Modulation of like-charge attraction by lipid and protein functionalized silica microparticles
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Controlling colloidal interactions continues to receive a great deal of attention due both to basic scientific interests as well as industrial applications. However, many aspects of interactions between microparticles remain poorly understood, including the attraction observed between particles with the same kind of charge (like-charge attraction). This situation hinders progress in the generation of colloidal self-assembled structures. This thesis focuses on measurements of pair interactions of functionalized silica microspheres and the resulting insights into colloidal interactions. Silica microparticles were functionalized in two ways. For one method, each particle was coated with a lipid inlayer membrane. The charge density of the particle surface can thereby be easily tuned by controlling the type or amount of charged lipids. For the other method, the cholera toxin subunit B protein (CTB) was bound to lipid-functionalized microparticles. To measure pair interactions, we invented a line optical trap that enables nearly free one-dimensional Brownian motion of particles. Pair interaction energies of functionalized particles above the bottom of the experimental chamber can be extracted via a Boltzmann relationship. Both lipid-only and lipid-plus-protein functionalized microparticles show tunable, attractive pair interactions. For lipid-only coatings, the attraction becomes stronger by increasing the fraction of positively charged lipids. There is a linear relationship between pair potential and molar percentage of positively charged lipids. For lipid-plus-protein coatings, attractive potentials were weakened monotonically by binding more CTB. Decompositions of potential curves allow identification of directly charge-dependent and charge-independent contributions to colloidal like-charge attraction. Analysis shows that the correlations between attraction strength and range are opposite in these two sets of particles. Moreover, the correlations between particle-wall separation and attraction strength in lipid-only and lipid-plus-protein functionalized particles are also opposite. These comparisons show that like-charge attraction may result from more than one mechanism. Finally, we measured pair potential energies of lipid functionalized silica particles above a lipid functionalized glass chamber bottom, which exhibit a quadratic relationship between the attraction strength and the fraction of positively charged lipids. Compared with the situation of particle functionalization only, this relation indicates that confinement-induced like-charge attraction can be modulated by altering electrostatic properties of the confining wall.