Quenching mechanism of Zn(salicylaldimine) by nitroaromatics

Nitroaromatics and nitroalkanes quench the fluorescence of Zn(Salophen) (H2Salophen = N,N'-phenylene-bis-(3,5-di- tert-butylsalicylideneimine); ZnL(R)) complexes. A structurally related family of ZnL(R) complexes (R = OMe, di-tBu, tBu, Cl, NO2) were prepared, and the mechanisms of fluorescence quenching by nitroaromatics were studied by a combined kinetics and spectroscopic approach. The fluorescent quantum yields for ZnL(R) were generally high (Phi approximately 0.3) with sub-nanosecond fluorescence lifetimes. The fluorescence of ZnL(R) was quenched by nitroaromatic compounds by a mixture of static and dynamic pathways, reflecting the ZnL(R) ligand bulk and reduction potential. Steady-state Stern-Volmer plots were curved for ZnL(R) with less-bulky substituents (R = OMe, NO2), suggesting that both static and dynamic pathways were important for quenching. Transient Stern-Volmer data indicated that the dynamic pathway dominated quenching for ZnL(R) with bulky substituents (R = tBu, DtBu). The quenching rate constants with varied nitroaromatics (ArNO2) followed the driving force dependence predicted for bimolecular electron transfer: ZnL* + ArNO2 --> ZnL(+) + ArNO2(-). A treatment of the diffusion-corrected quenching rates with Marcus theory yielded a modest reorganization energy (lambda = 25 kcal/mol), and a small self-exchange reorganization energy for ZnL*/ZnL(+) (ca. 20 kcal/mol) was estimated from the Marcus cross-relation, suggesting that metal phenoxyls may be robust biological redox cofactors. Electronic structure calculations indicated very small changes in bond distances for the ZnL --> ZnL(+) oxidation, suggesting that solvation was the dominant contributor to the observed reorganization energy. These mechanistic insights provide information that will be helpful to further develop ZnL(R) as sensors, as well as for potential photoinduced charge transfer chemistry.     

Fluorescent detection of nitroaromatics and 2,3-dimethyl 2,3-dinitrobutane (DMNB) by a zinc complex: (salophen)Zn

Fluorescent sensors for the detection of chemical explosives are in great demand. It is shown herein that the fluorescence of ZnL* (H2L=N,N'-phenylene-bis-(3,5-di-tert-butylsalicylideneimine)) is quenched in solution by nitroaromatics and 2,3-dimethyl-2,3-dinitrobutane (DMNB), chemical signatures of explosives. The relationship between the structure and fluorescence of ZnL is explored, and crystal structures of three forms of ZnL(base), (base=ethanol, tetrahydrofuran, pyridine) are reported, with the base=ethanol structure exhibiting a four-centered hydrogen bonding array. Solution structures are monitored by 1H NMR and molecular weight determination, revealing a dimeric structure in poor donor solvents which converts to a monomeric structure in the presence of good donor solvents or added Lewis bases to form five-coordinate ZnL(base). Fluorescence wavelengths and quantum yields in solution are nearly insensitive to monomer-dimer interconversion, as well as to the identity of the Lewis base; in contrast, the emission wavelength in the solid state varies for different ZnL(base) due to pi-stacking. Nitroaromatics and DMNB are moderately efficient quenchers of ZnL*, with Stern-Volmer constants KSV=2-49 M-1 in acetonitrile solution.     

Discrimination of liquor aromas by pattern recognition analysis of responses from a gas sensor array

A sensor array composed of six semiconductor gas sensors was applied to the discrimination of liquor aromas. A semi-automatic headspace concentrator utilizing porous polymers was used for pretreatment of sample aromas in order to remove excess amounts of ethanol and to standardize aroma introduction to the sensor array. The differences in response patterns for liquor samples were not so conspicuous because of the non-selectivity of the gas sensors. After normalizing the sensor responses to eliminate the effects of absolute magnitude, pattern recognition analysis was applied to the resulting six-dimensional data matrices. Cluster analysis succeeded in classifying eight liquors. Five spirits (cognac and four different brands of whisky) were correctly classified by both linear discriminant analysis and cluster analysis.     

