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

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 CH2Cl2, followed by forced N2 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 9.4·105. In 50 mM 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, μeo. Concentrations of SDS between 0.005 to 0.1% resulted in μeo ranging from 3 to 5·10−4 cm2 V−1 s−1. Addition of 1 to 2.3·10−4% (2.7–6.3 μM) CTAB caused flow reversal. CTAB concentrations between 3.5·10−4 and 0.05% (0.0014–1.37 mM) allowed control of μeo between −1·10−4 and −5.0·10−4 cm2 V−1 s−1. For both surfactants the added presence of 0.01% Brij 35 provided slowly varying changes in μeo with charged surfactant concentration.