Phenothiazine-based oligomers as novel fluorescence probes for detecting vapor-phase nitro compounds

To meet the need for rapid and low-cost chemical sensing of explosive, new fluorescence chemosensors based on oligophenothiazines for probing vapor-phase nitro compounds have been developed. The phenothiazine-based trimer P3 and pentamer P5 have been synthesized via Heck and Wittig reactions by convergent approach. It was found that they can detect the vapors of nitro compounds, including p-nitrotoluene (p-NT), 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT) with good sensitivity and reversibility. And the sensor of P3 film gave a linear fluorescence quenching response to 7-800 ppb TNT with the detection limit of 4 ppb. For DNT vapor, a linear working range of the sensor was 2-24 ppm with the detection limit of 40 ppb. Meanwhile, the interferents, including common organic solvents, p-nitrophenol and 2,4-dinitrophenylhydrazine cannot lead to obvious fluorescence quenching, meaning that the film based on oligophenothiazines exhibited good specificity of fluorescence response to explosive. Based on the fluorescence lifetime and UV-vis absorption measurements, we suggested that the fluorescence quenching of oligophenothiazine-based films exposed to the vapors of nitro compounds was due to the formation of non-fluorescent charge-transfer complex between oligophenothiazine and nitro compounds.     

Synthesis and characterization of polybinaphthalene incorporating chiral (R) or (S)-1,1′-binaphthalene and oxadiazole units by Heck reaction

Chiral conjugated polymers P-1 and P-2 were synthesized by the polymerization of (R)-3,3′-diiodo-2,2′-bisbutoxy-1,1′-binaphthalene ((R)-M-1) and (S)-3,3′-diiodo-2,2′-bisbutoxy-1,1′-binaphthalene ((S)-M-1) with 2,5-bis(4-vinylphenyl)-1,3,4-oxadiazole (M-2) under Pd-catalyzed Heck coupling reaction, respectively. Both monomers and polymers were analysed by NMR, MS, FT-IR, UV, DSC-TG, fluorescent spectroscopy, GPC and CD spectra. The chiral conjugated polymers exhibit strong Cotton effect in their circular dichroism (CD) spectra indicating a high rigidity of polymer backbone. CD spectra of polymers P-1 and P-2 are almost identical and have opposite signs for their position. These polymers have strong blue fluorescence.     

1-(4-Nitrophenoxycarbonyl)-7-pyridin-4-yl indolizine: a new versatile fluorescent building block. Application to the synthesis of a series of fluorescent β-cyclodextrins

The synthesis of a series of new fluorescent building blocks 1a-d incorporating a pyridinoindolizine unit and two potentially reactive sites is described. The reaction of 1a-d with the mono-6-amino-6-deoxy-β-cyclodextrin provides the corresponding fluorescent water soluble hosts 2a-d in good yield. The sensor properties of 2a-d in the presence of 1-adamantanol is described.     

A new fluorescent sensor for the detection of pyrophosphate based on a tetraphenylethylene moiety

We have developed a new fluorescent sensor 1-2Zn based on a tetraphenylethylene (TPE) moiety for the detection of PPi. This TPE-based chemosensor showed ‘turn-on’ fluorescence emission according to the concentration of PPi. The fluorescence enhancement upon binding of PPi to 1-2Zn resulted from the restriction of intramolecular rotation of phenyl rings in 1-2Zn.     

Complete suppression of the fluorophore fluorescence by combined effect of multiple fluorescence quenching groups: A fluorescent sensor for Cu with zero background signals

The reaction-based fluorescent sensors have attracted increasing attention in the past decades. However, the application of these sensors for accurate sensing was significantly retarded by the background fluorescence from the sensors themselves. In this work, we demonstrated a novel strategy that the background fluorescence of the sensor could be completely eliminated by the combined effect of multiple fluorescence quenching groups. Based on this new strategy, as proof-of-principle study, a fluorescent sensor (CuFS) for Cu²⁺ was judiciously developed. In CuFS, three types of fluorescence quenching groups were directly tethered to a commonly used coumarin fluorophore. The fluorescence of coumarin fluorophore in CuFS was completely suppressed by the combined effect of these fluorescence quenching groups. Upon treatment with 22 μM Cu²⁺, sensor CuFS achieved a dramatic fluorescence enhancement (fluorescence intensity enhanced up to 811-fold) centered at 469 nm. The detection limits was determined to be 12.3 nM. The fluorescence intensity enhancement also showed a good linearity with the Cu²⁺ concentration in the range of 12.3 nM to 2 μM. By fabricating test strips, sensor CuFS can be utilized as a simple tool to detect Cu²⁺ in water samples. Furthermore, the fluorescent sensor was successfully applied in detecting different concentration of Cu²⁺ in living cells.     

