Optical Titration Research Articles

Dioxygenase Chemistry in Nucleophilic Aldehyde Deformylations Utilizing Dicopper O2-Derived Peroxide Complexes.

The chemistry of copper-dioxygen complexes is relevant to copper enzymes in biology as well as in (ligand)Cu-O2 (or Cu2-O2) species utilized in oxidative transformations. For overall energy considerations, as applicable in chemical synthesis, it is beneficial to have an appropriate atom economy; both O-atoms of O2(g) are transferred to the product(s). However, examples of such dioxygenase-type chemistry are extremely rare or not well documented. Herein, we report on nucleophilic oxidative aldehyde deformylation reactivity by the peroxo-dicopper(II) species [Cu2II(BPMPO-)(O22-)]1+ {BPMPO-H = 2,6-bis{[(bis(2-pyridylmethyl)amino]methyl}-4-methylphenol)} and [Cu2II(XYLO-)(O22-)]1+ (XYLO- = a BPMPO- analogue possessing bis(2-{2-pyridyl}ethyl)amine chelating arms). Their dicopper(I) precursors are dioxygenase catalysts. The O2(g)-derived peroxo-dicopper(II) intermediates react rapidly with aldehydes like 2-phenylpropionaldehyde (2-PPA) and cyclohexanecarboxaldehyde (CCA) in 2-methyltetrahydrofuran at -90 °C. Warming to room temperature (RT) followed by workup results in good yields of formate (HC(O)O-) along with ketones (acetophenone or cyclohexanone). Mechanistic investigation shows that [Cu2II(BPMPO-)(O22-)]1+ species initially reacts reversibly with the aldehydes to form detectable dicopper(II) peroxyhemiacetal intermediates, for which optical titrations provide the Keq (at -90 °C) of 73.6 × 102 M-1 (2-PPA) and 10.4 × 102 M-1 (CCA). In the reaction of [Cu2II(XYLO-)(O22-)]1+ with 2-PPA, product complexes characterized by single-crystal X-ray crystallography are the anticipated dicopper(I) complex, [Cu2I(XYLO-)]1+ plus a mixed-valent Cu(I)Cu(II)-formate species. Formate was further identified and confirmed by 1H NMR spectroscopy and electrospray ionization mass spectrometry (ESI-MS) analysis. Using 18O2(g)-isotope labeling the reaction produced a high yield of 18-O incorporated acetophenone as well as formate. The overall results signify that true dioxygenase reactions have occurred, supported by a thorough mechanistic investigation.

The multistep oxidation of cholesterol to pregnenolone by human cytochrome P450 11A1 is highly processive

Cytochrome P450 (P450, CYP) 11A1 is the classical cholesterol side chain cleavage enzyme (P450scc) that removes six carbons of the side chain, the first and rate-limiting step in the synthesis of all mammalian steroids. The reaction is a 3-step, 6-electron oxidation that proceeds via formation of 22R-hydroxy (OH) and 20R,22R-(OH)2 cholesterol, yielding pregnenolone. We expressed human P450 11A1 in bacteria, purified the enzyme in the absence of nonionic detergents, and assayed pregnenolone formation by HPLC-mass spectrometry of the dansyl hydrazone. The reaction was inhibited by the nonionic detergent Tween 20, and several lipids did not enhance enzymatic activity. The 22R-OH and 20R,22R-(OH)2 cholesterol intermediates were bound to P450 11A1 relatively tightly, as judged by steady-state optical titrations and koff rates. The electron donor adrenodoxin had little effect on binding; the substrate cholesterol showed a ∼5-fold stimulatory effect on the binding of adrenodoxin to P450 11A1. Presteady-state single-turnover kinetic analysis was consistent with a highly processive reaction with rates of intermediate oxidation steps far exceeding dissociation rates for products and substrates. The presteady-state kinetic analysis revealed a second di-OH cholesterol product, separable by HPLC, in addition to 20R,22R-(OH)2 cholesterol, which we characterized as a rotamer that was also converted to pregnenolone at a similar rate. The first oxidation step (at C-22) is the slowest, limiting the overall rate of cleavage. d3-Cholesterol showed no kinetic deuterium isotope effect on C-22, indicating that C-H bond cleavage is not rate-limiting in the first hydroxylation step.

