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The dissociative (D) mechanistic water-exchange kinetics of aquated Al3+ in aqueous solution is systematically studied using the density functional theory-quantum chemical cluster model (DFT-CM) method. The modeled pathways include dehydration of the hexacoordinated Al(H2O)63+ and successive hydration of the pentacoordinated Al(H2O)53+. For the hydration of Al(H2O)53+, the attacking pathways corresponding to the second-shell solvent water molecules at different sites are investigated. The gas phase-supermolecule-polarizable continuum model (GP-SM-PCM) is used to simulate the explicit and bulk solvation effects. The reactant, transition state, and product geometries of the modeled reaction pathways are optimized at the B3LYP/6-311+G(d,p) level of theory. The real and apparent water-exchange reaction mechanisms of Al3+ are analyzed based on the Gibbs free energy changes for the dehydration and hydration pathways. The possible ligand competition with the solvent water molecules in the formation of aqueous Al...

In this study, the 27Al NMR chemical shifts and relative stabilities of monomeric Al3+ hydrolytic species with different coordination structures in aqueous solution are systematically investigated by using the density functional theory quantum chemical cluster model (DFT-CM) at the B3LYP/6-311+G(d,p) level. The main work includes: the static configurations of 20 possible existing monomeric Al3+ hydrolytic species from Al3+ to Al(OH)4– are optimized, and their 27Al NMR shieldings are calculated; the dehydration reaction pathways for typical monomeric Al3+ hydrolytic species are modeled, and the dominant forms of the intermediate hydrolytic species of Al(OH)2+, Al(OH)2+, and Al(OH)30 are analyzed based on the Gibbs free energy changes of the dehydration reactions. The important role of the tetracoordinated Al(H2O)(OH)30 in the formation mechanism of the polynuclear Keggin-Al13 is discussed. This work provides valuable references for further studying the formation and transformation mechanisms of the aqueous...

The stable geometries of a series of mononuclear bidentate chelating Al(III)-carboxylate complexes in aqueous solution are optimized and their transition-states for water-exchange reactions are modeled with the density functional theory – quantum chemical cluster model (DFT-CM) method. The studied carboxylates include oxalate, malonate, succinate, phthalate, salicylate and benzoate. Thermodynamic stability constants Kaq and water-exchange reaction rate constants kex of the tested Al(III)-carboxylate complexes are estimated from calculated Gibbs free energy changes. The estimated kex values suggest that the coordination of dicarboxylate and salicylate ligands to Al3+ leads to the labilization of inner-shell coordinated waters and that the cis waters of chelate rings are more labile than trans waters. From estimated log Kaq and log kex values of aqueous mononuclear Al(III)-carboxylate complexes, linear correlations between log Kaq, log kex values and experimental apparent rate constants kL of carboxylate ligand-promoted δ-Al2O3 mineral dissolution are established. Based on the calculation results, cis labilizing effects of the carboxylate ligands, strong “log kL ∼ log Kaq” correlations, and weak “log kL ∼ log kex” correlations are discussed. This work offers further insight into intrinsic relationships between mineral surface and aqueous metal complexes and furthers understanding of mineral dissolution kinetics at the molecular level.

The formation mechanisms, thermodynamic stabilities, and water-exchange reactivities of 1:1 monomer aluminum–salicylate (Al–salicylate) complexes in acidic aqueous solution are investigated using the density functional theory-quantum chemical cluster model (DFT-CM) method. (1) The formation pathways for possible monodentate and bidentate Al–salicylate configurations are modeled with the gas phase-supermolecule-polarizable continuum model (GP-SM-PCM). It shows that the formation pathways for the Al–salicylate complexes follow the Eigen-Wilkins mechanism, where the dissociation of an inner-shell coordinated water of Al3+ is the rate-determining step. (2) The formation constants Kaq for different Al–salicylate configurations are estimated based on the total Gibbs free energy changes ΔG° for their overall formation pathways. It is indicated that in the acidic aqueous solution at pH ∼ 3, the main existence form of the 1:1 monomer Al–salicylate complex is the phenol-deprotonated bidentate Al(Sal)(H2O)4+ with si...

The kinetic mechanism of spontaneous aluminum ion (Al3+) hydrolysis reaction in aqueous solution is investigated using the density functional theory–quantum chemical cluster model method. Three typical reaction pathways for the spontaneous Al3+ hydrolysis reaction are modeled, including (1) the traditional spontaneous proton dissociation on the Al3+ inner-shell coordinated waters; (2) the conventional bulk water-assisted proton dissociation; and (3) the second-shell water-assisted synergistic dissociation of the protons on the Al3+ inner-shell waters. The results show that the electrostatic effects between Al3+ and its coordinated waters alone cannot fully account for the proton loss on an inner-shell coordinated water. It is suggested that the main reaction pathway for natural hydrolysis of aqueous Al3+ is the second-shell water-assisted synergistic proton dissociation, in which the participation of the second hydration shell is crucially important. The calculated synergistic proton dissociation rate con...

Density functional theory (DFT) calculations combined with cluster models are performed at the B3LYP/6-311+G(d,p) level for investigating the solvent effects in Al(H2O)63+ water-exchange reactions. A “One-by-one” method is proposed to obtain the most representative number and arrangement of explicit H2Os in the second hydration sphere. First, all the possible ways to locate one explicit H2O in second sphere (Nm′ = 1) based on the gas phase structure (Nm′ = 0) are examined, and the optimal pathway (with the lowest energy barrier) for Nm′ = 1 is determined. Next, more explicit H2Os are added one by one until the inner-sphere is fully hydrogen bonded. Finally, the optimal pathways with Nm′ = 0–7 are obtained. The structural and energetic parameters as well as the lifetimes of the transition states are compared with the results obtained with the “Independent-minimum” method and the “Independent-average” method, and all three methods show that the pathway with Nm′ = 6 may be representative. Our results give a new idea for finding the representative pathway for water-exchange reactions in other hydrated metal ion systems.

