Another interesting application of self-assembled systems of nanoparticles is the performance mechanical tasks at the nanoscale. For example, colloidal magnetic nanoparticles can be moved easily by the application of an external magnetic field. It has been shown that linear assembly of magnetic nanoparticles can be used like magnetic stirring bars, but then at nanoscale. Yang et al. demonstrated that the hydrogenation of methylene blue can be achieved efficiently in microdroplets by using self-assembled magnetic nanoparticle (Fe3O4-NC-PZS-Pd) linear arrays as stirrers, Figure (a). Similarly, Chong et al. showed that nano-stirring by self-assembled chains of magnetic nanoparticles prevents the settling of the reaction suspension unlike larger micron sized stirrers, Figure (b) . Such nano-stirring systems are remarkable as the mass transfer resistance at nanoscale inside micro-droplets can be overcome without breaking of the droplets. A similar magnetically driven concept was demonstrated by Dong et al., where nanomotors were obtained by the self-assembly of different nanoparticles (Au, Pt, Fe3O4) over polymer single crystal (PSC) nanoplatforms . In this system the Fe3O4 nanoparticles respond to the magnetic field. Interestingly, the same self-assembled nanoparticle system can be moved by a catalytic reaction in 15% H2O2 solution due to the Pt nanoparticles that give rise to oxygen bubbles. The oxygen bubbles depart from the assembly pushing the assembly in the opposite direction by momentum exchange. Such catalytic propulsion has also been shown by others. Figure . (a) Linearly self-assembled Fe3O4-NC-PZS-Pd nanoparticles as nano-magnetic stirring bar for methylene blue hydrogenation in microdroplets. Reprinted with permission © 2015 John Wiley & Sons (b) Nanostirring bars of 75 nm wide keep the reaction suspension of 30 μL well mixed in contrast to larger magnetic bars. Reprinted with permission © 2013 John Wiley & Sons (c) Selfassembled system of Au, Pt, Fe3O4 nanoparticles on polymer single crystal nanoplatforms that can be moved in the solution phase by external magnetic field and catalytic oxygen evolution (in H2O2 solution).Reprinted with permission © 2013, American Chemical Society.