A Novel Red Fluorescent Probe for the “Off‐On‐Off” Detection of Al3+ and F− in Samples of Drinking Water and Toothpaste

AbstractA novel fluorescent sensor based on coumarin was developed for the sequential detection of Al3+ and F− ions in ethanol, exhibiting a large Stokes shift of 190 nm. The probe displayed “off‐on‐off” responses towards Al3+ and F− at the red region of 630 nm, with detection limits of 2.48×10−8 M and 2.34×10−8 M, respectively. The binding mechanism between the probe and Al3+ was elucidated using Job‘s plot, 1H NMR, optical titrations, and ESI‐MS. Additionally, the probe demonstrated an AIE property in DMSO/H2O solutions. Its successful application in measuring Al3+ and F− in water samples was observed. Moreover, with the convenience of “naked eye” detection, the probe could be effectively utilized for identifying F− in toothpastes.

A Fluorescein-Substituted Perbrominated Dodecaborate Cluster as an Anchor Dye for Large Macrocyclic Hosts and Its Application in Indicator Displacement Assays.

Perhalogenated boron clusters derived from B12Br122-, a superchaotropic dianion with a globular icosahedral shape, serve as inorganic cavity binders for cyclodextrins (CDs), in particular for large CDs (γ-CD and δ-CD), with high binding affinity (Ka > 106 M-1) in aqueous solution. This opens the door for applications of this anchoring moiety by linking it to organic residues, prominently fluorescent dyes. We report here the synthesis of a novel fluorescein-substituted perbrominated dodecaborate cluster by a copper(I)-catalyzed azide-alkyne click reaction. The formation of host-guest inclusion complexes between the dodecaborate-modified fluorescein dye and CDs can be readily followed by optical titrations, which afforded a binding constant of ∼1 × 104 M-1 with γ-CD; that is, the cluster functionalization allows binding of an otherwise nonbinding dye to the macrocycle ("anchor dye"). The formation of the 1:1 host-guest inclusion complex between the dye and γ-CD occurs over a broad range of pH values, which allows its application as a sensitive reporter pair according to the indicator displacement method, e.g., for drug detection. In addition, the substituted dye shows outer-wall binding to cucurbiturils through the dodecaborate moiety, leading to the formation of aggregates and significant fluorescence quenching of the dye.

Influence Mechanism of Surfactants on Wettability of Coal with Different Metamorphic Degrees Based on Infrared Spectrum Experiments.

Based on experiments, a numerical analysis is used to quantitatively explore the influence of coal and surfactant microstructures on wettability. First, based on an infrared spectrum experiment, the distribution of oxygen-containing functional groups, aromatic hydrocarbons, and aliphatic hydrocarbons of coal and surfactants was obtained. Second, the wettability relationship between coal and different surfactants was determined by optical titration, and the coal dust wettability and surfaces were optimized. The key factors of the active agent wetting ability affecting lignite wetting mainly depend on the carbonyl, ether, and carboxyl groups in the surfactant. The factors affecting non-stick coal and gas coal wetting mainly depend on the ether group and aromatic amine in the surfactant. The factors affecting fat coal wetting mainly depend on the ether group and hydroxyl group in the surfactant. Finally, the factors affecting coking coal and anthracite wetting mainly depend on the surfactant ether group, aliphatic amine, and aromatic amine. Then, combining the structural parameters with the coal wetting results, the quantitative mathematical relationship between coal dust wettability, the important influencing factors of the surfactant, and the wettability index was established. Finally, a perfect and reasonable wettability evaluation model between coal and the surfactants was established. The relative activity of methyl ether and aromatic ether is greater than that of methyl ether, and the influence on the lignite, coking coal, and anthracite wettability conforms to the model . The influence on the non-caking coal and fat coal wettability conforms to the model , while the influence on the gas coal wettability conforms to the model Z = A + B1X1 + B2X2. In general, this study provides scientific guidance for the compounding of high-efficiency and environmentally protective composite dust suppressors to realize clean mine production.

Micromechanism Analysis of Surfactant Wetting of Coal Based on 13C NMR Experiments.

