Multiphoton Imaging of Spatial Distribution, Coordination and Redox Environment of Uranium under Model Biogeochemical Conditions

Nuclear power is a sustainable zero-carbon energy source but requires generated radionuclide waste be remediated from contaminated lands. Legacy, mining and disposal activities all engender environmental contamination, particularly for uranium, which is the main component by mass being both radioactive and toxic. Microbial mediated redox transformations have been targeted as bioremediation techniques. Typically studied by bulk quantitative techniques or on fixed (dead) cells, an absence of techniques capable of quantitatively probing the distribution and environment of radionuclides in living cells has impeded repurposing such wastes. Here we demonstrate the use of two-photon luminescence microscopy utilizing the intrinsic optical properties of the uranyl cation (UVIO2 2+) to follow microbial processes at the sub-micron level in vivo. The fundamental multiphoton photophysical properties of key uranyl species have been determined, and two-photon imaging performed. The long-lived uranyl emission and inherent spatial control of two-photon excitation allows high-resolution, label free visualization of uranyl-containing biological material, while fluorescence lifetime mapping demonstrates the ability to visualize the microscopic redox conditions over the surface of uranyl-reducing bacterial cells.