Surface modification of poly(dimethylsiloxane) (PDMS) for microfluidic devices

Author: Jinwen Zhou

Zhou, Jinwen, 2012 Surface modification of poly(dimethylsiloxane) (PDMS) for microfluidic devices, Flinders University, School of Chemical and Physical Sciences

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Poly(dimethylsiloxane) (PDMS) is a popular material for microfluidic devices due to its relatively low cost, ease of fabrication, oxygen permeability and optical transmission characteristics. However, its highly hydrophobic surface is still the main factor limiting its wide application, in particular as a material for biointerfaces. This being the case, surface modification to tailor surface properties is required to render PDMS more practical for microfluidic applications. This thesis focuses on three different PDMS surface modification techniques, including 1) thermal assisted hydrosilylation; 2) self-assembled molecule (SAM) assisted templating and 3) a combination of Soxhlet-extraction and plasma treatment. The modified PDMS surfaces were then used for a series of analytical applications, including DNA hybridization and cocaine detection. Finally, the fabrication of native and surface modified PDMS-based microfluidic devices is also presented. The content in each chapter is outlined in the following. In Chapter 1, a comprehensive review of recent research regarding PDMS surface modification techniques is presented, including gas-phase processes, wet-chemical methods and the combination of gas-phase and wet-chemical methods. In addition, topographical and chemically patterned PDMS is discussed, as well as examples of the application of modified PDMS surfaces in microfluidics. Chapter 2 is the methodology chapter, which describes the three PDMS surface modification techniques used in this thesis. It also describes the fabrication process involved in the making of PDMS-based microfluidic devices. Moreover, details of the surface characterization techniques used for the analysis of the PDMS surfaces are described. These techniques include water contact angle (WCA) measurements, Fourier transform infrared-attenuated total reflection (FTIR-ATR) spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), streaming zeta-potential analysis, electroosmotic flow (EOF) measurements and fluorescence microscopy. Experimental details for the experiments involving DNA hybridization on modified PDMS are also described. In Chapter 3, we report on a cheap, easy and highly repeatable PDMS surface modification method by heating pre-cured PDMS with a thin film of undecylenic acid (UDA) at 80 degrees C in an oven. A hydrosilylation reaction between the UDA and the PDMS curing agent was induced during heating. The results showed the modified PDMS surfaces became more hydrophilic compared to native PDMS and showed a more or less constant WCA for up to 30 d storage in air. In addition, the stability of the modified PDMS surface was further improved by reducing the weight ratio of PDMS base and curing agent from 10:1 to 5:1. In Chapter 4, we present a chemical modification strategy for PDMS by curing a mixture of 2 wt % UDA in PDMS prepolymer on a pretreated gold coated glass slide. The pretreatment of the gold slide was achieved by coating the gold with a self-assembled monolayer of 3-mercaptopropionic acid (MPA). During curing of the UDA/PDMS prepolymer on the MPA/gold coated slide the hydrophilic UDA carboxyl moieties diffuse towards the hydrophilic MPA carboxyl moieties on the gold surface. This diffusion of UDA within the PDMS prepolymer to the surface is a direct result of surface energy minimization. Once completely cured, the PDMS was peeled off the gold substrate, thereby exposing the interfacial carboxyl groups. These groups were then available for subsequent attachment of 5'-amino-terminated oligonucleotides via amide linkages. Finally, fluorescently tagged complementary oligonucleotides were successfully hybridized to this surface, as determined by fluorescence microscopy. In Chapter 5, the surface modification of PDMS was carried out by using a 2-step plasma modification with Ar followed by acrylic acid (AAc). The stability of the modified PDMS surface was further improved by Soxhlet-extracting the PDMS with hexane prior to plasma treatment. 5'-amino-terminated oligonucleotides were covalently attached to the PAAc modified PDMS surface via carbodiimide coupling. Results show that the covalently tethered oligonucleotides can successfully capture fluorescein-labeled complementary oligonucleotides via hybridization, which were visualized using fluorescence microscopy. In Chapter 6, we report on an optical aptamer sensor for cocaine detection by first using minor groove binder based energy transfer (MBET) technique. First, a carboxyl-functionalized PDMS was prepared using SAM assisted templating as described in Chapter 4. A cocaine sensor was then fabricated on this carboxyl-functionalized PDMS surface by covalently immobilizing DNA aptamers via amide linkages. The cocaine sensitive fluorescein isothiocyanate (FITC)-labeled aptamer underwent a conformational change from partial single-stranded DNA to a double stranded T-junction in the presence of the target. The DNA minor groove binder Hoechst 33342 selectively bound to the double stranded T-junction, bringing the dye within the Forster radius of FITC. This process initiated MBET, thereby reporting on the presence of cocaine. In addition, this aptamer sensor was also implemented for cocaine detection in solution. In Chapter 7, the fabrication of microfluidic devices based on the native PDMS and/or the modified PDMS is described. First standard soft-lithography was used to produce PDMS microchannels. Then, the sealing of the microchannels was achieved with the assistance of thermal treatment or an O2 plasma. Finally, for the modified PDMS-based devices, the presence of reactive carboxyl groups from the initial UDA or AAc plasma treatment were verified by the immobilization of Lucifer Yellow CH dye in modified PDMS microchannels. In Chapter 8, an overall comparison between the three different PDMS surface modification methods is provided and the future perspectives are outlined.

Keywords: PDMS,surface modification,fabrication,microfluidic,cocaine detection
Subject: Nanotechnology thesis, Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2012
School: School of Chemical and Physical Sciences
Supervisor: Amanda V. Ellis