Chemoresistive Sensor Design
Chemoresistive Sensor Design
Typical configurations of chemoresistive sensors include tube, volume, chip, and liquid electrolyte types. Figure 1 shows a tubular chemoresistive sensor developed by the University of Kentucky, USA. It uses nanotube structure in its sensor chip.
A planar chemoresistive sensor is shown in Figure 2, the typical configuration for metal oxide sensors. The sensor has three main components: a sensing element, electrodes, and a heater. The sensing element, often made of a metal oxide material such as SnO2, WO3, or In2O3, reacts with the target molecules, causing changes in resistance. The electrodes, connected to the sensing element, measure the resistance of the sensing material. A heating element mounted under the substrate is used to regulate the sensor temperature (since metal oxide sensors operate at high temperature –200~600°C, and exhibit different gas response characteristics at different temperatures). An additional power supply is needed for the heater. In order to achieve high sensitivity for target detection, the sensing element is designed to have as high specific area (surface-area-to-volume ratio) as possible.
This is achieved in practice through using either thin films or thick, porous layers of partially sintered materials.
A typical polymer chemoresistor has a chip configuration. Figure 3 shows a carbon black polymer sensor. The sensor electrodes are the two square metal pads with two 96 × 96 μm open areas (indicated by the crosses “X”) to allow the sensing element, carbon black polymer, to be deposited onto the metal pads. The “nitride” functions as an insulator. The sensor has a baseline resistance of 10 kΩ, achieved by using the minimum pad dimensions, a 80 μm gap between the electrodes, and the proper polymer thickness with the right fraction of carbon black in the composite. If the sensing element is made of organic materials, the electrode spacing will be typically 5~100 μm with an applied voltage of 1~5 V.
The liquid electrolyte structure is commonly used in fuel cell CO sensors as shown in Figure 4. The sensor has three electrodes: working ( sensing), reference, and counter (output) electrodes. The gas molecules react at the sensing electrode as follows:The generated CO2 diffuses away into the air while the positively charged ions (H+) migrate into the electrolyte. The electrolyte facilitates H+ moving to the counter electrode, resulting in current flow between the sensing and the counter electrodes. The electrons generated (e−) are conducted through the counter electrode to the external measuring circuit. Thus, the change in conductivity is directly proportional to the concentration of CO. The aforementioned oxidation reaction is balanced by a corresponding reduction reaction at the counter electrode:The reference electrode is introduced to improve the performance of the sensor by maintaining a constant voltage at the sensing electrode. No current flows to or from the reference electrode. The CO diffuses into the cell through the barrier (capillary). A filter is installed in front of the sensing electrode to block unwanted gases. The most commonly used filter medium is activated charcoal. Properly selecting the filter medium can make a sensor more selective to its target gases.