Selective metal ion recognition using a fluorescent 1,8-diquinolylnaphthalene-derived sensor in aqueous solution

The use of anti-1,8-bis(2,2′-diisopropyl-4,4′-diquinolyl)naphthalene, 1, for metal ion-selective fluorescence recognition has been investigated. Employing CuCl₂, ZnCl₂, FeCl₂, and FeCl₃ in fluorescence titration experiments of 1 revealed formation of a bluegreen light emitting bimetallic complex. A dramatic red-shift of the fluorescence maximum of 1 and metal ion-selective quenching was observed in the presence of Cu(II), Fe(II), and Fe(III)chlorides in acetonitrile. By contrast, addition of ZnCl₂ was found to result in fluorescence enhancement, whereas Cu(I) did not induce any significant fluorescence change of 1. The sensor was found to undergo highly ion-selective fluorescence quenching in aqueous solution. Screening of main group and transition metal ions showed excellent selectivity for FeCl₃ even in the presence of competing metal ions.     

Ratiometric fluorescence chemosensors for copper(II) and mercury(II) based on FRET systems

A system based on FRET mechanism, comprising a coumarin donor and a rhodamine acceptor, was developed for the selective and quantitative detection of metal ions. Fluorescent chemosensors RCs, linked by 1,2-diethylamine, exhibit significant fluorescence enhancement and excellent selectivity toward Cu²⁺. Fluorescent probes CRB and CR6G, linked by hydrazide, function as ratiometric receptors for Cu²⁺ chromogentically and fluorogentically in organic-aqueous media. Furthermore, the characteristic rhodamine-based fluorescence response of CRB (excitation at 550 nm) exhibits high selectivity for Hg(II). The construction of this kind of universal FRET system opens a broader prospect for future design of ratiometric fluorescent probes.     

Chelation-enhanced fluorescence of phosphorus doped carbon nanodots for multi-ion detection

The present paper reports on a chelation enhanced fluorescence (CHEF) effect that is observed on addition of certain metal ions to phosphorus doped carbon nanodots (P-CNDs). The effect is accompanied by a large shortwave shift of the emission peak. Highly passivated P-CNDs with sizes of around 3 nm were prepared from lactose and phosphoric acid, using a one-pot low temperature solvothermal method. The nanoparticles were purified according to polarity and size. The extent of blue shift and strength of enhancement depend on metal ions and actual pH value. For instance, the P-CND complex with Al(III) has a fluorescence that is shifted to shorter wavelengths, and the fluorescence quantum yield is enhanced from 12% (for the free P-CNDs) to almost 62% at 490 nm. The fluorescence is also enhanced and shifted by the ions Zn(II) and Cd(II). It is quenched by the ions Fe(II), Fe(III), Hg(II), Cu(II) and Sn(II), among others. The enhancement is attributed to the chelation of metal ions with the passivated surface functional groups of P-CNDs, mainly those of phosphorus. Phosphorous free CNDs (prepared via HCl instead of H₃PO₄) and low-passivated P-CNDs (prepared for longer period of time; typically 8 h) show no enhancement. The metal ion induced enhancement led to the design of a fluorometric assay for the detection of these ions. The detection limits are 4 nM for Al(III) and 100 nM for Zn(II). The two ions were quantified in spiked pharmaceutical formulations. Recoveries typically are 102% (for n = 7).

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Graphical abstract The fluorescence emission of phosphorous doped carbon nanodots is significantly enhanced and tuned after binding to Al³⁺, Zn²⁺ and Cd²⁺. The enhancement mechanism is attributed to chelation enhanced fluorescence (CHEF).

     

Bimodal fluorescence signaling based on control of the excited-state conformational twisting and the ground-state protonation processes

Amino-based fluoroionophores 1 and 2 can selectively sense alkaline earth metal ions in MeCN under both neutral and acidic conditions by different signaling mechanisms. The fluoroionophoric behavior for the neutral probes is characterized by an ‘off-on’ photoinduced electron transfer (PET)-like fluorescence intensity response due to a switching from a twisted internal charge transfer (TICT) to a planar internal charge transfer (PICT) state. For the protonated probes (i.e., 1/H⁺ and 2/H⁺), the fluorescing species is the localized stilbene fluorophores, but dual fluorescence is induced upon metal-ion recognition through a deprotonation process.     

