© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Perfluoropentane (PFP) gas-filled biodegradable iron-doped silica nanoshells have been demonstrated as long-lived ultrasound contrast agents. Nanoshells are synthesized by a sol-gel process with tetramethyl orthosilicate (TMOS) and iron ethoxide. Substituting a fraction of the TMOS with R-substituted-trialkoxysilanes produces ultrathin nanoshells with varying shell thicknesses and morphologies composed of fused nanoflakes. The ultrathin nanoshells have continuous ultrasound Doppler imaging lifetimes exceeding 3 h, are twice as bright using contrast-specific imaging, and have decreased pressure thresholds compared to control nanoshells synthesized with just TMOS. Transmission electron microscopy shows that the R-group-substituted trialkoxysilanes can reduce the mechanically critical nanoshell layer to 1.4 nm. These ultrathin nanoshells have the mechanical behavior of weakly linked nanoflakes but the chemical stability of silica. The synthesis can be adapted for general fabrication of 3D nanostructures composed of nanoflakes, which have thicknesses from 1.4 to 3.8 nm and diameters from 2 to 23 nm.