Fluorescence Quenching Studies of Curcumin by Hydrogen Peroxide in Acetonitrile Solution

Steady state quenching studies of curcumin, 1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione, fluorescence by hydrogen peroxide were conducted in acetonitrile solution. A quenching rate constant, k_q, of 1.05×10¹⁰M^–1·s^–1 was obtained with a short fluorescence lifetime of 347ps. The reaction rate constant, which is within the diffusion-limited regime, is activation-controlled. The rate constant of deactivation of the thermally excited curcumin was 1.2 orders of magnitude more nonradiative (2.67×10⁹s^–1) than radiative (2.16×10⁸s^–1). The reaction was exothermic with a G° of –1.97eV and solvent reorganization energy of 1.37eV. These values indicate that the electron transfer reaction is solvent-mediated with electron transfer rate constant, k_ET, of 2.16×10¹⁰s^–1.     

Zr 5Ir 2In 4 – A Superstructure of the Lu 5Ni 2In 4 Type

Zr₅Ir₂In₄ was synthesized by reaction of the elements in a glassy carbon crucible in a water-cooled sample chamber of an induction furnace. The sample was characterized by X-ray diffraction on both powder and single crystals. Zr₅Ir₂In₄ crystallizes with a pronounced Lu₅Ni₂In₄ type subcell, space group Pbam, a=1739.5(6), b=766.3(2), c=338.9(2) pm. Weak additional reflections force a doubling of the subcell c axis. The superstructure of Zr₅Ir₂In₄ is of a new type: Pnma, a=1739.5(6), b=677.8(2), c=766.3(2) pm, wR2=0.0529, 1592 F² values, and 60 variable parameters. The group-subgroup scheme for the klassengleiche symmetry reduction is presented. The formation of the superstructure is most likely due to a puckering effect (size of the iridium atoms). The crystal chemistry of Zr₅Ir₂In₄ is briefly discussed.     

The Dissociation Constants of 1,1'-Bis (pyridinium-4-aldoxime)trimethylene dibromide

The dissociation constants of the two oxime groups of 1,1-bis(pyridinium-4-aldoxime)trimethylene dibromide (TMB-4) were determined using spectrophotometric data. Two numerical methods were applied to treat the overlapping equilibria. The results obtained by both agreed with each other and their mean values at 25°C corrected for the ionic strength of 0.05moldm^–3 are pK_a1=7.49±0.11 and pK_a2=8.96±0.09. These values were discussed in terms of the pK_as of 1,1-bis(pyridinium-4-aldoxime)oxydimethylene dichloride (Toxogonin), a similar dioxime, which were derived by extrapolation of literature data.     

EDA Complexes of 2,3-Dichloro-1,4-naphthoquinone with Some Substituted Anilines

Electron donor-acceptor (EDA) complexes of several substituted anilines with the -acceptor 2,3-dichloro-1,4-naphthoquinone (DClNQ) in chloroform, dichloromethane, and 1,2-dichloroethane were studied by a spectrophotometric method. Experimental data has been shown to confirm the 1:2 stoichiometry of the acceptor-donor in spite of the apparent linearity of the Benesi-Hildebrand plot for a 1:1 complex. The calculated values of oscillator strengths, transition moments, and thermodynamic parameters confirms the 1:2 (A:D) stoichiometry of the complexes. The formation constants and molar absorptivities for these AD₂ complexes have been evaluated. The stoichiometry was unaffected by the variation of the temperature over a small interval and also by the change in polarity of the solvent. The order of the donor strength followed the sequence: N-benzylaniline>p-chloroaniline>m-chloroaniline>o-chloroaniline.     

Exciton Chirality. (A) Origins of and (B) Applications from Strongly Fluorescent Dipyrrinone Chromophores

(A) The origin of exciton interaction and examples of its application to organic stereochemistry are presented. (B) N,N-Carbonyl-bridged dipyrrinones constitute a new class of highly fluorescent chromophores suitable for investigations of stereochemistry and absolute configuration. N,N-Carbonylxanthobilirubic acid esters are strongly fluorescent, with a fluorescence quantum yield (_F) 0.8, but produce only weak exciton CD from the trans-1,2-cyclohexanediol template. The ester of an analog with benzoic acid replacing propionic, N,N-carbonyl-8-(4-carboxyphenyl)-3-ethyl-2,7,9-trimethyl-(10H)-dipyrrin-1-one, exhibits strong fluorescence (_F=0.68, _em=493nm, _ex=422nm in CHCl₃) and UV-Vis absorption (21000 at 424nm) in organic solvents. Its diester with (1S,2S)-cyclohexanediol is fluorescent and exhibits exciton circular dichroism (=+15dm³·mol^–1·cm^–1, =432nm; =–4dm³·mol^–1cm^–1, =380nm) that correlates with the Exciton Chirality Rule.     

