Integration Of Biogenic Nanopore Membranes On Prefabricated Fluidic Support Substrates

Marine diatoms provide an alternative to machined silica nanopores, avoiding costly and slow throughput fabrication steps, while being able to achieve pore structures with diameters on the order of 40 nm. The hierarchical pore architecture makes these biogenic nanomembranes exceptionally mechanically stable, while maintaining a short pore length and a high porosity. The most prominent issue when replacing machined silica nanopore membranes with biogenic membranes is the initial random placement of the membranes on the solid substrate. This is also problematic when trying to accomplish a permanent fluidic seal around the membrane. In our study, we demonstrate the ability to localize and immobilize a 200μm-diameter biomineralized nanopore membrane structure from marine algae, Coscinodiscus wailesii, on pre-defined positions on micro-machined silicon substrates. The substrates feature micron-sized through-wafer channels that allow easy access to the nanopore membrane. Localization of the membrane structure is accomplished using patterning of 8 μm thick hydrophobic resin. The addition of poly-L-lysine to the surface before solution-depositing the nanopore membranes results in a strong electrostatic binding force between the oxidized silicon platform and the diatom membranes. Lift-off of the photoresist in acetone removes randomly placed nanopore membranes on the resist-coated area, not affecting diatoms adhering to the silicon surface. While poly-L-lysine provides an initial fluidic seal, permanent immobilization is accomplished by using UV-curable photoresist SU-8 and proximity photo lithography. Scanning electron micrographs after processing show intact diatoms without the presence of stress cracks. While initial electrochemical measurements indicate that some of the nanopores are clogged by residual epoxy resist after development, subsequent sulfuric-peroxide mixture (SPM) treatment removes the residual resist. Successful translocation experiments using polystyrene beads shows presence of unclogged pores, also indicating that the pore size of the biogenic silica nanomembranes can be modified by chemical treatment.