With the gradual improvement in coal mine mechanization and automation, the dust concentration at production sites is increasing significantly as the production efficiency improves, which not only poses a substantial threat to the occupational safety and health of workers but also affects the safe production and social stability of mines. At present, wet dust removal is the most economical and effective technical dust removal measure. However, most coal seams in China have poor wettability, unclear microscopic wetting mechanisms, and poor dust removal effects. Therefore, based on experiments and numerical analysis, this paper qualitatively studies the influencing factors of surfactants on coal wettability and quantitatively constructs an innovative evaluation model of the influence of the microstructure of coal and surfactants on wettability. First, based on 13C NMR experiments, the structural parameters of coal and several surfactants were obtained. Second, the wettability relationship between the coal and the surfactants was determined by optical titration, and the key factors affecting the wettability of coal dust and the wettability of the surfactants were selected. Then, using numerical analysis and function fitting analysis and combining the structural parameters with the coal wetting results, the relationship between the microstructure of the surfactants on different kinds of coal and the wettability of the coal samples was established. The results show that the ether group, the phenol or aromatic ether carbon, the fatty methyl group, and the aromatic methyl group in the surfactants have a substantial influence on the wettability. The research results can provide scientific guidance for the development of efficient and environmentally friendly compound dust suppressants to realize clean production in mines.

Two Colorimetric and Fluorescent Dual‐Channel Chemosensors for the Selective Detection of pH in Aqueous Solutions

AbstractFluorescent probe – which can detect metal ions, pH, and other analytical species through spectroscopic approaches – have recently attracted significant research attention in the fields of analytical and biological chemistry. For this study, two novel colorimetric and fluorescent probes, named RP1 and RP2, were designed and synthesized. Their structural confirmations were then achieved through elemental analysis, FT‐IR, NMR (1H and 13C) spectroscopies, and tandem MS spectrometry. To gain full understanding of the photophysical properties and chemosensory performances of these two dyes, both the UV‐Vis absorption and fluorescence titration experiments were carefully performed in aqueous media. Notably, upon treatment of RP1 and RP2 in solutions with each competitive metal ion, neither increases in absorption and fluorescence spectra above 400 nm nor any changes in color were observed. However, at lower pH (i. e.,<7.0), those two sensors exhibited a drastic change in color from colorless to pink, followed by broadened absorption bands. As expected, upon excitation at 510 nm, a strong enhancement of the fluorescence emissions was obtained. By conducting 1H NMR and optical titrations, the spirolactam‐ring opening reaction of these receptors, induced by H+ ions at lower pH, was approved. The results demonstrate that the fluorescent dyes RP1 and RP2 can be effectively used as reversible pH‐responsive chromogenic and fluorogenic “Off‐On” switches in real samples.

Evaluation of Luminogenic Substrates as Probe Substrates for Bacterial Cytochrome P450 Enzymes: Application to Mycobacterium tuberculosis

Several cytochrome P450 enzymes (CYPs) encoded in the genome of Mycobacterium tuberculosis (Mtb) are considered potential new drug targets due to the essential roles they play in bacterial viability and in the establishment of chronic intracellular infection. Identification of inhibitors of Mtb CYPs at present is conducted by ultraviolet-visible (UV-vis) optical titration experiments or by metabolism studies using endogenous substrates, such as cholesterol and lanosterol. The first technique requires high enzyme concentrations and volumes, while analysis of steroid hydroxylation is dependent on low-throughput analytical methods. Luciferin-based luminogenic substrates have proven to be very sensitive substrates for the high-throughput profiling of inhibitors of human CYPs. In the present study, 17 pro-luciferins were evaluated as substrates for Mtb CYP121A1, CYP124A1, CYP125A1, CYP130A1, and CYP142A1. Luciferin-BE was identified as an excellent probe substrate for CYP130A1, resulting in a high luminescence yield after addition of luciferase and adenosine triphosphate (ATP). Its applicability for high-throughput screening was supported by a high Z’-factor and high signal-to-background ratio. Using this substrate, the inhibitory properties of a selection of known inhibitors could be characterized using significantly less protein concentration when compared to UV-vis optical titration experiments. Although several luminogenic substrates were also identified for CYP121A1, CYP124A1, CYP125A1, and CYP142A1, their relatively low yield of luminescence and low signal-to-background ratios make them less suitable for high-throughput screening since high enzyme concentrations will be needed. Further structural optimization of luminogenic substrates will be necessary to obtain more sensitive probe substrates for these Mtb CYPs.

