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Future Applications of Isfets* Abstract the Application of Isfets Has Been Fo- Cused up to Now Mainly on Static Measure- Ments

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Future Applications of Isfets* Abstract the Application of Isfets Has Been Fo- Cused up to Now Mainly on Static Measure- Ments Sensors and Actuators B, 4 (1991) 125-133 125 Future Applications of ISFETs* P. BERGVELD University of Twente, Box 217, 7OOAE Enschede (The Netherlands) Abstract seen as directly of interest for the electrophys- iological experiments. The non-selectivity of The application of ISFETs has been fo- silicon dioxide has recently been confirmed cused up to now mainly on static measure- by independent measurements [ 31. ments, comparable with the conventional way With respect to the measurement of ion of applying ion-selective electrodes. The spe- fluxes, it should be noted that the related cific and unique properties of ISFETs, their current densities are in the order of some small dimensions and very fast response, are rnA/cm’, resulting in ionic transports in the not exploited explicitly in the case of static order of picomoles/cm2. However, by catch- measurements. These features will, however, ing the ionic flux in a very small cavity, which be explored more and more in the future, for is achieved by pressing the ISFET with its example in flow-through and FIA systems as surrounding ring of encapsulant firmly well as for ISFETs with an integrated coulo- against the nerve membrane, the ionic metric actuator. In the latter case the ISFET changes may rise to 0.1 millimol/litre, easily is part of an active microchemical reactor. detectable by an ISFET. The recorded Similar systems may in the future contain monophasic pulses [4], as shown in Fig. 1, bioactive materials, whose affinity to a bio- are therefore most probably indeed the result logical analyte can be measured by means of of very local ionic concentration changes, stimulus-response experiments. although this has never been proved in any later paper. In the context of the present paper, it is IntroduetIon important to notice that in the electrophysio- logical ISFET application as mentioned Relevant future applications of ISFETs above, the fast response of an ISFET is ex- cannot be discussed without considering the plicitly exploited, combined with its small original research, development and applica- dimensions. tion aims and the corresponding results as In another, rather isolated paper by the achieved up to now. present author, a chemically insensitive The author’s original intention in develop- device has been described, related to the ear- ing the ISFET was to measure the ionic lier ISFETs [5]. These devices were intended sodium influx and potassium efflux at nerve to be ISFETs, but, probably due to contami- and muscle membranes [ 1,2]. The silicon nation of the gate oxide by a very thin epoxy dioxide of a MOSFET, from which the gate layer, arising from the encapsulation of the metal was etched away, appeared to be sensi- devices, no sensitivity to ions was observed, tive for sodium and potassium ions, and thus at least not for an initial period of contact well suited for the intended application. Sen- with the extracellular liquid. The device oper- sitivity to pH was also observed, but was not ated as a purely capacitive electrode, similar to a MOSFET. Because the only difference from the MOSFET was the omission of the *Invited paper. gate metal, this device was called an OSFET. 09254005/91/$3.50 0 Elsevier Sequoia/Printed in The Netherlands 126 was not directed towards the electrophysio- logical application of ISFETs and related devices, but to the measurement of ionic con- stituents of various body liquids, mostly blood. This was due to the lack of any small conventional type of ion-selective electrode suitable for in vim application. This is still Fig. 1. Original registration of monophasic pulses with the reason that many ISFET projects at Uni- an ISFET pressed to the Locusta flexor tibialis. versity as well as industrial laboratories are devoted to extra- or intracorporeal measure- Nowadays such a device would be called a ments of blood chemical constituents. REFET. However, the original OSFET Application in analytical chemistry is less differed from present REFETs in that the gate pursued, because of the apparent adequacy was surrounded by a relatively large parasitic of conventional ion-selective electrodes and gate. The result was that extracellular action other analytical methods. This may, however, potentials could be measured from a muscle change in the future as soon as the specific fibre which contacted the actual gate as well as properties of ISFETs are more clearly recog- the parasitic gate. In fact the potentials as nized, such as their extremely short response caused by extracellular ionic currents were time, which is of great advantage in the case measured by means of a capacitive division of flow injection analysis (FIA) and other between the actual gate capacitance and the microchemical investigations, for example. parasitic gate capacitance. The recordings ex- Although the