Sedimentation and diffusion are important aspects of the behavior of colloidal nanoparticles in solution, and merit attention during the synthesis, characterization, and application of nanoparticles. Here, the sedimentation of nanoparticles is studied quantitatively using digital photography and a simple model based on the Mason–Weaver equation. Good agreement between experimental time‐lapse photography and numerical solutions of the model is found for a series of gold nanoparticles. The new method is extended to study for the first time the gravitational sedimentation of DNA‐linked gold nanoparticle dimers as a model system of a higher complexity structure. Additionally, simple formulas are derived for estimating suitable parameters for the preparative centrifugation of nanoparticle solutions.

Two-photon photoluminescence (TPPL) emission spectra of DNA-gold nanoparticle (AuNP) monoconjugates and the corresponding DNA-linked AuNP dimers are obtained by photon time-of-flight spectroscopy. This technique is combined with two-photon photoluminescence fluctuation correlation spectroscopy (TPPL-FCS) to simultaneously monitor the optical and hydrodynamic behaviour of these nano-assemblies in solution, with single-particle sensitivity and microsecond temporal resolution. In this study, the AuNPs have an average core diameter of 12 nm, which renders their dark-field plasmonic light scattering too weak for single-particle imaging. Moreover, as a result of the lack of plasmonic coupling in the dimers, the optical extinction, scattering and photoluminescence spectra of the DNA-AuNP complexes are not sufficiently different to distinguish between monomers and dimers. The use of TPPL-FCS successfully addresses these bottlenecks and enables the distinction between AuNP monomers and AuNP dimers in solution by measurement of their hydrodynamic rotational and translational diffusion.

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.