One of the big challenges of medicine today is to deliver drugs specifically to defected cells. Nanoparticulate drug carriers have the potential to answer to this call, as nanoparticles can cross physiological barriers and access different tissues, and also be provided in a targetable form aimed at enhancing cell specificity of the carrier. Recent developments within material science and strong collaborative efforts crossing disciplinary borders have highlighted the potential of mesoporous silica nanoparticles (MSNs) for such targeted drug delivery. Here we outline recent advances which in this sense push MSNs to the forefront of drug delivery development. Relatively straightforward inside-out tuning of the vehicles, high flexibility, and potential for sophisticated release mechanisms make these nanostructures promising candidates for targeted drug delivery such as ‘smart’ cancer therapies. Moreover, due to the large surface area and the controllable surface functionality of MSNs, they can be controllably loaded with large amounts of drugs and coupled to homing molecules to facilitate active targeting, simultaneously carrying traceable (fluorescent or magnetically active) modalities, also making them highly interesting as theragnostic agents. However, the increased relative surface area and small size, and flexible surface functionalization which is beneficially exploited in nanomedicine, consequently also includes potential risks in their interactions with biological systems. Therefore, we also discuss some safety issues regarding MSNs and highlight how different features of the drug delivery platform influence their behaviour in a biological setting. Addressing these burning questions will facilitate the application of MSNs in nanomedicine.

Towards multifunctional, targeted drug delivery systems using mesoporous silica nanoparticles – opportunities & challenges

Mesoporous non-siliceous oxides have attracted great interest due to their unique properties and potential applications. Since the discovery of mesoporous silicates in 1990s, organic–inorganic assembly processes by using surfactants or block copolymers as soft templates have been considered as a feasible path for creating mesopores in metal oxides. However, the harsh sol–gel conditions and low thermal stabilities have limited the expansion of this method to various metal oxide species. Nanocasting, using ordered mesoporous silica or carbon as a hard template, has provided possibilities for preparing novel mesoporous materials with new structures, compositions and high thermal stabilities. This review concerns the synthesis, composition, and parameter control of mesoporous non-siliceous oxides. Four synthesis routes, i.e. soft-templating (surfactants or block copolymers as templates), hard-templating (mesoporous silicas or carbons as sacrificial templates), colloidal crystal templating (3-D ordered colloidal particles as a template), and super lattice routes, are summarized in this review. Mesoporous metal oxides with different compositions have different properties. Non-siliceous mesoporous oxides are comprehensively described, including a discussion of constituting elements, synthesis, and structures. General aspects concerning pore size control, atomic scale crystallinity, and phase control are also reviewed.

Synthesis of non-siliceous mesoporous oxides

Inspired by natural processes, there is an enormous interest in light-driven water splitting to convert solar energy into electrical and chemical energy. This approach is thought to be able to eventually solve the main energy problem that society will face more dramatically in the near future. The water oxidation reaction is widely considered a major barrier for utilizing solar energy in artificial photosynthesis. Due to the relatively high overpotential and slow kinetics of the reaction, numerous efforts are made on the development of non-noble metal oxygen evolution catalysts such as transition metal oxides. Among them, cobalt-oxide-based materials have shown decent activity and thus present themselves as a promising candidate. In this perspective, we summarize the state of the art in synthesis of cobalt-oxide-based materials and application as water oxidation catalysts through electrochemical, photochemical, and photoelectrochemical approaches. Additionally, we state the future challenges that are crit...

Cobalt-Oxide-Based Materials as Water Oxidation Catalyst: Recent Progress and Challenges

2D-Co 3 O 4 and 3D-Co 3 O 4 catalysts were prepared by the hard template method, and nano-Co 3 O 4 was synthesized by precipitation method. The catalytic activity for the oxidation of formaldehyde over various types of catalysts was investigated. The 3D-Co 3 O 4 catalyst attained a 100% conversion rate of formaldehyde at 130 °C with a space velocity of 30,000 mL/(g h), while the 2D-Co 3 O 4 catalyst oxidized formaldehyde completely at 150 °C in the same space velocity conditions. The difference in activity is due to the clear channel structure of the mesoporous Co 3 O 4 prepared by the hard template method, which has large specific surface area and surface active species that allows the reactant to diffuse and undergo surface reactions. The 3D-Co 3 O 4 had the best performance of formaldehyde oxidation due to the three-dimensional porous channel structure, larger specific surface area, abundant active surface oxygen species and active Co 3+ cationic species on the exposed (2 2 0) crystal face. Complete conversion of formaldehyde had remained the same after 3D-Co 3 O 4 was observed for 160 h. Therefore, the 3D-Co 3 O 4 catalyst has the best catalytic activity and stability for formaldehyde, which might be a non-noble catalyst for catalytic removal of formaldehyde in practical application.

