Tailored Anisotropic Magnetic Chain Structures Hierarchically Assembled from Magnetoresponsive and Fluorescent Components

Abstract

The development of methods for controlling the organization of functional objects at a nanometer scale to build larger objects is of fundamental and technological interest. The unique electronic, magnetic, and optical properties of nanomaterials will be best utilized when they are well integrated into larger devices. A great deal of research is focused on such materials, particularly those with magnetic properties that can be exploited for the fabrication of ordered onedimensional (1D) chainlike assemblies. Pertinent targets include the synthesis of materials that have desirable anisotropic properties for electronic and optical devices. As this is a difficult task, different techniques have been employed including magnetic-field-induced (MFI) assembly, electric or magnetic dipole–dipole interactions, crystallographically specific orientation, non-uniform stabilizer distribution, templated synthesis to produce 1D nanostructured materials. However, an elegant approach would be to construct 1D chains consisting of structurally intricate units and functions. Suitable methods are still required to prepare aligned structures from nanoparticles suspended in an aqueous medium and to allow multifunctional properties to be imparted into such directional structures in a way that allows these additional properties to evaluated and exploited. Herein, we describe a method that provides opportunities for synthesizing materials that not only have a preferred 1D structure but that are also multifunctional. The strategy involves both designing and utilizing nanobuilding blocks to be linked together to generate aligned materials with anisotropic morphology and multifunctional properties. In the first step we assemble preformed nanoparticles (NPs) into a confined structure by using positively charged polypeptides as interparticle mediators while preserving the constituent nanoparticles functional properties. We have shown that the cationic polypeptides can undergo counterion condensation to form spherical aggregates by ionic cross-linking with either certain multivalent counteranions or nanoparticles which are capped with anionic species. The unique aggregation characteristics of these polycation–anion systems means that multiple nanoparticles can spontaneously assemble into microstructures. We have further shown that functionalities can also be integrated to give catalytic and optical properties. Herein, our approach is to use poly(l-lysine) (PLL) to cross-link with hydroxy pyrene trisulfonate (HPTS) and citrate-functionalized magnetic nanoparticles (MNPs) to afford magneto responsive fluorescent spheres (MFS). In a second step these spheres are magnetically aligned by virtue of magnetic dipole interactions to construct 1D anisotropic shapes. HPTS is a pyrene-based molecule and has been proved to be a versatile probe molecule in both chemistry and biology. The ability of HPTS, as an anionic multiple-point cross-linking center, to electrostatically bind cations on different polyelectrolyte chains can impart fluorescent properties to the resulting hybrid structures. To incorporate magnetic properties, we use citrate-functionalized Fe3O4 magnetic nanoparticles (MNPs) so that the ionic interaction of citrate with PLL can enable the formation of spherical MNP aggregates. The MNPs, synthesized by a co-precipitation method, are fully characterized to ensure their structure and the presence of the citrate functional group (Supporting Information, Figure S1). Scheme 1 illustrates our method to include both magnetic and fluorescent functions into the microspheres. Simultaneous columbic interactions between positively charged amine groups of PLL and the negatively charged carboxy groups on