In the present report, the use of negatively charged surfactants as modifiers of the background electrolyte is reported using poly(dimethylsiloxane) (PDMS) microchips. In particular, the use of anionic surfactants, such as sodium dodecyl sulfate, phosphatidic acid, and deoxycholate, was studied. When surfactants were present in the run buffer, an increase in the electroosmotic flow (EOF) was observed. Two additional effects were also observed: (i) stabilization of the run-to-run EOF, (ii) an improvement in the electrochemical response for several biomolecules. In order to characterize the analysis conditions, the effects of different surfactant, electrolyte, and pH were studied. EOF measurements were performed using either the current monitoring method or by detection of a neutral molecule. The first adsorption/desorption kinetics studies are also reported for different surfactants onto PDMS. The separation of biologically important analytes (glucose, penicillin, phenol, and homovanillic acid) was improved decreasing the analysis time from 200 to 125 s. However, no significant changes in the number of theoretical plates were observed.

The effect of successive multiple ionic layer (SMIL) coatings on the velocity and direction of EOF and the separation efficiency for PDMS electrophoresis microchips was studied using different polymer structures and deposition conditions. To date, the majority of SMIL studies have used traditional CE and fused-silica capillaries. EOF was measured as a function of polymer structure and number of layers, in one case using the same anionic polymer and varying the cationic polymer and in the second case using the same cationic polymer and varying the anionic polymer. In both situations, the EOF direction reversed with each additional deposited polymer layer. The absolute EOF magnitude, however, did not vary significantly with layer number or polymer structure. Next, different coatings were used to compare separation efficiencies on native and SMIL-coated PDMS microchips. For native PDMS microchips, the average separation efficiency was 4105 +/- 1540 theoretical plates. The addition of two layers of polymer increased the separation efficiency anywhere from two- to five-fold, depending on the polymer structure. A maximum separation efficiency of 12 880 +/- 1050 theoretical plates was achieved for SMIL coatings of polybrene (cationic) and dextran sulfate (anionic) polymers after deposition of six total layers. It was also noted that coating improved run-to-run consistency of the peaks as noted by a reduction of the RSD of the EOF and separation efficiency. This study shows that the use of polyelectrolyte coatings, irrespective of the polymer structure, generates a consistent EOF in the current experiments and dramatically improves the separation efficiency when compared to unmodified PDMS microchips.

Polydimethylsiloxane (PDMS) capillary electrophoresis microchips were modified using plasma‐enhanced chemical vapor deposition (PECVD), resulting in modified electroosmotic flow (EOF) values. Octafluoropropane (C3F8) and acrylic acid (AA) plasmas were chosen as initial test systems for device modification. Argon plasma pretreatments were used to improve adhesion of the fluorocarbon (FC) and AA films. Contact angle measurements and X‐ray photoelectron spectroscopy data demonstrated that the Ar/C3F8 plasma treatment of PDMS results in the deposition of a hydrophobic, crosslinked FC film, whereas the Ar/AA plasma treatment results in the deposition of a hydrophilic film with ionizable acid groups. The extent of plasma modification within the device channels was explored using scanning Auger microscopy and dye absorption measurements. EOF values were measured for plasma‐treated chips as a function of pH, and aging studies were performed to determine the durability of the plasma treatments. Results show that EOF is decreased in Ar/C3F8 plasma‐treated chips, and varies less with pH than untreated devices. Additionally, EOF measurements are constant for a minimum of 5 d. In contrast, EOF for Ar/AA plasma‐treated devices is dependent on pH. EOF measurements of C3F8 and AA treated chips without the Ar pretreatment are less stable, particularly in the AA case. In addition to improving adhesion, the Ar plasma treatment results in a decreased hydrophobic dye absorption into the PDMS, which is attributed to the physical crosslinking of the polymer by the Ar plasma.

Microchip CE coupled with electrochemical detection (MCE-EC) is a good method for the direct detection of many small molecule analytes because the technique is sensitive and readily miniaturized. Polymer materials are being increasingly used with MCE due to their affordability and ease of fabrication. While PDMS has become arguably the most widely used material in MCE-EC due to the simplicity of microelectrode incorporation, it suffers from a lack of separation efficiency, lower surface stability, and a tendency for analyte sorption. Other polymers, such as poly(methylmethacrylate) (PMMA) and poly(carbonate) (PC), have higher separation efficiencies but require more difficult fabrication techniques for electrode incorporation. In this report, thermoset polyester (TPE) was characterized as an alternative material for MCE-EC. TPE microchips were characterized in their native and plasma oxidized forms and after coating with polyelectrolyte multilayers (PEMs). TPE provides higher separation efficiencies when compared to PDMS microchips, while still using simple fabrication protocols. In this work, separation efficiencies as high as 295,000 N/m were seen when using TPE MCE-EC devices. Furthermore, the EOF was higher and more consistent as a function of pH for both native and plasma-treated TPE than PDMS. Finally, TPE is amenable to modification using simple PEM coatings as another way to control surface chemistry and surface charge.

Cover: A scanning Auger map of the FKLL signal of a capillary electrophoresis (CE) microchip that was treated with a C3F8 inductively‐coupled rf plasma is shown (right). The photograph in the article by I. T. Martin, B. Dressen, M. Boggs, Y. Liu, C. S. Henry, and E. R. Fisher* on page 414.