Mf 900 microforge

Schott glass capillaries (o.d. 1.5 mm, i.d. 1.0 mm, World Precision Instruments, Sarasota, FL) were cleaned by sonication in H₂O followed by ethanol, and dried at 60 °C for 1 h. Dried capillaries were pulled into pipets using a P-97 micropipette puller (Sutter Instrument Co., Novato, CA), and cut and fire polished using a MF-900 microforge (Narishige, East Meadow, NY) to an aperture i.d. of 15-20 μm. The pipet surface was cleaned with 1 M HNO₃ for 30 min, washed with H₂O and then dried at 110 °C for 1 h. Pipets were then silanized with PFDCS vapor for 10 min in a heated chamber.^{33 (link)} Modified pipets were sequentially washed with acetonitrile, acetone, H₂O and ethanol, then dried at 60 °C for 1 h prior to use.

Borosilicate capillaries (1.5 outer diameter and 1.1 mm inner diameter) were purchased from World Precision Instruments, (Sarasota, FL) and were fabricated into pipette apertures with 25 – 30 μm diameter using a P-97 micropipette puller (Sutter Instruments, Novato, CA) and fire polished with a model MF-900 microforge (Narishige, East Meadow, NY) for BLM formation. A schematic of the pipette fabrication process is shown in Supporting Information, Figure S1. Glass pipette apertures were silane functionalized using a solution phase method reported previously.^{11 (link)} Briefly, glass pipettes were filled and submerged in 0.1 M HNO₃ for 30 min followed by rinsing with H₂O at least 3 times. The pipettes were rinsed with acetone and dried on a hot plate at 80 – 100 °C for 5 min or in an oven at 70 °C for 15 – 30 min. The pipettes were filled with then submerged in 2% v/v silane solution in either ACN (for CPDCS) or toluene (for PFDCS) for 6 – 12 h. Pipettes functionalized with CPDCS were rinsed with ACN while PFDCS-modified pipettes were rinsed with toluene. Finally, the resulting CPDCS- or PFDCS-functionalized pipettes were rinsed with ethanol and H₂O. Only those silane-modified glass pipette apertures that showed a high success rate (> 80%) in the formation of BLMs, as monitored by repetitive formation and voltage-induced breakdown, were selected for use in further experiments.