Use of Confinement Effects in Mesoporous Materials to Build Tailored Nanoarchitectures

C.B. CONTRERAS; O. AZZARONI; G. J. A. A. SOLER ILLIA

ELSEVIER SCIENCE B. V.

ELSEVIER

Año: 2018

In the last two decades, a remarkable proficiency was achieved in both the synthesis and the physical characterization of mesoporousmaterials (MM), which present high surface areas, and organized arrays of monodisperse pores, with diameters tunable between 2 and50 nm. This length scale is somehow ?magic?, due to the relevant effects of the proximity of a surface, combined with the existence ofcondensed matter (solvent, analytes, reagents) in the mesopore space. Different MM have been thoroughly studied for the fundamentalinterest in chemistry and physics under confinement, as well as their possible applications in numerous fields such as heterogeneouscatalysis, sensing, optics, drug delivery, cell growth and bioimaging. The synthetic community has acquired a large expertise in controllingthe size and shape of nanoscopic cavities in inorganic and organic solids, and in their functionalization with a large variety of species,ranging from molecular groups to polymers, biomolecules or nanospecies.The field of MM has experienced an impressive growth since the first demonstration of supramolecular templating in the nineties[1?4]. Three advantages contributed to the success of MM as a dynamic and flourishing fundamental field of study: (a) The precise andindependent control over porosity, inorganic framework and surface/pore contents tailorability of these materials, (b) the simple, extendedreproducible and inexpensive experimental protocols derived from the combination of sol-gel techniques and self-assembly, and (c) thewide possibilities of advanced applications that exploit the high surface area, chemical functionalities and confinement. Furthermore, thepossibility of multiscale processing (i.e., control on materials shaping, inclusion in multiscale porous systems, etc.), makes MM suitablebuilding blocks for future technologies.The unique properties of MM derived from their framework composition, pore architectures and tailored pore surfaces represent keyfeatures with strong impact on a variety of technological applications. While the first efforts were directed to exploit the high surface areain straightforward applications towards catalysis, adsorption or sensing processes, there is a bright future in taking advantage of two lessutilized aspects: confinement effects due to the finite pore size and surface effects, and the consequent ability to use these effects to finelytune the positioning of chemical or nanostructured species within the walls, surfaces or pore interior.In this work, we will present and analyze examples from the literature that illustrate the effects of confining matter in these mesoporoussystems, such as solvent structuring, molecular or polymer conformation, or electrostatic effects. Interestingly, these confinement ? derivedeffects can be exploited to build up nanosystems with pre-determined function positioning. In turn, this possibility opens the path tocreating nanosystems with well-defined chemical and nanostructured building blocks arbitrarily placed in space. We will show that thevery existence of mesopores defines three spaces -the pore wall, the surface and the pore interior- that determine several properties such asaccessibility, reactivity and the distance between functions. These properties arise from the combination of architecture (at different lengthscales) and local chemical functions, which can be in turn advantageously used to control transport or local reactivity and, consequently,permit a directional mass or energy transfer. The elaborate nano-architectures resulting from this approach resemble more and more thebiological systems, in which local reactivity and transport between locally well-defined environments take place and generate complexphysical-chemical behavior.