  We present a versatile setup for investigating the nanofluidic behavior of
nanoparticles as a function of the gap distance between two confining surfaces.
The setup is designed as an open system which operates with small amounts of
dispersion of $\approx 20\,\mu$l, permits the use of coated and patterned
samples, and allows high-numerical-aperture microscopy access. Piezo elements
enable 5D relative positioning of the surfaces. We achieve a parallelization of
less than $1\,$nm vertical deviation over a lateral distance of $10\,\mu$m. The
vertical separation is tunable and detectable with subnanometer accuracy down
to direct contact. At rest, the gap distance is stable on a nanometer level.
Using the tool we measure the vertical position termed height and the lateral
diffusion of $60\,$nm charged Au nanospheres as a function of confinement
between a glass and a polymer surface. Interferometric scattering detection
results in sub $10\,$nm vertical and sub $5\,$nm lateral particle localization
accuracy, and a single particle illumination time below $40\,\mu$s. We measure
the height of the particles to be consistently above the gap center,
corresponding to a higher charge on the polymer substrate. In terms of
diffusion, we find a strong monotonic decay of the diffusion constant with
decreasing gap distance. This result cannot be explained by hydrodynamic
effects, including the asymmetric vertical position of the particles in the
gap. Instead we attribute it to an electroviscous effect. For strong
confinement of less than $120\,$nm gap distance, we detect an onset of
sub-diffusion which can be correlated to a motion of the particles along
high-gap-distance paths.
