Advancing atomic force microscopy-scanning electrochemical microscopy based sensing platforms for biological applications
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Combined atomic force microscopy-scanning electrochemical microscopy (AFM-SECM) is capable of providing simultaneous topographical and electrochemical imaging at sample surfaces. Integration of amperometric biosensors at tip-integrated electrodes recessed from the apex of the AFM tip further enhances the versatility of such bifunctional probes. Of particular interest to this work was the detection of adenosine triphosphate (ATP) at a cellular level, since ATP is involved in many biologically relevant processes. There are challenges concerning the integration of biosensors into bifunctional AFM-SECM probes. This thesis focuses on addressing and advancing several of these limitations. Thin insulation layers are important for AFM-SECM based applications to enhance AFM and SECM performance. Plasma-polymerized fluorocarbon membranes are introduced as novel thin film insulation materials for AFM-SECM probes. Insulation layers with a thickness of < 300 nm were found to exhibit excellent insulating properties and satisfying temporal stability for successful application in AFM-SECM experiments. Furthermore new approaches for increasing the electrode area in conventionally focused ion beam (FIB) fabricated AFM-SECM probes were implemented, since enhancement of the current response in conjunction with biosensing experiments is required. Ion beam induced deposition (IBID) was used to generate platinum carbon (PtC) deposits at AFM-SECM probes, thereby successfully increasing the tip-integrated electrode area. PtC composites were thoroughly characterized in terms of their physical and electrochemical properties. Since a high carbon fraction in the PtC composite was inhibiting the charge transfer kinetics at the electrode surface for certain analytes, several pre-treatment strategies were investigated including annealing, UV/ozone treatment, and FIB milling. FIB milling proved to be the most promising procedure improving charge transfer properties at the electrode along with fabrication compatibility at AFM-SECM probes. The last part of this thesis aimed at providing fundamental studies on AFM-SECM application at live epithelial cell monolayers. AFM was used in different imaging modes to characterize the topography of epithelial cells. ATP detection at epithelial cells was achieved with amperometric biosensors combined with non-invasive SECM. Biosensors were further miniaturized at batch-fabricated AFM-SECM probes enabling laterally-resolved detection of ATP at epithelial cells. Additionally, PtC composite materials were evaluated for applicability as transducer platforms for enzymatic biosensors.