Synthesis of Addressable NHC-linkers for gold surfaces

Abstract

In the last decades, tailoring surface properties has raised significant attraction in a variety of application fields, such as electronics, surface patterning and biosensing. Until recently, mostly self- assembled monolayers (SAMs) based on thiols were used to introduce functionalities on surfaces. However, these SAMs lack thermal and chemical stability limiting their applicability. In 2016, Crudden et. al. revolutionized the field by establishing a mild method for carbene-based SAM formation on gold surfaces, using single- source hydrogen carbonate NHC precursors. An alternative mild strategy to form NHC-based SAMs uses the corresponding benzimidazolium triflate salt in combination with a weak base. These mild approaches towards highly stable NHC-based SAMs on gold provide an essential toolbox to introduce addressable functional groups to the surface which otherwise would not be compatible with the reactivity of a free carbene.In this thesis, the synthesis of NHC- precursors with (bio-)orthogonally clickable functional groups (azides, (strained) alkynes and tetrazines) is described. The addressability in z-axis enables the introduction of various functionalities for applications, such as photoswitches and in biosensors (Figure 1a), while the ability of crosslinking the precursors in the x-axis (Figure 1b) increase the SAM stability and improves their efficiency in applications like biosensors or photoswitchable devices.After successful synthesis of these NHC- SAM precursors, different immobilization strategies with hydrogen carbonate or triflates as counterions are investigated. Furthermore, the obtained materials are tested towards their compatibility with functional groups during the immobilization process. Depending on the results of this study, an optimized strategy is used to introduce the NHC-based SAMs on the gold surface. In the final step, the newly formed SAM precursors are modified with a photoactive tag using click chemistry (CuAAC) enabling their application in switches. Exemplarily, selected compounds are tested as addressable NHC-precursors for applications in biosensor by comparing the protein coverage of NHC- and S-based SAMs on gold surfaces.