Novel fluorimetric bulk optode membrane based on 5,8-bis((5′-chloro-8′-hydroxy-7′-quinolinyl)methyl)-2,11-dithia-5,8-diaza-2,6-pyridinophane for selective detection of lead(II) ions

A novel fluorescence chemical sensor for the highly sensitive and selective determination of Pb²⁺ ions in aqueous solutions is described. The preliminary potentiometric and spectrofluorimetric complexation studies in solution revealed that the lipophilic ligand 5,8-bis((5′-chloro-8′-hydroxy-7′-quinolinyl)methyl)-2,11-dithia-5,8-diaza-2,6-pyridinophane (L2) forms a highly stable and selective [PbL2]²⁺ and [Pb(L2)₂]²⁺ complexes which results in a strong fluorescence quenching of the ligand. Thus, a novel fluorescence Pb²⁺ sensing system was prepared by incorporating L2 as a neutral lead-selective fluoroionophore in the plasticized PVC membrane containing tetrakis(p-chlorophenyl) borate as a liphophilic anionic additive. The response of the sensor is based on the strong selective fluorescence quenching of L2 by Pb²⁺ ions. At pH 5.5, the proposed sensor displays a calibration curve over a wide concentration range of 3.0 × 10⁻⁷ to 2.5 × 10⁻² M with a relatively fast response time of less than 5 min. In addition to high stability, reversibility and reproducibility, the sensor shows a unique selectivity towards Pb²⁺ ion with respect to common coexisting cations. The proposed fluorescence optode was successfully applied to the determination of lead in plastic toys and tap water samples.     

New regioisomeric naphthol–thiazole based ‘turn-on’ fluorescent chemosensor for Al

Two new reactive and highly selective turn-on fluorescent chemosensors based on the position of ring annulation of the naphthalene–thiazole moiety for aluminum ions in ethanol, were synthesized and investigated. It was found that sensors 2 and 4 exhibited a remarkable enhancement of emission upon complexation with Al³⁺. A TD-B3LYP/6-31G(d,p) calculation was performed to characterize the nature of the fluorescence behavior of sensors 2 and 4 upon Al³⁺ complexation. The mechanism of fluorescence was based on the cation promoted hydrolysis of ester and subsequent complexation. The combination of experimental and computational analyses provides a more complete understanding of the molecular level origin of these types of unique photophysical properties.     

Synthesis and evaluation of a novel pyrenyl-appended triazole-based thiacalix[4]arene as a fluorescent sensor for Ag ion

New fluorescent chemosensors 1,3-alternate-1 and 2 with pyrenyl-appended triazole-based on thiacalix[4]arene were synthesized. The fluorescence spectra changes suggested that chemosensors 1 and 2 are highly selective for Ag⁺ over other metal ions by enhancing the monomer emission of pyrene in neutral solution. However, other heavy metal ions, such as Cu²⁺, and Hg²⁺ quench both the monomer and excimer emission of pyrene acutely. The ¹H NMR results indicated that Ag⁺ can be selectively recognized by the triazole moieties on the receptors 1 and 2 together with the ionophoricity cavity formed by the two inverted benzene rings and sulfur atoms of the thiacalix[4]arene.     

Ratiometric fluorescent and colorimetric sensors for Cu based on 4,5-disubstituted-1,8-naphthalimide and sensing cyanide via Cu displacement approach

Two 4,5-disubstituted-1,8-naphthalimide derivatives 1 and 2 were synthesized as ratiometric fluorescent and colorimetric sensors for Cu²⁺, respectively. In 100% aqueous solutions of 1, the presence of Cu²⁺ induces a strong and increasing fluorescent emission centered at 478 nm at the expense of the fluorescent emission of 1 centered at 534 nm. Compound 2 senses Cu²⁺ by means of a colorimetric (primrose yellow to pink) method with a thorough quench in emission attributed to the deprotonation of the secondary amine conjugated to the naphthalimide fluorophore. 1-Cu²⁺ and 2-Cu²⁺ sense cyanide in ratiometric way via colorimetric and fluorescent changes.     

