Biogenic Silica Nanopore Membranes On Micromachined Silicon Substrates

Nanopore membranes are extremely valuable for applications such as molecular filtration, nanoparticle counting, and sizing studies. However, the fabrication of nanopore membranes using top-down silicon microfabrication technology requires using slow serial patterning processes, making it unsuitable for large-scale manufacturing. Marine diatoms on the other hand feature biomineralized silica shells with the smallest pore diameters on the order of 40 nm. Their hierarchical pore architecture makes these nanomembranes exceptionally mechanically stable, while maintaining a short pore length and a high porosity. In our study, we immobilized the biogenic silica nanomembranes on micromachined silicon substrates. These substrates feature micron-sized, through-wafer channels, enabling free fluidic access to the nanopore membrane. The diatom shells were mounted on top of the silicon microstructure using either poly-L-lysine or UV-polymerizable low-stress epoxy. The resulting microsystem allowed easy handling and mounting in a fluidic platform for nanoparticle transport studies. Using fluorescent nanoparticles we were able to verify that particles with a diameter larger than that of the nanopores were completely retained, while smaller particles, such as polystyrene beads or gold nanoparticles did permeate through the membrane. No evidence for leakage around the diatom was observed, indicating a successful seal around the perimeter of the membrane. When the particles passed through the membrane, the temporary blockage resulted in a reduction in ionic current corresponding to the ratio between the bead and pore size. The characteristic electrophoretic mobility of the beads allowed a characterization of nanoparticles of different origin. The large number of nanopores available for particle translocation (>200) makes them ideal size-selective filters with a low probability of clogging. The combination of biomineralized and microfabricated structures shows a pathway for integrating low-cost nanostructures with BioMEMS devices.