Photoinduced electron transfer (PET) based Zn2+ fluorescent probe: transformation of turn-on sensors into ratiometric ones with dual emission in acetonitrile

Fluorescent and Electrochemical Sensor Based on Basic Red 9 Dye Functionalised Graphene Oxide-Montmorillonite Composite for Selective Detection of Cerium (III) Ion

In this work, graphene oxide (GO) has been prepared from used dry cells using modified Hummer's method and encapsulated with montmorillonite clay. To enhance its electrical property, the GO-MMT composite has been functionalised with Basic Red 9 dye. The sensor was characterized by various spectroscopic techniques like FT-IR spectroscopy, PXRD, SEM analysis, etc. Basic Red 9 dye functionalised GO-MMT composite has been employed for fluorescent and electrochemical detection of Ce3+ ion. The fluorescent turn-on sensing is sensitive, reversible and free from interference from other metal ions. The detection of Ce3+ ion by the sensor was also conducted in bovine serum albumin (BSA) medium. Pt electrode modified with the hybrid sensor produces excellent electrochemical change in presence of Ce3+ ion through cyclic voltammetry and square wave voltammetry technique. The limit of detection (LOD) from fluorescence spectroscopy, cyclic voltammetry and square wave voltammetry were calculated to be 0.6556 × 10- 9 M, 1.232 × 10- 9 M and 1.923 × 10- 9 M respectively.

Lanthanide ternary complex as a fluorescent probe for highly sensitive and selective detection of copper ions based on selective recognition and photoinduced electron transfer

Copper ions have a very important role in human health, industrial and agricultural production. Herein, lanthanide ternary complex of 2,6-pyridinedicarboxylic acid (DPA)-Eu³⁺-polyethyleneimine (PEI) as a fluorescent probe was thus fabricated for highly sensitive and selective detection of copper ions. PEI itself is non-fluorescent, the PEI-Eu³⁺complex is also non-fluorescent, and PEI has specific recognition to copper ions due to its higher affinity ability to copper ion than other metal ions. It was found that Cu²⁺ ions cannot quench the characteristic fluorescence of Eu³⁺ in the DPA-Eu³⁺ system, while in the DPA-Eu³⁺-PEI system, Cu²⁺ ions can greatly quench the characteristic fluorescence of Eu³⁺ due to photoinduced electron transfer (PET). The luminescent and quenching mechanism was also discussed in detail. The DPA-Eu³⁺-PEI probe not only has high sensitivity and selectivity, but also has very rapid fluorescence response and the response time is only 1 min. A good linear relationship between the fluorescence ratios of F₀/F and the concentrations of Cu²⁺ was obtained in the range of 0.02 ∼ 10.0 μM (R² = 0.998), and the limit of detection (LOD) is 8.0 nM. The probe was successfully applied for the detection of Cu²⁺ ions in the lake and river water samples, wastewater and urine samples. This work may provide a new strategy for fabricating simple and effective fluorescence probe and a promising application for the rapid and on-site detection in environmental monitoring and biological fluids.

A "heat set" Zr-Diimide based Fibrous Metallogel: Multiresponsive Sensor, Column-based Dye Separation, and Iodine Sequestration

Sensing and monitoring hazardous contaminants in water and radioactive iodine sequestration is pivotal due to their detrimental impact on biological ecosystems. In this context, herein, a water stable zirconium-diimide based metallogel (Zr@MG) with fibrous columnar morphology is accomplished through the "heat set" method. The presence of diimide linkage with long aromatic chain manifests active luminescence properties in the linker as well as in the supramolecular framework structure. The as-synthesized Zr@MG xerogel can selectively detectCr₂O₇^2- (LOD = 0.52 ppm) and 2,4,6-trinitrophenol (TNP) (LOD = 80.2 ppb) in the aqueous medium. The Zr@MG paper strip-based detection for Cr₂O₇^2- and nitro explosive makes this metallogel reliable and an attractive luminescent sensor for practical use. Moreover, a column-based dye separation experiment was performed to show selective capture of positively charged methylene blue (MB) dye with 98 % separation efficiency from the mixture of two dyes. Also, the Zr@MG xerogel showed effective iodine sequestration from the vapor phase (232 wt%).

