Inner Surface Of Fused-silica Capillary Research Articles

Chemical vapour deposition in narrow capillaries: Electro-osmotic flow control in capillary electrophoresis

BackgroundIn capillary electrophoresis (CE), the inner surface of fused-silica capillaries is commonly covalently modified with liquid silanes to control electroosmotic flow (EOF). This liquid phase deposition (LPD) approach is challenging for long and narrow-diameter capillaries (≥1 m, ≤25 μm ID) inhibiting commercial production. Here, we use chemical vapour deposition (CVD) to covalently modify capillaries with different silanes. Using a home-built CVD device, capillaries were modified with neutral (3-glycidyloxypropyl) trimethoxysilane (GPTMS), the weak base (3-aminopropyl) trimethoxysilane (APTMS), the weak acid 3-mercaptopropyltrimethoxysilane (MPTMS) and the neutral hydrophobic trichloro(1H,1H,2H,2H-perfluorooctyl) silane (PFOCTS). Gas-phase modification of GPTMS with acid and ammonia allowed further modification of the surface prior to molecular layer deposition (MLD) of poly(p-phenylene terephthalamide) (PPTA) using the self-limiting sequential reaction between terephthalaldehyde (TA) and p-phenylenediamine (PD) vapours. ResultsCapillaries coated with GPTMS by CVD showed a greater reduction in EOF at all pH values than the conventional LPD. APTMS showed a reduction of the EOF at pH 9, with EOF reversal observed below pH 6. MPTMS provided a slightly lower EOF than an unmodified capillary at high pH, and a slightly higher EOF at lower pH. PFOCTS provided the most consistent EOF as a function of pH. The deposition of successive layers of PPTA resulted in increased surface coverage of the polymer and a greater reduction in EOF at pH higher than 5. The stability of a 10 μm ID GPTMS coated capillary was tested at pH 8.8 in a 200 mM CHES/Tris BGE for the separation of inorganic anions. Over 1.5 months of continuous operation (≈4130 runs), the reproducibility of the apparent mobilities for chloride, nitrite, nitrate and sulfate were 2.43%, 2.56%, 2.63% and 3.05%, respectively. The intra-day and inter-day column-to-column reproducibility and batch-to-batch reproducibility for all the coated capillaries ranged between 0.34% and 3.95%. SignificanceThe study demonstrates the superior performance of CVD coating for suppressing the EOF compared to LPD allowing the easy modification of long lengths of narrow capillary. The variation in silane, and the ability of MLD to modify and control the surface chemistry, provides a simple and facile method for surface modification. The stability of these coatings will allow long-term capillary electrophoresis monitoring of water chemistry, such as for monitoring fertiliser run-off in natural waters.

Platinum nanoparticles confined in imidazolium-based ionic polymer for assembling a microfluidic reactor with enhanced catalytic activity

A synthetic strategy is developed to grow Pt nanoparticles supported by imidazolium-based ionic polymers (Pt/ImIP-2BrB) on the inner surface of fused-silica capillary. The imidazolium-based ionic polymers ImIP-2BrB are prepared from condensation of tetrakis[4-(1-imidazolyl)phenyl]methane and 1,4-bis-bromomethyl-benzene and are employed to support Pt nanoparticles. The capillary coated with Pt/ImIP-2BrB is further assembled to achieve a catalytic microfluidic reactor. The catalytic activity is probed in either the reduction of nitrobenzene derivatives or the hydrosilylation of phenylacetylene with triethylsilane by flowing through the microfluidic reactor. The catalytic microfluidic reactor demonstrates significantly enhanced activity about 2–8 times in comparison with the corresponding reactions under batch conditions. The reactor greatly reduces the amount of Pt upon achieving similar yields. It also shows good recyclability without significant decrease of the activity.

