Insights into the intraparticle morphology of dendritic mesoporous silica nanoparticles from electron tomographic reconstructions

HYPOTHESIS

Although many synthetic pathways allow to fine-tune the morphology of dendritic mesoporous silica nanoparticles (DMSNs), the control of their particle size and mesopore diameter remains a challenge. Our study focuses on either increasing the mean particle size or adjusting the pore size distribution, changing only one parameter (particle or pore size) at a time. The dependence of key morphological features (porosity; pore shape and pore dimensions) on radial distance from the particle center has been investigated in detail.

EXPERIMENTS

Three-dimensional reconstructions of the particles obtained by scanning transmission electron microscopy (STEM) tomography were adapted as geometrical models for the quantification of intraparticle morphologies by radial porosity and chord length distribution analyses. Structural properties of the different synthesized DMSNs have been complementary characterized using TEM, SEM, nitrogen physisorption, and dynamic light scattering.

FINDINGS

The successful independent tuning of particle and pore sizes of the DMSNs could be confirmed by conventional analysis methods. Unique morphological features, which influence the uptake and release of guest molecules in biomedical applications, were uncovered from analyzing the STEM tomography-based reconstructions. It includes the quantification of structural hierarchy, identification of intrawall openings and pores, as well as the distinction of pore shapes (conical vs. cylindrical).

Morphology-transport relationships for SBA-15 and KIT-6 ordered mesoporous silicas

Quantitative morphology-transport relationships are derived for ordered mesoporous silicas through direct numerical simulation of hindered diffusion in realistic geometrical models of the pore space obtained from physical reconstruction by electron tomography. We monitor accessible porosity and effective diffusion coefficients resulting from steric and hydrodynamic interactions between passive tracers and the pore space confinement as a function of λ = d_tracer/d_meso (ratio of tracer diameter to mean mesopore diameter) in SBA-15 (d_meso = 9.1 nm) and KIT-6 (d_meso = 10.5 nm) silica samples. For λ = 0, the pointlike tracers reproduce the true diffusive tortuosities. For 0 ≤λ < 0.5, the derived hindrance factor quantifies the extent to which diffusion of finite-size tracers through the materials is hindered compared with free diffusion in the bulk liquid. The hindrance factor connects the transport properties of the ordered silicas to their mesopore space morphologies and enables quantitative comparison with random mesoporous silicas. Key feature of the ordered silicas is a narrow, symmetric mesopore size distribution (∼10% relative standard deviation), which engenders a sharper decline of the accessible-porosity window with increasing λ than observed for random silicas with their wide, asymmetric mesopore size distributions. As support structures, ordered mesoporous silicas should offer benefits for applications where spatial confinement effects and molecular size-selectivity are of prime importance. On the other hand, random mesoporous silicas enable higher diffusivities for λ > 0.3, because the larger pores carry most of the diffusive flux and keep pathways open when smaller pores have closed off.

[The value of walking exercise in treatment of intermittent claudication]

Physical training is one of the basic measures for all patients suffering from a generalized atherosclerosis. Specific training forms for PAD patients include home-based and structured, organized physical training programs; these are discussed and the results of meta-analyses presented. Whereas the published materials available for a home-based training is non-conclusive, structured training programs will lead to a statistically significant and clinically relevant improvement of a PAD patients' functional capacity.

[Altitude edema in the Swiss Alps. Observations on the incidence and clinical course in 50 patients 1980-1984]

In the Swiss Alps 50 patients with high altitude edema (high altitude pulmonary edema and/or high altitude cerebral edema) had to be rescued by helicopter during the period 1980-1984. The development, clinical picture and clinical course have been analyzed retrospectively. The patients were 49 men and one woman, generally in good health and well trained. They had ascended from the low-lands to an altitude above 2500 m and subsequently climbed higher. The climbers developed symptoms of acute mountain sickness on the second to third day of high altitude exposure and had to be evacuated by air on the fourth to fifth day. 70% of the cases occurred in the Valais Alps and the rest in the region of the Bernese Alps and the Bernina. The highest incidence of high altitude edema was observed in the Capanna Margherita (4559 m), where one of 588 climbers who stayed overnight had to be air-rescued. This ratio was about one in 4000 mountaineers at the Finsteraarhorn hut (3050 m) and the Monte Rosa hut (2795 m). Evacuation by air was the most successful therapeutic measure and resulted in immediate amelioration of clinical symptoms in 16 patients. 34 climbers had to be admitted to local hospitals due to severe high altitude pulmonary and/or cerebral edema. The pulmonary edema was bilateral in two thirds of these patients and unilateral in one third. Arterial blood gases showed moderate to severe limitation of oxygen diffusion capacity. All patients recovered completely within a few days.