Bergveld, Piet

Herber, S.

Olthuis, Wouter

van den Berg, Albert

English

In this paper is described how hydrogel is exploited as sensor material for the detection of carbon dioxide (CO2). A pH-sensitive hydrogel disc, which swells and deswells in response to pH changes, was clamped between a pressure sensor membrane and a porous metal screen together with a bicarbonate solution. Bicarbonate reacts with CO2 resulting in a pH change. The enclosed hydrogel will generate pressure as a response to the pH change. This pressure is a measure for the partial pressure of CO2. The main advantage of this sensor principle is the lack of a reference electrode as required for potentiometric sensors

University of Twente Research Information

Exploitation of a pH-sensitive hydrogel for CO2 detection  S. Herber, W. Olthuis, P. Bergveld, A. van den Berg  MESA+ Research Institute, University of Twente, P.O. Box 217,  7500 AE Enschede, The Netherlands email: S.Herber@el.utwente.nl url: http://www.bios.el.utwente.nl  Summary: In this paper is described how hydrogel is exploited as sensor material for the detection of carbon dioxide (CO2). A pH-sensitive hydrogel disc, which swells and deswells in response to pH changes, was clamped between a  pressure sensor membrane and a porous metal screen together with a bicarbonate solution. Bicarbonate reacts with CO2 resulting in a pH change. The enclosed hydrogel will generate pressure as a response to the pH change. This pressure is a measure for the partial pressure of CO2. The main advantage of this sensor principle is the lack of a reference electrode as required for potentiometric sensors.  Keywords: hydrogel, Pco2 sensor, Severinghaus Subject category: 5 (Chemical sensors) & 6 (Biosensors)  1. Introduction  Stimulus-sensitive hydrogels are cross-linked hydrophilic polymers which undergo a volume change in response to changes in stimuli, such as pH, temperature, electric field, ion concentration or light [1]. This research project aims at exploring the possibilities of detecting the pressure built-up generated by a hydrogel contained under isometric conditions for sensor applications.   2. Sensor design  The first goal is to develop a gastric Pco2 sensor for measuring the partial pressure of CO2 in the stomach to determine whether a person has Gastrointestinal Ischemia [2]. To realize this, a pH-sensitive hydrogel disc is used. This type of hydrogel will swell with a decrease in pH and deswell with  an increase in pH. A miniature pressure sensor is used to observe hydrogel swelling. In figure 1 a schematic representation of the hydrogel-based Pco2 sensor is shown.    Fig. 1. Schematic representation of the hydrogel-based Pco2 sensor.  CO2 will diffuse through the membrane and react with the bicarbonate solution resulting in a pH decrease (Severinghaus principle [3]). In response to the pH decrease the hydrogel would like to swell, but since its volume is fixed by the porous cover (isometric conditions), a pressure will be generated. This pressure is measured by the pressure sensor and is thus a measure for the partial pressure of CO2.  3. Experimental  The hydrogel disc was prepared from 2-hydroxyethyl methacrylate (HEMA, Acros) and 2-(dimethylamino)-ethylmethacylate (DMAEMA, Acros) by UV-polymerization. A monomer mixture of HEMA and DMAEMA was made with a mole ratio of 95:5 and to the total mole amount, 1.5% cross-linker tetraethyleneglycol diacrylate (TEGDA, Fluka) and 3% photoinitiator 2,2-dimethoxy-2-phenylaceto-phenone (DMPAP, Aldrich) was added. The solution was pipetted on a silicon mould with a cavity of 50 µm and covered with Mylar foil. On top a mask was placed with an aperture (diameter = 750 µm) through which the UV light can pass. The hydrogel was polymerized by 366 nm UV for 90 seconds. The principle is shown in figure 2. By using this method a hydrogel with a thickness of 50 µm can be made.    Fig. 2. Schematic representation of the setup that is used to obtain hydrogel discs with a diameter of 750 µm and a thickness of 50 µm.  335The hydrogel disc was placed on the pressure sensor membrane of a bare Honeywell pressure sensor.  The pressure sensor was glued on a PCB stick and wirebonded.  On top of the sensor a porous metal screen was glued (for the initial experiments a metal screen was used instead of a silicon cover).  4. Results  For the initial experiments the sensor was first placed in different 100 mM pH buffers to measure the pressure the hydrogel generates as a function of pH. The result is shown in figure 3. As expected the pressure increases with decreasing pH.  0,60,70,80,911,11,21,31,44 5 6 7 8 9 10pHPressure (bar) Fig. 3. Plot of the measured pressure versus pH.  Next, a CO2 measurement was performed. The sensor was placed in a 100 mM NaHCO3 solution together with a pH electrode. Gas was bubbled through the solution starting with 100% N2, then 100% CO2 and again 100% N2. During the experiment the pH and pressure were measured versus the time. The result is shown in figure 4.  0,650,70,750,80,850,90,9510 500 1000 1500 2000Time (sec)Pressure (bar)66,577,588,599,510pHPressurepH Fig. 4. Plot of  the pH change of a 100 mM sodium bicarbonate solution and the pressure change of the hydrogel based sensor as result of a gasflow  change  from 100% N2 to 100% CO2 and back to 100% N2.  As can be seen in the plot, the pH decreases first by the CO2 and then increases again by the N2 removing the CO2. The pressure at start is 0.68 bar caused by the clamping of the hydrogel. At t = 800 seconds the pH is decreased to 6.7 by the CO2. Due to this pH decrease, the hydrogel wanted to swell, but since it’s volume is fixed it generates an additional pressure of  0.29 bar resulting in a pressure of 0.97 bar. By interpolating the plot from figure 3 the pressure at pH 6.7 can be found which is around 0.95 bar. This agrees reasonable well with the pressure measured during the CO2 experiment.  Compared to the previous experiments with a Pco2 sensor holding hydrogel microspheres, the use of a very thin layer of hydrogel gave a faster response [4].  5. Conclusions  A sensor concept is demonstrated where the swelling of a thin pH-sensitive hydrogel layer is measured with a miniature pressure sensor. With the addition of bicarbonate it is shown to be possible to measure CO2. The principle showed in this article could also be used for other stimulus-sensitive hydrogels, such as a glucose-sensitive hydrogel to construct a glucose sensor.  References  [1] Heiko J. van der Linden, Sebastiaan Herber, Wouter Olthuis, Piet Bergveld, Stimulus-Sensitive Hydrogels And Their Applications In Chemical (Micro)Analysis, The Analyst, 2003, Accepted for publication. [2] A. Groeneveld , J. Kolkman, Gastointestinal ischemia and splanchnic tonometry: a review of physiology, methodology and clinical implications, J. Critical Care, 9, 1994, pp.198-210. [3] J. Severinghaus, F. Bradley, Electrodes for blood Po2 and Pco2 determination, J. Appl. Physiol., 13, 1958, pp.515-520. [4] S. Herber, W. Olthuis, P. Bergveld, A swelling hydrogel based carbon dioxide sensor, Sens. Actuators, B, 2003, Accepted for publication. 336

Exploitation of a pH-sensitive hydrogel for CO<sub>2</sub> detection

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Exploitation of a pH-sensitive hydrogel for CO<sub>2</sub> detection

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