The catalytic mechanism of electron-bifurcating electron transfer flavoproteins (ETFs) involves an intermediary complex with NAD+

Electron bifurcation plays a key role in anaerobic energy metabolism, but it is a relatively new discovery, and only limited mechanistic information is available on the diverse enzymes that employ it. Herein, we focused on the bifurcating electron transfer flavoprotein (ETF) from the hyperthermophilic archaeon Pyrobaculum aerophilum The EtfABCX enzyme complex couples NADH oxidation to the endergonic reduction of ferredoxin and exergonic reduction of menaquinone. We developed a model for the enzyme structure by using nondenaturing MS, cross-linking, and homology modeling in which EtfA, -B, and -C each contained FAD, whereas EtfX contained two [4Fe-4S] clusters. On the basis of analyses using transient absorption, EPR, and optical titrations with NADH or inorganic reductants with and without NAD+, we propose a catalytic cycle involving formation of an intermediary NAD+-bound complex. A charge transfer signal revealed an intriguing interplay of flavin semiquinones and a protein conformational change that gated electron transfer between the low- and high-potential pathways. We found that despite a common bifurcating flavin site, the proposed EtfABCX catalytic cycle is distinct from that of the genetically unrelated bifurcating NADH-dependent ferredoxin NADP+ oxidoreductase (NfnI). The two enzymes particularly differed in the role of NAD+, the resting and bifurcating-ready states of the enzymes, how electron flow is gated, and the two two-electron cycles constituting the overall four-electron reaction. We conclude that P. aerophilum EtfABCX provides a model catalytic mechanism that builds on and extends previous studies of related bifurcating ETFs and can be applied to the large bifurcating ETF family.

"Turn-On" Fluorescent and Colorimetric Detection of Zn2+ Ions by Rhodamine-Cinnamaldehyde Derivative.

We report the design and synthesis of a novel chemosensor (Rh6G-Cin) rhodamine-based indicator for selective detection of Zn2+ ion. Rh6G-Cin displayed high selectivity towards Zn2+ from various metal ions, including Ca2+, Ag+, Cd2+, Co2+, Cu2+, Al3+, Zn2+, Cr3+, Ba2+, Fe2+, Fe3+, Gd3+, Hg2+, Mg2+, Mn2+, Nd3+, Pb2+, Sr2+ and Ni2+, and the resultant complex is [Rh6G-Cin-Zn2+]. The obvious change from colorless to pink upon the addition of Zn2+ could make it a suitable "naked eye" indicator for Zn2+. More significantly, the sensor displayed a remarkable colorless to yellowish green fluorescence switch in the presence of Zn2+ ions. The ring-opening mechanism of the rhodamine spirolactam was induced by Zn2+ binding, and the 3:1 stoichiometric structure between Rh6G-Cin and Zn2+ was adequately supported by the Job's plot evaluation, optical titration and 1H NMR results.

Ionophore‐based Heterogeneous Calcium Optical Titration

AbstractRecently, the field of complexometric titration significantly progressed with the advent of ionophore‐based titration reagents. We present here for the first time an ionophore‐based heterogeneous optical titration method for calcium. The all‐in‐one reagent was composed of chromoionophore, ion‐exchanger and calcium ionophore in dichloromethane (DCM). The endpoint of this reverse titration scheme was determined by the color change of the DCM phase (blue to red). The ion‐exchange theory borrowed from ISE membranes was used to simulate the titration curve in a simplified manner. The phase separation speed during titration was found much faster at and beyond the endpoint. A mechanism was proposed for this phenomenon, with zeta‐potential measurements on ion‐selective nanospheres to support the hypothesis. Calcium titration in a urine sample was also successfully demonstrated and compared with EDTA titration with potentiometric ion‐selective electrodes as endpoint indicator. The proposed reagent serves as an alternative to the classical EDTA.

Novel Aryl Substituted Pyrazoles as Small Molecule Inhibitors of Cytochrome P450 CYP121A1: Synthesis and Antimycobacterial Evaluation.

