Chemists have long envied nature’s reaction flasks: enzymes, which create the perfect environment in which a particular reaction can take place. Chemists have tried to copy this strategy, using zeolites, nanoporous organic frameworks, and colloidal nanocrystal assemblies to create confined spaces where molecules might react. The problem with these strategies is that they rely on diffusion for reactants to enter spaces and for products to leave. Researchers led by Rafal Klajn of the Weizmann Institute of Science have come up with an alternative approach to trapping chemicals in confined spaces and then releasing them. The chemists have developed so-called nanoflasks that assemble in ultraviolet light and break apart in visible light (Nat. Nanotech. 2015, DOI: 10.1038/nnano.2015.256). Made of colloidal nanocrystals functionalized with light-responsive azobenzene ligands, the nanoflasks can trap reactants in a confined space to accelerate reactivity and then release the products into solution. Klajn’s team showed the nanoflasks are effective at speeding the hydrolysis of an acetal, as compared with the same reaction in bulk solution. They also demonstrated they could change the regioselectivity in the dimerization of anthracene by using the nanoflasks to perform the reaction.