Mechanism and Kinetics of Palladium Nanoparticles Electrochemical Formation onto Glassy Carbon, from a Deep Eutectic Solvent (Reline).

From electrochemical potentiodynamic and potentiostatic techniques, the electrodeposition mechanism and kinetics of palladium nanoparticles (PdNPs) onto a glassy carbon electrode (GCE), from Pd(II) ions dissolved in the choline chloride-urea deep eutectic solvent (reline) at 343 K, are reported for the first time. From the analysis of the potentiostatic current density transients, using the model developed by Palomar-Pardavé et al. [ Electrochim. Acta, 2005, 50, 4736-4745], it shows that the PdNPs electrodeposition occurs by multiple 3D nucleation and diffusion controlled-growth with the simultaneous reduction of residual water on the PdNPs growing surfaces. This model renders not just the quantification of the palladium nucleation kinetics parameters, but it effectively allows deconvolving the individual contributions to the total current and, thus, from the integration of the j-t plots of these contributions. It was demonstrated that the charge amount of each process depends on the deposition time and applied overpotential. From SEM images, it was possible to verify that the palladium deposits were constituted by PdNPs and from XPS measurements that these PdNPs were formed by a metallic palladium (core) and Pd(OH)2 (shell).

Spectro-electrochemical characterization and quantification of Rutin in aqueous media.

From UV-Vis spectrophotometric measurements the acidity constants of Rutin in aqueous media, at 25 °C and 0.1 M ionic strength, were determined as: pK1 = 4.392 ±â€¯0.167, pK2 = 7.130 ±â€¯0.050, pK3 = 8.661 ±â€¯0.042 and pK4 = 12.354 ±â€¯0.020 and the molar absorptivity coefficients of all the Rutin pH-dependent species were reported as a function of wavelength. Furthermore, the electrochemical behavior of Rutin at neutral pH was investigated using a bare carbon paste electrode, CPE. It was found that both: Rutin electrochemical oxidation and reduction are reversible, adsorption-controlled processes, involving 2 electron transfers. Moreover, the bare CPE was used for the electrochemical quantification of Rutin in neutral aqueous media, displaying the following features: (1.078 ±â€¯0.440) µM, (3.594 ±â€¯0.400) µM and (0.308 ±â€¯0.014) µA µM-1 for the detection and quantification limits and sensitivity, respectively, within the 1-11 µM linear range. Meanwhile the spectrophotometric method displayed the following analytical features: (3.385 ±â€¯1.318) µM, (11.283 ±â€¯3.114) µM and (0.0120 ±â€¯0.0001) AU µM-1 for the detection and quantification limits and sensitivity, respectively within the 11-110 µM linear range. In like manner, the bare CPE is also shown as a robust electrochemical sensor that allows Rutin quantification even in the presence of ascorbic acid, commonly found in Rutin samples.

Effect of platform subway depth on the presence of Airborne PM2.5, metals, and toxic organic species.

PM2.5 that have been related to public health risks, were collected during two seasons with High-Vol samplers in platforms of a Mexican subway station, which interconnects through transfers three lines having different depths. The objective was to study the influence of depth on the PM2.5 concentrations and their species. PM2.5 concentrations in cold-dry and warm-dry seasons presented statistical differences, being in average 57 and 66 µgm-3 respectively, in the shallower line 9; 90 µgm-3 and 111 µgm-3 in line 1; and 104 and 122 µgm-3in the deepest line 7. During the cold-dry season and warm-dry season PM2.5concentrations in the subway environment were respectively up to 3.5 times and up to 5 times greater than in the ambient air. Like PM2.5, metals analyzed with an OES-ICP presented higher concentrations in deeper lines as well as PAHs quantified with CG-MS, which ranged from 4.5 to 11.7 ngm-3. High PM2.5, metals and organic toxic concentrations found in deeper lines of the subway environment represent a risk for commuters endorsing the need for ventilation systems to reduce them. Zn, Pb, V and Ni in subway particles presented the highest solubility in artificial lysosomal fluid suggesting high bioavailability in the lung fluids.

Quercetin spectrofluorometric quantification in aqueous media using different surfactants as fluorescence promoters.

