Temperature sensitivity, ISFET

As basic sensor a pH-l FET chip was developed which can be used for different ISM depositions. It contains a dual ISFET structure (n-channel FET, LP-CVD Si3N4 gate membrane, gate size 16 x 400 pm) and a temperature sensitive diode. The chip is fabricated in a standard MOS>production line. For using the chip in a ISFET difference measuring mode without a conventional reference electrode an integrated pseudo reference electrode has been designed too. 

Another approach to the solution of this problem is the development of the REFET or reference FET (Fig. 8). Two ISFETs are manufactured on the same sensor chip and the response of both measured and compared. One of the ISFETs is chemically modified to render it less sensitive (or preferably totally insensitive) to ions present in the sample solution. The signal here is actually the difference between the two ISFETs, thereby meaning that effects due to temperature changes and other external effects tend to cancel out and be minimised. 

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)... 

Organophosphate pesticides studied in this work were the model low-toxic OPC trichlorfon, and some common organophosphate pesticides malathion, parathion, dichlorvos, and diazinon (Table I). Calibration curves for these pesticides (dependences of the sensor inhibition response on the analyte concentration) were obtained for all of these OPCs. These calibration curves were obtained under conditions (time of inhibition, pH and temperature) optimize with the model analyte trichlorfon. All of the pesticide calibration curves are similar and Fig. 4 illustrate the method by the example of malathion. The lowest concentration of pesticide samples assayed with 10 min. of incubation of the electrode in inhibitor containing solution was 5 ppb. This resulted in approximately 10 % of the relative inhibition signal. Fig. 4 predicts much better performance of our system compared with the literature data. For example, trichlorfon detection by means of ISFET had a reported limit of detection of ca 250 ppb (5), while conductometric sensor assay registered trichlorfon at ca. 25 ppb (5), still an order of magnitude higher than the described sensor. An amperometric sensor was used to detect dichlorvos with a limit of detection of 350 ppb (2J) and a potentiometric (pH-sensitive) sensor was shown to detect parathion at 39 ppm and diazinon at 35 ppb (9). 

Penicillinase is immobilized on a pH ISFET to obtain an ENFET that is sensitive to penicillin [7], The variation in local pH, arising from the hydrolysis of penicillin into penicilloic acid, is measured by the adjacent ISFET. The response time of this ENFET varies from 30 to SO seconds according to the thickness of the enzymatic membrane its sensitivity is related to the buffering capacity of the solution. A differential measurement compensates for variations in pH or temperature, and a concentration range of 10" to 10" M penicillin can be obtained. 

The majority of ENFETs use a pH ISFET as a basic transducer, which is very sensitive to the buffering capacity of the medium and to interference. The working ISFET must therefore be associated with a reference ISFET (without a bioreceptor), termed a REFET, which also reduces the sensitivity of the biosensor to variation in pH or temperature [196]. 

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