Clustering versus percolation in the assembly of colloids coated with long DNA

We report an experimental study in which we compare the self-assembly of 1 mu m colloids bridged through hybridization of complementary single-stranded DNA (ssDNA) strands (12 bp) attached to variable-length double-stranded DNA spacers that are grafted to the colloids. We considered three different spacer lengths: long spacers (48 500 bp), intermediate length spacers (7500 bp), and no spacers (in which case the ssDNA strands were directly grafted to the colloids). In all three cases, the same ssDNA pairs were used. However, confocal microscopy revealed that the aggregation behavior is very different. Upon cooling, the colloids coated with short and intermediate length DNAs undergo a phase transition to a dense amorphous phase that undergoes structural arrest shortly after percolation. In contrast, the colloids coated with the longest DNA systematically form finite-sized clusters. We speculate that the difference is due to the fact that very long DNA can easily be stretched by the amount needed to make only intracluster bonds, and in contrast, colloids coated with shorter DNA always contain free binding sites on the outside of a cluster. The grafting density of the DNA decreases strongly with increasing spacer length. This is reflected in a difference in the temperature dependence of the aggregates: for the two systems coated with long DNA, the resulting aggregates were stable against heating, whereas the colloids coated with ssDNA alone Would dissociate upon heating.