Universal scattering behavior of coassembled nanoparticle-polymer clusters

Adsorption of Organic Dyes on Magnetic Iron Oxide Nanoparticles. Part I: Mechanisms and Adsorption-Induced Nanoparticle Agglomeration

This series of two papers is devoted to the effect of organic dye (methylene blue, MB; or methyl orange, MO) adsorption on the surface of either bare or citrate-coated magnetic iron oxide nanoparticles (IONPs) on their primary agglomeration (in the absence of an applied magnetic field) and secondary field-induced agglomeration. The present paper (Part I) is focused on physicochemical mechanisms of dye adsorption and adsorption-induced primary agglomeration of IONPs. Dye adsorption to oppositely charged IONPs is found to be mostly promoted by electrostatic interactions and is very sensitive to pH and ionic strength variations. The shape of adsorption isotherms is correctly reproduced by the Langmuir law. For the particular MB/citrated IONP pair, the maximum surface density of adsorbed MB seems to correspond to the packing density of an adsorbed monolayer rather than to the surface density of the available adsorption sites. MB is shown to form H-aggregates on the surface of citrate-coated IONPs. The effective electric charge on the IONP surface remains nearly constant in a broad range of surface coverages by MB due to the combined action of counterion exchange and counterion condensation. Primary agglomeration of IONPs (revealed by an exponential increase of hydrodynamic size with surface coverage by MB) probably comes from correlation attractions or π-stacking aromatic interactions between adsorbed MB molecules or H-aggregates. From the application perspective, the maximum adsorption capacity is 139 ± 4 mg/g for the MB/citrated IONP pair (pH = 4-11) and 257 ± 16 mg/g for the MO/bare IONP pair (pH ∼ 4). Citrated IONPs have shown a good potential for their reusability in water treatment, with the adsorption efficiency remaining about 99% after nine adsorption/desorption cycles.

Kinetics of Aggregation and Magnetic Separation of Multicore Iron Oxide Nanoparticles: Effect of the Grafted Layer Thickness

In this work, we have studied field-induced aggregation and magnetic separation-realized in a microfluidic channel equipped with a single magnetizable micropillar-of multicore iron oxide nanoparticles (IONPs) also called "nanoflowers" of an average size of 27 ± 4 nm and covered by either a citrate or polyethylene (PEG) monolayer having a thickness of 0.2⁻1 nm and 3.4⁻7.8 nm, respectively. The thickness of the adsorbed molecular layer is shown to strongly affect the magnetic dipolar coupling parameter because thicker molecular layers result in larger separation distances between nanoparticle metal oxide multicores thus decreasing dipolar magnetic forces between them. This simple geometrical constraint effect leads to the following important features related to the aggregation and magnetic separation processes: (a) Thinner citrate layer on the IONP surface promotes faster and stronger field-induced aggregation resulting in longer and thicker bulk needle-like aggregates as compared to those obtained with a thicker PEG layer; (b) A stronger aggregation of citrated IONPs leads to an enhanced retention capacity of these IONPs by a magnetized micropillar during magnetic separation. However, the capture efficiency Λ at the beginning of the magnetic separation seems to be almost independent of the adsorbed layer thickness. This is explained by the fact that only a small portion of nanoparticles composes bulk aggregates, while the main part of nanoparticles forms chains whose capture efficiency is independent of the adsorbed layer thickness but depends solely on the Mason number Ma. More precisely, the capture efficiency shows a power law trend Λ ∝ M a−n, with n ≈ 1.4⁻1.7 at 300 < Ma < 10⁴, in agreement with a new theoretical model. Besides these fundamental issues, the current work shows that the multicore IONPs with a size of about 30 nm have a good potential for use in biomedical sensor applications where an efficient low-field magnetic separation is required. In these applications, the nanoparticle surface design should be carried out in a close feedback with the magnetic separation study in order to find a compromise between biological functionalities of the adsorbed molecular layer and magnetic separation efficiency.

(Ce-Al)-oxide pillared bentonite: A high affinity sorbent for plutonium

The ability of bentonite and montmorillonite pillared by Al-oxide and mixed (Ln-Al)-oxides (Ln = La, Ce) to remove ²³⁹plutonium solution species from water is comparatively investigated at pH 7 and pH 4. Small-angle scattering and neutron contrast variation with H₂O/D₂O mixtures is used to verify the ingress of water in the calcined products after hydrophilicity was introduced by an NH₃-H₂O vapor treatment. The size and shape of the (La/Ce)-Al oxo-hydroxy pillaring cations (2 nm spheres) is determined by small-angle x-ray scattering from the pillaring solutions. Not all of the oxide pillars improved Pu uptake compared with sodium montmorillonite. At neutral and acidic pH only (Ce-Al)-oxide pillared clays showed the ability to remove Pu over the concentration range studied (1.35 × 10^-8-8 × 10^-8 mol dm^-3) with distribution coefficient (K_D) values >10⁴. XPS analysis of the (Ce-Al)-oxide pillared clays indicates the presence of Ce⁴⁺ as cerium dioxide. The progressive improvement in sorption performance in the order of pillar type Al₂O₃ < La₂O₃-Al₂O₃ << CeO₂-Al₂O₃ reflects the increasing access of Pu solution species to the clay mineral layers by changes to the basal spacing and specific surface area, and also to the higher stability of the (Ce-Al)-oxide pillars.

