Gas microbubbles decorated with gold or magnetite nanoparticles: composite objects for multi-scale investigation of magnetic, magneto-optical and optical responses. – AirDeco

Elaboration of spherical objects decorated with nanoparticles (NPs) is an important challenge for the development of new devices with potential in photonics, electronics, and for high performance information storage. A number of hard and soft microspheres have been used as templates to self-assemble NPs; the final hollow spheres being obtained by removal of the template. However, an important limitation of all the existing systems is that the NPs are closely packed at the microsphere’s surface, forming a rigid, thick wall. The NPs are spatially localized at fixed positions, and their interdistances, hence their dipolar interactions, are constraint and not modular. The size of the NPs, their number and their interdistance control both magnetic (superparamagnetism, monodomain limit and dipolar interactions) and optical (spectral position and width of surface plasmon resonances) properties. A direct consequence is that some of the properties, which are expected to emerge by controlling of the NPs interdistance, could so far not be generated and investigated.
The general objective of the AirDeco project is to engineer gas microbubbles (MBs), coated by a fluid amphiphilic film, onto which Au or iron oxide (Fe3O4) NPs are grafted with a controlled density and mobile, flexible interactions. Two differently sized objects, the NPs (a few nm) and the MBs (a few µm) will thus be associated in our new modular hybrid superstructures. These new objects, dispersed in aqueous solutions or as isolated objects, will generate multi-scaled magnetic, magneto-optical and optical responses. To the best of our knowledge, such a search for multi-scale responses is presently impossible with any of the various NP-covered spheres described in the literature. 1. The MBs decorated with Fe3O4NPs will offer the possibility of determining the order of prevalence between the magnetic interactions that occur at three different levels: within the NPs, between NPs on the bubble’s surface, and between the magnetic bubbles. 2. The magneto-optical Kerr intensity will be measured on a single microbubble, which will allow determination of the magnetic moment of the interacting NPs. 3. MBs decorated with AuNPs will allow determination of the optical response at two different spatial scales, nanometric and micrometric, by measuring simultaneously the light scattering from the superstructure (sensitive to the size of the MB) and the resonant absorption of surface plasmons (sensitive to the size of the NPs).
The decorated microbubbles will be generated by forming the MBs from an aqueous dispersion of surfactants that carry the appropriate chemical functions that allow covalent grafting of the NPs. A first generation of MBs will be prepared from a dispersion of fluorinated amphiphiles. Fluorinated amphiphilic (phosphates or thiols) will be used, depending on the nature of the NPs to be grafted. The NPs will have affinity for any position on the bubble’s surface and their repartition on the bubble surface will depend on mutual interactions. Experimental evidence for the obtaining of Fe3O4-decoMBs is provided by a feasibility study. Justification for fitting the amphiphilic phosphates with perfluoroalkyl chains (which are known to promote self-assembly) is given by the obtaining of exceptionally stable MBs using one of the fluorinated phosphates proposed in this Project. The more ambitious second generation of decorated microbubbles aims at controlling the density of NPs using a combination of amphiphiles. Such control will be achieved by inducing two-dimensional lateral phase segregation between fluorinated and hydrogenated amphiphiles, and formation of separate domains within the bubble wall. The NPs will be then selectively grafted on the patches that exhibit the appropriate functions, i.e. thiol and phosphate groups. Preliminary experiments have shown that stable MBs have been obtained by using a ternary mixture of Twin, Span and a fluorinated phosphate.