Adsorption of a carboxylated silane on gold : characterization and application to PDMS-based electrochemical cells

Integrated sensing and biosensing microfluidic systems often require sealing between polydimethylsiloxane (PDMS), glass, and gold interfaces. Studying substances that can self-organize onto glass and gold surfaces may achieve these goals and pave the way for new technological advances. Work presented in this thesis focuses on characterizing the adsorption of N-[(3-trimethoxysilyl)propyl]ethylene-diamine triacetic acid (or TMS-EDTA) on Au and applying this knowledge to construct leak-free PDMS-based electrochemical cells. First, surface analysis of TMS-EDTA-modified Au surfaces was conducted using various techniques. Water contact angle measurements and X-ray photoelectron spectroscopy confirm that the carboxylated silane can chemically modify Au surfaces. Atomic force microscopy studies indicate that a uniform surface coverage with monolayer thickness is formed. Infrared spectroscopy studies indicate that there is little evidence of siloxane cross-linking. Surface plasmon resonance results suggest that the carboxylates on TMS-EDTA-modified Au are available for streptavidin immobilization. Second, electrochemistry was used to determine the Gibbs free energies of adsorption of TMS-EDTA on Au under aqueous conditions. Electrochemical differential capacitance measurements reveal that the potential-dependent free energies of adsorption are ∼ - 20 to - 30 kJ/mol (for potentials between - 0.5 and 0.2 V) in the complex electrolyte solution used. Furthermore, at highly negative potentials ( ∼ - 1.1 V), TMS-EDTA adsorbs minimally onto the Au surface. Third, PDMS surfaces were functionalized to present primary amino groups, and glass or gold slides were functionalized to present carboxyl groups. Strong bonding was achieved by bringing the two surfaces in contact and reacting at room temperature. Shear tests reveal that the novel carboxyl-amine bonding strategy achieved a comparable bond strength as the conventional methods. Subsequently, TMS-EDTA was applied to construct leak-free PDMS-based electrochemical cells. Pressure leak tests were conducted to provide a more realistic measure of the bond strengths under aqueous conditions. A method to electrochemically remove the adsorbed TMS-EDTA layer off of the Au electrode, while maintaining the sealed cell chamber, was also developed. The characterization studies and fabrication strategy presented have led to the development of leak-free PDMS-based electrochemical devices that are suitable for sensing and biosensing applications.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat