Protein separation and surfactant control of electroosmotic flow in poly(dimethylsiloxane)-coated capillaries and microchips

Abstract

A thermally pyrolyzed poly(dimethylsiloxane) (PDMS) coating intended to prevent surface adsorption during capillary electrophoretic (CE) [Science 222 (1983) 266] separation of proteins, and to provide a substrate for surfactant adsorption for electroosmotic mobility control was prepared and evaluated. Coating fused-silica capillaries or glass microchip CE devices with a 1% solution of 100 cSt silicone oil in CH 2Cl 2, followed by forced N 2 drying and thermal curing at 400 °C for 30 min produced a cross-linked PDMS layer. Addition of 0.01 to 0.02% Brij 35 to a 0.020 M phosphate buffer gave separations of lysozyme, cytochrome c, RNase, and fluorescein-labeled goat anti-human IgG Fab fragment. Respective plates/m typically obtained at 20 kV (740 V cm −1) were 2, 1.5, 1.25, and 9.4·10 5. In 50 m M ionic strength phosphate, 0.01% Brij 35 running buffer, the electroosmotic flow observed was about 25% of that in a bare capillary, and showed no pH dependence between pH 6.3–8.2. Addition of sodium dodecylsulfate (SDS) or cetyltrimethylammonium bromide (CTAB) to this running buffer allowed ready control of electroosmotic mobility, μ eo. Concentrations of SDS between 0.005 to 0.1% resulted in μ eo ranging from 3 to 5·10 −4 cm 2 V −1 s −1. Addition of 1 to 2.3·10 −4% (2.7–6.3 μ M) CTAB caused flow reversal. CTAB concentrations between 3.5·10 −4 and 0.05% (0.0014–1.37 m M) allowed control of μ eo between −1·10 −4 and −5.0·10 −4 cm 2 V −1 s −1. For both surfactants the added presence of 0.01% Brij 35 provided slowly varying changes in μ eo with charged surfactant concentration.