Selective and sensitive fluorescent turn‐off chemosensors for Fe3+

Two novel chemosensors (2a and 2b) were synthesized by facile condensation of the binding unit (l ‐histidine) and the fluorophores (anthracene and dansyl groups). Both of them displayed high selectivity and sensitivity towards Fe³⁺ over other metal ions in aqueous solution. The sensing mechanism was based on the paramagnetic property of Fe³⁺ that would lead to fluorescence quenching of the fluorophores when Fe³⁺ was bound to the recognition units. The results showed that l ‐histidine was a good coordination motif for Fe³⁺ and both the anthracene and dansyl groups can sensitively report the sensing information. Copyright © 2012 John Wiley & Sons, Ltd.     

A switch‐off fluorescence probe towards Pb(II) and cu(II) ions based on a calix[4]pyrrole bearing amino‐quinoline group

A new fluorescence receptor calix[4]pyrrole‐N‐(quinoline‐8‐yl) acetamide (CAMQ) containing a pyrrolic ring connected via the meso‐position was synthesized, purified and characterized by elemental analysis, NMR and mass spectroscopy. This compound was examined for its fluorescence properties towards different metal ions e.g. Ag(I), Hg(II), Co(II), Ca(II), Ni(II), Zn(II), Cr(II), Ba(II), Fe(II), Cu(II), Pb(II)and Mg(II) ions by spectrophotometry and spectrofluorometry. It was concluded that the compound (CAMQ) possessed significantly enhanced selectivity for Pb(II) and Cu(II) ions in dimethyl sulfoxide (DMSO) even at very low concentrations (1 μM). It exhibit ‘turn‐on’ fluorescence when exposed to Pb(II) and Cu(II) and did so in preference to other metal ions. The binding constants, stoichiometry and quantum yields have been determined. The quenching mechanism was assessed using the Stern–Volmer equation and was also discussed.     

Iron specificity of a biosensor based on fluorescent pyoverdin immobilized in sol-gel glass

Two current technologies used in biosensor development are very promising: 1. The sol-gel process of making microporous glass at room temperature, and 2. Using a fluorescent compound that undergoes fluorescence quenching in response to a specific analyte. These technologies have been combined to produce an iron biosensor. To optimize the iron (II or III) specificity of an iron biosensor, pyoverdin (a fluorescent siderophore produced by Pseudomonas spp.) was immobilized in 3 formulations of porous sol-gel glass. The formulations, A, B, and C, varied in the amount of water added, resulting in respective R values (molar ratio of water:silicon) of 5.6, 8.2, and 10.8. Pyoverdin-doped sol-gel pellets were placed in a flow cell in a fluorometer and the fluorescence quenching was measured as pellets were exposed to 0.28 - 0.56 mM iron (II or III). After 10 minutes of exposure to iron, ferrous ion caused a small fluorescence quenching (89 - 97% of the initial fluorescence, over the range of iron tested) while ferric ion caused much greater quenching (65 - 88%). The most specific and linear response was observed for pyoverdin immobilized in sol-gel C. In contrast, a solution of pyoverdin (3.0 μM) exposed to iron (II or III) for 10 minutes showed an increase in fluorescence (101 - 114%) at low ferrous concentrations (0.45 - 2.18 μM) while exposure to all ferric ion concentrations (0.45 - 3.03 μM) caused quenching. In summary, the iron specificity of pyoverdin was improved by immobilizing it in sol-gel glass C.     

Synthesis, photophysical characterisation and metal ion binding properties of new ligands containing anthracene chromophores

Two new fluorescent chemosensors for heavy metal ions have been synthesised and their photophysical properties have been investigated. They present a pyridyl-thioether-based binding site and the anthracene moiety as a chromophore. In the experimental conditions used, no evidence is found for the formation of complexes with Pb²⁺, Zn²⁺, Cd²⁺, and Ag⁺ ions. On the contrary, in acetonitrile solutions both ligands strongly bind Cu²⁺ and Hg²⁺ cations according to a 1:1 and a 1:2 (metal:ligand) stoichiometry. In these complexes, the intense luminescence typical of anthracene derivatives is almost completely quenched and this phenomenon can be mainly attributed to an intraligand electron transfer process from the anthracene chromophore to the complexed pyridine. These results are of interest for the development of new chemosensors for the design of efficient electronic tongues for the detection of transition metal ions.     

