Quantifying and Utilizing Electroosmotic Flow for Mechanical Measurements with the Scanning Ion Conductance Microscope

The scanning ion conductance microscope (SICM) is an emerging imaging technique for the investigation of delicate samples on the nanometer scale in liquid environments using ion current through a glass nanopipette. In recent years, the SICM has been increasingly applied to mechanical measurements, typically using a microfluidic flow in the nanopipette induced by hydrostatic pressure. Here, we introduce the use of electroosmotic flow (EOF) in mechanical SICM measurements. We show that the EOF in small SICM nanopipettes is comparable to the flow induced by commonly applied hydrostatic pressures. We quantify the electroosmotic mobility, which is a central parameter of EOF but strongly depends on experimental conditions, by measuring the streaming current independent of nanopipette geometry. Using decane microdroplets, we show that both EOF and hydrostatic pressure can be used to mechanically probe elastic samples on the nanometer scale. We then develop a numerical model to quantify the stiffness and the Young's modulus of elastic samples using EOF. Finally, we use EOF to map the Young's modulus of living cells, which gives similar results to the hydrostatic pressure method. We thereby demonstrate that EOF can be used to quantitatively probe sample stiffness with the SICM.