Electrokinetics in Micro-channeled Cantilevers: Extending the Toolbox for Reversible Colloidal Probes and AFM-Based Nanofluidics

Patrick Knödler,Thorsten Schumacher,Nicolas Helfricht,Matthias Karg,Markus Lippitz,Andreas Mark,Astrid Rauh,Binyang Du,Jinqiao Xue,Georg Papastavrou

doi:10.1038/s41598-019-56716-0

Abstract

The combination of atomic force microscopy (AFM) with nanofluidics, also referred to as FluidFM, has facilitated new applications in scanning ion conductance microscopy, direct force measurements, lithography, or controlled nanoparticle deposition. An essential element of this new type of AFMs is its cantilever, which bears an internal micro-channel with a defined aperture at the end. Here, we present a new approach for in-situ characterization of the internal micro-channels, which is non-destructive and based on electrochemical methods. It allows for probing the internal environment of a micro-channeled cantilever and the corresponding aperture, respectively. Acquiring the streaming current in the micro-channel allows to determine not only the state of the aperture over a wide range of ionic strengths but also the surface chemistry of the cantilever’s internal channel. The high practical applicability of this method is demonstrated by detecting the aspiration of polymeric, inorganic and hydrogel particles with diameters ranging from several µm down to 300 nm. By verifying in-situ the state of the aperture, i.e. open versus closed, electrophysiological or nano-deposition experiments will be significantly facilitated. Moreover, our approach is of high significance for direct force measurements by the FluidFM-technique and sub-micron colloidal probes.

Highlights

In the last three decades, the atomic force microscope (AFM) developed to an important analytical tool in colloid and interface science[1]
This cross-section has been obtained by focused ion beam milling (FIB) and illustrates the sandwich-like composition of the cantilever
In a number of separate experiments we demonstrated that the detection of aspiration events by streaming current can be applied to sub-micrometer colloidal particles and soft structures, such as hydrogel beads

Summary

Introduction

In the last three decades, the atomic force microscope (AFM) developed to an important analytical tool in colloid and interface science[1]. A new type of cantilever has been presented that bears a micro-channel in its interior and allowing for combining nanofluidics with AFM12 This approach is often referred to as FluidFM-technique and represents arguably a revolutionary step forward in terms of versatility for many of the aforementioned applications of AFM13–16. The here-presented new method would be a direct approach that is based on electrical signals These can be evaluated directly and be made visible to the operator of the AFM. As they can be interfaced to the control electronics, the implementation of combinatoric methods for direct force measurements would become feasible. The implementation of methods inspired by electrophysiology allows to unlock the full potential of the FluidFM-technology

Methods

Results

Conclusion