Capillary electrochromatography and preconcentration of neutral compounds on poly(dimethylsiloxane) microchips

Capillary electrochromatography (CEC) and preconcentration of neutral compounds have been realized on poly(dimethylsiloxane) (PDMS) microchips. The channels are coated with polyelectrolyte multilayers to avoid absorption of hydrophobic analytes into PDMS. The structures of a microchip include an injector and a bead chamber with integrated frits, where the particles of the stationary phase are completely retained. Dimensions of the frit structures are 25 micro mx20 micro m, and the space between the structures is 3 micro m. A neutral compound, BODIPY, that is strongly absorbed into native PDMS, is successfully and selectively retained on octadecylsilane-coated silica beads in the bead chamber with a concentration enhancement of up to 100 times and eluted with elution buffer solution containing 70% acetonitrile. Preconcentrations and CEC separations of coumarins have been conducted with the same device and achieved complete separations in less than 50 s.     

Capillary electrochromatography of peptides in a microfabricated system.

Reversed-phase liquid chromatography of tryptic peptides is shown in the capillary electrochromatography mode using microfabricated columns. Although selectivity is different, a mixture of tryptic peptides from ovalbumin appears to be as easily separated in the CEC as HPLC mode. The major difference between a separation in the macrofabricated CEC column and conventional separations in the HPLC mode is that separations are more readily achieved in the isocratic mode in the lower surface area microfabricated CEC columns.     

Catalytic hydrolysis of peptides by cerium(IV)

Oligopeptides are efficiently hydrolyzed by Ce(IV) to the corresponding amino acids under mild conditions. The pseudo first-order rate constants for the hydrolysis of H-Gly-Phe-OH and H-Gly-Gly-OH at pH 7.0 and 50 degrees C are 3.5 x 10(-1) and 2.8 x 10(-1) h(-1), with [Ce(NH4)2(NO3)6]0=10mM (the half-lives are 2.0 and 2.5 h). The catalytic activity of the Ce(IV) is far greater than those of other lanthanide ions and non-lanthanide ions. No oxidative cleavage was observed under the reaction conditions. Catalytic turnover of the Ce(IV) was also evidenced. The hydrolysis is fast especially when the substrates have no metal-coordinating side chains. Tripeptides and tetrapeptides are hydrolyzed at the similar rates as the dipeptides. In the hydrolysis of tripeptides, the amide linkage near the N-terminus is preferentially hydrolyzed. Neither the N-carbobenzyloxy derivative nor the amide of H-Gly-Phe-OH is hydrolyzed to a measurable extent, showing that both the terminal amino group and the carboxylate are coordinated to the Ce(IV) ion. This complexation is further confirmed by 1H NMR spectroscopy. The Ce(IV) ion is therefore one of the most active catalysts for peptide hydrolysis.     

Stability of microfabricated high aspect ratio structures in poly(dimethylsiloxane)

The stability of structures microfabricated in soft elastomeric polymers is an important concern in most applications that use these structures. Although relevant for several applications, the collapse to the ground of high aspect ratio structures (ground collapse) is still poorly understood. The stability of soft microfabricated high aspect ratio structures versus ground collapse was experimentally assessed, and a new model of ground collapse involving adhesion was developed. Sets of posts with diameters from 0.36 to 2.29 microm were fabricated in poly(dimethylsiloxane) and tested in air or immersed in water and ethanol to change the work of adhesion. The critical aspect ratio (the highest length-to-width ratio for which a post is not at risk of collapsing) was determined as a function of the diameter. The critical aspect ratio in air ranged from 2 to 4 and increased with the diameter. Work of adhesion was found to be determinant for and inversely correlated to stability. These results highlight the role played by adhesion and offer the possibility of improving stability by reducing the work of adhesion. The ground collapse model developed accounted for the main features of structure stability. The results indicate that ground collapse can be a limiting factor in the design of soft polymer structures.     

