On-chip electrodes

In another report, fractionation of DNA samples was achieved on a Si-Pyrex device using on-chip electrodes (see Figure 6.38). The DNA fragments migrate and separate in the main gel-filled separation channel. As the individual band of interest migrates into the intersection formed by the separation channel and side channel, an electric field perpendicular to the direction of the separation field is applied, and the target band can then be attracted to the capture electrode in the side channel out of the gel [96]. 

Significant counterion transport can rapidly deplete the counterions on one side of the membrane. To sustain electroneutrality, the co-ions also deplete rapidly to produce an ion-depleted zone. Sufficiently high DC fields (>100 V/cm) can deionize a 100 pm neighborhood (the depletion zone) near the membrane. The depletion layer with low interfacial ionic strength produces the maximum possible ion current without convection and exhibits a distinct limiting-current plateau in the polarization I-V or cyclic voltammetry spectrum (Fig. 2b). This nonlinear I-V polarization is not due to electron-transfer reactions but bulk-to-membrane ion flux across the extended and depleted interfacial double layer. Its sensitivity to the interfacial charge in the depleted double layer allows sensitive conduction/ capacitance detection of hybridization with the same actuation on-chip electrodes that drive the ion current. 

For the protein deposition experiments a platinum wire was used as a counter electrode. For batch measurements an SCE reference electrode (Metrohm, Switzerland) was used together with the on-chip counter electrode, and for the Flow Injection Analysis (FIA) measurements all three on-chip electrodes were utilized. 

As mentioned in previous chapters, CP actuators need a counter and preferably even a reference electrode to operate. As a first step towards an all integrated or all-polymer system, microactuators with all electrodes integrated on a single chip have been also developed (Jager et al. 1999). The on-chip electrodes permit electronic control of the movable microelectrodes, allowing their movement out of a surface. Thus, it is possible to control the approach of one electrode to a particular object. This tool is expected to be useful in fields such as single-cell studies or neurophysiology. 

With the introduction of modern electronics, inexpensive computers, and new materials there is a resurgence of voltammetric techniques in various branches of science as evident in hundreds of new publications. Now, voltammetry can be performed with a nano-electrode for the detection of single molecular events [1], similar electrodes can be used to monitor the activity of neurotransmitter in a single living cell in subnanoliter volume electrochemical cell [2], measurement of fast electron transfer kinetics, trace metal analysis, etc. Voltammetric sensors are now commonplace in gas sensors (home CO sensor), biomedical sensors (blood glucose meter), and detectors for liquid chromatography. Voltammetric sensors appear to be an ideal candidate for miniaturization and mass production. This is evident in the development of lab-on-chip... 

The counter electrode is the current carrying electrode and it must be inert and larger in dimension. Platinum wire or foil is the most common counter electrode. For work with micro- or ultramicroelectrode where the maximum current demand is of the order of few microamperes, the counter electrode is not necessary. At very low current, a two-electrode system with the reference electrode can function as the current-carrying electrode with very little change in the composition of the reference electrode. Many commercial glucose sensors and on-chip microcells have such electrode configuration. 

It is more than probable that arrays for 50-100 DNA sequences will be needed for some clinical applications. Although it is not difficult to design electrode pads with reproducible dimensions of a micron or less, the electrochemical readout requires mechanical connections to each individual electrode. Therefore, the construction of very large, multiplexed arrays presents a major engineering challenge. Electronic switches in the form of an on-chip electronic multiplexer may provide a possible solution for this problem. 

R.M. Guijt, E. Baltussen, G. van der Steen, H. Frank, H. Billiet, T. Schalkhammer, F. Laugere, M. Vellekoop, A. Berthold, L. Sarro and G.W.K. van Dedem, Capillary electrophoresis with on-chip four-electrode capacitively coupled conductivity detection for application in bioanalysis, Electrophoresis, 22 (2001) 2537-2541. 

Use of conventional reference electrodes is a limiting factor in reducing the size of the various CHEMFETs. This could be solved by incorporating the reference electrode into the CHEMFET chip. An example of this is the on-chip fabrication of an Ag/AgCl electrode containing a gel-filled cavity sealed with a porous silicon plug [84]. Unfortunately, sensor lifetime can be limited by leakage of the reference solution. 

Another approach is the use of the potentiometric principle with planar thin film electrodes on a separate chip but in close vicinity to a FET input amplifier. Glucose and urea chips are now on the market commercialized by the company i-stat. These sensors are based on ion selective electrodes. The problems of stability are circumvented by a simple on chip calibration procedure and by the use of such microelectronic electrodes as disposable single shot probes for measuring Na, K, Cl, BUN, Glucose,iCa,pH,pC02 and Hct [56,57]. 

To facilitate leak-free fusion bonding of chips consisting of metal electrodes, the metal layers were deposited in pre-etched recesses on chips [141,276-280,747,1127],... 

There are numerous advantages of amperometric detection on-chip. These include (1) the detector has a minimal dead volume (2) preparation of electrodes (working) is compatible with the planar micromachining technology (3) the electrodes can be miniaturized without compromising LOD and (4) the EC detection has a short response time [746]. 

Conductivity detection has also been used for on-chip measurements. In one report, the solution (e.g., NaCl) was in contact with the Pt electrodes on a Si-Pyrex chip [770]. Conductivity detection was made possible on a Pt electrode sputtered on a PMMA chip [631]. 

Transverse IEF was also conducted in a pressure-driven flow for BSA and soybean lectin separation on-chip [1040], Here, Pd electrodes were used (in preference to Au) because of the non-gassing character of Pd. In addition, the protein sample was sandwiched between two buffer streams and was prevented from direct contact with the channel wall (and hence the electrode), a process akin to hydrodynamic focusing [1040],... 

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