A novel detector using a fluorescent sensor array and discrimination of pesticides

A novel, simple, and rapid detector using a fluorescent sensor array for discrimination and quantification of different concentrations (ppb level) of pesticides was proposed in this paper. Employing porphyrin, porphyrin derivatives, and chemically responsive dyes as the sensing elements, the developed sensor array based on a cross-responsive mechanism showed a unique pattern of fluorescence changes upon the reaction that lasted just 10 min. The eigenvalues from raw fluorescence spectra were analyzed via a pattern recognition algorithm, including hierarchical cluster analysis (HCA), principal component analysis (PCA), and back-propagation neural network (BPNN). The results showed that HCA, which were used to assess the feasibility and effectiveness of discrimination of the fluorescent sensor array, revealed a distinct separation between different pesticides. PCA and BPNN were used for automatically predicting the concentration of pesticides, and the recovery was 91.29–109.81 % while the lowest relative standard deviation was up to 3.12 %. It indicates a detector based on the fluorescent sensor array is a rapid and feasible sensing platform for the discrimination and quantitative analysis of pesticides, and also shows the possibilities in the related fields of pesticides identification and detection.     

Ratiometric fluorescent sensor proteins with subnanomolar affinity for Zn(II) based on copper chaperone domains

The ability to image the concentration of transition metals in living cells in real time is important for further understanding of transition metal homeostasis and its involvement in diseases. The goal of this study was to develop a genetically encoded FRET-based sensor for copper(I) based on the copper-induced dimerization of two copper binding domains involved in human copper homeostasis, Atox1 and the fourth domain of ATP7B (WD4). A sensor has been constructed by linking these copper binding domains to donor and acceptor fluorescent protein domains. Energy transfer is observed in the presence of Cu(I), but the Cu(I)-bridged complex is easily disrupted by low molecular weight thiols such as DTT and glutathione. To our surprise, energy transfer is also observed in the presence of very low concentrations of Zn(II) (10(-)(10) M), even in the presence of DTT. Zn(II) is able to form a stable complex by binding to the cysteines present in the conserved MXCXXC motif of the two copper binding domains. Co(II), Cd(II), and Pb(II) also induce an increase in FRET, but other, physiologically relevant metals are not able to mediate an interaction. The Zn(II) binding properties have been tuned by mutation of the copper-binding motif to the zinc-binding consensus sequence MDCXXC found in the zinc transporter ZntA. The present system allows the molecular mechanism of copper and zinc homeostasis to be studied under carefully controlled conditions in solution. It also provides an attractive platform for the further development of genetically encoded FRET-based sensors for Zn(II) and other transition metal ions.     

A 2D Cd(II)-MOF as a multifunctional luminescencent sensor for nitroaromatics,iron(III) and chromate ions

A Cd(II)-MOF, {[Cd(L)(4,4′-bipy)]·H₂O·DMF}_n (1) (L = nicotinic acid (2,4-dihydroxybenzylidene)-hydrazide and 4,4′-bipy = 4,4′-bipyridine), has been synthesized and characterized by microanalyses, FTIR, TGA, and single-crystal X-ray diffraction. Additionally, powder X-ray diffraction was performed to check the phase purity of the synthesized compound. Single-crystal X-ray diffraction reveals that 1 has a 2D grid network. Photoluminescent sensing of nitrobenzene, Fe(III) and CrO₄^2? ions indicates that 1 could be a candidate for developing selective luminescent sensors for these species. Theoretical calculations have been performed to gain insight into the possible mechanism of quenching effect in emission on addition of nitrobenzene in 1 which supports the mechanism operating through ground state charge transfer between 1 and nitrobenzene.     

A pyrene-based fluorescent sensor for Zn2+ ions: a molecular 'butterfly'

A simple pyrene-based triazole receptor has been synthesised and shown to self-assemble in the presence of ZnCl(2) in an exclusively 2:1 ratio, whereas a mixture of 2:1 and 1:1 ratios are observed for other Zn(2+) salts. The pyrene units are syn in orientation; this is supported by a strong excimer signal observed at 410 nm in the presence of ZnCl(2) in acetonitrile. DFT calculations and 2D NMR support the proposed structure.     