Multilayer poly(vinyl alcohol)-adsorbed coating on poly(dimethylsiloxane) microfluidic chips for biopolymer separation

A poly(dimethylsiloxane) (PDMS) microfluidic chip surface was modified by multilayer-adsorbed and heat-immobilized poly(vinyl alcohol) (PVA) after oxygen plasma treatment. The reflection absorption infrared spectrum (RAIRS) showed that 88% hydrolyzed PVA adsorbed more strongly than 100% hydrolyzed one on the oxygen plasma-pretreated PDMS surface, and they all had little adsorption on original PDMS surface. Repeating the coating procedure three times was found to produce the most robust and effective coating. PVA coating converted the original PDMS surface from a hydrophobic one into a hydrophilic surface, and suppressed electroosmotic flow (EOF) in the range of pH 3-11. More than 1,000,000 plates/m and baseline resolution were obtained for separation of fluorescently labeled basic proteins (lysozyme, ribonuclease B). Fluorescently labeled acidic proteins (bovine serum albumin, beta-lactoglobulin) and fragments of dsDNA phiX174 RF/HaeIII were also separated satisfactorily in the three-layer 88% PVA-coated PDMS microchip. Good separation of basic proteins was obtained for about 70 consecutive runs.     

Fabrication of poly(methyl methacrylate) microfluidic chips by redox-initiated polymerization

In this report, a method based on the redox-initiated polymerization of methyl methacrylate (MMA) has been developed for the rapid fabrication of poly(methyl methacrylate) (PMMA) microfluidic chips. MMA containing 2-2'-azo-bis-isobutyronitrile was allowed to prepolymerize in a water bath to form a viscous prepolymer solution that was subsequently mixed with MMA containing a redox-initiation couple of benzoyl peroxide/N,N-dimethylaniline. The dense molding solution was sandwiched between a silicon template and a piece of 1-mm-thick PMMA plate. The polymerization could complete within 50 min under ambient temperature. The images of raised microfluidic structures on the silicon template were precisely replicated into the synthesized PMMA substrate during the redox-initiated polymerization of the molding solution. The chips were subsequently assembled by the thermal bonding of the channel plates and the covers. The new fabrication approach obviates the need for special equipment and significantly simplifies the process of fabricating PMMA microdevices. The attractive performance of the novel PMMA microchips has been demonstrated in connection with contactless conductivity detection for the separation and detection of ionic species.     

Capillary electrochromatography of therapeutic peptides on mixed-mode butylmethacrylate monoliths

In this study, a porous mixed-mode n-alkyl methacrylate-based monolith has been used in the separation of therapeutic peptides. While the sulfonic acid (SCX) moiety derived from 2-acrylamido-2-methyl-1-propanesulfonic acid supports the generation of a stable electroosmotic flow (EOF) at both acidic and basic pH values, the butyl ligands provide the nonpolar sites for chromatographic resolution. The performance of the monolith was evaluated regarding the influence of pH on chromatographic resolution of peptides. The suitability of the butylmethacrylate/SCX monolith for the analysis of therapeutic peptides containing basic centres, for example arginine, at moderately high pH 9.5 and the stability to repeat injections of a mixture of peptides was demonstrated. Separations with efficiencies as high as 5.0 x 10(5) plates/m were obtained and the migration behaviour of the peptides at both low (2.8) and high (9.5) pH values could be rationalised based on their charge, molecular mass/shape and relative hydrophobicities.     

Vinyl polymerization of acrylonitrile by the cerium(IV)-propane-1,2-diol system

The polymerization of acrylonitrile (M) initiated by the Ce(IV)-propane-1,2-diol (R) redox system has been studied in aqueous sulphuric acid under nitrogen in the temperature range 30 to 40°. The rate of polymerization is proportional to [M]², [R] and [Ce(IV)]^?1 and the rate of ceric ion disappearance is proportional to [R], [Ce(IV)]. The effects of certain salts, acid, solvent and temperature on both rates have been investigated. A kinetic scheme has been proposed, and various rate and energy parameters evaluated.     

Capillary zone electrophoresis of amino acids on a hybrid poly(dimethylsiloxane)-glass chip

Poly(dimethylsiloxane) (PDMS)-PDMS and hybrid PDMS-glass devices have been characterized and compared in terms of current-voltage linearity, contact angle, electroosmotic velocity, electroosmotic mobility, and electrokinetic potential in dependence on the surface treatment. The hybrid PDMS-glass microfluidic devices have further been tested as on-chip capillary electrophoresis systems for the separation of fluorescently labeled amino acids. It has been demonstrated that different methods of surface pretreatment of the PDMS-glass devices result in significantly different separation performance, with plate numbers varying from 650 to 57 000 in dependence on the surface state and the nature of the amino acids. Electrophoretic separations of amino acids have been achieved within tens of seconds with detection limits of less than 2 microM (approximately 2 x 10(-16) to 2.5 x 10(-16) mol quantities at injection volumes of 110-120 pL). The detected amounts of fluorescein isothiocyante (FITC)-amino acids are at least ten times lower, since the amino acid:FITC ratio is 10:1 mol. The results demonstrate the perspective of such hybrid PDMS-glass microfluidic systems and the methods to modify their surfaces for on-chip separation methods for biomolecules.     

