(bio-)chemical and physical parameters

Field-effect based multifunctional hybrid sensor module for the determination of both (bio-)chemical and physical parameters Michael J. Schöning*a,b, Arshak Poghossian**a, J.W. Schultze***c, Hans Lüth**a, a Institute of Thin Films and Interfaces, Research Centre Jülich GmbH b University of Applied Sciences Aachen c Institute of Physical Chemistry and Electrochemistry, University Düsseldorf ABSTRACT Sensor systems for multi-parameter detection in fluidics usually combine different sensors, which are designed to detect either a physical or (bio-)chemical parameter. Therefore, such systems include a more complicated fabrication technology and measuring set-up. In this work, an ISFET (ion-sensitive field-effect transistor), which is well known as a (bio-)chemical sensor, is utilised as transducer for the detection of both (bio-)chemical and physical parameters. A multifunctional hybrid module for the determination of two (bio-)chemical parameters (pH, penicillin concentration) and three physical parameters (temperature, flow velocity and flow direction) using only two sensor structures, an ion generator and a reference electrode, is realised and its performance has been investigated. Here, a multifunctionality of the sensor system is achieved by means of different sensor arrangements and/or different operation modes. A Ta2O5gate ISFET was used as transducer for all sensors. A novel time-of-flight type ISFET-based flow-velocity (flow rate) and flow-direction sensor using in-situ electrochemical generation of chemical tracers is presented. Due to the fast response of the ISFET (usually in the millisecond range), an ISFET-based flow sensor is suitable for the measurement of the flow velocity in a wide range. With regard to practical applications, pH measurements with this ISFET were performed in rain droplets. Keywords: field effect, ISFET, hybrid module, (bio-)chemical sensor, physical sensor

1. INTRODUCTION Miniaturised systems for (bio-)chemical analysis, like µTAS (micro total analysis system) [1,2] and „laboratory on a chip“ [3-5] have become increasing importance in the fields of environmental monitoring, analytical chemistry, biotechnology, food control, medicine, industrial process control, etc.. The most critical and important part of microanalysis systems is the detection unit, intended for the simultaneous measurement of different (bio-)chemical and physical parameters in a liquid flow such as the concentration of ions, dissolved gases, enzymes, conductivity, temperature, flow rate and so on. Similar to microelectronics, a sensor system (detector unit) for multi-parameter detection can be built-up monolithically or hybrid. Due to the simplicity, the relative low production costs, and the replacibility, hybrid-assembled modules are more suitable to make a small series production of flexible multisensor systems [4,6]. Different kinds of approaches for designing a sensor module for chemical sensing have been described in literature, e.g., the so-called „zero order“ concept that is based on different chemically sensitive layers and the same transducer principle, or the „high order“ concept, which implies more than one transducer principle for the same chemically sensitive layer [7-9]. However, a hybrid module for multi-parameter detection contains besides the (bio-)chemical sensors also physical sensors for the control of physical parameters of the fluid flow such as conductivity, temperature, flow rate, etc.. The design (structure, sensitive layer, transducer principle and measuring set-up) of the physical sensors usually differ _________________________________ * [email protected]; phone 49-2461-612973; fax 49-2461-612940; http://www.fz-juelich.de/isg/sensorik; University of Applied Sciences Aachen, Ginsterweg 1, D-52428 Jülich, Germany; ** [email protected]; phone 49-2461-612605; fax 492461-612940; http://www.fz-juelich.de/isg/sensorik; Institute of Thin Films and Interfaces, Research Centre Jülich GmbH, D-52425 Jülich, Germany; *** [email protected], phone 49-211-8114750; fax 49-211-8112803; http://phys-chem2.uniduesseldorf.de; Institute of Physical Chemistry and Electrochemistry; University Düsseldorf, Universitätstr. 1, D-40225 Düsseldorf, Germany

Advanced Environmental Sensing Technology II, Tuan Vo-Dinh, Stephanus Büttgenbach, Editors, Proceedings of SPIE Vol. 4576 (2002) © 2002 SPIE · 0277-786X/02/$15.00

