addressable elements for two-dimensional concentration mapping. T. Hermesa .... is used to form the electrolyte matrix [5] to complete the operating sensor array ...
ELSEVIER
Sensors and Actuators B 21 (1994) 33-37
An amperometric microsensor array with 1024 individually addressable elements for two-dimensional concentration mapping T. Hermesa , M. Biihnera, S. Biichera, C. Sundermeierb, C. Dumschatb, M. Borchardtb, K. Cammannb, M. KnoW,b 'Pachhochschule MUnster, Labor fiir Mikrosensorik wu1 Halb1eitenechnologie, Stegerwaldstrasse 39, D-48565 Sleinfurt, Germany blnstitut fiir Chemo- wu1 Bicsensorik (ICB) Wilhelm-KJemm-Strasse 8, D-48149 MUnster, Germany
Received 16 September 1993; accepted 19 November 1993
Abstract
A microsensor array with 32 rows and 32 columns of 1024 individual addressable amperometric cells on a 30 mm x30 mm Si substrate has been developed. The performance of the two-dimensional measuring system is described with an example of a Clark-type oxygen-measurement system, leading to a novel nearly real-time oxygen-concentration mapping important for transplantation medicine and biosensor applications. Special attention has been paid to an easy, reliable mass-production process with integrated electrolyte and membrane immobilization, resulting in low-cost devices. Parameters influencing the measurements can be chosen by a specially developed software that allows nearly real-time amperometric measurements to be realized after an automated calibration of the whole array. As a sub-sensor for biosensors based on oxygen consumption or hydrogen peroxide production, exciting new possibilities for two-dimensional concentration mapping of many other analytes open new horizons in analytical chemistry. Keywords: Microsensor array; Oxygen sensors; Two-dimensional mapping
1, Introduction
Conventional amperometric measurements with single working electrodes yield results only about the global concentration of the sample. But for some applications two-dilnensional concentration profiles are required. Especially in the field of transplantation medicine, pathology and biochemistry, there is a strong need for information on the two-dimensional concentration profile. The oxygen concentration of the surface of organs to be transplanted delivers more reliable information on the state and the quality of this vital 'device' than a silnple finger-touch analysis by the surgeon. Up to now, tilne-consuming multiple sampling procedures with conventional oxygen electrodes of the Oark type have been employed. Here the parallel measurement of up to 1024 different surface locations drastically improves this quality-control step. 0925·4005/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDl 0925-4005(93)01209-M
On the other hand, since more than 100 biosensors based on oxygenases consuming oxygen are known [1], a correspondingconcentration mapping of these analytes (e.g., glucose, alcohol and many others) becomes possible with this device. In the field of immunosensors, g1ucose-oxidase-marked antibodies allow the surface distribution of an unlimited number of different analytes to be measured with excellent sensitivities far better than those obtained with fluorescence markers. If one is not interested in a two-dimensional concentration mapping of a corresponding analyte, parallel measurements will increase the accuracy by statistical data treatments or multi-analyte biosensors for nominally 1024 different analytes become possible. As a sub-sensor (transducer) for biosensors, the lowcost approach of the array described below allows throw-away biosensors and/or an encapsulated sensor reserve that is contacted by the sample only after the biosensor in use has lost its function to be achieved.
