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We present dielectrophoresis (DEP) and electrorotation (ROT) measurements of gold-coated polystyrene microspheres as a function of frequency and for several electrolyte conductivities. Particle rotation was counterfield with a maximum rotation rate observed at a single characteristic frequency. Negative DEP was observed for frequencies lower than this characteristic frequency and positive DEP for signal frequencies higher than this. These experimental observations are in agreement with predictions for the force and torque on the induced dipole of a perfectly polarizable metal sphere. We present a theoretical model for this case, and good agreement is found for both ROT and DEP measurements if we take into account the viscous friction for a spherical particle near a wall. From the characteristic frequency for rotation, we obtain the capacitance of the electrical double layer at the electrolyte-particle interface. Remarkably, no effect of induced charge electroosmosis around the particles can be inferred from DEP measurements.

The synthesis of a tetrabiotinylated porphyrin ligand, 5,10,15,20-tetrakis{α-[4-(biotinylamidomethyl)-pyridinium bromide]-p-tolyl}porphyrin (TBPP), is described and its monolayer behavior at the air-water interface investigated. Monolayers of the pure salt were found to be unstable. The effects of complexing with different mole ratios of sodium octadecyl sulfate (ODS) were studied. The TBPP-ODS (1 :4) complex gave stable isotherms from which it was deduced that porphyrin molecules lay flat on the water surface, with biotin moieties pointing into the subphase. From STM images it was deduced that the high deposition ratio (∼1.5) during vertical dipping probably arose from a spontanous self-assembly of the complex into rodlike stacks. Fluorescence microscopy showed that, in the gaseous phase, the complex assembled into a network of associated porphyrin molecules surrounding circular voids of different size. Addition of active avidin or streptavidin caused a significant expansion ofthe isotherm that was accompanied by the appearance of a domain-like structure in the monolayer. That no such changes were observed with inactive proteins confirms the specific nature of the interactions being investigated.