Introduction to soft matter and neutron scattering

As an opening lecture to the French-Swedish neutron scattering school held in Uppsala (6th to 9th of December 2016), the basic concepts of both soft matter science and neutron scattering are introduced. Typical soft matter systems like self-assembled surfactants in water, microemulsions, (co-)polymers, and colloids are presented. It will be shown that widely different systems have a common underlying physics dominated by the thermal energy, with astonishing consequences on their statistical thermodynamics, and ultimately rheological properties – namely softness. In the second part, the fundamentals of neutron scattering techniques and in particular small-angle neutron scattering as a powerful method to characterize soft matter systems will be outlined.

Extensive characterization of magnetic microrods observed using optical microscopy

The usage of micro or nanorods is steadily increasing in various applications from fundamental research to industry. Therefore their geometrical, mechanical and eventually magnetic properties need to be well determined. Here, using an optical microscope equipped with magnetic tweezers, we report an experimental procedure to obtain all those information on a single magnetic rod. In particular, we measure magnetic susceptibility χ by analyzing the deformation of a rod subjected to a uniform magnetic field. To do so, we refine a theoretical model which takes into account the variation of χ with the internal field. We prove experimentally that this model yields consistent measurements, at any value of the field strength and the incidence angle. From the combination of the different measurements, we also deduce the number of iron oxide nanoparticles which are embedded within the polymer matrix of the superparamagnetic rods under study.

Structural and multi-scale rheophysical investigation of diphasic magneto-sensitive materials based on biopolymers

We present a structural and a multi-scale rheophysical investigation of magneto-sensitive materials based on biopolymers, namely aqueous solutions of sodium alginate incorporating magnetic maghemite nanoparticles, functionalized with adsorbed negative citrate ions. The large alginate ionic strength impacts the structure and the rheology of these nanocomposites in zero magnetic field. In given physico-chemical conditions, the system is fluid and homogeneous on macroscopic scales while it is diphasic on microscopic ones, containing micro-droplets coming from the demixion of the system. These micro-droplets are liquid and deformable under magnetic field. Their under-field elongation and their zero-field relaxation are directly observed by optical microscopy to determine their interfacial tension, their magnetic susceptibility and their internal viscosity. A structural analysis of the solutions of alginate chains and of the phase-separated mixtures of alginate and nanoparticles by Small Angle Scattering completes the local description of the system.

Ligand-dominated temperature dependence of agglomeration kinetics and morphology in alkyl-thiol-coated gold nanoparticles

The stability of nanoparticle suspensions and the details of their agglomeration depend on the interactions between particles. We study this relationship in gold nanoparticles stabilized with different alkyl thiols in heptane. Temperature-dependent interactions were inferred from small-angle x-ray scattering, agglomeration kinetics from dynamic light scattering, and agglomerate morphologies from transmission electron microscopy. We find that the particles precipitate at temperatures below the melting temperatures of the dry ligands. Agglomerates grow with rates that depend on the temperature: Around precipitation temperature, globular agglomerates form slowly, while at lower temperatures, fibrilar agglomerates form rapidly. All agglomerates contain random dense packings rather than crystalline superlattices. We conclude that ligand-ligand and ligand-solvent interactions of the individual particles dominate suspension stability and agglomeration kinetics. The microscopic packing is dominated by interactions between the ligands of different nanoparticles.

Controlling electrostatic co-assembly using ion-containing copolymers: from surfactants to nanoparticles

In this review, we address the issue of the electrostatic complexation between charged-neutral diblock copolymers and oppositely charged nanocolloids. We show that nanocolloids such as surfactant micelles and iron oxide magnetic nanoparticles share similar properties when mixed with charged-neutral diblocks. Above a critical charge ratio, core-shell hierarchical structures form spontaneously under direct mixing conditions. The core-shell structures are identified by a combination of small-angle scattering techniques and transmission electron microscopy. The formation of multi-level objects is driven by the electrostatic attraction between opposite charges and by the release of the condensed counterions. Alternative mixing processes inspired from molecular biology are also described. The protocols applied here consist in screening the electrostatic interactions of the mixed dispersions, and then removing the salt progressively as an example by dialysis. With these techniques, the oppositely charged species are intimately mixed before they can interact, and their association is monitored by the desalting kinetics. As a result, sphere- and wire-like aggregates with remarkable superparamagnetic and stability properties are obtained. These findings are discussed in the light of a new paradigm which deals with the possibility to use inorganic nanoparticles as building blocks for the design and fabrication of supracolloidal assemblies with enhanced functionalities.