Comparison of the performance for oxidation of formaldehyde on nano-Co3O4, 2D-Co3O4, and 3D-Co3O4 catalysts

Metal nanoparticles hold great promise for heterogeneous catalysis due to their high dispersion, large concentration of highly undercoordinated surface sites, and the presence of quantum confinement effects, which can drastically alter their reactivity. However, the poor thermal stability of nano-sized particles limits their use to low temperature conditions and constitutes one of the key hurdles towards industrial application. The present perspective paper briefly reviews the mechanisms underlying nanoparticle sintering, and then gives an overview of emerging approaches towards stabilizing metal nanoparticles for heterogeneous catalysis. We conclude by highlighting the current needs for further developments in the field.

Stabilizing metal nanoparticles for heterogeneous catalysis

We report here a detailed study on the surface topology of well-known ordered mesoporous silica (SBA-15, MCM-41, and KIT-6) and a series of nanocast Co3O4, Co3O4/CoFe2O4 composites by high resolution scanning electron microscopy (HR-SEM). Images of the MCM-41 structure were obtained at a resolution of the pore size, as well as a real space image of the gyroid silica surface of KIT-6 for two different aging temperatures, clearly revealing the differences of the aging procedures. By using the low voltage HR-SEM technique with extremely high resolution, we could very clearly show the influence of the template properties on the structure of the nanocast metal oxides.

Direct Imaging of Surface Topology and Pore System of Ordered Mesoporous Silica (MCM-41, SBA-15, and KIT-6) and Nanocast Metal Oxides by High Resolution Scanning Electron Microscopy

Advances in scanning electron microscopy (SEM) enable the high-resolution imaging of single nanoparticles (NPs) with sizes well below 10 nm. The SEM analysis in transmission mode (T-SEM) of NPs on thin film supports has many benefits when compared to the analysis of NPs on bulk substrates. The enhanced material (mass - thickness) contrast of the T-SEM imaging mode is well suited for in-depth and, particularly valuable, to very accurate, traceable, lateral dimensional measurements of NPs. Compared to samples prepared on bulk substrates, T-SEM with energy dispersive X-ray spectroscopy (EDS) achieves a drastically improved spatial resolution of the emitted X-rays. The poor signal-to-noise ratio of the X-ray spectra emitted by a single nanoparticle (NP) can be improved by the use of high-sensitivity (high collection solid angle) silicon drift (SDD), energy-dispersive X-ray spectrometers (EDS). The EDS spectral imaging of a single NP with a spatial resolution below 10 nm has become possible. This is demonstrated by means of various examples of nanostructures. Advanced data processing of T-SEM/EDS results sets the stage for the automated classification of NPs by feature analysis. This method combines the detection of morphological structures of interest by image processing of T-SEM micrographs with the chemical classification by EDS.

https://iopscience.iop.org/article/10.1088/1757-899X/109/1/012006/pdf

Characterisation of nanoparticles by means of high-resolution SEM/EDS in transmission mode

The histidine−metallochelating lipid complex is one of the smallest high affinity binding units used as tools for rapid noncovalent binding of histidine tagged molecules, especially recombinant proteins. The advantage of metallochelating complex over protein−ligand complexes (e.g., streptavidine−biotin, glutathiontransferase−glutathion) consists in its very low immunogenicity, if any. This concept for the construction of surface-modified metallochelating microbubbles was proved with recombinant green fluorescent protein (rGFP) containing 6His-tag. This protein is easy to be detected by various fluorescence techniques as flow cytometry and confocal microscopy. Microbubbles (MB) composed of DPPC with various contents of metallochelating lipid DOGS-NTA-Ni were prepared by intensive shaking of the liposome suspension under the atmosphere of sulfur hexafluoride. For this purpose, the instrument 3M ESPE CapMix was used. Various techniques (static light scattering, flow cytometry, and optical microscopy) were co...

Roland Schmidt

Papers

Direct Imaging of Surface Topology and Pore System of Ordered Mesoporous Silica (MCM-41, SBA-15, and KIT-6) and Nanocast Metal Oxides by High Resolution Scanning Electron Microscopy

Characterisation of nanoparticles by means of high-resolution SEM/EDS in transmission mode

Preparation of metallochelating microbubbles and study on their site-specific interaction with rGFP-HisTag as a model protein.