electrophysiological experi- celled in a very good signal-to-noise ratio as ments, as mentioned in the Introduction, al- compared to the usual recordings with metal ready showed from the beginning that ion microelectrodes and also the broad frequency fluxes occurring within milliseconds could range (0 <f < 45 kHz) was remarkable. Un- easily be recorded, this fact was hardly recog- fortunately the excellent properties of record- nized at that moment. The reason is most ing features as mentioned above have never probably that directly after the first descrip- been exploited since its first publication, al- tion of ISFETs, these devices were seen as a though recently the possibilities have been modern solid-state appearance of the more rediscovered [ 61. bulky conventional (glass) membrane elec- In view of the present paper it should be trodes and thus should replace this type of noted that the parasitic gate in fact operated electrode. Much effort was and still is there- as a capacitive reference electrode. Due to its fore spent on static measurements with the size, its capacitance was much larger than the drawbacks of drift and lack of a stable solid- gate capacitance which in fact acted as a state reference electrode. The benefits of these micro-electrode. The capacitive voltage divi- investigations are the gain in knowledge of sion therefore resulted in a nearly unattenu- the actual ISFET operation and the applica- ated conversion of the extracellular potentials. tion of better inorganic gate materials, such It should be kept in mind that a capacitive as A120g, S&N4 and Ta,O,. Nevertheless it is reference might in the future also be explored, still doubtful, regardless of further improve- especially with respect to dynamic ISFET ments in ISFET technology, whether it will applications such as those described below. ever be possible to prepare drift-free ISFETs. The reason is that any device with an insulat- ing layer exposed to an electric field will drift, From Static to Dynamic Measurements as long as mobile charges in the insulator cannot be abandoned completely. In this re- The follow-up of the initial work, as spect ISFETs can hardly compete with their briefly summarized in the previous Section, conventional counterparts, whose operation 127 relies on ion exchange mechanisms at con- tional liquid-filled reference electrode, with ducting membranes [7]. Furthermore a stable the well-known problems of the liquid junc- solid-state, liquid-free reference electrode is tion potential which can also react very fast still not developed. to a change in ionic composition, this poten- More successful applications of ISFETs tial change possibly coinciding with the can, in the opinion of the author, sooner be ISFET response to the sample. Moreover, expected in the area of dynamic measure- especially if the sample is whole blood, con- ments than in the competitive field of static tamination of the reference electrode frit can- measurements. The first examples are already not be excluded. Only the use of heparinized given in the literature and will be reviewed in calibration (and rinsing) liquids can prevent the following Sections. Not only are the spe- this contamination, but it makes the system cific ISFET properties, namely small dimen- more complicated [9]. A solution to the refer- sions and fast response, more explicitly ence problem is the application of a differen- explored, but also the reference problem can tial measuring set-up, as described in the be solved much more easily in the case of following Sections. dynamic measurements. Differential Measurements Application of ISFETs in, Flow-through Cells To solve the reference electrode problem, a The fast response of an ISFET has already differential measurement set-up has been pro- been exploited in flow-through cells, where a posed [ 1 l] in which case the ISFET response sample is injected into a carrier stream, as is measured with respect to a REFET [ 121, schematically drawn in Fig. 2. while the analyte potential is defined by a The first system based on this approach platinum wire, as shown schematically in was published in 1980 [ 81, while Sibbald in- Fig. 3. troduced it for on-line clinical analysis of The unstable electrolyte/platinum elec- blood ionic composition, using a flow- trode potential is a common signal to the through cell with four integrated membrane- differential system and will thus not interfere covered ISFETs [9]. Especially adapted to the with the final output signal. Therefore the flow-through cell configuration, backside platinum electrode is usually called a pseudo- contacted ISFETs can be used with great reference electrode. advantage for this type of measurement, as However, for static measurements such recently shown by Alegret [lo]. systems require that the REFET is at least as In all the examples mentioned above, the stable as the ISFET, while furthermore the reference electrode consisted of a conven- electrical characteristics of the ISFET and Fig. 2. Schematic representation of single ISFET flow- Fig. 3. ISFET/REFET differential measurement with through cell arrangement.
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