RESUMO

The relationship among the standard reaction Gibbs free energy ΔG°, the standard reduction potential E°, and the atomic structure parameters of radius, nuclear charge, and isoelectronic orbitals nl is accomplished through the attraction electric force F elec. In relationship with E°, it was necessary to define two new reference scales: E 0 ° with a final state of E° in the element, which allowed to have a parabolic trend of ΔG° versus F elec, and E °,0 whose final state is the ion with a more negative charge (e.g., -1, -2, -3). The relationship with ΔG° is related to the concept of chemical stability, and the relationship with E °,0 is more related to the concept of electronegativity. In relationship with ΔG°, it was necessary to predict the values of possible new cations and noncommon cations in order to find a better trend of ΔG° versus F elec, whose stability is analyzed by Frost diagrams of the isoelectronic series. This dependence of ΔG° on F elec is split into two terms. The first term indicates the behavior of the minimum of ΔG° for each isoelectronic orbital nl, while the second term deals with the parabolic trend of this orbital. For the minima of the configuration np6, a hysteresis behavior of the minima of ΔG° is found: an exponential behavior from periods 1 and 2 and a sigmoidal behavior from periods 5 and 4 to interpolate period 3. It is also found that the proximity of unfilled np or (n + 1)s orbitals induces instability of the ion in configurations ns2/nd2/4f2 and nd10/nd8(n + 1)s2, respectively. On the contrary, the stability of the orbitals np6 does not depend on the neighboring empty (n + 1)s0 orbitals. Both phenomena can be explained by the stability of the configuration of noble gas np6 and the nd10(n + 1)s2 configuration. We have also found that it is possible to increase the reduction potential E °,0 (macroscopic electronegativity), although the electric force F elec decreases because the orbital overlap influences the electronegativity.

RESUMO

This study was conducted to measure the activity of the enzyme glutathione S-transferase (GST) in saliva and to compare the activity of this enzyme in children with and without dental fluorosis in communities with different concentrations of naturally fluoridated water. A total of 141 schoolchildren participated in this cross-sectional study. Children were selected from two communities: one with a low (0.4 ppm) and the other with a high (1.8 ppm) water fluoride concentration. Dental fluorosis was evaluated by applying the Thylstrup and Fejerskov Index (TFI) criteria. Stimulated saliva was obtained, and fluoride concentration and GST activity were measured. The GST activity was compared among children with different levels of dental fluorosis using multinomial logistic regression models and odds ratios (OR). The mean age of the children was 10.6 (±1.03) years. Approximately half of the children showed dental fluorosis (52.5 %). The average GST activity was 0.5678 (±0.1959) nmol/min/µg. A higher concentration of fluoride in the saliva was detected in children with a higher GST activity (p = 0.039). A multinomial logistic regression model used to evaluate the GST activity and the dental fluorosis score identified a strong association between TFI = 2-3 (OR = 15.44, p = 0.007) and TFI ≥ 4 (OR = 55.40, p = 0.026) and the GST activity level, compared with children showing TFI = 0-1, adjusted for age and sex. Schoolchildren with higher levels of dental fluorosis and a higher fluoride concentration in the saliva showed greater GST activity. The increased GST activity most likely was the result of the body's need to inactivate free radicals produced by exposure to fluoride.

Assuntos

RESUMO

The entrapping of physicochemical active molecules inside mesoporous networks is an appealing field of research due to the myriad of potential applications in optics, photocatalysis, chemical sensing, and medicine. One of the most important reasons for this success is the possibility of optimizing the properties that a free active species displays in solution but now trapped inside a solid substrate. Additionally it is possible to modulate the textural characteristics of substrates, such as pore size, specific surface area, polarity and chemical affinity of the surface, toward the physical or chemical adhesion of a variety of adsorbates. In the present document, two kinds of non-silicon metal alkoxides, Zr and Ti, are employed to prepare xerogels containing entrapped tetrapyrrolic species that could be inserted beforehand in analogue silica systems. The main goal is to develop efficient methods for trapping or binding tetrapyrrole macrocycles inside TiO2 and ZrO2 xerogels, while comparing the properties of these systems against those of the SiO2 analogues. Once the optimal synthesis conditions for obtaining translucent monolithic xerogels of ZrO2 and TiO2 networks were determined, it was confirmed that these substrates allowed the entrapment, in monomeric form, of macrocycles that commonly appear as aggregates within the SiO2 network. From these experiments, it could be determined that the average pore diameters, specific surface areas, and water sorption capacities depicted by each one of these substrates, are a consequence of their own nature combined with the particular structure of the entrapped tetrapyrrole macrocycle. Furthermore, the establishment of covalent bonds between the intruding species and the pore walls leads to the obtainment of very similar pore sizes in the three different metal oxide (Ti, Zr, and Si) substrates as a consequence of the templating effect of the encapsulated species.

