A recent study establishes that several recent reports are indicating that small nanoscale spikes or nanopillars of various compositions can kill growing bacteria, most likely by impaling them and presumably causing leakage of cytoplasmic components and collapse of cells’ proton motive force, all leading to cell death.

While the latter findings are notable, they seem reasonable for growing bacterial cell killing by black silicon (BSI) nanopillars, given the silicon’s hardness and the relatively low elasticity of bacterial cells. They were also able to kill adsorbed dormant spores of the bacterium Bacillus subtilis. There have been several recent reports indicating that small nanoscale spikes or nanopillars of various compositions can kill growing bacteria.

Most likely by impaling them and presumably causing leakage of cytoplasmic components and collapse of cells’ proton motive force, all leading to cell death. While the latter findings are notable, they seem reasonable for growing bacterial cell killing by black silicon (BSI) nanopillars, given the silicon’s hardness and the relatively low elasticity of bacterial cells. However, it was reported that some of these surfaces, nanopillars of BSI as well as on dragonfly wings, were also able to kill adsorbed dormant spores of the bacterium Bacillus subtilis.

The viability of dormant and germinated spores and growing cells recovered from BSI and control wafers and the starting spores and growing cells was determined by spotting duplicate 10 μl aliquots of serial 10-fold dilutions in PBS on LB plates with antibiotics if appropriate.  The plates were incubated for 24 hr at 37 °C (B. subtilis strains) or 30 °C (B. megaterium and B. cereus), and colonies were counted.  The BSI wafer preparation is a lithography-free mixed reactive ion etching (RIE) process, where etching due to F radicals and passivation from O2 oxidation occurs at the same time.

The process produces a homogeneously distributed layer of needle-like nanopillars across the full wafer surface. The average spacing between nanopillars is estimated to be around 200 nm. The new results indicate that dormant spores are neither impaled nor killed by incubation or even drying on a bed of black silicon nanopillars.

To test the possible killing mechanism of cells by nanopillar-induced impaling, we are performing molecular dynamics (MD) simulations of nanoindentation and nanocontact on biological membranes and cell walls, with a focus on organic-inorganic interfaces.

A potential extension of this research is to explore a new strategy to enhance the bactericidal efficiency of BSI. One hypothesis is that water nanojets generated by rapidly collapsing nanobubbles near cells can produce sufficiently large stresses to cause poration in the cell wall and subsequent interaction with BSI nanopillars will be more effective in destroying the integrity of the cell wall. MD simulations of nanoindentation can test this hypothesis in conjunction with nanobubble collapse.