The fascinating world of nanoparticle research

Nanoparticle research is a fascinating branch of science. The strongly size-related properties of nanoparticles offer uncountable opportunities for surprising discoveries. The often unexpected and unprecedented behavior of nanoparticles bears great potential for innovative technological applications, but also poses great challenges to the scientists. They have to develop highly controllable synthesis approaches, more sensitive characterization tools and finally new models and theories to explain the experimental observations. In this review, we discuss a personal selection of papers dedicated to nanoparticle research, which we believe provide an illustrative overview of current research directions in this rapidly developing field. We have structured the text in five sections: introduction, nanoparticle synthesis, formation mechanisms, nanoparticle assembly, and applications. The chosen examples within these sections are not directly related to each other, but reflect the remarkable broadness of nanoparticle research covering historical aspects, basic and applied science as well as commercial applications.ISSN:1369-7021ISSN:1873-410

Anatase-silica composite aerogels: a nanoparticle-based approach

Herein we present the synthesis of anatase–silica aerogels based on the controlled gelation of preformed nanoparticle mixtures. The monolithic aerogels with macroscopic dimensions show large specific surface areas, and high and uniform porosities. The major advantage of such a particle-based approach is the great flexibility in pre-defining the compositional and structural features of the final aerogels before the gelation process by fine-tuning the properties of the titania and silica building blocks (e.g., size, composition and crystallinity) and their relative ratio in the dispersion. Specific surface functionalization enables control over the interaction between the nanoparticles and thus over their distribution in the aerogel. Positively charged titania nanoparticles are co-assembled with negatively charged Stoeber particles, resulting in a binary aerogel with a crystalline anatase and amorphous silica framework directly after supercritical drying without any calcination step. Titania–silica aerogels combine the photocatalytic activity of the anatase nanoparticles with the extensive silica chemistry available for silica surface functionalization.ISSN:0928-0707ISSN:1573-484

Improved Nonaqueous Synthesis of TiO2 for Dye-Sensitized Solar Cells

Nonaqueous synthesis routes have emerged as a powerful platform for directly obtaining diverse metal oxide nanoparticles with high crystallinity and tunable compositions. The benzyl alcohol (BA) route, for example, has been applied toward dozens of oxides including binary, ternary, and even more complex multimetal systems. Here we compare anatase nanoparticles made from the BA route with the traditional hydrothermal route. XPS measurements Indicated that the BA route resulted in more reduced Ti states, corresponding to additional oxygen vacancies. These defects resulted in additional trap states, slower recombination, and slower charge transport. The performance of BA anatase was improved by incorporating niobium intended to suppress oxygen vacancies. The higher performance Nb-containing films were post-treated to yield a 7.96% power conversion efficiency (AM 1.5), similar to the state-of-the-art hydrolytic TiO2 in the same configuration

Colloidal silica nanoparticles for use in click chemistry-based conjugations and fluorescent affinity assays

Silica nanoparticles (SiNPs) with an average diameter of 25 nm were prepared by a one-pot method that involves the formation of a silica core that is subsequently covered with a shell functionalized with either azido groups or alkyne groups for use in copper(I)-catalyzed click reactions. Respective triethoxysilane reagents are presented. The SiNPs were (a) rendered fluorescent by click conjugation to fluorophores of various colors, and (b) made bioconjugatable by introducing maleimide groups (that covalently bind thiols) and biotin (a widely used bioaffinity reagent that binds streptavidin). Particles were characterized by transmission electron microscopy, infrared spectroscopy, fluorescence, and light scattering. The fluorescently labeled SiNPs carrying maleimido groups were conjugated to the thiol group of bovine serum albumin (BSA) labeled with a fluorophore, and fluorescence resonance energy transfer was shown to occur between the labeled SiNPs and the labeled BSA. This is considered to represent a new approach towards nanoparticle-based fluorescent bioassays

