Self Assembled Monolayers for Quartz Crystal Microbalance based Biosensing

Sammanfattning: The work in this thesis has been focused on developing surfaces for use in biosensor systems, especially for quartz crystal microbalances. The surfaces were prepared by adsorption of organosulfur molecules onto gold substrates, so called self assembled monolayers (SAMs). By chemical synthesis these thiols can be specifically tailored to provide surfaces with desired properties. The investigated surfaces were all based on thiols terminated with carboxylic acid groups to render hydrophilic surfaces onto which desired proteins can be covalently attached.In order to increase the performance of two dimensional carboxyl surfaces, a method for improving the immobilization of proteins to the surface was investigated. The immobilization levels of antibodies were increased by using N-hydroxysulfo-succinimide (sulfo-NHS), instead of N-hydroxy-succinimide (NHS), as stabilizer of the amine reactive intermediate formed by reaction of the carboxyl groups with 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC). The negatively charged sulfo-NHS intermediate promotes the attraction of overall positively charged proteins and enables immobilization also at low pH. In addition, the orientation of the immobilized antibodies was shown to be dependent on the pI of the antibody and to have a profound effect on the subsequent interaction with the antigen.The organization of carboxyl terminated SAMs can be poor due to the repellation between the polar terminal groups. By using acidified ethanol as solvent during the assembly step of monolayer formation, the organization in carboxyl terminated alkyl and oligo(ethylene glycol) SAMs was improved. However, the carboxyl groups were found to be converted to ethyl esters, the rate being related to the acid strength. Furthermore, the long-term stability of carboxyl oligo(ethylene glycol) SAMs was investigated. Here, the effect of alkyl chain length on the storage stability was of interest. A short alkyl chain was shown to have a profound negative effect on the storage stability of the SAM, resulting in decomposition and loss of functionality over time compared to when thiols with longer alkyl chains were studied.