Magnetically governed self-assembly of soft matter : A look into interfacial layering, crystallization and percolation

Self-organisation is the key route for assembling colloidal particles into well-defined structures. Decisive for this are the interactions between the constituents, which are amongst others steric, electrostatic or magnetic. A deep knowledge on the underlying physical processes during self-assembly is crucial for the design and fabrication of well-defined hierarchical architectures from a nanometer scale as well as for realizing smart, functional or stimuli responsive synthetic materials. In this dissertation, the self-assembly of colloidal magnetic particles into organized and multi-layered structures is studied. Particular emphasis is given to solid-liquid boundaries and the response to applied magnetic fields. Particle coatings with specific functional molecules stabilize the nanoparticles (NPs) in the solvent and can simultaneously promote their assembly at a substrate. An example in this context is N-hydroxysuccinimide interacting with (3-aminopropyl)triethoxy silane at the substrate. As a result of this chemical affinity, uniform and densely packed particle wetting layers are seeded which then instate the layering process. As an alternative to chemical binding, the magnetic stray field of a ferrimagnetic (Tb15Co85 film) deposited on a substrate induces particle self-assembly with dense layers as well. The application of an external magnetic field further promotes densification, particle layering and leads to variations in the assembly characteristics such as quasi-domain formation of closely packed layers. At an interface with a magnetic field applied in the plane of the interface, Brownian motion and Neel relaxation of the NPs are decisive for the layering and give raise to these domains. For a magnetic field oriented along the surface normal similar structural layering but denser packing is found. The self-assembly is a relatively slow process and evolves over hours and is maximized, most ordered and dense for superparamagnetic NPs which are single domain and having a large remanent moment and reduced thermal mobility. Small quantities of magnetic micelles in a hybrid magnetic polymer nanocomposite, facilitate the crystallization of Pluronic F127 micelles dissolved in water into single crystalline structures via a micro-shear effect under applied magnetic field. Also, a magnetic field applied to a colloidal dispersion of conducting magnetic and non-magnetic polystyrene microbeads suspended in an oil-based ferrofluid can lead to percolated structures. This allows current transmission and switching. A working contact for possible applications in automotive, switchboard and telecommunications is demonstrated

Magnetically governed self-assembly of soft matter : A look into interfacial layering, crystallization and percolation

Self-organisation is the key route for assembling colloidal particles into well-defined structures. Decisive for this are the interactions between the constituents, which are amongst others steric, electrostatic or magnetic. A deep knowledge on the underlying physical processes during self-assembly is crucial for the design and fabrication of well-defined hierarchical architectures from a nanometer scale as well as for realizing smart, functional or stimuli responsive synthetic materials. In this dissertation, the self-assembly of colloidal magnetic particles into organized and multi-layered structures is studied. Particular emphasis is given to solid-liquid boundaries and the response to applied magnetic fields. Particle coatings with specific functional molecules stabilize the nanoparticles (NPs) in the solvent and can simultaneously promote their assembly at a substrate. An example in this context is N-hydroxysuccinimide interacting with (3-aminopropyl)triethoxy silane at the substrate. As a result of this chemical affinity, uniform and densely packed particle wetting layers are seeded which then instate the layering process. As an alternative to chemical binding, the magnetic stray field of a ferrimagnetic (Tb15Co85 film) deposited on a substrate induces particle self-assembly with dense layers as well. The application of an external magnetic field further promotes densification, particle layering and leads to variations in the assembly characteristics such as quasi-domain formation of closely packed layers. At an interface with a magnetic field applied in the plane of the interface, Brownian motion and Neel relaxation of the NPs are decisive for the layering and give raise to these domains. For a magnetic field oriented along the surface normal similar structural layering but denser packing is found. The self-assembly is a relatively slow process and evolves over hours and is maximized, most ordered and dense for superparamagnetic NPs which are single domain and having a large remanent moment and reduced thermal mobility. Small quantities of magnetic micelles in a hybrid magnetic polymer nanocomposite, facilitate the crystallization of Pluronic F127 micelles dissolved in water into single crystalline structures via a micro-shear effect under applied magnetic field. Also, a magnetic field applied to a colloidal dispersion of conducting magnetic and non-magnetic polystyrene microbeads suspended in an oil-based ferrofluid can lead to percolated structures. This allows current transmission and switching. A working contact for possible applications in automotive, switchboard and telecommunications is demonstrated

