Self-supported gold/chitosan nanocatalyst for chemoselective hydrogenation in π-conjugated C C C O system

A novel electrochemical platform based on mesoporous silica/titania and gold nanoparticles for simultaneous determination of norepinephrine and dopamine

An amorphous and mesoporous silica/titania (SiTi) material was synthesized by sol-gel method and its surface was modified with gold nanoparticles (AuNP) previously stabilized in a chitosan solution. The presence of small AuNP, with diameter lower than 10 nm was confirmed by transmission electron microscopy (TEM) and UV-Vis spectroscopy. Carbon paste electrodes were prepared to test the electrochemical properties by using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in [Fe(CN)₆]^3-/4- solution probe whereby the material silica-titania/gold nanoparticles (SiTi/AuNP) showed a huge improvement in the redox peak current and low charge transfer resistance. This electrode presented a good response for both norepinephrine and dopamine by means of square wave voltammetry (SWV) measurements; great sensitivity for both analytes, in an extensive linear range, was obtained. The limits of detection were 0.35 μmol L^-1 and 0.57 μmol L^-1 for norepinephrine and dopamine, respectively. Additionally, this electrode showed high selectivity for both analytes and it was applied in the simultaneous determination of norepinephrine and dopamine. The sensor was also tested in simulated biological fluids presenting a good recovery. The SWV electrochemical response of norepinephrine was also investigated in the presence of possible interferers such as uric acid, ascorbic acid and glucose and there was no significant interference. The prepared electrode also exhibits good reproducibility for norepinephrine detection, with relative standard deviation of 5.19%.

Immobilization of pectinase on chitosan-magnetic particles: Influence of particle preparation protocol on enzyme properties for fruit juice clarification

Magnetic-chitosan particles were prepared following three different protocols enabling the preparation of particles with different sizes - nano (Nano-CMag, Micro (Micro-CMag) and Macro (Macro-CMag) - and used for pectinase immobilization and clarification of grape, apple and orange juices. The particle size had a great effect in the kinetic parameters, Nano-CMag biocatalyst presented the highest V_max value (78.95 mg. min^-1), followed by Micro-CMag and Macro-CMag, with V_max of 57.20 mg.min^-1 and 46.03 mg.min^-1, respectively. However, the highest thermal stability was achieved using Macro-CMag, that was 8 and 3-times more stable than Nano-CMag and Micro-CMag biocatalysts, respectively. Pectinase immobilized on Macro-CMag kept 85% of its initial activity after 25 batch cycles in orange juice clarification. These results suggested that the chitosan magnetic biocatalysts presented great potential application as clarifying catalysts for the fruit juice industry and the great importance of the chitosan particles preparation on the final biocatalyst properties.

Tunneling effects in confined gold nanoparticle hydrogenation catalysts

Clean surface gold nanoparticles (AuNPs) of ∼6.6 nm that were confined in ionic liquid (IL) cages of hybrid γ-alumina (γ-Al₂O₃) displayed hydrogenation pathways in the reduction of trans-cinnamaldehyde distinct from those imprinted directly onto γ-Al₂O₃. Hydrogen activation proceeded via homolytic activation in IL-encapsulated AuNPs and via heterolytic cleavage for IL-free supported AuNPs. Higher negative apparent entropy (ΔS_app) values were obtained for the IL-confined AuNPs compared to the non-hybrid catalyst (Au/γ-Al₂O₃), suggesting a decrease in the number of microstates induced by the nano-confined environment. High kinetic isotope effect (KIE) values (k_H/k_D = 2.5-2.9 at 273 K) and Arrhenius convex curves were observed. Furthermore, differences of 5.6 and 6.2 kJ mol^-1 between the apparent activation energies of the deuteration and hydrogenation reactions (E-E) associated with pre-exponential factor ratios (A^D/A^H) of 4.6 and 5.1 provided strong evidence of the possible involvement of a tunneling pathway in the case of the confined AuNPs.