A quinazolin-based Schiff-base chemosensor for colorimetric detection of Ni2+ and Zn2+ ions and 'turn-on' fluorometric detection of Zn2+ ion

Herein, we have reported a novel quinazolin-based Schiff base chemosensor (E)-2-benzamido-N'-(1-(pyridin-2-yl)ethylidene)benzohydrazide (L). L has been designed, synthesised and characterised by ¹H-NMR, IR spectroscopy, ESI-MS spectrometry and theoretical studies. The receptor showed appreciable colorimetric λ _max shift for both Ni²⁺ and Zn²⁺ ions and fluorometric "turn on" response in presence of only Zn²⁺ ion. The Jobs plot analysis revealed that receptor forms 2 : 1 complex with both the ions Ni²⁺ and Zn²⁺, further confirmed by ESI-MS analysis. The single crystal structure of L-Ni²⁺ complex (1) has also been determined. The colorimetric detection limits were calculated to 7.9 nM and 7.5 nM respectively for Ni²⁺ and Zn²⁺ in methanol-Tris-HCl buffer medium (10 mM, pH 7.2, 1 : 1 v/v). The chemosensor L can be applied for the recovery of contaminated water samples.

A Highly Selective Turn-On Fluorescent Probe for the Detection of Zinc

A novel turn-on fluorescence probe L has been designed that exhibits high selectivity and sensitivity with a detection limit of 9.53 × 10⁻⁸ mol/L for the quantification of Zn²⁺. ¹H-NMR spectroscopy and single crystal X-ray diffraction analysis revealed the unsymmetrical nature of the structure of the Schiff base probe L. An emission titration experiment in the presence of different molar fractions of Zn²⁺ was used to perform a Job’s plot analysis. The results showed that the stoichiometric ratio of the complex formed by L and Zn²⁺ was 1:1. Moreover, the molecular structure of the mononuclear Cu complex reveals one ligand L coordinates with one Cu atom in the asymmetric unit. On adding CuCl₂ to the ZnCl₂/L system, a Cu-Zn complex was formed and a strong quenching behavior was observed, which inferred that the Cu²⁺ displaced Zn²⁺ to coordinate with the imine nitrogen atoms and hydroxyl oxygen atoms of probe L.

A highly selective sequential recognition probe for Zn2+ and HSO4-/H2PO4- based on a diarylethene chemosensor

A novel diarylethene derivative chemosensor DTP-o connected to Schiff base unit for fluorescent detection of Zn²⁺ and relay-detection of HSO₄^-/H₂PO₄^- was designed and synthesized successfully. DTP-o displayed excellent photochromism and fluorometric sensing toward Zn²⁺ to form DTP-o-Zn²⁺ complex in acetonitrile with the detection limit of 5.62 × 10^-7 M. And the form of DTP-o combined with Zn²⁺ could further be verified by Job's plot titrations and mass spectrometry analysis. Furthermore, the complex of DTP-o-Zn²⁺ showed an excellent characteristic of fluorescent relay-response toward HSO₄^- and H₂PO₄^- with high sensitivity and selectivity. The detection limits for HSO₄^- and H₂PO₄^- were as low as 3.04 × 10^-8 M and 3.41 × 10^-8 M, respectively. Moreover, the sensor DTP-o could also be applied to detect Zn²⁺ on practical samples and test strips with high accuracy.

Smart Bimodal Imaging of Hypochlorous Acid In Vivo Using a Heterobimetallic Ruthenium(II)-Gadolinium(III) Complex Probe

A unique heterobimetallic Ru(II)-Gd(III) complex, Ru-AN-Gd, is reported to serve as an effective probe for bimodal phosphorescence-magnetic resonance (MR) imaging of hypochlorous acid (HClO) in vitro and in vivo. The probe was designed by incorporating a MR contrast agent, Gd-DOTA, into a HClO-responsive bipyridine-Ru(II) complex derivative. The specific reaction between Ru-AN-Gd and HClO triggers the cleavage of an ether bond in the probe molecule, resulting in phosphorescence turn-on and MR turn-off responses to HClO. The integration of MR and phosphorescence detection modes allows the probe to be employed for detecting HClO in a quite wide concentration range (0.6-2000 μM) and for imaging HClO at various resolutions ranging from the subcellular level to the whole body without a depth limit. Its applicability was demonstrated by phosphorescence imaging of lysosomal HClO in live cells, visualization of HClO generation in a mouse arthritis model, and bimodal phosphorescence-MR imaging of HClO in drug-induced acute liver and kidney injury of a mouse. The research achievements suggested the potential of Ru-AN-Gd for diagnosis and treatment monitoring of HClO-related disease.

