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

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.

Contactless calibration of microchanneled AFM cantilevers for fluidic force microscopy

“…For all microchanneled cantilevers, the width of the channel was 27 ± 3 µm and length 1100 ± 110 µm. 65 The channel height was 0.95 ± 0.05 µm for cantilevers with 2 N/m and 0.5 ± 0.05 µm for 0.3 N/m, respectively. Nominal aperture sizes of 2, 4, and 8 µm have been used.…”

“…Microchanneled cantilevers with different channel heights and aperture diameters have been purchased from Cytosurge AG. For all microchanneled cantilevers, the width of the channel was 27 ± 3 µm and length 1100 ± 110 µm 65 . The channel height was 0.95 ± 0.05 µm for cantilevers with 2 N/m and 0.5 ± 0.05 µm for 0.3 N/m, respectively.…”

“…62,63 The internal flow of liquid in the cantilever can be followed by particle image tracking 64 or by streaming potential measurements. 65 Cantilevers with an internal microchannel for fluidic force microscopy are commercially available and have also been purposely constructed. [66][67][68][69] To allow for an internal channel, commonly, a sandwich type of structure is chosen.…”

“…Thereby, various types of experimental procedures can be implemented for applications in cell biology, 25,53–57 colloidal science, 16–18 the structuring of hydrogels and interfaces, 58,59 scanning ion conductance microscopy (SICM), 60,61 and the deposition of metals and colloidal particles 62,63 . The internal flow of liquid in the cantilever can be followed by particle image tracking 64 or by streaming potential measurements 65 …”

Development and application of modern biophysical techniques for single-cell and microbead adhesion measurements

“…As with any new technology, in the case of FluidFM, new solutions entail new challenges. Ideally, fixing the bead or cell to the aperture with a negative pressure creates an airtight physical contact, however this is impossible to confirm without using complex streaming potential measurements [108]. Non-tight connection might cause the grabbed object to move during the approach or retraction phase, generating artifacts in the FD curve.…”