Collapse and folding of flexible colloidal polymers

The self-assembly of colloids can yield novel materials with unusual optical or mechanical properties. Unlike periodically repeating structures, here we introduce a new paradigm of self-assembly in which flexible colloidal polymers find a 'fold', similar to the way polypeptides fold into proteins. We have previously shown how emulsion droplets decorated with DNA sticky ends self-assemble into freely-jointed polymers. Here, we demonstrate the addition of secondary, switchable interactions along the length of a self-assembled polymer. These interactions are triggered by a temperature quench, whose rate governs the folding pathways from the extended to the collapsed states. The folding time scales with the length of the chain, in agreement with the increase in entropy. Using the simplest form of sequence control, we show that labeling alternating particles with secondary interactions eliminates some folding pathways visited when all droplets along the chain interact, narrowing the ensemble of folded configurations. This result opens up the prospect of programmable sequence design to achieve a unique stable structure, as the Anfinsen's dogma for protein folding proposes. This work was supported by the NSF MRSEC Program (DMR-0820341).