Covalent Coatings Research Articles

ESI-MS Compatible Permanent Coating of Glass Surfaces Using Poly(ethylene glycol)-Terminated Alkoxysilanes for Capillary Zone Electrophoretic Protein Separations

Thin poly(ethylene glycol) silane (PEG-silane) coatings formed from N-(triethoxysilyl propyl)-O-poly(ethylene oxide) urethane with different chain lengths of poly(ethylene glycol) (MW 750 and 4000-5000) are used to modify glass microfluidic channels and fused-silica capillaries for electrophoretic separations of proteins. These coatings combine three important properties, which make them favorable for proteomic analyses including reduction of protein adsorption, compatibility with mass spectrometry due to their stability, and the ability to control the magnitude of electroosmotic flow (EOF). The coatings have been successfully used in microfluidic chips and fused-silica capillaries for separation of protein sample mixtures under low EOF conditions. The long-chain and mixed PEG-silane coatings suppress electroosmotic flow by more than 90%, whereas the short-chain PEG silane suppresses EOF by 65-75% at pH values of 3-9. The long-chain and mixed PEG-silane coatings are suitable for low EOF applications or for cases where negative effects of EOF are to be minimized. Efficient separations of unlabeled basic proteins at low pH and FITC-labeled proteins at high pH were achieved, as well as excellent stability for at least 200 electrophoretic runs. Additionally, these covalent coatings produce no detectable background ions in ESI-MS, making them compatible with on-line mass spectrometry.

Separation of 4-color DNA sequencing extension products in noncovalently coated capillaries using low viscosity polymer solutions.

A low viscosity (ca. 75cP) solution using polydimethylacrylamide (PDMA) was developed for separating DNA sequencing extension products by capillary electrophoresis (CE). This medium gave a length-of-read (LOR) value of approximately 600 bases in about 2 h using four-color sequencing in 50 microm capillary at 42 degrees C under a field of 160 V/cm. This medium also works in bare capillaries by noncovalently coating the surface to suppress both electroosmotic flow (EOF) and DNA-capillary wall interactions, and eliminates the need for complicated covalent coatings. At least 100 successive sequencing runs were performed in the same capillary by simply pumping fresh medium after every run, without requiring any reconditioning of the capillary surface between runs. The thermal stability of the noncovalent coating can be improved by adding small amounts of high molecular weight PDMA to the separation medium. The advantages of low viscosity separation media and uncoated capillaries are of paramount importance to develop high-throughput instruments for DNA sequencing.

Hot Corrosion Behavior of Coated Covalent Ceramics

Ceramic thin films were deposited on hot-pressed silicon nitride and silicon carbide substrates to provide corrosion protection to the underlying material. High-purity silicon nitride, silicon carbide, and silicon oxynitride (SiO{sub 2}/Si{sub 3}N{sub 4} composite) coatings were deposited by plasma-enhanced chemical vapor deposition and rf sputtering to form an effective diffusion barrier for cation migration into the silica corrosion product. Preliminary results indicated that the extent of corrosion in Na{sub 2}SO{sub 4} melts at T = 900{degree}, 950{degree}, and 1,000{degree}C was dependent on the outward diffusion of cations in the grain-boundary phase and the crystallinity of the grain-boundary phase in the uncoated ceramics. Accelerated corrosion occurred in these ceramics because of breakdown of the protective silica film formed on the surface. Corrosion rates in Na{sub 2}SO{sub 4} melts were reduced by separating the oxide grain-boundary phases from the corrosion products with high-purity covalent ceramic coatings.