The comparative study of two new Schiff bases derived from 5-(thiophene-2-yl)isoxazole as "Off-On-Off" fluorescence sensors for the sequential detection of Ga3+ and Fe3+ ions

Two new Schiff bases, TIC ((E)-N'-(2-hydroxybenzylidene)-5-(thiophene-2-yl)isoxazole-3-carbohydrazide) and TIE ((E)-N'-(3-ethoxy-2-hydroxybenzylidene)-5-(thiophene-2-yl)isoxazole-3-carbohydrazide), have been designed and synthesized as chemosensors for distinct recognition of Ga3+ and Fe3+ ions. TIE demonstrated a prominent "turn on" response characterized by clear distinguished fluorescence when coordination with Ga3+ ions in the DMSO/H2O buffer solution. In comparison, TIC also showed "turn on" response of blue fluorescence which was more selective and sensitive than that of TIE due to the steric hindrance of ethoxy group of TIE. The newly formed complexes TIC-Ga3+ and TIE-Ga3+ may act as selective "turn-off" fluorescent probes towards Fe3+ ions. Limits of detection of TIC and TIE towards Ga3+ ions were 7.8809 × 10-9 M and 2.6277 × 10-8 M, respectively. Limits of detection of TIC-Ga3+ and TIE-Ga3+ towards Fe3+ ions were 8.6562 × 10-9 M and 3.3764 × 10-7 M, respectively. The molar ratio of the complex between the sensor and Ga3+ or Fe3+ ions were all 1:2 determined through Job's Plot, mass spectrometry, and theoretical calculations. Both sensors were utilized for the determination of target ions in environment water samples, and the portable paper sensors for detecting Ga3+ ions have been successfully developed.

A novel ethylene linkage-based covalent organic framework for turn-on fluorescence sensing for Al3+ with excellent selectivity and sensitivity

Covalent organic Framework (COFs) has become a new platform for functional research and material design. A novel covalent organic framework (CN-COF) was first synthesized with p-xylylene dicyanide and 2-hydroxy-1,3,5-benzenetrialdehyde through the Knoevenagel condensation reaction. CN-COF is a porous crystal material with strong thermal and chemical stability. CN-COF exhibits a selective "turn-on" fluorescence response to Al3+ in ethanol with blue-shifted emission spectra over the other tested metal ions. The color changes from pink to earth yellow, and the fluorescence effect is clearly visible. The fluorescence intensity of CN-COF was linearly related to the concentration of Al3+, and the detection limit was 1.815 μM. Importantly, CN-COF exhibits a satisfactory recovery for detecting Al3+ in drinking water and fish samples. CN-COF also showed the intuitive semi-quantitative detection ability for Al3+ via the color change with the naked eyes. The special pore structure is conducive to allow Al3+ enter to coordinate with O and N atoms on the wall of CN-COF scaffold. The revisable fluorescence change upon the selective addition of Al3+ and XRD, EDTA, XPS and DFT results demonstrated the complex process. The inhibition of the photoinduced electron transition from O atoms to Al3+ induced the fluorescence enhancement. This study not only presents a synthesis idea for a new structural organic framework, but also offers a highly selective and sensitive fluorescence chemical sensor for the identification and detection of Al3+.

A Poly(carbazole-alt-triazole) with Thiabendazole Side Groups as an "On-Off-On" Fluorescent Probe for Detection of Cu(II) Ion and Cysteine

A novel conjugated polymer PCZBTA-TBZ containing thiabendazole as recognition unit was synthesized via Suzuki coupling reaction, and its structural characterization, spectroscopic analysis and photophysical properties were investigated. In the metal ion response study, the addition of Cu²⁺ led to the occurrence of the photoinduced electron transfer (PET) mechanism, which significantly quenched the fluorescence of the polymer PCZBTA-TBZ with a quenching effect of 98%. Furthermore, I^- can significantly quench the fluorescence of the polymer, but other anions have no such effect. According to the density functional theory calculation, compared with other polycarbazoles or other alternative copolymers containing carbazole, with alternating carbazole and triazole enhances the electron mobility and reduces the energy band gap of the polymer. Due to the strong coordination ability between Cu²⁺ and Cys, the adding Cys competes the Cu²⁺ in the [PCZBTA-TBZ-Cu²⁺] complex, blocking the occurrence of PET, and the fluorescence intensity of PCZBTA-TBZ is restored. The addition of other amino acids caused almost no change. The polymer is expected to be used for dual fluorescence detection of specific metal ions and Cys.