The solvation effect on the kinetic rattling behaviour of the hydrated excess proton H+(aq) in water is theoretically modeled by using density functional theory (DFT) and the quantum chemical cluster model (CM). To test the solvation effects on the proton morphology and rattling kinetics, different solvation models for the proton are constructed based on the gas phase (GP) Zundel cation, which include the gas phase-polarizable continuum model (GP-PCM), the gas phase-supermolecule model (GP-SM), and the gas phase-supermolecule-polarizable continuum model (GP-SM-PCM). These solvation models consider either one or both of the short- and long-range solute-solvent interactions. Meanwhile, 1 to 6 explicit solvent water molecules (Nm' = 1-6) are added around the GP Zundel cation to test different explicit solvation environments. The calculation results show that the solvation environment has an important influence on the morphology and rattling kinetics of H+(aq). The proton rattling pathways are obtained only under the condition that both symmetrical explicit solvation environments and implicit bulk solvents are present. The zero-point contribution reduces the reaction energy barrier and enables the rattling to occur spontaneously at room temperature. The theoretical modeling results provide new insights into the microscopic kinetic behaviour of proton rattling in water at the molecular level, which are helpful in studying the proton transfer mechanism in aqueous systems.

Density functional theory (DFT) calculations combined with cluster models are performed at the B3LYP/6-311+G(d,p) level for investigating the solvent effects in Al(H

Density functional theory (DFT) calculations were performed on the structures and water-exchange reactions of aqueous Al(III)–salicylate complexes. Based on the four models (gas phase (GP); polarizable continuum model (PCM), which estimates the bulk solvent effect; supermolecule model (SM), which considers the explicit solvent effect, and supermolecule–polarizable continuum model (SM–PCM), which accounts for both types of solvent effects), we systematically conducted this study by examining three different properties of the complexes. (1) The microscopic properties of the aqueous Al(III)–salicylate complexes were studied by optimizing their various structures (including the possible 1:1 mono- and bidentate complexes, cis and trans isomers of the 1:2 bidentate complexes and 1:3 bidentate complexes) at the B3LYP/6-311+G(d, p) level. (2) The 27 Al and 13 C NMR chemical shifts were calculated using the GIAO method at the HF/6-311+G(d, p) level. The calculation results show that the values obtained with the SM–PCM models are in good agreement with the experimental data available in the literature, indicating that the models we employed are appropriate for Al(III)–salicylate complexes. (3) The water-exchange reactions of 1:1 mono- and bidentate Al(III)–salicylate complexes were simulated using supermolecule models at the B3LYP/6-311+G(d, p) level. The logarithm of the water-exchange rate constant (log k ex ) of the 1:1 bidentate complex predicted using the “log k ex – d Al–OH2 ” correlation is 4.0, which is in good agreement with the experimental value of 3.7, whereas the calculated range of log k ex of the 1:1 monodentate complexes is 1.3–1.9. By effectively combining the results for the thermodynamic static structures with the simulations of the kinetic water-exchange reactions, this work promotes further understanding of the configurations and formation mechanism of Al(III)–salicylate complexes.

Theoretical modeling and analyzing of reversible electrode reaction coupled with Langmuir adsorption is presented using cyclic reciprocal derivative chronopotentiometry (CRDCP) with symmetrical programmed current applied, in which the finite difference method was firstly used to solve the boundary value problem corresponding to the electrode processes. The algorithm described in this paper is available to solve analogous difference equations with nonlinear boundary conditions. Distinct features of dt/dE–E curves for typical models of electrode processes are obtained. If the adsorption coefficients of the electroactive species are of different orders of magnitude, the predominant peak splitting of the dt/dE–E curves can be observed, corresponding to the adsorption peaks and diffusion peaks. The sequence of the diffusion and adsorption peaks can indicate the strong adsorption of either reactant or product.

The oxidation of 1, 4-nicotinamide adenine dinucleotide (NADH) to β-nicotinamide adenine dinucleotide (NAD+) coupled with converting of pyruvic acid (PA) to lactate catalyzed by lactate dehydrogenase (LDH), NADH + PA + H+ ⇌ LDH NAD+ + Lactate, was widely adopted to quantify the cell's death, membrane infiltration and proliferation induced by potential toxins. The differential pulse voltammetry (DPV) cathodic signal of NAD+ at a hanging mercury drop electrode (HMDE) showed LDH activity decreased with the elevating dosages of and the pre-contact time (tc) with multi-walled carbon nanotubes (MWCNTs). Comparison of kinetic rate constant of above enzymatic reaction (ER) was able to sensitively assay the adverse influence of MWCNTs. Toxic concentration of altering relative LDH activity by 50% (TC50) of MWCNTs was derived to be 40 mg/L. TC50 values indicated a decrease toxicity order Al (III) > MWCNTs > nano-Al13 > 50 nm-Al2O3 ≥ 1000 nm-Al2O3. The negatively charged surfaces of these nanoparticles (NPs) might be a main cause for the decrement of LDH activity. This decrement was capable to result in the underestimation of the toxicity of NPs in classic LDH assays. This observation would highlight to settle down contradictory medium dependent toxicity of MWCNTs among the literature.