T. Hermes et aL / Sensors and Actwtors B 21 (1994) 33-37
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2. The l024·element sensor array The 30 rom X 30 rom sensor array contains 1024 individually addressable amperometric sensor cells with 40 pm X 80 pm platinum working electrodes and a nonpolarizable AglAgCl reference electrode with equal dimensions. These two-electrode sensor cells are arranged in a matrix with 32 rows and 32 columns on an Si substrate. Working electrodes and reference electrodes are directly connected with metal leads. Thus, terminal pads, leads and sensor electrodes can be formed in the same fabrication step. Hence, the number of masks and fabrication steps can be reduced and for that reason the production costs are minimized. References electrodes are connected to columns and working electrodes to rows. Every row and every column ends in a connecting pad at the extreme left or right side of the silicon substrate. By means of this pad, every sensor element can be addressed by two contact bars that fix the sensor array during the measurements. Fig. 1 shows the layout of the sensor array and the measurement scheme. 3. Technology Three inch (111)-oriented silicon wafers (from Wacker Chemie, Burghausen, Germany) are used. After passivation by wet thermal oxidation, a 40 nm chromium adhesive layer [2] and a 300 nm platinum layer are deposited onto the water. Then the wafer is sputter etched through a mask consisting of a photoresist layer to form the working electrode layer. As interlevel dielectric, a methyl siloxane sol-gel is spun on and
tempered in a nitrogen atmosphere. Windows for connecting pads and sensor electrodes are etched with 2% buffered HF solution. In the next step the second metallization (a chromium adhesive layer and a 900 nm silver layer) is deposited onto the substrate by sputtering. This layer forms the reference electrodes and is structured by etching with a conventional silver etching solution [3], followed by the etching of the chromium adhesive layer by an etching solution containing KMn04 as oxidant [4]. Finally, a protective coating against mechanical and chemical influences is deposited. It consists of a highly tempered photoresist layer. Windows for sensor cells and connecting pads are formed by lithographic processes. In the last step, 2-hydroxyethylmethacrylate (HEMA) is used to form the electrolyte matrix [5] to complete the operating sensor array. It is dispensed by an automatic dispensing system (Automove 402, Asymtek Co.) onto each sensor electrode using a drain tube of 150 JLm inner diameter. For the hydrogel, 2 g of HEMA are doped with 60 mg tetraethyleneglycoldiacrylate, 80 mg dimethoxyphenylacetophenone, 150 mg of the adhesion promoter Silan A 174 (Merck) and 120 mg of polyvinylpyrolidone to improve viscosity [6,7]. This hydrogel is used to dispense several layers of drops by the following process: (i) dispensing a drop onto every amperometric cell; (ii) polymerizing the gel cocktail by 3 min of UV irradiation in a nitrogen atmosphere; (iii) dispensing the next layer, and so on. The HEMA membranes are about 70D-8oo JLm in diameter; they are in a non-contact configuration to avoid stray currents.
columns with reference-electrodes signal current
rows with working-
I I
electrodes
amplifiers
multiplexer
a-d converter multiplexer
l-
--,""'-~_ _---'
resistor
Fig. 1. Layout of the sensor array and measurement scheme. The signal current is converted into a proportional voltage, amplified and converted into a digital signal by the analog-digital converter (ADC).
T. Hermes et aL I Sensor:! alld Actuator:! B 21 (1994) 33-37 sol-gel interlevel HE dielectric gel e1ect/olyte pia num ng electlode
35
sol-gel interlevel dielectric
WOI
$I subs la e
A
liiii~tAglAgC',
reference electtode
B
interface
-----f digital signals
)
analog signals
Fig. 3. Logical diagram of analog and digital signal flows. Fig. 2. Cross section and plan view of a sensor element.
Before actual oxygen measurements. the hydrogel has to be swollen in the electrolyte solution. A 0.1 M potassium chloride solution mixed 1:1 with a pH 10 buffer solution (Merck) was used for this purpose. After this the silver electrode surfaces have to be chloridized by connecting the sensor cells for some seconds to a ceIl voltage of 690 mY. A cross section and a plan view of a sensor element are shown in Fig. 2.
signal current
to multiplexer and
+------1
+
a..d converter
•
-~/T
o--------~
4. Set.up for measurements The measuring equipment described below consists of an amperometric sensor array, an external hardware and supporting unit containing contact bars. multiplexers. digital-analog converter (DAC) and analog-digital converter (ADC), a slot card interface and a personal computer. The logical diagram of analog and digital data flows is shown in Fig. 3. The signal transformation is divided into three steps: conversion from signal current into a proportional voltage. amplification and AD conversion. Each of the amperometric ceIls is connected by two metal pads (on~ for the column and the other for the row metallization). Contact is made by two movable contact bars on the supporting unit fitted to these pads. The main function of these bars, serving also as a support device for the array, is to supply a cell voltage of 690 mV to the sensor cells of the array via springloaded contacts. This voltage is provided by a DAC placed in the external hardware unit, which can be manipulated by a computer via a specially developed interface. A sensor cell is exactly addressable by applying the ceIl voltage to the corresponding column and observing
Fig. 4. Active filter to convert and amplify the signal current of the sensor cells.
