Advanced microscopy and imaging techniques are essential for investigating cellular and subcellular architecture and chemical composition. In this thesis, Photoemission Electron Microscopy (PEEM) is developed and applied as a surface- and chemically sensitive imaging modality for biological systems, with a particular focus on neutrophils. Neutrophils are the most abundant white blood cells in humans and constitute a first line of defence in the innate immune system. Upon activation, they perform key antimicrobial functions, including phagocytosis, degranulation, and the release of neutrophil extracellular traps (NETs), which trap, immobilize, and neutralize invading pathogens using DNA and antimicrobial agents.

In this work, NETs formation in combination with iron oxide (FeO_x) nanoparticles is investigated, including the magnetically guided assembly of linear and cross-shaped NETs-FeO_x nanoparticle μ-threads, induced by magnetic nanoparticles and externally applied magnetic fields. These engineered extracellular structures hold potential for materials with intrinsic antibacterial properties and, under magnetic control, exhibit a high degree of orientational order. The ability to impose controlled macroscopic alignment on DNA-based structures further suggests opportunities for the development of robust, oriented macromolecular systems, with relevance for structured organic and conjugated materials.

Work-function mapping based on surface-sensitive contrast in PEEM is a powerful and well-established technique in surface physics and materials science; however, its application in the life sciences, particularly for subcellular imaging, remains largely unexplored. To increase the information content of cellular imaging, this thesis introduces a photoemission-based strategy that integrates three-dimensional spatial reconstruction with pixel-resolved spectral (work-function) contrast, enabling quantitative insight into cellular composition and organization. Local variations in the work function provide intrinsic contrast between subcellular structures based on their molecular composition, allowing visualization of the polylobulated nuclei, intracellular granules, and membrane structures of neutrophils.

The thesis further includes the development and surface modification of gadolinium-incorporated cerium oxide nanoparticles for use as contrast agents in magnetic resonance and X-ray imaging. Two functionalization strategies are presented to enable targeting and therapeutic functionality. X-ray Photoemission Spectroscopy (XPS) is employed for chemical characterization of both nanoparticles and biological structures, highlighting the broader potential of photoemission-based methods within the life sciences.