Square wave adsorptive cathodic stripping voltarnmetry automated by sequential injection analysis - Potentialities and limitations exemplified by the determination of methyl parathion in water samples

This paper describes the development and evaluation of a sequential injection method to automate the determination of methyl parathion by square wave adsorptive cathodic stripping voltammetry exploiting the concept of monosegmented flow analysis to perform in-line sample conditioning and standard addition. Accumulation and stripping steps are made in the sample medium conditioned with 40 mmol L-1 Britton-Robinson buffer (pH 10) in 0.25 mol L-1 NaNO3. The homogenized mixture is injected at a flow rate of 10 mu Ls(-1) toward the flow cell, which is adapted to the capillary of a hanging drop mercury electrode. After a suitable deposition time, the flow is stopped and the potential is scanned from -0.3 to -1.0 V versus Ag/AgCl at frequency of 250 Hz and pulse height of 25 mV The linear dynamic range is observed for methyl parathion concentrations between 0.010 and 0.50 mgL(-1), with detection and quantification limits of 2 and 7 mu gL(-1), respectively. The sampling throughput is 25 h(-1) if the in line standard addition and sample conditioning protocols are followed, but this frequency can be increased up to 61 h(-1) if the sample is conditioned off-line and quantified using an external calibration curve. The method was applied for determination of methyl parathion in spiked water samples and the accuracy was evaluated either by comparison to high performance liquid chromatography with UV detection, or by the recovery percentages. Although no evidences of statistically significant differences were observed between the expected and obtained concentrations, because of the susceptibility of the method to interference by other pesticides (e.g., parathion, dichlorvos) and natural organic matter (e.g., fulvic and humic acids), isolation of the analyte may be required when more complex sample matrices are encountered. (C) 2007 Elsevier B.V. All rights reserved.

Amyloid beta-peptide activates nuclear factor-kappa B through an N-methyl-D-aspartate signaling pathway in cultured cerebellar cells

Amyloid P-peptide (A beta) likely causes functional alterations in neurons well prior to their death. Nuclear factor-kappa B (NF-kappa B), a transcription factor that is known to play important roles in cell survival and apoptosis, has been shown to be modulated by A beta in neurons and glia, but the mechanism is unknown. Because A beta has also been shown to enhance activation of N-methyl-D-aspartate (NMDA) receptors, we investigated the role of NMDA receptor-mediated intracellular signaling pathways in A beta-induced NF-kappa B activation in primary cultured rat cerebellar cells. Cells were treated with different concentrations of A beta 1-40 (1 or 2 mu M) for different periods (6, 12, or 24 hr). MK-801 (NMDA antagonist), manumycin A and FTase inhibitor 1 (farnesyltransferase inhibitors), PP1 (Src-family tyrosine kinase inhibitor), PD98059 [mitogen-activated protein kinase (MAPK) inhibitor], and LY294002 [phosphatidylinositol 3-kinase (PI3-k) inhibitor] were added 20 min before A beta treatment of the cells. A beta induced a time- and concentration-dependent activation of NF-kappa B (1 mu M, 12 hr); both p50/p65 and p50/p50 NF-kappa B dimers were involved. This activation was abolished by MK-801 and attenuated by manumycin A, FTase inhibitor 1, PP1, PD98059, and LY294002. AP at 1 mu M increased the expression of inhibitory protein I kappa B, brain-derived neurotrophic factor, inducible nitric oxide synthase, tumor necrosis factor-alpha, and interleukin-1 beta as shown by RTPCR assays. Collectively, these findings suggest that AP activates NF-kappa B by an NMDA-Src-Ras-like protein through MAPK and PI3-k pathways in cultured cerebellar cells. This pathway may mediate an adaptive, neuroprotective response to A beta. (c) 2007 Wiley-Liss, Inc.