Influence of annealing on structure and optical properties of Mn-substituted ZnO nanoparticles

The influence of annealing on crystal growth and the micro- and mesoscopic structures of Mn-substituted nanocrystalline zinc oxide (ZnO) has been investigated using X-ray diffraction, high-resolution transmission electron microscopy and small-angle neutron scattering (SANS). Average particle sizes and their distributions have been estimated from scattering experiments as well as a microscopy study, and found to be in the nanometre range. The SANS study indicates that the fractal dimension, which describes the nature of the agglomerate, is almost unchanged up to an annealing temperature of 1023 K. However, at 1223 K, the fractal dimension increases to 3. An attempt has also been made to understand the influence of the annealing temperature on the growth of the structural morphology of the aggregates by performing a simulation based on a diffusion-limited aggregate model. By showing the variation of the band gap estimated from UV–visible absorption spectra with particle size, an attempt has been made to establish a structure–property correlation.

Tuning of nanoparticle-surfactant interactions in aqueous system

The interaction of charged (anionic) silica nanoparticles with ionic and nonionic surfactants has been studied using small-angle neutron scattering (SANS). The surfactants used are anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethyl ammonium bromide (DTAB) and nonionic decaoxyethylene n-dodecylether (C(12)E(10)). The measurements are carried out at fixed concentration (1 wt%) of silica nanoparticles and with surfactant concentration varied in the range 0-2 wt%. It is found that there is no direct interaction between the nanoparticles and the surfactant (SDS) when they both are similarly charged. Both the silica nanoparticles and micelles coexist individually with no significant change in the structure of the micelles with respect to that in the pure surfactant solution. On the other hand, the presence of oppositely charged surfactant (DTAB) leads to the aggregation of silica nanoparticles even with very low surfactant concentration. The aggregation of silica nanoparticles is characterized by fractal structure and its fractal dimension remains constant with the increase in the surfactant concentration. In the case of nonionic surfactant, it interacts with the individual silica nanoparticles. The interaction is examined using two models: one that considers the surfactant layer coating on silica nanoparticles and a second one where the surface of the nanoparticles is decorated by the micelles. Contrast variation SANS measurements confirm the uniform decoration of nonionic micelles on the nanoparticles.

Buckling-driven morphological transformation of droplets of a mixed colloidal suspension during evaporation-induced self-assembly by spray drying

Morphological transformation during evaporation-induced self-assembly of a mixed colloidal suspension in micrometric droplets has been investigated. It has been demonstrated that a buckling-driven shape transition of drying droplets of mixed colloidal suspension takes place during evaporation-induced self-assembly. Further, it is also shown that the distortion modulations get significantly amplified with enhancement in volume fraction of anisotropic soft colloidal component of the mixed colloids. It has been argued that the reduction in elastic modulus of formed shell, at the boundary of a drying droplet, and the anisotropic nature of one of the colloidal components facilitate the deformation process. Hierarchical structures of these assembled colloidal grains have been probed using electron microscopy and scattering techniques.

Microfluidics with on-line dynamic light scattering for size measurements

We present a detailed investigation on the feasibility of on-line dynamic light scattering measurements of colloidal sizes in a pressure-driven microfluidic flow. We review some theoretical arguments showing that such experiments are difficult to perform due to the Poiseuille flow that induces interferences of different Doppler shifts. Such a theoretical approach is however very useful to figure out the range of parameters where on-line size measurements are possible. We then build a dynamic light scattering setup around a microfluidic chip that enables us to estimate the size of Brownian scatterers flowing in PDMS-based microchannels, thus validating experimentally the theoretical estimations. We finally present a microfluidic chip that can mix two reactants in approximately 200 ms, and allows size measurements using dynamic light scattering at about 300 ms after complete mixing. Two applications are presented: the continuous monitoring of the viscosity of a two-fluid mixture, and the electrostatic co-assembly of oppositely charged nanoparticles and block copolymers.

Superparamagnetic magnetite nanocrystal clusters: a sensitive tool for MR cellular imaging

Silica coated, PEI and citric acid hybrid superparamagnetic magnetite nanocrystal clusters (SMNC) were synthesized using either a mini-emulsion/sol-gel method or a polyol technique. After careful characterization of the size, structure, composition, and magnetic properties, the as-synthesized SMNC were used for cell labeling while the MR detection sensitivity of cells labeled with silica SMNC was performed with a 3 T whole body MR scanner. TEM investigations revealed that the sizes of the SMNC were about 200 nm and the SMNC mainly consisted of magnetite nanoparticles imbedded in a PEI, citric acid or polystyrene scaffold. Silica and citric acid SMNC were highly negatively charged and PEI SMNC were positively charged. Relaxometry measurements revealed that these SMNC possessed a very high MR sensitivity (silica SMNC: r(2) = 299 s(-1) mM(-1), PEI SMNC: r(2) = 124 s(-1) mM(-1)), especially for the citric acid SMNC (r(2) = 360 s(-1) mM(-1)). Furthermore, when used for cell (RAW264.7 cells) labeling, the SMNC had no adverse effect on cell viability, and the cell uptake of the SMNC show a dose- and time-dependent feature. MR imaging of cells labeled with silica SMNC indicated that cells with a concentration as low as 10 x 10(3) cells ml(-1) could be detected with a 3 T MRI scanner. Our study demonstrated that superparamagnetic magnetite nanocrystal clusters are a sensitive tool for cell imaging.