Self-assembled and covalent capillary coating of diazoresin and D-Glucurone for protein analysis in capillary electrophoresis

ABSTRACTA simple, environmental and economical capillary electrophoresis method has been developed for the analysis of four model proteins by employing a photoactive and non-toxic diazoresin (DR), D-Glucurone modification of the inner surface of fused-silica capillary. DR/D-Glucurone coated capillary not only effectively eliminated the wall adsorption of proteins, but also showed good stability and repeatability. Instead of using the moisture sensitive and toxic silane coupling agent, the application of photosensitive DR may provide a flexible and environmentally friendly strategy to prepare novel covalently coated capillaries. Thus, DR/D-Glucurone coated capillary has great potential in capillary electrochromatography and may provide a new method for biological compounds separation.

Surface initiated polymerization of a cationic monomer on inner surfaces of silica capillaries: Analyte separation by capillary electrophoresis versus polyelectrolyte behavior

[2-(Methacryloyl)oxyethyl]trimethylammonium chloride was successfully polymerized by surface-initiated atom transfer radical polymerization method on the inner surface of fused-silica capillaries resulting in a covalently bound poly([2-(methacryloyl)oxyethyl]trimethylammonium chloride) coating. The coated capillaries provided in capillary electrophoresis an excellent run-to-run repeatability, capillary-to-capillary and day-to-day reproducibility. The capillaries worked reliably over 1 month with EOF repeatability below 0.5%. The positively charged coated capillaries were successfully applied to the capillary electrophoretic separation of three standard proteins and five β-blockers with the separation efficiencies ranging from 132,000 to 303,000 plates/m, and from 82,000 to 189,000 plates/m, respectively. In addition, challenging high- and low-density lipoprotein particles could be separated. The hydrodynamic sizes of free polymer chains in buffers used in the capillary electrophoretic experiments were measured for the characterization of the coatings.

Polymeric ionic liquid as a dynamic coating additive for separation of basic proteins by capillary electrophoresis

A simple and economical capillary electrophoresis method has been developed for the analysis of four model basic proteins by employing a polymeric ionic liquid (PIL), poly(1-vinyl-3-butylimidazolium) bromide, as the dynamic coating additive. When a small amount of PIL was present in the background electrolyte, a cationic coating on the inner surface of fused-silica capillary was established. These PIL modified capillaries not only generated a stable reversed electroosmotic flow, but also effectively eliminated the wall adsorption of proteins. Several important parameters such as the PIL concentration in the background electrolyte, pH values and concentrations of the background electrolyte were optimized to improve the separation of basic proteins. Consequently, under the optimum conditions, a satisfied separation of basic proteins with peak efficiencies ranging from 247,000 to 540,000 (plates m −1) had been accomplished within 11 min. The run-to-run RSDs ( n = 3) of the migration times for the four basic proteins were all less than 0.37%.

Cationic amylopectin derivatives as additives for analysis of proteins in capillary electrophoresis

Positively charged amylopectin, which is a major constituent of cationic starch, was used to modify the inner surface of fused-silica capillaries by addition to the running solution, which was subsequently employed in CE. Capillaries filled with cationic amylopectin derivatives were shown to generate a stable reversed EOF in the investigated range of pH 4-8. Among the additives studied, quaternary ammonium amylopectin derivatives with high amino and low hydroxypropyl groups showed fast electroosmotic mobility and very effectively suppressed the adsorption of proteins. The run-to-run and batch-to-batch repeatability of the procedures were satisfactory with RSDs of 0.5% and 2.4%, respectively. A basic protein, alpha-chymotrypsinogen, migrated within 6 min and the theoretical plate number of it reached 560 000 plates/m.

Cationic starch derivatives as dynamic coating additives for protein analysis in capillary electrophoresis

Positively charged starch derivatives were used to modify the inner surface of fused-silica capillaries by addition to running buffer, which were subsequently employed in capillary electrophoresis (CE). Capillaries coated with the cationic starch derivatives were shown to generate a stable, reversed electroosmotic flow (EOF) in the investigated pH range of 3–9. The presented coating procedure was fast, based on a simple rinsing protocol where the polymer created a physically adsorbed, cationic polymer layer. Among the additives studied, a quaternary ammonium starch derivative showed a fast EOF mobility and effectively suppressed the adsorption of proteins. The intra- and inter-day reproducibility of the coating referring to the EOF mobility were satisfactory with relative standard deviation (RSD) of 0.27 and 1.67%, respectively. The coating enabled separation of some protein mixtures including basic proteins within l3 min with efficiencies up to 280,000 plates/m. In addition, this cationic starch derivative possessed a good solubility (about 100 mg/mL), and it does not significantly contribute to the background adsorption in the UV region of 190–400 nm.