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from that of (bio-)chemical sensors (see e.g. [5,10]). It is obvious that a sensor system combined of different sensors, which are designed to detect only one physical or (bio-)chemical parameter, will include a more complicated fabrication technology and measuring set-up. To circumvent this drawback, recently [11] we have suggested a novel approach for a multifunctional hybrid sensor module using the same transducer principle and structure for both (bio-)chemical and physical sensors. In this hybrid module, the same chemical sensor can also serve as a physical sensor and thus, the number of obtained (bio-)chemical and physical information can be higher than the number of the sensors needed („high order“ system). Fig. 1 shows the schematic configuration of such a hybrid module with an ISFET (ion-sensitive field-effect transistor) as transducer. Each element or sub-module built-up of several sensors or sensor arrays, is fabricated on a separate chip and set as a discrete component on a common printed circuit board. On the basis of this module one can realise at least three (bio-)chemical sensors (pH, ion and/or enzyme sensor), four physical sensors (temperature, flow-rate, flow-direction and conductivity sensor), and one actuator (ion or gas generator). Here, the multifunctionality is achieved by means of different sensor arrangements and/or different operation modes. Moreover, it allows to minimise or fully compensate the influence of various disturbing factors by using a differential measuring setup.

printed circuit board Fig. 1: Schematic configuration of the multifunctional hybrid module with an ISFET as the transducer. The module consists of two ISFET structures, an ion generator and a reference (RE) or a counter electrode (CE), which are set on the same printed circuit board (BioFET: biologically-sensitive field-effect transistor).

In this work, a hybrid module for the determination of two (bio-)chemical parameters (pH, penicillin concentration) and three physical parameters (temperature, flow velocity and flow direction) using only two sensor structures, an ion generator and a reference electrode, is presented. A pH-sensitive Ta2O5-gate-ISFET is used as transducer for all sensors.

2. EXPERIMENTAL 2.1 Hybrid module design Fig. 2 shows the videomicroscopic picture of the developed hybrid module with two pH ISFETs (or one pH ISFET and one PenFET (penicillin-sensitive field-effect transistor)) and an ion generator representing the generator electrode (Au film) and the counter electrode (Pt wire of 0.3 mm in diameter). An additional Pt wire, also included in the module, can serve as second generator electrode or as pseudo reference electrode when utilising the differential measuring set-up. All elements were placed on a printed circuit board, electrically bonded and the whole module, except the sensitive regions was encapsulated using a commercially available epoxy compound (Epo-Tek 77, Epoxy Technology). The resulting sensitive area of the ISFETs was about 0.5-1 mm2.

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Pt w ire

epoxy

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Pt w ire

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Fig. 2: Videomicroscopic picture of the developed hybrid module with two pH ISFETs (or one pH

ISFET and one PenFET (penicillin-sensitive field-effect transistor)) and an ion generator combining the generator electrode (Au film) and the counter electrode (Pt wire of 0.3 mm in diameter). An additional Pt wire is also attached to the module, to serve as second generator electrode or as pseudo reference electrode. 2.2 ISFET preparation The pH ISFETs used were n-channel devices, fabricated from a p-type boron-doped silicon wafer with (100)-orientation and 20 Ωcm specific resistivity. The channel is approximately 1 mm wide and 30 µm long. The source was connected to the bulk. For easy encapsulation of the device, the pH-sensitive gate was separated from the bonding pads as far as it was possible. The ISFET-chip size was approximately 2.2 mm in width, and 3.6 mm in length. The gate composite insulator consists of an approximately 80 nm thermally grown silicon dioxide layer, prepared in dry oxygen atmosphere at 1150°C that is covered by a Ta2O5 film (also 80 nm). The pH-sensitive Ta2O5 films were prepared by thermal oxidation of sputtered Ta in an oxygen atmosphere at 530°C for about 2 h [11,12]. The Ta2O5-gate pH-ISFETs were used as transducers for the development of a PenFET as well as for an ISFET-based temperature, flow-velocity and flow-direction sensor. 2.3 Measuring set-up The characterisation of the pH, flow-velocity and flow-direction sensor was performed using a specifically developed flow-through cell presented in Fig. 3. The hybrid module is vertically placed in the middle of the rectangular flow channel. The dimension of the flow channel is 15 mm in width and 2 mm in height. A Ag/AgCl liquid-junction miniature reference electrode was placed in the same flow-through cell at a certain distance from the ISFETs. The measured solution was pumped through the channel by the pump „MS-1 Reglo 160“ (Ismatec, Wertheim-Mondfeld, Germany) providing a flow rate in the range from 0.005 to 45 ml/min. The characterisation of the penicillin and temperature sensors was carried out in static solutions. All measurements were performed by means of the constant charge mode (CCM) with a grounded reference electrode using a home-made four channel laboratory ISFET meter (Research Centre Juelich, Germany) [12]. The CCM with the grounded reference electrode allows to carry out simultaneous measurements of the dynamic characteristics of multisensors by employing a common reference electrode. The measuring system is completely driven by a personal computer. The measurements were carried out at room temperature (except the temperature measurements). Some details of the sensors function principle, preparation and measuring procedure will be described in the corresponding sections.



