Schottky field emitter

Scanning electron microscopy (SEM) was implemented to study the cell-nanorod interactions after centrifugation as well as the pristine nanostructured electrodes using a scanning electron microscope (LEO 1530 Gemini with Schottky field emitter, Zeiss). S. oneidensis WT cells were loaded on AAO and PC nanostructured electrodes according to the method presented above in the ‘fluorescence microscopy analysis’ section. A lower cell density culture (i.e. OD₆₀₀ = 0.1) was used to improve the visualisation of individual cells on the nanostructured surfaces. SEM sample preparation was performed by first fixing the cells in a 4% formaldehyde solution dissolved in M4 salt buffer (see Table S3 in the supplemental information for composition) for 1 hour at 4°C. A dilution series in increasing concentrations of ethanol was then applied to remove the water content (i.e. 0, 25 and 50% ethanol) after which the sample was frozen at −80°C overnight and then freeze dried at −80°C and under vacuum for approx. 8 hours. The samples were sputter coated with a 5 nm conductive Pt layer prior to SEM analysis. Post analysis of the SEM images was performed using the open source imaging software Fiji.

Two laboratories carried out the NP measurements with SEM. The first instrument (SEM-1) is a Zeiss-Ultra+ field-emission gun (FEG) microscope equipped with a GEMINI column and an in-lens secondary electron detector (Zeiss, Oberkochen, Germany) particularly useful for measuring nano-objects due to its high sensitivity to the surface morphology. The Zeiss-Ultra+ resolution is specified by the manufacturer as 1.7 nm at 1 kV and 1.0 nm at 15 kV. The second instrument (SEM-2) is also a Zeiss microscope of type Supra 40 equipped with a Schottky field emitter (Zeiss, Oberkochen, Germany). Regarding both SEMs, the measurements were performed with the secondary electron in-lens detector.