinc (II) cation is a good candidate for building supramolecular smart tools. The review summarizes the recent achievements of zinc-based CPs as stimuli-responsive materials able to provide a chromic response. An overview of the past five years has been organised, encompassing 1, 2 and 3D responsive zinc-based CPs; specifically zinc-based metallorganic frameworks and zinc-based nanosized polymeric probes. The most relevant examples were collected following a consequential and progressive approach, referring to the structure-responsiveness relationship, the sensing mechanisms, the analytes and/or parameters detected. Finally, applications of highly bioengineered Zn-CPs for advanced imaging technique have been discussed.

Recent progress in cadmium fluorescent and colorimetric probes

Cadmium is a heavy metal which exists widely in industrial and agricultural production and can induce a variety of diseases in organisms. Therefore, its detection is of great significance in the fields of biology, environment and medicine. Fluorescent probe has been a powerful tool for cadmium detection because of its convenience, sensitivity, and bioimaging capability. In this paper, we reviewed 98 literatures on cadmium fluorescent sensors reported from 2017 to 2021, classified them according to different fluorophores, elaborated the probe design, application characteristics and recognition mode, summarized and prospected the development of cadmium fluorescent and colorimetric probes. We hope to provide some help for researchers to design cadmium fluorescent probes with higher selectivity, sensitivity and practicability.

Cadmium is a heavy metal which exists widely in industrial and agricultural production and can induce a variety of diseases in organisms.

Synthesis of an unprecedented H-stitched binuclear crystal structure based on selective fluorescence recognition of Zn2+ in newly synthesized Schiff base ligand with DFT and imaging application in living cells

Single hydrogen atom stitched giant binuclear crystal.

Recent advances of BINOL-based sensors for enantioselective fluorescence recognition

Enantioselective fluorescent sensors show large potential for fast, real-time, and highly sensitive measurement of the concentration and enantiomeric composition of chiral molecules. Among all of the sensors, BINOL-based sensors have been actively investigated and extensively used to carry out highly enantioselective, sensitive recognition of chiral α-hydroxycarboxylic acids, amino acids, amino acid derivatives, amino alcohols and amines. In this manuscript, the recent progress of chiral BINOL-based sensors for enantioselective fluorescence recognition of different substrates is reviewed and discussed. The structure of BINOL is tuned by introducing various groups or molecules which systematically changed its fluorescence properties and offered potential for rapid assays of chiral organic molecules. From the development of this area, we gain fresh insight into the challenges and chances of BINOL-based sensors.

A Fluorescent Chemosensor Based on Schiff Base for the Determination of Zn2+, Cd2+and Hg2

Metal complexes were obtained by the reaction of zinc, cadmium and mercury(II) salts with Schiff base HL (N(salicylidene)benzylamine). HL was synthesized by the condensation reaction of benzylamine and 2-hydroxybenzaldehyde. The fluorescence properties of the Schiff base and its metal complexes were studied in ethanol-water solutions. HL was examined for its utility as a fuorescent chemosensor for the determination of Zn²⁺, Cd²⁺ and Hg²⁺ in aqueous samples. The HL chemosensor was found to be sensitive to Zn²⁺, Cd²⁺ and Hg²⁺ than some metal ions and its complexes emitted strong fluorescence at 452 nm for Zn²⁺ at 474 nm for Cd²⁺ and at 491 nm for Hg²⁺, respectively. It was determined that HL forms complexes with a ratio of 2:1 for Zn²⁺ and Hg²⁺ and with a ratio of 1:1 for Cd²⁺ by Job plots. For the detection of Zn²⁺, Cd²⁺ and Hg²⁺ in aqueous samples, pH, solvent type and ligand concentration were optimized for an analytical method based on HL chemosensor. HL gave a wide range of linearity with Zn²⁺, Hg²⁺ and Cd²⁺, the limit of detection was found to be 2.7 × 10^-7 M, 7.5 × 10^-7 M and 6.0 × 10^-7 M, respectively.

A fluorescence probe based on 6-phenylimidazo[2,1-b]thiazole and salicylaldehyde for the relay discerning of In3+ and Cr3+

A new fluorescence probe, (E)-N′-(2-hydroxybenzylidene)-6-phenylimidazo[2,1-b]thiazole-3-carbohydrazide (LB1), based on 6-phenylimidazo[2,1-b]thiazole and salicylaldehyde was designed and synthesized.

