XRM 2024, Lund, Sweden (11.08.2024 - 16.08.2024)

Poster contribution "Redox-properties of environmental biofilms probed by echem-STXM

CSC, Winnipeg, Canada (02.06.2024 - 06.02.2024)

Presentation on "Operando single particle electrochemistry in STXM" 

Pablo Ingino¹, Kerstin Hockmann¹, Haytham Eraky², Chunyang Zhang², Adam P. Hitchcock², Martin Obst¹

¹ Experimental Biogeochemistry, BayCEER, University of Bayreuth, D-95448 Bayreuth, Germany
² Chemistry & Chemical Biology, McMaster University, Hamilton, ON, Canada.

Biogeochemical redox processes control the cycling and fate of elements in the environment including nutrients toxic elements like heavy metals. Gradients between oxic and anoxic environments are abundant, e.g. in wetlands or at mining sites. They provide characteristic habitats wherein redox-active microorganisms thrive and form biofilms. Despite the microorganisms these biofilms consist of biopolymers produced by the microorganisms and minerals particles that are trapped or precipitated within the biofilms.

Some biofilm-forming Fe(II)-oxidizing bacteria excrete sophisticated polymer structures such as twisted stalk or sheaths with unique properties: Previous measurements indicated the presence of redox-active functional groups such as quinone and phenol moieties that can mediate redox reactions, e.g. by interacting with dissolved Fe²⁺ ions. Furthermore, charged functional groups such carboxylic groups can complex Fe²⁺ or Fe³⁺ ions, and template the precipitation of Fe oxyhydroxides. These Fe minerals, in turn, can interact with the redox-functional organic groups.

To study such complex interactions, we use a combination of electrochemical manipulation of the sample and soft X-ray spectromicroscopy to identify redox reactions and to quantify their contributions at defined electrochemical potentials.

This required the development and optimization of an electrochemical cell for in-operando applications in a scanning transmission X-ray microscope (STXM). Single microbial biopolymer structures such as sheaths or stalks of approximately 2 µm in diameter and several tens of µm in length were deposited onto a gold working electrode on a Si₃N₄ window using a micromanipulator. A second window was sealed on the electrode window using epoxy resin. A microfluidic device was plasma-bond to the Si₃N₄chips and allowed for filling the measurement cell with electrolyte and for rapid in-operando electrolyte changes that are required to introduce redox-mediators at certain stages of an experiment.

Image stacks of single microbial structures were successfully acquired in the pre-edge of the O1s absorption edge and both Fe oxyhydroxides and quinone moieties of the organic polymers show specific absorption peaks in this energy region. Intensities of specific absorption peaks in this energy region changed reversibly with the applied potentials, indicating reversible redox reactions of fractions of the respective moieties.

Despite various experimental challenges including the handling of microfluidic liquid cells in a vacuum-environment, the developed setup allows for electrochemical X-ray absorption studies of single organic-inorganic composite materials at a spatial resolution of tens of nm. The optimized setup allows for studies at energies in the water window between the C1s edge and the onset of the absorption of water at the O1s absorption edge as well as at any soft X-ray energy higher than including the Cu 2p edge.