Assuntos

RESUMO

We report the heptapeptide-mediated biomineralization of titanium dioxide nanoparticles from titanium alkoxides. We evaluated the influence of pH on the biomineralized products and found that nanostructured TiO2 was formed in the absence of external ions (water only) at pH ~ 6.5. Several variants (mutants) of the peptides with different properties (i.e., different charges, isoelectric points (pIs), and sequences) were designed and tested in biomineralization experiments. Acid-catalyzed experiments were run using the H1 (HKKPSKS) peptide at room temperature, which produced anatase nanoparticles (~5 nm in size) for the first time via a heptapeptide and sol-gel approach. In addition, the peptide H1 was used to synthesize SiO2 nanoparticles. The influence of the pH and the added ions were monitored: at higher pH levels (8-9), SiO2 nanoparticles (20-30 nm in size) were obtained. In addition, whereas borate and Tris ions allowed the formation of colloidal systems, phosphate ions were unable to produce sols. The results presented here demonstrate that biomineralization depends on the sequence and charge of the peptide, and ions in solution can optimize the formation of nanostructures.

Assuntos

RESUMO

The crossed and linked histories of tetrapyrrolic macrocycles, interwoven with new research discoveries, suggest that Nature has found in these structures a way to ensure the continuity of life. For diverse applications porphyrins or phthalocyanines must be trapped inside solid networks, but due to their nature, these compounds cannot be introduced by thermal diffusion; the sol-gel method makes possible this insertion through a soft chemical process. The methodologies for trapping or bonding macrocycles inside pristine or organo-modified silica or inside ZrO2 xerogels were developed by using phthalocyanines and porphyrins as molecular probes. The sizes of the pores formed depend on the structure, the cation nature, and the identities and positions of peripheral substituents of the macrocycle. The interactions of the macrocyclic molecule and surface Si-OH groups inhibit the efficient displaying of the macrocycle properties and to avoid this undesirable event, strategies such as situating the macrocycle far from the pore walls or to exchange the Si-OH species by alkyl or aryl groups have been proposed. Spectroscopic properties are better preserved when long unions are established between the macrocycle and the pore walls, or when oligomeric macrocyclic species are trapped inside each pore. When macrocycles are trapped inside organo-modified silica, their properties result similar to those displayed in solution and their intensities depend on the length of the alkyl chain attached to the matrix. These results support the prospect of tuning up the pore size, surface area, and polarity inside the pore cavities in order to prepare efficient catalytic, optical, sensoring, and medical systems. The most important feature is that research would confirm again that tetrapyrrolic macrocycles can help in the development of the authentic pore engineering in materials science.

Assuntos

RESUMO

Ordered surfactantless self-assembled, mesoporous SnO(2) adsorbents, consisting of tubular voids of nanometric sizes, are prepared by the sol-gel processing of tin (IV) tetra-tert-amyloxide, Sn(OAm(t))(4), whose molecules have been previously chelated with acetylacetone in the absence of water, to modulate their reactivity and to promote an incipient self-assembling of -O-Sn-O oligomeric species; ultimately, the necessary amount of water to induce the hydrolysis-condensation reactions is added to this aged sol, then producing tubular pore templates within the SnO(2) xerogel network. A collection of mesoporous SnO(2) xerogels of assorted structural properties has been obtained after calcination in air of precursory gels proceeding from an aged mixture of Sn(OAm(t))(4) and acetylacetone at temperatures in the range 200-1000 degrees C. N(2) sorption isotherms measured on these SnO(2) solids evidence mesoporous structures of diverse textural characteristics (i.e. pore widths of 3-50 nm and surface areas of 10-140 m(2) g(-1)) in which voids virtually behave as if they are independent cylindrical pores during capillary condensation and evaporation.