Fluorescence emission control and switching of oxymethylcrowned spirobenzopyrans by metal ion

Oxymethylcrowned spirobenzopyran 1 and pyrenylspirobenzopyran 2 were synthesized, and fluorescence emission of their corresponding merocyanine form was examined in the presence of metal ions. For 2, fluorescence emission derived from the pyrene moiety was completely quenched by photoinduced electron transfer (PET) of the nitrogen atom when the merocyanine form was not produced, namely, without metal ions. However, when 2 was converted to the merocyanine form by the complexation of its crown ether with a metal ion, fluorescence resonance energy transfer (FRET) from the pyrene to the merocyanine moieties took place to produce fluorescence emission. This result demonstrates that the spirobenzopyran isomerization can function as a fluorescence emission switch. Fluorescence quantum yield measurement for 1 and 2 showed that fluorescence emission depends on the binding metal ion in which the fluorescence quantum yield generally increased with the increase of metal ion radius.     

Fe(III)- and Hg(II)-selective dual channel fluorescence of a rhodamine-azacrown ether conjugate

A rhodamine-azacrown ether conjugate (1) demonstrates Fe(III)-selective green fluorescence, while showing Hg(II)-selective orange fluorescence. This is the first example of rhodamine-based fluorescent probe that shows dual channel fluorescence for two different metal cations.     

Metal induced enhancement of fluorescence and modulation of two-photon absorption cross-section with a donor-acceptor-acceptor-donor receptor

A metal ion sensing fluorophore L that exhibits a large two-photon absorption cross-section has been synthesized in good yields. The influences of different metal ion inputs, on the one- and two-photon spectroscopic properties of L, have been investigated. The ligand itself does not show any fluorescence although in presence of a metal ion like Zn(II), Cd(II), Mg(II) or Ca(II), a ∼25 time enhancement of fluorescence is observed. The ligand with symmetrical “donor-acceptor-acceptor-donor” characteristics exhibits a large two-photon absorption cross-section measured by femtosecond open-aperture Z-scan technique at 880 nm. However, presence of any of the above metal ions lowers its two-photon absorption cross-section (δ) to different extents at 880 nm. Theoretical calculation carried out in DFT formalism on the ligand and its Zn(II) complex corroborate experimental results.     

A turn-on Schiff base fluorescence sensor for zinc ion

A simple Schiff base type fluorescent receptor 1 was prepared and evaluated for its fluorescence response to heavy metal ions. Receptor 1 exhibits an ‘off-on-type’ mode with high selectivity in the presence of Zn²⁺ ion. The selectivity of 1 for Zn²⁺ is the consequence of combined effects of chelation-enhanced fluorescence (CHEF), CN isomerization, and inhibition of photoinduced electron transfer (PET).     

Morphological consequences of metal ion-peptide vesicle interaction

The triskelion peptide conjugate 1, having a Trp-Trp dipeptide unit on the three arms, was synthesized and studied for the interaction of peptide-based soft structures with metal ions, by fluorescence and microscopic analyses. We observed that fluorescence was significantly quenched upon addition of Cu(II) metal ions, whereas the addition of other metal ions also caused moderate to insignificant changes in the fluorescence emission, suggesting specificity of this triskelion peptide 1 for Cu(II) ions. The addition of Cu(II) and other metal ions also altered the morphology of preformed vesicles obtained from triskelion peptide 1 in a concentration-dependent fashion, as observed from microscopic analysis. Such metal-responsive soft structures may find potential use as novel materials for delivery and sensing applications.     