Performance of a physically adsorbed high-molecular-mass polyethyleneimine layer as coating for the separation of basic proteins and peptides by capillary electrophoresis

A simple method for the preparation of a polyethyleneimine (PEI) coating on the inner surface of fused-silica capillaries for capillary electrophoresis (CE) is reported. The PEI layer can be coated on the silica surface by just flushing the capillary with a solution containing high-molecular-mass PEI. The physically adsorbed layer appears to be very stable and can be used in a pH range of 3–11. In comparison to described methods to fabricate an immobilized PEI layer, the proposed method does not require an immobilization step, is simple and the preparation time of the coating is less than two hours. Good reproducibilities of migration times of basic proteins and peptides were obtained on the same PEI-coated capillary as well as on different PEI-coated capillaries. For basic proteins efficiencies ranging from 300 000–500 000 plates per meter were normally found.

Enantiomer separation by complexation gas and supercritical fluid chromatography on immobilized polysiloxane-bonded nickel(II) bis[(3-heptafluorobutanoyl)-10-methylene-(1 R)-camphorate] (Chirasil-nickel)

The synthesis of a polysiloxane containing chemically bonded chiral metal complex derived from nickel(II) bis[(3-heptafluorobutanoyl)-10-methylene-(1 R)-camphorate] (Chirasil-nickel) and its effective immobilization on the inner surface of fused-silica capillaries by thermal treatment is described. As expected, the immobilization properties were found to be dependent on the content of residual reactive SiR groups in the polymer backbone (R = OCH 3, H). The immobilized Chirasil-nickel stationary phase was employed for the analytical enantiomer separation of coordinating solutes by high-resolution capillary gas and supercritical fluid chromatography. The increase in the relative retention monitored for racemic test solutes after rinsing the columns indicates a slight increase in the effective complex concentration which does not affect the chiral separation factor α. The temperature limit of analysis was raised to 170–180°C for temperature-programmed runs and to 140–150°C under iso thermal conditions, thus extending the scope of complexation gas chromatography considerably, as demonstrated by various enantiomer separations performed at elevated temperatures. When Chirasil-nickel is immobilized on the inner surface of short, narrow-bore capillaries (50 μm I.D.), it can be employed in complexation supercritical fluid chromatography, combining the high solvation strength of supercritical carbon dioxide with the benefit of low operating temperatures, enhancing enantioselectivity.

Synthesis and evaluation of epoxy polymer coatings for the analysis of proteins by capillary zone electrophoresis

This paper reports a diol-epoxy coating process to deactivate the inner surface of fused-silica capillaries for use in capillary zone electrophoresis. The most effective coating consists of an epoxy polymer covalently bonded to the silica surface through glycerylpropyl silane which eliminates a number of the negatively charged silanol groups. The epoxy polymer phase also further masks residual surface silanols and sterically limits interaction of proteins with the inorganic surface of the capillary. Application of the coating was a multi-step process. In the initial step, glycidylpropyltrimethoxy silane was used to derivatize the surface of fused-silica capillaries. This was followed by deposition at the capillary wall of a film of multifunctional oxirane. Polymerization was achieved by either boron trifluoride or tertiary amine. These coatings were of sufficient thickness and hydrophilicity to reduce protein adsorption but still allow sufficient electroosmotic pumping to carry both positive and negative species past the detector. The coatings provided capillaries with lifetimes exceeding 120 h. Proteins were resolved quickly and efficiently with high recovery of both cationic and anionic species between pH 5 to 10.