Magnetic Particle Self-Assembly at Functionalized Interfaces.

Apurve Saini,Kathryn L. Krycka,Katharina Theis-Bröhl,Alexandros Koutsioubas,Max Wolff,Julie A. Borchers

doi:10.1021/acs.langmuir.0c03235

Abstract

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.

Highlights

The formation of ordered nanoparticle (NP) structures can be realized by self-assembly
One interesting class of materials in this context are magnetic nanoparticles (NPs) dispersed in a solvent, since they can self-assemble and are responsive to external stimuli. This enables a range of applications such as magnetic sealing and magnetic memory or in biomedicine.[5−7] These applications make use of the ability of colloidal magnetic NPs to form structures such as linear or branched chains, clusters, or rings in an applied magnetic field.[8−10] Similar applications are considered for thin films of magnetic NPs with the additional advantage that the self-assembling structure can be prepatterned and grown from a substrate
We have investigated the assembly of 11 nm Fe3O4 particles dispersed in D2O/H2O at a SiO2/Si surface under the influence of magnetic field and shear in a vertical sample geometry.[19]

Summary

Introduction

The formation of ordered nanoparticle (NP) structures can be realized by self-assembly. One interesting class of materials in this context are magnetic nanoparticles (NPs) dispersed in a solvent, since they can self-assemble and are responsive to external stimuli (magnetic field) This enables a range of applications such as magnetic sealing and magnetic memory or in biomedicine.[5−7] These applications make use of the ability of colloidal magnetic NPs to form structures such as linear or branched chains, clusters, or rings in an applied magnetic field.[8−10] Similar applications are considered for thin films of magnetic NPs with the additional advantage that the self-assembling structure can be prepatterned and grown from a substrate. From the specularly reflected intensity, nuclear and magnetic density profiles across interfaces can be extracted with high precision.[13−15]

Methods

Results

Discussion

Conclusion