Zinc selective chemosensor based on pyridyl-amide fluorescence

Chemosensors are developed to image zinc ions. Fluorescence enhancement due to Zn²⁺ binding is an excellent way to detect its presence. A chemosensor for Zn²⁺ based on dipicolylamine (DPA) groups connected by a pyridyl amide backbone has been synthesized. Addition of 2-chloroacetyl chloride to 2,6-diaminopyridine affords 2,6-bis(chloroethylamido)pyridine, which is converted to the sensor BADPA-P by 2,2′-dipicolylamine displacement of chlorine. This compound along with two others, the mono-DPA, ADPA-P and the benzyl in place of pyridyl, BADPA-B, present three potential Zn²⁺ sensors. It was found that BADPA-P in the presence of Zn²⁺ shows a large increase in fluorescence, whether in polar organic or aqueous environments. Its fluorescence in the presence of Cd²⁺, unlike with Zn²⁺, is not enhanced when excited at longer wavelengths. Proton NMR measurements, indicate two Zn²⁺ ions bind to BADPA-P. Also, Zn²⁺ enhances fluorescence even when other metal ions are present.     

The development of a fluorescence turn-on sensor for cysteine, glutathione and other biothiols. A kinetic study

Two fluorescence probes for the detection of cysteine (Cys), glutathione (GSH) and other biothiols, such as homocysteine (Hcy) and cysteinyl-glycine (Cys-Gly), were developed. These molecular probes are coumarin-based derivatives containing a chalcone-like moiety that reacts with biothiols through a Michael addition reaction, leading to strong fluorescence enhancements. The reactivity of the tested biothiols toward both probes (ChC1 and ChC2) follows the order Cys > GSH > Hcy > Cys-Gly, ChC1 being less reactive than ChC2. Possible interference with other amino acids was assessed. ChC1 and ChC2 display a highly selective fluorescence enhancement with thiols, allowing these probes to be used for fluorimetric thiol determination in SH-SY5Y cells.     

A Zn(II) ion selective fluorescence sensor that is not affected by Cd(II)

Comparison of sensors sodium-2,6-diamino-(N,N,N′,N′-tetraacetate)-4-methylanisole 1 and sodium-2,6-diamino-(N,N,N′-triacetate)-4-methylanisole 2 reveal that the loss of an acetyl group in 2 leads to a more selective Zn(II) induced fluorescence enhancement and shows no response to any other metal ions including Cd(II). Structural modifications and AM1 calculations indicate that the sensor uses the three acetyl groups and the 3° amino nitrogen for binding the metal ion. AM1 calculations imply a trigonal bipyramidal coordination for Zn(II) with the solvent molecule occupying one of the axial positions.     

Salicylaldehyde hydrazones as fluorescent probes for zinc ion in aqueous solution of physiological pH

Salicylaldehyde hydrazones of 1 and 2 were synthesized and their potential as fluorescent probes for zinc ion was investigated in this paper. Both of the probes were found to show fluorescence change upon binding with Zn²⁺ in aqueous solutions, with good selectivity to Zn²⁺ over other metal ions such as alkali/alkali earth metal ions and heavy metal ions of Pb²⁺, Cd²⁺ and Hg²⁺. They showed 1:2 metal-to-ligand ratio when their Zn²⁺ complex was formed. By introducing pyrene as fluorophore, 2 showed interesting ratiometric response to Zn²⁺. Under optimal condition, 2 exhibited a linear range of 0-5.0 μM and detection limit of 0.08 μM Zn²⁺ in aqueous buffer, respectively. The detection of Zn²⁺ in drinking water samples using 2 as fluorescent probe was successful.     

A “turn-on” fluorescent chemosensor for zinc ion with facile synthesis and application in live cell imaging

Compound 1 was facilely synthesized through a one step reaction from commercially available materials. As a sensitive and selective “turn-on” fluorescent chemosensor for Zn(II), 1 exhibits a 40-fold fluorescence enhancement response to Zn(II) over other physiological relevant metal ions in aqueous solution at neutral pH. Furthermore, 1 could be efficiently delivered to live cells for bioimaging of Zn(II).     

