Combined AFM — Nanopipette cartridge system for actively dispensing femtolitre droplets

We present a method to actively dispense droplets with volumes in the range of a few femtolitres by means of a nanopipette cartridge, sealed off by a soft-lid with integrated thermal pump. This sealing prevented premature evaporation of the reagent for more than 40 hours and enabled active dispensing. The cartridge is suitable for mounting in commercial AFMs. The operation of the device was tested and analyzed under an optical microscope using water with an added fluorescent marker. To clearly see the pumping effect, we selected a nanopipette with a hydrophobic channel to prevent self-filling by the capillary effect. Using a current of around 300 mA for heating the thermal pump was sufficient to evaporate liquid inside the reservoir, causing a high enough pressure to overcome the resistance of the capillary pressure, which was estimated to be 0.87 bar. The transparent cantilever allowed realtime visualization of the advancing meniscus and revealed a velocity of up to ~2.4 μm/s corresponding to a pumping speed of about 19.5 femtolitres per second. Measuring changes in the mechanical resonance of the nanopipette allowed us estimating the deposited mass, from which we could calculate a typical droplet volume of 30 femtolitres.