Carla C. S. Cavalheiro

Interações entre corantes e argilas em suspensão aquosa

Adsorption of cationic dyes on clays can be used as a model for the interactions between organic compounds and these minerals. Cationic dyes like methylene blue are used to study these interactions because of the spectroscopic changes observed when their molecules are adsorbed on clay surfaces. Depending on the structure and layer charge of the clay particles several processes may occur, like adsorption of dye monomers and aggregates on the external and internal surfaces of the clay tactoids, migration towards internal surfaces, protonation, etc. Under certain conditions the deaggregation-aggregation of the clay particles are accelerated trapping dye species during these processes. A general scheme is proposed for the processes occurring between clays and dyes in aqueous suspensions, which can be used to explain the behaviour of specific systems.

The UV/H2O2 - photodegradation of poly(ethyleneglycol) and model compounds

O mecanismo geral para a fotodegradacao do polietilenoglicol (PEG) usando o sistema H2O2/UV foi determinado por GPC e HPLC, analisando o comportamento de moleculas modelo, como por exemplo os etilenoglicois de baixa massa molar (tetra-, tri-, di-, e etilenoglicol). Apos 30 min de irradiacao, a massa molar ponderal media (Mw) do PEG degradado, analisada por GPC, cai para metade do seu valor inicial com um aumento concomitante na polidispersidade e numero medio de quebra de cadeia (S). Essa queda acentuada caracteriza uma quebra de cadeia aleatoria, formando oligomeros e etilenoglicois de menor massa molar. A analise da fotodegradacao de etilenoglicois modelo por HPLC permitiu sugerir um mecanismo que envolve processos consecutivos, em que etilenoglicois maiores originam menores, sucessivamente. A fotodegradacao do etilenoglicol formou acidos carboxilicos de baixa massa molar, como por exemplo os acidos glicolico, oxalico e formico.

Determinação espectrofotométrica de vitamina B2 (riboflavina) em formulações farmacêuticas empregando sistema de análises por injeção em fluxo

A flow injection spectrophotometric procedure exploiting merging zones is proposed for determining vitamin B2 (riboflavin) in pharmaceutical preparations. The determination is based on the red-colored complex formation between vitamin B2 and silver(I) which was measured at 520 nm. Vitamin B2 was determined in four pharmaceutical preparations in the 1.0 to 50.0 mg L-1 concentration range, with a detection limit of 0.5 mg L-1. The recovery from three samples ranged from 98.0 to 104.0 %. The analytical frequency was 42 h-1 and r.s.d. were lower than 1% for solutions containing 10.0, 30.0 and 50.0 mg L-1 vitamin B2 (n= 10). The results obtained in commercial formulations using the FIA procedure were in good agreement with those obtained by using a conventional fluorimetric procedure (r=0.9998) and also with the label values (r= 0.9997).

Efeitos da história térmica nas propriedades do polímero pet: um experimento para ensino de análise térmica

Polyethilenetherephtalate (PET) is commonly used in manufacturing of beverage bottles representing a didactic appeal in thermal analysis education due to its presence in the students day life. Additionally such polymer presents well defined thermal analytical curves and a well known thermal behavior. TGA curve is used to present the thermal stability. The thermal history effects in the thermal properties of a PET sample from a soft drink bottle are used to demonstrate the effect of different heating/cooling conditions on glass transition, melting, crystallization and crystalline degree using DSC curves.

The use of magnesium silicate (talc) in a potentiometric sensor for hydrogen ions

The construction and analytical evaluation of a pH sensor based in a matrix containing 20% of [(Mg6Si8O20)](OH)4, a magnesium silicate, talc and 30% graphite dispersed in an epoxy resin (50%) is described. The data obtained from various acid–base titrations were compared with those using a glass electrode in the same conditions. The proposed electrode presented a linear response in the 1–12 pH range with a slope of � 39.9F0.3 mV/pH (at 25 jC) and ca. 15 s of response time. The lifetime of this electrode was higher than 8 months (ca. 1500 determinations) and it showed a good performance for pH determination and end-point indication in the potentiometric titrations of acids and bases.