Mn(II) azamacrocyclic bromide complexes with different nuclearities

The coordination behaviour of a series of pyridyl azamacrocyclic ligands, some of them containing cyanomethyl and cyanoethyl pendant-arms, towards Mn(II) ion was studied. All the complexes were characterized by microanalysis, LSI mass spectrometry, IR, UV-Vis spectroscopy and magnetic measurements. Crystal structures of [MnL¹][MnBr₄] (1), [MnL³][MnBr₄] · 2CH₃CN (3), [Mn₂L⁵Br₄] · 2CH₃CN (5) and [Mn₂L⁶Br₄] (6) complexes have been determined. The X-ray studies show the presence of an ionic mixed octahedral-tetrahedral complex for 1 and 2, with the manganese ion of the cation complex, endomacrocyclicly coordinated by the six nitrogen donor atoms from the macrocyclic backbone in a distorted octahedral geometry. Instead, the complexes 5 and 6 are dinuclear, and both manganese ions are coordinated by one pyridinic and two amine nitrogen atoms from the macrocyclic backbone and two bromide ions, being the geometry around the metal better described as distorted square pyramidal. In all cases, the nitrile pendant-arms do not show coordination to the metal ion.     

Imidazole and imine coated ZnO nanoparticles for nanomolar detection of Al(III) and Zn(II) in semi-aqueous media

Imidazole and imine-linked dipodal receptors were decorated on the surface of ZnO nanoparticles. Cation binding assays of these chemosensors showed that receptors 5 and 6 had high selectivity for Al(III) and Zn(II) ions, respectively. The detection limit for Al(III) with receptor 5 was 11 nM, and the detection limit for Zn(II) ion with receptor 6 was 23 nM.     

Three different fluorescent responses to transition metal ions using receptors based on 1,2-bis- and 1,2,4,5-tetrakis-(8-hydroxyquinolinoxymethyl)benzene

The dipod 1,2-bis(8-hydroxyquinolinoxymethyl)benzene (3) and tetrapod 1,2,4,5-tetrakis(8-hydroxyquinolinoxymethyl)benzene (5) have been synthesized through nucleophilic substitution of respective 1,2-bis(bromomethyl)benzene (2) and 1,2,4,5-tetra(bromomethyl)benzene (4) with 8-hydroxyquinoline (1). For comparison, 1,3,5-tris(8-hydroxyquinolinoxymethyl)benzene derivatives (7a and 7b) have been obtained. The complexation behavior of these podands towards Ag⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, and Cd²⁺ metal ions has been investigated in acetonitrile by fluorescence spectroscopy. The sterically crowded 1,2,4,5-tetrapod 5 displays unique fluorescence ‘ON-OFF-ON’ switching through fluorescence quenching (λ_max 395 nm, switch OFF) with <1.0 equiv of Ag⁺ and fluorescence enhancement (λ_max 495 nm, switch ON) with >3 equiv Ag⁺ and can be used for estimation of two different concentrations of Ag⁺ at two different wavelengths. The addition of Cu²⁺, Ni²⁺, and Co²⁺ metal ions to tetrapod 5 causes fluorescence quenching, i.e., ‘ON-OFF’ phenomena at λ_max 395 nm for <10 μM (1 equiv) of these ions but addition of Zn²⁺ and Cd²⁺ to tetrapod 5 results in fluorescence enhancement with a gradual shift of λ_em from 395 to 432 and 418 nm, respectively. Similarly, dipod 3 behaves as an ‘ON-OFF-ON’ switch with Ag⁺, an ‘ON-OFF’ switch with Cu²⁺, and an ‘OFF-ON’ switch with Zn²⁺. The placement of quinolinoxymethyl groups at the 1,3,5-positions of benzene ring in tripod 7a-b leads to simultaneous fluorescence quenching at λ_max 380 nm and enhancement at λ_max 490 nm with both Ag⁺ and Cu²⁺. This behavior is in parallel with 8-methoxyquinoline 8. The rationalization of these results in terms of metal ion coordination and protonation of podands shows that 1,2 placement of quinoline units in tetrapod 5 and dipod 3 causes three different fluorescent responses, i.e., ‘ON-OFF-ON’, ‘ON-OFF’, and ‘OFF-ON’ due to metal ion coordination of different transition metal ions and 1, 3, and 5 placement of three quinolines in tripod 7, the protonation of quinolines is preferred over metal ion coordination. In general, the greater number of quinoline units coordinated per metal ion in 5 compared with the other podands points to organization of the four quinoline moieties around metal ions in the case of 5.     