Calixarene and Resorcarene Based Receptors: From Structural and Thermodynamic Studies to the Synthesis of a New Mercury(II) Selective Material

Materials used in current technological approaches for the removal of mercury lack selectivity. Given that this is one of the main features of supramolecular chemistry, receptors based on calix[4]arene and calix[4]resorcarene containing functional groups able to interact selectively with polluting ions while discriminating against biologically essential ones were designed. Thus two receptors, a partially functionalized calix[4]arene derivative, namely, 5,11,17,23-tetra-tert-butyl [25-27-bis(diethyl thiophosphate amino)dihydroxy] calix[4]arene (1) and a fully functionalized calix[4]resorcarene, 4,6,10,12,16,18,22,24-diethyl thiophosphate calix[4]resorcarene (2) are introduced. Mercury(II) was the identified target due to the environmental and health problems associated with its presence in water Thus following the synthesis and characterization of 1 and 2 in solution ((1)HNMR) and in the solid state (X-ray crystallography) the sequence of experimental events leading to cation complexation studies in acetonitrile and methanol ((1)H NMR, conductance, potentiometric, and calorimetric measurements) with the aim of assessing their behavior as mercury selective receptors are described. The cation selectivity pattern observed in acetonitrile follows the sequence Hg(II) > Cu(II) > Ag(I). In methanol 1 is also selective for Hg(II) relative to Ag(I) but no interaction takes place between this receptor and Cu(II) in this solvent. Based on previous results and experimental facts shown in this paper, it is concluded that the complexation observed with Cu(II) in acetonitrile occurs through the acetonitrile-receptor adduct rather than through the free ligand. Receptor 2 has an enhanced capacity for uptaking Hg(II) but forms metalate complexes with Cu(II). These studies in solution guided the inmobilization of receptor 1 into a silica support to produce a new and recyclable material for the removal of Hg(II) from water. An assessment on its capacity to extract this cation from water relative to Cu(II) and Ag (I) shows that the cation selectivity pattern of the inmobilized receptor is the same as that observed for the free receptor in methanol. These findings demonstrate that fundamental studies play a critical role in the selection of the receptor to be attached to silicates as well as in the reaction medium used for the synthesis of the new decontaminating agent.

Copper(II) and zinc(II) complexes with 2-benzoylpyridine-methyl hydrazone

2-Benzoylpyridine-methyl hydrazone (HBzMe) has been obtained as well as its copper(II) [Cu(HBzMe)Cl(2)] (1) and zinc(II) [Zn(HBzMe)Cl(2)] (2) complexes. Upon re-crystallization in 1 - 9 DMSO:acetone conversion of I into dimeric [Cu(BzMe)Cl](2) (1a) occurred. The crystal structures of HBzMe, 1, 1a, and 2 were determined. HBzMe adopts the ZE conformation in the solid. In all complexes the hydrazone adopts the E configuration to attach to the metal through the N(py)-N2-O chelating system. In 1 and 2 a neutral hydrazone coordinates to the metal center while in 1a deprotonation occurs with coordination of an anionic ligand. la presents a dimeric structure. having two copper(II) ions per asymmetric unit. Two chlorides are also present in the copper coordination sphere, which act as bridging ligands and connect the copper centers to each other. (C) 2008 Elsevier B.V. All rights reserved.

Structure-Activity Relationship for Quaternary Ammonium Compounds Hybridized with Poly(methyl methacrylate)

Hybrid films from poly (methylmethacrylate) (PMMA) and dioctadecyldimethylammonium bromide (DODAB), cetyltrimethylammonium bromide (CTAB), or tetrapropylammonium bromide (TPAB) were characterized by determination of wettability, ellipsometry, atomic force microscopy, active compounds diffusion to water, X-ray photoelectron spectroscopy (XPS) with determination of atomic composition on the films surface, and biocidal activity against Pseudomonas aeruginosa or Staphylococcus aureus. QAC mobility in the films increased from DODAB to CTAB to TPAB. Diffusion and optimal hydrophobic hydrophilic balance imparted the highest bioactivity to CTAB. DODAB sustained immobilization at the film surface killed bacteria upon contact. TPAB ability to diffuse was useless because of its unfavorable hydrophobic hydrophilic balance for bioactivity.

