Figure 2.

Comparison between the ion current during approach of a cell and the noise levels of the MoNa and SICM system (A,B) Ion current during approach and penetration of an U2OS cell with the MoNa system (current range: ±10 nA) (A) and the SICM system (B). The blue area indicates decreasing current, as the pipette tip approaches and comes in contact with the membranes. After penetration, the current stops dropping notably and enters the red area with anything between a slight up or down slope of ion current. This principle can be observed for both outer cell membrane and nuclear membrane. Although the signal acquired by the commercial setup is much smoother and contains less noise, the injection features are clearly visible in the current data of our system as well. For both experiments, the electrode voltage was set to +100 mV and the approach speed was ~1 µm/s. (C) Mean noise (RMS, root mean square) levels and standard deviations of different range settings of the operational amplifier and for comparison the original SICM (pre-) amplifier configuration. These values cover the entire signal chain from electrodes to amplifier to A/D converter to computer. The measurements were performed under typical approach conditions i.e. phosphate buffered saline (PBS) was used as medium and the applied voltages ranged from −200 mV to +200 mV in 100 mV increments. As expected, the SICM system delivered the lowest noise level at (8.08 ± 0.24 pA). The custom setup gives (82 ± 11 pA), (239 ± 34 pA), (3.31 ± 0.17 nA) and (9.68 ± 0.12 nA) dependent on the selected current range.