Using a sol-gel method, we have fabricated poly(dimethylsiloxane) (PDMS) microchips with SiO2 particles homogeneously distributed within the PDMS polymer matrix. These particles are approximately 10 nm in diameter. To fabricate such devices, PDMS (Sylgard 184) was cast against SU-8 molds. After curing, the chips were carefully removed from the mold and sealed against flat, cured pieces of PDMS to form enclosed channel manifolds. These chips were then solvated in tetraethyl orthosilicate (TEOS), causing them to expand. Subsequently, the chips were placed in an aqueous solution containing 2.8% ethylamine and heated to form nanometer-sized SiO2 particles within the cross-linked PDMS polymer. The water contact angle for the PDMS-SiO2 chips was approximately 90.2 degrees compared to a water contact angle for Sylgard 184 of approximately 108.5 degrees . More importantly, the SiO2 modified PDMS chips showed no rhodamine B absorption after 4 h, indicating a substantially more hydrophilic and nonabsorptive surface than native PDMS. Initial electroosmotic mobilities (EOM) of (8.3+/-0.2)x10(-4) cm2/(V.s) (RSD=2.6% (RSD is relative standard deviation); n=10) were measured. This value was approximately twice that of native Sylgard 184 PDMS chips (4.21+/-0.09)x10(-4) cm2/(V.s) (RSD=2.2%; n=10) and 55% greater than glass chips (5.3+/-0.4)x10(-4) cm2/(V.s) (RSD=7.7%; n=5). After 60 days of dry storage, the EOM was (7.6+/-0.3)x10(-4) cm2/(V.s) (RSD=3.9%; n=3), a decrease of only 8% below that of the initially measured value. Separations performed on these devices generated 80,000-100,000 theoretical plates in 6-14 s for both tetramethylrhodamine succidimidyl ester and fluorescein-5-isothiocyanate derivatized amino acids. The separation distance was 3.5 cm. Plots of peak variance vs analyte migration times gave diffusion coefficients which indicate that the separation efficiencies are within 15% of the diffusion limit.

Microdialysis sampling probes were interfaced to a segmented flow system to improve temporal resolution for monitoring concentration dynamics. Aqueous dialysate was segmented into nanoliter plugs by pumping sample stream into the base of a tee channel structure microfabricated on a PDMS chip that had an immiscible carrier phase (perfluorodecalin) pumped into the cross arm of the tee. Varying the oil flow rate from 0.22 to 6.3 μL/min and sample flow rate from 42 to 328 nL/min allowed control of plug volume, interval between plugs, and frequency of plug generation between 6 to 28 nL, 0.6 to 10 s, and 0.1 to 1.7 Hz, respectively. Temporal resolution of the system, determined by measuring fluorescence in individual sample plugs following step changes of fluorescein concentration at the sampling probe surface, was as good as 15 s. Temporal resolution was independent of both sampling flow rate and distance that samples were pumped from the sampling probe. This effect is due to the prevention of Taylor dispersion of the sample as it was transported by segmented flow. In contrast, without flow segmentation temporal resolution was worsened from 25 to 160 s as the detection point was moved from the sampling probe to 40 cm downstream. Glucose was detected by modifying the chip to allow enzyme assay reagents to be mixed with dialysate as sample plugs formed. The resulting assay had a detection limit of 50 μM and a linear range of 0.2 to 2 mM. This system was used to measure glucose in the brain of anesthetized rats. Basal concentration was 1.5 ± 0.1 mM (n = 3) and was decreased 60% by infusion of high K + solution through the probe. These results demonstrate the potential of microdialysis with segmented flow to be used for in vivo monitoring experiments with high temporal resolution.