Fabrication and evaluation of a carbon-based dual-electrode detector for poly(dimethylsiloxane) electrophoresis chips

The first carbon-based dual-electrode detector for microchip capillary electrophoresis (CE) is described. The poly(dimethylsiloxane) (PDMS)-based microchip CE devices were constructed by reversibly sealing a PDMS layer containing separation and injection channels to another PDMS layer containing carbon fiber working electrodes. End-channel amperometric detection was employed and the performance of the chip was evaluated using catechol. The response was found to be linear between 1 and 600 microM with an experimentally determined limit of detection (LOD) of 500 nM and a sensitivity of 30 pA/microM. Collection efficiencies for catechol ranged from 36.0 to 43.7% at field strengths of 260-615 V/cm. The selectivity that can be gained with these devices is demonstrated by the first CE-based dual-electrode detection of a Cu(II) peptide complex. These devices illustrate the potential for a rugged and easily constructed microchip CE system with an integrated carbon-based detector of similar scale.     

Sorption studies of radionuclides on a modified mesoporous cerium(IV) silicate

Five different samples of a new sorbent, modified mesoporous cerium(IV) silicate have been prepared with various mole ratios of Si/Ce and Cetyltrimethylammonium bromide (CTMABr) as template. XRD, nitrogen sorption, SEM, IR, thermogravimetry and sorption of radionuclides have been studied. Separation of Hg(II)-Th(IV), Hg(II)-Zr(IV) and Rb(I)-Zr(IV) have been developed on columns of this novel sorbent.     

Study on the separation of amino acids in modified poly(dimethylsiloxane) microchips

A mixture of five amino acids including arginine, histidine, phenylalanine, serine and glutamic acid was successfully separated in microchip capillary electrophoresis and detected with laser-induced fluorescence (LIF) detector. These amino acids were labeled with 5-(4, 6-dichloro-s-triazin-2-ylamino) fluorescein (DTAF). The analyses were performed on two kinds of modified poly(dimethylsiloxane) (PDMS) microchips. One kind of chip was simply treated with oxygen plasma (OP-chip), and the other was further modified by coating double layers of non-ionic polymer poly(vinyl alcohol) (PVA) after plasma oxidization (PVA-chip). The derivatization condition of amino acids by DTAF was optimized. The properties of the two modified PDMS microchips were studied and separation conditions, such as the buffer pH, buffer concentration and separation voltage, were also optimized. The column efficiencies of the two microchips were in the range of 193,000–1,370,000 plates/m. The DTAF-labeled amino acids were sufficiently separated within 50 s and 90 s in 2.5 cm channels on OP-chip and PVA-chip, respectively.     

Indirect fluorescence detection of simple sugars via high-pH electrophoresis in poly(dimethylsiloxane) microfluidic chips

This article describes the successful electrophoretic separation of simple carbohydrates in a polymeric microfluidic chip. The device fabricated in poly(dimethylsiloxane) (PDMS) is found to be stable in high-pH solutions. This allows sugars to be separated electrophoretically at pH values at or above their pK(a) using indirect fluorescence detection. Signal-to-noise values greater than 10:1 were obtained using a mercury arc lamp excitation source and a fluorescein-containing mobile phase for the detection of sugars at concentrations as low as 5 mM. The results obtained compare favorably with published results for the same system using a traditional fused-silica capillary. Analysis of the data revealed a significant experimental sensitivity of the migration times measured in these PDMS devices, an aging effect that leads to considerable systematic drift over the course of a series of replicate measurements. These experiments highlighted the importance of the surface chemistry of PDMS, especially as it pertains to its ability to support stable electroosmotic flow within the separation device. Channel priming at high pH provides a necessary, but by itself insufficient, means by which this instability can be minimized.     

