Assembly and Characterization of Protein Resistant Planar Bilayers in PDMS Microfluidic Devices
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Assembly and Characterization of Protein Resistant Planar Bilayers in PDMS Microfluidic Devices K. Scott Phillips and Quan Cheng* Department of Chemistry University of California Riverside, California 92521 Corresponding author: [email protected] Tel: (909) 787-2702
ABSTRACT We report a method using vesicle fusion techniques that result in long-standing hydrophilic and protein-resistant PDMS surfaces. The PDMS was oxidized in plasma followed by treatment with various concentrations of phospholipid vesicles to form planar bilayer membranes in the microchannels. Contact angle measurements showed that a freshly oxidized hydrophilic surface began hydrophobic recovery immediately, but PC (phosphatidylcholine) treated surfaces exhibited prolonged hydrophilic properties up to two hours after bilayer modification, and cationic DOPC+ treated surfaces had a contact angle of ~70ยบ. FRAP experiments confirmed fusion of vesicles to form bilayers with lateral mobility. Protein adsorption studies in microchannels showed 2-3 orders of magnitude decrease in fluorescence from non-specific adsorption of dye-conjugated avidin and BSA. Coulombic interaction was found to play an important role in determining the amount of non-specific adsorption. Dehydration of the membranes resulted in increased protein adsorption after rehydration for PC but not for the synthetic lipid DOPC+, which has a positively charged headgroup. INTRODUCTION Microfluidic devices are in demand because they have the potential to reduce analysis time, reagent use, and cost. The advent of rapid-prototyping with polydimethylsiloxane (PDMS) [1] has resulted in swift development of microfluidic platforms that now span a wide range of applications in sensing, separations, and reactions. However, the hydrophobic polymer surface of PDMS exhibits poor aqueous sample loading, non-specific adsorption, and unstable electroosmotic flow (EOF). These properties are major drawbacks for real applications such as chip based CE [2], optical detection methods [3], and analysis of low concentrations of proteins and other hydrophobic bio-molecules [4]. Attempts to improve the PDMS surface have focused on changing the surface through methods such as oxidation, coatings, and grafting. Each type of surface treatment has inherent drawbacks. Oxidation is a convenient way to render the surface hydrophilic for a short time, but recovers hydrophobicity quickly [5]. Coatings last slightly longer than oxidation, but are constantly being exchanged with other hydrophobic molecules in competition for surface binding sites [6]. Grafting is more complicated, involves an overnight washing step, and may affect transparency [7]. There is not one surface treatment that is suitable for every purpose, but for microfluidic biosensing a promising alternative to the above protocols is the fusion of phospholipid vesicles to form a supported planar bilayer (SPB) on channel surfaces. SPBs are inspired by the structure of the cell membrane, which is a good example of a surface that can resist non-specific
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