In this article we investigated the effect of solvents (CCl4, CH3CN, DMF, ethanol, ethanol–H2O and H2O) on self-assembly of Thioctic acid (TA) and Mercaptohexanol (MCH) on gold by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Electrochemical characteristics of TA and MCH self-assembled monolayers (SAMs) formed in different solvents were evaluated by inspecting the ions permeability (interfacial capacitance C and phase angle Ф1 Hz) and electron transfer capability (current density difference Δi and charge transfer resistance Rct). Experimental results indicated that the ability of solvents availing the ordering of SAMs was: for TA, CCl4 > ethanol > CH3CN > ethanol–H2O > DMF; for MCH, H2O > ethanol–H2O ≈ CCl4 > ethanol ≈ CH3CN > DMF. Through relating the C, Ф1 Hz, Δi and Rct of SAMs (TA and MCH) with parameters of solvent (polarity ETN, solubility parameter δ and octanol/water partition coefficients logPow), it was found that solvents with bigger logPow (smaller ETN and δ) availed the ordering of TA-SAMs but the effect of solvents on MCH self-assembly was complex and MCH-SAMs formed in H2O (the biggest ETN, δ and the smallest logPow) and CCl4 (the smallest ETN, δ and the biggest logPow) were more ordered than in other solvents.

The mechanisms for the substitution of an aqua ligand with F(-) in monomeric Al complexes were studied with density functional theory (DFT). Typical mechanisms are modeled to determine the preferred substitution pathway according to the activation energy barriers. The present computational results are in favor of interchange associative (I(a)) mechanism for the substitution of F(-) into Al(H(2)O)(6)(3+), whereas interchange dissociative (I(d)) mechanism is preferred for the substitution into Al(H(2)O)(5)(OH)(2+), which is in agreement with the previous experimental findings. This implies the mechanistic changeover from I(a) to I(d) induced by the spectator hydroxyl ligand. Like the water-exchange reaction, the substitution rate is accelerated by OH(-) ligand. The difference of the computational and experimental activation enthalpy values is interpreted as the DFT errors in energy and the deviation of transmission coefficient from unity.

Theoretical modeling and analyzing of electrode process in the presence of adsorptive reactants is presented using cyclic reciprocal derivative chronopotentiometry (CRDCP) with symmetrical programmed current applied. Electrode process proposed based on the assumption of the electrolysis sequence of diffusion and adsorbed electroactive species is described as “on first, off last” and denoted as Onfol model. The numerical simulated (d t /d E )– E waves obtained from the analytical potential-time expressions of the Onfol model present peaks during both “diffusion processes” and “adsorption processes”. Influences of maximum surface excess, adsorbed monolayer thickness, adsorption coefficient and current steps on CRDCP characteristic parameters are discussed. CRDCP behavior allows us to distinguish the form of the electroactive species contributing to the system current and electrolysis sequence can therefore be determined.

Raman frequency calculations of Al ( H 2 O ) 6 3 + and Al ( OH ) 4 - species in different models were carried out with density functional theory (DFT). Gas-phase model (GP), polarizable continuum model (PCM), supermolecule model (SM) and supermolecule-polarizable continuum model (SM-PCM) were considered and frequencies over the wavenumber range from 0 to 1000 cm−1 were discussed. The calculated frequencies of the two species in SM-PCM model are in fair agreement with the observed frequencies in aqueous solution and the average deviation is 6 cm−1. Based on the results of Al ( H 2 O ) 6 3 + and Al ( OH ) 4 - , the DFT method and the SM-PCM model were used to study the Al–O Raman bands for some other monomeric and dimeric aluminiums. The monomeric aluminiums Al(OH)2+, Al ( OH ) 2 + and Al ( OH ) 6 3 - as well as the dimer Al 2 O ( OH ) 6 2 - were included. As a result, we predict the main Al–O skeleton vibrations of Al ( OH ) ( H 2 O ) 5 2 + at 685 cm−1, trans- Al ( OH ) 2 ( H 2 O ) 4 + at 542 cm−1, Al ( OH ) 6 3 - at 510 cm−1 and Al 2 O ( OH ) 6 2 - at 510 and 699 cm−1 respectively in Raman spectra of aqueous solution. It can also be concluded that the calculated frequencies of Al 2 O ( OH ) 6 2 - are close to the experimental values and the existence of Al ( OH ) 6 3 - in aluminate solution cannot be ruled out.

In this article we systematically investigated self-assembly of short-chain thiols of thioctic acid (TA) and mercaptohexanol (MCH) on gold under potential control, Edc (−0.4, +0.4 and +0.7 V) and compared the results obtained with open circuit potential (EOCP). Effect of Edc on thiol self-asembly was inspected based on the changes in electrochemical parameters including interfacial capacitance (C), phase angle (Φ1 Hz), current density difference (Δi), charge transfer resistance (Rct) through cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Experimental results showed that Edc could not obtain stable short-chain self-assembled monolayers (SAMs) (TA and MCH) in a short time. Both TA and MCH had slow self-assembly dynamics and needed a long time (> 24 h) to achieve adsorption equilibrium. Furthermore, the negative potential Edc (−0.4 V) did not facilitate the ordering of SAMs. The ordering of TA-SAMs was found to be the best when assembled under Edc (+0.4 V), whereas that of MCH-SAMs was almost the same when assembled under either EOCP or Edc (+0.4, +0.7 V). We considered that permeation of ions and water molecules perhaps dominated the slow self-assembly dynamics of short-chain thiols (TA and MCH) under Edc and mutual interaction between adjacent chains of thiols played an important role in the ordering of SAMs.