Competitive hydrogen bonding influences of fluorophore- urea-adenine system in water: Photophysical and photochemical approaches

Photophysical and photochemical investigation of photoinduced electron transfer (PET)-based acridinedione dye (ADR1) with urea in the presence of a nitrogenous base (adenine) were carried out in water. Urea suppresses the PET resulting in a fluorescence enhancement and the extent of binding is correlated and governed by the number of urea molecules surrounding the close vicinity of dye. On the contrary, adenine forms a true 1:2 complex with dye. Presence of adenine in dye-urea microenvironment results in the displacement of dye from the vicinity of urea molecules. The stability of dye-urea network in the presence of adenine reveals that the microenvironment of dye is governed and influenced by both urea and adenine. Introduction of adenine to dye-urea results in the formation of several hydrogen bonding assemblies that are competitive and influences the excited state characteristics of ADR1 dye. The micro assemblies comprise dye-urea (DU), dye-adenine (DA), urea-adenine (UA), urea-water (UW), urea-urea (UU), and adenine-water (AW) framework and the existence of several competitive hydrogen bonding results in a large variation in fluorescence properties of ADR1 dye. The presence of several assemblies also signifies that no confined phase selectively of DU or DA assemblies exist in any stoichiometric proportion in the aqueous phase. The binding constant, the variation in the fluorescence lifetime and its relative amplitude of DA in the presence of urea authenticate that the binding nature of dye-urea-adenine (DUA) is dependent on the several hydrogen bonding assemblies that coexist at any concentration. The extent of hydrogen bonding of DA is found to be entirely different from that of urea. Further, urea resulted in changes in the transient absorption peak of dye with a large variation in lifetime and shift of the transient absorption peaks. Fluorescence spectral techniques are used as an efficient tool in elucidating the binding nature of DU framework in the presence of non-fluorescent hydrogen-bonding solute like adenine.

Two similar Schiff-base receptor based quinoline derivate: Highly selective fluorescent probe for Zn(II)

As is known, Zn²⁺ plays a vital role in a variety of biological processes but excessive exposure of Zn²⁺ to human beings can cause toxicity, inducing a series of overt poisoning symptoms and neurodegenerative disorders. Thus, we designed and synthesized two quinoline-derived Schiff-bases HL₁ and HL₂, and investigated the fluorescence emission responses of these two Schiff-bases to various metal ions. A significant enhancement in fluorescence emission band centered at 450 nm was observed in the ethanolic solution of HL₁ with addition of Zn²⁺, while remarkably lower fluorescence emission enhancement was obtained in the case of HL₂ in which one methyl group was introduced to the azomethine carbon. In addition, HL₁ showed good selectivity and high sensitivity towards Zn²⁺ in the existence of other various interfering metal ions, and the reversibility and regeneration of HL₁ were also perfect for extending its applications in environmental and biological systems. Therefore, HL₁ could be identified as a fluorescent probe for sensing Zn²⁺ environmentally and biologically.

Cu(II) Ion Adsorption by Aniline Grafted Chitosan and Its Responsive Fluorescence Properties

