The effect of sedimentation and diffusion on cellular uptake of gold nanoparticles

In vitro experiments typically measure the uptake of nanoparticles by exposing cells at the bottom of a culture plate to a suspension of nanoparticles, and it is generally assumed that this suspension is well-dispersed. However, nanoparticles can sediment, which means that the concentration of nanoparticles on the cell surface may be higher than the initial bulk concentration, and this could lead to increased uptake by cells. Here, we use upright and inverted cell culture configurations to show that cellular uptake of gold nanoparticles depends on the sedimentation and diffusion velocities of the nanoparticles and is independent of size, shape, density, surface coating and initial concentration of the nanoparticles. Generally, more nanoparticles are taken up in the upright configuration than in the inverted one, and nanoparticles with faster sedimentation rates showed greater differences in uptake between the two configurations. Our results suggest that sedimentation needs to be considered when performing in vitro studies for large and/or heavy nanoparticles. The cellular uptake of gold nanoparticles is sensitive to the way cells are positioned and the effects of nanoparticle sedimentation on uptake should be considered in future in vitro studies of large and/or heavy nanoparticles.