Chemoresistive Sensor Applications

Saturday, November 25th, 2017 - Chemical, Resistive Transducers

Chemoresistive Sensor Applications

Chemoresistors and their arrays have been widely used for gas or vapor detection of substances such as H2, O2, CO, CO2, NO, NO2, CH4, pH, and H2O. Following are a few examples of their applications.

Hygristor

A hygristor (from the words “hygro-” and “resistor”) is a humidity sensor that uses a moisture-dependent resistor to measure humidity. It is made of hygroscopic materials (e.g., conductive polymer, silica gel, paper, or treated substrate) whose specific resistivity depends largely on the concentration of absorbed water molecules. The resistance changes inversely and exponentially as humidity changes. A typical hygristor contains a substrate and two silkscreen-printed conductive electrodes, which are usually covered with a hygroscopic semiconductive gel.

The most important attributes of resistive humidity sensors—small size, low cost, interchangeability, and long-term stability—have made them suitable for industrial, commercial, and residential applications. Some resistive humidity sensors also resist chemical and physical contaminants and have the ability to recover from repeating condensations. The life expectancy of humidity sensors is about 5 years for residential and commercial applications, but exposure to chemical vapors and other contaminants (e.g., oil mist) may shorten the sensors’ life span. Resistive humidity sensors are also significantly influenced by temperature. If they are used in environments with large temperature fluctuations (>10°F), temperature compensation should be incorporated into their design to ensure accurate measurement.

Groundwater Monitoring System

A four-chemoresistor array has been developed for groundwater monitoring wells at Edwards Air Force Base and for the chemical waste landfill at Sandia National Laboratories [50]. The sensor is fitted with a GORE-TEX®  membrane to allow chemical vapors to go through to the sensor while protecting internal electronics and wiring from contaminants. The chemoresistors are incorporated into a single die with the circuitry to measure sensor resistance.

Electronic Nose

An electronic nose (e-nose) has been widely used to analyze volatile organic compounds, monitor vehicle emissions, detect explosives, and perform clinical diagnoses. Many chemoresistors can be integrated in an array to form an e-nose, including metal oxide semiconductors, conducting polymers, nanotubes or nanowires, etc.

These sensing elements or materials form a sensitive layer and allow a wide variety of chemical compounds to be detected. An e-nose also includes a pattern-recognition mechanism (chemometrics) that compares the patterns from the measurements to the known patterns for identification. An example of an e-nose’s response to different gases is shown in Figure 1. The different output pattern from this eight-sensor array e-nose indicates a different gas. If the array is “trained” properly, it can recognize an individual gas in mixtures.

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Figure 1. An eight-sensor array e-nose’s output patterns corresponding to different gases

An increasing research interest is to integrate chemoresistors into robotic systems for radiation detection or toxin sensing without risking human exposure. If chemical sensors are mounted in multirobot systems (e.g., swarm robots), a mobile sensor network can be established to perform more powerful functions.

I hope this information about “Chemoresistive Sensor Applications” is useful.