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

The need to develop alternative agricultures that preserve soil health with reduced contribution to climate change has led to growing interest in soil's microscale structure and dynamics. Microscale X-ray computed tomography (μX-CT) can image soil mineral particles at high-resolution but does not readily distinguish low-density aqueous and organic phases, nor image fluid and nutrient transport. Here we have developed polymer-templated gold nanoparticles as a contrast agent to label the aqueous phase in soil, selecting gold for low toxicity. Nanoparticles are created by templated synthesis inside block copolymer micelles with a stabilizing corona: poly(2-(dimethylamino)ethyl methacrylate)-block-poly[poly(ethylene glycol)methyl ether methacrylate]. These gold-polymer nanoparticles (gold core diameter 12 nm; overall hydrodynamic diameter 44 nm) are generated at high concentrations and in large volumes for soil imaging. They show exceptional colloidal stability (to ≥1.5 M ionic strength), and are stable in the soil microenvironment, with no adsorption to mineral particles. In situ μX-CT imaging distinguished nanoparticle-labelled soil aqueous phase from unlabelled aqueous phase, at ∼5 mg ml−1 Au. Dynamic imaging determined the nanoparticle bulk diffusion constant in soil to be 1.1 ± 0.3 × 10−10 m2 s−1, much slower than typical single-ion transport in soil. We propose these nanoparticles as an effective contrast agent for flow and transport imaging in living soil.