Biosensor surface chemistry for oriented protein immobilization and biochip patterning

Abstract: This licentiate thesis is focused on two methods for protein immobilization to biosensor surfaces for future applications in protein microarray formats. The common denominator is a surface chemistry based on a gold substrate with a self-assembled monolayer (SAM) of functionalized alkanethiolates. Both methods involve photochemistry, in the first case for direct immobilization of proteins to the surface, in the other for grafting a hydrogel, which is then used for protein immobilization.Paper I describes the development and characterization of Chelation Assisted Photoimmobilization (CAP), a three-component surface chemistry that allows for covalent attachment and controlled orientation of the immobilized recognition molecule (ligand) and thereby provides a robust sensor surface for detection of analyte in solution. The concept was demonstrated using His-tagged IgG-Fc as the ligand and protein A as the analyte. Surprisingly, as concluded from IR spectroscopy and surface plasmon resonance (SPR) analysis, the binding ability of this bivalent ligand was found to be more than two times higher with random orientation obtained by amine coupling than with homogeneous orientation obtained by CAP. It is suggested that a multivalent ligand is less sensitive to orientation effects than a monovalent ligand and that island formation of the alkanethiolates used for CAP results in a locally high ligand density and steric hindrance.Paper II describes the development of nanoscale hydrogel structures. These were photografted on a SAM pattern obtained by dip-pen nanolithography (DPN) and subsequent backfilling. The hydrogel grew fast on the hydrophilic patterns and slower on the hydrophobic background, which contained a buried oligo(ethylene glycol) (OEG) chain. Using IR spectroscopy, it was found that the OEG part was degraded during UV light irradiation and acted as a sacrificial layer. In this process other OEG residues were exposed and acted as new starting points for the self-initiated photografting and photopolymerization (SIPGP). A biotin derivative was immobilized to the hydrogel density pattern and interaction with streptavidin was demonstrated by epifluorescence microscopy.