Aspiration through hollow cantilever-based nanopipette by means of evaporation

Presented is a method for aspirating liquids into a hollow atomic force microscope (AFM)-cantilever through a 350 nm-wide nozzle near the tip apex. The cantilever was made of transparent SiO₂ and connected a fluidic reservoir to an evaporation cell. The nanopipette-chip is suitable for mounting the microfluidic system into commercial AFMs. The channel inside the lever spontaneously filled with liquid by capillary forces upon which evaporation started and continuously pumped liquid from the reservoir. The resonance frequency of the cantilever was found to be 153.946 kHz when empty and a frequency shift of 92 Hz was measured when filled. The cantilever´s transparency allowed visualisation of the advancing meniscus in real time and confirmed the presence of aspirated, fluorescently labelled liquid. An aspiration rate of ~230 aL/s was measured. This value represents the flow rate in the microfluidic system when operated under ambient conditions (21°C temperature, 43% relative humidity). The estimated volume that has been aspirated in total was ~ 85.42 aL. The aspiration capability of the device was tested and analysed under an optical microscope using an aqueous solution of fluorescently labelled nanobeads. The nanopipetting experiments represent an extension over the authors´ earlier work which concentrated on the dispensing and imaging capabilities of a similar system.