Porous microfabrication

Ikariyama [2] described a unique method for the preparation of a glucose oxidase (GOD) electrode in their work. The method is based on two electrochemical processes, i.e. electrochemical adsorption of GOD molecules and electrochemical growth of porous electrode. GOD immobilized in the growing matrix of platinum black particles is employed for the microfabrication of the enzyme electrode. It demonstrated high performance with high sensitivity and fast responsiveness. 

Zhang, Advani, and Prasad [51,52] also used microfabrication techniques in order to develop a thin, perforated copper foil and use it as a cathode DL in a PEMLC. In addition to the metal DL, an "enhancement" layer was used that consisted of a porous material locafed befween the perforated copper foil and fhe LF plate (CLP was used in fhis study). This layer improved the overall short-term performance and wafer managemenf of fhe cell. Flowever, the authors did not discuss any possible long-term issues related to contaminahon of the membrane due to the use of a copper DL. 

Microfluidics and miniaturization hold great promise in terms of sample throughput advantages [100]. Miniaturization of analytical processes into microchip platforms designed for micro total analytical systems (/i-TASs) is a new and rapidly developing field. For SPE, Yu et al. [123] developed a microfabricated analytical microchip device that uses a porous monolith sorbent with two different surface chemistries. The monolithic porous polymer was prepared by in situ photoinitiated polymerization within the channels of the microfluidic device and used for on-chip SPE. The sorbent was prepared to have both hydrophobic and ionizable surface chemistries. Use of the device for sorption and desorption of various analytes was demonstrated [123]. 

FIGURE 3.7 Diagram of the microfabricated electroosmotic pumping system. (1) Open-channel electroosmotic pump, (2) micropump inlet reservoir, (3) micropump outlet reservoir, (4) double-T sample injection element, (5) channel for sample infusion or separation, (6) sample inlet reservoir, (7) sample waste reservoir, (8) channels for sample inlet, (9) channels for sample outlet, (10) ESI emitter to a MS detector. The inset shows an expanded view of the micropump outlet reservoir (3) containing the porous glass disk [115]. Reprinted with permission from the American Chemical Society. 

The chemical component of CMP slurry creates porous unstable oxides or soluble surface complexes. The slurries are designed to have additives that initiate the above reactions. The mechanical component of the process removes the above-formed films by abrasion. In most planarization systems the mechanical component is the rate-limiting step. As soon as the formed porous film is removed, a new one is formed and planarization proceeds. Therefore, the removal rate is directly proportional to the applied pressure. To achieve practical copper removal rates, pressures greater than 3 psi are often required. These pressures should not create delamination, material deformation, or cracking on dense or relatively dense dielectrics used in silicon microfabrication on conventional dielectrics. However, the introduction of porous ultra-low-fc (low dielectric constant) materials will require a low downpressure (< 1 psi) polishing to maintain the structural integrity of the device [7-9]. It is expected that dielectrics with k value less than 2.4 will require a planarization process of 1 psi downpressure or less when they are introduced to production. It is expected that this process requirement will become even more important for the 45-nm technology node [10]. 

Preconcentration is another commonly encountered sample pretreatment method that has been successfully integrated onto a CE chip. Ramsey and coworkers incorporated a porous membrane structure into a microfabricated injection valve, enabling electrokinetic concentration of DNA samples using homogeneous buffer conditions [5]. Sample preconcentration in nonhomogeneous buffer systems — a technique known as sample stacking —has also been achieved on-chip [6]. 

Khandurina, J., Jacobson, S. C., Waters, L. C., Foote, R. S., and Ramsey, J. M. Microfabricated porous membrane structure for sample concentration and electrophoretic analysis. Anal. Chem. 71 1815-1819, 1999. 

Foote and coworkers [120] developed a microfabricated system with the ability to electrophoretically preconcentrate fluorescently labeled proteins prior to their separation (see Fig. 6). The authors were able to preconcentrate the proteins using a porous silica membrane situated between adjacent microchannels that allowed for the passage of buffer ions, but excluded larger migrating molecules, such as proteins. Preconcentration factors of 600-fold were achieved using this on-chip format followed by an electrophoretic separation of proteins with SDS-PAGE. Using this chip, fluorescently labeled ovalbumin was detected at concentrations as low as 100 fmol by a combination of field-amplified injection and preconcentration at the membrane prior to microchip electrophoresis. 

In addition to specifically increasing the complexity of the pore architecture in the submicrometer range, such porous networks can also be structured three-dimensionally on much larger length scales than the pore size by microfabrication techniques in a top-down approach. This is of major importance for device fabrication and interfacing of the nanoscopic structures with the macroscopic world. 

Insulating materials, such as glass, quartz, alumina, and porous silicon, and their microfabrication methods to create high-aspect-ratio structures compatible with wafer bonding... 

As illustrated by the examples given in Table 3, the application of labs-on-chips to real samples is still limited. This is partly due to the fact that the analytical assay is only the final step of the whole procedure, which includes sample pretreatment protocols such as filtration, analyte cleanup, or analyte preconcentration. However, also the integration of corresponding microfabricated elements is described. Filtration was achieved by porous membranes or arrays of thin channels preventing particulates to enter the analytical device. Analyte preconcentration in combination with removal of other sample constituents is achieved by solid-phase extraction modules, which are either capillaries or beads coated with a suitable adsorbent, such as a Cl 8 phase originating from coating with octa-decyltrimethoxysilane, from which the analyte is... 

Jong et al. [143]. Porous microfluidic devices were fabricated by phase separation micromolding. Oil-in-water emulsions with 190 pm size and polydispersity 2.5% were produced inside 150 pm diameter porous microchannels. Sugiura and co-workers [144—148] proposed a microfabricated silicon geometry containing at least 400 microchannels of small dimensions (typical cross-section 6x13 pm) (Figure 8.25). Not all the channels were initially active, but as the pressure of the... 

Multiphase packed-bed or trickle-bed microreactor [29, 30] Standard porous catalysts are incorporated in silicon-glass microfabricated reactors consisting of a microfluidic distribution manifold, a single micro-channel reactor or a microchannel array and a 25-pm microfllter. The fluid streams come into contact via a series of interleaved high aspect ratio inlet chaimels. Perpendicular to these chaimels, a 400-pm wide channel is used to deliver catalysts as a slurry to the reaction chaimel and contains two ports to allow cross-flow of the slurry. High maldistribution, pressure drop and large heat losses may occur... 

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