How to remove air bubbles from your nanopipettes

Researchers at Kanazawa University have reported an efficient method for filling a batch of nanopipettes with a pore opening of no more than 10 nm.

The function of a nanopipette is usually to enable the transport, and their detection, of nanometre-sized objects (in solution) through the pipette pore. Completely filling a nanopipette with a solution is challenging, however; because of the capillary force, an ‘air bubble’ is nearly always present in the pipette’s tip. Removing the air bubble has proven to be especially problematic for nanopipettes with a pore opening of 10 nm or less.

Now, Shinji Watanabe and colleagues have found a simple but efficient way to completely fill a batch of many nanopipettes with a pore opening of 10 nm. Described in the journal Analytical Chemistry, their method is based on the application of a temperature gradient to the nanopipette tips so that residual air bubbles are driven out.

The scientists applied their ‘thermally driven method’ to a batch of 94 pipettes, aligned lengthwise next to each other, all with a pore diameter of around 10 nm. The pipettes were put on a metal plate kept at a temperature of 80°C, with their tips protruding from the plate, resulting in a temperature gradient. Time-lapsed optical microscopy images of the filling process of the nanopipettes showed that after 1200 seconds, the tips were completely filled with solution and the air bubbles were driven out of the pipettes.

In order to double-check that the pipettes were indeed bubble-free, the colleagues performed so-called I–V measurements. Every pipette was filled with a solution of potassium chloride (KCl), which is conducting. Both pipette ends were then contacted with electrodes. If an electrical current runs between the ends — specifically, if the pipette has an electrical conductivity below a few GΩ — then filling with the solution is complete. The researchers observed electrical currents and therefore filling for the whole batch of pipettes.

The scientists also performed transmission electron microscopy (TEM) measurements of pipettes with pore diameters below 10 nm. Although the thermally driven method leads to good electrical contacts, particle-like structures were observed inside the tips of the nanopipettes — demonstrating that, according to the researchers, “TEM observation without inducing pipette deformation is important for accurately determining the characteristics of sub-10-nm nanopipettes.”

The colleagues concluded that their method is very practical and easy to introduce in nanopipette fabrication. They said their study “will provide a significant contribution to various fields of nanoscience using nanopipettes”.

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