Characterization of a fluorescence assay to monitor changes in the aqueous volume of lipid vesicles

The fluorescent compound, 4',5'-bis[N,N-bis(carboxymethyl)aminomethyl] fluorescein (calcein) has been characterized for use in lipid vesicle studies. Particularly useful is its reaction with Co2+, which results in fluorescence quenching. This is accompanied by about a 10-nm blue shift in the uv absorbance bands and a small reduction in the visible absorbance band. For vesicle studies, Co2+ may be combined with citrate, which does not significantly hinder calcein quenching by Co2+. It does augment the absorbance of the metal ion. No significant interaction of citrate X Co2+ with phosphatidylserine vesicles was observed. Zn2+ is capable of displacing Co2+ and restoring calcein fluorescence. Fluorescence quenching due to formation of the calcein X Co2+ complex can also be reversed with EDTA. Thus, calcein is the basis of some simple reactions which can be used to assay changes in the aqueous volume of lipid vesicles.     

Performance of 4,5-diphenyl-1H-imidazole derived highly selective ‘Turn-Off’ fluorescent chemosensor for iron(III) ions detection and biological applications

Two fluorescent chemosensors, denoted as chemosensor 1 and chemosensor 2 , were synthesized and subjected to comprehensive characterization using various techniques. The characterization techniques employed were Fourier-transform infrared (FTIR), proton (¹H)- and carbon-13 (¹³C)-nuclear magnetic resonance (NMR) spectroscopy, electrospray ionization (ESI) mass spectrometry, and single crystal X-ray diffraction analysis. Chemosensor 1 is composed of a 1H-imidazole core with specific substituents, including a 4-(2-(4,5-c-2-yl)naphthalene-3-yloxy)butoxy)naphthalene-1-yl moiety. However, chemosensor 2 features a 1H-imidazole core with distinct substituents, such as 4-methyl-2-(4,5-diphenyl-1H-imidazole-2-yl)phenoxy)butoxy)-5-methylphenyl. Chemosensor 1 crystallizes in the monoclinic space group C2/c. Both chemosensors 1 and 2 exhibit a discernible fluorescence quenching response selectively toward iron(III) ion (Fe³⁺) at 435 and 390 nm, respectively, in dimethylformamide (DMF) solutions, distinguishing them from other tested cations. This fluorescence quenching is attributed to the established mechanism of chelation quenched fluorescence (CHQF). The binding constants for the formation of the 1 + Fe³⁺ and 2 + Fe³⁺ complexes were determined using the modified Benesi–Hildebrand equation, yielding values of approximately 2.2 × 10³ and 1.3 × 10⁴ M⁻¹, respectively. The calculated average fluorescence lifetimes for 1 and 1 + Fe³⁺ were 2.51 and 1.17 ns, respectively, while for 2 and 2 + Fe³⁺, the lifetimes were 1.13 and 0.63 ns, respectively. Additionally, the applicability of chemosensors 1 and 2 in detecting Fe³⁺ in live cells was demonstrated, with negligible observed cell toxicity.     

Lead complexation behaviour of root exudates of salt marsh plant Salicornia europaea L

Abstract

Root exudates are considered to have an important role in mobility and bioavailability of heavy metals. High molecular weight (HMW) substances are the main components of root exudates, however, knowledge about their interactions with heavy metals is lacking. In the present study, Pb(II) complexation of the HMW fluorescent fractions in root exudates from Salicornia europaea L. was investigated using excitation emission matrix (EEM) fluorescence spectroscopy. Two protein-like fluorescence peaks were identified in the EEM spectrum of root exudates. The fluorescence of both peaks was clearly quenched by Pb(II). The values of conditional stability constants, log K_a, for these two protein-like fluorescence peaks were 4.14 and 3.79. This indicates that the fluorescent substances are strong Pb(II) complexing organic ligands.     

