Prof. Dr. Georg Papastavrou
Chair of Physical Chemistry II, University of Bayreuth, Germany
Atomic force microscopy (AFM) has developed in the last decades to a ‘classical’ surface analytical technique. Based on the simple idea of ‘scratching’ with a sharp tip over a surface, AFM became the origin of a plethora of imaging modes and combinations with other techniques. These AFM-based techniques impressively demonstrate the potential of and the need for further analytical techniques with high lateral resolution.
Here, some applications of the combination of AFM with nanofluidic techniques, also known as Fluid Force Microscopy (FluidFM), is presented. The basic idea is to fabricate AFM-cantilevers with an internal channel and an aperture in the micro- to nanometer range at the end. Originally this technique was intended to be used in the biomedical field, for example to manipulate single cells and bacteria. However, the possibility to eject and aspirate fluids, respectively, is much more universal and can be applied also in material or colloid science.
The colloidal probe technique, which is based on AFM, revolutionized direct force measurements. Unfortunately, for many applications, the surface chemistry and the minimum dimensions for the probe particles restricted the general use of this technique. The FluidFM-approach allowed for the first time to overcome these limitations. We demonstrated the use of 300 nm sized carboxylate-modified latex particles as well as sub-micron core-shell particles with a soft exterior layer as probes. Moreover, by utilizing electrokinetic effects in the interior channel, it became possible to detect the aspiration of small colloidal particles to the cantilever’s aperture in order to automatize the process.
Furthermore, the FluidFM-technique allows for structing or deposition of materials on the micrometer-level. We demonstrate a proof-of-principle for the etching of patterns in very soft hydrogel films. The resulting ‘chemical writing’ process has been studied in detail and the influence of various parameters, such as applied pressure and time, has been validated.
 Meister, A. et al. FluidFM: Combining Atomic Force Microscopy and Nanofluidics in a Universal Liquid Delivery System for Single Cell Applications and Beyond. Nano Letters 9, 2501-2507 (2009).
 Mark, A. et al. The Next Generation of Colloidal Probes: A Universal Approach for Soft and Ultra‐Small Particles. Small 15, 1902976 (2019).
 Mark, A. et al. Electrokinetics in Micro-channeled Cantilevers: Extending the Toolbox for Reversible Colloidal Probes and AFM-Based Nanofluidics. Scientific Reports 9, 20294 (2019).
 Helfricht, N. et al. Writing with Fluid: Structuring Hydrogels with Micrometer Precision by AFM in Combination with Nanofluidics. Small 13, 1700962 (2017).