Switchable self-protected attractions in DNA-functionalized colloids.

Surface functionalization with DNA is a powerful tool for guiding the self-assembly of nanometre- and micrometre-sized particles. Complementary 'sticky ends' form specific inter-particle links and reproducibly bind at low temperature and unbind at high temperature. Surprisingly, the ability of single-stranded DNA to form folded secondary structures has not been explored for controlling (nano) colloidal assembly processes, despite its frequent use in DNA nanotechnology. Here, we show how loop and hairpin formation in the DNA coatings of micrometre-sized particles gives us in situ control over the inter-particle binding strength and association kinetics. We can finely tune and even switch off the attractions between particles, rendering them inert unless they are heated or held together--like a nano-contact glue. The novel kinetic control offered by the switchable self-protected attractions is explained with a simple quantitative model that emphasizes the competition between intra- and inter-particle hybridization, and the practical utility is demonstrated by the assembly of designer clusters in concentrated suspensions. With self-protection, both the suspension and assembly product are stable, whereas conventional attractive colloids would quickly aggregate. This remarkable functionality makes our self-protected colloids a novel material that greatly extends the utility of DNA-functionalized systems, enabling more versatile, multi-stage assembly approaches.