Investigation of catalytic reaction with spectrofluorimetric method

Numerous derivatives of nicotinic acid hydrazids are suitable for sensitive fluorescence determination of metal ions. The reaction proceeds upon ultraviolet illumination in the presence of an oxidizing agent and catalyst forming a fluorescent product. Therefore, the catalyst metal ion can be quantitatively determined by fluorescence methods. We have studied the reaction of nicotinic acid (di-pyridin-2-yl-methylene)-hydrazide (NADPMH), catalyzed by Mn(II)ion, and determined the optimal parameters of metal ion method.     

Synthesis, characterization, fluorescence and computational studies of new Cu, Ni and Hg complexes with emissive thienylbenzoxazolyl-alanine ligands

The interaction of four fluorescent compounds containing thiophene and benzoxazole moieties combined with an alanine residue with alkaline, alkaline-earth, transition and post-transition metal ions was explored. The highly fluorescent heterocyclic alanine derivatives are strongly quenched in the solid state after complexation with the paramagnetic metal ions Cu²⁺ and Ni²⁺, and with the diamagnetic Hg²⁺. Absorption and steady-state fluorescence titrations reveal a selective interaction with Cu²⁺, Ni²⁺ and Hg²⁺. In all cases the formation of mononuclear or dinuclear metal complexes in solid state and in solution are postulated. DFT calculations on the mercury(II) complexes confirm the formation of dinuclear species. Our results suggest that one metal ion is coordinated by the chelate group formed by the amine and the protonated carboxylic groups present in the amino acid residue while a second metal ion is directly linked to the chromophore. As parent compound, L4 shows no interaction with Cu²⁺ and Ni²⁺ salts. However, the interaction with Hg²⁺ induces a strong quenching and a red shift of the fluorescence emission.     

Synthesis and characterisation of anthracene-based fluorophore and its interactions with selected metal ions

An anthracene-based novel ligand (L), 9,10-bis((4,6-dimethylpyrimidin-2-ylthio)methyl)anthracene, was synthesised and fully characterised. Interactions of the ligand with selected metal ions, Hg(II), Cu(II), Ag(I), Pb(II), Zn(II), Ni(II), Co(II), and Cr(III), were spectroscopically investigated. Of the examined metal ions, both Hg(II) and Cu(II) showed responses in both UV-Vis and fluorescent spectroscopy towards the ligand in acetonitrile solution. Spectroscopic titration indicated that the ligand forms complexes with the two metal ions in 1:1 and 1:2 ratios, respectively. DFT calculations revealed that Hg(II) binds possibly with two pairs of donor-set {SN} of the ligand to form a mononuclear complex in a distorted planar geometry whereas Cu(II) forms likely a binuclear complex in a tetrahedral geometry in which each Cu(II) is further coordinated with possibly two acetonitrile molecules.     

Effect of bis-triazoles on a ribose-based fluorescent sensor

We synthesized two ribosyl-based fluorescent sensors. Both sensors have an anthracene as the fluorophore, but they differ in the recognition site for metal ions. One (3) has two ribosyl esters, and the other (6) has two triazole groups linked to two ribosyl esters. Among the metal ions examined in MeOH, compound 3 displayed a large chelation-enhanced fluorescence (CHEF) effect with Hg(2+) and Cu(2+) ions, and compound 6 displayed a large chelation-quenched fluorescence (CHQF) effect with Cu(2+) and Ni(2+) ions. The results demonstrated that the absence (sensor 3) and presence (sensor 6) of an incorporated bis-triazole group in a ribosyl-based fluorescent sensor conferred different preferences and distinct binding modes for metal ions.     

