Steffi Rades

On the role of surface composition and curvature on biointerface formation and colloidal stability of nanoparticles in a protein-rich model system.

The need for a better understanding of nanoparticle-protein interactions and the mechanisms governing the resulting colloidal stability has been emphasised in recent years. In the present contribution, the short and long term colloidal stability of silica nanoparticles (SNPs) and silica-poly(ethylene glycol) nanohybrids (Sil-PEG) have been scrutinised in a protein model system. Well-defined silica nanoparticles are rapidly covered by bovine serum albumin (BSA) and form small clusters after 20min while large agglomerates are detected after 10h depending on both particle size and nanoparticle-protein ratio. Oppositely, Sil-PEG hybrids present suppressive protein adsorption and enhanced short and long term colloidal stability in protein solution. No critical agglomeration was found for either system in the absence of protein, proving that instability found for SNPs must arise as a consequence of protein adsorption and not to high ionic environment. Analysis of the small angle X-ray scattering (SAXS) structure factor indicates a short-range attractive potential between particles in the silica-BSA system, which is in good agreement with a protein bridging agglomeration mechanism. The results presented here point out the importance of the nanoparticle surface properties on the ability to adsorb proteins and how the induced or depressed adsorption may potentially drive the resulting colloidal stability.

High-resolution imaging with SEM/T-SEM, EDX and SAM as a combined methodical approach for morphological and elemental analyses of single engineered nanoparticles

The combination of complementary characterization techniques such as SEM (Scanning Electron Microscopy), T-SEM (Scanning Electron Microscopy in Transmission Mode), EDX (Energy Dispersive X-ray Spectroscopy) and SAM (Scanning Auger Microscopy) has been proven to be a powerful and relatively quick characterization strategy for comprehensive morphological and chemical characterization of individual silica and titania nanoparticles. The selected “real life” test materials, silica and titania, are listed in the OECD guidance manual as representative examples because they are often used as commercial nanomaterials. Imaging by high resolution SEM and in the transmission mode by T-SEM allows almost simultaneous surface and in-depth inspection of the same particle using the same instrument. EDX and SAM enable the chemical characterization of bulk and surface of individual nanoparticles. The core–shell properties of silica based materials are addressed as well. Titania nominally coated by silane purchased from an industrial source has been found to be inhomogeneous in terms of chemical composition.

Sample loss in asymmetric flow field-flow fractionation coupled to inductively coupled plasma-mass spectrometry of silver nanoparticles

In this work, sample losses of silver nanoparticles (Ag NPs) in asymmetrical flow field-flow fractionation (AF4) have been systematically investigated with the main focus on instrumental conditions like focusing and cross-flow parameters as well as sample concentration and buffer composition. Special attention was drawn to the AF4 membrane. For monitoring possible silver depositions on the membrane, imaging laser ablation coupled to inductively coupled plasma mass spectrometry (LA-ICP-MS) was used. Our results show that the sample residue on the membrane was below 0.6% of the total injected amount and therefore could be almost completely avoided at low sample concentrations and optimized conditions. By investigation of the AF4 flows using inductively coupled plasma mass spectrometry (ICP-MS), we found the recovery rate in the detector flow under optimized conditions to be nearly 90%, while the cross-flow, slot-outlet flow and purge flow showed negligible amounts of under 0.5%. The analysis of an aqueous ionic Ag standard solution resulted in recovery rates of over 6% and the ionic Ag content in the sample was found to be nearly 8%. Therefore, we were able to indicate the ionic Ag content as the most important source of sample loss in this study.

Complementary methodical approach for the analysis of a perovskite solar cell layered system

Loss in efficiency of perovskite solar cells may be caused by structural and/or chemical alterations of the complex layered system. As these changes might take place either in the bulk and/or on the surface of the stratified material, analytical tools addressing both key issues are selected and combined. SEM/EDX combined with XPS were chosen as appropriate methodical approach to characterise perovskite laboratory cells in depth and complementary on top, before and after light exposure. The layered perovskite system investigated here is based on glass covered with fluorine doped tin oxide (FTO), followed by three porous thin films of TiO2, ZrO2 and a thick monolithic carbon. The TiO2 film is subdivided into a dense layer covered by a porous one constituted of nanoparticles with a truncated bipyramidal shape. This layered system serves as the matrix for the perovskite. After infiltration of perovskite solution and annealing, EDX spectral maps on cross-sections of the specimen have been measured. The distribution of relevant elements – Si, Sn, Ti, Zr and C – correlates conclusively with layers visible in the acquired SEM images. Lead and iodine are distributed throughout the porous layers C, ZrO2 and TiO2. In a SEM micrograph taken of the cross-section of a sample after illumination, the glass substrate and all layers FTO, TiO2, ZrO2 as well as C are clearly identified. By EDX it was found that several weeks of ambient daylight did not change significantly the qualitative elemental composition of lead and iodine throughout the solar cell system. It was confirmed with EDX that nanoparticles identified in high-resolution SEM micrographs contain mainly Pb and I, indicating these to be the perovskite crystals. However, a time-dependent compositional and chemical altering was observed with XPS for the near-surface region of the outermost ~10 nm after two months of illumination.

Improved Spatial Resolution of EDX/SEM for the Elemental Analysis of Nanoparticles

The interest in nanoparticles remains at a high level in fundamental research since many years and increasingly, nanoparticles are incorporated into consumer products to enhance their performance. Consequently, the accurate and rapid characterization of nanoparticles is more and more demanded. Electron microscopy (SEM, TSEM and TEM) is one of the few techniques which are able to image individual nanoparticles. It was demonstrated recently that the transmission electron microscopy at a SEM can successfully be applied as a standard method to characterize accurately the size (distribution) and shape of nanoparticles down to less than 10 nm [1, 2].

