DNA molecules are polymers consisting of four kinds of nucleotides: adenine (A), thymine (T), cytosine (C) and guanine (G). These nucleotides interact specifically: A only binds to T with two hydrogen bonds; C to G with three bonds. A DNA strand only binds to the one with complementary nucleotides. Complementary strands hybridize into a double-stranded DNA (dsDNA). A dsDNA strand can be dehybridized into a pair of single-stranded DNA (ssDNA) by breaking hydrogen bonds with heat. Hence DNA hybridization is not only specific but also thermal-reversible.
When DNA strands are coated on colloids, colloidal self-assembly can be directed. DNA-coated colloids only bind to the ones coated with complementary strands.
When a colloid is coated with more than one kind of DNA, it is complementary to multiple kinds of colloids, or polygamous. For example, coating a colloid (D) with four different kinds of DNA (S1, S2, S3 and S4) enables it to recognize four different kinds of colloids (H, F, G and E) coated with complementary strands (S1‘, S2‘, S3‘and S4‘).
When such a polygamous particle is bathed with its complementary particles, it can hold them simultaneously.
Polygamous particles enable us to create materials whose phase depends on their kinetic paths. The material consists of three different kinds of particles: X, Y and Z. X and Z do not interact but they both bind to Y, a polygamous particle. The binding originates from inter-particle DNA strands hybridization; it depends on temperature. Y-X bonds melt at TYX ≈ 47 ℃; Y-Z bonds at TYZ ≈ 41 ℃.
At a temperature higher than both melting temperatures, e.g. 52 ℃, DNA bonds are suppressed; particles do not interact. Quenching the system to room temperature, e. g. 23 ℃, activates X-Y and Y-Z bonds simultaneously; particles aggregate randomly, forming a mutually-locked gel network (a). Instead of quenching, annealing activates X-Y and Y-Z bonds sequentially, driving X to be bound by Y’s first, followed by Z’s piling up. The particles self-assemble into two-shell-like clusters. The clusters outer shells consist of only Z’s. Z’s do not interact; the clusters are inert to each other. The system is mobile, behaving like a fluid (b). Hence by utilizing polygamous particles we create dual-phase materials: gel when quenched; fluid when annealed.
Coating colloids with DNA creates specific, thermal-reversible inter-particle bonds. We have demonstrated that multiple kinds of DNA can be coated on one particle, enabling it to be polygamous. Polygamous particles can be utilized to create unprecedented materials such as dual-phase materials. Utilizing DNA in colloidal systems advances colloidal self-assembly as well as material science to the next level. In the future, we will introduce DNA in active matter, creating temperature-controlled active matter. Toward this end, we are looking forward to collaborating with experimentalists who have experiences on conjugating proteins with DNA as well as synthesizing DNA-coated emulsions.
For more details, see Wu, et al. PNAS 109, 18731 (2012).