Reference ISFET

The reference ISFET can be modified by chemically reacting the SiOH groups with trimethoxysilanes [7]. An alternative method involves deposition of a hydrophobic polymer on the surface, for example by thermal deposition of parylene [85]. These layers tend to be chemically bound to the SiOH surface, leading to enhanced stability and long lifetimes. Also the layers are very thin, which is essential because of decreased electrical sensitivity as the insulator thickness increases [9]. For ion-blocking layers, a stable attachment has been realized by plasma deposition [86,87]. 

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, 5 Reaction chamber with (I) reference ISFET, (2) indicator ISFET, (3) pseudo-ref electrode, (4) actuator, (5) titration volume and (6) spacer with inlet and outlet. 

An acetylcholine receptor may be immobilized in the place of the antibody on the ISFET. In the presence of the positively-charged acetylcholine, the potential difference between the sensing element and that of a reference ISFET (REFET) without a receptor changes in the same direction, indicating that the acetylcholine is strongly bound to its receptor. This is not the case for other proteins (catalase or BS A) when they are fixed in the place of the acetylcholine receptor. 

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]. 

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. 

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. 

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 ... 

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. 

Ever since an ISFET that was chemically modified by a valinomycin-containing PVC membrane was reported [141], there has been general consensus on the advantages of this type of microsensor over conventional ISEs. Some serious problems have also been acknowledged, though e.g. the low mechanical stability of the membranes, the interference of COj in the potentiometric response, the lack of a stable micro-reference electrode and the relatively high drift rate of ISFETs). Attachment of the membrane can... 

Ion-Selective Field Effect Transistors [22b,c,d] An ion-selective field effect transistor (ISFET) is a hybrid of an ion-selective electrode and a metal-oxide semiconductor field effect transistor (MOSFET), the metal gate of the MOSFET being replaced by or contacted with a thin film of a solid or liquid ion-sensitive material. The ISFET and a reference electrode are immersed in the solution containing ion i, to which the ISFET is sensitive, and electrically connected as in Fig. 5.37. A potential which varies with the activity of ion i, o(i), as in Eq. (5.38), is developed at the ion-sensitive film ... 

Fig. 5.37 An ion-selective field-effect transistor (ISFET). 1, drain 2, source 3, substrate 4, insulator 5, metal lead 6, reference electrode 7, solution 8, membrane 9, encapsulant [22b].

Ion-selective membranes can be used in two basic configurations. If the solution is placed on either side of the membrane, the arrangement (e.g., Fig. 6.16a) is symmetrical. It is found in conventional ion-selective electrodes in which the internal contact is realized by the solution in which the internal reference electrode is immersed. In the nonsymmetrical arrangement (Fig. 6.16b), one side of the membrane is contacted by the sample (usually aqueous), and the other side is interfaced with some solid material. Examples of this type are coated wire electrodes and Ion-Sensitive Field-Effect Transistors (ISFETs). 

It contains parameters related to the solid-state (i.e., Ex) as well as to the chemical part(i.e., namL, jcq) of the sensor. Because the reference electrode is physically separate from the ISFET, its potential is not included in the threshold voltage. However, that choice is rather arbitrary. Whether the actual output follows (6.65) or (6.66) depends on the externally applied gate voltage Eg, which has nothing to do with the... 

Fig. 6.26 Individual and differential response of the NaCI ion-sensitive field-effect transistor (ISFET) sensor (a) response of Na-ISFET and (b) Cl ISFET both measured against regular reference electrode (c) differential current measurement of concentration of NaClin 0.01MMgSO4 solution (adapted from Bezegh et al., 1987)...

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). 

The first documented description of an ISFET contained the sensational claim that such a device could be used for the measurement of ion activities without a reference electrode. 

The electrical current flows from the source, via the channel, to the drain. However, the channel resistance depends on the electric field perpendicular to the direction of the current and the potential difference over the gate oxide. Should this surface be in contact with an aqueous solution, any interactions between the silicon oxide gate and ions in solution will affect the gate potential. Therefore, the source-drain current is influenced by the potential at the Si02/aqueous solution interface. This results in a change in electron density within the inversion layer and a measurable change in the drain current. This means we have an ion-selective FET (an ISFET), since the drain current can be related to ion concentration. Usually these are operated in feedback mode, so that the drain current is kept constant and the change of potential compared to a reference electrode is measured. 

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. 

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 ... 

In these sensors the technology developed for ISFET construction is used in conventional electrodes. Links between the membrane and internal reference are metallic (ohmic contact), by deposition of the metal on the membrane (solid state membranes), or by deposition of an ion-selective membrane on a metal. This latter is an integrated sensor. 

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). 

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