The Electrochemical Behavior of p-Aminophenol at a ω-Mercaptopropionic Acid Self-Assembled Gold Electrode

A -mercaptopropionic acid (MPA) self-assembled monolayer modified electrode (MPA/SAM/Au) on a gold electrode has been fabricated. The characterization of the MPA/SAM/Au was investigated using attenuated total reflection-fourier transform infrared (ATR-FTIR) and A.C. impedance. The electrochemical behaviors of p-aminophenol (p-AP) were studied at the MPA/SAM/Au by cyclic voltammetry and semi-derivative voltammetry (SDV) in BR buffer solution. The modified electrode shows excellent electrocatalytic activity for the redox of p-AP and accelerates the electron transfer rate. The diffusion coefficient (D) is 4.55×10^–6cm²s^–1. The oxidative peak current increases linearly with the concentration of p-AP in the range of 4.0×10^–88×10^–6molL^–1 and 1.0×10^–52×10^–4molL^–1 by square wave voltammetry response, respectively. The detection limit (three times the signal blank/slope) is up to 1.2×10^–8molL^–1. The modified electrode is able to eliminate the interference of p-benzenediol, o-benzenediol and o-AP at a 40-, 90- or 70-fold concentration of p-AP, and it has been satisfactorily used for the determination of the real sample.     

Determination of Heavy Metal Ions in Chinese Herbal Medicine by Microwave Digestion and RP-HPLC with UV-Vis Detection

A new method for the simultaneous determination of heavy metal ions in Chinese herbal medicine by microwave digestion and reversed-phase high-performance liquid chromatography (RP-HPLC) has been developed. The Chinese herbal medicine samples were digested by microwave digestion. Lead, cadmium, mercury, nickel, copper, zinc, and tin ions in the digested samples were pre-column derivatized with tetra-(4-chlorophenyl)-porphyrin (T₄-CPP) to form the colored chelates which were then enriched by solid phase extraction with C₁₈ cartridge and eluted from the cartridge with tetrahydrofuran (THF). The chelates were separated on a Waters Xterra RP₁₈ column by gradient elution with methanol (containing 0.05molL^–1 pyrrolidine-acetic acid buffer salt, pH=10.0) and THF (containing 0.05molL^–1 pyrrolidine-acetic acid buffer salt, pH=10.0) as mobile phase at a flow rate of 0.5mLmin^–1 and detected with a photodiode array detector in the range of 350–600nm. In the original samples the detection limits of lead, cadmium, mercury, nickel, copper, zinc and tin are 4ngL^–1, 3ngL^–1, 6ngL^–1, 5ngL^–1, 2ngL^–1, 6ngL^–1, and 4ngL^–1, respectively. This method was applied to the determination of lead, cadmium, mercury, nickel, copper, zinc and tin in Chinese herbal medicine samples with good results.     

Solid-Phase Extraction with Slurry Injection of the Resin Into ETAAS for Trace Determination of Thallium in Natural Water

Thallium in natural water samples was determined by electrothermal atomic absorption spectrometry after 1000-fold enrichment by mini solid-phase extraction from a 100-mL sample solution. A Tl-pyrrolidine-1-carbodithioate complex formed in a sample solution of pH 1.6 was extracted on fine particles of a cellulose nitrate resin dispersed in the sample solution. The cellulose nitrate resin was then collected on a membrane filter (25mm^ø) by filtration under suction using a glass funnel with an effective filtration area of 0.64cm². As a result, a circular thin layer of the resin phase with a diameter of 9mm was obtained. Then the resin phase was carved out by an acrylate resin puncher with a 10-mm^ø hole to put it into a sample cup containing 100µL of 10mM HNO₃ containing 0.5mM NaCl. The resin phase was suspended in the solution by ultrasonication. 1000-fold enrichment was thus attained within 15min, and the suspension was delivered to electrothermal atomic absorption spectrometry. The linear calibration graph was obtained in the range of 0–4ng of Tl in 100mL of a sample solution. The detection limit obtained by 3 method was 0.19ng. The proposed method was applied to the determination of Tl in natural water samples. The results showed the concentration of Tl in seawater was 12.1±1.8pgmL^–1 for the calibration graph method and 12.6±1.4pgmL^–1 for the standard addition method. A snowmelt sample contained 20.7±1.0pgmL^–1 of Tl.     