Anion detection by fluorescent Zn(II) complexes of functionalized polyamine ligands

The Zn(2+) coordination chemistry and luminescent behavior of two ligands constituted by an open 1,4,7-triazaheptane chain functionalized at both ends with 2-picolyl units and either a methylnaphthyl (L1) or a dansyl (L2) fluorescent unit attached to the central amino nitrogen are reported. The fluorescent properties of the ZnL1(2+) and ZnL2(2+) complexes are then exploited toward detection of anions. L1 in the pH range of study has four protonation constants. The fluorescence emission from the naphthalene fluorophore is quenched either at low or at high pH values leading to an emissive pH window centered around pH = 5. In contrast, in L2 the fluorescence emission from the dansyl unit occurs only at basic pH values. In the case of L1, a red-shifted band appearing in the visible region was assigned to an exciplex emission involving the naphthalene and the tertiary amine of the polyamine chain. L1 forms Zn(2+) mononuclear complexes of ZnH(p)L1((p+2)+) stoichiometry with p = 1, 0, -1. Formation of the ZnL1(2+)species produces a strong enhancement of the L1 luminescence leading to an extended emissive pH window from pH = 5 to pH = 9. Addition of several anions to this last complex leads to a partial quenching effect. On the contrary, the fluorescence emission of L2 is partially quenched upon complexation with Zn(2+) in the same pH window (5 < pH < 9). The lower stability of ZnL2(2+) with respect to ZnL1(2+) suggests a lack of involvement of the sulfonamide group in the first coordination sphere. However, there is spectral evidence for an interesting photoinduced binding of the sulfonamide nitrogen to Zn(2+). While addition of diphosphate, triphosphate, citrate, and D,L-isocitrate to a solution of ZnL2(2+) restores the fluorescence emission of the system (lambda max ca. 600 nm), addition of phosphate, chloride, iodide, and cyanurate do not produce any significant change in fluorescence. Moreover, this system would permit one to differentiate diphosphate and triphosphate over citrate and d, l-isocitrate because the fluorescence enhancement observed upon addition of the first anions is much sharper. The ZnL2(2+) complex and its mixed complexes with diphosphate, triphosphate, citrate, and D,L-isocitrate have been characterized by (1)H, (31)P NMR, and Electrospray Mass Spectrometry.     

Selective dendritic fluorescent sensors for Zn(II)

A series of dendritic 8-hydroxyquinoline (8-HQ) and 5-dialkyl(aryl)aminomethyl-8-HQ derivatives were synthesized and their fluoroionophoric properties toward representative alkali, alkaline earth, group IIIA and transition metal ions were investigated. Among the selected ions, Zn(II) enhanced the fluorescence of N-di-(methoxycarbonylethyl)aminoethyl-3-[4-(8-hydroxyquinolin-5-ylmethyl)piperazin-1-yl]-propanoic amide] (7) by 31-fold, while Al(III) caused enhancement to some extent. The absence of any significant fluorescence enhancement by the other ions examined renders 7a highly useful Zn(II)-selective fluorescent sensor.     

Highly sensitive detection of nitroaromatic explosives using an electrospun nanofibrous sensor based on a novel fluorescent conjugated polymer

An electrospun nanofibrous explosive sensor was first constructed based on a newly developed fluorescent conjugated polymer P containing heteroatom polycyclic units. Electrospinning by doping polymer P as a fluorescent probe in a polystyrene supporting matrix afforded a fluorescence nanofibrous film with unique porous structures, and effectively avoided the aggregation of polymer P. The novel explosive sensor exhibited stable fluorescence property, satisfactory reversibility with less than 5% loss of signal intensity after four quenching–regeneration cycles, and good reproducibility among three batches with a relative standard deviation of 2.8%. Such fabricated sensor also showed remarkable sensitivity toward a series of trace nitroaromatic explosive vapors, including picric acid (parts-per-trillion level) and 2,4,6-trinitrotoluene vapor (parts-per-billion level), as well as good selectivity with less than 10% response to typical interferents. Therefore, the present strategy extends the application of different kinds of conjugated polymers for the construction of optical chemosensors.     

Highly sensitive sensing of zinc(II) by development and characterization of a PVC-based fluorescent chemical sensor

A sensor membrane with excellent performance based on 1-methyl-1-phenyl-3-[1-hydroxyimino-2-(succinimido)ethyl]cyclobutane has been developed for the determination of zinc(II) ions. The sensing membrane is capable of determining zinc(II) with an outstanding high selectivity over a dynamic range between 8.0×10(-8) and 1.6×10(-4) mol L(-1) with a limit of detection of 2.5×10(-8) mol L(-1) (1.6 μg L(-1)). It can be easily and completely regenerated by using 0.1 mol L(-1) EDTA solution. The optical sensor developed here was found to be stable, cost effective, easy to prepare, and has unique selectivity towards Zn(2+) ion with respect to common metal ions. The proposed sensor was then applied for the determination of zinc in tap water and hair samples with satisfactory results.     

