To evaluate the performance of the micelle-templated nanoparticles as contrast agents, the same soil was rehydrated with water and alternatively with an aqueous solution of the nanoparticles (54 mg(Au)/ml), in two otherwise identically prepared tubes. These were then imaged side-by-side using a X-CT (Zeiss XRM-510 instrument with 80 keV X-ray, voxel . (Note that greyscale value in the X-CT images represents the level of X-ray attenuation in arbitrary units, so that more strongly attenuating pixels are shown as brighter, which is the standard convention.) It can be observed that the aqueous phase exhibits a substantially higher grayscale value (on an 8-bit scale) in the sample where it is labelled with gold nanoparticles, in comparison with the sample where it is not. This is evidently due to the higher average electron density of the gold-labelled area, arising from the fact that gold's atomic mass is so much higher than the light elements otherwise present in this phase. The relative greyscale (X-ray attenuation) values of the different phases are quantitatively compared in Fig. . It can be seen that the nanoparticle-labelled aqueous phase sits between the values of the unlabelled aqueous phase, and soil, and can be distinguished from both. Indeed, although the value of the labelled water phase is clearly closer to that of soil than the unlabelled water, it is still possible to segment the two phases (Fig S1 ).