the voltage drop at a resistor related to the selected row (see Fig. 1). By connecting all cells of a column to the ceIl voltage at the same time, it is guaranteed that almost exclusively the signal current of only one sensor cell contributes to the voltage drop and the possibility of stray currents is excluded. The sensor cells of a column are measured sequentially by means of a multiplexer. The time needed for the measurement of one sensor ceIl is about 0.3-1.2 ms. The voltage drop at the resistor is amplified by a first-order active filter with a low impedance output according to Fig. 4. The amplification factor of the active filter is 0.2 ILA!V and the cut-off frequency is about 2 Hz. So that every row can be supplied with voltage at the same time, the signal transformation unit has to contain 32 resistors and 32 op-amp circuits. The outputs are connected to eight 4-in-l analog multiplexers and one 8-channel ADC. The digital conversion of the signal voltage is acquired upon acknowledgement via the interface on the databus.
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T. Hennes et aL I Sensors and Actuators B 21 (1994) 33-37 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000
This interface serves as a buffer and is equipped with tri-state drivers. To display the data, software has been developed that provides a three-dimensional diagram of the concentration mapping. Other features of this software are: changing the value of the cell voltage; choosing the time between applying voltage to the whole array and measurement (polarization time) and between applying the cell voltage to a column and measurement of this column of sensor cells (column time).
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5. Measurements (a)
First, the sensor array is calibrated by exposing it to a pure nitrogen atmosphere or to air. The data obtained are treated by the software in order to standardize the zero current of the sensor cells. When the measurement is performed as descnbed below, the differences between the calibration values and the values obtained during the measurement are displayed. In order to perform a two-dimensional oxygen-concentration mapping, two different experiments for the generation of oxygen profiles were chosen. In the first experiment a moisturized oxygen flow is directed by a thin drain tube to the upper right edge of the square sensor array. The scheme of the oxygen flow and the obtained two-dimensional concentration profile of this experiment is shown in Fig. 5(a,b). The polarization time has been chosen to be 10 s and the column time 1 s. The maximum current is found to be 40 nA. In the second experiment (Fig. 6) a cylindrical tube with 15 rom diameter is placed directly on top of the array. Pure moist oxygen is blown through this tube. The polarization time and column time are identical to those in the first experiment. The maximum current is about 60 nA. The response time of the sensor array is about a few seconds. About 60-70 s are needed for the measurement of the whole array. From Figs. 5(b) and 6(b) it is obvious that the new sensor is suitable for determining oxygen-concentration profiles in gaseous samples. The lifetime of the array is limited by the drying out of the HEMA layer to about 15 min in dry air. This will be prevented by the next generation of the sensor array, which will then be tested with medical samples.
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Ilgnal current
Fig. 5. (a) Drain tube with sensor array. (b) Concentration mapping of an O2 distribution.
(a)
6. Conclusions An amperometric sensor array for measuring the two-dimensional distribution of oxygen in gaseous samples and on biological surfaces was introduced. The
(b)
cOlumn
Fig. 6. (a) 15 mm tube on sensor array. (b) Concentration mapping of an O2 distribution. Light shades correspond to high currents.
T. Hennes et al. I Senson and Actuaton B 21 (1994) 33-37
open architecture of the measuring system should allow it to be used for a multitude of chemical analysis problems. Furthermore, because of the short response time an application in flow-injection analysis (FIA) is possible, leading to nearly real-time multiple analyte detection if an array of amperometric biosensors is built up. In this case the quadratic form will be replaced by a rectangular sensor arrangement covered by a thin sample channel. This sensor array could be a technological base for developing a total biochemical microanalysis system. The production technology is simple and allows easy mass production at low cost, as described elsewhere
[8].
