Nanoporous hybrid core–shell nanoparticles for sequential release

In this article, a new type of core–shell nanoparticle is introduced. In contrast to most reported core–shell systems, the particles presented here consist of a porous core as well as a porous shell using only non-metal materials. The core–shell nanoparticles were successfully synthesized using nanoporous silica nanoparticles (NPSNPs) as the starting material, which were coated with nanoporous phenylene-bridged organosilica, resulting in a total particle diameter of about 80 nm. The combination of a hydrophilic nanoporous silica core and a more hydrophobic nanoporous organosilica shell provides regions of different chemical character and slightly different pore sizes within one particle. These different properties combined in one particle enable the selective adsorption of guest molecules at different parts of the particle depending on the molecular charge and polarity. On the other hand, the core–shell make-up of the particles provides a sequential release of guest molecules adsorbed at different parts of the nanoparticles. As a proof of concept, loading and release experiments with dyes were performed using non polar fluorescein and polar and charged methylene blue as model guest molecules. Non polar fluorescein is mostly adsorbed on the hydrophobic organosilica shell and therefore quickly released whereas the polar methylene blue, accumulated in the hydrophilic silica core, is only released subsequently. This occurs in small doses for an extended time corresponding to a sustained release over at least one year, controlled by the organosilica shell which acts as a diffusion barrier. An initial experiment with two drugs — non polar ibuprofen and polar and charged procaine hydrochloride — has been carried out as well and shows that the core–shell nanoparticles presented here can also be used for the sequential release of more relevant combinations of molecules

Deformation Dynamics of Nanopores upon Water Imbibition

Capillarity-driven transport in nanoporous solids is ubiquitous in nature and is of increasing importance for the functionality of modern liquid-infused engineering materials. During imbibition, highly curved menisci are driven by negative Laplace pressures of several hundred atmospheres, exerting an enormous contractile load on an increasing portion of the porous matrix. Due to the challenge of simultaneously monitoring imbibition and deformation with high spatial resolution, the resulting coupling of solid elasticity to liquid capillarity has remained largely unexplored. Here, we study water imbibition in mesoporous silica using optical imaging, gravimetry, and high-resolution dilatometry. In contrast to an expected Laplace pressure-induced contraction, we find a square-root-of-time expansion and an additional abrupt length increase when the menisci reach the top surface. The final expansion is absent when we stop the imbibition front inside the porous medium in a dynamic imbibition-evaporation equilibrium, as is typical for water transport and transpiration in plants. These peculiar deformation behaviors are validated by single-nanopore molecular dynamics simulations and described by a continuum model that highlights the importance of expansive surface stresses at the pore walls (Bangham effect) and the buildup or release of contractile Laplace pressures as nanoscale menisci collectively advance, arrest, or disappear. Our model predicts that these observations are valid not only for water imbibition in silica, but for any imbibition process in nanopores, regardless of the liquid/solid combination. This also suggests that simple deformation measurements can be used to quantify surface stresses and Laplace pressures or transport in a wide variety of natural and artificial porous media.Comment: 11 pages, 4 figures. Supplementary information and 3 movies are stored as ancillary file

Translational and rotational diffusion coefficients in nanofluids from polarized dynamic light scattering

Nanofluids representing nanometer-sized solid particles dispersed in liquids are of interest in many fields of process and energy engineering, e.g., heat transfer, catalysis, and the design of functionalized materials [1]. The physical, chemical, optical, and electronic properties of nanofluids are strongly driven by the size, shape, surface potential, and concentration of the nanoparticles. For the analysis of diffusive processes in nanofluids allowing access to, e.g., particle size and its distribution, dynamic light scattering (DLS) is the state-of-the-art technique. It is based on the analysis of microscopic fluctuations originating from the random thermal movement of particles in the continuous liquid phase at macroscopic thermodynamic equilibrium. For anisotropic particles or particle aggregates, besides translational diffusion also rotational diffusion occurs. To obtain the sum of the orientation-averaged translational (DT) and rotational (DR) diffusivities by depolarized DLS [2], a homodyne detection scheme is usually applied which can hardly be fulfilled in the experimental realization. Furthermore, the experiments are restricted to limited ranges for temperature, particle concentration, and viscosity

Comparative Gas Sorption and Cryoporometry Study of Mesoporous Glass Structure: Application of the Serially Connected Pore Model

Nitrogen sorption and melting and freezing of water in a small pore size mesoporous glass with irregular pore structure is studied. The analysis of the experimentally obtained data is performed using the recently developed serially connected pore model (SCPM). The model intrinsically incorporates structural disorder by introducing coupling between nucleation and phase growth mechanisms in geometrically disordered mesopore spaces. It is shown that, in contrast to the independent pore models prevailing in the literature, SCPM self-consistently describes not only boundary transitions, but also the entire family of the scanning transitions. The scanning behavior is shown to be very sensitive to microscopic details of the fluid phase distribution within the porous materials, hence can be used to check the validity of the thermodynamic models and to improve the structural analysis. We show excellent quantitative agreement between the structural information evaluated from the cryoporometry and gas sorption data using SCPM

Modeling adsorption in metal-organic frameworks with open metal sites : propane/propylene separations

We present a new approach for modeling adsorption in metal-organic frameworks (MOFs) with unsaturated metal centers and apply it to the challenging propane/propylene separation in copper(II) benzene-1,3,5-tricarboxylate (CuBTC). We obtain information about the specific interactions between olefins and the open metal sites of the MOP using quantum mechanical density functional theory. A proper consideration of all the relevant contributions to the adsorption energy enables us to extract the component that is due to specific attractive interactions between the pi-orbitals of the alkene and the coordinatively unsaturated metal. This component is fitted using a combination of a Morse potential and a power law function and is then included into classical grand canonical Monte Carlo simulations of adsorption. Using this modified potential model, together with a standard Lennard-Jones model, we are able to predict the adsorption of not only propane (where no specific interactions are present), but also of propylene (where specific interactions are dominant). Binary adsorption isotherms for this mixture are in reasonable agreement with ideal adsorbed solution theory predictions. We compare our approach with previous attempts to predict adsorption in MOFs with open metal sites and suggest possible future routes for improving our model

A New Role of the Surfactant in the Synthesis of Mesostructured Phases: Dodecyl Phosphate as Template and Reactant for Aluminophosphates

Lamellar mesostructured aluminophosphates were synthesized from aluminum triisopropoxide and phosphoric acid; monododecyl phosphate surfactant was used as structure-directing template. Depending on the relative Al/P ratio in the samples, variable relative amounts of tetrahedrally and octahedrally coordinated Al are found, indicating that both aluminophosphate and aluminum oxide species (as thermodynamically favored) are being formed in the syntheses. This is investigated quantitatively by Al K-edge XANES spectroscopy. The same syntheses were carried out without phosphoric acid, resulting in similar lamellar structures. The inorganic lamellae of these products consist to a significant extent of aluminophosphate rather than exclusively of aluminum oxide, which means that the phosphate headgroups of the surfactant molecules become incorporated into the inorganic network. Thus, for the first time, the surfactant serves as both template and reactant