Covalent immobilisation of proteins for biomaterial and biosensing applications

Author: Endre Jozsef Szili

Szili, Endre Jozsef, 2008 Covalent immobilisation of proteins for biomaterial and biosensing applications, Flinders University, School of Chemical and Physical Sciences

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This thesis focuses on surface science and bioengineering investigations, first for the development of an improved biomaterial for orthopaedic implant applications, and second, for the development of a biosensor device for biomedical diagnostics. A key component considered in this thesis was the covalent linkage of proteins to the material’s surface for retaining the protein’s immunological and biological activities and for generating a functional interface. Part 1 of this thesis investigated surface modification procedures for improving the bioactivity of titanium substrates. Titanium is first coated with a bioactive silica film grown by plasma enhanced chemical vapour deposition (PECVD), referred to as PECVD-Si-Ti. In previous studies, the bone-implant integration process was enhanced 1.6-fold for titanium implants coated with PECVD-Si films compared to uncoated titanium implants in vivo. However, in vitro studies carried out in this thesis showed that the growth of MG63 osteoblast-like cells was 7-fold higher on uncoated titanium compared to PECVD-Si coated titanium. Therefore, to improve cell growth on the surface and, by inference, the integration of PECVD-Si-Ti implants into bone tissue, the implant’s surface was functionalised with a mitogenic factor, insulin-like growth factor-1 (IGF-1). This was accomplished by modifying the PECVD-Si-Ti surface with an alkoxysilane, 3-isocyanatopropyl triethoxysilane (IPTES), and then by covalent bioconjugation of IGF-1 through isocyanate-amino chemistry. After 72 h of in vitro cell culture in serum-free medium, the growth of MG63 cells was enhanced 1.9-fold on IPTES functionalised PECVD-Si-Ti, which was loaded with covalently immobilised IGF-1 compared to IPTES functionalised PECVD-Si-Ti without IGF-1 (isocyanate reactive groups were quenched with ethanolamine hydrochloride). The attachment and adhesion of MG63 cells were also enhanced on PECVD-Si-Ti by the covalently immobilised IGF-1 in serum-free cell culture conditions. Therefore, the bioactivity of PECVD-Si-Ti was improved by covalently linking IGF-1 to the substrate surface through isocyanate-amino chemistry. Part 2 of this thesis involved the development of a new optical interferometric biosensor. The biosensor platform was constructed from electrochemically-prepared thin films of porous silicon that acted as a sensing matrix and transducer element. By reflective interferometry using white light, an enzyme-catalysed reaction was discovered (horseradish peroxidase (HRP) mediated oxidation of 3,3’,5,5’-tetramethylbenzidine (TMB)), which led to an acceleration in the rate of porous silicon corrosion and represented the biosensor’s readout signal. We discovered that another substrate, which is also oxidised by HRP, OPD, produces an even more pronounced readout signal. The HRP-OPD system was used in an immunoassay for detecting human IgG from an Intragam solution. An important part in the design of the biosensor was the surface functionalisation approach where anti-human IgG, referred to as the capture antibody, is immobilised on the porous silicon surface. The readout signal (produced from the capture of human IgG) was enhanced 4-fold on the porous silicon biosensing platform functionalised with covalently linked anti-human IgG through isocyanate-amino chemistry compared to the porous silicon biosensing platform functionalised with adsorbed anti-human IgG. The optimised biosensor was used to detect IgG from a total human protein concentration of Intragam to a sensitivity of 100 ng/ml. In summary, isocyanate-amino bioconjugate chemistry was used to covalently link either IGF-1 to PECVD-Si-Ti for improving the biological activity of the orthopaedic implant and to covalently link IgG to porous silicon for developing a sensitive biosensor for the detection of proteins. This surface chemistry approach is very useful for biomaterial and biosensing applications.

Keywords: Biomaterial,plasma enhanced chemical vapour deposition (PECVD),biosensor,titanium,osteoblast,bioconjugation,IGF-1,IgG,porous silicon

Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2008
School: School of Chemical and Physical Sciences
Supervisor: Nicolas H. Voelcker