N-heterocyclic carbene enabled synthesis of conjugated polymers

The Suzuki polycondensation of a dihalogenated 4,5-diphenylimidazole (2) with a fluorenyl diboronic acid diester followed by methylation afforded a conjugated poly(imidazolium) copolymer (P2) in 93% yield. Upon exposure to strong base, P2 was converted in situ to the corresponding poly(N-heterocyclic carbene) P3, as evidenced by ¹H NMR spectroscopy and a trapping experiment involving sulfur that afforded the corresponding poly(thiourea) P4. Similarly, treating a solution of P2 with KOtBu and [Ir(1,5-cyclooctadiene)Cl]₂ afforded a conjugated polymer bearing pendant Ir complexes (P5) in 63% yield. Thermal and photophysical analyses of the aforementioned polymers revealed that they were thermally stable with tunable fluorescence properties, features which poise them for use in various electronic and sensing applications. The presented methodology is expected to facilitate the synthesis of a broad range conjugated organometallic polymers from a common and readily accessible precursor.     

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.     

Substituent effects on the properties of photochromic diarylethenes

Photochromic symmetrical diarylethenes 1o-5o bearing different electron-donating or electron-withdrawing substitutents have been synthesized, and the structures of 1o, 2o, 4o, and 5o were determined by single-crystal X-ray diffraction analysis. Substitutent effects on their optoelectronic properties, including photochromism, fluorescence, and electrochemical properties were investigated in detail. The electron-withdrawing substituents can shift significantly the absorption maxima of the diarylethenes to a longer wavelength and increase their cyclization quantum yield, while the molar absorption coefficients increased with an increasing electron-donating ability. Diarylethenes 1, 2, and 4 show good photochromism both in solution and in the single crystalline phase; however, diarylethenes 5 show no photochromism in the crystalline phase because the distance between the reactive carbons become larger than 4.2 Å. Diarylethenes 1-3 exhibited good fluorescent switching upon alternating irradiation with UV and visible light, and their fluorescent conversions in the photostationary state were all larger than 80% in hexane. In addition, cyclic voltammetry tests showed that different electron-donating and electron-withdrawing substituents had a remarkable effect on the electrochemical behaviors of these diarylethenes.     

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.     

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.     

Highly enantioselective fluorescent recognition of amino acid derivatives by unsymmetrical salan sensors

Novel unsymmetrical salan fluorescent sensors 2a and 2b have been designed and synthesized. The chiral recognition of N-Boc-protected amino acids by 2a and 2b has been investigated. Sensor 2a possesses higher sensitivity and enantioselectivity than sensor 2b does. Job analysis and nonlinear regression results show that 2a can form a 1:1 stoichiometric complex with a N-Boc-protected amino acid. The obtained response selectivities and the association constants indicate that 2a is a highly enantioselective and sensitive fluorescent sensor toward N-Boc-protected amino acids.     

Naphthalimide-based fluorescent Zn chemosensors showing PET effect according to their linker length in water

We have developed naphthalimide-based fluorescent chemosensors that exhibit fluorescence enhancement upon binding Zn²⁺ ion in 10 mM HEPES buffer (pH 7.4) at 25 °C. The fluorescence enhancement was induced by a PET inhibition process in which electron transfer from the nitrogen lone pair electrons of the Dpa unit to naphthalimide was blocked upon the binding of the sensor to Zn²⁺. The longer the linker length (n = 1-3) of the sensor, the less the PET efficiency becomes. Among the sensors (1, 2, and 3) examined, 1 shows the highest selectivity and sensitivity for Zn²⁺ over other transition metal ions and alkali metal ions in water.     

