CHEMFET sensitive

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. 

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. 

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

The ion controlled diode was an initial attempt to isolate the active electronics from the chemical solution by producing a metallic-like via that allows the isolation of the chemically sensitive region from an area where electronic components could be deposited (41,42). However, the limited precision of the non-standard microfabrication techniques made this process difficult and costly. Since this device is still essentially a capacitive membrane-insulator-semiconductor structure like the chemfet, the same problems of hermetic isolation of the gate remain. 

Figure 3. Ionic leakage paths in chemfet structures a.Schematic illustration of ionic leakage paths around the chemically sensitive membrane. Leakage through the membrane also occurs but is not illustrated b. Schematic illustration of leakage at the surface of a standard ion sensitive field effect transistor.

Other authors have produced multi-ion chemfets. Matsuo and Esashi have demonstrated that multi-ion ehemfet structures can be planar processed in a two-ion needle shaped form (48). Their ingenious needle shaped design permits them to incorporate two or possibly three sensitive elements at the tip. However, the demands of electrical and chemical isolation become increasingly difficult as the number of ion sensitive elements increases in these devices. Pace has demonstrated that a pH sensitive element, combined with a multilayer composite structure, can be used to detect a wide range of chemical species (56). This concept is quite versatile and has been proposed as the means to extend the capability of the pH sensitive... 

Table 2. Effect of polymeric membrane comp "ition on the K+ sensitivity of CHEMFETs containing valinomycin...

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). 

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. 

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... 

The basic building block of all CHEMFETs is the ion-sensitive field effect transistor (ISFET), introduced in 1970 by Bergveld [4]. In an ISFET the gate metal of a metal oxide semiconductor field effect transistor (MOSFET) is removed and the resulting gate oxide surface (2) is directly exposed to the electrolyte solution (Figure 1). 

Four examples are known of calix[6]arene derivatives which have been used as receptor molecules in potentiometric measuring devices. Calix[6]arene 36 functionalized with six ethylester moieties showed a moderate selectivity for hexylammo-nium ions in the presence of smaller primary alkylammonium ions (logA = —1.0 to — 1.4) in ion-selective electrodes [140]. The same derivative 36 also proved to be sensitive towards Cs ions [141]. CHEMFETs with calix[6]arenes, functionalized with three diethylamido moieties (37) or with three phosphate moieties (38), are selective for guanidinium ions [142]. The fourth example is calix[6]arene 39 functionalized with two thiophosphoryl groups which is selective for Pb " ions... 

Chemosensory applications will normally take place in an environment of complex composition. Humidity and other varying ambient conditions are in sharp contrast to the well-defined environment most typically found in related applications of imprinted polymers. Moreover, the trend in sensor technology towards miniaturisation, with the aim of future nano-scale dimensions, is a primary reason for rising perturbation sensitivity, such as new interfering forces that can be neglected in the macro range. Chemical sensors can be influenced by numerous factors, such as electrostatic effects (ChemFETs) or non-specific adsorption (SAW, surface plasmon resonance). 

CHEMFET Chemically sensitive field effect transistor... 

The field of molecular electronics may be considered to encompass much more than molecular electronic devices. In its broadest context, molecular electronics may be regarded as simply the application of molecules, primarily organic molecules, to electronics. This definition would include such areas as liquid crystalline materials, piezoelectric materials such as poly(vinylidine fluoride), chemically sensitive field-eflFect transistors (CHEMFET), and the whole range of electroactive polymers. These applications are beyond the scope of this book and are covered in other reviews 34, 33). However, given the basic tenet of molecular electronics, namely, the ability to engineer and assemble molecular structures into a useful device, the broader definition raises the question of whether organic molecules can be specifically assembled or engineered for unique applications in electronics. 

Figure 1.23. Cross-sections of FET structures employing LB films as gate insulators. The top diagram illustrates a thin-film FET using an amorphous silicon source and drain structures. The bottom diagram illustrates a chemically sensitive FET (CHEMFET) in which the electrical characteristics of the LB gate insulator are sensitive...

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