In this paper, the structure of the Al 30 O 8 (OH) 56 (H 2 O) 26 18+ (Al 30 ) polyoxocation in aqueous solution is investigated, including an exploration of its water-exchange reaction using a supramolecular model. Thirty-one solvent water molecules were explicitly included in the supramolecular model to approximate the influence of the solvent. The calculated results indicated that both the gas-phase and the supramolecular models could correctly reproduce the structure of the Al 30 polyoxocation, but the supramolecular model described the structure more accurately. Using the supramolecular model, we calculated the 27 Al NMR chemical shifts of various aluminum atoms using HF and GIAO methods, and they compared well to the chemical shifts determined experimentally. The water-exchange reaction of the Al 30 polyoxocation could not be simulated with the gas phase model because of a proton-transfer reaction that is induced by the highly positive charge of the Al 30 polyoxocation. However, the inclusion of an explicit second solvation sphere lowered the acidity of the coordinated water molecules and allowed simulation of the water exchange reaction.

The formation mechanism of Al 30 O 8 (OH) 56 (H 2 O) 26 18+ (Al 30 ) has been investigated by the density functional theory based on the supermolecule model and kinetic analysis on the 27 Al nuclear magnetic resonance (NMR) experimental results in monitoring Al 30 synthesis process. The theoretical chemistry calculations on the four possible schemes show that δ-Na–Al 13 is the reasonable intermediate followed by the substitution of Na with Al to form δ-Al 14 , and Na + plays an important role in stabilizing the intermediate (δ-Na–Al 13 ) in the transformation. The kinetic analysis on the 27 Al NMR experimental data indicates that e-Al 13 decomposes and isomerizes in the formation of Al 30 , while Al monomers facilitate the decomposition of e-Al 13 and so the isomerization of e-isomers to δ-isomers effectively. The favorable formation mechanism of Al 30 includes three steps: (1) e-Al 13 decomposes and rearranges into the isomer δ-Al 13 ; (2) Na + reacts with δ-Al 13 to stabilize the intermediate δ-Na–Al 13 , followed by Al monomers replacing Na to form δ-Al 14 ; (3) δ-Al 14 reacts with the Al monomers in the solution to finally form Al 30 . Both Al monomers and Na + are important in the transformation. Al monomers are the basic building units and helpful to the isomerization while Na + can well stabilize the isomer δ-Al 13 to yield intermediate δ-Na–Al 13 . The results also show that other isomers of e-Al 13 (β-Al 13 and α-Al 13 ) form in the formation of Al 30 , and their calculated 27 Al NMR tetrahedral resonance shifts are consistent with the experimental 27 Al NMR tetrahedral signals in the preparation process of Al 30 .

A simple and sensitive indirect determination method for sulfide in water samples by anodic stripping voltammetry (ASV) using mercury-film electrode (MFE) has been developed, which is based on the determination of residual cadmium ion after reaction of Cd2+ with S2−. The linear range is adjustable depending on Cd2+ concentration, for example, the determination of S2− can be achieved in the range of 1.5–7.0 × 10−6 mol L−1 by selecting 3.0 × 10−6 mol L−1 Cd2+. The detection limit is 1.3 × 10−8 mol L−1 under optimum conditions, and the relative standard deviation (RSD, n = 10) for 2.0 × 10−6 mol L−1 S2− is 0.7%. Compared with other methods, this method has the following prominent advantages: with low detection limit, easy to operate and less interference. The proposed method has been successfully applied to the determination of S2− in synthetic wastewater, lake water, beverage, spring water and real wastewater samples.

Cyclic reciprocal derivative chronopotentiometry (CRDCP) with successive symmetrical programmed currents applied has been extended to describe irreversible electrode reactions where Henry adsorption occurs at the electrode/solution interface. The numerical simulated (d t /d E )– E curves are sensitive to electrode reactions coupled with adsorption processes. Effects of parameters on (d t /d E )– E curves presented in this work involve adsorption constants ( K Ox and K Red ) and kinetic parameters ( k s 0 and α ). Characteristic parameters of (d t /d E )– E curves depend on the adsorption strength markedly and afford effective criteria to determine different cases. The characteristic behavior of (d t /d E )– E curves with successive currents applied give distinct descriptions between two limiting cases – the “surface nature” and “diffusion character”. Comparisons are made with reversible charge transfer reaction with Henry adsorption and uncomplicated, irreversible electrode reaction.

The interaction between double-strand calf thymus gland DNA (ds-DNA) and Al(III) was studied by using differential pulse voltammetry (DPV) at a hanging mercury drop electrode (HMDE), Raman spectrometry and circular dichroism (CD) spectra. It was shown that at neutral pH ds-DNA did not produce any cathodic peak at the HMDE in the potential window from -550 to -2000 mV vs. SCE. However, in the presence of Al(III), a cathodic peak was generated at about -1660 mV, which is ascribed to a reduction of adenine and cytosine residues of single denatured DNA (sd-DNA). It was concluded that ds-DNA was completely denatured to sd-DNA by Al(III) at a neutral pH. The apparent denaturing kinetic velocity constants of ds-DNA by Al(III) were derived from linear increases of the cathodic peak currents with time. When [Al(III)]x[OH(-)](3) > or = 2 x 10(-26), the precipitation of Al(OH)(3) was observed and identified by the Raman spectrum, and inductively coupled plasma atomic emission spectrometry (ICP-AES). CD spectra showed that the B-type of structure conformations of ds-DNA and related sd-DNA did not change with the increment of Al(III) from 5.0 x 10(-7) to 1.0 x 10(-5) M, but the corresponding absorption strengths increased. The related physiological significances and possible applications of the observations were considered.

This paper reports the preparation of dodecanethiol self-assembled monolayers (C 12 SH-SAMs) on polycrystalline gold electrodes in a magnetic field. The qualities of C 12 SH-SAMs were characterized by both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results show that highly dense and well-ordered SAMs could form in a relatively short time (3 h), indicating that the rate of SAMs formation increased under an external magnetic field compared with the natural self-assembly process. Moreover, the results of CV and EIS measurements also suggested that the presence of a magnetic field had influenced the qualities of the SAMs; the stronger magnetic intensity can help to obtain much denser and well-ordered SAMs.

