Compelling justification: Hard-core repulsion is the simplest interaction in Nature yet it drives the self-organization of many complex fluids. To investigate how enthalpy impacts upon entropy-dominated liquid crystalline states, we introduce a highly localized and tunable directional attractive interaction (or "patch") on one of the tips of rod-shaped colloids. Our experiments and computer simulations show that increasing the patch attraction dramatically stabilizes the lamellar phase, a structure desired in materials science due to its outstanding mechanical and optical properties. Our work demonstrates that introducing patches in anisotropic nanoparticles adds to the control of their self-assembly.Abstract: Dispersions of rod-like colloidal particles exhibit a plethora of liquid crystalline states, including nematic, smectic A, smectic B, and columnar phases. This phase behavior can be explained by presuming the predominance of hard-core volume exclusion between the particles.We show here how the self-organization of rod-like colloids can be controlled by introducing a weak and highly localized directional attractive interaction between one of the ends of the particles. This has been performed by functionalizing the tips of filamentous viruses by means of regioselectively grafting fluorescent dyes onto them, resulting in a hydrophobic patch whose attraction can be tuned by varying the number of bound dye molecules. We show, in agreement with our computer simulations, that increasing the single tip attraction stabilizes the smectic phase at the expense of the nematic phase, leaving all other liquid crystalline phases invariant. For sufficiently strong tip attraction the nematic state may be suppressed completely to get a direct isotropic liquid-tosmectic phase transition. Our findings provide insights into the rational design of building blocks for functional structures formed at low densities.

We report on the construction of multiarm colloidal molecules by tip-linking filamentous bacteriophages, functionalized either by biological engineering or chemical conjugation. The affinity for streptavidin of a genetically modified vector phage displaying Strep-tags fused to one end of the viral particle is measured by determining the dissociation constant, K. In order to improve both the colloidal stability and the efficiency of the self-assembly process, a biotinylation protocol having a chemical yield higher than 90% is presented to regioselectively functionalize the cystein residues located at one end of the bacteriophages. For both viral systems, a theoretical comparison is performed by developing a quantitative model of the self-assembly and interaction of the modified viruses with streptavidin compounds, which accurately accounts for our experimental results. Multiarm colloidal structures of different valencies are then produced by conjugation of these tip-functionalized viruses with streptavidin activated nanoparticles. We succeed to form stable virus-based colloidal molecules, whose number of arms, called valency, is solely controlled by tuning the molar excess. Thanks to a fluorescent labeling of the viral arms, the dynamics of such systems is also presented in real time by fluorescence microscopy.