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Abstract

This thesis focuses on the properties and applications of ultrathin poly(ethylene glycol) (PEG) films and freestanding nanosheets, fabricated by thermally-induced crosslinking of amine/epoxy decorated STAR-PEG precursors. In addition, my research was also extended to another kind of molecular films – self-assembled monolayers (SAMs), for which the performance of a model aromatic-aliphatic tripodal SAM on Au(111) in the context of electron beam lithography (EBL) and fabrication of carbon nanomembranes (CNMs) – another kind of nanosheets – was tested.

As the first subproject, the effect of ultraviolet (UV) light (254 nm) on the PEG films was explored. UV irradiation was shown to result in progressive decomposition of the PEG material followed by desorption of released fragments, while preserving the original chemical composition and properties of the films, which offers potential for 3D patterning of PEG materials with retaining bioinert and hydrogel properties. As the second subproject, the effect of molecular weight (MW) of the precursors (2000-20000 g/mol) on the properties of the PEG films and nanosheets was studied. These systems exhibited pronounced biorepulsive, hydrogel, and elastic properties which varied with the MW. The MW affected in particular the swelling behavior and permeability of the PEG films as well as elastic properties of the PEG nanosheets. As the following subproject, the effect of other relevant parameters, such as a deviation from the equilibrium 1:1 composition of the precursors and electron and UV (additional studies) treatment were studied. In particular, the elasticity and stability of the PEG nanosheets were found to be strongly deteriorated by electron irradiation. In contrast, UV irradiation (254 nm) did not affect their elastic properties, in agreement with my previous results on this subject. Within the further subprojects, PEG films were used as a porous and bioinert matrix for DNA sensing, relying on immobilization and hybridization of single-stranded DNA (ssDNA) in the PEG matrix. The immobilization of the probe ssDNA was based on either NHS-ester-amine or thiol-epoxy linkage to the free amine or epoxy groups in the specifically prepared PEG matrix, respectively. In both cases, efficient immobilization of the probe ssDNA and high selectivity and hybridization efficiency of the resulting 3D ssDNA arrays with respect to the target strands were demonstrated. As a suitable transduction technique for the DNA sensing, requiring no ssDNA labeling, electrochemical impedance spectroscopy was employed.

Within a further subproject, a series of thin (80-100 nm) PEG-fullerene (C60) composite films were prepared by immersion, one-pot, and reflux methods. These films exhibited distinct optical and electrochemical properties of C60, merged with some favorable characteristics of the PEG matrix, resulting, in particular, in good electrochemical conductivity and high elasticity. It was demonstrated that these films can be detached from the primary substrate to form free-standing composite nanosheets, having potential for various applications such as flexible electronics, photodetectors, and electrochemical biosensors.

The final subproject was the electron-beam-induced treatment of a triptycene-based SAM (Trip-T1). Upon electron irradiation, this monolayer was found to exhibit behavior similar to that of monopodal aromatic monolayers, showing a clear dominance of intermolecular crosslinking. It was demonstrated that the Trip-T1 SAM can serve as a negative resist in EBL, similar to the reference, monopodal benzylthiol (PT1) SAM. Finally, robust and defect-free CNMs could be successfully fabricated from the Trip-T1 SAM, which, however, required a somewhat higher dose (80 mC/cm2) than for the reference PT1 monolayer (40 mC/cm2). These CNMs correspond to the low limit of material density for such objects.