Three series of biarylpyrazole imidazole and triazoles are described, which vary in the linker between the biaryl pyrazole and imidazole/triazole group. The imidazole and triazole series with the short −CH2– linker displayed promising antimycobacterial activity, with the imidazole–CH2– series (7) showing low MIC values (6.25–25 μg/mL), which was also influenced by lipophilicity. Extending the linker to −C(O)NH(CH2)2– resulted in a loss of antimycobacterial activity. The binding affinity of the compounds with CYP121A1 was determined by UV–visible optical titrations with KD values of 2.63, 35.6, and 290 μM, respectively, for the tightest binding compounds 7e, 8b, and 13d from their respective series. Both binding affinity assays and docking studies of the CYP121A1 inhibitors suggest type II indirect binding through interstitial water molecules, with key binding residues Thr77, Val78, Val82, Val83, Met86, Ser237, Gln385, and Arg386, comparable with the binding interactions observed with fluconazole and the natural substrate dicyclotyrosine.

Determination of the Absolute Enantiomeric Excess of the Carbon Nanotube Ensemble by Symmetry Breaking Using the Optical Titration Method.

Symmetry breaking of single-walled carbon nanotubes (SWNTs) has profound effects on their optoelectronic properties that are essential for fundamental study and applications. Here, we show that isomeric SWNTs that exhibit identical photoluminescence (PL) undergo symmetry breaking by flavin mononucleotide (FMN) and exhibit dual PLs and different binding affinities (Ka). Increasing the FMN concentration leads to systematic PL shifts of SWNTs according to structural modality and handedness due to symmetry breaking. Density gradient ultracentrifugation using a FMN-SWNT dispersion displays PL shifts and different densities according to SWNT handedness. Using the optical titration method to determine the PL-based Ka of SWNTs against an achiral surfactant as a titrant, left- and right-handed SWNTs display two-step PL inflection corresponding to respective Ka values with FMN, which leads to the determination of the enantiomeric excess (ee) of the SWNT ensemble that was confirmed by circular dichroism measurement. Decreasing the FMN concentration for the SWNT dispersion leads to enantiomeric selection of SWNTs. The titration-based ee determination of the widely used sodium cholate-based SWNT dispersion was also demonstrated by using FMN as a cosurfactant.

Substituent Effects on the Coordination Chemistry of Metal-Binding Pharmacophores.

A combination of XAS, UV-vis, NMR, and EPR was used to examine the binding of a series of α-hydroxythiones to CoCA. All three appear to bind preferentially in their neutral, protonated forms. Two of the three clearly bind in a monodentate fashion, through the thione sulfur alone. Thiomaltol (TM) appears to show some orientational preference, on the basis of the NMR, while it appears that thiopyromeconic acid (TPMA) retains rotational freedom. In contrast, allothiomaltol (ATM), after initially binding in its neutral form, presumably through the thione sulfur, forms a final complex that is five-coordinate via bidentate coordination of ATM. On the basis of optical titrations, we speculate that this may be due to the lower initial pKa of ATM (8.3) relative to those of TM (9.0) and TPMA (9.5). Binding through the thione is shown to reduce the hydroxyl pKa by ∼0.7 pH unit on metal binding, bringing only ATM's pKa close to the pH of the experiment, facilitating deprotonation and subsequent coordination of the hydroxyl. The data predict the presence of a solvent-exchangeable proton on TM and TPMA, and Q-band 2-pulse ESEEM experiments on CoCA + TM suggest that the proton is present. ESE-detected EPR also showed a surprising frequency dependence, giving only a subset of the expected resonances at X-band.

Electroanalytical and spectral investigation of organic receptors as colorimetric and absorption ratiometric anion chemosensor

Colorimetric receptors R1 and R2 have been designed and synthesized by Schiff base condensation and characterised by standard spectroscopic techniques. Anion binding ability of the receptors have been investigated quantitatively through optical, electrochemical and 1HNMR titration studies. UV-vis spectra of receptor R1 and R2 exhibited a significant red shift for F− and AcO− ions with a visual color response. Receptor R1 exhibited selective response towards AcO− ion in the presence of HEPES buffer media. Incremental color change of receptor R2 with the higher equivalence of AcO− ions clearly represent the ratiometric response. Cyclic voltammetric studies of R1 and R2 exhibits shift in oxidation and reduction peak with successive addition of AcO− ions indicating the anion induced oxidation of -NH and reduction of the keto group and nitro species. Electrooptical and 1H NMR titration studies of R2 collectively reflects the anion induced change of chromophore from C=N to N=N indicative of azo-hydrazone tautomeric signaling in the presence of AcO− ions. Lower detection limit of 2.1 and 0.41 ppm achieved with sodium salt of AcO− ion with R1 and R2 reflects their utility as colorimetric chemosensor.