Novel RP-HPLC based assay for selective and sensitive endotoxin quantification

The paper presents a novel instrumental analytical endotoxin quantification assay. It uses common analytical laboratory equipment (HPLC-FLD) and allows quantifying endotoxins (ETs) in different matrices from about 109 EU / mL down to about 40 EU / mL (RSE based). Test results are obtained in concentration units (e.g. ng ET / mL), which can then be converted to commonly used endotoxin units (EU / mL) in case of known pyrogenic activity. During endotoxin hydrolysis, the endotoxin specific rare sugar acid KDO is obtained quantitatively. After that, KDO is stoichiometrically reacted with DMB, which results in a highly fluorescent derivative. The mixture is separated using RP-HPLC followed by KDO-DMB quantification with a fluorescence detector. Based on the KDO content, the endotoxin content in the sample is calculated. The developed assay is economic and has a small error. Its applicability was demonstrated in applied research. ETs were quantified in purified bacterial biopolymers, which were produced by Gram-negative bacteria. Results were compared to LAL results obtained for the same samples. A high correlation was found between the results of both methods. Further, the new assay was utilized with high success during the development of novel endotoxin specific depth filters, which allow efficient, economic and sustainable ET removal during DSP. Those examples demonstrate that the new assay has the potential to complement the animal-based biological LAL pyrogenic quantification tests, which are accepted today by the major health authorities worldwide for the release of commercial pharmaceutical products

Self-Assembly of Metal and Metal Oxide Nanoparticles and Nanowires into a Macroscopic Ternary Aerogel Monolith with Tailored Photocatalytic Properties

Self-assembly processes represent the most powerful strategy to produce complex materials with unique structural and compositional sophistication. Here we present such a self-assembly route to a three-component aerogel from preformed nanoparticle building blocks. Starting with a mixture of gold and anatase nanoparticles and tungsten oxide nanowires, controlled cogelation resulted in the formation of a macroscopic aerogel monolith with high specific surface area and porosity, remarkable transparency, and excellent crystallinity. The modular approach enables us to fine-tune the composition of the aerogels, and thus their properties, by choosing the appropriate building blocks and their relative concentration ratios. As an illustrative example, we show the targeted tailoring of the photocatalytic activity: the gold nanoparticles and the tungsten oxide nanowires both add their specific beneficial effects to the anatase aerogel matrix, leading to a superior performance of the three-component system

Size-Dependent Luminescence in HfO₂ Nanocrystals: Toward White Emission from Intrinsic Surface Defects

Defect engineering operated on metal oxides by chemical and structural modifications may strongly affect properties suitable for various applications such as photoelectrochemical behavior, charge transport, and luminescence. In this work, we report the tunable optical features observed in undoped monoclinic HfO₂ nanocrystals and their dependence on the structural properties of the material at the nanoscale. Transmission electron microscopy together with X-ray diffraction and surface area measurements were used to determine the fine structural modifications, in terms of crystal growth and coalescence of crystalline domains, occurring during a calcination process in the temperature range from 400 to 1000 °C. The fit of the broad optical emission into spectral components, together with time-resolved photoluminescence, allowed us to identify the dual nature of the emission at 2.5 eV, where an ultrafast defect-related intrinsic luminescence (with a decay time of a few nanoseconds) overlaps with a slower emission (decay of several microseconds) due to extrinsic Ti-impurity centers. Moreover, the evolution of intrinsic visible bands during the material transformation was monitored. The relationship between structural parameters uniquely occurring in nanosized materials and the optical properties was investigated and tentatively modeled. The blue emissions at 2.5 and 2.9 eV are clearly related to defects lying at crystal boundaries, while an unprecedented emission at 2.1 eV enables, at relatively low calcination temperatures, the white luminescence of HfO₂ under near-UV excitation

Nanoparticle-Based Magnetoelectric BaTiO₃–CoFe₂O₄ Thin Film Heterostructures for Voltage Control of Magnetism

Multiferroic composite materials combining ferroelectric and ferromagnetic order at room temperature have great potential for emerging applications such as four-state memories, magnetoelectric sensors, and microwave devices. In this paper, we report an effective and facile liquid phase deposition route to create multiferroic composite thin films involving the spin-coating of nanoparticle dispersions of BaTiO₃, a well-known ferroelectric, and CoFe₂O₄, a highly magnetostrictive material. This approach offers great flexibility in terms of accessible film configurations (co-dispersed as well as layered films), thicknesses (from 100 nm to several μm) and composition (5–50 wt % CoFe₂O₄ with respect to BaTiO₃) to address various potential applications. A detailed structural characterization proves that BaTiO₃ and CoFe₂O₄ remain phase-separated with clear interfaces on the nanoscale after heat treatment, while electrical and magnetic studies indicate the simultaneous presence of both ferroelectric and ferromagnetic order. Furthermore, coupling between these orders within the films is demonstrated with voltage control of the magnetism at ambient temperatures