Optical devices for the detection of cyanide in water based on ethyl(hydroxyethyl)cellulose functionalized with perichromic dyes

Films of three polymers, based on ethyl(hydroxyethyl)cellulose functionalized with protonated perichromic dyes, were used for anion sensing. The polymer functionalized with protonated Brookers merocyanine acts as a chromogenic/fluorogenic system for the selective detection of cyanide in water. An increase of >28 times was verified for the fluorescence lifetime of the sensing units in the polymer in comparison with protonated Brookers merocyanine in water. Moreover, an increase in the pKa values was verified for the sensing units in the polymers. Data suggest that the hydrocarbonic polymeric chains provide an adequate microenvironment to protect the sensing unit from bulk water. The other polymer, functionalized with an iminophenol, also showed high selectivity for cyanide (detection limit=9.36×10-6molL-1 and quantification limit=3.12×10-5molL-1). The polymer functionalized with azophenol units is unable for the detection of cyanide, due to the low pKa value verified for its chromogenic units.

Curing, monomer leaching and water sorption of TEGDMA/BisGMA photopolymerized copolymers

The aim of this work was to correlate the degree of conversion with residual monomer release and water absorption of 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy) phenyl]propane (BisGMA) and triethyleneglycol dimethacrylate (TEGDMA) (TB copolymers) obtained by photopolymerization, using canforquinone/ethyl 4-dimethylaminobenzoate (CQ/EDB) photoinitiator. The monomer release from the copolymers was evaluated by immersion in ethanol/water and HPLC (high performance liquid chromatography) analysis. Water absorption from artificial saliva was determined by mass variation. The results show that the composition of the TB copolymers influences the polymerization kinetics, the amount of the released monomers, the degree of conversion and the amount of adsorbed water. A clear correlation can be found between these properties.

Photochemical Synthesis of Gold Nanoparticles by Irradiation of Gold Chloride with the 2nd Harmonic of a Nd:YAG Laser

A synthesis method was developed capable of obtaining pristine gold nanoparticles (AuNPs) using the lowest possible amount of auxiliary compounds. An aqueous solution of HAuCl4.3H2O was irradiated, in the absence of any stabilizing species, with the 532-nm radiation of a Nd:YAG laser. Spherical shaped AuNPs were produce by this method with sizes below 10 nm. The crystalline structures were stable for up to 28 days. A mechanism for this process is also proposed based on the initial cleavage of water molecules by the laser irradiation followed by the formation of hydrogen peroxide that reduces the gold species Au. The resulting species of these reactions (H2 and O2) were identified by mass spectrometry. This new method suggests that laser irradiation is a feasible alternative to produce ready-to-use AuNPs, for drug delivery, diagnostics or bio-imaging.

3D printing of natural organic materials by photochemistry

In previous works, we have used two-photon induced photochemistry to fabricate 3D microstructures based on proteins, anti-bodies, and enzymes for different types of bio-applications. Among them, we can cite collagen lines to guide the movement of living cells, peptide modified GFP biosensing pads to detect Gram positive bacteria, anti-body pads to determine the type of red blood cells, and trypsin columns in a microfluidic channel to obtain a real time biochemical micro-reactor. In this paper, we report for the first time on two-photon 3D microfabrication of DNA material. We also present our preliminary results on using a commercial 3D printer based on a video projector to polymerize slicing layers of gelatine-objects.

Changes in Molecular Weight of Poly(Styrenesulfonate) Initiated by Thioxanthone: Photolysis and Photo-Oxidation

The presence of dyes in colored polymers can cause a series of reactions that promote their photodegradation under irradiation. The aim of this study was to investigate the kinetic behavior of the photodegradation of poly (sodium 4-styrenesulfonate) (PSS) in aqueous solution or in the presence of thioxanthone (TX) under UV radiation in different atmospheres. These systems were monitored by SEC. The polydispersity decreased in all systems studied indicating that there was a breach of the polymer main chain. Only the PSS / TX / air atmosphere system peaked, which occured at 43h of irradiation. However, this system showed slower kinetics of polymeric scission due to the dye suppressing a transfer of energy from the triplet state to the polymer. The photodegradation of solutions of PSS / air atmosphere showed higher efficiency under ultraviolet radiation in relation to the PSS in the presence or absence of the dye under an inert atmosphere.