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PCB w ith two ISFETs and ion generator RE inlet

outlet

Fig. 3: Videomicroscopic picture of the flow-through cell with the hybrid sensor module

and a reference electrode (RE). PCB: printed circuit board.

3. RESULTS AND DISCUSSIONS 3.1 pH ISFET The basic characteristics of the developed pH-sensitive Ta2O5-gate ISFETs in static pH buffer solutions were already reported in [11,13]. In the present work, we have tested the flow-through behaviour of the pH ISFET that is important for the pH-ISFET based flow-velocity and flow-direction sensor. For this purpose, flow-through pH measurements using standard pH buffer solutions were performed. Fig. 4 shows a flow-through dynamic response of two ISFETs at different pH values, reaching from pH 3 to pH 7. The flow rate was 0.5 ml/min. The sensor output signal was almost the same in the upward and downward series of the measurement. In contrast to the pH response in static-solution measurements [11], a small oscillation of the sensor signal is observed during the flow-through measurement that is caused by the pump functioning and possible bubble formation near by the ISFET gate region. The respective calibration curves of two ISFETs were linear with a nearly-Nernstian slope of about 55 mV/pH. One of the most promising field of ISFET applications is environmental monitoring. The analysis of chemical components in single raindrops is expected to give a new and interesting information about air pollution that requires microanalytical approaches and microsensors. The developed Ta2O5-gate pH-ISFETs were utilised for the pH measurements in bulk rain samples as well as in single rain droplets (about 3-5 µl volume) of different rain events. The reference pH measurements have been also performed in bulk samples of the same rain events with a conventional pHglass electrode. The bulk rain samples and raindrops were collected from two different areas of Germany, Jülich as a typical rural site and Essen as an urban site. For experimental details see [12,14]. As an example, Fig. 5 shows the dynamic response of the pH ISFET in rain droplets for 11 different rain events (R1R11). The pH of the bulk rain samples varied between 4.56 and 6.33. In most cases, we found a relatively good agreement between the pH values measured by the ISFET and the conventional glass electrode. Future applications will prove the acidic compounds of single droplets by means of in-situ rain drop monitoring.

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Fig. 5: Dynamic response of the pH ISFET in raindrops of 11 different rain events (R1-R11). The gray numbers correspond to the pH values in the bulk rain samples measured with the conventional pH-glass electrode. The black numbers correspond to the pH values in the raindrops of the same rain events measured with the pH ISFET.



