Fabrication and characterisation of spin coated oxidised PMMA to provide a robust surface for on-chip assays

A method to prepare a highly stable carboxylic acid functional surface on various substrates for use in bioassays is reported. A thin layer of a poly(methylmethacrylate) (PMMA) is achieved by spin coating dissolved PMMA onto a variety of underlying substrates at various thicknesses in a range of c. 5-27 nm. Varying the PMMA concentration, time and spinning speed controls the thickness of the spin coated layer. The root-mean-squared roughness values of the spin coated PMMA are less than 1.5 nm, resulting in smooth and uniform layer. Substrate functionalisation is carried out by either ultraviolet/ozone (UV/O₃) or oxygen plasma oxidation. Both techniques result in initially stable, highly functional films as demonstrated by the covalent attachment of amino-modified oligonucleotides, however longevity studies comparing the stability of films attached following oxidative activation show better stability for UV/O₃ activated substrates when compared to oxygen plasma activated substrates. PMMA films activated by UV/O₃ yield highly stable (for up to 24 days) functional surfaces that retain immobilised biomolecules after several extended wash steps. In contrast, films attached to surfaces pre-treated with oxygen plasma discharge lose their functionality within 5 days of oxidation. Direct DNA and sandwich antibody assays were successfully demonstrated on the UV/O₃ functionalised surfaces, showing a low level of non-specific binding. Furthermore, the quenching of fluorescently labelled biomolecules bound to PMMA-coated gold-coated slides is shown to be dependent on the PMMA thickness, indicating potential usage in surface-plasmon resonance-based assays.