Modulating affinities of di-2-picolylamine (DPA)-substituted quinoline sensors for zinc ions by varying pendant ligands

Selective off-on fluorescent chemosensor for detection of Fe3+ ions in aqueous media

A Fe(3+) chemosensor L1 was successfully synthesized with a quinoline moiety bound to rhodamine 6G hydrazide. The sensor L1 shows high selectivity and sensitivity to Fe(3+) in aqueous solution in the presence of other trace metal ions in organisms, abundant cellular cations and prevalent toxic metal ions in the environment. In addition, biological imaging and micro computed tomography (MCT) technology studies have demonstrated that L1 could act as a turn-on fluorescent chemosensor for Fe(3+) in living cells.

Single fluorescent probe displays a distinct response to Zn2+ and Cd2+

Single probe, multiple targets: the design and synthesis of a new probe is described that can readily discriminate Zn(2+) and Cd(2+) with different fluorescence signals. The studies of Job's plot, mass spectroscopy, DFT calculations, and (1)H NMR spectroscopy reveal that the new probe has possible distinct binding modes to Zn(2+) and Cd(2+). This work may open up new opportunities for the development of single fluorescent probes that can sense multiple targets of interest.

A highly selective fluorescent sensor for distinguishing cadmium from zinc ions based on a quinoline platform

A fluorescent sensor, N-(quinolin-8-yl)-2-(quinolin-8-yloxy)acetamide (HL), based on 8-aminoquinoline and 8-hydroxyquinoline platforms has been synthesized. This sensor displays high selectivity and sensitive fluorescence enhancement to Cd(2+) in ethanol. Moreover, sensor HL can distinguish Cd(2+) from Zn(2+) via two different sensing mechanisms (photoinduced electron transfer for Cd(2+); internal charge transfer for Zn(2+)). The composition of the complex Cd(2+)/HL or Zn(2+)/L(-) has been found to be 1:1, based on the fluorescence/absorption titration and further confirmed by X-ray crystallography.

Anion recognition and sensing with Zn(II)-dipicolylamine complexes

This critical review covers the developments in anion recognition and sensing using Zn(II)-dipicolylamine functionalized receptors over the past decade with emphasis on recent rapid advances in the last five years.

Highly selective fluorescent chemosensor for Zn2+ derived from inorganic-organic hybrid magnetic core/shell Fe3O4@SiO2 nanoparticles

Magnetic nanoparticles with attractive optical properties have been proposed for applications in such areas as separation and magnetic resonance imaging. In this paper, a simple and novel fluorescent sensor of Zn2+ was designed with 3,5-di-tert-butyl-2-hydroxybenzaldehyde [DTH] covalently grafted onto the surface of magnetic core/shell Fe3O4@SiO2 nanoparticles [NPs] (DTH-Fe3O4@SiO2 NPs) using the silanol hydrolysis approach. The DTH-Fe3O4@SiO2 inorganic-organic hybrid material was characterized by transmission electron microscopy, dynamic light scattering, X-ray power diffraction, diffuse reflectance infrared Fourier transform, UV-visible absorption and emission spectrometry. The compound DTH exhibited fluorescence response towards Zn2+ and Mg2+ ions, but the DTH-Fe3O4@SiO2 NPs only effectively recognized Zn2+ ion by significant fluorescent enhancement in the presence of various ions, which is due to the restriction of the N-C rotation of DTH-Fe3O4@SiO2 NPs and the formation of the rigid plane with conjugation when the DTH-Fe3O4@SiO2 is coordinated with Zn2+. Moreover, this DTH-Fe3O4@SiO2 fluorescent chemosensor also displayed superparamagnetic properties, and thus, it can be recycled by magnetic attraction.

Zinc-specific fluorescent response of tris(isoquinolylmethyl)amines (isoTQAs)

Isoquinoline-based tetradentate ligands with C(3)-symmetry, tris(1- or 3-isoquinolylmethyl)amine (1- or 3-isoTQA), have been prepared and their zinc-induced fluorescence enhancement was investigated. Upon excitation at 324 nm, 1-isoTQA shows very weak fluorescence (ϕ = ∼0.003) in DMF/H(2)O (1/1) solution. In the presence of zinc ion, 1-isoTQA exhibits fluorescence increase (ϕ = 0.041) at 359 and 470 nm. This fluorescence enhancement at 470 nm is specific for zinc. However, 3-isoTQA exhibited a smaller fluorescence enhancement upon zinc complexation (ϕ = 0.017, λ(em) = 360 and 464 nm) compared with 1-isoTQA. Crystal structures of zinc complexes of isoTQAs demonstrate the diminished steric crowding and shorter Zn-N(aromatic) distances compared with isoTQENs (N,N,N',N'-tetrakis(isoquinolylmethyl)ethylenediamines) leads to a higher fluorescent response toward zinc relative to cadmium.

Balance between fluorescence enhancement and association affinity in fluorescent heteroditopic indicators for imaging zinc ion in living cells

A fluorescent heteroditopic indicator for the zinc(II) ion possesses two different zinc(II) binding sites. The sequential coordination of zinc(II) at the two sites can be transmitted into distinct fluorescence changes. In the heteroditopic ligand system that our group developed, the formations of mono- and dizinc(II) complexes along an increasing gradient of zinc(II) concentration lead to fluorescence enhancement and an emission bathochromic shift, respectively. The extents of these two changes determine the sensitivity and, ultimately, the effectiveness of the heteroditopic indicator in quantifying zinc(II) ion over a large concentration range. In this work, a strategy to increase the degree of fluorescence enhancement upon the formation of the monozinc(II) complex of a heteroditopic ligand under simulated physiological conditions is demonstrated. Fluorination of the pyridyl groups in the pentadentate N,N,N'-tris(pyridylmethyl)ethyleneamino group reduces the apparent pK(a) value of the high-affinity site, which increases the degree of fluorescence enhancement as the monozinc(II) complex is forming. However, fluorination impairs the coordination strength of the high-affinity zinc(II) binding site, which in the triply fluorinated ligand reduces the binding strength to the level of the low-affinity 2,2'-bipyridyl. The potential of the reported ligands in imaging zinc(II) ion in living cells was evaluated. The subcellular localization properties of two ligands in five organelles were characterized. Both benefits and deficiencies of these ligands were revealed, which provides directions for the near future in this line of research.

Solvent dependent fluorescent properties of a 1,2,3-triazole linked 8-hydroxyquinoline chemosensor: tunable detection from zinc(II) to iron(III) in the CH3CN/H2O system

A triazole-containing 8-hydroxyquinoline (8-HQ) ether 2 was efficiently synthesized in two steps from the "click" strategy. Compound 2 gave a strong fluorescence (Φ = 0.21) in nonprotic solvent like CH(3)CN, and a weak fluorescence (Φ = 0.06) in protic solvent like water. In water, a more than 100 nm red shift of the fluorescence maximum was observed for compound 2 in comparison with that in CH(3)CN. This fluorescence difference may be attributed to the intermolecular photoinduced proton transfer (PPT) process involving the protic solvent water molecules. Similarly, this intermolecular PPT process was also observed in the high-water-content CH(3)CN aqueous solution (e.g., CH(3)CN/H(2)O = 5/95, v/v). The water content in the CH(3)CN/H(2)O binary solvent mixture greatly affected the fluorescence intensity (e.g., Φ = 0.06 and 0.25 when CH(3)CN/H(2)O = 5/95 and 95/5, v/v, respectively) and emission wavelength. Using this interesting property, by simple variation of the water content in the CH(3)CN aqueous solution, compound 2 was tuned from a selective "turn-on" fluorescent sensor for Zn(2+) (CH(3)CN/H(2)O = 5/95, v/v) to a ratiometric one for Zn(2+) and a selective "turn-off" one for Fe(3+) (CH(3)CN/H(2)O = 95/5, v/v) over a wide range of pH value. In high-water-content (CH(3)CN/H(2)O = 5/95, v/v) aqueous solution compound 2 shows a selective "turn-on" response toward Zn(2+), with a 10-fold enhancement in the fluorescence intensity at 428 nm and a 62 nm blue shift of the emission maximum (490 to 428 nm) due to the inhibition of intermolecular PPT process upon chelating with Zn(2+). However, in a less polar solvent (CH(3)CN/H(2)O = 95/5, v/v) in which compound 2 has high fluorescence (quantum yield =0.25), it shows a ratiometric response toward Zn(2+), with a continuous decrease of the fluorescence intensity at 399 nm and an increase at 423 nm. More interestingly, in this case, it also exhibits a very sensitive, selective, and ratiometric fluorescence quenching in the presence of Fe(3+), with an 81 nm red shift of the emission maximum (399 to 480 nm) in a wide range of pH through a metal ligand charge transfer (MLCT) effect.