Unexpected highly selective fluorescence ‘turn-on’ and ratiometric detection of Hg based on fluorescein platform

Methylated fluorescein 1 was explored for fluorescence ‘turn-on’ and ratiometric detection of Hg²⁺ in THF and CH₂Cl₂/MeOH (v/v=9:1), respectively, with unexpected high selectivity. In the presence of Hg²⁺, characteristic structured absorption band of 1 diminished and a new sharp band appeared at 445 nm. Meanwhile a blue shifted and enhanced emission was observed. The ratio of the fluorescence intensity at 559 and 478 nm increased linearly with [Hg²⁺], and solution color changing from yellow to cyan under irradiation at 365 nm in CH₂Cl₂/MeOH. Job plot indicated a 1:1 stoichiometry for 1-Hg²⁺ complex in solution. ¹H NMR titration and IR spectra suggested the coordination of carbonyl group in xanthene moiety to Hg²⁺, affording its spectral behavior. Compound 2 bearing two triazolyl amino esters in place of methyl group showed quite similar behavior to Hg²⁺, which indicated that substituents did not interfere with the specific binding behavior of fluorescein platform. Our work presents a new way to explore xanthene dyes as new chemosensors by modulating electron density on the xanthene ring through non-covalent interactions with carbonyl group.     

A selective fluorescent sensor for Pb(II) in water

A new fluorescent sensor (1) containing bis(2-pyridylmethyl)amine group as a binding moiety for Pb²⁺ was developed. Compound 1 shows selective response to Pb²⁺ over other metal ions in pH 7.0 HEPES buffer solution. The fluorescence intensity enhancement was ascribed to the complex formation between Pb²⁺ and 1 which blocked the photo-induced electron transfer process.     

Selective metal detection in an unsymmetrical 1,3-alternate calix[4]biscrown chemosensor

Calix[4]crown-based chemosensors 1, 2, and reference 3 have been synthesized. The weak fluorescence intensity of 1,5-naphthalene of 1 suggests that the benzene rings of the calix[4]arene as well as the oxygen atoms of the crown-5 ring take part in PET. The complexation of two K⁺ ions by both crown-5 and 1,5-naphthalene-crown-6 loops of 1 caused fluorescence enhancement of the naphthalene unit by CHEF. Pb²⁺ acted as a quenching metal ion.     

Exploiting the deprotonation mechanism for the design of ratiometric and colorimetric Zn fluorescent chemosensor with a large red-shift in emission

The design, synthesis, and photophysical evaluation of a new naphthalimide-based fluorescent chemosensor, N-butyl-4-[di-(2-picolyl)amino]-5-(2-picolyl)amino-1,8-naphthalimide (1), were described for the detection of Zn²⁺ in aqueous acetonitrile solution at pH 7.0. Probe 1 showed absorption at 451 nm and a strong fluorescence emission at 537 nm (Φ_F=0.33). The capture of Zn²⁺ by the receptor resulted in the deprotonation of the secondary amine conjugated to 1,8-naphthalimide so that the electron-donating ability of the N atom would be greatly enhanced; thus probe 1 showed a 56 nm red-shift in absorption (507 nm) and fluorescence spectra (593 nm, Φ_F=0.14), respectively, from which one could sense Zn²⁺ ratiometrically and colorimetrically. The deprotonated complex, [(1-H)/Zn]⁺, was calculated at m/z 619.1800 and measured at m/z 618.9890. In contrast to these results, the emission of 1 was thoroughly quenched by Cu²⁺, Co²⁺, and Ni²⁺. The addition of other metal ions such as Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe³⁺, Mn²⁺, Al³⁺, Cd²⁺, Hg²⁺, Ag⁺, and Pb²⁺ produced a nominal change in the optical properties of 1 due to their low affinity to probe 1. This means that probe 1 has a very high fluorescent imaging selectivity to Zn²⁺ among metal ions.     

Syntheses, spectroscopic and structural characterizations and metal ion binding properties of Schiff bases derived from 4′-aminobenzo-15-crown-5

A series of benzyloxybenzaldehyde derivatives (1-4) were synthesized by the reactions of 4-(bromomethyl)benzonitrile with 4-hydroxy-3-methoxybenzaldehyde (vanillin), 2-hydroxy-3-methoxybenzaldehyde (o-vanillin), 2-hydroxy-4-methoxybenzaldehyde and 2-hydroxy-5-methoxybenzaldehyde. Condensation reactions among the new benzyloxybenzaldehyde derivatives (1-4) with 4′-aminobenzo-15-crown-5 yielded the new Schiff base compounds (5-8). Sodium complexes (5a-8a) and potassium complexes (5b-8b) were prepared with NaClO₄ and KI, respectively. All of these synthesized compounds were characterized on the basis of FT-IR, ¹H and ¹³C NMR, mass spectrometry and elemental analyses data. The solid state structures of compounds 8 and 5a were determined by X-ray crystallography. The extraction abilities of compounds 5-8 were also evaluated in CH₂Cl₂ by using several main group and transition metal picrates, such as Na⁺, K⁺, Pb²⁺, Cr³⁺, Ni²⁺, Cu²⁺ and Zn²⁺.     

