Uptake of gold nanoparticles in HeLa cells observed by confocal microscopy shows dependency on particle size and shape

Uptake of gold nanoparticles by HeLa cells fixed at different incubation times of up to eight hours was investigated using 3D confocal laser scanning microscopy followed by image analysis. The cell bodies were characterized by fluorescence labeling, whereas the gold nanoparticles were identified by light scattering. The amount of nanoparticles uptaken at different times was evaluated with Fiji according to a dedicated protocol. We assessed the effect of particle size (80 and 150 nm diameter spheres) and shape (spheres vs "urchins") on their uptake. The large spherical nanoparticles presented approximately fourfold higher levels of uptake than the nanourchins. Also, the spheres presented a clearly increasing uptake in the time profile, reaching a maximum around seven hours and then showing a slight decrease. We ascribe this behavior to a lower aptitude of the cells for taking up nanoparticles with either urchin shape or smaller size and to the possible presence of competing kinetics for exocytosis at the longest times. Live experiments confirmed for the time profile a relatively flat cell response to the urchins and an increasing response to the spheres, yet with a final plateau. However, in the live case, the internalization levels were as low for the spheres as for the urchins.