Conjugated microporous organic polymer as fluorescent chemosensor for detection of Fe3+ and Fe2+ ions with high selectivity and sensitivity

A conjugated microporous organic polymer (TPA-Bp) comprised of triphenylamine (TPA) and 2,2'-bipyridine-5,5'-diformaldehyde (Bp) was prepared via the Schiff-base reaction under ambient conditions. TPA-Bp is an amorphous and microporous spherical nanoparticle with very high stability. TPA-Bp suspension in DMF displayed strong fluorescence emission and selective fluorescence quenching response towards Fe³⁺ and Fe²⁺ ions. The fluorescence intensity of TPA-Bp at 331 nm presents linear relationship with the concentrations of both Fe³⁺ and Fe²⁺ with low detection limits of 1.02 × 10^-5 M for Fe³⁺ and 5.37 × 10^-6 M for Fe²⁺. The results of X-ray photoelectron spectroscopy (XPS) and Fourier Transform infrared spectroscopy (FTIR) confirm the selective coordination of N atoms of pyridine unit with Fe ions. The fluorescence quenching of TPA-Bp upon the addition of Fe³⁺/Fe²⁺ ions can be attributed to the absorption competition quenching (ACQ) mechanism and the energy transfer between TPA-Bp and Fe³⁺/Fe²⁺ ions. This work demonstrates that the conjugated microporous polymers are promising candidates as luminescent sensor for detection of the special analytes in practical applications.

A Benzothiazole-Based Fluorescence Turn-on Sensor for Copper(II)

A new benzothiazole-based chemosensor BTN (1-((Z)-(((E)-3-methylbenzo[d]thiazol-2(3H)-ylidene)hydrazono)methyl)naphthalen-2-ol) was synthesized for the detection of Cu²⁺. BTN could detect Cu²⁺ with "off-on" fluorescent response from colorless to yellow irrespective of presence of other cations. Limit of detection for Cu²⁺ was determined to be 3.3 μM. Binding ratio of BTN and Cu²⁺ turned out to be a 1:1 with the analysis of Job plot and ESI-MS. Sensing feature of Cu²⁺ by BTN was explained with theoretical calculations, which might be owing to internal charge transfer and chelation-enhanced fluorescence processes.

A hydrophilic naphthalimide-based fluorescence chemosensor forCu2+ ion: Sensing properties, cell imaging and molecular logic behavior

In this work, a hydrophilic naphthalimide-based fluorescence chemosensor (sensor 1) was synthesized for Cu²⁺ recognition, in which 2-(2-aminoethoxy)ethanol was introduced to improve the hydrophily and Schiff base acted as the multidentate ligand for Cu²⁺. The effect factors, sensing mechanism and regenerability of sensor 1 for Cu²⁺ sensing were systematically investigated. It was found that sensor 1 displayed a long emission wavelength of 532 nm upon excited in visible light region (436 nm), and the good water solubility made it utilized in aqueous media. It could selectively react with Cu²⁺ over other common metal ions to form a 2:1 complex within 1 min and result in significant fluorescence quench. The fluorescence change was linear to 0.5-10.0 μmol L^-1 of Cu²⁺ with a low detection limit of 3.74 × 10^-8 mol L^-1. Sensor 1 has been successfully utilized for analyzing Cu²⁺ in water samples as well as imaging cellular Cu²⁺. Moreover, in view of fluorescence "on-off-on" switches of sensor 1 induced by Cu²⁺ and EDTA, an IMPLICATION logic gate was constructed based on fluorescence mode with Cu²⁺ and EDTA as inputs.

Fluorescent determination of zinc by a quinoline-based chemosensor in aqueous media and zebrafish

A quinoline-based fluorescence sensor QDTD was developed for Zn²⁺. QDTD can detect Zn²⁺ by fluorescence turn-on. Detecting limit (0.27 μM) of QDTD for Zn²⁺ was far below WHO standard (76.0 μM). For the practical application, compound QDTD could be used to determine Zn²⁺ in real samples and applied to the test kit. More importantly, QDTD was expertly applied for Zn²⁺ imaging in HeLa cells and zebrafish with good membrane-permeability. Detection mechanism of Zn²⁺ ion by compound QDTD was suggested through the analytical tools like ¹H NMR titration, ESI-MS, Job plot, fluorescent and UV-vis titration, and theoretical calculations, and through the synthesis and applications of a model compound AAQA.

