Effect of capillary action and gravitational force on resistive pulse sensing with nanopipettes

In Resistive Pulse Sensing, nanoparticles dispersed in solution are individually detected and characterized during their translocation through a narrow pore or channel. Electrophoretic force and fluid flow can be precisely adjusted to direct nanoparticles toward the sensing zone. The impact of various factors on nanoparticle translocation dynamics, including solution ionic strength, pH, applied potential difference, and pipette tip geometry, has been extensively investigated. In this work, we focus on the role of pipette filling height, an experimental parameter often overlooked despite its significant impact on the overall pressure gradient and the resulting flow through the pipette tip. We used a solution of NaCl 150 mM plus 0.1 % v/v Triton X-100 at pH 7.2, a pipette with radius of approximately 200 nm and a voltage of ± 200 mV. Our findings reveal that the pipette filling height emerges as the critical factor dictating the translocation direction of negatively charged 160 nm PMMA particles and surpassing the combined effect of electrokinetic forces. Ultimately, our results indicate that considering the pipette filling level could enhance the accurate interpretation of experimental results, offering an additional parameter for fine-tuning nanoparticles dynamics, thus providing a valuable tool to researchers in this field.