A nanoparticle contrast agent enables dynamic microscale X-ray computed tomography imaging of the soil aqueous phase

The current need to develop alternative agricultures that preserve soil health with a reduced contribution to climate change, has led to a growth of interest in understanding natural processes within soil. This creates a demand for 3D imaging techniques that dynamically image soil processes such as fluid and nutrient transport with high resolution. Microscale X-ray computed tomography (X-CT) delivers high-contrast and high-resolution (down to ~1 m) imaging of soil mineral phases. However it does not readily distinguish low-density aqueous and organic phases, nor image water transport. Here we have developed polymer-templated gold nanoparticles as a contrast agent to label the aqueous phase in soil, with gold selected for low toxicity. Nanoparticles are generated by templated synthesis within pre-assembled block copolymer micelles, poly(2-(dimethylamino)ethyl methacrylate)-block-poly[poly(ethylene glycol) methyl ether methacrylate)], poly(DEAEMA-b-PEGMA), to intrinsically exhibit a stabilizing PEG-bottlebrush corona. Block copolymers are generated by group transfer polymerization (GTP). Overall, this process generates gold nanoparticles at high concentrations and in large volumes for soil imaging. The nanoparticles show exceptional colloidal stability (to at least 4M ionic strength), and are stable in the challenging soil environment, showing no adsorption to the soil mineral phase. X-CT imaging within soil distinguished aqueous phase labelled with the nanoparticle contrast agent from unlabelled aqueous phase, at ~ 5 mg/ml Au. In a transport experiment, we determined the effective bulk diffusion constant of the nanoparticle system in water to be 1.1 0.3 10-10 m2s-1. Intriguingly, this is greater than the single particle diffusion constant in dilute solution, suggesting a role for crowding effects. Taken together, these results demonstrate this nanoparticle system as an effective and practical contrast agent for imaging flow and transport processes in living soil.