A simple click generated probe for highly selective sequential recognition of Cu(II) and pyrophosphate

A click generated quinoline derivative (1) has been synthesized and used as a fluorescent probe for sequential recognition of Cu²⁺ and pyrophosphate (PPi) in DMSO/H₂O (1:1, v/v, HEPES 20 mM, pH = 7.4) solution. Probe 1 displays high selectivity to Cu²⁺ ions, and the in-situ prepared probe 1-Cu²⁺ exhibits high selectivity toward pyrophosphate (PPi) with emission recovery of probe 1. Therefore, 1-Cu²⁺ complex can be applied as a fluorescence turn-on probe for PPi with high selectivity and sensitivity.     

Tetrathiafulvalene-flavin dyads: electron transfer promoted by metal cations

Tetrathiafulvalene-flavin dyads 1 and 2 are reported. Both absorption and ESR spectral studies show that the intramolecular electron transfer occurs from TTF to flavin units in dyads 1 and 2 in the presence of Pb²⁺/Sc³⁺. But, the electron transfer is more efficient for dyad 1 in the presence of Pb²⁺/Sc³⁺. Electrochemical studies manifest that coordination of dyads 1 and 2 with Pb²⁺/Sc³⁺ play an important role in facilitating the electron transfer within dyads 1 and 2.     

A new Fe fluorescent chemosensor based on aggregation-induced emission

A new tetraphenylethylene (TPE)-based sensor M1 bearing double 2-methylpyridyl-2-methylthiophenylamino units linked with triazole moieties was reported. Both UV–vis and fluorescence spectroscopic studies demonstrated that M1 was highly sensitive and selective toward Fe³⁺ over other metal ions in THF/H₂O solution based on the aggregation-induced emission quenching mechanism. The lowest detection limit of M1 for Fe³⁺ is 0.7 μM. The detailed fluorescent titration study suggested that the binding stoichiometry of the M1–Fe³⁺ complex was 1:2, and the structure between M1 and the Fe³⁺ complex was confirmed by the ¹H NMR titration.     

The synthesis of luminescent lanthanide-based chemosensors for the detection of zinc ions

Herein, we report the synthesis of two lanthanide-based chemosensors, Tb-5 and Eu-6, designed to sense free zinc ions (Zn²⁺) in aqueous solutions. The Tb-5 complex features a bis(2-pyridinylmethyl)amine moiety as a zinc(II)-responsive lanthanide-sensitising ‘antenna’, while Eu-6 incorporates a quinoline-based moiety for this purpose. Luminescence enhancements of 210% and 340% are observed upon addition of Zn²⁺ ions to Tb-5 and Eu-6, respectively. Both sensors are selective for Zn²⁺ ions over several other cations of environmental significance.     

Ratiometric fluorescence probe for two-photon bioimaging of Cr in living cells

We presented a ratiometric fluorescent probe dansylamide–rhodamine dyad (DANS–RB) for selectively detecting Cr³⁺ in semi-aqueous solution. The detection mechanism relies on the fluorescent resonance energy transfer (FRET) process from the dansylamide (energy donor) to the rhodamine (energy acceptor) after the addition of Cr³⁺. The cell-permeability of DANS–RB was confirmed by the two-photon fluorescence microscopy experiments, which demonstrated DANS–RB was a good candidate for monitoring the intracellular Cr³⁺ level with the ratiometric fluorescent method. Combining the excellent selectivity, the ratiometric quantitative detection, and the cell-permeability, DANS–RB may find a broad application in the investigation on biologically relevant species in living cells.     