A Novel Thiophene-Based Fluorescent Chemosensor for the Detection of Zn2+ and CN-: Imaging Applications in Live Cells and Zebrafish

A novel fluorescent turn-on chemosensor DHADC ((E)-3-((4-(diethylamino)-2-hydroxybenzylidene)amino)-2,3-dihydrothiophene-2-carboxamide) has been developed and used to detect Zn2+ and CN-. Compound DHADC displayed a notable fluorescence increase with Zn2+. The limit of detection (2.55 ± 0.05 μM) for zinc ion was far below the standard (76 μM) of the WHO (World Health Organization). In particular, compound DHADC could be applied to determine Zn2+ in real samples, and to image Zn2+ in both HeLa cells and zebrafish. Additionally, DHADC could detect CN- through a fluorescence enhancement with little inhibition with the existence of other types of anions. The detection processes of compound DHADC for Zn2+ and CN- were demonstrated with various analytical methods like Job plots, 1H NMR titrations, and ESI-Mass analyses.

A coumarin derivative as a "turn-on" fluorescence probe toward Cd2+ in live cells

A novel coumarin-derived Schiff base fluorescence probe (CTB) has been successfully designed and synthesized through exploiting tris-(2-aminothyl)-amine moiety as a recognition unit for the highly selective and sensitive detection of Cd²⁺. It is based on CN isomerization and the photo-induced electron transfer (PET) mechanism. The investigation into the sensing processes showed that CTB exhibited an excellent selectivity for Cd²⁺. The sensitivity exceeded that of other competing metal ions, and had a high sensitivity, a detection limit of 1.16 × 10^-7 M with the association constants of 1.37 × 10¹¹ M^-2. The experiments including Job's plot, UV-Vis titration, ¹H NMR titration and ESI-MS spectrum established that the probe CTB binds to Cd²⁺ in a 1:2 ratio. Further studies also demonstrated that probe CTB can be successfully applied to the fluorescence imaging of Cd²⁺ in HepG-2 cells.

A Highly Selective Turn-on Fluorescent Probe for the Detection of Aluminum and Its Application to Bio-Imaging

Aluminum is the most abundant metallic element in the Earth's crust and acts as a non-essential element for biological species. The accumulation of excessive amounts of aluminum can be harmful to biological species. Thus, the development of convenient and selective tools for the aluminum detection is necessary. In this work, a highly selective aluminum ion fluorescent probe N'-(2,5-dihydroxybenzylidene)acetohydrazide (Al-II) has been successfully synthesized and systemically characterized. The fluorescence intensity of this probe shows a significant enhancement in the presence of Al³⁺, which is subject to the strong quench effects caused by Cu²⁺ and Fe³⁺. The binding ratio of probe-Al³⁺ was determined from the Job's plot to be 1:1. Moreover, the probe was demonstrated to be effective for in vivo imaging of the intracellular aluminum ion in both living Drosophila S2 cells and Malpighian tubules.

A novel aluminum-sensitive fluorescent chemosensor based on 4-aminoantipyrine: An experimental and theoretical study

A practical and an efficient Schiff base fluorescent chemosensor, salicylidene-4-aminoantipyrinyl-4-aminophenol (A2) has been synthesized through the condensation procedure of 1-phenyl-2,3-dimethyl-4-(N-2-hydroxybenzylidene)-3-pyrazoline-5-one and 4-aminophenol. Compound A2 has displayed a considerable fluorescence enhancement with high selectivity and sensitivity toward Al³⁺ ion and exhibited an emission band at 484 nm, which contained a low detection limit (LOD) of 1.06 × 10^-7 M. In accordance to the experimental study, DFT, TDDFT calculations, and the enhancement of fluorescence intensity might be attributed to the inhibition of Photoinduced Electron Transfer (PET) along with the Excited-State Intramolecular Proton Transfer (ESIPT). As it has been specified by Job's plot and DFT calculations, the binding stoichiometries of A2 with Al³⁺ are 1:1, while the association constant (Ka) of Al³⁺ has been calculated and observed to be 2.67 × × 10⁵ M^-1. Furthermore, the binding behavior and sensing mechanism of A2 with Al³⁺ have been confirmed through the experiments of ¹H NMR titration.

A fluorescent and colorimetric Schiff base chemosensor for the detection of Zn2+ and Cu2+: Application in live cell imaging and colorimetric test kit

A novel Schiff base chemosensor HMID, ((E)‑1‑((2‑hydroxy‑3‑methoxybenzylidene)amino)imidazolidine‑2,4‑dione), have been designed and synthesized. Sensor HMID showed a selectivity to Zn²⁺ through fluorescence enhancement in aqueous solution. Its detection limit was analyzed as 11.9 μM. Importantly, compound HMID could be applied to image Zn²⁺ in live cells. Detection mechanism of Zn²⁺ by HMID was suggested to be an effect of chelation-enhanced fluorescence (CHEF) by DFT calculations. Moreover, HMID could detect Cu²⁺ with a change of color from colorless to pink. The selective detection mechanism of Cu²⁺ by HMID was demonstrated to be the promotion of intramolecular charge transfer band by DFT calculations. Additionally, HMID could be employed as a naked-eye colorimetric kit for Cu²⁺. Therefore, HMID has the ability as a 'single sensor for dual targets'.