Reaction pathways, solvent effects and energy barriers have been investigated for the water exchange of the polyoxocation GaO 4 Al 12 (OH) 24 (H 2 O) 12 7+ (K–GaAl 12 ) in aqueous solution by means of supermolecule density functional theory calculations. In the proposed reaction pathway, the supermolecular reactant K–GaAl 12 15H 2 O first loses a water ligand to form an intermediate with a five-coordinated aluminum atom, and then the incoming water molecule in the second coordination sphere attacks the intermediate with a five-coordinated aluminum atom to produce the reaction product. Our calculated results indicate that the water exchange of K–GaAl 12 proceeds via a dissociative mechanism, and that the reverse reaction of Step II is the most favorable dissociative pathway, with a barrier height of 31.3 kJ mol −1 . The calculated transition-state rate for the favorable dissociative pathway is much larger than the experimental rate constant, but is close to the data calculated for Al 30 by molecular dynamics. The transmission coefficient was also predicted on the basis of both the calculated transition-state rate and the experimental rate. Our calculated results also indicate that both the explicit solvent effect and the bulk solvent effect have obvious effects on the barrier heights of the water exchange reaction of K–GaAl 12 . By comparison, the water exchange mechanism for K–GaAl 12 was found to be more similar to that for mineral surfaces than that for monomeric aluminum species.

This paper reports the study of the solvent (tetrachloromethane, toluene, tetrahydrofuran, acetone, N , N -dimethylformamide, acetonitrile, ethanol, ethanol–water mixture (1:1, v:v) and neat dodecanethiol liquid) effect on the quality of self-assembled monolayers of dodecanethiol (C 12 SH-SAMs) on polycrystalline gold. The quality of the C 12 SH-SAMs was investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), and the results show that highly dense and well-ordered C 12 SH-SAMs can be prepared from ethanol–water mixture and neat dodecanethiol liquid. The property parameters of the solvents, such as polarity, solubility parameter, molecular diameter, octanol–water partition coefficients and viscosity were correlated with their ability to influence the quality of the C 12 SH-SAMs.

The effects of nanometer-sized tridecameric aluminum polycation (nano-Al13) on lactate dehydrogenase (LDH) activity have been investigated using differential pulse voltammetry (DPV) at the molecular level. The effects of nano-Al13 on LDH activity at different temperature and pH values were investigated. The results showed that these factors had quite different influences. The characteristic parameters of Michaelis constant Km and maximum velocity vmax were evaluated by monitoring the DPV reduction current of NAD+ (β-nicotinamide adenine dinucleotide) and compared with the other known inhibitors of LDH. The proposed method might be hypothesized to be a simple and convenient approach to evaluate the toxic and biological effects of nanoparticles on the environment.

The promotion effect of titania nanoparticles (nano-TiO(2)) on the direct electron transfer between lactate dehydrogenase (LDH) and the silica sol-gel modified gold electrode was investigated by adding nano-TiO(2) (50 nm) in the modification process. This nano-TiO(2)-LDH electrode showed a pair of quasi-reversible cyclic voltammetry peaks with the formal potential of 70 mV (vs. SCE). Compared to the previous result of LDH modified electrode with only an irreversible cathodic peak, an anodic peak appeared and the cathodic peak potential shifted to the positive direction on this nano-TiO(2)-LDH electrode, which demonstrated that the direct electrochemistry of LDH was enhanced by nano-TiO(2). We supposed that the direct electrochemistry of LDH may be due to the redox reaction of some electroactive amino acids in the LDH molecule. The surface morphologies of electrodes characterized by SEM indicated that LDH was successfully immobilized on the sol-gel matrix and also had some interactions with nano-TiO(2). This electrode can be used as a biosensor for the determination of lactic acid. The calibration range of lactic acid was from 1.0 to 20 micromolL(-1) and the detection limit was 0.4 micromolL(-1). Meanwhile, the small K(m)(app) value (2.2 micromolL(-1)) suggested that LDH possessed high enzymatic activity and good affinity to lactic acid owing to the promotion effect of nano-TiO(2).

By using Eriochrome Cyanine R modified anion-exchange resin as solid phase extraction cartridge coupled with ICP-AES, a novel method for the determination of Al fractionation in soil extracts has been developed. The proposed method possesses the following distinct advantages: (1) Good selectivity. The Eriochrome Cyanine R modified resin shows good recognizing ability for Al fractions. The total monomeric Ala can be adsorbed on the modified resin in the pH range of 6 ∼ 10 by column procedure, and the organic monomeric Alo can be determined by batch procedure rapidly while no adsorption of inorganic monomeric Ali was found in 30 min at least. Thus Ali can be obtained by subtraction between Ala and Alo. Major matrix's interference (Fe, Mn and organic substances) in soil extracts can be avoided. (2) Simple manipulation. The modified resin is very easy to prepare just simply by impregnating the resin with a solution containing Eriochrome Cyanine R. This method has been applied to the determination of ...

