ISFET electrodes

Fi . 13.17. Solid-state ISFET electrode. (Courtesy of IQ Scientific Instruments, Inc.)... 

Accelerated and microwave extraction and digestion Solid-state ISFET electrodes... 

The ISFET is an electrochemical sensor based on a modification of the metal oxide semiconductor field effect transistor (MOSFET). The metal gate of the MOSFET is replaced by a reference electrode and the gate insulator is exposed to the analyte solution or is coated with an ion-selective membrane as illustrated in Fig. 

Future trends may include the commercialization of ISE s for other clinically significant ions such as bicarbonate, magnesium and phosphate. Solid contact electrodes and ISFET s may allow for mass production of smaller, less expensive devices. However, a high standard of performance must be achieved before conventional electrodes become obsolete. 

Selectivity coefficients values for K - and Na -ISFETs with the optimized ion-sen-sing membranes encapsulating valinomycin and bis(12-crown-4) are summarized in Fig. 9. The selectivity coefficient for with respect to Na in the K -ISFET is 2 x 10 " and that for Na with respect to in the Na -ISFET is 3 x 10. The selectivity coefficient values are similar to those for the ISFETs and ion-selective electrodes with the previous membrane materials containing the same neutral carriers. The high sensitivity and selectivity for the neutral-carrier-type ISFETs based on sol-gel-derived membranes can last for at least 3 weeks. 

For the drain current IA of the ISFET, again eqn. 2.101 is valid there is a freedom of choice as to the drain-bulk voltage Vjb> but once its value has been chosen one has to calibrate with buffers. In fact, a reference electrode is not essential, but it contributes to more stable results. 

Continuous flow analysis (CFA) (Skeggs), since 1960 Segmented flow Improved ISEs Tubular electrodes (Blaedel) Adapted ISFETs Special sampling requirements in plant and environment control (Sections 5.5 and 5.6)... 

Although a few amperometric pH sensors are reported [32], most pH electrodes are potentiometric sensors. Among various potentiometric pH sensors, conventional glass pH electrodes are widely used and the pH value measured using a glass electrode is often considered as a gold standard in the development and calibration of other novel pH sensors in vivo and in vitro [33], Other pH electrodes, such as metal/metal oxide and ISFETs have received more and more attention in recent years due to their robustness, fast response, all-solid format and capability for miniaturization. Potentiometric microelectrodes for pH measurements will be the focus of this chapter. 

Besides, potentiometric sensors with ion-selective ionophores in modified poly(vinyl chloride) (PVC) have been used to detect analytes from human serum [128], Cellular respiration and acidification due to the activity of the cells has been measured with CMOS ISFETS [129], Some potentiometric methods employ gas-sensing electrodes for NH3 (for deaminase reactions) and C02 (for decarboxylase reactions). Ion-selective electrodes have also been used to quantitate penicillin, since the penicillinase reaction may be mediated with I or GST. 

In principle the ISFET is derived from a MOSFET, where the metal is replaced by the couple solution-reference electrode and where a CIM (Chemically Interactive Material) is deposited on the S1O2, the gate oxide. 

Figure 8. cross sectional view of an ISFET in a solution with the reference electrode. 

Any charge change occurring only between the reference electrode and the semiconductor is a candidate for a change of Ids. In particular one of the most important points is the surface potential at the oxide-solution interface (surface potentials between the CIM and the solution and the potential between the SiC>2 and the CIM, in the presence of a given CIM. The ISFET operation may be represented by the following changes-flow which may be considered as superimposed on the quiescent point determined by the reference electrode potential ... 

Explain the main mechanistic differences between a glass membrane electrode and an ion-sensitive field effect transistor (ISFET). 

In ion-selective FETs (ISFETs), an ion-selective membrane replaces the gate electrode. When an enzyme-loaded gel is combined with the membrane, the device can be used to measure substrates which enzymically generate charged species. 

The ion-selective field-effect transistor (ISFET) represents a remarkable new construction principle [7, 63], Inverse potentiometry with ion-selective electrodes is the electrolysis at the interface between two immiscible electrolyte solutions (ITIES) [28, 55],... 

Ion-selective electrodes are systems containing a membrane consisting basically either of a layer of solid electrolyte or of an electrolyte solution whose solvent is immiscible with water. The membrane is in contact with an aqueous electrolyte solution on both sides (or sometimes only on one). The ion-selective electrode frequently contains an internal reference electrode, sometimes only a metallic contact, or, for an ion-selective field-effect transistor (ISFET), an insulating and a semiconducting layer. In order to understand what takes place at the boundary between the membrane and the other phases with which it is in contact, various types of electric potential or of potential difference formed in these membrane systems must first be defined. 

For correct function of the ISFET, a sufficiently large gate voltage, Vq, must be applied between the leads to the reference electrode and to the substrate, so that a sufficiently large potential difference is formed between the surface and the interior of the substrate for formation of the n-type conductive channel at the insulator/substrate interface. This channel conductively connects drain 1 and source 2, which are connected with the substrate by a p-n transition. On application of voltage Vj between the drain and the source, drain current /p begins to pass. Under certain conditions the drain current is a linear function of the difference between Vq and the Volta potential difference between the substrate and the membrane. 

In the bypass position, the carrier solution flows through the bypass loop and across the ISFET. The sample is injected into the sampling valve and is introduced into the carrier solution. The bypass loop has a high hydrodynamic resistance and thus the solution proceeds to the detector. The reference electrode is always immersed only in the carrier solution and is electrically connected with the ISFET through the solutioa The apparatus is regularly calibrated by K, Ca and pH standard solutions. 

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