Sérgio Alves

Mercury determination by FI-CV-AAS after the degradation of organomercurials with the aid of an ultrasonic field : The important role of the hypochlorite ion

Due to new findings, the methodology based on room-temperature ultrasonic irradiation (sonolysis) for conversion of organomercurials into inorganic mercury [J.L. Capelo, I. Lavilla, C. Bendicho, Anal. Chem. 72 (2000) 4979-4984.] is further investigated. Inorganic mercury is selectively determined by Flow Injection-Cold Vapour Atomic Absorption Spectrometry (FI-CV-AAS) using SnCl(2)/HCl. Complete oxidation of methyl-mercury can be accomplished within 90 s whilst phenyl and diphenyl-mercury can be degraded within 10s using a 50% sonication amplitude (100 W nominal power) provided by a probe ultrasonic device (20.5 kHz frequency) and a 1 mol L(-1) HCl liquid medium with the presence of hypochlorite ion. The importance of hypochlorite in reduction of organomercurials by stannous chloride is highlighted. Oxidation kinetics indicated a pseudo first-order reaction for methyl-mercury, phenyl-mercury, and diphenyl-mercury.

Particles decorated by an ionizable thermoresponsive polymer brush in water: experiments and self-consistent field modeling.

We have synthesized anionic multistimuli responsive core-shell polymer nanoparticles with low size dispersity composed of glassy poly(methyl methacrylate) (PMMA) cores of ca. 40 nm radius and poly(N-isopropylacrylamide) (PNIPAM) anionic brush-like shells with methacrylic acid comonomers. Using dynamic light scattering, we observed a volume phase transition upon an increase in temperature and this response was pH and ionic strength dependent. Already at room temperature we observed a pronounced polyelectrolyte effect, that is, a shift of the apparent pKa extracted from the degree of dissociation of the acids as a function of the pH. The multiresponsive behavior of the hydrophobic polyelectrolyte brush has been modeled using the Scheutjens-Fleer self-consistent field (SF-SCF) approach. Using a phenomenological relation between the Flory-Huggins χ parameter and the temperature, we confront the predicted change in the brush height with the observed change of the hydrodynamic radius and degree of dissociation and obtain estimates for the average chain lengths (number of Kuhn segments) of the corona chains, the grafting density and charge density distributions. The theory reveals a rich internal structure of the hydrophobic polyelectrolyte brush, especially near the collapse transition, where we find a microphase segregated structure. Considering this complexity, it is fair to state that the theoretical predictions follow the experimental data semiquantitatively, and it is attractive to attribute the observed disparity between theory and experiments to the unknown polydispersity of the chains, the unknown distribution of the charges, or other experimental complications. More likely, however, the deviations point to significant problems of the mean field theory, which focuses solely on the radial distributions and ignores the possibility of the formation of lateral (local) inhomogeneities in partially collapsed polyelectrolyte brushes. We argue that the PNIPAM brush at room temperature is already behaving nonideally.

A new optical boron detection method

The detection of boron, either as boric acid or phenylboronic acid, in natural or residual water, still faces a number of challenges. Here, we propose the use of 2,3,6,7,10,11-hexahydroxytriphenylene as an optical sensor for boron, using spectrofluorimetry or UV-visible spectrophotometry detection. We evaluate this sensor for the quantification of both boric acid and phenylboronic acid in aqueous solution. The limit of detection of the sensor using spectrofluorimetry is 10 ppb of boron for boric acid and 6 ppb of boron for phenylboronic acid, with the interference of metal cations efficiently eliminated by using a chelating agent such as EDTA. The proposed method involves a very simple experimental procedure and is easily amenable for use in field work.

Temperature-responsive fibres of cellulose-based copolymers

Well-defined temperature-responsive cellulose acetate (CA) grafted with a temperature-responsive polymer (TRP) was synthesized using a grafting-to approach. The TRP obtained by reversible addition–fragmentation chain transfer polymerization (RAFT), with Mn = 17 700 and dispersity Đ = 1.10, is a random copolymer of di(ethylene glycol) methyl ether methacrylate (MEO2MA) and oligo(ethylene glycol) methyl ether methacrylate (OEGMA), containing terminal azide groups introduced by addition of an azide-modified methacrylate monomer in the final stage of the polymerization. For the comonomer ratio used, the volume phase transition temperature of the TRP is TVPT = 33 °C. After modification of CA with propargyl groups using the remaining hydroxyl groups of the glucopyranose rings, the TRP was grafted to the CA by an azide-propargyl click reaction. The CA-g-TRP copolymer was then used to prepare fibers by electrospinning, for application as temperature-responsive scaffolds in cell culture. The fibers were found to change their average diameter from 2.0 ± 0.4 μm to 1.3 ± 0.3 μm upon heating in water, above the TVPT, and subsequently drying. When cooled in water below the TVPT, the average diameter increases to 1.8 ± 0.3 μm. Experiments in thin films of the same copolymer show that the change in temperature does not translate into a significative change in the hydrophobic character, possibly due to a conformation rearrangement of the copolymer chains.

Optical sensing of aqueous boron based on polymeric hydroxytriphenylene derivatives

The detection of boron in natural water and wastewaters is still limited to a few methods, requiring a compromise between sensitivity, reliability and accessibility. Here we present a novel polymeric fluorescent boron sensor with excellent water solubility, boron sensitivity, ease of handling, which can be easily recovered and reused. The new off–on fluorescent boron sensor is able to detect and quantify ppb amounts of boron in water, with a limit of detection of 10 ppb of boron, both for boric acid and phenylboronic acid. The sensor was prepared by copolymerizing methacrylic acid with a polymerizable asymmetric hexa-substituted triphenylene, obtained by oxidative cyclization of biphenyl and catechol precursors.