Anwar Ahniyaz

Dispersion and surface functionalization of oxide nanoparticles for transparent photocatalytic and UV-protecting coatings and sunscreens

Abstract This review describes recent efforts on the synthesis, dispersion and surface functionalization of the three dominating oxide nanoparticles used for photocatalytic, UV-blocking and sunscreen applications: titania, zinc oxide, and ceria. The gas phase and liquid phase synthesis is described briefly and examples are given of how weakly aggregated photocatalytic or UV-absorbing oxide nanoparticles with different composition, morphology and size can be generated. The principles of deagglomeration are reviewed and the specific challenges for nanoparticles highlighted. The stabilization of oxide nanoparticles in both aqueous and non-aqueous media requires a good understanding of the magnitude of the interparticle forces and the surface chemistry of the materials. Quantitative estimates of the Hamaker constants in various media and measurements of the isoelectric points for the different oxide nanoparticles are presented together with an overview of different additives used to prepare stable dispersions. The structural and chemical requirements and the various routes to produce transparent photocatalytic and nanoparticle-based UV-protecting coatings, and UV-blocking sunscreens are described and discussed.

Magnetic field-induced assembly of oriented superlattices from maghemite nanocubes

Tailoring the structure of nanocrystal superlattices is an important step toward controlled design of novel nanostructured materials and devices. We demonstrate how the long-range order and macroscopic dimensions of magnetic nanoparticle arrays can be controlled by the use of a modulated magnetic field. Inducing a dipolar attraction during the initial stage of the drying-mediated self-assembly process was sufficient to assemble the superparamagnetic oleate-capped maghemite nanocubes into large and defect-free superstructures with both translational and orientational order. The characteristic dimensions of the superlattice are controlled by the particle concentration as well as the duration of the applied magnetic field. The superparamagnetic maghemite nanocubes assemble into large and highly oriented thin arrays by applying the magnetic field perpendicular to the substrate surface only during the initial phase of drying-mediated self-assembly. Micrometer-sized and thick three-dimensional mesocrystals are obtained when the drying dispersion is subjected to an external magnetic field of moderate strength for the entire duration of the assembly process. The discovery of how translational and orientation order of nanocrystal superlattices can be induced by a temporal modulation of an anisotropic interparticle force offers new insight on the importance of the initial nucleation stage in the self-assembly process and suggests new routes for controlled self-assembly of dipolar nanocrystals.

Low temperature preparation of β-LiFe5O8 fine particles by hydrothermal ball milling

Abstract Lithium ferrite, β-LiFe 5 O 8 , fine particles with a 100–120 nm size were successfully prepared by hydrothermal ball milling at the low temperature of 170 °C for 2 h without any post annealing. The lithium ferrite fine particle formation was confirmed by characterization using XRD, SEM, XPS and magnetization measurements. The experimental results strongly support the idea that ball milling can accelerate the reaction not only between the solid and solid phases but also between the solid and liquid phases. We believe hydrothermal ball milling can open a new era for the low temperature synthesis and fabrication of homogeneous fine particles both in composition and size.

Low-temperature hydrothermal synthesis of spinel-type lithium manganese oxide nanocrystallites

Abstract A mild hydrothermal method has been developed to synthesize spinel-type lithium manganese oxide nanocrystallites directly from commercial LiOH, Mn(NO 3 ) 2 and H 2 O 2 at 90–110 °C for 8 h. The as-prepared products were characterized by X-ray diffraction (XRD), inductively coupled plasma–atomic emission spectroscopy and chemical analysis. The analysis results indicated that spinel-type lithium manganese oxides with the compositions of Li 1+ x Mn 2− x O 4− δ (0.09≤ x ≤0.28, δ 1.28 Mn 1.72 O 3.99 were investigated by thermogravimetric analysis (TG) and differential thermal analysis (DTA).

Precise control over shape and size of iron oxide nanocrystals suitable for assembly into ordered particle arrays

Abstract Here we demonstrate how monodisperse iron oxide nanocubes and nanospheres with average sizes between 5 and 27 nm can be synthesized by thermal decomposition. The relative importance of the purity of the reactants, the ratio of oleic acid and sodium oleate, the maximum temperature, and the rate of temperature increase, on robust and reproducible size and shape-selective iron oxide nanoparticle synthesis are identified and discussed. The synthesis conditions that generate highly monodisperse iron oxide nanocubes suitable for producing large ordered arrays, or mesocrystals are described in detail.

