title: New Techniques for Interface Characterization by Combining Scanning Probe Microscopy with Electrochemistry and Nanofluidics
Creators: name: Mark, Andreas
Publisher: name: University of Bayreuth
Year: 2020
Subjects: subject: Colloidal Interactions
subject: Atomic Force Microscopy, FluidFM
subject: Nanoparticles
subject: Electrochemistry
subject: Electrokinetics
subject: Surface Characterization
subject: 500
subject: 540

description: Upcoming challenges in energy storage systems and the growing demand for automatized high-throughput screening assays initiated the development of new approaches in atomic force microscopy (AFM). A better understanding of the structure-activity relationship in such systems is a prerequisite for further rational design. However, it requires an in-depth understanding of the interfacial properties on a fundamental level. In this respect, AFM provides the possibility to locally probe and manipulate the interface. Novel concepts in probe microscopy incorporate nanofluidics or analytical techniques such as electrochemistry, which allow to simultaneously study multiple material properties. However, a major prerequisite for novel AFM concepts are suitable cantilever probes. The objective of this thesis covers the development and evaluation of combinatoric AFM methods and the understanding of physico-chemical interactions and processes taking place at the probe tip. In the first part of the thesis, AFM cantilevers bearing a nanoelectrode tip have been used to sense locally resolved faradaic processes on electrodes with heterogeneous reactivity. Additionally, this approach provides access to the topography of the sample and surface properties such as adhesion. As size and surface properties of the nanoelectrode tip dramatically influence its resolution, test samples are a major prerequisite for monitoring the quality of the tip. Here, three different and simple to prepare micro-structured electrode samples have been evaluated, which can be fabricated without the need of expensive or complicated equipment. It was demonstrated that these type of electrochemical probes are able to resolve heterogeneities in redox-reactivity with a lateral resolution &lt; 100 nm. The AFM additionally allows to study macroscopic aggregation and adsorption processes of colloids on chemically heterogeneous surfaces by directly measuring interaction forces on the single particle level. In this project the colloidal probe technique has been used to study direct interaction forces of latex particles on orthogonally functionalized gold nano-mesh electrodes. By laterally scanning the chemically heterogeneous surface with the latex probe, electrostatic interactions and adhesion forces have been measured as a function of the pH. The measured electrostatic focusing effect towards the nano-holes properly explains the macroscopic self-assembly of these particles on the nano-mesh electrode. Since the preparation of 'classical' colloidal probes is restricted to the µm-level, industrial relevant nanoparticles were so far hardly accessible for direct force measurements. Due to recent progresses in micro-fabrication techniques, it became possible to construct micro-channeled AFM cantilevers with a defined aperture. These cantilevers allow for combining the force-sensing capabilities of an AFM with nanofluidic techniques, for example to aspirate or eject fluids at the aperture by an externally applied pressure. Furthermore, the so-called FluidFM technology allows the reversible aspiration of particles at the aperture, while forming an exchangeable colloidal probe. Here, for the first time direct force measurements with individual nanoparticles of diameters down to 330 nm could be reported. Moreover, delicate core-shell particles consisting of a soft Poly(N-isopropylacrylamide) (PNIPAM) shell and a rigid silica core could be used as probe particles. The thermo-responsive nature of the PNIPAM hydrogel shell has also been studied in terms of its impact on particle size, adhesion properties and hydrogel stiffness. The handling of nanoparticles with the FluidFM cantilever is a challenging task, as it cannot be followed anymore by optical microscopy. Here, two new concepts were developed that make use of electrical signals in order to detect whether the cantilever aperture is open or blocked by an aspirated particle. By means of electrodes integrated in the cantilever's reservoir and the bath solution, both ionic-conductivity and flow-induced streaming current have been monitored. Both methods allow to directly test the status of the aperture. However, the conductivity measurements require an elaborate insulation of the connection to the electrode in order to avoid parasitic leakage currents. By contrast, the streaming current approach demands no further insulation effort and additionally enables to determine the surface chemistry of the micro-channel. As the FluidFM technology can handle well-defined flow rates with pL-precision combined with an accurate control of the loading force, novel concepts for chemical lithography could be implemented. This is especially relevant for soft hydrogels that so far could not be processed. Here, a pH-responsive hydrogel has been formed on electrodes by an electrochemically induced gelation process. By means of a micro-channeled cantilevers alkaline solution has been dispensed on reversibly formed hydrogel films leading to a local dissolution of the hydrogel. For the first time, soft hydrogels have been in-situ structured with sub-µm resolution by subtractive processing.