This paper reports a new solvent, room-temperature ionic liquid (RTIL), for the preparation of dodecanethiol self-assembled monolayers (C12SH-SAMs) on polycrystalline gold. The quality of C12SH-SAMs was characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). From CV experiments, we find that the differential capacitance Cd values of the C12SH-SAM prepared in RTIL, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF6) containing 10 μL neat C12SH for 24 h (C12SH-SAMs[BMIM]PF6,10μL,24h) are independent of the scan rate, the effective thickness deff value and the average cant angle φ value of this monolayer are 18 ± 1 Å and 27 ± 4°, respectively. The difference value of the current density at −0.2 and 0.5 V (Δip) is only 0.73 ± 0.18 μA cm−2. EIS experiments show that the phase angle value at 1 Hz Φ1Hz, the charge transfer resistance Rct value and surface coverage θ value of this C12SH-SAM are 88.2 ± 0.7°, 3.44 ± 1.91 MΩ cm2 and 99.998 ± 0.001%, respectively. These results indicate that high-quality C12SH-SAMs can be formed in [BMIM]PF6. In addition, the rate of formations of high-quality C12SH-SAMs in RTIL can be substantially improved by ultrasound. Keywords: Self-assembled monolayer, Dodecanethiol, Room-temperature ionic liquids, Cyclic voltammetry, Electrochemical impedance spectroscopy

This article reports a rapid method of preparing self-assembled monolayers of dodecanethiol (C12SH-SAMs) on polycrystalline gold by microwave irradiation (MWI, 650 W, duty cycle is 10%). The qualities of C12SH-SAMs were characterized by both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results show that the C12SH-SAMs formed by MWI in 120 s (C12SH-SAMsMWI,120 s) have low ionic permeability (the differential capacitance Cd values are independent of the scan rate and phase angle at 1 Hz Φ1 Hz = 89 ± 0.9°), excellent electrochemical blocking ability towards the redox probe (the current iMWI,120 s obtained from CV is lowest when compared to other SAMs and charge transfer resistance Rct = (1.15 ± 0.19) × 106 Ω cm2), and high surface coverage (99.996 ± 0.001%). Keywords: Rapid formation, Self-assembled monolayer, Dodecanethiol, Microwave irradiation, Cyclic voltammetry, Electrochemical impedance spectroscopy

Self-assembled monolayers of dodecanethiol (C 12 SH-SAMs) on polycrystalline gold were prepared under ultrasonic irradiation at 100 W (the actual ultrasonic power intensity is about 0.1 W cm −2 including the heat loss) for different time and investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CV experiments show that the differential capacitance C d values of the C 12 SH-SAM prepared under ultrasonic irradiation at 100 W (0.1 W cm −2 ) for 15 min are independent of the scan rate, the thickness d value of this monolayer is 17.5 A, the tilt angle φ value of the molecules in this monolayer from the gold surface normal was calculated to be 30° and the difference value of the current density at −0.2 and 0.5 V (Δ i p ) is only 0.69 μA cm −2 . From the EIS experiments, we find that the phase angle value at 1 Hz Φ 1 Hz of the C 12 SH-SAM prepared under ultrasonic irradiation at 100 W (0.1 W cm −2 ) for 15 min is 89°, the charge transfer resistance R ct value of this monolayer is 1.40 × 10 6 Ω cm 2 and the surface coverage θ value of this monolayer was calculated to be 99.997% from R ct . These results indicate that the C 12 SH-SAM of almost defect-free structure and very low ionic permeability can be formed under ultrasonic irradiation at 100 W (0.1 W cm −2 ) in a short time (15 min).

A novel method for recognition and indirect determination of Al III by using biological molecules has been established based on the quenching of RRS intensity. In the weak acidic medium, the reaction of ethyl violet (EV) and DNA would result in great enhancement of RRS intensity. However, the presence of Al III would lead to the decrease of the RRS intensity owing to the competition coordination of Al with DNA. The decreased intensity of RRS is directly proportional to the concentration of Al III in the range of (0.1–2.5) × 10 −6 and (0.30–4.5) × 10 −5 M, respectively. The method has high sensitivity and its detection limit (3 σ ) is 3.6 × 10 −8 M. The characteristics of RRS spectra of the system, the optimum conditions of the reaction, and the reaction mechanism have been investigated. The method can recognize Al III selectively owing to its strong binding to the phosphate backbone of DNA, and has been applied to the determination of Al III concentration in synthetic biological samples with satisfactory results. Therefore, the proposed method is promising as an effective means for selective recognition and sensitive determination in situ of Al III . Furthermore, this study would contribute to further understanding of the biological significance of Al neurotoxicity.

We present a novel method for the study of the aggregation of protein induced by metal ion aluminum(III) using resonance Rayleigh scattering (RRS) technique. In neutral Tris–HCl medium, the effect of this aggregation of protein results in the enhancement of RRS intensity and the relationship between the enhancement of the RRS signal and the Al concentration is nonlinear. On this basis, we established a new method for the determination of the critical induced-aggregation concentrations ( C CIAC ) of metal ion Al(III) inducing the protein aggregation. Our results show that many factors, such as, pH value, anions, salts, temperature and solvents have obvious effects. We also studied the extent of aggregation and structural changes using ultra-violet spectrometry, protein intrinsic fluorescence and circular dichroism to further understand the exact mechanisms of the aggregation characteristics of proteins induced by metal ion Al(III) at the molecular level, to help us to develop effective methods to investigate the toxicity of metal ion Al, and to provide theoretical and quantitative evidences for the development of appropriate treatments for neurodementia such as Parkinson's disease, Alzheimer's disease and dementia related to dialysis.

In this paper, we firstly report the direct voltammetric recognition and determination of dopamine (DA) by using Al(III)-DA complexes at the hanging mercury drop electrode (HMDE). A new sensitive cathodic peak of Al(III)-DA can be detected at -900 mV (vs. SCE) in 0.1 M NH(4)Cl-NH(3).H(2)O-0.1 M KCl buffer solution at pH 8.5. This unique -900 mV cathodic peak arises from the specific interaction between Al(III) and DA on the HMDE, whereas other substances with similar structures, such as L-dopa, epinephrine (EP), norepinephrine (NE), catechols, caffeic acid (CA), trihydric phenols and tiron, do not yield any new peak on the voltammograms in the potential range from -100 to -1200 mV when Al(III) is added. The distinct voltammetric characteristic of the recognition of DA can effectively inhibit the interferences of both ascorbic acid and uric acid in the DA determination by the direct electrochemistry, which is a major difficulty when a solid electrode is used. The proposed method can be anticipated as an effective means for the recognition of DA in the elucidation of the mechanisms of Parkinson's disease (PD) and Alzheimer's disease (AD) in the presence of Al(III).