A simple quinolone Schiff-base containing CHEF based fluorescence 'turn-on' chemosensor for distinguishing Zn2+ and Hg2+ with high sensitivity, selectivity and reversibility

A new simple 'dual' chemosensor MQA ((E)-2-methoxy-N-((quinolin-2-yl)methylene)aniline) for distinguishing Zn²⁺ and Hg²⁺ has been designed, synthesized and characterized. The sensor showed excellent selectivity and sensitivity with a fluorescence enhancement to Zn²⁺/Hg²⁺ over other commonly coexisting cations (such as Na⁺, Mg²⁺, Al³⁺, K⁺, Mn²⁺, Fe²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Ga³⁺, Cd²⁺, In³⁺ and Pb²⁺) in DMSO-H₂O solution (1/99 v/v), which was reversible with the addition of ethylenediaminetetraacetic acid (EDTA). The detection limit for Zn²⁺/Hg²⁺ by MQA both reached the 10^-8 M level. The 1 : 1 ligand-to-metal coordination patterns of the MQA-Zn2+ and MQA-Hg2+ were calculated through a Job's plot and ESI-MS spectra, and were further confirmed by X-ray crystal structures of complexes MQA-Zn2+ and MQA-Hg2+. This chemosensor can recognize similar metal ions by coherently utilizing intramolecular charge transfer (ICT) and different electronic affinities of various metal ions. DFT calculations have revealed that the energy gap between the HOMO and LUMO of MQA has decreased upon coordination with Zn(ii)/Hg(ii).

A new fluorene-based Schiff-base as fluorescent chemosensor for selective detection of Cr3+ and Al3

2-((9H-fluoren-2-ylimino) methyl)phenol (F3) was synthesized by condensation reaction of 9H-fluoren-2-amine and 2-hydroxybenzaldehyde in EtOH and characterized by its melting point, ¹H-,¹³C NMR and molecular mass. F3 exhibits a high selectivity for detection of Cr³⁺ and Al³⁺ ions as a fluorescent chemosensor and showed a single emission band at 536nm upon excitation at 333nm according to fluorescence emission studies. The addition of Cr³⁺ and Al³⁺ make a significant increase in fluorescent intensity at 536nm in CH₃CN, while other metal ions have almost no influence on the fluorescence. The fluorescence enhancement was attributed to the inhibited CN isomerization and the obstructed excited state intra-molecular proton transfer (ESIPT) of compound F3. Job's plot and DFT calculations data showed that the binding stoichiometries of F3 with Cr³⁺ and Al³⁺ are 2:1. The association constants (K_a) for Cr³⁺ and Al³⁺ were calculated and found to be 8.33×10⁴M^-1 and 5.44×10⁴M^-1, respectively. The detection limits were also calculated for Cr³⁺ and Al³⁺ and found to be 2.5×10^-7mol/L and 3.1×10^-7mol/L, respectively.

Origin of green photoluminescence in four-ring bent-core molecules with ESIPT, selective sensing of zinc ions by turn-on emission and their liquid crystal properties

Fluorescent four-ring symmetrical/unsymmetrical molecules containing alkyl chains of a varied number of –CH₂– groups with a bent-core have been synthesized to explore their liquid crystalline (LC) and photophysical properties.

A biomimetic fluorescent chemosensor for highly sensitive zinc(ii) detection and its application for cell imaging

To fabricate a novel biomimetic fluorescent chemosensor, PSaAEMA-co-PMPC was synthesized via atom transfer radical polymerization, and this copolymer could be used for the detection of zinc(ii) and cell imaging. A series tests with various metal ions verified the specific fluorescence response behavior. This novel biomimetic fluorescent chemosensor exhibits excellent selectivity for Zn²⁺ ions over a wide range of tested metal ions in an aqueous solution. Moreover, cytotoxicity and bio-imaging tests were conducted to study the potential bio-application of the chemosensor. Owing to the biomimetic portion (phosphorylcholine), this copolymer possesses outstanding biocompatibility and could clearly image cells. The results indicated that PSaAEMA-co-PMPC has great potential for application in zinc(ii) detection and cell imaging.

To fabricate a novel biomimetic fluorescent chemosensor, PSaAEMA-co-PMPC was synthesized via atom transfer radical polymerization, and this copolymer could be used for the detection of zinc(ii) and cell imaging.

