Switchable self-protected attractions in DNA-functionalized colloids (original) (raw)

Surface functionalization with DNA is a powerful tool for guiding the self-assembly of nanometre-and micrometresized particles 1-11. 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 12-14. 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. In many DNA-functionalized systems, the particle association and structural organization are equilibrium processes that depend solely on the system temperature, relative to the particles' DNA-dependent dissociation temperature. This is, for instance, demonstrated by our observations on mixtures of beads that form normal Watson-Crick pairs of complementary C N /C N sticky ends (interaction scheme Ia, Fig. 1). Figure 2a shows the fraction of nonassociated particles, or singlet fraction, as a function of time in an experiment where we first lowered the temperature from 52 to 20 • C (t < 810 s) and then ramped it back up (t > 810 s). Clearly, as soon as we go below the particles' dissociation temperature (T dis ≈ 40 • C), the singlet fraction quickly drops to zero, and the particles come together in extensive structures; conversely, when we increase the temperature above T dis the aggregates quickly dissociate. The rate of temperature change determines how fast T dis is reached, but it does not change the qualitative shape of the curves. Much more flexibility is gained if the sticky ends possess secondary conformations, such as hairpins and loops due to intra-particle complementarity (for example, interaction scheme II, Fig. 1). Such secondary structures form in fractions of a microsecond, as estimated from the rotational diffusion time of singlestranded DNA with an end-to-end distance of ∼14 nm. This should