Different modulation mechanisms of attractive colloidal interactions by lipid and protein functionalization.

The nature of attractive interactions observed between like-charged microparticles near a confining wall remains an outstanding puzzle in colloidal science. The shortage of experimental systems that provide tunable attractions contributes to the lack of progress in solving this mystery. We have recently shown that the functionalization of microspheres with lipid membranes allows simple control of interparticle interactions as a function of membrane composition (Kong, Y.; Parthasarathy, R. Soft Matter 2009, 5, 2027-2032). Here we introduce a new approach to biomembrane-mediated control in which varying amounts of a peripheral membrane protein, cholera toxin subunit B, are bound to the surface of lipid-functionalized silica particles. Protein functionalization again provides a family of tunable attractive pair interactions, measured using an optical line trap. Surprisingly, however, the form of interactions is strikingly different for particles with protein-plus-lipid membranes than for particles with lipid-only membranes, displaying opposite correlations between the depth of the attractive potential well and the spatial range of the interaction as well as between the well depth and the distance to the confining wall. Our findings and their distinctiveness from previous membrane-functionalized systems not only demonstrate an orthogonal route to the practical control of colloidal assembly but also, more fundamentally, show that multiple physical mechanisms or mechanisms that are especially sensitive to particle surface chemistries may be responsible for governing like-charge attraction in colloidal systems.