Soft probes for bio-electrochemical imaging

The aim of bioimaging is to visualize properties and processes of living objects or biological samples and extracts image-related information for revealing important physical structures or map the distribution of specific biomolecules in tissues for clinical purposes. However, current bioimaging methods which mainly rely on the optical detection methods can suffer from optical interferences and thus bias the imaging results. Bio-electrochemical imaging with micrometer resolution represents a promising alternative tool since electrochemical signals, e.g. faradaic currents, depend exclusively on the redox reactions occurring at the sample surface and a sensing probe. Scanning electrochemical microscopy (SECM) is a scanning probe technique that is composed of a micro- or nanoelectrode that can be positioned or scanned in close proximity to an interface. Faradaic current signals can be recorded due to the flux of redox active species between the sample and an amperometric SECM probe. SECM can be used to image the topography and reactivity of biological specimens for mapping localized biochemical activity. Although SECM has been applied to different biological systems, SECM studies of tissues are still under exploration. The reason is due to the shape and high roughness of such real samples and requires overcoming major drawbacks in conventional SECM instrumentation when scanning large, i.e. square centimeter sized areas with irregular surface keeping a constant working distance. This thesis aims to develop various reliable SECM bioimaging techniques for the study of the antioxidant defense system of fruit peels, distribution of biomarkers and nanomaterials in thin and thick animal samples, as well as human melanoma. Particularly the last is of major importance, because melanoma is the most lethal form of skin cancer striking thousands of people around the world. The survival rate depends on the stage of the cancer when it is diagnosed. Therefore, reliable methodologies for early diagnosis and unequivocal identification of cancer stages are of high relevance. One of the well-known melanoma biomarkers is « tyrosinase » which is the key enzyme involved in the biosynthesis of melanin and fruit maturation. Different tyrosinase SECM detection strategies were developed for the analysis of the spatial distribution of tyrosinase in melanoma as well as in banana samples. It demonstrated in this thesis that that SECM could improve the diagnosis and understanding of different melanoma stages based on highly resolved maps of the tyrosinase distribution while being immune against optical interferences, e.g. from the presence of melanin in the skin samples. Spider probe composed of eight independent microelectrodes developed in this thesis allowed the large area scanning over thin and thick animal tissues in contact mode. The redox active proteins inside the entire mouse heart was imaged for the first time by spider probe. In addition, the distributions of injected conductive graphene nanoribbons (GONRs) for drug delivery were studied by Soft-Probe-SECM. Through the mapping of feedback mode currents over conductive GONRs, the GONRs were found concentrated inside lobules, which are hexagonal microstructures in the liver. Finally, this thesis describes a non-invasive electrochemical strategy for mapping the antioxidant (AO) activity of apple peels using Soft-Probe-SECM. The global AO activity in the apple peel including lenticels and regions with artific