A porous metal-insulator-metal sensor system was developed with the ultimate goal of enhancing the sensitivity of electrochemical sensors by taking advantage of redox cycling of electro active molecules between closely spaced electrodes. The novel fabrication technology is based on thin film deposition in combination with colloidal self-assembly and reactive ion etching to create micro-or nanopores. This cost effective approach is advantageous compared to common interdigitated electrode arrays (IDA) since it does not require high definition lithography technology. Spin-coating and random particle deposition, combined with a new sublimation process are discussed as competing strategies to generate monolayers of colloidal spheres. Metal-insulator-metal layer systems with low leakage currents < 10 pA and an insulator thickness as low as 100 nm were obtained at high yield (typically > 90%). We also discuss possible causes of sensor failure with respect to critical fabrication processes. Short circuits which could occur during or as a result of the pore etching process were investigated in detail. Infrared microscopy in combination with focused ion beam etching/SEM were used to reveal a defect mechanism creating interconnects and increased leakage current between the top and bottom electrodes. Redox cycling provides for amplification factors of >100. A general applicability for electrochemical diagnostic assays is therefore anticipated.
Control of wetting behaviour of fluidic microsystems was achieved by selective chemical surface modification using either a solution-based procedure or micro-contact printing. The modification procedures were designed in such a way as to obtain optimum wetting in fluid channels, sample reservoirs and nozzles, while at the same time preventing intermixing of fluids from different nozzle exits and thus enabling multiple long-term stable delivery of nano-litre droplets. Analysis included thickness measurements by ellipsometry, contact angle measurements and fluorescence microscopy. Selectively coated TopSpot dosage chips as are used for the fabrication of DNA and protein micro-arrays exhibit superior performance over uncoated dosage chips. Mixed and back-filled silane coatings show enhanced stability versus hydrolysis in basic and acidic solutions, as has been determined from measurements of contact angle as a function of immersion time.
A novel adhesive bonding technology has been developed based on the preparation of ultra-thin adhesive layers between precision machined cylinders and roll-to-surface print transfer onto micro-machined substrates. In contrast to many existing bonding technologies, this process is compatible with bio-functionalized devices since it operates at low temperatures and does not rely on cover plates previously pre-coated with adhesive. The process was initially developed for the bonding of glass/SU-8 structures to glass cover plates for the fabrication of micro-fluidic devices with integrated 3D-micro-electrode arrays. The precision of alignment is usually <2 µm. In addition, larger (6 inch) polymeric substrates with micro-machined channels have also been successfully bonded using this method.
The disruption force of specific biotin-streptavidin bonds was determined using DNA oligomers as force tags. Forces were generated by an electric field acting on a biotinylated fluorescently labeled DNA oligomer. DNA oligomers were immobilized via biotin-streptavidin bonds on the walls of microfluidic channels. Channel layout and fluid-based deposition process were designed to enable well-defined localized deposition of the oligomers in a narrow gap of the microchannel. Electric fields of up to 400 V/cm were applied and electric field induced desorption of DNA oligomers was observed. At T approximately 30 degrees C, field-induced desorption of both a 12 mer as well as a 48 mer yielded a streptavidin-biotin disruption force of 75 fN. Streptavidin-functionalized surfaces remained intact and could be reloaded with biotinylated oligomers. At approximately 20 degrees C, however, no field-induced unbinding of the oligomers was observed at electric field strength of up to 400 V/cm, indicating a significant temperature dependence of the bond strength.
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