Structure and spectral properties of dinuclear zinc complex containing semicarbazonate ligands

The dinuclear Zn(2+) complex [Zn(HSSC)OAc](2)·2DMF (H(2)SSC=salicylaldehyde semicarbazone; HOAc=acetic acid; DMF=N,N-dimethylfomamide) was prepared and structurally characterized by single crystal X-ray. The basic structural unit of the complex is a dinuclear complex [Zn(HSSC)OAc](2) in which the semicarbazone ligand adopts the phenol-imine form. The deprotonated phenol group forms a one-atom bridge between the two zinc centers, and both of the zinc centers are five-coordinated. The local coordination environment of Zn(2+) can be approximately considered as square pyramidal. UV spectral studies show that the H(2)SSC provides strong binding of Zn(2+) in a 1:1 ratio in solution. The conditional binding constant of the complex is lgK(Zn-L)=12.89±0.76 in 0.05M Tris-HCl buffer at pH 7.4. The H(2)SSC exhibits an enhanced fluorescence effect by the addition of Zn(2+), and affords an excellent selectivity for Zn(2+) under physiological conditions.

Methoxy-substituted isoTQEN family for enhanced fluorescence response toward zinc ion

Previously, we have reported that 1- and 3-isoTQENs (N,N,N',N'-tetrakis(1- or 3-isoquinolylmethyl)ethylenediamines) exhibit a specific fluorescence enhancement toward zinc ion. In this study, three methoxy-substituted derivatives of 1-isoTQEN were synthesized and their fluorescent response toward zinc ion was studied. The substitution pattern of the methoxy group significantly changes the solubility of compounds in aqueous DMF, λ(max) in the absorption spectra, excitation/emission wavelengths and fluorescence intensity of zinc complexes. In the presence of zinc ion, 7-MeO-1-isoTQEN exhibits higher fluorescence intensity and longer excitation/emission wavelengths (λ(ex) = 342 nm, λ(em) = 526 nm) than 6-MeO-1-isoTQEN (λ(ex) = 303 nm, λ(em) = 469 nm) and 5,6,7-triMeO-1-isoTQEN (λ(ex) = 340 nm, λ(em) = 504 nm). The fluorescence intensity of a zinc complex of 7-MeO-1-isoTQEN (ϕ = 0.122) is four times higher than the parent 1-isoTQEN (ϕ = 0.034) under the same conditions. The crystal structure of 7-MeO-1-isoTQEN-Zn complex reveals that all six nitrogen atoms participate to the metal coordination with ideal octahedral geometry, affording significantly high metal binding affinity comparable with TPEN (N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine). 7-MeO-1-isoTQEN detects zinc ion concentration change in cells by fluorescence microscopic analysis.