Development of a sequential injection-square wave voltammetry method for determination of paraquat in water samples employing the hanging mercury drop electrode

This work describes the development and optimization of a sequential injection method to automate the determination of paraquat by square-wave voltammetry employing a hanging mercury drop electrode. Automation by sequential injection enhanced the sampling throughput, improving the sensitivity and precision of the measurements as a consequence of the highly reproducible and efficient conditions of mass transport of the analyte toward the electrode surface. For instance, 212 analyses can be made per hour if the sample/standard solution is prepared off-line and the sequential injection system is used just to inject the solution towards the flow cell. In-line sample conditioning reduces the sampling frequency to 44 h(-1). Experiments were performed in 0.10 M NaCl, which was the carrier solution, using a frequency of 200 Hz, a pulse height of 25 mV, a potential step of 2 mV, and a flow rate of 100 mu L s(-1). For a concentration range between 0.010 and 0.25 mg L(-1), the current (i(p), mu A) read at the potential corresponding to the peak maximum fitted the following linear equation with the paraquat concentration (mg L(-1)): ip = (-20.5 +/- 0.3) Cparaquat -(0.02 +/- 0.03). The limits of detection and quantification were 2.0 and 7.0 mu g L(-1), respectively. The accuracy of the method was evaluated by recovery studies using spiked water samples that were also analyzed by molecular absorption spectrophotometry after reduction of paraquat with sodium dithionite in an alkaline medium. No evidence of statistically significant differences between the two methods was observed at the 95% confidence level.

Spectroscopic and theoretical studies of some N-methoxy-N-methyl-2-[(4 `-substituted) phenylsulfonyl]propanamides

The analysis of the IR carbonyl band of the N-methoxy-N-methyl-2-[(4`-substituted)phenyisulfonyI]propanamides Y-PhSO(2)CH(Me)C(O)N(OMe)Me (Y = OMe 1, Me 2, H 3, Cl 4, NO(2) 5), supported by HF/6-31G(d,p) calculations of 3,, indicated the existence of two gauche conformers (g(1) and g(2)), the g, conformer being the most stable and the least polar one (in gas phase and in solution). Both conformers are present in solution of the non polar solvent (CCl(4)) for 1-5 and in solution of the more polar solvents (CHCl(3)) for 1. 4, 5 and (CH(2)Cl(2)) for 5, while only the g(1) conformer is present in solution of the most polar solvent (CH(3)CN) for 1-5. The g, and g2 conformers correspond to the enantiomeric pairs of diastereomers (diast(1) and diast(2)) whose relative configurations are [C(3)(R)N(R)]/[C(3)(s)N(s)] and [C(3)(R)N(s)]/[C(3)(s)N(R)], respectively. The computed carbonyl frequencies for g(1) (diast(1)) and g(2) (diast(2)) stereoisomers of3 match well the experimental values. The NBO analysis, for 3 shows the important role of the orbital interactions in conformer stabilization and the overall balance of these interactions corroborates that the g, conformer is more stable than the 92 one. The observed abnormal solvent effect on the relative intensities of the carbonyl doublet components is attributed to the molecular crowding in the g2 conformer which hinders its solvation in comparison to the g, conformer (diast(1)). X-ray single crystal analysis performed for 3 shows the existence Of two 92, and g(1b) conformers of diastereomers (diast2, and diast(1b)) whose absolute configurations are [C(3)(R)N(s)] and [C(3)(R)N(R)], respectively. The larger population and. thus, the larger stabilization of the g(2), conformer over the gib form in the crystals may be associated with a larger energy gain deriving from dipole moment coupling in the former conformer along with a series of C-H center dot center dot center dot O electrostatic and hydrogen bond interactions, (C) 2009 Elsevier B.V. All rights reserved.

Spectroscopic and theoretical studies of some N-methoxy-N-methyl-2-[(4 `-substituted) phenylthio]propanamides

