The effect of ionic strength and pH on phosphatidylcholine (PC) adsorption from vesicles on silica nanoparticles was investigated over a range of NaCl concentrations (0.1-150 mM) at pH 6.3 and 7.4 from determination of adsorption isotherms, colloid stability, particle sizing, and zeta-potentials. At and above 10 mM ionic strength, pH 6.3, high-affinity adsorption isotherms with limiting adsorption indicative of one-bilayer deposition on each silica particle were obtained. At 10 mM ionic strength, adsorption isotherms indicated lower affinity between PC and silica at pH 7.4 than at pH 6.3, suggesting a role of hydrogen bonding between silanol on silica and phosphate on PC in promoting bilayer deposition at low pH. Under conditions where high affinity and bilayer deposition were achieved, silica sedimentation documented from photographs was absent, suggesting particle stabilization induced by bilayer coverage. However, at physiological (150 mM NaCl) or close to physiological ionic strength (140 mM NaCl), the large colloid stability similarly achieved at pH 6.3 or 7.4 suggested the major role of van der Waals attraction between the PC bilayer vesicle and silica particle in determining bilayer deposition. The effect of increasing ionic strength was increasing van der Waals attraction, which caused PC vesicle disruption with bilayer deposition and bilayer-induced silica stabilization.

The challenge of breaking open rigid bilayer vesicles upon contact with silica particles was circumvemted by using a dioctadecyldimethylammonium bromide (DODAB) dispersion consisting of open, nanosized bilayer fragments instead of vesicles. At low ionic strength and pH 6.5, DODAB adsorption from bilayer fragments on silica was quantified from adsorption isotherms at 0, 0.1, 0.5, 1.0, and 10.0 mM KCl. Adsorption increased as a function of KCl concentration and surface charge density on particles. The isotherm shapes were typical of competitive adsorption with a maximum possibly due to hydrophobic attraction between adsorbed and free bilayer fragments. Upon adsorption as flat patches on particles, the fragments did not apparently seal into a continuous and closed bilayer surrounding the particle, instead interacted via hydrophobic edges with free fragments in dispersion. At pH 6.5, over a range of DODAB (0−1.0 mM) and KCl concentrations (0.1−10.0 mM), from particle sizing, ζ-potential analysis, photographs of the mixtures, and particle sedimentation kinetics, the colloid stability of the particles in the mixtures was governed by the ratio R of the total surface areas for bilayers A b and particles A p, R = A b/A p. At R ∼ 0.5, the mean ζ-potential was zero, the mean particle diameter (Dz) was at maximum, sedimentation was rapid, and colloid stability was at minimum; at R > 1, ζ was positive, Dz was minimized, sedimentation was absent, and colloid stability was high. At low ionic strength ([KCl] < 10 mM), high colloid stability for particles in the presence of cationic bilayer fragments was achieved at or above R = 1, i.e., from the equivalence of total surface areas for bilayer fragments and particles.

Biomimetic particles supporting lipid bilayers are becoming increasingly important to isolate and reconstitute protein function. Cholera toxin (CT) from Vibrio cholerae, an 87-kDa AB5 hexameric protein, and its receptor, the monosialoganglioside GM1, a cell membrane glycolipid, self-assembled on phosphatidylcholine (PC) bilayer-covered silica particles at 1 CT/5 GM1 molar ratio in perfect agreement with literature. This receptor-ligand recognition represented a proof-of-concept that receptors in general can be isolated and their function reconstituted using biomimetic particles, i.e., bilayer-covered silica. After incubation of colloidal silica with small unilamellar PC vesicles in saline solution, pH 7.4, PC adsorption isotherms on silica from inorganic phosphorus analysis showed a high PC affinity for silica with maximal PC adsorption at bilayer deposition. At 0.3 mM PC, fluorescence of pyrene-labeled GM(1) showed that GM(1) incorporation in biomimetic particles increased as a function of particles concentration. At 1 mg/mL silica, receptor incorporation increased to a maximum of 40% at 0.2-0.3 mM PC and then decreased as a function of PC concentration. At 5 microM GM(1), 0.3 mM PC, and 1 mg/mL silica, CT binding increased as a function of CT concentration with a plateau at 2 mg bound CT/m2 silica, which corresponded to the 5 GM(1)/1 CT molar proportion and showed successful reconstitution of receptor-ligand interaction.

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At 2 % cationic lipid and 98 % phospholipid, pH 6.3, in pure water, high affinity adsorption isotherms with limiting adsorption indicative of one bilayer deposition on each silica particle were obtained whereas for the other molar proportions tested, limiting lipid adsorption was either above or below the level expected for bilayer deposition. This suggested a modulating role for cationic lipid allowing control of total amount of lipid adsorbed on silica. Silica sedimentation documented from photographs was almost absent over a range of lipid concentrations where bilayer deposition was achieved.