Determination of Attogram Quantities of Silver via Quenching of the Solid Substrate Room-Temperature Phosphorescence of Rhodamine B Based on the Catalysis of its Oxidation by Potassium Persulfate

A new method of SS-RTP for the determination of trace silver has been established. This method is based on the fact that Ag⁺, when activated by ,-bipyridyl (bipy) in a media of HAc–NaAc (pH=4.9), can catalyze the reaction of Rhodamine B (RhoB) oxidized by K₂S₂O₈, thus causing the Solid Substrate Room-Temperature Phosphorescence (SS-RTP) of RhoB to be quenched. The activating efficiency of bipy is 6.7 times higher than that of o-phenanthroline (phen). The reduction of the phosphorescence intensity (I_p) of RhoB is directly proportional to the concentration of Ag⁺ ions in the range of 1.6016.0agspot^–1 (0.40µLspot^–1). The regression equation of the working curve can be expressed as I_p=18.78+5.100m_Ag+ (agspot^–1) (r=0.9994, n=6), the detection limit is 0.28agspot^–1. This rapid, accurate and sensitive method has been successfully applied to the determination of trace silver in tea and human hair samples, and the results agree well with the Atomic Absorption Spectroscopy (AAS) method. The mechanism of the catalyzing reaction is also discussed.     

Fluorescence Quenching Studies of Curcumin by Hydrogen Peroxide in Acetonitrile Solution

Summary. Steady state quenching studies of curcumin, 1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione, fluorescence by hydrogen peroxide were conducted in acetonitrile solution. A quenching rate constant, k_q, of 1.05×10¹⁰M^–1·s^–1 was obtained with a short fluorescence lifetime of 347ps. The reaction rate constant, which is within the diffusion-limited regime, is activation-controlled. The rate constant of deactivation of the thermally excited curcumin was 1.2 orders of magnitude more nonradiative (2.67×10⁹s^–1) than radiative (2.16×10⁸s^–1). The reaction was exothermic with a G° of –1.97eV and solvent reorganization energy of 1.37eV. These values indicate that the electron transfer reaction is solvent-mediated with electron transfer rate constant, k_ET, of 2.16×10¹⁰s^–1.     

Non-Aqueous Capillary Electrophoresis with Indirect Photometric Detection for Direct Analysis of Pb2+, Zn2+ and Cd2+ Ions

A new approach, based on non-aqueous capillary electrophoresis separation and indirect photometric detection, was established for the determination of the transition metal ions Pb²⁺, Zn²⁺ and Cd²⁺. Under optimized conditions, the method produced baseline separation of these three metal ions. The linear range and detection limits were 1050µM, 1.9µM for Cd²⁺; 1050µM, 2.1µM for Zn²⁺; and 20100µM, 3.8µM for Pb²⁺, respectively.     

Mixed ligand complexes involving aminoacid dithiocarbamates,substituted phosphines and nickel(II)

Mixed ligand complexes involving four aminoacid dithiocarbamates (RRdtc=glydtc^– (R=H; R=H), methdtc^– (R=H; R=C₃H₇S), sardtc^– (R=Me; R=H), trydtc^– (R=H; R=C₉H₈N), substituted phosphines [PPh₃, Ph₂PCH₂CH₂PPh₂(dppe)] and nickel(II) are reported. All are diamagnetic. Thermal analyses of the complexes are in keeping with the proposed formulae. Thermal decomposition of the dithiocarbamate moiety proceeds through the formation of Ni(SCN)₂. PPh₃ and dppe are lost in the initial decomposition stages.     

Energy Transfer Electrogenerated Chemiluminescence for the Determination of Sulfite

A simple and novel electrogenerated chemiluminescence (ECL) method for the determination of sulfite has been developed based on the energy transfer ECL process. It was found that a weak ECL signal of sulfite was electrochemically generated on a platinum electrode in neutral aqueous solution. The signal was strongly enhanced by rhodamine B as an energy receptor and further enhanced by the neutral surfactant Tween 80. In 0.10M phosphate buffer solution (pH=7.5) containing 2.0×10^–6gmL^–1 rhodamine B and 0.4% (v/v) Tween 80, the ECL response to the concentration of sulfite at a potential of 0.82V was linear over a range of 1.0×10^–7gmL^–1 to 8.0×10^–6gmL^–1, and the detection limit was 5×10^–8gmL^–1. The relative standard deviation (n=11, 1.0×10^–6gmL^–1) was 3.8%. The proposed method has been successfully applied to the determination of sulfite in pharmaceutical injections and white sugar samples.     