Theoretical expressions and mathematical analysis in cyclic reciprocal derivative chronopotentiometry (CRDCP) are presented for totally irreversible electrode processes corresponding to the application of symmetrical and unsymmetrical programmed currents. For two successive unsymmetrical programmed currents, the effect of the currents ratio b ( b = | I 2 ( t )/ I 1 ( t )|) on the (d t /d E )– E curves is discussed. The electrochemical behavior of totally irreversible electrode processes has been studied corresponding to the application of the unique unsymmetrical programmed Φ m ( I 0 ) proposed recently. CRDCP characteristic parameters obtained for totally irreversible electrode processes are different from those of reversible electrode processes. Therefore, a comparison of CRDCP between both mechanisms is presented. Based on the mathematical derivation, alternative methods for kinetic measuring are described. It is prospected that CRDCP is convenient and applicable for studying the reversibility of the electrode processes in form of CRDCP characteristic parameters.

A novel and reliable high-performance liquid chromatographic method for trace short-chain organic acids measurement in wheat root exudates under aluminum stress has been optimized and validated. The chromatographic separation of the short-chain organic acids (citric, oxalic, malonic, succinic, tartaric, malic, and acetic acids) was achieved with Bio-rad Aminex HPX-87H cation exchange resin column. These seven organic acids were identified and quantified in 25 min. Well-shaped peaks were obtained for trace organic acids using dilute sulfuric acid as mobile phase. Under optimum conditions, Bio-rad Aminex HPX-87H column showed distinct advantages of the ability to well separate different short-chain organic acids (especially for tartaric and malic acids, as well as malonic and acetic acids) in wheat root exudates under aluminum stress, and offered accurate and precise results for the analysis of these organic acids. This HPLC method can efficiently eliminate the aluminum’s interference and is quite suitable to the trace detection of trace organic acids in wheat root exudates under aluminum stress.

The existence of dimethylselenium (DMSe) and dimethyldiselenium (DMDSe) in some environmental samples can cause serious interference on Se(IV) determination by hydride generation atomic fluorescence spectrometry (HG-AFS) due to their contribution on HG-response. A flow injection separation and preconcentration system coupled to HG-AFS was therefore developed by on-line coprecipitation in a knotted reactor (KR) for eliminating interference subjected from organoselenium. The sample, spiked with lanthanum nitrate, was merged with an ammonium buffer solution (pH 8.8), which promoted coprecipitation of Se(IV) and quantitative collection by 150cm PTFE KR. DMSe and DMDSe, however, were unretained and expelled from the KR. An air flow was introduced to remove the residual solution from the KR, then a 1.2moll(-1) HCl was pumped to dissolve the precipitates and merge with KBH(4) solution for HG-AFS detection. The interference of DMSe and DMDSe on the Se(IV) determination by conventional HG-AFS and its elimination by the developed separation and preconcentration system were evaluated. With optimal experimental conditions and with a sample consumption of 12.0ml, an enhancement factor of 18 was obtained at a sample frequency of 24h(-1). The limit of detection was 0.014mugl(-1) and the precision (R.S.D.) for 11 replicate measurements of 1.0mugl(-1) Se(IV) was 2.5%. The developed method was successfully applied to the determination of inorganic selenium species in a variety of natural water samples.

Phytotoxicity of aluminum (Al) has become a serious problem in inhibiting plant growth on acid soils. Under Al stress, the changes of rhizosphere pH, root elongation, absorption of Al by wheat roots, organic acids exuded from roots, and some main factors related to Al-tolerant mechanisms have been studied using hydroponics, fluorescence spectrophotometry, and high performance liquid chromatography (HPLC). Two wheat cultivars, Brevor and Atlas66, differing in Al tolerance are chosen in the study. Accordingly, the rhizosphere pH has a positive effect on Al tolerance. Atlas66 (Al-tolerant) has higher capability to maintain high rhizosphere pH than Brevor (Al-sensitive) does. High pH can reduce Al3+ activity and toxicity, and increase the efficiency of exuding organic acids from the roots. More inhibition of root elongation has been found in Brevor because of the exposure of roots to Al3+ solution at low pH. Brevor accumulate more Al in roots than Atlas66 even at higher pH. Al-induced exudation of malic and citric acids has been found in Atlas66 roots, while no Al-induced organic acids have been found in Brevor. These results indicate that the Al-induced secretion of organic acids from Atlas66 roots has a positive correlation with Al tolerance. Comprehensive treatment of Al3+ and H+ indicates that wheat is adversely influenced by excess Al3+, rather than low pH.

The direct voltammetry of catalase (CAT) immobilized in silica sol-gel film in the presence of cysteine on gold electrode was investigated. The CAT electrode showed a pair of well-defined and quasi-reversible cyclic voltammetry peaks. It can be used as an electrochemical biosensor for the determination of hydrogen peroxide. The calibration range of H(2)O(2) was from 1 to 30 micromolL(-1) and the detection limit was 0.4 micromolL(-1) at a signal-to-noise ratio of 3. The interaction of CAT and aluminum ion was also investigated based on the CAT-modified electrode. The electrochemical activity of the CAT-modified electrode was increased with the addition of Al(3+). The experimental results of voltammetry and fluorescence spectroscopy indicated that the conformation of CAT molecule was altered by the formation of Al-CAT complex with Al(3+), which may influence the activity of CAT.