Synthesis,photoluminescence properties and theoretical insights on 1,3‐diphenyl‐5‐(9‐anthryl)‐2‐pyrazoline and ‐1H‐pyrazole

1,3‐Diphenyl‐5‐(9‐anthryl)‐2‐pyrazoline and 1,3‐diphenyl‐5‐(9‐anthryl)‐1H‐pyrazole with an anthryl chromophore were synthesized and characterized using ¹H NMR, ¹³C NMR, FT‐IR, mass spectrometry and elemental analysis. Their optical properties were characterized by UV–vis absorption and fluorescence spectroscopy. It was observed that the absorption and fluorescence spectra of the two compounds showed a red shift with respect to that of anthracene. Pyrazole exhibited high fluorescent quantum yields (Φ_f = 0.90 in toluene) while pyrazoline showed nearly no fluorescence in solution. The significant fluorescence divergence of the two similar compounds was investigated theoretically through density functional theory (DFT) calculations. The energetically lowest‐lying state S₁ in the pyrazoline exhibited both characteristics of locally excited and electron‐transfer states that resulted in the fluorescence quenching of anthryl chromophore whereas the S₁ state in the pyrazole corresponded to an optically allowed state that led to high fluorescence quantum yields in solutions. Copyright © 2012 John Wiley & Sons, Ltd.     

Metal ion accessibility of histidine-modified superfolder green fluorescent protein expressed in Escherichia coli

Green fluorescent protein (GFP) is frequently utilized for metal ion detection and quantification. To improve the metal binding potential of GFP, three residues (N146, F165, and L201) were substituted to histidines. Each variant responded differently upon interaction with metal ions. More than 80% of N146H, having the most accessible surface area, could bind to immobilized metal ions. However, only F165H exhibited significant differences in quenching by soluble metal ions (22% fluorescence decrease) in comparison with the template protein (12%). These findings can be utilized for designing GFP variants for metal binding and sensor applications.     

Structure-based differences between the metal ion selectivity of two siderophores desferrioxamine B (DFB) and desferricoprogen (DFC): why DFC is much better Pb(II) sequestering agent than DFB?

Complexation of desferrioxamine B (DFB) and desferricoprogen (DFC) with Cd(II) and Pb(II) toxic ions as well as complexation of DFC with Ca(II) and Mg(II) essential metals have been investigated and the results have been compared to those with other metal ions. The two siderophores have moderate Cd(II)-binding ability, but both, and especially DFC, bind Pb(II) in high stability complexes. Surprisingly, significant differences exist between Pb(II)-complexation of DFB and DFC. Namely, a maximum of two hydroxamate groups of a DFB coordinate to a Pb(II) ion, the third one binds to another metal ion with high preference and the formation of a trinuclear species, [Pb₃(DFBH)₂]²⁺, is predominant even at 1:1 metal to ligand ratio in this system. On the contrary, DFC forms mononuclear complex, [ML], with much higher stability and the formation of the trinuclear complex is negligible compared to DFB. The 6s² electron-pair of Pb(II), which remains always inert during complexation with hydroxamic acids and also with DFB, seems to become active in the DFC complexes (due to the effect of the double bonds in β-position to each hydroxamate), what, at least in some extent, allows the coordination of all the three hydroxamates of DFC to the same Pb(II) ion. This way of interaction (unique with a hydroxamate-based compound) results in significant stability increase, and, as a consequence, DFC is much better Pb(II)-chelating agent than DFB. Although DFC forms unexpectedly high stability complexes with Mg(II) compared to Ca(II), but even Mg(II), compared to many other metals, is not an efficient DFC-binding metal. Therefore, any sequestration of this biologically very important metal is not likely from a living organism by DFC.     