Freshwater dispersion stability of PAA-stabilised cerium oxide nanoparticles and toxicity towards Pseudokirchneriella subcapitata

An aqueous dispersion of poly (acrylic acid)-stabilised cerium oxide (CeO₂) nanoparticles (PAA-CeO₂) was evaluated for its stability in a range of freshwater ecotoxicity media (MHRW, TG 201 and M7), with and without natural organic matter (NOM). In a 15 day dispersion stability study, PAA-CeO₂ did not undergo significant aggregation in any media type. Zeta potential varied between media types and was influenced by PAA-CeO₂ concentration, but remained constant over 15 days. NOM had no influence on PAA-CeO₂ aggregation or zeta potential. The ecotoxicity of the PAA-CeO₂ dispersion was investigated in 72 h algal growth inhibition tests using the freshwater microalgae Pseudokirchneriella subcapitata. PAA-CeO₂ EC₅₀ values for growth inhibition (GI; 0.024 mg/L) were 2-3 orders of magnitude lower than pristine CeO₂ EC₅₀ values reported in the literature. The concentration of dissolved cerium (Ce(3+)/Ce(4+)) in PAA-CeO₂ exposure suspensions was very low, ranging between 0.5 and 5.6 μg/L. Free PAA concentration in the exposure solutions (0.0096-0.0384 mg/L) was significantly lower than the EC10 growth inhibition (47.7 mg/L) value of pure PAA, indicating that free PAA did not contribute to the observed toxicity. Elemental analysis indicated that up to 38% of the total Cerium becomes directly associated with the algal cells during the 72 h exposure. TOF-SIMS analysis of algal cell wall compounds indicated three different modes of action, including a significant oxidative stress response to PAA-CeO₂ exposure. In contrast to pristine CeO₂ nanoparticles, which rapidly aggregate in standard ecotoxicity media, PAA-stabilised CeO₂ nanoparticles remain dispersed and available to water column species. Interaction of PAA with cell wall components, which could be responsible for the observed biomarker alterations, could not be excluded. This study indicates that the increased dispersion stability of PAA-CeO₂ leads to an increase in toxicity compared to pristine non-stabilised forms.

Aminopolycarboxylic acids as a versatile tool to stabilize ceria nanoparticles – a fundamental model experimentally demonstrated

An extremely stable water dispersion of cerium oxide nanoparticles was prepared by colloidal synthesis, using nitrilotriacetic acid (NTA) as a stabilizer. Based on FT-IR measurements, the surface characteristics of NTA-stabilized ceria nanoparticles are clarified and a fundamental stabilization mechanism is proposed. The mechanism is based on the combination of the ionic nature of cerium oxide surface and the inner-sphere complexation model. From an application perspective it is remarkable that ceria nanoparticle dispersions stabilized by NTA are stable at neutral pH, which makes them a potential successful additive in UV screening applications.

Polymer/Iron Oxide Nanoparticle Composites—A Straight Forward and Scalable Synthesis Approach

Magnetic nanoparticle systems can be divided into single-core nanoparticles (with only one magnetic core per particle) and magnetic multi-core nanoparticles (with several magnetic cores per particle). Here, we report multi-core nanoparticle synthesis based on a controlled precipitation process within a well-defined oil in water emulsion to trap the superparamagnetic iron oxide nanoparticles (SPION) in a range of polymer matrices of choice, such as poly(styrene), poly(lactid acid), poly(methyl methacrylate), and poly(caprolactone). Multi-core particles were obtained within the Z-average size range of 130 to 340 nm. With the aim to combine the fast room temperature magnetic relaxation of small individual cores with high magnetization of the ensemble of SPIONs, we used small (<10 nm) core nanoparticles. The performed synthesis is highly flexible with respect to the choice of polymer and SPION loading and gives rise to multi-core particles with interesting magnetic properties and magnetic resonance imaging (MRI) contrast efficacy.

Hybrid acrylic/CeO2 nanocomposites using hydrophilic spherical and high aspect ratio CeO2 nanoparticles

A dispersion of CeO2 nanoparticles and nanorods stabilized with nitrilotriacetic acid (NTA) and a 4,4′-azobis(4-cyanovaleric acid) (V-501) initiator has been used to initiate the emulsion polymerization of acrylic monomers, yielding stable hybrid CeO2 nanoparticle-nanorod/polyacrylate latexes for the first time. Films cast from these hybrid latexes are transparent due to the very homogenous distribution of the polymer compatibilized CeO2. Furthermore, it has been proven that the UV-Vis absorption capacity of the hybrid latexes is enhanced with the incorporation of the nanorods.

Mesoporous Solid Carrier Particles in Controlled Delivery and Release

Microencapsulation of active ingredients can be realized in a multitude of different ways. Frequently, the microencapsulation carrier is tailored to the active ingredient. In this chapter, we describe how a mesoporous carrier material can be used for controlled delivery and release of different active ingredients. The loading strategies related to the properties of the active ingredient are described. Methods for analyzing the release of the active ingredients are described and related to the properties of the active ingredient and the release media. The effects of loading method, solvent choice, and the characteristics of the active on the loading capacity and the release behavior are further discussed.