The detection and removal of heavy metal species in aquatic environments is of continued interest to address ongoing efforts in water security. This study was focused on the preparation and characterization of aniline grafted chitosan (CS-Ac-An), and evaluation of its adsorption properties with Cu(II) under variable conditions. Materials characterization provides support for the grafting of aniline onto chitosan, where the kinetic and thermodynamic adsorption properties reveal a notably greater uptake (>20-fold) of Cu(II) relative to chitosan, where the adsorption capacity (Q_m) of CS-Ac-An was 106.6 mg/g. Adsorbent regeneration was demonstrated over multiple adsorption-desorption cycles with good uptake efficiency. CS-Ac-An has a strong fluorescence emission that undergoes prominent quenching at part per billion levels in aqueous solution. The quenching process displays a linear response over variable Cu(II) concentration (0.05–5 mM) that affords reliable detection of low level Cu(II) levels by an in situ “turn-off” process. The tweezer-like chelation properties of CS-Ac-An with Cu(II) was characterized by complementary spectroscopic methods: IR, NMR, X-ray photoelectron (XPS), and scanning electron microscopy (SEM). The role of synergistic effects are inferred among two types of active adsorption sites: electron rich arene rings and amine groups of chitosan with Cu(II) species to afford a tweezer-like binding modality.

Design and Applications of Small Molecular Probes for Calcium Detection

The physiological significance of calcium ions such as the role in cellular signalling, cell growth, etc. have driven the development of methods to detect and monitor the level of Ca²⁺ ions, both in vivo and in vitro. Although various approaches for the detection of calcium ions have been reported, methods based on small molecular fluorescent probes have unique advantages including small probe size, easy monitoring of detection processes and applicability in biological systems. In this review article, we will discuss the progress in the development of Ca²⁺ -binding fluorescent probes by taking into account the types of chelating groups that have been employed for Ca²⁺ binding.

Fluorescence Spectral Studies on the Interaction of Alanine and Valine with Resorcinol-Based Acridinedione Dyes in Aqueous Solution: A Comparative Study with Glycine

Photophysical studies were carried out for simple amino acids like alanine and valine with resorcinol-based aqueous acridinedione (ADDR) dyes. ADDR dyes exhibit interesting excited-state characteristics on altering the substituents at the 9th and 10th sites (Scheme 1). The longest-wavelength absorption maxima remain the same on adding the amino acids to the fluorophore, whereas the excited-state behavior varies significantly mostly based on the nature of the substituent at the 9th position. The absence of fluorescence enhancement was observed with addition of β-alanine, l-alanine, and l-valine to ADDR1 dye (photoinduced electron transfer, PET), whereas addition of glycine exhibits enhancement accompanied with a shift toward a longer-wavelength region. Interestingly, the addition of amino acids to non-PET dyes results in a fluorescence quenching accompanied with a larger shift toward the shorter-wavelength region. The properties of fluorophore and nonfluorophore dyes in the presence of alanine or valine are found to be entirely different from those of glycine. The interaction of alanine with ADDR dyes is predominantly through H-bonding, but the structural aspects of H-bonding interactions of alanine and water are completely different from those of glycine and water. The time-correlated single-photon counting method portrays the existence of fluorophore in two distinguishable microenvironments in the presence of amino acids. The fluorescence spectral technique used as a tool in elucidating the mode of interaction of dye with neutral amino acids in aqueous solution is illustrated in the present study.

A highly sensitive fluorescent probe with different responses to Cu2+ and Zn2

We report a highly sensitive fluorescent probe based on p-dimethylaminobenzoyl derivatives (probe L) for the detection of Cu²⁺ and Zn²⁺. In this work, the probe L exhibited a fluorescent turn-on sensing model to Cu²⁺ and Zn²⁺ with a distinct fluorescent color change from colorless to green and yellow respectively. Probe L exhibited high selectivity as a fluorescent Cu²⁺/Zn²⁺ probe with a limit of detection (LOD) of 45 nM/17 nM. The results of ¹H NMR titrations revealed that the response of L to Cu²⁺ and Zn²⁺ was triggered by the interaction of the thiophene unit and the metal ion. Furthermore, the fluorescence titrations and Job's plot curves displayed the binding ratio of 1:2 for Cu²⁺ and 1:1 for Zn²⁺ metal-L complex formation respectively. Density functional theory calculation also demonstrated the possibility of molecular luminescence and the process of metal-L complex formation. Additionally, fluorescent test strips have been prepared for convenient detection of Cu²⁺ and Zn²⁺, which means the convenient and rapid assay in real samples can be achieved.