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Sillce Bacher was born in 1970 in Hasbergen, Germany. She received a diploma in electronic engineering from the Fachhochschule Munster in 1993, where she worked on the development of an amperometric sensor array. Christian Sundermeier graduated in electrical engineering at the Fachhochschule Munster in 1989. Between 1989 and 1990 he worked in the Department of Semiconductor Devices of the Fernuniversitat Gesamthochschule Hagen. From 1990 to 1991 he worked in the Department of Semiconductor Devices of the Fachhochschule Munster. Since 1991 he has been engaged in the research and development of solid-state chemical sensors and micromechanical devices. He is currently working at the Institut flir Chemo- und Biosensorik (ICB) in Munster.
References [1] F. Scheller and F. Schubert, Biosensoren, Akademieverlag, Berlin, 1989. [2] D.J. Elliott, Integrated Circuit Fabrication Technology, McGraw Hill, New York, 1989. [3] G. Petzow, MetaUographisches Atzen, Gebriider Borntrager, Berlin, 1976. [4] W.E. Beadle, J.e. Tsai and R.D. Plummer (eds.), Quick Reference Manual f07 Silicon Integrated Circuit Technology, Wiley, New York, 1989. [5] M.L. Davies and B.J. Tighe, The potential of 'hydrogen polymers in sensor applications, Selective Electrode Rev., 13 (1991) 159--226. [6] B. Ross, K. Cammann, W. Mokwa and M. Rospert, VI· tramicroelectrode arrays as transducers for new amperometric oxygen sensors; Senson and Actuaton B, 7 (1992) 758--762. [7] P. van der Wal. The development of a durable potassium sensor based on PET technology, Ph.D. Thesis, Vniversity of Twente, Netherlands, 1990. [8] R. Kakerow, Y. Manoli, W. Mokwa, M. Rospert, K. Cam· mann, J. Krause and H. Meyer, A monolithic sensor array for measuring chemical and biochemical parameters, Proc. 7th Int. Conf. Solid-State Sensors and Actuators (Transducen '93), Yokohama, Japan, June 7-10, 1993.
Biographies
Thomas Hermes was born in 1970 in Munster, Germany. He received his diploma degree in electronic engineering from the Fachhochschule Munster in 1993. His current research activity concerns chemosensor and biochemosensor arrays. Mario Bahner was born in 1971 in Ibbenburen, Germany. He received a diploma in electronic engineering at the Fachhochschule Munster in 1993, where his main interest was the development of a special hardware unit for the evaluation of measurements of a Oarktype sensor array.
Christa Dumschat graduated in chemistry at the Technical University Leuna-Merseburg in 1986 and received the Dr. rer. nat. degree in 1991 from this university. She is currently working at the ICB in Munster. Her research interests are chemically sensitive membranes for chemically sensitive semiconductor devices (especially membrane deposition by the use of photolithographic techniques) and disposable chemical sensors. Michael Borchardt received his diploma in electrical engineering in 1991 from the Fachhochschule Munster. Since 1991 his main research interests have been the realization and characterization of disposable chemical sensors at the ICB in Miinster. KJIrl Cammann, born in 1939, received his Ph.D. at the University of Munich in 1975. Since 1979 he has been professor of analytical chemistry at the Universities of Ulm and Munich (Germany) and guest professor at the University ofDelaware (USA). In 1987 he received the Oce-van-der-Grinten Environmental Prize for Technology Transfer from the Ministry of Science and Education, Germany. Now he is full professor at the University of Munster and head of the ICB. He is mainly engaged in the development of chemical sensors and biosensors and in the development of spectroscopic and chromatographic methods. Meinhard Knoll received a diploma in electrical engineering in 1979 and his Dr.-Ing. degree in 1983 from the Technische Universitat Berlin. Between 1980 and 1985 he worked at the Hahn-Meitner-Institut fUr Kernforschung Berlin in the field of radiation effects in semiconductor devices. Since 1985 he has been professor at the Fachhochschule Munster, where he works in the fields of semiconductor devices, semiconductor technology and solid-state chemical sensors.
