Electron tomographic analysis of spherical gold nanoparticles. Figure 1A represents the micrograph gray value range that served for setting the threshold. The threshold was set at -15106.4, that is somewhere between the two peaks. Figure 1B shows a representative electron tomographic 3D-reconstruction of spherical gold NP. Bar: 50 nm. Figure 1C and Figure 1D show the correlation between the calculated and measured volumes and areas of ten electron tomograms.