CHEMFETs

CHEMFET with antibody It has been shown that the immunological coupling response of some of these electrodes might be a minor component of the overall response, which would make these sensors difficult to use as immunoelectrodes In general, these electrodes as yet have insufficient sensitivity for most practical immunoassays. 

Fig. 1. Schematic diagram of the first reported pH responsive CHEMFET...

In the early days of CHEMFET development, the expectations for successful application to a variety of biomedically important sensing applications were high. This was in part due to the fact that CHEMFET s are easily miniaturized (<2 mm surface area) and so are obvious candidates for in vivo applications. This enthusiasm has largely been tempered by the reality that although field-effect tran-... 

Fig. 2. Comparison of the typical configuration used for pH measurements (left) with a CHEMFET system (right)...

Since the initial report of the pH responsive CHEMFET in 1970, CHEMFET s for other species such as Ca , Na", and penicillin have been descril d. In addition, some of these devices have been tested for in vivo or on-line continuous whole blood monitoring. While problems associated with mass production of the more complex CHEMFET s such as those employing enzymes (for example, with the penicillin CHEMFET) have not yet been fully solved, the technology for mass production of the relatively simple pH CHEMFET is api rently now available and problems noted with early devices attributable to irreversible SiO changes and... 

Despite the advances in CHEMFET s and other chemically sensitive electronic devices, they have not yet achieved commercial success. Assuming the performance (precision, accuracy, response time, thermal sensitivity, durability, etc.) of these devices can match or exceed that of conventional pH electrodes, the only issue concerning their viability as alternatives is cost. With the apparent successes in automation of the entire CHEMFET process for pH devices it seems likely that some degree of commercialization will be achieved if attractive preliminary performance claims associated with some recently reported CHEMFET devices are corroborated. 

Comparable is the CHEMFET (Chemical Field Effect Transistor), a chemical sensor on a FET, e.g., for H , Na, K and Ca2+ in blood, four CHEMFETs had been mounted on one plate [Clin. Chem., 30 (1984) 1361. 

Sensors involving interaction with the surface of a semiconductor, or ceramic layer, e.g.(CHEMFETS and other electrochemical sensors) Low cost. Can measure total exposure over time, if a non-reversible reaction is used. Poisoning can occur. May exhibit non-reversible behaviour, which may be undesirable. May consume analyte. 

Chemically modified waxes, 26 220 Chemically resistant fibers, 13 389 Chemically sensitive field-effect transistors (ChemFETs), 22 269. See also Field effect transistors (FETs)... 

Low-cost, disposable, Si02/Si3N4 chemical field effect transistor (ChemFET) microsensors have been fabricated for pH measurements and adapted to biochemical applications by using polyvinyl alcohol (PVA) enzymatic layers deposited and patterned... 

Cobben et al. [151] designed and tested a wall-jet and a flow-through cell of this type. The wall-jet cell (Fig. 4.19.A) consisted of two parts, A and B. Part A was a Perspex block of 24 x 24 x 20 mm (1) furnished with two resilient hooks (3) for electrical contact. The hooks were pressed on the surface of the contact pads of the CHEMFET (4), the back of which lay on the Perspex surface. In this way, the sensor gate was positioned in the centre of the Perspex block, which was marked by an engraved cross. Part B was... 

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).

Figure 4.20.A shows a more recent cell reported by Cobben et al. It consists of three Perspex blocks, of which two (A) are identical and the third (B) different. Part A is a Perspex block (1) furnished with two pairs of resilient hooks (3) for electrical contact. With the aid of a spring, the hooks press at the surface of the sensor contact pads (4), the back side of which rests on the Perspex siuface, so the sensor gate is positioned in the centre of the block, which is marked by an engraved cross as in the above-described wall-jet cell. Part B is a prismatic Perspex block (2) (85 x 24 x 10 mm ) into which a Z-shaped flow channel of 0.5 mm diameter is drilled. Each of the wedges of the Z reaches the outside of the block. The Z-shaped flow-cell thus built has a zero dead volume. As a result, the solution volume held between the two CHEMFETs is very small (3 pL). The cell is sealed by gently pushing block A to B with a lever. The inherent plasticity of the PVC membrane ensures water-tight closure of the cell. The closeness between the two electrodes enables differential measurements with no interference from the liquid junction potential. The differential signal provided by a potassium-selective and a sodium-selective CHEMFET exhibits a Nemstian behaviour and is selective towards potassium in the presence of a (fixed) excess concentration of sodium. The combined use of a highly lead-selective CHEMFET and a potassium-selective CHEMFET in this type of cell also provides excellent results.

Figure 4.19 — (A) Detail of a wall-jet cell in (Al) top view and (A2) side view (1,2) Perspex blocks (3) contact wire (hook) (4) CHEMFET (5) glass (6) ring (7) PTFE tubing. (B) Side view of the flow-through cell (1,2) Perspex blocks (3) contact wire (4) CHEMFET. (Reproduced from [151] with permission of Elsevier Science Publishers).

Figure 4.20 — (A) Detailed side view of a double-sensor flow-cell 1,2 Perspex 3 contact wire 4, CHEMFET. (B) Scheme of a microlitre coulometric sensor. (Rqtroduced from [152] and [154] with permission of Elsevier Science Publishers).

Successful operation of potentiometric chemosensors opened up the possibility for the fabrication of chemical field-effect transistors (chemFETs) and ion-selective field-effect transistors (ISFETs). A sensing element in these devices, i.e. the MIP film loaded with the molecular, neutral or ionic, respectively, imprinted substance is used to modify surface of the transistor gate area. Apparently, any change in the potential of the film due to its interactions with the analyte alters the current flowing between the source and drain. 

Fig. 6.36 Response of PANI/CSA/IL WF CHEMFET to ammonia (adapted from Saheb, 2008)...

Implicit in this equation is the assumption of constancy of the selectivity coefficient K, and the assumption of constancy of the term Vq. Those assumptions put this otherwise useful equation in the category of empirical relationships, the same as the Nikolskij-Eisenman equation. An example of such a response is shown in Fig. 6.36 in which WF CHEMFET with doped polyaniline/camphorsulfonic/ionic liquid gate was exposed to stepwise changes of ammonia concentration. In this case ammonia acts as an electron donor, thus lowering the work function of the selective layer. The value of 5g determined from the slope of (6.101) was found to be 0.6. 

Fig. 8.12 Dual purpose (a) CHEMFET, (b) OFET structure used for chemical sensing, (c) Schematic of the device and (d) response to ammonia...

The surface field effect can be realized in a number of ways. The semiconductor can be built into a capacitor and an external potential applied (IGFET), or the field can arise from the chemical effects on the gate materials (CHEMFET). In both cases, change in the surface electric field intensity changes the density of mobile charge carriers in the surface inversion layer. The physical effect that is measured is the change in the electric current carried by the surface inversion layer, called the drain current. 

Pd MOS STRUCTURES The Pd MOS device (capacitor and field effect transistor) has been extensively studied as a model chemical sensor system and as a practical element for the detection of hydrogen molecules in a gas. There have been two outstanding reviews of the status of the Pd MOS sensor with primary emphasis on the reactions at the surface (7,8). In this section, the use of the device as a model chemical sensor will be emphasized. As will be seen, the results are applicable not only to the Pd based devices, they also shed light on the operation of chemfet type systems as well. Because of its simplicity and the control that can be exercised in its fabrication, the discussion will focus on the study of the Pd-MOSCAP structure exclusively. The insights gained from these studies are immediately applicable to the more useful Pd-MOSFET. 

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