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3.2 Penicillin sensor The PenFET (penicillin-sensitive FET) is obtained by covering the gate of a pH ISFET with the enzyme penicillinase. The operation principle of the penicillin biosensor is as follows: the PenFET detects variations in the H+-ion concentration resulting from the catalysed hydrolysis of penicillin by the enzyme penicillinase. A resulting local pH decrease near the gate region leads to a change in the surface charge of the pH-sensitive layer (here, Ta2O5) and consequently, to a change in the drain current of the PenFET. In this study, the enzyme penicillinase (EC 3.5.2.6., Bacillus cereus from Sigma, specific activity: 1650 units/mg protein) was adsorptively immobilised onto the Ta2O5 gate of a pH ISFET. Usually, penicillin biosensors have a certain blank pH response due to the change of the pH value of the bulk solution [15]. In order to eliminate the influence of possible pH variations as well as to compensate such effects that are caused by disturbing factors, like temperature and sensor drift, etc., the penicillin measurements were carried out in a differential measuring set-up [11] between the PenFET and a reference pH ISFET with nearly identical intrinsic sensor characteristics. In this case, the resulting differential sensor output signal is only dependent on the penicillin concentration. In addition, the Au or Pt electrode can be utilised as pseudo reference electrode. As a working buffer, a 0.2 mM polymix buffer, pH 8, was used. A detailed description of the immobilisation procedure as well as the measuring and storing conditions of the PenFETs is given in [16,17]. The specifications of the developed penicillin biosensor are summarised in the Table 1. The penicillin biosensor with the adsorptively immobilised penicillinase possesses a high average sensitivity of about 120 mV/mM in the concentration range from 0.05 to 1 mM penicillin G. Moreover, the sensor has a extremely low detection limit of about 5 µl, a low hysteresis within 4 mV and a long life-time of more than 1 year. The upper detection limit is about 20 mM. Table 1. Specifications of the developed penicillin FET. Sensitivity Linear range Lower detection limit Upper detection limit Hysteresis Response time Lifetime

120 !10 mV/mM 0.05 – 1 mM 5 µM 20 mM < 4 mV 0.5 – 3 min > 1 year

3.3 ISFET-based temperature sensor The temperature behaviour of an ISFET-based sensor is a complex function related to the reference electrode, the electrolyte, interfacial potentials and the solid-state device itself [18-20]. The sign and the amount of the temperature sensitivity of the ISFET depends on the working point, i.e. the value of the drain current. At a certain working point, the so-called isothermal point, the drain current (or the gate voltage) is nearly independent of the temperature. When the drain current ID is below the drain current of the isothermal point IDo, the temperature coefficient of the gate voltage dVG/dT is negative. If ID is higher than IDo, one can observe a positive temperature coefficient. In spite of this distinct temperature dependence, a single pH ISFET can not be used as a temperature sensor, because the sensor output signal depends on the pH value of the solution, too. In this case, a distinction between the temperature and/or pH effect is not possible. Moreover, the position of the isothermal point also depends on the pH value of the solution [18-20]. However, we have demonstrated that a pH ISFET-based temperature sensor can be realised by simply using a differential measuring set-up consisting of two identical pH ISFETs, which are operated at different working points [11]. Therefore, the disturbing factors, like a possible pH change of the solution and a drift of the sensor signal can be compensated and thus, the differential output signal should only record the temperature as the measuring signal, similar as for the pH ISFET/PenFET dual chip. The difference of the temperature sensitivity of two ISFETs operating at different working points with different drain currents allows to measure the exact temperature. As one example, Fig. 6 shows the temperature dependence of the gate-voltage difference ∆VG=VG1(ID1)–VG2(ID2) for

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two different working points ID1=0.5 mA and ID2=0.33 mA. The measurements were carried out in a standard buffer solution, pH 5, in the temperature range from 22°C to 37°C using a thermostatic chamber (RTE-110, Neslab). The temperature dependence of ∆VG is linear with a slope of about 2.24 mV/°C and practically independent on the pH of the solution. Hence, a direct correlation between the sensor output signal and the temperature can be achieved.