A photoswitchable Zn (II) selective spiropyran-based sensor

A spiropyran-based fluorescent and photoregenerable receptor, that is, selective towards zinc(II) ions over a series of biologically and environmentally relevant cations has been designed and synthesized. The complex formation gives rise to colour changes that are visible to the naked eye and reversible upon visible light irradiation. ¹H NMR studies confirm that the closed form is converted to the open trans-merocyanine complex upon addition of zinc ions.     

Tetrakis(2-quinolinylmethyl)ethylenediamine (TQEN) as a new fluorescent sensor for zinc

A new fluorescent sensor for zinc that binds 1 equivalent of zinc ion, N,N,N',N'-tetrakis(2-quinolylmethyl)ethylenediamine (TQEN), has been prepared and characterized. Zinc-bound TQEN exhibits fluorescence around 383 nm upon excitation at 317 nm, while free TQEN emits very weak fluorescence. UV-Vis and 1H NMR spectral changes also detected the binding of TQEN with zinc ion. The crystal structure of zinc complex with TQEN was determined by X-ray crystallography and compared with that of the TPEN-Zn complex (TPEN =N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine). The binding affinity of TQEN with zinc ion is very high (Kd < 1 microM in aqueous dmf solution). Competition experiments reveal that the zinc-binding affinity of TQEN is lower than the parent, strong metal ion chelator TPEN, and comparable to EGTA (EGTA = ethylene glycol-bis(2-aminomethyl)-N,N,N',N'-tetraacetic acid).     

Novel highly selective fluorescent chemosensors for Zn(II)

Two novel fluorescent Zn²⁺ chemosensors were synthesized in four steps from inexpensive starting materials. They exhibited very strong fluorescence responses to Zn²⁺ and had remarkably high selectivity to Zn²⁺ than other metal ions including Mg²⁺, Ca²⁺, Ni²⁺, Cu²⁺, and Cd²⁺. These two new molecules could be used as low-priced yet high-quality Zn²⁺ chemosensors.     

Specific detection of D-glucose by a tetraphenylethene-based fluorescent sensor

A conceptually new "light-up" biosensor with a high specificity for d-glucose (Glu) in aqueous media has been developed. The emission from a tetraphenylethene (TPE)-cored diboronic acid (1) was greatly boosted when the fluorogen was oligomerized with Glu because of restriction of the intramolecular rotations of the aryl rotors of TPE by formation of the oligomer. Little change in the light emission was observed when 1 was mixed with D-fructose, D-galactose, or D-mannose, as these saccharides are unable to oligomerize with the fluorogen.     

Binding of H+ and Zn(II) ions with a new fluorescent macrocyclic phenanthrolinophane

The new macrocyclic ligand 1,9(4,7)-diphenanthroline-3,7,11,15-tetraazacyclohexadecaphane (L) was synthesized by a 2?:?2 reaction of 1,10-phenanthroline-4,7-dialdehyde with 1,3-diaminopropane, followed by reduction with NaBH(4). L contains two phenanthroline groups linked together by two 1,3-diaminopropane chains in such a way that the heteroaromatic nitrogen atoms point outside the ligand cavity. The ligand structure defines two pairs of identical compartments displaying a specific ability in the binding of protons (1,3-diaminopropane) and metal ions (phenanthroline). Protonation and Zn(II) coordination were studied by means of potentiometric and spectroscopic ((1)H NMR, UV-vis, fluorescence) techniques. Both protonation and Zn(II) coordination consistently affect the fluorescence emission properties of L, giving rise to enhancement or quenching of the emission, depending on the species involved. L becomes emissive upon protonation, but the formation of the highly protonated species, in particular the fully protonated [H(6)L](6+), quenches the emission. The mono- and dinuclear Zn(II) complexes of the unprotonated ligand are non-emissive, like free L, while Zn(II) binding to [HL](+) activates the emission. The most interesting aspect, however, is the chelation enhancement of quenching (CHEQ) observed upon Zn(II) binding to [H(2)L](2+) and [H(4)L](4+), being among the few examples of CHEQ effect observed for Zn(II) complexes. Hydrogen bonding between a metal coordinated water molecule and a phenanthroline group seems to be responsible for the CHEQ observed for [ZnH(2)L](4+).