Camila M. Maroneze

Ion-Exchange Properties of Imidazolium-Grafted SBA-15 toward AuCl4– Anions and Their Conversion into Supported Gold Nanoparticles

Imidazolium groups were successfully prepared and grafted on the surface of SBA-15 mesoporous silica. The ion-exchange properties of the functionalized porous solid (SBA-15/R(+)Cl(-)) toward AuCl(4)(-) anions were evaluated through an ion-exchange isotherm. The calculated values of the equilibrium constant (log β = 4.47) and the effective ion-exchange capacity (t(Q) = 0.79 mmol g(-1)) indicate that the AuCl(4)(-) species can be loaded and strongly retained on the functionalized surface as counterions of the imidazolium groups. Subsequently, solids containing different amounts of AuCl(4)(-) ions were submitted to a chemical reduction process with NaBH(4), converting the anionic gold species into supported gold nanoparticles. The plasmon resonance bands, the X-ray diffraction patterns, and transmission electron microscopy images of the supported gold nanoparticles before and after thermal treatment at 973 K indicate that the metal nanostructures are highly dispersed and stabilized by the host environment.

Development of a sensor for L-Dopa based on Co(DMG)2ClPy/multi-walled carbon nanotubes composite immobilized on basal plane pyrolytic graphite electrode

L-Dopa is the immediate precursor of the neurotransmitter dopamine, being the most widely prescribed drug in the treatment of Parkinsons disease. A sensitive and selective method is presented for the voltammetric determination of L-Dopa in pharmaceutical formulations using a basal plane pyrolytic graphite (BPPG) electrode modified with chloro(pyridine)bis(dimethylglyoximato)cobalt(III) (Co(DMG)(2)ClPy) absorbed in a multi-walled carbon nanotube (MWCNT). Scanning Electron Microscopy and Fourier Transform Infrared Spectroscopy were used to characterize the materials. The electrocatalytical oxidation of L-Dopa using the Co(DMG)(2)ClPy/MWCNT/BPPG electrode was investigated by cyclic voltammetry and square wave voltammetry. The parameters that influence the electrode response (the amount of Co(DMG)(2)ClPy and of MWCNT, buffer solution, buffer concentration, buffer pH, frequency and potential pulse amplitude) were investigated. Voltammetric peak currents showed a linear response for L-Dopa concentration in the range of 3 to 100 μM, with a sensitivity of 4.43 μAcm(-2)/μM and a detection limit of 0.86 μM. The related standard deviation for 10 determinations of 50 μM L-Dopa was 1.6%. The results obtained for L-Dopa determination in pharmaceutical formulations (tablets) were in agreement with the compared official method. The sensor was successfully applied for L-Dopa selective determination in pharmaceutical formulations.

Surface functionalization of SBA-15 and a nonordered mesoporous silica with a 1,4-diazabicyclo[2.2.2]octane derivative: Study of CuCl2 adsorption from ethanol solution

This work describes the preparation and characterization of postfunctionalized ordered (SBA-15) and nonordered (SMD) mesoporous silicas with n-propyl-1,4-diazoniabicycle[2.2.2]octane chloride (DbCl) moiety. The main interest is based on the fact that these materials are excellent adsorbents due to the ability of functional groups to retain copper chlorides on their surfaces as anionic complexes CuCl(2+n)(n-). The specific surface areas (S(BET)) and average pore diameters (d(pore)) for SBA-15 and SMD are SBA-15, S(BET)=944 m(2) g(-1), d(pore)=9.0 nm; SMD, S(BET)=710 m(2) g(-1), d(pore)=11 nm. On functionalization with DbCl, reductions in the specific surface areas of the resulting materials (SBA-15/DbCl and SMD/DbCl) are observed and the following functionalization degrees (ϕ) were determined: SBA-15/DbCl, S(BET)=247 m(2) g(-1), ϕ=0.95 mmol g(-1); SMD/DbCl, S(BET)=83 m(2) g(-1), ϕ=1.2 mmol g(-1). The adsorption equilibria of CuCl(2) in ethanol were characterized, and the heterogeneous stability constants, β(1) and β(2), corresponding to formation of CuCl(4)(2-) and CuCl(3)(-) anionic species adsorbed on the surface were found. Also, the effective sorption capacities (t(Q)) were determined: SBA-15/DbCl, log β(1)=4.46, log β(2)=7.10, t(Q)=0.80 mmol g(-1); SMD/DbCl, log β(1)=4.95, log β(2)=7.52, t(Q)=0.75 mmol g(-1). Regeneration of the adsorbents requires a very simple procedure consisting of their immersion in aqueous solution followed by immediate release to the solution phase of the Cu(OH(2))(n)(2+) species, followed by chloride anions as the counterions.

Electrooxidation of nitrite on a silica-cerium mixed oxide carbon paste electrode.

A silica-cerium mixed oxide (SiCe) was prepared by the sol-gel process, using tetraethylorthosilicate and cerium nitrate as precursors and obtained as an amorphous solid possessing a specific surface area of 459 m(2) g(-1). Infrared spectroscopy of the SiCe material showed the formation of the Si-O-Ce linkage in the mixed oxide. Scanning electron microscopy/energy dispersive spectroscopy indicated that the cerium oxide particles were homogenously dispersed on the matrix surface. X-ray diffraction and (29)Si solid-state nuclear magnetic resonance implied non-crystalline silica matrices with chemical environments that are typical for silica-based mixed oxides. X-ray photoelectron spectroscopy showed that Ce was present in approximately equal amounts of both the 3+ and 4+ oxidation states. Cyclic voltammetry data of electrode prepared from the silica-cerium mixed oxide showed a peak for oxidation of Ce(3+)/Ce(4+) at 0.76 V and electrochemical impedance spectroscopy equivalent circuit indicated a porous structure with low charge transfer resistance. In the presence of nitrite, the SiCe electrode shows an anodic oxidation peak at 0.76 V with a linear response as the concentration of the analyte increases from 3×10(-5) at 3.9×10(-3) mol L(-1).