V G 1 (I D1 ) - V G 2 (I D2 ) (V)

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3.4 ISFET-based flow-velocity and flow-direction sensor The schematic structure and functional principle of the time-of-flight type ISFET-based flow-velocity and flowdirection sensor is depicted in Fig. 7. The sensor presents a chemical sensor-actuator system, consisting of one ion generator (e.g., Pt, Au for H+- or OH--ion generation) and one detector (e.g., ISFET) for the in-situ electrochemically generated ions. The generator, the ISFET detector as well as an additional reference and/or counter electrode are placed in the flow-through channel. The corresponding electrochemical reactions for the H+- and OH--ion generation due to the electrolysis are demonstrated in Fig. 7 (a). Dependent on the current direction between the generator electrode and counter electrode, the H+ ions (anodic electrolysis) or OH- ions (cathodic electrolysis) are produced. The in-situ generated chemical tracers (H+- or OH- ions) flow to the downstream-placed ISFET, where they are immediately detected. The ISFET output signal (detector signal) will be determined by the pH near the gate region. The time difference ∆t=td-tg (time of flight) between the detection and generation of the ions (see Fig. 7 (b)) is then, inversely proportional to the mean velocity of the fluid. If the geometry of the flow channel is given, the flow rate can be calculated, too. Moreover, a design with an upstream and a downstream ISFET and an ion generator placed between them (see Fig. 2) allows to realise a flow-direction sensor at the same time. Since the ISFET usually has a short response time (millisecond range), it can be utilised as an excellent detector for the measurement of the flow velocity in a wide application range. Moreover, since this measuring method is a dynamic method, the detector´s drift, nonlinearity, temperature instability and sensitivity discrepancy between the various ISFETs are not relevant, because only relative changes in the sensor´s output signal will be evaluated.



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Fig. 7: ISFET-based flow velocity and flow direction sensor (schematically): a) functional principle, b) time of flight. The corresponding electrochemical reactions for the H+- and OH- ion generation through the electrolysis are presented (the reference and/or counter electrodes are not shown for simplicity). At the moment tg the generator produces a chemical tracer impulse, which is detected by the downstream placed ISFET at the moment td. (∆t=td-tg is the time of flight (or transit time); s is the distance between the generator and the ISFET).

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Flow-velocity and flow-direction measurements were performed using the flow-through cell with the hybrid module, already presented in Fig. 3. A potential of ~1.7 V was applied to the Au-generator electrode for the H+-ion generation. The duration of the voltage impulse is about 20 s. During the measurement, unbuffered 0.1 M KNO3 water solution, pH ~7, was pumped through the flow channel by a peristaltic pump. Fig. 8 shows the calibration curve of developed ISFET-based flow-velocity sensor, evaluated for different flow rates from 0.5 to 2.5 ml/min. A good linearity between the flow velocity measured with the ISFET and the delivered flow rate of the pump is observed.

Fig. 8: Calibration curve of the ISFET-based flow-velocity sensor, evaluated for different flow rates from 0.5 to 2.5 ml/min.

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Fig. 9 demonstrates an example of a flow-direction measurement. In this case, OH- ions were produced. The generator electrode is placed between the upstream and downstream pH ISFET (see Fig.2 and Fig 7 (a)). The flow rate was 0.5 ml/min. As can be seen, the output signal of the downstream ISFET shows a peak due to a change of the OH--ion concentration close to the gate region, while for the upstream ISFET practically no change in the output signal is observed. Thus, besides the flow velocity the flow direction can be determined, too.

CONCLUSIONS 1. A multifunctional hybrid module using an identical transducer principle for both (bio-)chemical and physical sensors is presented. Here, an ISFET, which is well known as a (bio-)chemical sensor, also serves as a physical sensor. Multiparameter detection is achieved by means of different sensor arrangements or different operation modes. 2. The hybrid module for the determination of two (bio-)chemical parameters (pH, penicillin concentration) and three physical parameters (temperature, flow velocity and flow direction) is realised and its performance has been investigated. A Ta2O5-gate ISFET was used as transducer for all sensors. 3. A novel time-of-flight type ISFET-based flow-velocity (flow rate) and flow-direction sensor using in-situ electrochemical generation of chemical tracers is developed. 4. With regard to practical applications, the possibility of an ISFET application for the pH determination in raindrops is demonstrated.

ACKNOWLEDGMENTS The authors thank H. Emons and A. Baade for valuable discussions and L. Berndsen for technical support. Part of this work was supported from the Ministerium für Schule und Weiterbildung, Wissenschaft und Forschung des Landes Nordrhein-Westfalen, Germany (project ELMINOS).

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