Functional luminescent lanthanide complexes: Modulation of visible luminescence from europium complexes

The syntheses of three new ligands (L^1-3), which are based upon a DO3A core and appended with additional receptor sites for metal cations, are described, together with their corresponding Eu(III) complexes (Eu-L^1-3). The complexes are visibly luminescent in aqueous solution, following sensitization via the pyridine chromophore, showing characteristic narrow line-like emission from Eu(III). The luminescence properties show that water is effectively excluded from the inner coordination sphere of europium (q = 0). Each of the complexes showed perturbed luminescence properties upon addition of a variety of d-block metal ions. For example, emission quenching was observed for each complex following addition of Cr(III) and Cu(II). Selectivity towards Hg(II) (over Cd(II), Cu(II) and Zn(II)) was demonstrated with Eu-L³, which possesses a receptor site incorporating a softer thiophene moiety. More specifically, Hg(II) binding resulted in changes in the form of the steady state emission spectrum, together with a corresponding reduction of the luminescence lifetime in water, which can be attributed to an increase in inner sphere hydration (q = 2) and thus enhanced non-radiative deactivation of the ⁵D₀ state by proximate O-H oscillators.     

Tetrahedral silicon-centered imidazolyl derivatives: Promising candidates for OLEDs and fluorescence response of Ag (I) ion

A series of novel tetrahedral silicon-centered imidazolyl derivatives, Bis(4-(imidazol-1-yl)phenyl)dimethylsilane (1), Tri(4-(imidazol-1-yl)phenyl)methyl silane (2), Bis(4-(imidazol-1-yl)phenyl)diphenylsilane (3), Tri(4-(imidazol-1-yl)phenyl)phenylsilane (4), [Bis(4-(imidazol-1-yl)phenyl)](4-bromophenyl)phenylsilane (5) and [Tri(4-(imidazol-1-yl)phenyl)](4-bromophenyl)silane (6) have been synthesized and characterized by FTIR, ¹H NMR, ¹³C NMR and mass spectroscopy. They all display high thermal stability, are fluorescent with emission in the region of violet to blue, and possess large HOMO-LUMO energy gaps ranging from 4.9585 to 5.1879 eV, which could be potentially used as blue emitters or hole blocking materials in OLEDs. Moreover, the metal ion titrations based on Ag (I) and compounds 1-4 reveal that these ligands show distinguishable fluorescence response with increasing of Ag (I) ions.     

Different geometrical arrangements in carboxylate coordination polymers of flexible dicarboxylic acid

Dicarboxylate coordination polymers (1-5) of Mn(II), Ni(II), Cu(II), Zn(II) and Cd(II), respectively, derived from (7-carboxymethoxy-naphthalen-2-yloxy)-acetic acid (L₁H₂) are synthesized and characterized. Depending on the coordination sites around the metal centers and coordination mode of the ligand, dimensionality of these polymers varies. The dicarboxylates adopt three spatial orientations: in-plane linear coordination, out-of-plane cis coordination and out-of-plane trans coordination mode. Both the cis and trans out-of-plane coordination modes are found to exist only if the ancillary ligand pyridine is coordinated to the metal ion. When the aquoligand coordinates the in-plane linear coordination mode of L₁ predominates. The coordination polymers 4 and 5 show photoluminescence in solution. The dicarboxylate of (5-carboxymethoxy-naphthalen-1-yloxy)-acetic acid (L₂H₂) does not form coordination polymer under ambient conditions, but prefers to remain as uncoordinated anion providing hydrophobic confinement to hexa-aquometal(II) cation. Compound 3 crystallizes in P2₁ space group and it shows broadband ultra-violet fluorescence centered at 352.9 nm on focusing 632.8 nm He:Ne laser.