Adsorptive Catalytic Voltammetry of Physcion in the Presence of Dissolved Oxygen at a Carbon Paste Electrode

A novel electroanalytical method for the determination of physcion is described for the first time. Physcion yields an adsorption catalytic voltammetric peak at –0.74V (vs. SCE) in 0.4molL^–1 NH₄Cl–NH₃·H₂O buffer solution (pH 10.5) at a carbon paste electrode (CPE). The experimental results indicated that physcion is efficiently accumulated at a CPE by adsorption. In the subsequent potential scan, physcion was reduced to a homologous anthrahydroquinone compound. The compound was then immediately oxidized to physcion by the dissolved oxygen in the solution, and then physcion was again reduced at the CPE. As a result, a cyclic catalytic reaction was established. The second-order derivative peak current is proportional to the physcion concentration in the ranges of 2.0×10^–104.0×10^–9molL^–1 (accumulation 90s) and 4.0×10^–92.0×10^–8molL^–1 (accumulation 60s). The limit of detection is 8×10^–11molL^–1 (S/N=3) for a 120s accumulation time. The method was applied to the direct determination of physcion in the medicinal plant polygonum multiflorum Thumb with satisfactory results.     

Zr<Subscript>5</Subscript>Ir<Subscript>2</Subscript>In<Subscript>4</Subscript> – A Superstructure of the Lu<Subscript>5</Subscript>Ni<Subscript>2</Subscript>In<Subscript>4</Subscript> Type

Summary. Zr₅Ir₂In₄ was synthesized by reaction of the elements in a glassy carbon crucible in a water-cooled sample chamber of an induction furnace. The sample was characterized by X-ray diffraction on both powder and single crystals. Zr₅Ir₂In₄ crystallizes with a pronounced Lu₅Ni₂In₄ type subcell, space group Pbam, a=1739.5(6), b=766.3(2), c=338.9(2) pm. Weak additional reflections force a doubling of the subcell c axis. The superstructure of Zr₅Ir₂In₄ is of a new type: Pnma, a=1739.5(6), b=677.8(2), c=766.3(2) pm, wR2=0.0529, 1592 F² values, and 60 variable parameters. The group-subgroup scheme for the klassengleiche symmetry reduction is presented. The formation of the superstructure is most likely due to a puckering effect (size of the iridium atoms). The crystal chemistry of Zr₅Ir₂In₄ is briefly discussed.     

Electrochemical Determination of 4-Nitrophenol Using a Single-Wall Carbon Nanotube Film-Coated Glassy Carbon Electrode

A single-wall carbon nanotubes (SWNT) film coated glassy carbon electrode (GCE) was fabricated for the direct determination of 4-nitrophenol (4-NP). The electrochemical behaviors of 4-NP at the SWNT-film coated GCE were examined. In 0.1M phosphate buffer with a pH of 5.0, 4-NP yields a very sensitive and well-defined reduction peak at the SWNT-modified GCE. It is found that the SWNT film exhibits obvious electrocatalytic activity towards the reduction of 4-NP since it not only increases the reduction peak current but also lowers the reduction overpotential. Based on this, an electrochemical method was proposed for the direct determination of 4-NP. The reduction peak current varies linearly with the concentration of 4-NP ranging from 1×10^–8 to 5×10^–6M, and the detection limit is 2.5×10^–9M after 3min of open-circuit accumulation. The relative standard deviation at 2×10^–7M 4-NP was about 6% (n=10), suggesting excellent reproducibility. This new method was successfully employed to determine 4-NP in several lake water samples.     

Determination of Caffeine Using a Simple Graphite Pencil Electrode with Square-Wave Anodic Stripping Voltammetry

A simple commercial graphite pencil electrode (GPE) was utilized for monitoring caffeine using the square-wave anodic stripping voltammetry (SWASV) method. This method was applied to determine the caffeine levels in several tea samples, which yielded a relative error of 1% in the concentrations. Caffeine was deposited at 0.0V (vs. Ag/AgCl), then reduced at +1.40V to strip it on the GPE. Optimal experimental conditions for the analysis were found to be as follows: pH value of 9 for the medium; deposition potential of 0.0V; deposition time of 120s; SW frequency of 25Hz; SW amplitude of 45mV, and step potential of 6mV. Given these optimum conditions, a linear range was observed within the concentration of 0500mgL^–1. At caffeine concentrations of 50.0, 250.0, and 500.0mgL^–1, the relative standard deviations in measured concentrations (n=12) were 0.19, 0.09, and 0.11%, respectively. The detection limit was found to be 9.2mgL^–1, which is comparable with the result obtained using a carbon paste electrode, equivalent to 8.2mgL^–1.     

