Sensors ChemFET

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. 

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. 

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 4.20.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 4.20 — (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).

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. 

The majority of the devices mentioned thus far rely on the Hofmeister series for anion selectivity. However, for anions that deviate from this series, organometallic receptors can be utilised. The type of ligand or metal centre will influence the sensor selectivity due to the characteristics of the electron acceptance of the complex. An interesting development that is being explored here is the use of calixarenes. These have previously found use as cation-selective species, but with suitable substitution are now being incorporated within anion-selective devices. Compounds suitable as receptors for halides [61],benzoate [61] and acetate [62] have been developed. Reinhoudt and his co-workers have reported the production of a POj-selective CHEMFET based on a uranyl cation immobilised within a salophene ligand (Fig. 5), which shows selectivity over more lipophilic anions such as Br" and NOj [63]. 

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. 

Mode 2 devices which rely on a different detection principle are the Kelvin probe sensor and the CHEMFET. In the first case, a vibrating capacitor measures the change of the work function (see Figure 2), while in the second case the interaction is detected in the field-effect transistor mode.29 31... 

In this chapter our work is described that deals with the development of chemically modified Field Effect Transistors (CHEMFETs) that are able to transduce chemical information from an aqueous solution directly into electronic signals. The emphasis of this part of our work will be on the materials that are required for the attachment of synthetic receptor molecules to the gate oxide surface of the Field Effect Transistor. In addition the integration of all individual components into one defined chemical system will be described. Finally, several examples of cation selective sensors that have resulted from our work will be presented. 

For the synthesis of a photopolymerizable receptor molecule that has a high selectivity for K+, we have modified the hemispherand synthesis according to Scheme 4. The final polymer membrane is represented by Figure 8. The response to a variation in K+ concentration (10 s - 10"1 M) in the presence of 0.1 M NaCl is given in Figure 9. The potentiometric selectivities (log K ) determined by the mixed solution method are -3.0 (Na+), -3.5 (Ca2+), -4.0 (Mg2+), and -0.7 (NH/). This renders this CHEMFET an excellent K+-sensor with a longterm stability t> 100 days). 

Finally, a-Si H CHEMFET sensors can also be made, and preliminary results in this direction are encouraging. Figure 16 shows the schematic of a Pd MOSFET used by the authors to measure H2 concentrations, and Fig. 16 also shows the first results obtained by plotting Id versus the gate voltage for constant drain-to-source voltage, in air and in the presence of 0.5% H2 in H2 + N2 flux. 

ISEs and CHEMFETs with calix[4]arene 32, bearing two thioether functionalities, are highly selective for Ag+ in the presence of alkaline earth ions (K" ", Ca ) and transition metal ions (Cu, Cd ), with selectivities log/f gj varying from —4.2 to —4.7 [133,135]. The CHEMFETs with 32 also showed selectivity for Ag in the presence of Hg " " ions (log g g = —2.7), which is quite unique since both ions have the same coordination number and preference for a linear interaction with soft donor atoms [133]. McKervey et al. reported Ag selectivity for calix[4]arene 33 in PVC membrane based ISEs [136]. These ISEs show a moderate Ag" selectivity (—1.1 to -2.1). Due to the presence of the hard donating carbonyl atoms in the molecule, a sub-Nemstian response (50mV decade ) was obtained in the presence of alkaline metal ions [137]. Moreover, the sensor was not selective for Ag in the presence of Hg " ions. 

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