Rationally designed fluorescence turn-on sensors: a new design strategy based on orbital control

Herein, we explore a new strategy in the chemo-sensor field for fluorescence amplification upon binding with metal ions based on controlled participation of the nitrogen lone pair orbital. The basic architecture of the sensor entails a fluorophore, the sp(2) hybridized nitrogen lone pair (-C═N-), and a chelator site referred to as the control part. Though nonplanar and nonfluorescent, compound IC1 achieved pseudo planarity from binding with Zn(2+) as indicated by the increased fluorescence signal. Its other analogue (IC2) is also planar, and unlike IC1-Zn(2+) was fluorescent with a lack of binding affinity to metal ions. The time-dependent density functional theory (TDDFT) calculations revealed that the fluorescence amplification was due to the blocking of the nitrogen lone pair orbital; unlikely geometrical rearrangements were insignificant. This could indicate a breakthrough concept in the future design of fluorescent turn-on sensors.

Crystal structure of an oxalate-bridged tetranuclear 8-hydroxyquinoline Zn(II) cluster: [Zn4Q6(Ox)]0.5n

The chain structure of a tetranuclear zinc(II) cluster [Zn4Q6(Ox)]0.5n ([Zn4 (C9H6NO)6(C2O4)]0.5n) (1) (Q = 8- hydroxyquinoline anion, Ox = oxalate dianion) was determined by X-ray crystallography and characterized by elemental analysis, IR spectroscopy and thermal analysis. It crystallizes in the monoclinic system, space group P21/n (No. 14), with the lattice parameters a = 13.2222(15) ?, b = 11.0566(12) ?, c = 16.2224(18) ?, ? = 92.1770(10)?, V = 2369.9(5) ?3, Z = 4, Mr = 607.23 g mol-1, Dc = 1.702 g cm-3. The tetranuclear zinc(II) clusters form 1D polymeric chains parallel to the b-axis. The ?-? stacking interactions involving aryl rings support the formation of the 1D polymeric structure. The neighboring polymeric chains are connected by C-H???? interactions.

Fluorescent PET (photoinduced electron transfer) sensors as potent analytical tools

Fluorescent sensors are an important part of the analytical scientist's toolbox. The use of fluorescent PET (Photoinduced Electron Transfer) sensors has seen particular growth in recent times. This Critical Review discusses recent growth areas in fluorescent PET sensors by emphasizing the modular features of the 'fluorophore-spacer-receptor' design. The occurrence of the dipicolylamine receptor in PET sensor designs is critically examined as a case in point.

A ratiometric fluorescent sensor for zinc ions based on covalently immobilized derivative of benzoxazole

In the present paper, we describe the fabrication and analytical characteristics of fluorescence-based zinc ion-sensing glass slides. To construct the sensor, a benzoxazole derivative 4-benzoxazol-2'-yl-3-hydroxyphenyl allyl ether (1) with a terminal double bond was synthesized and copolymerized with 2-hydroxyethyl methacrylate (HEMA) on the activated surface of glass slides by UV irradiation. In the absence of Zn(2+) at pH 7.24, the resulting optical sensor emitted fluorescence at 450 nm via excited-state intramolecular proton transfer (ESIPT). Upon binding with Zn(2+), the ESIPT process was inhibited resulting in a 46 nm blue-shift of fluorescence emission. Thus, the proposed sensor can behave as a ratiometric fluorescent sensor for the selective detection of Zn(2+). In addition, the sensor shows nice selectivity, good reproducibility and fast response time. Cd(2+) did not interfere with Zn(2+) sensing. The sensing membrane demonstrates a good stability with a lifetime of at least 3 months. The linear response range covers a concentration range of Zn(2+) from 8.0x10(-5) to 4.0x10(-3) mol/L and the detection limit is 4.0x10(-5) mol/L. The determination of Zn(2+) in both tap and river water samples shows satisfactory results.