An effective method for in vivo chemical monitoring is to couple sampling probes, such as microdialysis, to on-line analytical methods. A limitation of this approach is that in vivo chemical dynamics may be distorted by flow and diffusion broadening during transfer from sampling probe to analytical system. Converting a homogenous sample stream to segmented flow can prevent such broadening. We have developed a system for coupling segmented microdialysis flow with chip-based electrophoresis. In this system, the dialysis probe is integrated with a PDMS chip that merges dialysate with fluorogenic reagent and segments the flow into 8-10 nL plugs at 0.3-0.5 Hz separated by perfluorodecalin. The plugs flow to a glass chip where they are extracted to an aqueous stream and analyzed by electrophoresis with fluorescence detection. The novel extraction system connects the segmented flow to an electrophoresis sampling channel by a shallow and hydrophilic extraction bridge that removes the entire aqueous droplet from the oil stream. With this approach, temporal resolution was 35 s and independent of distance between sampling and analysis. Electrophoretic analysis produced separation with 223,000 ± 21,000 theoretical plates, 4.4% RSD in peak height, and detection limits of 90-180 nM for six amino acids. This performance was made possible by three key elements: 1) reliable transfer of plug flow to a glass chip; 2) efficient extraction of aqueous plugs from segmented flow; and 3) electrophoretic injection suitable for high efficiency separation with minimal dilution of sample. The system was used to detect rapid concentration changes evoked by infusing glutamate uptake inhibitor into the striatum of anesthetized rats. These results demonstrate the potential of incorporating segmented flow into separations-based sensing schemes for studying chemical dynamics in vivo with improved temporal resolution.

A method for sampling and electrophoretic analysis of aqueous plugs segmented in a stream of immiscible oil is described. In the method, an aqueous buffer and oil stream flow parallel to each other to form a stable virtual wall in a microfabricated K-shaped fluidic element. As aqueous sample plugs in the oil stream make contact with the virtual wall coalescence occurs and sample is electrokinetically transferred to the aqueous stream. Using this virtual wall, two methods of injection for channel electrophoresis were developed. In the first, discrete sample zones flow past the inlet of an electrophoresis channel and a portion is injected by electroosmotic flow, termed the "discrete injector". With this approach at least 800 plugs could be injected without interruption from a continuous segmented stream with 5.1% RSD in peak area. This method generated up to 1,050 theoretical plates; although analysis of the injector suggested that improvements may be possible. In a second method, aqueous plugs are sampled in a way that allows them to form a continuous stream that is directed to a microfluidic cross-style injector, termed the "desegmenting injector". This method does not analyze each individual plug but instead allows periodic sampling of a highfrequency stream of plugs. Using this system at least 1000 injections could be performed sequentially with 5.8% RSD in peak area and 53,500 theoretical plates. This method was demonstrated to be useful for monitoring concentration changes from a sampling device with 10 s temporal resolution.

We report the coating of poly(dimethylsiloxane) (PDMS) microchannels using transition metal sol-gel chemistry and the subsequent characterization of the coatings. The channels were created using soft polymer lithography, and three metal alkoxide sol-gel precursors were investigated, titanium isopropoxide, zirconium isopropoxide, and vanadium triisobutoxide oxide. The metal alkoxides were diffused into the sidewalls of a PDMS channel and subsequently hydrolyzed using water vapor. This procedure resulted in the formation of durable metal oxide surfaces of titania, zirconia, or vanadia. The resulting surfaces were characterized using contact angle, X-ray photoelectron spectroscopy (XPS), Raman, transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and electroosmotic mobility (EOM) measurements. All of the metal oxide-modified PDMS surfaces were significantly more hydrophilic than native PDMS. Contact angles for the coatings were 90 degrees for PDMS-ZrO2, 61 degrees for PDMS-TiO2, and 19 degrees for PDMS-vanadia. XPS showed the presence of titania, zirconia, and vanadia on the PDMS surface. XPS spectra also showed no chemical modification of the PDMS after the in situ deposition of the particles either in the Si-O, Si-C, or C-H bonds of the PDMS. The particles deposited in situ were imaged with TEM and were found to be homogeneously distributed throughout the bulk of the PDMS. EOM measurements of the inorganic coatings were stable over a period of at least 95 days. Both cathodic and anodic EOMs could be generated depending upon buffer pH used. The points of net zero charge for PDMS-TiO2, PDMS-ZrO2, and PDMS-vanadia channels were calculated using EOM versus pH measurements and were found to be 4.1 +/- 0.25, 6.1 +/- 0.2, and 7.0 +/- 0.43, respectively. In addition to modifying PDMS channels with inorganic coatings, these inorganic coatings were derivatized with various organic functionalities including oligoethylene oxide (OEO), amino, perfluoro, or mercapto groups using silane chemistry. Contact angle measurements for perfluoro, mercapto, amino, and OEO-coated surfaces yielded contact angles of 120 degrees , 76 degrees , 45 degrees , and 23 degrees , respectively. These contact angles did not change over the period of 95 days. OEO-coated channels reduced the EOM by 50% from native PDMS-TiO2 to 0.9 +/- 0.05 x 10(-4) cm2/V.s (n = 5, 5.5% RSD).