Beyond Shape Engineering of TiO2 Nanoparticles: Post-Synthesis Treatment Dependence of Surface Hydration, Hydroxylation, Lewis Acidity and Photocatalytic Activity of TiO2 Anatase Nanoparticles with Dominant {001} or {101} Facets

TiO2 anatase nanoparticles are among the relevant players in the field of light-responsive semiconductor nanomaterials used to face environmental and energy issues. In particular, shape-engineered TiO2 anatase nanosheets with dominant {001} basal facets gained momentum because of the possibility to exploit different and/or improved functional behaviors with respect to usual bipyramidal TiO2 anatase nanoparticles, mainly exposing {101} facets. Nevertheless, such behavior depends in a significant extent on the physicochemical features of surfaces exposed by nanosheets. They can vary in dependence on the presence or removal degree of capping agents, namely, fluorides, used for shape-engineering, and experimental investigations in this respect are still a few. Here we report on the evolution of interfacial/surface features of TiO2 anatase nanosheets with dominant {001} facets from pristine nanoparticles fluorinated both in the bulk and at their surface to nanoparticles with F– free surfaces by treatment in a ...

Organic surface modification and analysis of titania nanoparticles for self-assembly in multiple layers

The characteristics of TiO2 coatings can greatly influence their final performance. In the EU/FP7 project SETNanoMetro (http://www.setnanometro.eu/), different deposition procedures are being set for applying films of TiO2 nanoparticles (NPs) with defined and homogenous thickness on supports of interest for large-scale applications [1]. The selected substrates are the following: (i) silica glasses for photocatalytic measurements, (ii) Ti-alloys for orthopedic and/or dental prostheses, and for cell cultures, and (iii) conductive glasses (e.g. Fluorine doped Tin Oxide, FTO) for dye-sensitized solar cells (DSSC). From the different film deposition procedures, self-assembly [2] of TiO2 NPs in multiple layers was selected for systematic characterization. For this, surface modification of the substrate and of TiO2 NPs with e.g. silane coupling agents [3] is a prerequisite.

Analysis of fluorine traces in TiO2 nanoplatelets by SEM/EDX, AES and TOF-SIMS

Hydrothermal synthesis of anatase TiO2 nanosheets with a high fraction of exposed {001} facets and related high photocatalytic activity - as an alternative to bipyramidal anatase TiO2 nanoparticles mainly exposing the {101} facets. The scope of the material preparation work is the thermal reduction of residual fluorides from HF (capping agent) induced during the synthesis of TiO2 nanosheets by calcination at 873K. The analytical task consists of detection and localization of fluorine present at the surface and/or in the bulk of TiO2 nanosheets before and after calcination by SEM/EDX, Auger electron spectroscopy and ToF-SIMS.

Dispersion of Nanomaterials in Aqueous Media: Towards Protocol Optimization

The sonication process is commonly used for de-agglomerating and dispersing nanomaterials in aqueous based media, necessary to improve homogeneity and stability of the suspension. In this study, a systematic step-wise approach is carried out to identify optimal sonication conditions in order to achieve a stable dispersion. This approach has been adopted and shown to be suitable for several nanomaterials (cerium oxide, zinc oxide, and carbon nanotubes) dispersed in deionized (DI) water. However, with any change in either the nanomaterial type or dispersing medium, there needs to be optimization of the basic protocol by adjusting various factors such as sonication time, power, and sonicator type as well as temperature rise during the process. The approach records the dispersion process in detail. This is necessary to identify the time points as well as other above-mentioned conditions during the sonication process in which there may be undesirable changes, such as damage to the particle surface thus affecting surface properties. Our goal is to offer a harmonized approach that can control the quality of the final, produced dispersion. Such a guideline is instrumental in ensuring dispersion quality repeatability in the nanoscience community, particularly in the field of nanotoxicology.

Need for Large-Area EDS Detectors for Imaging Nanoparticles in a SEM Operating in Transmission Mode

Accurate characterization of nanoparticles (NPs) with a high-resolution SEM with respect to morphology, shape and size distribution is carried out meanwhile routinely in more and more laboratories [1,2]. The valuable result of the metrological measurement of the NP size distribution -including traceability to the SI length unit was also recently demonstrated by exploiting the transmission mode in a SEM, i.e. TSEM [1,3]. Especially for NPs prepared on thin foil supports like the typical TEM grids the transmission mode at low voltages (up to 30 kV) supplies a high imaging contrast so that sharp boarders of the NPs enable accurate lateral dimensional measurements of NPs size. There are just a few analytical methods which are able to characterize physico-chemical properties of individual NPs. In most cases a SEM has attached an EDS detector so that information on the elemental composition of the sample to be analyzed becomes possible. The addressed sample volume typically in the micrometer range for bulk specimens is considerably reduced if thin, electron transparent support films are used. Hence, one improves the spatial resolution however, at the cost of lower signal-to-noise ratios of the X-ray spectra, due to the tiny amount of substance excited [2,4,5]. One decisive technological development in the manufacturing of EDS detectors, namely the large-area SDD EDS, meanwhile available commercially in various designs, compensates this analytical drawback. If collection solid angles of 1-2 msr have been typically available for almost four decades, recent technological improvements in SDD EDS size and design have resulted in solid angles in the sr range.