Reference CHEMFET

The differential signal of the reference CHEMFET and the heavy metal ion CHEMFET in the flow cell yields a stable signal, free of drift and noise effects (Figure 4). 

Figure 4.18 — (A) Schematic diagram of a flow-injection system for potassium (1) carrier solution (2) injection valve (3,4) flow cell (5) pseudo-reference electrode (6) waste. (B) Detail of the flow-cell (3,4) CHEMFETs (5) pseudo-reference electrode. (Reproduced from [150] with permission of Elsevier Science Publishers).

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. 

Flow-injection analysis is a versatile technique to evaluate the performance of a detector system. CHEMFETs may have an advantage over ISEs because of their small size and fast response times. We have tested our K+-sensitive CHEMFETs in a wall-jet cell with a platinum (pseudo-)reference electrode. One CHEMFET was contineously exposed to 0.1 M NaCl and the other to a carrier stream of 0.1 M NaCl in which various KC1 concentrations in 0.1 M NaCl were injected. The linear response of 56 mV per decade was observed for concentrations of KC1 above 5 x 10"5 M (Figure 9). When we used this FIA cell (Figure 10) for determination of K+ activities in human serum and urine samples, excellent correlations between our results and activities determined by flame photometry were obtained (Figure 11). 

Because of its particular technological characteristics, the discussion of the ICD shall remain confined to those made of c-Si therefore it will not be considered in this section. On the other hand, only those structures having a configuration adaptable to a-Si H thin-film technology will be taken into consideration. Two kinds of FETs are referred to in the literature the ion-selective FET (ISFET) and the gas-sensitive FET (CHEMFET). 

Fig. 14. (a) Schematic ISFET structure without reference. Source and drain are protected from solution the oxide is hydrated, (b) Schematic of an ISFET structure with reference electrode. Source and drain are protected, (c) Schematic of a Pd gate CHEMFET. Source and drain are protected. 

A solid-state - semiconductor device operated as a reference electrode. Typically a pH-sensitive -> CHEMFET is filled with pH-buffered gel on the pH-sensitive membrane attached to the gate of the FET keeping the operating conditions of the FET and thus the associated potential constant. 

Semiconductor electrodes ion-selective field effect transistors (ISFETs) are semiconductor devices used to measure ionic species in solution. They are sometimes called chemical field-effect transistors or ChemFETs. The transistor is coated with silicon nitride, which is in contact with the test solution via an analyte-sensing membrane and also connected to a reference electrode. A variation in the concentration of the analyte ions changes proportionally with the voltage of the ISFET. ISFETs are rugged, have a faster response time than membrane electrodes and can be stored dry. 

The chemically sensitive MISFET, called CHEMFET (insulator coated with a sensitive membrane) or ISFET (ion-selective FET, usually with only an ion-active insulator), is arranged in such a way that the reference electrode, measured solution, and sensitive membrane or insulator surface are inserted between the gate of the original MISFET and the gate dielectric (see Fig. 7A). The metallic ISFET gate is now represented by a contact of the reference electrode and the voltage Uqs is identical to a constant or controlled reference voltage, Ur, applied to the reference electrode ... 

By substituting and Uqs = Ur into Eqs. (9) and (10), the dependencies of the drain current, on the activity of the ions are obtained. An analogous expression holds for CHEMFETs with another ion-selective membrane, ENFETs (enzyme-layer FETs), IMFETs (immuno-FETs), BIOFETs (with attached bioreactor containing bacteria, whole cells, tissue slices etc.) and REFETs (ISFETs with a constant activity of ions used as a reference to indicating ISFETs). 

The circuit depicted in Fig. 7E works more precisely. Here the reference electrode is again used to set up the optimal parameters of both CHEMFET and REFET by means of an applied Ur voltage. Two differentially connected current-to-voltage converters provide a voltage signal proportionally dependent on the difference between the measured and reference currents. 

Another approach to eliminate the inner filling solution of conventional ISEs was introduced also in the 1970s and is based on the use of field effect transistors (FETs). These devices are referred to as ISFETs, that is, ion-sensitive FETs, and belong together with enzyme FETs (EnFETs) and gas sensitive FETs to the larger category of ChemFETs (chemically sensitive FETs). In the case of the ISFET, the ISE membrane is applied to the Si3N4... 

ChemFETs (chemical field effect transistors) combined with a reference electrode. Although the transducer principle varies from the potentiometric sensors, the result is comparable. 

A new type of flow cell has been developed for detection of heavy metal ions in aqueous solutions, based on the flow injection method The cell contains two chemically modified field effect transistors (CHEMFETs) of which one is selective for the supporting electrolyte ion (in this case a 0 1 M potassium ion solution) and the other is selective for the heavy metal ion to be detected (Cd ", Pb " ) The differential signal of the reference electrode CHEMFET and the heavy metal ion CHEMFET in the flow cell system which has been presently developed will be evaluated for miniaturization and further integration in a micro total analysis system... 

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