A fluorescence-quencher pair for DNA hybridization studies based on hydrophobic base surrogates

The synthesis of the novel, fluorescent 3-aminobiphenyl-C-nucleoside M as well as the corresponding building block for oligodeoxynucleotide synthesis is described. M was incorporated into oligodeoxynucleotides via standard phosphoramidite chemistry and the thermal stabilities of duplexes with one and three consecutive M-M, M-P, and M-O pairs, where P denotes an unmodified biphenyl C-nucleoside and O a 3,5-dinitrobiphenyl-C-nucleoside, were determined. It was found that duplexes containing three consecutive M-O pairs were the most stable in the series, notably more stable than a duplex with one additional natural G-C pair instead of the modified residues. Furthermore it was found that the fluorescence of M is efficiently quenched in a duplex when placed opposite to the dinitrophenyl unit O. Thus M and O constitute a novel fluorophore/quencher pair that is orthogonal to natural base pairs in its recognition properties enabling its use as highly specific tags with reporting properties.     

Synthesis and photophysical properties of novel push-pull-type solvatochromic 7-deaza-2′-deoxypurine nucleosides

We have synthesized two novel push-pull-type fluorescent 7-deazapurine nucleosides, ^CNZA and ^CNZG, and investigated their photophysical properties. In particular, ^CNZA was found to exhibit a remarkable solvatofluorochromicity (Δλ_fl.max = 60 nm). We incorporated ^CNZA into oligonucleotides and found that ^CNZA can form a stable base pair with both thymine and cytosine. Such environmentally sensitive fluorescent nucleosides have a potential as a fluorescence sensor for structural studies of nucleic acids.     

Turn-on fluorescence sensor based on the aggregation of pyrazolo[3,4-b]pyridine-based coumarin chromophores induced by Hg

We developed a new fluorescent sensor (PPC-S) for Hg²⁺ based on the aggregation-induced emission (AIE) of pyrazolo[3,4-b]pyridine-based coumarin chromophore (PPC-O). Given the desulfurization reaction with Hg²⁺, AIE inactive PPC-S can be transformed into PPC-O with AIE activity, which can be employed for the fluorescence turn-on detection of Hg²⁺ with satisfactory selectivity and sensitivity in aqueous solutions.     

Intramolecular charge transfer dual fluorescent sensors from 4-(dialkylamino)benzanilides with metal binding site within electron acceptor

Three fluoroionophores (2a-c) were designed as the intramolecular charge transfer (CT) dual fluorescent sensors for metal cations with metal binding site within the electron acceptor. These sensors were derived from 4-dialkylaminobenzanilides (alkyl=methyl, ethyl, and n-butyl) with the amido phenyl ring being an arm of 15-crown-5 thus bearing binding site for alkaline and alkaline earth metal cations. Compounds 2a-c were expected to have two possible CT channels of opposite direction. The absorption and fluorescence spectra of 2a-c and their crown-ether free model molecules 3a-c in a variety of solvents were recorded. Dual fluorescence was observed with 2a-c and was assigned to the LE and the CT states, respectively. In nonpolar or less polar solvents the CT occurring with 2a-c was identified as that occurred with benzanilides (BA) with the amido anilines being the electron donor (the BA-like CT), while in polar solvents such as acetonitrile (ACN), the CT was still mainly the BA-like. In the presence of alkali and alkaline earth metal cations in ACN, the CT dual fluorescence underwent substantial changes so as increased total quantum yield, red-shifted LE band and enhanced CT to LE intensity ratio. Binding of the metal cations at the 15-crown-5 moiety of 2a-c was shown to turn the CT direction that the dialkylamino group in the binding complexes being the electron donor while the benzo-15-crown-5 moiety now being within the electron acceptor. The occurrence of this CT enhances metal cation binding to 15-crown-5 ether in 2a-c, which was confirmed by the observed higher metal binding constants. Compounds 2a-c as the CT dual fluorescent sensors were shown to operate under the mechanism of the metal cation binding induced switching of the CT character from the BA-like to that occurred with 4-(dimethylamino)benzamides (the DMABA-like). Compounds 2a-c therefore represent successful examples for the CT dual fluorescent sensors for cations with the metal binding site within the electron acceptor and can be employed as sensitive ratiometric fluorescent sensors for metal cations of improved sensing performance.     