Rhodamine based derivative and its zinc complex: synthesis and recognition behavior toward Hg(II)

A simple fluorescent probe, which contains rhodamine and aminoquinoline moieties, was designed and prepared for selective detection of Hg²⁺ in acetonitrile. RbQ exhibited high selectivity and sensitivity toward Hg²⁺ over other common metal ions. The recognition of RbQ toward Hg²⁺ can be detected by fluorescence spectra, absorption spectra, and even by naked eyes. The binding ratio of the RbQ–Hg²⁺ complex was found to be 1:1 according to Job plot experiment, and the limit of detection was 1.05×10⁻⁷ M. Moreover, the prepared complex RbQ–Zn²⁺ (RbQZ) could detect Hg²⁺ in a ratiometric way and showed lower limit of detection (2.95×10⁻⁸ M) than RbQ in the same condition. Finally, we also demonstrated that the aminoquinoline–zinc complex could be served as a new and effective FRET donor for rhodamine derivatives.     

Novel highly selective anion chemosensors based on 2,5-bis(2-hydroxyphenyl)-1,3,4-oxadiazole

2-(2-Hydroxyphenyl)-5-phenyl-1,3,4-oxadiazole 1 and 2,5-bis(2-hydroxyphenyl)-1,3,4-oxadiazole 2 were used as anion fluorescent and colorimetric chemosensors with high selectivity for H₂PO₄⁻ and F⁻ over Cl⁻, while 2 can even distinguish H₂PO₄⁻ from F⁻.     

Highly selective detection of Hg ion by push–pull-type purine nucleoside-based fluorescent sensor

Highly selective detection of Hg²⁺ ion has been achieved using the push–pull-type purine nucleoside-based fluorescent sensor L1. The sensor L1 incorporating aza-18-crown-6 at C6 position of purine nucleoside, is highly sensitive and selective toward Hg²⁺ ion in CH₃CN–H₂O mixture (92/8, v/v). The detection limit for the fluorescent sensor L1 toward Hg²⁺ ion is 7.8 × 10⁻⁸.     

Probing Pb cation via the iridium based phosphorescent dye

We demonstrate the concept of Pb²⁺ cation sensing using the emissive Ir(III) complex (1) based on the associated decrease of room-temperature phosphorescence upon forming the 1:1 adduct 1-Pb²⁺. Complex 1 bears two cyclometalated N-phenyl pyrazoles with pyrazoles residing at the mutual trans dispositions as well as one 3,5-di(pyridyl) pyrazolate chelate. X-ray structural analyses on the adduct 1-Pb²⁺ confirm the key function of 3,5-di(pyridyl) pyrazolate as it forms chelate interaction with the metal analytes, while quenching of phosphorescent emission is probably due to the Pb²⁺ induced perturbation, which increases the intersystem crossing to another lower-lying triplet state for the host chromophore via an enhanced spin–orbit coupling.     

Mercury assisted fluorescent supramolecular assembly of hexaphenylbenzene derivative for femtogram detection of picric acid

Aggregates of hexaphenylbenzene derivatives 3, having pyrene groups form network of fluorescent nanofibres in presence of mercury ions. Further, fluorescent nanofibres of 3-Hg²⁺ supramolecular ensemble exhibit sensitive and pronounced response towards the picric acid. In addition, the solution coated paper strips of 3-Hg²⁺ supramolecular ensemble can detect picric acid in the range of 2.29 fg/cm², thus, providing a simple, portable and low cost method for detection of picric acid in solution and in contact mode.     

A squaraine-based chemosensor for Hg and Pb

A novel unsymmetrical cationic squaraine dye (USqH⁺), absorbing in the near-infrared region (600-850 nm), was synthesized by reacting a semi-squaric acid derivative 3-[4-(N,N-dioctylamino)phenyl]-4-hydroxycyclobutene-1,2-dione with the squarylium dye, bis(3-methylbenzothiazol-2-ylidene)squaraine. Addition of micromolar amounts of Hg²⁺ and Pb²⁺ to solutions of USqH⁺ in dichloromethane brings about significant changes in its absorption spectrum, resulting in a change in colour of the solution from green to pink. The absorption spectrum of the dye is relatively insensitive to other environmentally relevant metal ions, making it possible to visually detect Hg²⁺ and Pb²⁺ in the presence of other metal ions. Molecular modeling of the complexes at the density functional theory (DFT) and semiempirical PM3 levels strongly suggests that the oxygen atoms are the most likely choice for cation binding. The calculations also indicate a high affinity of Hg²⁺ and Pb²⁺ toward the formation of 2:1 complexes, which is in good agreement with the experimental results.