Quercetin spectrofluorometric quantification was carried out in aqueous media (pH 7) using micelles of surfactants, namely: CTAB, SDS and TX100 as promoters of quercetin fluorescence, having a critical micelle concentration, CMC, of: 0.94 ± 0.03, 7.7 ± 0.6 and 0.18 ± 0.3 mM, respectively, measured through UV-Vis spectrophotometry. The thermodynamic binding constant, K, for the quercetin-surfactants' micelles supramolecular complex was estimated to be (log(K/M-1)) = 2.87 ± 0.02, 2.78 ± 0.04 and 2.80 ± 0.04, respectively, from fluorescence spectrophotometry. With the aid of these parameters it was possible to construct species distribution diagrams and choose the experimental conditions where quercetin can be quantified in aqueous media from fluorescence measurements. The best lowest limit of detection (0.244 ± 0.092) µM was achieved with CTAB micelles while the best sensitivity (2.919 ± 0.054) M-1 corresponded to SDS.

New insights on the spectrophotometric determination of melatonin pKa values and melatonin-ßCD inclusion complex formation constant.

Using UV-Vis spectrophotometry a stability study of melatonin at different pH values was done in aqueous media, finding that at acidic pH melatonin is unstable when interacting with the environment, however it becomes stable protecting it from light and oxygen. From the UV-Vis spectra and SQUAD software, melatonin pKa values, in a completely protected aqueous medium, were estimated as 5.777±0.011 and 10.201±0.024. Using the same techniques, the melatonin and ß-cyclodextrin inclusion complex formation constants were assessed at pH3, 7 and 11.5, giving the values of log ß=(3.07±0.06), (2.94±0.01) and (3.07±0.06) M-1, respectively. From the global acidity formation constants and the complexes' formation constants, the molar fractions were determined for each species of MT and MT-ßCD, to build the molar fraction-[ßCD]-pH 3D diagram and the molar fraction-pH 2D diagrams, where it was possible to observe the predominance of the MT species with and without ßCD. A voltammetric study at pH3, allowed obtaining a value of log ß=(3.15±0.01) M-1, which corroborates that obtained through UV-Vis spectrophotometry, supporting strongly the rationale behind using simple, straightforward techniques.

Spectrophotometric quantification of the thermodynamic constants of the complexes formed by dopamine and Cu(II) in aqueous media.

The thermodynamic constants of the complex Cu(II)-dopamine in aqueous solution were evaluated from spectrophotometric data using the software SQUAD. It was found that there exist Cu(II):DA complexes with 1:1 and 1:2 stoichiometries and that their predominance depends on both the solution pH and the [Cu(II)]/[DA] ratio. Moreover, it is shown that the solubility of Cu(OH)2(s) increases drastically when these complexes are thermodynamically stable.

Enzyme entrapment by ß-cyclodextrin electropolymerization onto a carbon nanotubes-modified screen-printed electrode.

A novel enzyme entrapment approach based on an electropolymerization process utilizing multi-walled carbon nanotubes (MWCNT), ß-cyclodextrin (ß-CD) and glucose oxidase (GOx) is shown. Dopamine (DA) quantification is presented using a screen-printed electrode modified by electropolymerization of cyclodextrin with glucose oxidase, SPE/MWCNT/ß-CD-GOx. In order to show the relevance of the enzyme entrapment strategy controlled by electropolymerization to develop a specific and efficient biosensor, the various parts composing the electrode: SPE, SPE/ß-CD, SPE/GOx, SPE/ß-CD/GOx, SPE/MWCNT/ß-CD, SPE/MWCNT/GOx and SPE/MWCNT/ß-CD/GOx were tested separately. It was shown that although DA determination can be achieved with all of them, the electrodes modified with MWCNT presented better analytical features that those built without MWCNT, the best being the one including all components. This biosensor displayed good reproducibility, repeatability, and prolonged life-time under cold storage conditions. Its DA limit of detection (LOD) was 0.48±0.02 µA in a linear range of 10-50 µM with a sensitivity of 0.0302±0.0003 µA µM(-1) that makes it comparable or even better than many other electrodes reported in the literature. Moreover, it was also shown that using this electrode, DA quantification can be done in the presence of interfering agents such as ascorbic and uric acid. These findings demonstrate that the approach employed is feasible for enzyme entrapment and may find applications in other biosensing systems, where better sensitivity, stability and fast response are required.

Nucleation and growth kinetics of electrodeposited sulfate-doped polypyrrole: determination of the diffusion coefficient of SO(4)(2-) in the polymeric membrane.

