ISFET sodium

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]. 

Examples of the use of FIA with ISE detection involve the determination of nitrate and total nitrogen in environmental samples [48, 49, 125, 166], potassium, sodium [125], calcium [51] and urea [124] in serum or major nutrients in fertilizers [73]. An interesting combination of an ISFET sensor with the FIA principle [52] is shown in fig. 5.17. This is a simultaneous determination of potassium, calcium and pH in serum during dialysis on an artificial kidney. 

Fig. 6.25 Dual ion-sensitive field-effect transistor (ISFET) for measurement of sodium chloride concentration...

Because the two ISFETs are on the same chip, and the reference electrode is common to both, subtraction of (6.72) from (6.71) yields the value of activity of sodium chloride salt with double the slope (Fig. 6.26). 

This probe has been fabricated and used for the determination of sodium chloride in perspiration, as with the test for detection of cystic fibrosis (Bezegh et al., 1988) The constant in (6.73) accounts for nonideality and differences in the two ISFETs. 

Sodium ion-selective field-effect transistors (Na+ ISFETs) were prepared by using three different types of polymeric matrix materials, such as polyvinyl chloride, bio-compatible polymer (polyurethane) and Urushi (natural oriental lacquer). Their electrochemical characteristics were discussed in connection with their characteristics of polymeric matrix membranes. 

Two types of Na+ ISFET have been reported so far. One was an inorganic sodium-aluminum-silicate (NAS) glass ISFET, which was fabricated by the hydrolysis of a mixed solution of metal alcoholates, followed by thermal treatment (2), or by the ion implantation technique (3,4). The other type of Na+ ISFET was prepared by coating with so-called solvent polymeric membrane, such as polyvinyl chloride (PVC) membrane. 

Preparation of sodium PVC/ISFET. Na+ PVC/ISFETs were obtained by dipcoating with the tetrahydrofuran (THF) solution composed of the mixture of 7.1 wt. % of Na ionophote (1,1,1 -tris(l,-(2/-oxa-4/-oxo-5/-aza-5 -methyl)dodecanyl)propane ... 

ETH 227, Fluka AG), 63.9 wt. % of 2-nitrophenyloctylether (NPOE Dojin Research Laboratories Co. Ltd.), 0.4 wt. % of sodium tetraphenylborate (NaBPh4, Dojin Research Laboratories Co. Ltd.) and 28.6 wt. % of PVC (Dojin Research Laboratories Co. Ltd.) onto the Si3N4 gate of the ISFET devices (0.5 mm x 5.5 mm x 0.2 mm catheter type ISFET donated by Shindengen Electric Mfg. Co. Ltd.) at several times to avoid pin-holes. The resulting Na+ PVC/ISFETs were allowed to dry overnight. The thickness of the membrane was approximately 0.1 mm. 

Preparation of sodium Urushi/ISFET. A mixture of 5 wt. % of ETH 227, 45 wt. % of di-2-ethylhexylphthalate (DOP Kishida Chemical Co. Ltd.) containing 0.5 wt. % of potassium tetrakis(4-chlorophenyl)borate (Dojin Research Laboratories Co. Ltd.) and 50 wt. % of Urushi (Saito Urushi Co. Ltd.) was coated on the FET devices and then the resulting Na+ Urushi membranes were hardened for 10 days at 30 °C and 90% relative humidity. The thickness of the membrane was approximately 0.1 mm. The surface of the Urushi matrix membrane was lustrous, smooth and adhesive to the gate of the device. The hardening mechanisms were discussed in detail elsewhere (6). 

Measurements with sodium ISFETs. After the prepared ISFETs were conditioned in 10 3 M NaCl solution for a few hours to stabilize the potential response, their potential response was measured vs. Ag/AgCl reference electrode with a source-follower circuit (ISFET mV/pH meter Shindengen Electric Mfg. Co. Ltd.) at 25 °C in the dark. The frozen horse serum (Working Certified Reference Serum for ISEs ... 

Selectivity of sodium ISFETs. The selectivity of the Na+ ISFETs were evaluated... 

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. 

Stability of sodium ISFETs. The PVC and KP-13 matrix ISFETs have some drift characteristics of a few mV per hour. The lifetimes of the both ISFETs are about 1 week. It is considered that the drift and durability is caused by the poor membrane adhesion to the ISFET device (9). The Urushi matrix ISFETs exhibited a drift <0.1 mV per hour and durability > 1 month because of the strong adhesion of the Na+ sensing membrane to the ISFET device. 

Preliminary characterization of the FIM prototype was carried out using ISFETs as microsensors for pH, nitrate, ammonium, sodium and potassium. The sensibilization was made by either ion implantation or casting of a polymer solution. 

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