Binding of the Carcinogen 4-Nitroquinoline-1-Oxide to phiX174 DNA - Selectivity and Distortion

Optical titration studies, NMR studies and studies with DNA oligonucleotides have suggested that the carcinogen 4-nitroquinoline-1-oxide [NQO] binds to DNA with marked sequence selectivity and cooperativity and that the binding affects the reactivity of enzymes with DNA. To further examine the sequence selectivity of NQO binding to DNA, we have examined the binding of NQO [at NQO/DNA bp ratios 0.1-3] to various locations on phiX174RF DNA using restriction enzyme activity assays employing seven restriction enzymes chosen to have different reaction sequences and flanking sequences. Alterations in restriction enzyme activity were observed with the restriction enzymes Alw44, BssH II, Dra I, Stu I and Xho I while no effect was observed for cleavage by Mlu I and Pst I. While the cleavage sites for the affected enzymes differ, the flanking sequences contain similar motifs [TCTT, TGATTG, TGTTG, TCCTTG, TGTTG]. This motif type is not found in the vicinity of the unaffected enzymes. This motif type was previous shown, with oligomers, to be a hot spot for NQO binding and to be a hot spot for the binding of the carcinogen N-acetoxy-N-acetyl-2-aminofluorene. The alterations in DNA cleavage [enhanced cleavage by Alw44 and inhibition by the other enzymes] provide further indication that NQO alters DNA structure upon binding. The effects of NQO binding on DNA structure were probed using E.coli topoisomerase I assays and mung bean nuclease assays. The topoisomerase I assays suggest that NQO unwinds the DNA upon binding. NQO/DNA bp ratios from 0.07 to 0.3 produced enhancement in mung bean nuclease activity. NQO/DNA bp ratios from 0.7 to 3 produced inhibited cleavage. This change in effect with concentration may indicate cooperativity in NQO binding. These assays provide further indication that NQO distorts the structure of DNA - which could lead to the changes in restriction enzyme activity. The results presented provide further insight into the modes of carcinogen attack on DNA.

Acid-Generating and Leaching Potential of Soils in a Coal Waste Rock Pile in Northeastern China

ABSTRACTThe present study involved the assessment of potential generation of acid drainage and also metal leaching from an abandoned large (175 m) low sulfur waste rock pile—the dominant mine waste at the site—at the Haizhou coal mine. Laboratory-based static and column leaching tests on waste rock samples were conducted. The static tests were done for 8 composite samples collected from different parts of pile. A column study was performed using mixture of waste rock soil samples to assess metals attenuation process of background soils in vicinity of pile when rainwater reacts with low-sulfide waste rocks. Total concentration of major elements in solid samples was determined using dispersive X-ray fluorescence spectrometry (XRF) and chemical analysis of leachate were measured by inductively coupled plasma optical emission spectrometry (ICP-OES), ion chromatography and titration methods. The inverse geochemical modeling using PHREEQC was applied to explain possible mass transfer processes between column leachates of waste pile and background soils. Static tests (including acid–base accounting (ABA) and net acid generation (NAG)) and mineralogical information suggest that the waste rock is non-acid generating. The large amount of aluminosilicate minerals and probably trace amounts of carbonates with respect to low sulfur content of waste rock pile represents a potentially large source of neutralization potential according to static test results. It was also found that presence of inherent neutralizing materials in waste rock and also in surrounding background soils provide sufficient neutrality and possibly immobilize the trace and heavy metal contents of waste rocks and potentially protects water resources.