Anthracene-labeled pyridinium-based symmetrical chiral chemosensor for enantioselective recognition of l-tartrate

A new anthracene-based chiral chemosensor 1 has been designed and synthesized. l-Valine has been used as the chiral source in the design. The chemosensor 1 has been established as an efficient enantioselective sensor for l-tartrate. While in the presence of l-tartrate the fluorescent sensor 1 in DMSO exhibits considerable increase in emission, the isomeric tartrate brings relatively small change. The enantiomeric fluorescence difference ratio (ef) has been determined to be 29.38.     

A novel BINOL-based cyclophane via click chemistry: synthesis and its applications for sensing silver ions

A novel BINOL-based cyclophane 1 incorporating two triazole moieties was synthesized via click chemistry and characterized. Among the metal ions screened, only Ag⁺ was found to have the ability to quench the fluorescence of 1 in methanol solution. The 1:1 binding mode of 1-Ag⁺ was confirmed. The competitive experiment showed that compound 1 can be used as a specific fluorescent sensor for Ag⁺ over a wide range of competing cations.     

Intensely blue-fluorescent 2,5-bis(benzoimidazol-2-yl)pyrazine dyes with improved solubility: their synthesis, fluorescent properties, and application as microenvironment polarity probes

In order to improve the solubility of intensively fluorescent 2,5-bis(benzimidazol-2-yl)pyrazine (BBIP), we synthesized new BBIP derivatives (2, 3a,b, and 5a,b) possessing two alkyl chains at the N-1 and N-1′ positions of the two benzimidazole moieties. Characterization of these compounds demonstrated that they exhibit high fluorescence intensity even in protic solvents, as well as solvatochromic fluorescence, in which their fluorescence maxima in aqueous methanol exhibited bathochromic shift with increasing ?_r value of the medium. We utilized 5a as a microenvironment polarity probe to indicate the variation in polarity around the backbone of the temperature-sensitive poly(N-isopropylacrylamide) by measuring the spectral change caused by the thermal phase transition of the polymer.     

Degradation and calcification in vitro of new bioresorbable terpolymers of lactides with an improved degradation pattern

Experimental implants produced from poly(l-lactide/?-caprolactone/glycolide) 80/10/10% (polymer 1), poly(l-lactide/dl-lactide/glycolide) 80/10/10% (polymer 2) and poly(l-lactide/dl-lactide/glycolide) 80/5/15% (polymer 3) were subjected to in vitro degradation in a buffer solution at 37 °C and pH = 7.4 and calcification in vitro was performed at 37 °C in a simulated body fluid for 4, 8, 12, 16, 24, 28, 32 and 36 weeks. In vitro degradation was performed in static and pseudodynamic modes. Samples from poly(l-lactide) were used as a control. The changes in the materials during the course of degradation were assessed from the measurements of molecular weight, mechanical properties and crystallinity. The changes in the appearance of the materials upon degradation and calcification were observed using a scanning electron microscope with an EDAX attachment. The decrease of molecular weight at 4 weeks was 66% for polymer 1, 56% for polymer 2 and 20% for polymer 3. Samples retained 55% of their bending strength at 4 weeks (polymer 1), 50% at 12 weeks (polymer 2) and 99% at 12 weeks (polymer 3). The bending modulus of polymer 3 remained practically unchanged during the first 12 weeks of degradation. Subsequently it increased by 44% at week 16 and remained unchanged up to 24 weeks and next decreased to 33% of the initial value at the end of the experiment at week 32. The bending modulus of polymer 2 decreased 35% at week 8 and subsequently increased to 44% of the initial value at week 16 and remained at this level until week 20. Next the modulus decreased to 84% of the initial value at week 24. The bending modulus of polymer 1 progressively decreased over the first 12 weeks of degradation to 40% of the initial value. The maximum crystallinity attained by the samples at the end of the experiments was 60% for polymer 1 and 38% for polymers 2 and 3. In the static mode the pH remained constant up to week 8 for polymer 1, week 20 for polymer 2 and week 28 for polymer 3. It decreased to 3.8 at weeks 12, 20 and 36 for polymers 1, 2 and 3, respectively. All the samples underwent calcification from week 16 of the experiments with the Ca/P ratio ranging from 0.92 to 1.20.