Electrochemical Determination of Cinnamtannin B1 with a Pyrolytic Graphite Electrode

Cinnamtannin B1 (trimeric proanthocyanidin), which is identified and isolated from the effective fraction of the root of Lindera aggregata (Sims) Kosterm, is one kind of condensed tannin used as an effective antipyrotic and antitumor agent. Its electrochemical response can be obtained at a pyrolytic graphite electrode. Consequently, an easily performed and sensitive method for the determination of cinnamtannin B1 is developed. The detection limit is estimated to be 1.0×10^–7M with the linear determination range of 2.0×10^–7M to 1.8×10^–6M. Five replicate analyses of 1.0×10^–6M cinnamtannin B1 yields an RSD value of 2.1%. Since the working electrode does not need to be modified with any other species, it is very stable, repeatable and easily treated, and this method therefore potentially useful in real sample analysis.     

Oxidation of Substituted Benzyl Alcohols by Quinoxalinium Dichromate. A Kinetic Study

Summary. Quinoxalinium dichromate (QxDC) oxidizes benzyl alcohol and substituted benzyl alcohols smoothly in dimethyl sulfoxide (DMSO) and in the presence of acid to the corresponding aldehydes. The reaction has unit dependence on each of the alcohol, QxDC, and acid concentrations. Electron-releasing substituents accelerate the reaction, whereas electron-withdrawing groups retard the reaction, and the rate data obey Hammetts relationship. The reaction constant was –1.09±0.01 at 303K. Oxidation of ,-dideuteriobenzyl alcohol indicated the presence of a substantial primary kinetic isotope effect (k_H/k_D=6.78 at 303K). The reaction failed to induce the polymerization of acrylonitrile. The rates of oxidation were determined at different temperatures and the activation parameters were evaluated. The analysis of the dependence of the kinetic isotope effect on temperature indicated that the reaction involves a symmetrical cyclic transition state. A suitable mechanism is proposed.     

Synthesis, Structure, and Magnetic Properties of the Silicides REIrSi ( RE = Ce, Pr, Er, Tm, Lu) and SmIr 0.266(8)Si 1.734(8)

The equiatomic rare earth metal–iridium–silicides REIrSi (RE=Ce, Pr, Er, Tm, Lu) were prepared by arc-melting of the elements and subsequent annealing. All silicides were characterized through their X-ray powder patterns. The structures of CeIrSi, ErIrSi, and LuIrSi were refined from X-ray single crystal diffractometer data: LaIrSi type, P2₁3, a=629.15(2)pm, wR2=0.1232, 280F² values, and 11 variable parameters for CeIrSi; TiNiSi type, Pnma, a=673.4(1), b=416.07(5), c=744.88(9)pm, wR2=0.0705, 339F² values, and 20 variable parameters for ErIrSi, and a=664.0(3), b=412.9(1), c=742.6(1)pm, wR2=0.0398, 496F² values, and 20 variable parameters for LuIrSi. The iridium and silicon atoms in CeIrSi, ErIrSi, and LuIrSi build three-dimensional [IrSi] networks where the iridium atoms have three (CeIrSi, Ir–Si 229pm) and four (ErIrSi, Ir–Si 247–258pm; LuIrSi, Ir–Si 245–256pm) silicon neighbors. The [IrSi] networks leave larger channels in which the cerium, erbium, and lutetium atoms are located. Temperature dependent susceptibility data for LuIrSi indicate Pauli paramagnetism. CeIrSi shows Curie-Weiss paramagnetism above 100K with an experimental magnetic moment of 2.56(2)_B/Ce atom. With samarium as rare earth metal component the silicide SmIr_0.266(8)Si_1.734(8) with -ThSi₂ type structure was obtained: I4₁/amd, a=409.3(1), c=1397.2(5)pm, wR2=0.0575, 161F² values, and 9 variable parameters. Within the three-dimensional [Ir_0.266Si_1.734] network the Ir/Si–Ir/Si distances range from 230 to 237pm.