The analysis of the IR carbonyl band of the N-methoxy-N-methyl-2-[(4`-substituted)phenylthio]propanamides Y-PhSCH(Me)C(O)N(OMe)Me (Y=OMe 1, Me 2, H 3, Cl 4, NO(2) 5), supported by B3LYP/cc-pVDZ calculations of 3, indicated the existence of two gauche conformers (g(1) and g(2)), the g(1) conformer being the more stable and the less polar one (in gas phase and in solution). Both conformers are present in solution of the polar solvents (CH(2)Cl(2) and CH(3)CN) for 1-5 and in solution of the less polar solvent (CHCl(3)) for 1-4, while only the g(1) conformer is present in solution of non polar solvents (n-C(6)H(14) and CCl(4)) and in solution of CHCl(3) for 5. NBO analysis shows that both the sigma(C-S) -> pi*(C=O) (hyperconjugative) and the pi(C=O) -> sigma*(C-S) orbital interactions contribute almost to the same extent for the stabilization of g(1) and g(2) conformers. The pi*(C=O) -> sigma*(C-S), n(S) -> pi*(C=O) and the n(S) -> pi*(C=O) orbital interactions stabilize more the g(1) conformer than the g(2) one. Moreover, the suitable geometry of the g(1) conformer leads to its stabilization through the LP(O2) -> sigma*(C8-H11) orbital interaction (hydrogen bond) along with the strong O([CO])(delta-) center dot center dot center dot H([O-Ph])(delta+) electrostatic interaction. On the other hand, the appropriate geometry of the g(2) conformer leads to its stabilization by the LP(O22) -> sigma*(C9-H13) orbital interaction (hydrogen bond) along with the weak O([OMe])(delta-) center dot center dot center dot H([o`-Ph])(delta+) electrostatic static interaction. As for the 4`-nitro derivative 5 the ortho-phenyl hydrogen atom becomes more acidic, leading to a stronger O([CO])(delta-) center dot center dot center dot H([o-Ph])(delta+) interaction and, thus, into a larger stabilization of the g(1) conformer in the whole series. This trend is responsible for the unique IR carbonyl band in CHCl(3) solution of 5. The larger occupancy of the pi*(C=O) orbital of the g(1) conformer relative to that of the g(2) conformer, along with the O([CO])(delta-) center dot center dot center dot H([o-Ph])(delta+) electrostatic interaction (hydrogen bond) justifies the lower carbonyl frequency of the g(1) conformer with respect to the g(2) one, in gas phase and in solution. (C) 2008 Elsevier B.V. All rights reserved.

The reaction of methyl radical with nitrogen atom on the triplet potential energy surface: A CCSD(T)/CBS characterization

CCSD(T)/cc-pVnZ (n = D, T, Q) calculations followed by extrapolations to the CBS limit are used to characterize stationary states of species participating in the N((4)S) + CH(3) (2A ``) reaction on the triplet PES. A mechanistic model is investigated and reaction rates are computed for every step and the overall reaction. Our best CBS estimate (1.93 x 10(10) cm(3) molecule(1) s(1)) for the overall rate constant leading to the formation of H(2)CN + H compares well with the experimental values (8.5 x 10 (11) and 1.3 x 10(10) cm(3) molecule(1) s(1)), thus reducing significantly the discrepancy of a previous theoretical result (9.1 x 10(12) cm(3) molecule(1) s(1)). (C) 2008 Elsevier B.V. All rights reserved.

Evaluation of a carbon paste electrode modified with organofunctionalised SBA-15 nanostructured silica in the simultaneous determination of divalent lead, copper and mercury ions

The performance of a carbon paste electrode (CPE) modified with SBA-15 nanostructured silica organofunctionalised with 2-benzothiazolethiol in the simultaneous determination of Pb(II), Cu(II) and Hg(II) ions in natural water and sugar cane spirit (cachaca) is described. Pb(II), Cu(II) and Hg(II) were pre-concentrated on the surface of the modified electrode by complexing with 2-benzothiazolethiol and reduced at a negative potential (-0.80 V). Then the reduced products were oxidised by DPASV procedure. The fact that three stripping peaks appeared on the voltammograms at the potentials of -0.48 V (Pb2+), -0.03 V (Cu2+) and +0.36 V (Hg2+) in relation to the SCE, demonstrates the possibility of simultaneous determination of Pb2+, Cu2+ and Hg2+. The best results were obtained under the following optimised conditions: 100 mV pulse amplitude, 3 min accumulation time, 25 mV s(-1) scan rate in phosphate solution pH 3.0. Using such parameters, calibration graphs were linear in the concentration ranges of 3.00-70.0 x 10(-7) mol L-1 (Pb2+), 8.00-100.0 X 10(-7) mol L-1 (Cu2+) and 2.00-10.0 x 10(-6) mol L-1 (Hg2+). Detection limits of 4.0 x 10(-8) mol L-1 (Pb2+), 2.0 x 10(-7) mol L-1 (Cu2+) and 4.0 x 10(-7) mol L-1 (Hg2+) were obtained at the signal noise ratio (SNR) of 3. The results indicate that this electrode is sensitive and effective for simultaneous determination of Pb2+, Cu2+ and Hg2+ in the analysed samples. (C) 2008 Published by Elsevier B.V.

