Membrane-covered ISFET

The processing of the devices and the casting of the membranes is identical to the steps as described in Section A.2, except for the additional deposition of a platinum actuator electrode around the gate of the ISFET. A cross-sectional view of the membrane-covered ISFET with actuator is shown in Fig. 8. Also in this case, the membrane consists of an 8 gm thick layer of polystyrene beads with a diameter of 0.1 /tm and agarose. 

FIG. 19 Measured response on a membrane-covered ISFET to an ion step as a function of incubation time in 3 I0-5 M lysozyme. The incubation times were (from high to low transient response) 0, l, 5, 15, 30. 60, and 120 min, respectively. 

We have presented four alternative methods for volumetric protein titration, based on a membrane-covered ISFET, in which proteins can be incubated. These methods are... 

Nicotinic receptor from Torpedo californica Receptor was fixed into a cross-linked poly(vinylbutyral) membrane covering the gate of an ISFET which was mounted in a sample cell with a reference ISFET and Ag-AgCl reference electrode. Sensitive to ACh, H +, Na + and K +. Steady state reached after 2 min. Initial rate of change of the voltage was a rectilinear function of log ACh concentration was a from 0.1 to 10 pM. When receptor was immobilized in the membrane by using lecithin an amplified response was obtained which was due to Na + flux across receptor channel. [65]... 

FIG. 8 Cross section of the measuring device with the actuator electrode deposited around the ISFET gate and the membrane covering both. The typical thickness of the polystyrene beads/agarose membrane is 8 /am. The gate is the pH-sensitive part of the ISFET. 

ISFETs Sensitive to Other Ions. Deposition of an ion-selective membrane on top of the gate insulator opens the way to measurement of ions other than H . Most of the sensitive materials for ISE applications described in Section 28,2.3.1,1 have been used in conjunction with ISFETs, providing sensors covering a wide variety of species. Thus, ISFETs with solid membranes have been described with AgCl-AgBr membranes sensitive to Ag . Cr, and Br [149], for example, or with LaFs membranes sensitive to fluoride [150], Ion-sensitive polymer-matrix membranes (liquid membranes with a polymer matrix) have also been used as sensitive membranes for ISFETs. The first experiments involved mainly PVC membranes [151]. [152], but these membranes show poor adhesion and poor mechanical strength. To improve membrane adhesion, modified PVC was utilized as a matrix material [153], The use of silicones as matrix materials has made it possible to prepare very durable ISFETs with polymer-matrix membranes [154], Other reports describe the use of photopolymerized polymers as matrix materials [155], [156],... 

The hydrolysed surface of Ihe Si3N4 insulator functions as a pH-sensitive membrane [90, 105, 116, 179]. A penicillin-sensitive ISFET is based on this membrane that is covered by an immobilized layer of penicillinase, converting penicillin into the penicillanic acid anion with liberation of hydrogen ions [24]. Another version of pH-sensitive ISFETs has membrane gates made of TajOs [3] or of a suitable glass [39]. The latter ISFET with a gate made of alumino- or borosilicate glass is sensitive to sodium ions. Other ISFETs are sensitive to halide ions [22, 153, 178], [105, 115, 130] and Ca [90, 105]. 

Matrix mechanisms of sodium Urushi and PVC/ISFETs. The electrochemical characteristics, such as linear response range, sensitivity, selectivity and response time of the Urushi matrix ISFETs are similar to those of the PVC matrix ISFETs. The reason of the same characteristics is discussed from the standpoint of matrix mechanisms as follows. The obtained results indicate that these characteristics are mainly determined not by polymeric matrix materials but by sodium-sensing materials, including the membrane solvent (NPOE etc.). Therefore, it is considered that the polymeric matrix materials, such as PVC and Urushi only act as a hydrophobic support polymer and that the major part of surface of the matrix membrane should be covered with the membrane solvent containing the Na ionophore. 

Ca + Sensitive ISFET Figure 6 shows the reproducibility of the response of three ISFETs covered with double photopolymerized membranes tested by titrating... 

Figure 6. Response of three different calcium sensitive ISFETs covered with photopolymerized polyHEMA and polysiloxane membranes upon titration of demineralized water with CaCl2- T = 25 °C. (Reproduced with permission from ref. 15. Copyright 1991 Sens, and Actuators.)...

The first example of chemically modified field-effect transistors (CHEMFETs) was reported by Janata et al. [15] in 1978 for ion-sensitive field-effect transistors (ISFETs) in which the gate oxide was covered with a PVC membrane containing... 

Field-effect Transistors Enzyme FETs and immuno FETs (IMFETs) are based on principles similar to those valid in potentiometric membrane biosensors. The enzyme is immobihzed on top of the ion-selective membrane on the gate of the FET. For construction of ENFETs, usually double-gate FETs are used employing one gate as a reference system, covered only with a layer of the immobilization matrix, and allowing for the real-time compensation of pH modulations, temperature, and drift. Mostly, pH-sensitive FETs (ISFET)... 

Semiconductor processing technologies have often been used to produce ISEs, particularly as field-effect transistors (FETs) with ion-selective layers like silicon oxide over the gate region. Such ion-selective FETs (ISFETs) are, in principle, solid ISEs, although sometimes the dielectric over the gate is covered with a second, liquid membrane-type layer to achieve different selectivities. 

The CHEMFET is a special modification of the ion sensitive field effect transistor (ISFET) in which the gate oxide is covered with an ion selective membrane [2]. The ion selectivity is introduced by incorporation of ion-selective receptor molecules in a hydrophobic polymer membrane. Between the sensing membrane and the gate oxide of the ISFET a buffered hydrogel is introduced which serves as inner electrolyte solution and eliminates pH sensitivity and CO2 interference (Figure 1) [3],... 

The sensitive component of an ISFET operates along the same principles as a potentiometric electrode. In both systems, there is no consumption of reaction product, and the interaction of ions with the sensitive component leads to a membrane potential. This potential is measured directly in the potentiometric electrode and indirectly in the field-effect transistor. The measurement of the activity of the ions is generally made at a constant current the measured output potential is thus a function of the ionic activity of the solution. Once the ISFET and the potentiometric electrode are covered with enzymatic membranes to give an ENFET and an enzyme electrode, respectively, the two biosensors differ only in their method of transduction. 

The first dielectric material that was used on open gates of field effect transistors (ion-sensitive field effect transistor, ISFET) was silicon dioxide by Bergveld in 1970. The problem in the ISFET structure is the poor insulation between the device and the solutions. The ISFET structure was improved by using ion-sensitive electrodes (ISE). With field effect transistors the gate area can be extended by using a conductive wire covered with the sensitive membrane. This new approach helps in enhancing the stability, sensitivity, and flexibility in shaping i.e., miniaturization of pH-sensitive devices. 

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