Ternary heterocoagulation

4.2.1. Equally sized colloids

In order to show that also a three-micro-sphere, i.e. ternary system can be introduced using DNAcc far from equilibrium, a system of three different colloids of the same size (= 1µm), but of different DNA coatingα,β and γ was investigated.

4. Static Structure Formation by DNA coated colloids It is expected, that on addition of the linker strands αβ and γβ, α- and γ-spheres both start to bind to β-spheres leading to a competitive binding process. Consequently, in the compact regime (Xγ−β +Xα−β ≥ 30), predominantly compact isolated aggregation seeds should be found that have a composition that reflects the stoichiometries αβ and γβ (see figure 4.7A).

The data indeed shows that a balanced stoichiometry ofγ- andα-spheres (Xγ−β = 1) leads to an equally bound number of these two colloidal species to β-spheres (see figure 4.7B).

Decreasing the stoichiometry Xγ−β results in a depletion of γ-spheres, until there is on average only oneγ-sphere bound to aβ-sphere. As the maximum number of bound majority spheres to minority spheres is nmax= 6.8 (see figure 4.6B), the stoichiometry at which β-spheres are on average covered by only one γ and several α -spheres equals Xγ−α = _6.8¹ = 0.147. Experimentally Xγ−α = 0.12 is found, which is in good agreement with the expected value. Consequently, this value reflects the stoichiometry at which the coagulates consist of oneγ-sphere and severalα-spheres, hence exhibiting polar order (see figure 4.7B, lowest red dashed line). Increasing the stoichiometry Xγ−α above 1 symmetrically leads to the same result. It is obvious that at a ratio of Xγ−α = 6.8 the average cluster contains only one α-sphere (see figure 4.7B, highest red dashed line)

Figure 4.7.: Ternary aggregation of equally sized microspheres in the compact regime. (A) Introducing two microsphere speciesα and γ that can exclusively bind toβ-spheres results in competetive binding that allows for controlling the internal structure of the clusters.

(B) A balanced stoichiometry of white γ- and green α-spheres (Xγ−β = 1) leads to an equally bound number of these two colloidal species to a redβ-sphere (centered red dashed line). Decreasing the stoichiometry Xγ−β results in a depletion of γ-spheres, until there is on average only one γ-sphere bound to a β-sphere (lower red dashed line), reflection a stoichiometry at which the coagulates exhibit polar order. Increasing the stoichiometry Xγ−β above 1 symmetrically leads to the same result.

4.2.2. Differently sized colloids for asymmetry

As it has been shown in section 4.2.1 ternary heterocoagulation of equally sized micro spheres can be used to control the composition of spheres within clusters in the compact

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4. Static Structure Formation by DNA coated colloids

regime. Also polar objects could be obtained, offering an opportunity for directional binding to these clusters. However, these polar clusters only emerge at a narrow range of stoichiome-triesXγ−α, thus yielding very limited options for further mutli-component self-organization processes. In order to further enhance the polar regime of compact ternary heterocoagula-tion, it is possible to extend ternary heterocoagulation to differently sized spheres. Instead usingα- β- and γ-spheres of the same size, Γ-spheres of the double the size (2µm) were used. By this, a geometrical constraint is introduced, which results in an even more effective

Figure 4.8.: Ternary aggregation of differently sized microspheres. (A) Adding a a third, larger spherical particle enables the formation of polar objects at varying stoichiometries.

Besides the two isotropic regimes (blue area and blue markers, green area and green mark-ers), that correspond to binary heterocoagulation described in section 4.1, a regime of polar structures (purple area, red and purple markers) is found, where the majority type of formed clusters is of polar order. (B) Representative confocal image purified solution of polar clusters. Purification was performed using a magnetic separation technique (see sec-tion 3.1.1.3). (C) Change in cluster distribution illustrated by four example stoichiometries.

Whereas in the blue and green regimes more than 80% of the clusters are isotropic (I, IV), the majority of clusters remains polar in the purple regime (II, III). The exact position in the phase diagram is relevant for the polar substructure (II, III). The shown pictograms of single clusters are 3D renderings of confocal data.

blocking of potential binding sites on the sameβ-hemisphere for the α-spheres, resulting in

4. Static Structure Formation by DNA coated colloids a large polar regime. At XΓ−α ≪1, only isotropic clusters are observed, as the ternary ag-gregation process is basically reduced to a binary problem (see figure 4.8A, green markers).

However, increasing XΓ−α leads to the cross-over to a regime where bothα- andΓ-spheres can bind to a β-sphere, yielding true ternary clusters. The majority of the resulting clus-ters are polar and harbor only one large Γ-sphere and multiple α-spheres (see figure 4.8A, purple markers). Consistently, a further increment ofXΓ−α leads to the binding of not only one but two Γ-spheres to one β-sphere. In contrast to equally sized γ-spheres, the use of larger Γ-spheres preserves polarity also in this regime (see figure 4.8A, red markers). Only if the concentration ofΓ-spheres is drastically increased toXΓ−α ≫1, so that three or four Γ-spheres are bound to oneβ-sphere, polarity is lost again due to an effective screening of the α-spheres (see figure 4.8, blue markers).

The data show that a wide range of concentration of α- and Γ-spheres can be used to design polar structures with distinct composition (see figure 4.8C). This variety of compact junction-type structures opens up the possibility to aim for higher-order structures based on a hierarchical DLCA process. However, this idea of self-organization far from equilibrium in the compact regime also leaves behind the majority of majority types spheres, as they are not part of the assembly and therefore remain unbound (see also section 4.1.3). To remove these free majority type spheres, magnetic particle separation techniques can be applied (see also section 3.1.1.3), resulting in a higher purity of the assembled structures (see figure 4.8B). As an alternative, instead of treating them as waste, the unbound ma-jority type spheres can be included in the subsequent more complex assembly process (see section 4.3).