Ex Situ Scanning Electrochemical Microscopy (SECM) Investigation of Bismuth- and Bismuth/Lead Alloy Film-Modified Gold Electrodes in Alkaline Medium

Scanning electrochemical microscopy (SECM) in feedback mode was employed to characterise the reactivity and microscopic peculiarities of bismuth and bismuth/lead alloys plated onto gold disk substrates in 0.1 molL(-1) NaOH solutions. Methyl viologen was used as redox mediator, while a platinum microelectrode was employed as the SECM tip. The metal films were electrodeposited ex situ from NaOH solutions containing either bismuth ions only or both bismuth and lead ions. Approach curves and SECM images indicated that the metal films were conductive and locally reactive with oxygen to provide Bi(3+) and Pb(2+) ions. The occurrence of the latter chemical reactions was verified by local anodic stripping voltammetry (ASV) at the substrate solution interface by using a mercury-coated platinum SECM tip. The latter types of measurements allowed also verifying that lead was not uniformly distributed onto the bismuth film electrode substrate. These findings were confirmed by scanning electron microscopy images. The surface heterogeneity produced during the metal deposition process, however, did not affect the analytical performance of the bismuth coated gold electrode in anodic stripping voltammetry for the determination of lead in alkaline media, even in aerated aqueous solutions. Under the latter conditions, stripping peak currents proportional to lead concentration with a satisfactory reproducibility (within 5% RSD) were obtained.

Stereochemical and electronic interaction studies of some N-methoxy-N-methyl-2-[(4 `-substituted)phenylsulfinyl]propanamides

The (1)H NMR spectra of N-methoxy-N-methyl-2-[(4`-substituted)phenylsulfinyl]-propanamides [Y-Ph-S(O)CH(Me)C(O)N(OMe)Me; Y = OMe 1, Me 2, H 3. Cl 4, NO(2) 5] along with the X-ray diffraction analysis of the nitro-derivative (5). have shown the existence of two pairs of diastereomers (racemic mixture) [C(R)S(S)/C(S)S(R) (diast(1)) and C(R)S(R)/C(S)S(S) (diast(2))] in the ratio of ca. 7:3. respectively. The v(CO) IR analysis of the title compounds supported by HF and B3LYP/6-31G** calculations of 3 and of the parent N-methoxy-N-methyl-propanamide (6) by HF, have shown that diast(1) exists in an equilibrium between the two more polar and more stable quasi-cis (q-c(1) and q-c(2)) conformers and the gauche(g) conformer. The population of the g conformer in the equilibrium increases with the increase in the solvent polarity, which is attributed to a larger solvation effect on the carbonyl and sulfinyl groups. Diast(2) of compound 3 occurs in the gas phase as an equilibrium between the most stable quasi-gauche (q-g) conformer and the quasi-cis (q-c) conformer, both presenting very similar dipole moments. The former is stabilized by electrostatic and charge transfer interactions, which results in a less solvated spatial arrangement. Moreover, all conformers of both diastereomers are stabilized by several intramolecular hydrogen bonds. X-ray single crystal analysis performed for diast(1) and for diast(2) of 5 indicates that both stereoisomers assume, in the solid state, the anti-clinal (gauche) conformation. For the crystal packing, diast(1) of 5 is made up of three molecules joined through two centro-symmetric H center dot center dot center dot O hydrogen bonds. (C) 2008 Elsevier B.V. All rights reserved.

Fluorescent probes with malononitrile side group in methyl methacrylate copolymers

Fluorescent probes derivated from auramine, 1-aminopyrene, and 9-aminoacridine containing a malononitrile group are copolymerized with methyl methacrylate. These new fluorescent polymeric materials are studied in solution of different solvents by steady-state and time-resolved emission techniques. Their spectroscopic properties and excited state dynamics are driven by charge transfer from the aromatic group to the electron withdrawing CN groups, and this factor is responsible for the non-exponential emission decay behavior. (c) 2008 Elsevier B.V. All rights reserved.