Volume and concentration dosing in picolitres using a two-channel microfluidic AFM cantilever

We introduce a two-channel microfluidic atomic force microscopy (AFM) cantilever that combines the nanomechanical sensing functionality of an AFM cantilever with the ability to manipulate fluids of picolitres or smaller volumes through nanoscale apertures near the cantilever tip. Each channel is connected to a separate fluid reservoir, which can be independently controlled by pressure. Various systematic experiments with fluorescent liquids were done by either injecting the liquids from the on-chip reservoir or aspirating directly through the nanoscale apertures at the tip. A flow rate analysis of volume dosing, aspiration and concentration dosing inside the liquid medium was performed. To understand the fluid behaviour, an analytical model based on the hydrodynamic resistance, as well as numerical flow simulations of single and multi-phase conditions were performed and compared. By applying pressures between -500 mbar and 500 mbar to the reservoirs of the probe with respect to the ambient pressure, flow rates ranging from 10 fl s-1 to 83 pl s-1 were obtained inside the channels of the cantilever as predicted by the analytical model. The smallest dosing flow rate through the apertures was 720 fl s-1, which was obtained with a 10 mbar pressure on one reservoir and ambient pressure on the other. The solute concentration in the outflow could be tuned to values between 0% and 100% by pure convection and to values between 17.5% and 90% in combination with diffusion. The results prove that this new probe enables handling multiple fluids with the scope to inject different concentrations of analytes inside a single living cell and also perform regular AFM functionalities.