A new and straightforward method for screening highly catalytically active silver nanoparticle-polymer composites derived from branched polyethyleneimine (PEI) is reported. The one-step systematic derivatization of the PEI scaffold with alkyl (butyl or octyl) and ethanolic groups led to a structural diversity correlated to the stabilization of silver nanoparticles and catalysis. Analysis of PEI derivative libraries identified a silver nanoparticle-polymer composite that was able to efficiently catalyze the p-nitrophenol reduction by NaBH(4) in water with a rate constant normalized to the surface area of the nanoparticles per unit volume (k(1)) of 0.57 s(-1) m(-2) L. Carried out in the presence of excess NaBH(4), the catalytic reaction was observed to follow pseudo-first-order kinetics and the apparent rate constant was linearly dependent on the total surface area of the silver nanoparticles (Ag-NPs), indicating that catalysis takes place on the surface of the nanoparticles. All reaction kinetics presented induction periods, which were dependent on the concentration of substrates, the total surface of the nanoparticles, and the polymer composition. All data indicated that this induction time is related to the resistance to substrate diffusion through the polymer support. Hydrophobic effects are also assumed to play an important role in the catalysis, through an increase in the local substrate concentration.
Colloidal silver nanoparticles (Ag-NPs) with a mean diameter of 6.1 nm and a narrow size distribution were prepared by reduction of the correspondent metal salt with injection of NaBH(4), in the presence of dextran, and characterized by UV-vis, TEM, and DLS. The concentration of all reactants involved in the formation of the nanoparticles was optimized with the use of a new multivariate method, which revealed a significant reduction in the number of experiments when compared with the vast majority of univariate methods described in the literature. The Ag-NPs-dextran composite was able to efficiently catalyze the p-nitrophenol reduction in water by NaBH(4) with a rate constant normalized to the surface area of the nanoparticles per unit volume (k(1)) of 1.41 s(-1) m(-2) L, which is higher than values ever reported for Ag-NPs catalytic systems.
On
the basis of in situ selection utilizing a reaction
kinetics parameter, a new and straightforward method for screening
the formation of catalytically active silver-PVP nanoparticles is
reported. The method utilizes a multivariate analysis for the optimization
of the reactant concentrations in the synthesis of nanoparticles with
an analytical response based on the ability of the nanoparticles formed
to catalyze the reduction of p-nitrofenol in situ. The best synthetic conditions were selected, the
nanoparticles fully characterized, and their catalytic properties
with regard to the reduction of five nitroaromatic compounds, possessing
different substituents at the para position, were
determined. The kinetics analysis was based on the Langmuir–Hinshelwood
semi-heterogeneous model. The results showed the greater ability of
substrates to adsorb onto the nanoparticle surface compared with that
of borohydride ions and that the substrates possessing an electron-withdrawing
substituent are more catalytically favored. These differences are
discussed in terms of substrate adsorption and of a linear free-energy
relationship based on the Hammett plot.
Oligosaccharide-based amphiphiles were readily prepared by click chemistry from ω-azido-hexanoic or dodecanoic acids with propargyl-functionalized maltoheptaose or xyloglucanoligosaccharides. These amphiphilic compounds were used as capping/stabilizer agents in order to obtain highly stable catalytic silver glyconanoparticles (Ag-GNPs) through the in situ reduction of silver nitrate with NaBH4. With a view to long-term storage, the stabilization was optimized using a multivariate approach, and the nanoparticles were characterized by UV-vis, TEM, SAXS, and DLS. In order to explore the functionality of the Ag-GNPs in catalysis, a full kinetic analysis of the reduction of p-nitrophenol by NaBH4 in water and in water/ethanol mixtures was performed under semi-heterogeneous and quasi-homogeneous conditions. A pseudomonomolecular surface reaction was performed, and the kinetic data obtained were treated according to the Langmuir model. The Ag-GNPs were very active, and both substrates adsorbed onto the surface of the nanoparticles. For comparison purposes, the reaction was also performed in the presence of silver-sodium dodecanoate nanoparticles, which showed catalytic activity similar to that of the glyconanoparticles, supporting the choice of the carboxyl group as the stabilizing agent, although it provided much lower temporal stability. Finally, by combining kinetic and water/ethanol surface tension data it was possible to observe the effect of the addition of the less polar solvent (ethanol) to the reaction medium.
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