This investigation establishes a system of gold nanoparticles
that show good colloidal stability as an X-ray computed
tomography (XCT) contrast agent under soil conditions. Gold
nanoparticles offer numerous beneficial traits for experiments in
biology including: comparatively minimal phytotoxicity, X-ray
attenuation of the material and the capacity for functionalization.
However, soil salinity, acidity and surface charges can induce
aggregation and destabilize gold nanoparticles, hence in
biomedical applications polymer coatings are commonly
applied to gold nanoparticles to enhance stability in the in vivo
environment. Here we first demonstrate non-coated
nanoparticles aggregate in soil-water solutions. We then show
coating with a polyethylene glycol (PEG) layer prevents this
aggregation. To demonstrate this, PEG-coated nanoparticles
were drawn through flow columns containing soil and were
shown to be stable; this is in contrast with control experiments
using silica and alumina-packed columns. We further
determined that a suspension of coated gold nanoparticles
which fully saturated soil maintained stability over at least 5
days. Finally, we used time resolved XCT imaging and image
based models to approximate nanoparticle diffusion as similar to
that of other typical plant nutrients diffusing in water. Together,
these results establish the PEGylated gold nanoparticles as
potential contrast agents for XCT imaging in soil.