Poly(dimethylsiloxane) (PDMS) has become one of the most widely used materials for microchip capillary electrophoresis and microfluidics. The popularity of this material is the result of its low cost, simple fabrication, and rugged elastomeric properties. The hydrophobic nature of PDMS, however, limits its applicability for microchip CE, microfluidic patterning, and other nonelectrophoresis applications. The surface of PDMS can be made hydrophilic using a simple air plasma treatment; however, this property is quickly lost through hydrophobic recovery caused by diffusion of unreacted oligomer to the surface. Here, a simple approach for the generation of hydrophilic PDMS with long-term stability in air is presented. PDMS is rendered hydrophilic through a simple two-step extraction/oxidation process. First, PDMS is extracted in a series of solvents designed to remove unreacted oligomers from the bulk phase. Second, the oligomer-free PDMS is oxidized in a simple air plasma, generating a stable layer of hydrophilic SiO2. The conversion of surface-bound siloxane to SiO2 was followed with X-ray photoelectron spectroscopy. SiO2 on extracted-oxidized PDMS was stable for 7 days in air as compared to less than 3 h for native PDMS. Furthermore, the contact angle for modified PDMS was reduced to <40 degrees and remained low throughout the experiments. As a result of the decreased contact angle, capillary channels self-wet through capillary action, making the microchannels much easier to fill. Finally, the modification significantly improved the performance of the devices for microchip electrophoresis. The electroosmotic flow increased from 4.1 x 10(-4) to 6.8 x 10(-4) cm(2)/V.s for native compared to oxidized PDMS. Separation efficiencies for electrochemical detection also increased from 50 000 to 400 000 N/m for a 1.1-nL injection volume. The result of this modification is a significant improvement in the performance of PDMS for microchip electrophoresis and microfluidic applications.

A simple and sensitive electrode design for microchip capillary electrophoresis/electrochemistry (CE-EC) is presented. The system employs metal microwires as the working electrodes for electrochemical detection. Two general approaches for integration of electrodes in microchip CE-EC are commonly used, end-channel and microfabrication. The end-channel approach allows electrode cleaning and the use of chemically modified electrodes; however, the designs generally lack portability and the ability to incorporate multiple electrodes. Microfabrication allows the incorporation of multiple electrodes on-chip and is easily made portable; however, it requires the use of expensive metallization and clean room facilities, and integration of more than one electrode material is challenging. The reported approach aligns a solid metal microwire through the separation channel allowing integration of multiple electrodes and the use of different electrode materials without sacrificing the portability. A detection limit of 100 nM for dopamine was achieved without the use of a decoupler as a result of a higher collection efficiency with the new design.

There is a need to develop broadly applicable, highly sensitive detection methods for microchip CE that do not require analyte derivatization. LIF is highly sensitive but typically requires analyte derivatization. Electrochemistry provides an alternative method for direct analyte detection; however, in its most common form, direct current (DC) amperometry, it is limited to a small number of easily oxidizable or reducible analytes. Pulsed amperometric detection (PAD) is an alternative waveform that can increase the number of electrochemically detectable analytes. Increasing sensitivity for electrochemical detection (EC) and PAD requires the isolation of detection current (nA) from the separation current (muA) in a process generally referred to as current decoupling. Here, we present the development of a simple integrated decoupler to improve sensitivity and its coupling with PAD. A Pd microwire is used as the cathode for decoupling and a second Au or Pt wire is used as the working electrode for either EC or PAD. The electrode system is easy to make, requiring no clean-room facilities or specialized metallization systems. Sensitive detection of a wide range of analytes is shown to be possible using this system. Using this system we were able to achieve detection limits as low as 5 nM for dopamine, 74 nM for glutathione, and 100 nM for glucose.

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.

A total internal reflection photoluminescence (TIRPh) device employing an easily fabricated PMMA/PDMS waveguide system provides a detection limit comparable to the best reported results but without using an excitation filter. The optical mechanism is similar to total-internal-reflection-fluorescence (TIRF) but uses a ruthenium-based phosphorescent dye (Ru(dpp)3) deposited on the PMMA core, motivating the generalized term of photoluminescence to include both fluorescence and phosphorescence. An enzymatic hydrogen peroxide (H2O2) biosensor incorporating catalase was fabricated on the TIRPh platform without photolithography or etching. The O2-sensitive phosphorescence of Ru(dpp)3 was used as a transduction mechanism and catalase was used as a biocomponent for sensing. The H2O2 sensor exhibits a phosphorescence to scattered excitation light ratio of 76 ± 10 without filtering. The unfiltered device demonstrates a detection limit of (2.2 ± 0.6) μM with a linear range of 0.1 mM to 20 mM. The device is the first total internal reflection photoluminescence based enzymatic biosensor platform, and is promising for cost-effective, low excitation interference, field-portable sensing.