Production of poly(methyl methacrylate) particles by dispersion polymerization with mercaptopropyl terminated poly(dimethylsiloxane) stabilizer in supercritical carbon dioxide

Poly(methyl methacrylate) (PMMA) particles were produced by dispersion polymerization of methyl methacrylate in the presence of mercaptopropyl terminated poly(dimethylsiloxane) (MP-PDMS) in supercritical carbon dioxide at about 30 MPa for 24 h at 65 °C. The particle diameter could be controlled in a size range of submicron to micron by varying MP-PDMS concentration. The MP-PDMS worked as not only a chain transfer agent but also a colloidal stabilizer, which was named tran stab.Part CCLI of the series Studies on suspension and emulsion     

Oxidation of alcohols using cerium(IV) alkyl phosphonate modified silica

The oxidation of a range of alcohols to ketones or carboxylic acids proceeds in good yield using catalytic quantities of cerium(IV) phosphonate modified silica and sodium bromate as the re-oxidant.     

Capillary electrophoresis coupled to mass spectrometry from a polymer modified poly(dimethylsiloxane) microchip with an integrated graphite electrospray tip

Hybrid capillary-poly(dimethysiloxane)(PDMS) microchips with integrated electrospray ionization (ESI) tips were directly fabricated by casting PDMS in a mould. The shapes of the emitter tips were drilled into the mould, which produced highly reproducible three-dimensional tips. Due to the fabrication method of the microfluidic devices, no sealing was necessary and it was possible to produce a perfect channel modified by PolyE-323, an aliphatic polyamine coating agent. A variety of different coating procedures were also evaluated for the outside of the emitter tip. Dusting graphite on a thin unpolymerised PDMS layer followed by polymerisation was proven to be the most suitable procedure. The emitter tips showed excellent electrochemical properties and durabilities. The coating of the emitter was eventually passivated, but not lost, and could be regenerated by electrochemical means. The excellent electrochemical stability was further confirmed in long term electrospray experiments, in which the emitter sprayed continuously for more than 180 h. The PolyE-323 was found suitable for systems that integrate rigid fused silica and soft PDMS technology, since it simply could be applied successfully to both materials. The spray stability was confirmed from the recording of a total ion chromatogram in which the electrospray current exhibited a relative standard deviation of 3.9% for a 30 min run. CE-ESI-MS separations of peptides were carried out within 2 min using the hybrid PDMS chip resulting in similar efficiencies as for fused silica capillaries of the same length and thus with no measurable band broadening effects, originating from the PDMS emitter.     

Oxidative breakdown of polymeric plutonium(IV) hydroxide by cerium(IV)

The breakdown of polymeric plutonium(IV) hydroxide (plutonium colloid) by cerium(IV) has been studied spectrophotometrically by monitoring the formation of plutonium(VI). Reaction rate variations were studied with changes in cerium(IV), acid and plutonium colloid concentrations. A reaction mechanism involving the formation of [Ce₂O(OH)₂]⁴⁺ and its reaction with the polymer surface to produce two plutonium(V) ions is postulated to explain the observed kinetic data, in particular the maximum in the reaction rate at around 0.3 M nitric acid.     

Production of submicron-sized poly(methyl methacrylate) particles by dispersion polymerization with a poly(dimethylsiloxane)-based azoinitiator in supercritical carbon dioxide

 Submicron-sized, comparatively monodisperse poly (methyl methacrylate) particles were produced by dispersion polymerization of methyl methacrylate with a poly(dimethylsiloxane)-based azoinitiator in supercritical carbon dioxide at 30 MPa for 24 h at 65 °C. The initiator operated not only as a radical initiator but also as a colloidal stabilizer, and was named an “inistab”. Received: 13 February 2001 Accepted: 20 June 2001     

Photolithographically patterned surface modification of poly(dimethylsiloxane) via UV-initiated graft polymerization of acrylates

Patterned surface modification of poly(dimethylsiloxane) (PDMS) is achieved by combining ultraviolet-initiated graft polymerization (UV-GP) and photolithography. Poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) patterns were grafted onto PDMS with micrometer-scale feature edge resolution. The morphology and chemical composition of the grafted layers were assessed by optical and atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and XPS imaging. AFM section analyses demonstrated the deposition of 33 +/- 1 and 62 +/- 8 nm thick patterned films of PAA and PMAA, respectively. Spatially resolved C 1s XPS provided images of carboxylic acid functionalities, verifying the patterned deposition of acrylate films on PDMS. These observations demonstrate the general usefulness of UV-GP and photolithography for micropatterning.