An ‘OFF–ON’ fluorescent chemosensor based on rhodamine 6G-2-chloronicotinaldehyde for the detection of Al3+ ions: Part II

A novel, optical rhodamine-2-chloronicotinaldehyde-type chemosensor (R6CN) was designed, synthesized and characterized as a reversible switch. R6CN displayed high selectivity toward Al3+ from various metal ions, including Al3+, Li+, Na+, K+, Cs+, Mg2+, Ca2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+, Hg2+, Pb2+ and the resultant complex [R6CN-Al3+]. The ring-opening mechanism of the rhodamine spirolactam was induced by Al3+ binding, and the 1:1 stoichiometric structure between R6CN and Al3+ was adequately supported by the Job-plot evaluation, optical titration, FT-IR and 1H NMR results. Theoretical calculations and modeling simulations were performed using Material Studio 4.3 suite (VAMP), and the results supported the formation of a 1:1 complex between R6CN and Al3+. The fluorescence quantum yield of R6CN-Al3+ (Φf=92.33%) was very high compared to that of the bare ligand. The detection limit for Al3+ was 4.28×10−9M, and a significant color change from almost colorless to pale-pink occurred in the presence of Al3+. In turn, the R6CN-Al3+ complex acted as a selective chemosensor toward N3− among various anions, including F−, Cl−, Br−, I−, NO3−, CH3COO−, ClO4−, CN−, SCN−, HSO4−, HPO4− and PF6−, in acetonitrile media. Moreover, the R6CN-Al3+ complex also exhibited a high selectivity and sensitivity toward the azide anion upon the addition of Al3+, and the color reversed back to colorless when the two ions were present together in solution. At last, R6CN was productively applied to the PEGDMA polymer to sense Al3+ ions, which was analyzed using FT-IR, fluorescence confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) images.

A novel urea-based “turn-on” fluorescent sensor for detection of Fe3+/F− ions with high selectivity and sensitivity

A novel urea-based “turn-on” fluorescent sensor 1,3-bis(4-(tert-butyldimethylsilyloxy)phenyl)urea (TBSU) has been synthesized and its sensing properties are explored. This sensor exhibited highly selective and sensitive detection of Fe3+ and F− ions in aqueous DMSO solution (DMSO/H2O, 95/5, v/v) with a significant fluorescence and absorption enhancement. The 1:1 stoichiometric structure between TBSU and Fe3+ was adequately supported by Job-plot evaluation, optical titration and FT-IR analysis. The recognition process of the chemosensor TBSU for Fe3+ is chemically reversible on the addition of CN− anion. The Fe3+/F− detection was not affected by the coexistence of other competitive ions. In addition, the detection limit was found to be as low as 5.87×10−8M for Fe3+ and 4.84×10−8M for F−, respectively. Thus, TBSU could be a highly promising fluorescent sensor for the direct determination of Fe3+/F− ions in aqueous DMSO medium.

Dual sensing performance of a rhodamine-derived scaffold for the determination of Cu2+ and Ce4+ in aqueous media

A rhodamine 6G-based probe bearing a carbazole unit (R6G-CB) was developed as a fluorescent chelation agent for Cu2+ and a chemodosimeter for Ce4+ in acetonitrile:aqueous (5:5, v/v) media. The R6G-CB chemosensor exhibited selectivity toward Cu2+ ions over other metal cations by showing “off-on”-type changes in its absorption and emission spectra, which were attributed to the transformation of the non-fluorescent and colorless spirolactam derivative to the ring-opened, fluorescent, pink amide. The Cu2+-induced chelation-enhanced fluorescence (CHEF) is associated with the spirolactam ring opening of the rhodamine unit. This R6G-CB chemosensor also showed a fast response (<1min) to an irreversible Ce4+-promoted oxidation reaction, responding instantaneously at room temperature with a change that is observable by the naked eye. This through-bond energy transfer (TBET)-based sensor is also highly selective and sensitive to Ce4+. Spectroscopic analysis revealed that different structural substitution patterns of the probe resulted in different sensitivities and selectivities for specific metal ions. The optical titration, Job's plot estimation, mass spectroscopy, and Fourier transform infrared (FT-IR) results indicated that 1:2 and 1:1 complexes formed between R6G-CB and Cu2+ and between R6G-CB and Ce4+, respectively. The reversibility of the R6G-CB chemosensor was verified via its spectral response to Cu2+ and EDTA disodium salt titration experiments. All of these features make these sensors advantageous for Cu2+/Ce4+ test strip applications. Due to the significant absorption intensity changes resulting from the addition of Cu2+ and Ce4+ ions, the novel system can act as a single-output combinational logic circuit with two chemical inputs. Finally, density functional theory (DFT) calculations were performed to gain insight into the structures and electronic properties of R6G-CB, R6G-CB-Cu2+ complexes and R6G-CB-Ce4+ adducts.