Fluorenone based fluorescent probe for selective "turn-on" detection of pyrophosphate and alanine

To sense biologically important entities with different size and dimensions, a fluorenone based fluorescent receptor was designed and synthesized. Probe 1 displayed a distinct fluorescence enhancement emission at 565nm for pyrophosphate and 530nm for alanine in polar solvent. The fluorescence titration experiments confirm 1:1 stoichiometric ratio with high-binding constant and very low limit of detection (LoD) values. Receptor 1 showed a highly selective and sensitive recognition to HP₂O₇^3- and to alanine over other competitive anions and amino acids. In addition, the fluorescence lifetime measurement and reversible binding study results support the practical importance of 1.

A Fluorescent Chemosensor for Zn2+ Based on a C3-Symmetrical and Pre-Organized 2,2′,2″-Nitrilotribenzoic Acid Material

A C3-symmetrical 4,4″,4⁗-nitrilotris(2′-methyl-[1,1′-biphenyl]-3-carboxylic acid) (4) derived from nitrilotriacetic acid (NTA) was found to selectively bind Zinc(ii) ions both in DMSO or MeOH. A synergistic effect of the anionic counter ion SO42− on the sensing behaviour of 4 to metal ions was clearly observed in DMSO. Interestingly, 4 showed a rapid hypochromatic shift in emission ascribed to the deprotonation and the concomitant formation of a 4–metal complex upon the addition of Zn2+ ions, instead of the bathochromic shift and emission enhancement attributed to the SO42−-involved hydrogen-bonding interaction for Ni2+, Li+, Mg2+, and Na+ ions at ratios below 1:1 in DMSO. The observed sensing process of sulfate salts associated with the SO42−-involved hydrogen-bonding interaction, deprotonation, and the concomitant complexation can also be clearly monitored by titration methods utilising UV-vis, fluorescence, and NMR spectroscopy in solution. In comparison with 4, compound 1 showed an obvious difference in the binding interaction with zinc sulfate in MeOH, probably owing to the decreased acidity. Anion-induced hydrogen-bonding interactions and deprotonation of the COOH protons in the excited state also endowed 4 versatile spectroscopic properties. The addition of F− and SO42− anions resulted in a remarkable enhancement probably related with a rigidifying effect. 2,2′,2″-Nitrilotribenzoic acid can be utilised as a potential scaffold to build a series of conjugated fluorescent sensors by its chelation effect owing to the rigid cavity pre-organised by the triphenylamine moiety and the carboxylic groups and the conjugation extension in the 4,4′,4″ positions.

Bulk Aggregation Based Fluorescence Turn-On Sensors for Selective Detection of Progesterone in Aqueous Solution

Steroids are polycyclic compounds that share tetracyclic ring as core scaffold, and selective detection of a steroid is challenging owing to their structural similarities. The discovery of chemosensors that recognize progesterone by alteration of self-aggregation state is described, and these show significant fluorescence turn-on. A self-aggregated 48-membered dansyl library was screened against a series of metabolites in aqueous buffer and discovered two compounds (PG-1, PG-2) exhibited exceptional selectivity for progesterone. Following studies of aggregation properties of probes using dynamic light scattering and transmission electron microscopy supports progesterone recognition lead to the generation of bulk aggregates that induce fluorescence enhancement. Though many fluorescence sensing mechanisms have been proposed, a sensing mode based on the bulk aggregate formation of fluorophore has never been reported, and this may open a new avenue of chemosensor design.

Progressive structural modification to a zinc-actuated photoinduced electron transfer (PeT) switch in the context of intracellular zinc imaging

Zinc-targeting sensors that contain an anilino-based electron donor (aka, the PeT switch) are reported.

Recent progress in the design and applications of fluorescence probes containing crown ethers

Crown ethers, discovered by the winner of the Nobel Prize Charles Pedersen, are cyclic chemical compounds that consist of a ring or multiple rings containing several ether groups that are capable of binding various ions.

The development of fluorescence turn-on probe for Al(III) sensing and live cell nucleus-nucleoli staining

The morphology of nucleus and nucleolus is powerful indicator of physiological and pathological conditions. The specific staining of nucleolus recently gained much attention due to the limited and expensive availability of the only existing stain "SYTO RNA-Select". Here, a new multifunctional salen type ligand (L1) and its Al3+ complex (1) are designed and synthesized. L1 acts as a chemosensor for Al3+ whereas 1 demonstrates specific staining of nucleus as well as nucleoli. The binding of 1 with nucleic acid is probed by DNase and RNase digestion in stained cells. 1 shows an excellent photostability, which is a limitation for existing nucleus stains during long term observations. 1 is assumed to be a potential candidate as an alternative to expensive commercial dyes for nucleus and nucleoli staining.