Preparation of supported AuPd nanoalloys mediated by ionic liquid-like functionalized SBA-15: structural correlations concerning its catalytic activity

Noble metal nanoalloys are very important in catalysis, sensing, electrochemistry, and plasmonics. Based on the importance of these materials and in order to overcome the synthetic limitations for the in situ synthesis of supported nanoalloys in porous supports, we extended a synthetic protocol to achieve supported AuPd nanoalloys within SBA-15 pores modified with an ionic liquid-like alkoxysilane. The synthesized materials form very small nanoparticles with non-passivated surfaces, which are highly active as heterogeneous catalysts for the reduction of 4-nitrophenol. The anion exchange ability of the functionalized SBA-15 was used to adsorb Au and Pd anionic complexes. The adsorbed species were then reduced and converted into supported monometallic nanoparticles or nanoalloys. Nitrogen physisorption isotherms showed that the synthetic process does not damage the mesoporous support nor block the pores. TEM/EDS and UV-vis analyses were used to prove alloy formation in the bimetallic materials through the concomitant presence of Au and Pd in the same particles and through the disappearance of the gold plasmon band as the palladium content increased. Finally, the catalytic activity of the materials increased as the palladium content increased, showing that it is possible to control the catalytic performance by tuning the material composition during the anion exchange step.

Synthesis and structural characterization of nanometric ceria highly dispersed in SBA-15 with oxygen exchange capacity

Nanometric ceria-decorated SBA-15 was prepared using a route involving the impregnation of SBA-15 pores by a solution of cerium(III) 2-ethylhexanoate, followed by its thermal decomposition. According to XRF analysis, the number of successive impregnation–decomposition cycles (IDC) allows control of the CeO2/SiO2 ratio in the final material, and also the tailoring of the nanoparticle size of the fluorite CeO2nanoparticles supported in the SBA-15, as confirmed by XRD, Raman and UV-Vis spectroscopies. The mean pore size of the SBA-15 decreases with successive IDC, as observed by N2 adsorption–desorption, suggesting that CeO2 nanoparticles are located inside the SBA-15 mesopores, as confirmed by TEM and HRTEM analyses. The degree of oxygen storage capacity (OSC) was measured by the number of hydrogen uptake from the temperature programmed reduction (H2-TPR). It was found that the value of hydrogen uptake of SBA-15 submitted to one IDC corresponds to 3344 μmol of O2 per gram of CeO2, whereas those of SBA-15 submitted to five and ten IDC were 1324 and 2769 μmol of O2 per gram of CeO2, respectively.

Size controlled synthesis of highly dispersed anatase/rutile nanoparticles with photocatalytic activity toward salicylic acid degradation

TiO2 nanocrystals supported in SBA-15 pores were prepared by a simple route of successive cycles of impregnation of SBA-15 with titanium(IV) di-(n-propoxy)-di-(2-ethylhexanoate) followed by thermal treatment. Anatase and rutile phases were identified by XRD in the materials obtained after cycles 1, 3, 5, 7 and 10. Raman scattering combined with XRD and TEM shows that titania is in a nanometric regime. Increases in the size of the titania nanocrystals, as well as increases in the TiO2 content in the final material (XRF), occur after successive cycles. SAXS and N2 sorption indicate that the nanocrystals are formed inside the SBA-15 mesopores, which remain unblocked even after 10 cycles. UV-Vis (DRS) spectroscopy indicates important changes in electronic properties. The ability of SBA-15-supported titania nanocrystals to photodegrade salicylic acid was tested. There is a clear relation between the TiO2 nanoparticle size and the photocatalytic activity. Among all samples tested, the best result was obtained for the materials with the highest band-gaps and with the smallest sized titania.

Applications of MN4 Macrocyclic Metal Complexes in Electroanalysis

The rich and wide redox chemistry of N4 metal macrocyclic complexes allied to the several strategies for immobilizing them on the surface of different types of solids and electrode materials have opened innumerous possibilities of using these molecules as electron mediators in electrochemical devices. This chapter describes and discusses the main aspects related to the molecular anchoring of metallomacrocyclic complexes on the surface of inorganic solids (silica and carbon-based materials) and conventional electrodes through different types of chemical and physical interactions, as well as the application of these systems in the electroanalysis of compounds with technological, biological, and environmental interest. Particular attention was focused on the discussions of the properties and structure-activity relationships that have allowed designing materials with interesting sensing ability in distinct types of electrochemical interfaces and configurations, ranging from chemically modified electrodes with hybrid solid materials and thin films to a low-cost paper-based analytical device. Our emphasis was not to make comparisons between particular examples with respect to sensor performances, but rather to focus on the potentiality of N4 metal macrocyclic complexes to be applied in the design and development of platforms useful in electroanalysis, highlighting the diversity of support materials suitable for immobilizing the active redox mediator as well as the broad range of analytes that can be analyzed by electroanalytical methods.