Nano-LEGO® – assembling functional nanoparticles
Thomas Nann et al. Nanomaterials and their mesoscopic properties have fascinated many scientists in recent years. The general public associates Nanotechnology probably more with microscopic robots as depicted in Figure 1. Figure 1: Artistic view of a micro-submarine in a blood vessel. CONEYL JAY / SCIENCE PHOTO LIBRARY To date, there is still a huge gap between applications such as the micro-sub (they only exist in the fantasy of some artists) and what nanoscience and –technology can deliver. More specifically, nanoscientists are able to synthesise a very wide range of mono-disperse (equally sized and shaped) nanoparticles with new and useful mesoscopic properties, but the options to assemble them into a larger nano-architecture are extremely limited. For a number of years, the Nann research group pursued the goal (one amongst others of course) to find a method to link two different, randomly chosen nanoparticles ino one nano-object – a nanoparticle hetero-dimer, the simplest form of a “complex” nano-architecture. The first step towards a hetero-dimer was to control the number of functional/accessible sites on the surface of the nanoparticles. In order to do so, we attached the nanoparticles of choice to a solid substrate to separate a small contact area from the bulk of the nanoparticles’ surface as schematically shown in Figure 2.
Figure 2: Schematic depiction of “mono-functionalisation” strategy. This idea is not entirely new and is similar to the “Merrifield” peptide synthesis and other solid support synthesis methods in chemistry. However, polymers are unsuitable solid supports for our purpose, because they “wrap” around the nanoparticles, thus creating more than one contact area. The solution was to synthesise a second type of nanoparticle as solid support. Figure 3 shows a transmission electron micrograph (TEM) of gold nanoparticles attached to a solid silica support particle.
Figure 3: “Strawberry” particle – gold nanoparticles attached to a silica solid support. Figure from . Functionalising and cleaving the nanoparticles off their solid support opens up a number of interesting options already: the resulting mono-functional particles (particles with functionality at just one spot) can be used to reversibly dimerise particles, which may have potential applications in biosensing and other areas . The final step linking two different nanoparticles includes first mono-functionalising both types of particles with different surface-functionality and then linking these particles chemically. Figure 4 shows TEM micrographs of a hetero-dimer comprising of a gold and a magnetite nanoparticle. Figure 4 B. shows the corresponding scanning TEM image, where gold atoms are shown inblue and iron in green.
Figure 4: Gold/magnetite hetero-dimer. TEM picture from . One might argue that this work comprises a nice academic exercise, but still lacks far behind the vision shown in Figure 1. However, linking two different nanoparticles (or even the same type of nanoparticles) offers a number of advantages that can be exploited for various applications already. For example, by combining a gold and a magnetic nanoparticle, the resulting object has the combined properties of both. This could be used in the area of nanomedicine as multi-modal contrast agent or – if the linkage is reversible – an in-vivo biosensor. The future work will look at new methods to assemble increasingly more complex nano-architectures and to use these structures for various applications. References  M. R. Dewi, T. A. Gschneidtner, S. Elmas, M. Ranford, K. Moth-Poulsen, T. Nann, ACS Nano 2015, 9, 1434–1439.  M. Dewi, G. Laufersky, T. Nann, Microchim Acta 2015, 1–6.