Silicon-based Plasmonic Surfaces and Flow Microreactors

Abstract

There is a need for streamlining certain laboratory techniques in synthetic chemistry for the discovery of novel molecules. This applies to many fields, especially to drug discovery, which relies on the cost-efficient design and synthesis of drug candidates. As the requirements for new materials and molecules in medicinal chemistry and other fields get more complex, new technologies have received great attention in recent years. New chemical reactor types that benefit from process automation, continuous manufacturing and miniaturisation have been developed for that purpose. Miniaturised flow microreactors can provide exceptionally uniform reaction conditions and possibly increase the space-time yield compared to batch reactors—in particular for large reaction screenings. They also facilitate multistep reactions without the need for working up the intermediates. Significant contributions have been made from microsystems technology in the design and fabrication of Si- and glass-based microfluidic devices as flow microreactors. Due to their high chemical resistance and their compatibility with micro- and nanofabrication processing, they are well suited as materials in lab-on-a-chip systems for synthetic chemistry. The aim of this work was to establish the basis for a self-optimising reaction screening device on a chip. The system consists of multiple connected flow microreactor compartments populated with different heterogeneous organocatalysts. These molecular catalysts are immobilised as monolayers on the channel surfaces and the reactors are therefore called "wall-coated flow microreactors". Several fundamental aspects of this aim were investigated. The viability of wall-coated flow microreactors for this purpose was studied by designing various reactor types and developing a versatile fabrication protocol using microfabrication. This fabrication process involved the metallisation of the microchannel surfaces with a Au thin film. This served to bind thiol-linked organocatalysts as self-assembled monolayers (SAMs) to the reactor surfaces. The detection and characterisation of such SAMs was a major focus of this work. In the context of the wall-coated flow microreactors, methods for the assessment of the presence and stability of the catalyst monolayers are important. Field-enhancing, nanopatterned surfaces can enable the detection of immobilised molecular monolayers, e.g. by surface-enhanced Raman spectroscopy (SERS). Therefore, a fabrication protocol was developed to generate a large-area plasmonic surface patterning in a Au thin film on Si, which consisted of regular nanohole arrays in the metallic layer. This plasmonic surface delivers a uniform field-enhancement over the entire patterned area for fast SERS spectra acquisition of the molecular monolayers. For the deposition of different catalyst monolayers in connected microfluidic compartments, methods for the site-selective functionalisation of the reactor surfaces and the chemical modification of these molecular monolayers on the surface are required. Therefore, this project also focused on detecting the chemical and electrochemical modifications of SAMs. The SERS monitoring of a two-step solid-phase synthesis in a SAM could be demonstrated using the developed plasmonic nanohole arrays. For the site-selective surface functionalisation, a fabrication process was developed to integrate the Au nanohole arrays in thin-film electrodes. This allows first the electrochemically triggered functionalisation of the Au thin-film electrode, followed by characterisation of the deposited molecular layer by SERS. Substantial contributions were thus made with the described research projects towards reaction screening using Si-based wall-coated flow microreactor networks