Metal-based optical chemosensors for CN- detection

This critical review focuses on recent advances (2010-2015) in the detection of cyanide anion via metal-based optical chemosensors in which a change in colour and/or fluorescence intensity (or emission wavelength) of a molecular metal complex is determined by the direct interaction of the metal centre with this anion.

Several hemicyanine dyes as fluorescence chemosensors for cyanide anions

Four hemicyanine dyes as chemosensors for cyanide anions were synthesized easily. Their photophysical properties and recognition properties for cyanide anions were investigated. The results indicate that all the dyes can recognize cyanide anions with obvious color, absorption and fluorescence change. The recognition mechanism analysis basing on in situ (1)H NMR and Job plot data indicates that to the compounds with hydroxyl group, the recognition mechanism is intramolecular hydrogen bonding interaction. However, to the compounds without hydroxyl group, cyanide anion is bonded to carbon-carbon double bond in conjugated bridge and induces N(+)CH3 to neutral NCH3. Fluorescence of the compounds is almost quenched upon the addition of cyanide anions.

Fluorescent oligomer as a chemosensor for the label-free detection of Fe(3+) and dopamine with selectivity and sensitivity

In this article, a sensitive and selective turn-off fluorescence chemosensor, Tyloxapol (one kind of water soluble oligomer), was developed for the label-free detection of Fe(3+) ions in aqueous solution. Fluorescence (FL) experiments demonstrated that Tyloxapol was a sensitive and selective fluorescence sensor for the detection of Fe(3+) directly in water over a wide range of metal cations including Na(+), K(+), Ag(+), Hg(2+), Cd(2+), Co(2+), Cu(2+), Cr(3+), Mn(2+), Ba(2+), Zn(2+), Ni(2+), Mg(2+), Ca(2+), and Pb(2+). Moreover, the fluorescence intensity of Tyloxapol has shown a linear response to Fe(3+) in the concentration range of 0-100 μmol L(-1) with a detection limit of 2.2 μmol L(-1) in aqueous solution. Next, based on a competition mechanism, another turn-on sensing application of the Tyloxapol/Fe(3+) platform to probe dopamine (DA) against various other biological molecules such as other neurotransmitters or amino acids (norepinephrine bitartrate, acetylcholine chloride, alanine, valine, phenylalanine, tyrosine, leucine, glycine, histidine) were also investigated. It is expected that our strategy may offer a new approach for developing simple, cost-effective, rapid and sensitive sensors in biological and environmental applications.

A novel "off-on" colorimetric and fluorescent rhodamine-based pH chemosensor for extreme acidity

A novel "off-on" colorimetric and fluorescent rhodamine analogue was synthesized and characterized, and used to monitor extreme acidity (below pH 3.5) via the photophysical response to pH. The colorless spirocyclic structure at high pH (pH⩾7.0) opened to the colored and highly fluorescent form at very low pH (pH<3.0). This sensitive pH probe was characterized with short response time, good reversibility and no interaction with interfering metal ions, and the quantitative relationship between the fluorescence intensity and pH value was consistent with the equilibrium equation pH=pKa-log[(Imax-I)/(I-Imin)]. The fluorescent response to strong acidity was further verified by fluorescent imaging of bacteria, Escherichia coli, which contributed to the development of more useful colorimetric and fluorescent sensors based on the rhodamine platform for measuring intracellular pH in extremely acidic conditions.

Selective and sensitive detection of aluminium ions in water via fluorescence "turn-on" with both solid and water soluble sensory polymer substrates

A solid substrate comprised of a cross-linked polymer network is shaped as a film with gel-like behaviour and is used to detect aluminium ions in water; concurrently, a water soluble sensory polymer synthesised towards the same purpose is also discussed. The detection in both systems was achieved via fluorescence "turn-on". The limits of detection for Al(III) were 1.6 and 25ppb for the former and latter materials, respectively; these levels are significantly lower than the EPA recommendations for drinking water.