Electric fields can induce various types of motion in liquid suspensions of colloidal nanoparticles. These electrokinetic phenomena depend on the parameters of the electric field (frequency, amplitude, 3D topology), the particles (size, shape, composition) and the suspending liquid (polarizability, ionic strength, pH). In particular, the dielectrophoretic force on submicron colloidal particles is dependent on the properties of the electric double layer (the "ion cloud") around these particles. This dependence provides a mechanism for detecting and quantifying interactions between biomolecules and these nanoparticles, which can be combined with optical and spectroscopic measurements. Here, we report on functionalized plasmonic nanoparticles that are tracked inside microfluidic systems by dark-field video-microscopy. A high-gradient AC electric field is set up using transparent micro-electrodes. Electrohydrodynamic motion of the entire fluid and dielectrophoretic trapping of individual particles can be analyzed quantitatively by numerical methods. By switching the electric field synchronously with the video acquisition, the effect of biomolecules on the electrokinetic trapping can be quantified. The electromicrofluidic devices allow also for rapid measurement of diffusion coefficients.