A kinetic study for the electrosynthesis of polypyrrole (Ppy) doped with SO(4)(2-) ions is presented. Ppy films were electrochemically polymerized onto a graphite-epoxy resin electrode. Experimental current density transients (j-t) were obtained for three different potentiometric behaviors: anionic, cationic, and a combination. Theoretical models were used to fit the experimental j-t data to determine the nucleation and growth processes controlling the polymer synthesis. It was encountered that, in all cases, pyrrole electropolimerization involves two concomitant processes, namely, a Ppy diffusion limited multiple 3D nucleation and growth and pyrrole electro-oxidation on the growing surface of the Ppy nuclei. SEM analysis of the electrodes surfaces reveals that Ppy deposition occurred over most of the electrode surface by multiple nucleation of hemispheres, as the theoretical model used for the analysis of the current transients required. Hemispherical particles formed the polymeric film displaying different sizes. The order for the particle size was as follows: anionic > anionic-cationic > cationic. These results are congruent with those obtained by theoretical analysis of the corresponding current transients. Analysis of the impedance measurements recorded on the anionic Ppy film, immersed in an aqueous solution with different sulfate ion concentrations evidenced that SO(4)(2-) ions diffuse through the Ppy film provoking a decrease of its electrical resistance and an increase of its dielectric constant. From the Warburg impedance coefficient, the sulfate coefficient of diffusion in the Ppy film was 1.38 x 10(-9) cm(2) s(-1).

Study on the stability of noradrenaline and on the determination of its acidity constants.

Noradrenaline is a catecholamine which has been largely recognised to play a very important role in biological systems. In view of the neurotransmitter's alleged importance, this work aimed at showing the influence of time on its spectral behaviour using different analytical methods and determining its acidity constants through spectrophotometric titration and by the so-called point-by-point analysis, where the samples are freshly prepared for each pH value investigated at the instant required. Because the catecholamines are light-sensitive and likely to react with the oxygen in the surrounding air, both methods used preclude its incidence onto the samples being analysed under the presence of a nitrogen atmosphere maintained over the solutions. The constants obtained through point-by-point analysis were log beta1 = 30.71+/-0.16, log beta2 = 22.00+/-0.15 and log beta3 = 11.69+/-0.16.

Cd(II) and Pb(II) separation from aqueous solution using clinoptilolite and Opuntia ectodermis.

This work presents the conditions for Pb(II) and Cd(II) removal from aqueous solution using two different sorbent materials, namely, clinoptilolite and Opuntia ectodermis, which were characterized before and after entering into contact with the metal-containing aqueous media, using scanning electron microscopy (SEM), thermal analysis (TGA) and surface area (BET). Metal removal was found to depend on the initial metal concentration in aqueous solution, pH and the mass/volume ratio. The Pb(II) and Cd(II) uptake process was maximum at pH 4 for both sorbents, which showed an adsorption capacity that was adequately described by the Freundlich adsorption isotherm. The Thomas model was used to describe the adsorption data from column studies, the sorption capacity was 12.21 Pb(II) mg per gram of Opuntia ectodermis, while 7.71 Pb(II) mg per gram of clinoptilolite were adsorbed; lower values were obtained for Cd(II) removal. It was demonstrated that the use of low cost materials for the treatment of Pb(II), and Cd(II), containing wastewater is an effective and economical alternative method.

Study on the stability of the serotonin and on the determination of its acidity constants.

The present work aimed at describing the spectral behaviour of the serotonin and to evaluate its acidity constants using three different methods, using two spectrophotometry titrations and a third method that involved point-by-point analysis, which permitted to monitor closely and determine the evolution of the serotonin species in solution as a function of time. The three methods allowed estimation of three acidity constants associated to the same number of functional groups that form part of the molecule. The results given by the point-by-point analysis were: log(beta1) = 24.95 +/- 0.12; log(beta2) = 20.20 +/- 0.10; log(beta3) = 10.89 +/- 0.018.

Study on the stability of adrenaline and on the determination of its acidity constants.

In this work, the results are presented concerning the influence of time on the spectral behaviour of adrenaline (C(9)H(13)NO(3)) (AD) and of the determination of its acidity constants by means of spectrophotometry titrations and point-by-point analysis, using for the latter freshly prepared samples for each analysis at every single pH. As the catecholamines are sensitive to light, all samples were protected against it during the course of the experiments. Each method rendered four acidity constants corresponding each to the four acid protons belonging to the functional groups present in the molecule; for the point-by-point analysis the values found were: log beta(1) = 38.25 +/- 0.21, log beta(2) = 29.65 +/- 0.17, log beta (3) = 21.01 +/- 0.14, log beta(4) = 11.34 +/- 0.071.

