Nanofabrication and Surface Characterization for Biosensors

Abstract

This thesis is focused on surface characterization and nanofabrication techniques to be exploited potentially for use in biosensors and is divided into two articles. The first article presents an investigation of the non-specific adsorption of bovine serum albumin (BSA) labeled with fluorescein isothiocyanate (FITC) to microfluidic material surfaces through fluorescence microscopy. The tested and characterized material surfaces include the three commercially available grades of the fluoropolymer CYTOP (poly[perfluoro(4-vinyloxy-1-butene)]), M, A and S, as well as thermally grown silica on Si, and SU-8. In addition, surface roughness and surface wettability measurements were performed on the tested surfaces and related to the non-specific adsorption results. The lowest adsorption of BSA occurred on SU-8 likely due to its hydrophilicity, followed by S-grade CYTOP despite being hydrophobic. The second article introduces a new nanofabrication process: helium ion beam lithography and liftoff. The process produces negligible proximity effects and exploits the higher resist sensitivity to helium ions compared to a conventional electron beam. The fabrication process targeted the realisation of nanoantenna arrays on fused silica substrates; however, the process was successfully applied to multiple substrates including Si, silver films on Si and MgO substrates with minimal alteration. The fabricated structures imaged via helium ion microscopy and AFM showed good overall agreement with the target layout dimensions. In addition, the optical transmittance response of the fabricated arrays was experimentally measured and compared to 3D-finite-difference time domain (FDTD) simulations revealing excellent agreement. Isolated gold lines, 20 nm thick and of width as small as 14 nm, were realised on Si, which matches the width achieved using state-of-the-art electron beam lithography systems.