Size-Dependent Catalytic Activity of Oxo-Hydroxo Titanium Sub-Nanoislets Grafted on Organically Modified Mesoporous Silica

An experimental approach for controlling confinement effects at catalyst interfaces

Catalysts are conventionally designed with a focus on enthalpic effects, manipulating the Arrhenius activation energy. This approach ignores the possibility of designing materials to control the entropic factors that determine the pre-exponential factor. Here we investigate a new method of designing supported Pt catalysts with varying degrees of molecular confinement at the active site. Combining these with fast and precise online measurements, we analyse the kinetics of a model reaction, the platinum-catalysed hydrolysis of ammonia borane. We control the environment around the Pt particles by erecting organophosphonic acid barriers of different heights and at different distances. This is done by first coating the particles with organothiols, then coating the surface with organophosphonic acids, and finally removing the thiols. The result is a set of catalysts with well-defined "empty areas" surrounding the active sites. Generating Arrhenius plots with >300 points each, we then compare the effects of each confinement scenario. We show experimentally that confining the reaction influences mainly the entropy part of the enthalpy/entropy trade-off, leaving the enthalpy unchanged. Furthermore, we find this entropy contribution is only relevant at very small distances (<3 Å for ammonia borane), where the "empty space" is of a similar size to the reactant molecule. This suggests that confinement effects observed over larger distances must be enthalpic in nature.

Lower methane combustion temperature on palladium nanoparticles anchored on TiOx subnano-islets in stellate mesoporous silica nanospheres

Highly accessible and active palladium nanoparticles stabilized on TiO_x subnano-islets in monodispersed mesoporous silica nanospheres for methane combustion at low temperature.

Mesoporous Silica Containers and Programmed Catalytic Hairpin Assembly/Hybridization Chain Reaction Based Electrochemical Sensing Platform for MicroRNA Ultrasensitive Detection with Low Background

In this work, based on mesoporous silica containers (MSNs) with the programmed enzyme-free DNA assembly amplification of catalytic hairpin assembly (CHA) and hybridization chain reaction (HCR), an ultrasensitive electrochemical sensing platform with low background is developed for the detection of microRNA (miRNA). Herein, the electrochemical reporter methylene blue (MB) was sealed in the pores of MSNs by the double-stranded DNA (dsDNA) gate of hairpin DNA H1 and anchor DNA. In the absence of target, neither the CHA nor the HCR process happened, which enabled a low background. After target was added, DNA H1 was displaced from the MSNs surface and participated in the CHA process with the assistance of hairpin DNA H2, which accelerated the release of MB from the MSNs pore. Meanwhile, the CHA products H1-H2 were hybridized with the capture probes (SH-CP) on the electrode surface, which further initiated the HCR process. The released MB from the MSNs will effectively intercalate into long dsDNA polymers of HCR products, resulting in a significant electrochemical response. Taking miRNA-21 as the model target, the proposed sensing platform achieves a satisfactory detection limit down to 0.037 fM, which is lower than that of electrochemical assay with amplification methods. In addition, the strategy shows good selectivity against other miRNAs and is capable in practical analytes. Benefitting from the features of being label-free and enzyme-free and having low background, high sensitivity, and selectivity, this strategy shows great potential in bioanalysis and clinical diagnostics.