Evaluation of the acidity constants of the 4-hidroxy-5-[salicylideneamino]-2-7-naphthalenedisulfonic acid (Azomethine-H) using UV-vis spectrophotometry.

The time stability of the azomethine-H species was determined not to be better than 10 min in the absence of oxygen and light, however under phosphate buffered conditions the azomethine-H species remained stable for longer periods, as indicated by the spectrophotometric behaviour. Nevertheless, the analysis time still exceeded the stability allowance. Therefore, the determination of the acidity constants of the Azomethine-H species was studied by means of UV-vis spectrophotometry in buffered media by means of the point-by-point analysis and data processing with SQUAD to refine the resulting constants, which were: pK(a1) = 3.39, pK(a2) 7.36 and pK(a3) 8.73. The latter were associated to the corresponding acid-base equilibria of the amine and hydroxy groups constituting the molecule.

Facilitated transport of Hg(II) through novel activated composite membranes.

The results presented in this work deal with the prime application of activated composite membranes (ACMs) for the transport of Hg(II) ions in a continuous extraction-re-extraction system using di-(2-ethylhexyl)dithiophosphoric acid (DTPA) as carrier. The effects of variables such as the pH, the nature of the acid and the concentration of the casting solutions on the transport of Hg(II) are also investigated. When the ACM was prepared with a 0.5 M DTPA solution and when the feed solution contained 2.5x10(-4) M Hg(II) in 0.1 M HCl, the amount of mercury extracted was greater than 76%. The re-extracted mercury was subsequently recovered by means of a stripping phase comprising 0.3 M thiourea solution in 2 M H2SO4, yielding 54% of the initial amount of mercury after transport had taken place for 180 min.

Cr(VI) removal from wastewater using low cost sorbent materials: roots of Typha latifolia and ashes.

This work presents conditions for hexavalent chromium (Cr(VI)) removal from aqueous solution using different sorbent materials, namely: pyrolytic ashes of an industrial sludge from wastewater treatment and roots of Typha latifolia. The sorbent materials were characterized using scanning electron microscopy (SEM) and surface area using the Brunauer-Emmett-Teller (BET) technique, before and after the contact with the chromium-containing aqueous media. An overall Cr(VI) concentration reduction of 45% was achieved using the roots of Typha latifolia whereas in the case of pyrolytic ashes a 60% removal was observed. The percentage removal was found to depend on the initial Cr(VI) concentration in aqueous solution, pH and temperature. The Cr(VI) uptake process was maximum at pH 2 and a temperature of 40 degrees C for both sorbents. These materials showed a Cr(VI) adsorption capacity that was adequately described by the Langmuir adsorption isotherm. It was demonstrated that the use of waste materials for the treatment of Cr(VI)-containing wastewater is an effective and economical alternative method.

Spectrophotometric study on the stability of dopamine and the determination of its acidity constants.

The interest in determining the acidity constants of the catecholamines stems from the fact that they play rather an important biological role. The present work reveals the effect of different parameters such as oxygen, light, analysis time and pH on the dopamine oxidation process, where oxygen has an effect on the dopamine oxidation of 40% and up to 20% is attributed to exposure to light as a function of the pH. The application of adequate control on the said parameters (which ensured stability of the dopamine) facilitated the determination of the corresponding three acidity constants, 9.046+/-0.147, 10.579+/-0.148 and 12.071+/-0.069.

Determination of lead and cadmium using a polycyclodextrin-modified carbon paste electrode with anodic stripping voltammetry.

This work reports the use of alpha and beta-cyclodextrin-modified carbon paste electrodes (CPE(alpha-CD) and CPE(beta-CD)) to determine simultaneously Pb(II) and Cd(II) by means of the electrochemical technique known as anodic stripping voltammetry (ASV). Both modified electrodes displayed good resolution of the oxidation peaks of the said metals. Statistic analysis of the results strongly suggests that the CPE(beta-CD )exhibited a better analytical response that the CPE(alpha-CD), while the detection limits obtained for Pb(II) were 6.3 x 10(-7) M for the CPE(alpha-CD) and 7.14 x 10(-7) M for the CPE(beta-CD), whereas for Cd(II) they were 2.51 x 10(-6) M for the CPE(alpha-CD) and 2.03 x 10(-6) M for the CPE(beta-CD).