Magnetic Particle Self-Assembly at Functionalized Interfaces

We study the assembly of magnetite nanoparticles in water-based ferrofluids in wetting layers close to silicon substrates with different functionalization without and with an out-of-plane magnetic field. For particles of nominal sizes 5, 15, and 25 nm, we extract density profiles from neutron reflectivity measurements. We show that self-assembly is only promoted by a magnetic field if a seed layer is formed at the silicon substrate. Such a layer can be formed by chemisorption of activated N-hydroxysuccinimide ester-coated nanoparticles at a (3-aminopropyl)triethoxysilane functionalized surface. Less dense packing is reported for physisorption of the same particles at a piranha-treated (strongly hydrophilic) silicon wafer, and no wetting layer is found for a self-assembled monolayer of octadecyltrichlorosilane (strongly hydrophobic) at the interface. We show that once the seed layer is formed and under an out-of-plane magnetic field further wetting layers assemble. These layers become denser with time, larger magnetic fields, higher particle concentrations, and larger moment of the nanoparticles

Self-Assembly of Magnetic Nanoparticles in Ferrofluids on Different Templates Investigated by Neutron Reflectometry

In this article we review the process by which magnetite nanoparticles self-assemble onto solid surfaces. The focus is on neutron reflectometry studies providing information on the density and magnetization depth profiles of buried interfaces. Specific attention is given to the near-interface "wetting" layer and to examples of magnetite nanoparticles on a hydrophilic silicon crystal, one coated with (3-Aminopropyl)triethoxysilane, and finally, one with a magnetic film with out-of-plane magnetization

Axonic Au Tips Induced Enhancement in Raman Spectra and Biomolecular Sensing

International audienceAxonic Au stars with sharp crystalline tips were synthesized via reduction of HAuCl4 capped with cetyl trimethylammonium bromide (CTAB). A seed-mediated method was adopted in which Au seeds were used in the growth solution containing Au+ ions, for initiating the nucleation with CTAB and with ascorbic acid acting as a mild reducing agent. Transmission Electron Microscopy image clearly reveals the synthesis of gold stars. The gold star dispersions display a well-defined optical response as observed by the presence of a sharp surface plasmon resonance centered at 525 nm. The Au stars functionalized with 11-mercaptoundecanoic acid and immobilized with urease showed improved urea-sensing behavior in comparison to pure gold disk in attenuated total reflection surface plasmon resonance (ATR SPR) mode. Surface-enhanced Raman spectroscopy performed using the functionalized Au stars showed a significant enhancement in Raman signals for a wider range of urea concentration (2–20 mg/ml)

Layering of magnetic nanoparticles at amorphous magnetic templates with perpendicular anisotropy

We reveal the assembly of magnetite nanoparticles of sizes 5 nm, 15 nm and 25 nm from dilute water-based ferrofluids onto an amorphous magnetic template with out-of-plane anisotropy. From neutron reflectometry experiments we extract density profiles and show that the particles self-assemble into layers at the magnetic surface. The layers are extremely stable against cleaning and rinsing of the substrate. The density of the layers is determined by and increases with the remanent magnetic moment of the particles.Changes in author lineup since manuscript version in thesis: Sebastian George added, Max Wolff now last author.</p

Synthesis and SERS application of SiO2@Au nanoparticles

International audienceIn this letter, we report a chemical route for synthesizing SiO2@Au core-shell nanoparticles. The process includes four steps: i) preparation of the silica cores, ii) grafting gold nanoparticles over SiO2 cores, iii) priming of the silica-coated gold nanoparticles with 2 and 10 nm gold colloids and finally iv) formation of complete shell. The optical extinction spectra were experimentally measured and compared to numerical calculations in order to confirm the dimensions deduced from SEM images. Finally, the potential of such core-shell nanoparticles for biosensing was probed by means of Surface Enhanced Raman Scattering measurements and revealed higher sensitivities with much lower gold quantity of such core-shell nanoparticles compared to Au nanoparticles exhibiting similar diameters