Acridine derivative as a “turn on” probe for selective detection of picric acid via PET deterrence

A new acridine based photoinduced electron transfer based fluorescent chemosensor for the detection of nitroaromatic compounds is reported. The probe exhibited high selectivity and sensitivity for the detection of picric acid over trivial explosive in CH₃OH.

DFT/TD-DFT calculations on the sensing mechanism of a dual response near-infrared fluorescent chemosensor for superoxide anion and hydrogen polysulfides: photoinduced electron transfer

Fluorescence quenching by the PET process in HCy-FN.

A turn-on fluorescent sensor for relay recognition of two ions: from a F−-selective sensor to highly Zn2+-selective sensor by tuning electronic effects

A simple ion sensor bearing quinoline and an amide group was designed and synthesized, which showed both colorimetric detection for F⁻ and a fluorescence turn-on response for Zn²⁺. Moreover, sensor L2 can distinguish F⁻ and Zn²⁺via different sensing mechanisms.

Synthesis and characterization of monometallic rhenium(i) complexes and their application as selective sensors for copper(ii) ions

Monometallic Re(i) complexes, selective and sensitive sensor of Cu(ii) ion, show substantial enhancement in the emission intensity, quantum yield, and lifetime due to the restriction of CN isomerization.

Engineering a DNA-cleaving DNAzyme and PCR into a simple sensor for zinc ion detection

The development of a simple sensor (9NL27-Zn) based on DNAzyme and PCR and aimed at the detection of low concentrations of zinc (II) ions is described. A specific Zn(II)-dependent DNAzyme (9NL27) with DNA-cleaving activity was employed. In the presence of zinc (II), the DNAzyme hydrolyzed DNA substrate into two pieces (5' and 3' fragments), forming 3'-terminal hydroxyl in the 5' fragment and 5'-phosphate in the 3' fragments. Subsequently, the 5' fragment left the DNAzyme and bound a short DNA template. The 5' fragment was used as a primer and extended a single-stranded full-length template by Taq polymerase. Finally, this full-length template was amplified by PCR. The amplified products had a quantitative relationship with Zn(II) concentration. Under our experimental conditions, the DNA sensor showed sensitivity (10 nM) and high specificity for zinc ion detection. After improvement of the DNA sensor, the detection limit can reach 1 nM. The simple DNA sensor may become a DNA model for the detection of trace amounts of other targets.

A novel chromo- and fluorogenic dual sensor for Mg2+and Zn2+with cell imaging possibilities and DFT studies

A diformyl-p-cresol-8-aminoquinoline based probe exhibits dual colorimetric and fluorogenic properties on selective binding towards Mg²⁺and Zn²⁺. This probe could be made selective towards Mg²⁺over Zn²⁺in the presence of TPEN.

Studies on photocatalytic CO(2) reduction over NH2 -Uio-66(Zr) and its derivatives: towards a better understanding of photocatalysis on metal-organic frameworks

Metal-organic framework (MOF) NH2 -Uio-66(Zr) exhibits photocatalytic activity for CO2 reduction in the presence of triethanolamine as sacrificial agent under visible-light irradiation. Photoinduced electron transfer from the excited 2-aminoterephthalate (ATA) to Zr oxo clusters in NH2 -Uio-66(Zr) was for the first time revealed by photoluminescence studies. Generation of Zr(III) and its involvement in photocatalytic CO2 reduction was confirmed by ESR analysis. Moreover, NH2 -Uio-66(Zr) with mixed ATA and 2,5-diaminoterephthalate (DTA) ligands was prepared and shown to exhibit higher performance for photocatalytic CO2 reduction due to its enhanced light adsorption and increased adsorption of CO2 . This study provides a better understanding of photocatalytic CO2 reduction over MOF-based photocatalysts and also demonstrates the great potential of using MOFs as highly stable, molecularly tunable, and recyclable photocatalysts in CO2 reduction.