A systematic series of fluorescence chemosensors with multiple binding sites for Hg(ii) based on pyrenyl-functionalized cyclotriphosphazenes and their application in live cell imaging

A systematic series of fluorescence chemosensors based on cyclotriphosphazene derivatives were designed, synthesized, and evaluated for their sensing behaviors toward metal ions.

Synthesis, characterization, second and third order non-linear optical properties and luminescence properties of 1,10-phenanthroline-2,9-di(carboxaldehyde phenylhydrazone) and its transition metal complexes

The requirement for materials which exhibit good second and third order non-linear optical properties and also for materials which could sense metals in trace quantities has kindled renewed investigations. Organometallics and coordination compounds show a lot of promise as new NLO materials combining the variety of organic moieties with the strength and variable oxidation states of metals. Especially ligands which selectively detect industrial pollutants like Cd and biologically significant metals like Zn are necessary. In the current work the ligand 1,10-phenanthroline-2,9-di(carboxaldehyde phenylhydrazone) (L) and its Ni2+, Co2+, Fe2+, Zn2+, Cd2+ and Ir3+ complexes were synthesized. These were characterized by UV-Vis, FT-IR, 1H NMR, MS and CHN microanalysis techniques. The complexes were shown to have the formula [ML]2+. The second and third order NLO of the ligand and its complexes were recorded These new compounds were found to have same order of third order nonlinear optical susceptibility as that of CS2 and their second hyperpolarizability was an order of magnitude greater than that of C60. Furthermore the ligand also displays selective luminescence sensing of metals ions Fe2+ and Ir3+ even in the presence of other metal ions.

New approach for the quantification of metallic species in healthcare products based on optical switching of a Schiff base possessing ONO donor set

A new method is reported for the quantification of some metallic components of healthcare products utilizing a Schiff base chelator derived from 2-hydroxyacetophenone and ethanolamine. The Schiff base chelator recognizes some metallic species such as iron, copper and zinc (important components of some healthcare products), and cadmium (common contaminant in healthcare products) giving colorimetric/fluorimetric response. It coordinates with Fe²⁺/Fe³⁺ and Cu²⁺ ions via ONO donor set and switches the colour to bright red, green and orange, respectively. Similarly, it switches 'ON' a fluorometric response when coordinates with Zn²⁺ and Cd²⁺ ions. In the present approach, detailed studies on the colorimetric and fluorimetric response of ONO Schiff base is investigated in detail. The Job plot for the complexation of ONO switch with various metal ions suggested formation of 1:1 (metal-chelator) complex with Fe²⁺, Fe³⁺, and Cu²⁺ while 1:2 (metal-chelator) for Zn²⁺ and Cd²⁺ ions. The limit of detection, limit of quantification are 6.73, 18.0, 25.0, 0.65, 1.10μM and 27.0, 72.0, 100.0, 2.60 and 4.40μM for Fe²⁺, Fe³⁺, Cu²⁺, Zn²⁺ and Cd²⁺ ions, respectively. Under the optimized conditions, chelator was used for the quantification of important metals present in healthcare products via direct dissolution and furnace treatment during sample preparation. The results were found precise and accurate for both sample preparation techniques using the developed method.

Non-conjugated fluorescent molecular cages of salicylaldehyde-based tri-Schiff bases: AIE, enantiomers, mechanochromism, anion hosts/probes, and cell imaging properties

Non-conjugated fluorescent molecular cages of salicylaldehyde-based tri-Schiff bases with a trimethylamine bridge have a small π-conjugated system but exhibit strong aggregation- and anion-induced emission.

A new turn-on benzimidazole-based greenish-yellow fluorescent sensor for Zn2+ions at biological pH applicable in cell imaging

A newly designed and structurally characterized non-cytotoxic benzimidazole containing quinazoline derivative (HL) acts as a ‘turn-on’ greenish-yellow fluorescent sensor selective for Zn²⁺ions at as low as 39.91 nM in 5 mM HEPES buffer (DMSO/water: 1/5, v/v) at biological pH.

A Colorimetric and Fluorescent Chemosensor for the Selective Detection of Cu2+ and Zn2+ Ions

A new bi-functional chemosensor 1 based on 3,5-dichlorosalicylaldehyde and 2-(methylthio)aniline has been synthesized. It can detect Cu²⁺ with a color change from pale yellow to dark yellow in aqueous solution. The selective mechanism of 1 for Cu²⁺ was proposed to be the enhancement of the intramolecular charge transfer (ICT) band, which was explained by theoretical calculations. The sensor 1 could be used to detect and quantify Cu²⁺ in water samples. In addition, the sensor 1 displayed "turn-on" fluorescence response only to Zn²⁺, based on an effect of chelation-enhanced fluorescence (CHEF). Therefore, 1 can serve as a 'single sensor for two different targets' with dual modes.