METAL‐INDUCED REACTIVE OXYGEN SPECIES PRODUCTION IN CHLAMYDOMONAS REINHARDTII (CHLOROPHYCEAE)1

Toxic effects of metals appear to be partly related to the production of reactive oxygen species (ROS), which can cause oxidative damage to cells. The ability of several redox active metals [Fe(III), Cu(II), Ag(I), Cr(III), Cr(VI)], nonredox active metals [Pb(II), Cd(II), Zn(II)], and the metalloid As(III) and As(V) to produce ROS at environmentally relevant metal concentrations was assessed. Cells of the freshwater alga Chlamydomonas reinhardtii P. A. Dang. were exposed to various metal concentrations for 2.5 h. Intracellular ROS accumulation was detected using an oxidation‐sensitive reporter dye, 5‐(and‐6)‐carboxy‐2′,7′‐dihydrodifluorofluorescein diacetate (H₂DFFDA), and changes in the fluorescence signal were quantified by flow cytometry (FCM). In almost all cases, low concentrations of both redox and nonredox active metals enhanced intracellular ROS levels. The hierarchy of maximal ROS induction indicated by the increased number of stained cells compared to the control sample was as follows: Pb(II) > Fe(III) > Cd(II) > Ag(I) > Cu(II) > As(V) > Cr(VI) > Zn(II). As(III) and Cr(III) had no detectable effect. The effective free metal ion concentrations ranged from 10^?6 to 10^?9 M, except in the case of Fe(III), which was effective at 10^?18 M. These metal concentrations did not affect algal photosynthesis. Therefore, a slightly enhanced ROS production is a general and early response to elevated, environmentally relevant metal concentrations.     

Fluorescence quenching and bonding properties of some hydroxamic acid derivatives by iron(III) and manganese(II)

Spectrophotometric investigations of highly fluorescent metal chelating molecules are of relevance due to their potential application in novel, selective fluorescence‐based sensors. Benzene and naphthalene chromophores are highly fluorescent while hydroxamic acids are widely used as ligands for complexation of transition metals. In order to develop fluorescence probes, several phenyl derivatives of N‐phenylbenzohydroxamic acid and an aminodihydroxamic acid linked with a naphthalene chromophore were synthesized and their selective ionophoric properties towards iron(III) and manganese(II) ions were investigated using fluorescence and absorption spectroscopy. Both methods confirm the formation of 1:1 and 1:2 complexes for iron(III) and a 1:1 complex for manganese(II). The complex that is formed depends on the concentration of the ligand and pH of the medium. The amino dihydroxamic acid exhibits a prominent selectivity towards iron(III) with a two‐step 1:1 and 1:2 quenching mechanism at pH 3 and towards manganese(II) with a 1:1 quenching mechanism at a probe concentration of 1 × 10^?5 mol dm^?3 at pH 9.5 The logarithm of overall formation constants of 1:1 and 1:2 complexes of iron(III) were estimated as 3.30 and 9.05, respectively. Copyright © 2008 John Wiley & Sons, Ltd.     

Regulating the fluorescence intensity of an anthracene boronic acid system: a B-N bond or a hydrolysis mechanism?

An anthracene-based fluorescent boronic acid system developed by the Shinkai group has been widely used for the preparation of fluorescent sensors for carbohydrates. Such application is based on the significant fluorescence intensity increase of this system upon binding with a carbohydrate. The mechanism through which this fluorescence intensity change happens was originally proposed to go through a B-N bond formation mechanism, which masks the nitrogen lone pair electrons. However, our own fluorescence studies suggest a possible alternative mechanism for the fluorescence change upon the formation of a boronic acid (1a) complex with diols. In this new proposed mechanism, complex formation induces solvolysis, which results in the protonation of the amine nitrogen if the reactions are carried out in a protic solvent such as water. This protonation prevents the photoinduced electron transfer, resulting in reduced quenching of the anthracene fluorescence. Such a solvolysis mechanism is supported by evidence from various types of experiments and theoretical calculations.     

Types of interaction of meso‐tetraphenylporphyrin with alkali and alkaline‐earth metal ions

The cavity in a porphyrin can accommodate metal ions through electron donor–acceptor (EDA) interaction in acetonitrile media without any specially designed fabrication with the porphyrin subunit. Alkali metal ion forms a complex with meso‐tetraphenylporphyrin (TP) in 2:1 stoichiometry, while the bivalent Mg²⁺ ion follows a 1:1 stoichiometry. A fluorescence interaction study indicated that TP can behave like a chemosensor for these ions present in the blood electrolytes. Specifically, for the alkali metal ions intensity‐based sensing was observed, due to inhibition of photoinduced electron transfer (PET), entailing enhancement of fluorescence intensity, and for the alkaline‐earth Mg²⁺ a mixed quenching was observed. Na⁺ and K⁺ ions can be differentiated depending upon the extent of fluorescence enhancement. Copyright © 2011 John Wiley & Sons, Ltd.