A novel non-chromatographic approach for direct speciation of mercury, based on the selective retention inorganic mercury and methylmercury on the inner wall of a knotted reactor by using ammonium diethyl dithiophosphate and dithizone as complexing agents respectively, was developed for flow injection on-line sorption preconcentration coupled with chemical vapor generation non-dispersive atomic fluorescence spectrometry. With the sample pH kept at 2.0, the preconcentration of inorganic mercury on the inner walls of the knotted reactor was carried out based on the exclusive retention of Hg–DDP complex in the presence of methylmercury via on-line merging the sample solution with ammonium diethyl dithiophosphate solution, and selective preconcentration methylmercury was achieved with dithizone instead of ammonium diethyl dithiophosphate. A 15% (v/v) HCl was introduced to elute the retained mercury species and merge with KBH 4 solution for atomic fluorescence spectrometry detection. Under the optimal experimental conditions, the sample throughputs of inorganic mercury and methylmercury were 30 and 20 h − 1 with the enhancement factors of 13 and 24. The detection limits were found to be 3.6 ng l − 1 for Hg 2+ and 2.0 ng l − 1 for CH 3 Hg + . The precisions (RSD) for the 11 replicate measurements of each 0.2 μg l − 1 of Hg 2+ and CH 3 Hg + were 2.2% and 2.8%, respectively. The developed method was validated by the analysis of certified reference materials (simulated natural water, rice flour and pork) and by recovery measurements on spiked samples, and was applied to the determination of inorganic mercury and methylmercury in biological and environmental water samples.

In this paper, the theoretical study of cyclic reciprocal derivative chronopotentiometry (CRDCP) for totally reversible electrode processes with symmetrical and unsymmetrical programmed currents is presented. The main viewpoints are: (1) for symmetrical programmed current, the amplitudes of the successive currents are the same, i.e. I ( t ) = (−1) i +1 I 0 , whereas the transition times of each current step, τ i , have different values. The properties of the d t /d E – E curves corresponding to each current cycle also differ a lot. (2) For unsymmetrical programmed current proposed in this work for the first time, the applied current successively reversed at each transition time steps to different amplitude. The use of this unique programmed current is advantageous versus symmetrical programmed current since the transition times obtained are equal to each other anticipatively, and the d t /d E – E curves also coincide with each other for the successive cycles. In this case, the results obtained in CRDCP are almost similar to those of cyclic voltammetry (CV). The characteristic parameters obtained in the d t /d E – E curves in both cases are quantitatively related to the species concentrations adjacent to the electrode surface and afford simple diagnosis criteria to characterize the reversibility of electrode processes. Properties of reversible electrode processes have also been further studied through the use of more than one current cycle.

The biological effects of aluminum (Al) have received much attention in recent years. Al is of basic relevance as concern with its reactivity and bioavailability. In this paper, the electrochemical behaviors of norepinephrine (NE) in the absence and presence of Al III at the hanging mercury drop electrode have been studied and applied to the practical analysis. Highly selective catalytic cathodic peak of NE is yielded by linear scan voltammetry (LSV) at −1.32 V (vs. SCE). A linear relationship holds between the cathodic peak current and the Al III concentration. It has been successfully applied to the determination of Al III in real waters and synthetic biological samples with satisfying results, which are in accordance with those obtained by ICP-AES method. The electrochemical properties and the mechanisms of the peaks in the presence and absence of Al III have been explored. The results show that they are irreversible adsorptive hydrogen catalytic waves. These studies not only enrich the methods of determining Al, but also lay foundations of further understanding of the mechanisms of neurodementia.

Since the study of Al3+ ion on the enzyme activity by using of electrochemical techniques was rarely found in available literatures, the differential-pulse polarography (DPP) technique was applied to study the effects of Al3+ ion on the glutamate dehydrogenase (GDH) activity in the catalytical reaction of α-KG +NADH+NH4 + ⇔ L-Glu+NAD++H2O by monitoring the DPP reduction current of NAD+. At the plant and animal physiologically relevant pH values (pH=6.5 and 7.5), the GDH enzyme activities were strongly depended on the concentrations of the metal ion in the assay mixture solutions. In the lower Al (III) concentration solutions (80μM), the inhibition effects of Al (III) were shown again. The cyclic voltammetry of NAD+ and NAD+-GDH in the presence of Al (III) can help to explain some biological phenomena. According to the differential-pulse polarography and cyclic voltammetry experiments, the present research confirmed that the electrochemical technique is a convenient and reliable sensor for accurate determination of enzyme activity in biological and environmental samples.

In weakly acidic buffer medium, the interaction of amikacin with calf thymus DNA, yeast RNA and denatured DNA has been investigated by using resonance Rayleigh scattering (RRS) technique. The result shows that calf thymus DNA is capable of enhancing the RRS intensity of the amikacin, while yeast RNA and denatured DNA have very little enhancement effect. Based on the characteristics, a sensitive assay for detecting double-stranded DNA in the presence of denatured DNA and yeast RNA has been developed. The enhancement of the RRS signal is directly proportional to the concentration of double-stranded DNA in the range 0.02–12.0 μg ml −1 for calf thymus DNA and its detection limit (3 σ ) is 2.5 ng ml −1 . The method shows a wide linear range and high sensitivity, and almost no interference can be observed from RNA, denatured DNA, amino acid and most of the metal ions. The trace amounts of nucleic acid in synthetic samples and practical samples are determined with satisfactory results. Therefore, the proposed method is promising for as an effect means for recognition in vivo and determination in situ of double-stranded DNA.