Chemiresistors
by Shan
By Oz Agar, Shan Gao, Dena Molnar
Click the images:
Chemiresistor Intro:
Chemiresistors are sensor components that detect chemicals, typically vapors, through changes in chemical resistance, typically of a conductive particle filled polymer. These chemiresistors are generally small, rugged, cheap, reusable and reasonably sensitive (~1/1000 of the vapor saturation resolution), with low power requirements. Their cost, size, reusability and ruggedness are advantageous over other methods of detecting chemical vapors such as gas chromatography-mass spectrometry (GCMS), though the resolution, even improved by a pre-concentrator, is less ideal. For an idea of sizing, compare a GCMS machine with a reasonable Chemiresistor size (the red square).
GCMS machine [Wikipedia]
The main application for chemiresistors under industrial development is as a cheap, easily used sensor to detect chemical toxins and warfare agents in the air (for first responders, military, factory workers), soil (near oil pipelines, factories), and water (factory run off, near farms, oceans). There is also investigation of chemiresistors for process control in chemical engineering.
The ability to print chemiresistors is interesting, and could conceivably be used in advanced 3D printing/rapid prototyping involving other printed mixtures involving solvents to detect when a device had finished curing, or to detect the presence of environmental adulterants that could damage printed circuits or devices.
Basic, basic chemistry
Essentially, a chemiresistor has to have some interaction with whatever it’s meant to detect, and that interaction has to change the resistive quality of the chemiresistor. Furthermore, this interaction should be reversible. There are two types of detector, threshold and continuous:
Threshold detectors: Above some threshold value, there is or is not resistance. Below, there is the opposite. In other words, some step function involving concentration of chemical. Less frequently used, as there are many limitations as to likely scenarios, since can only detect whether the threshold condition is being met. Might see something like this for continuous liquid testing.
Continuous detectors: No step function. Can measure concentration of chemical over a broad range of values. There will be a resolution issue on the low end of detectable concentration, and at the high end saturation or even breakdown of the chemiresistor can occur depending on circumstances.
There isn’t some overall general formula to make a chemiresistor, unfortunately. Depending on what you’re testing for, and how specific you want to be, there are very specific chemistry considerations for the detection reaction. That said, there is a general concept to a chemiresistor, which is that the resistance changes depending on the presence of chemicals. This device should not be totally rugged in conductive performance; rather, it should have significant interactions with one specific environment. Conductive particles in polymer matrixes are good candidates to detect organic volatiles, a common scenario. The polymers experience swelling as they take up the volatile organics and add them to the material matrix; this disturbs the conductive particles, resulting in a resistance change that can be measured and used to calculate concentration of volatile organics.
Left: a basic polymer/conductive particle device. Right: device in presence of volatile organic[1]
Further Reading:
[1]:http://prod.sandia.gov/techlib/access-control.cgi/2003/033410.pdf This is a comprehensive paper about chemiresistors, from fabrication to usage, test cases, etc. carried out by Sandia National Labs (Lockheed Martin under contract to USDoEnergy). Image taken from page 16
http://www.sciencedirect.com/science/article/pii/S0003267008018667 Inkjet printed gold nanoparticle chemiresistor. Gives some ideas about printing possibilities, non polymeric chemiresistors
Safety
Composition / Stability of the Composites
poly(vinylacetate) (PVAc) and
poly(methyl methacrylate) (PMMA)
CB/PBMA composites
(Carbon Black/ monomer, butyl methacrylate (BMA),
initiator, benzoyl peroxide (BPO)
Carbon black filled polymer composite sensing materials are prepared by melt-mixing or solution-mixing. This process is potentially toxic.
Carbon Black- Cabot Safety, Health, & Environmental Information
USE- 90% of carbon black is used in rubber applications, 9% as a pigment, and the remaining 1% as an essential ingredient in hundreds of diverse applications.
Fire- Carbon blacks in the powder or pellet form burn slowly (smolder) and sustain combustion that may not be visible as flames or smoke. t. Combustion gases generated during smoldering include carbon monoxide (CO), carbon dioxide (CO2), and oxides of sulfur.
There has been no recorded industry experience to suggest that carbon black dust concentrations pose an explosion hazard . Under certain circumstances, however, it may be possible for smoldering carbon black to produce a sufficient concentration of combustible gases
Carbon black dust may be small enough to penetrate electrical boxes and other electrical devices, possibly creating electrical hazards resulting in equipment failure. Electrical devices that may be exposed to carbon black dust should be tightly sealed or purged with clean air, periodically inspected, and cleaned, as required.
Some grades of carbon black may be less electrically conductive, permitting a build-up of static energy during handling. Grounding of equipment and conveying systems may be required under certain conditions.
INHALATION- Carbon black dust spreads easily in air through virtually any air current or movement
Carbon black is classified by the International Agency for Research on Cancer (IARC) as a Group 2B carcinogen (possibly carcinogenic to humans). Studies have demonstrated that regular exposure to carbon black and other poorly soluble particles may play a role in declining lung capacity
HUMAN STUDIES- The health effects of carbon black exposure have been studied in the United States and Western
Europe for over 60 years. These studies indicate no evidence of significant clinical health effects due to occupational exposure
Test Organic Vapors
Use- It is found that the effect of swelling on the composite’s conductivity notably depends on the organic solvent nature, namely, at least one polymer in the composites should be capable of absorbing the organic solvent vapor by a dissolution
process.
Test Solvents -
cyclohexane, carbon tetrachloride, methyl methacrylate, styrene, ethyl acetate, benzene, toluene, chloroform, dichloromethane, 1,2-dichloroethane, tetrahydrofuran, acetone, pyridine and methanol,were reagent of grades and used as received. All of these can be volatile.
Organic Vapors
The safety of organic vapors being tested is dependent on what concentrations one is testing for.
Possibility of Future Application
Health and security
- The detection of dangerous and harmful bacteria
- The quality control of food products as it could be conveniently placed in food packaging to clearly indicate when food has started to rot.
- Warn of the presence of natural gas, for those who had anosmia or a weak sense of smell.
- A technology of Electronic Nose may someday change late diagnosis.
A smell print of the breath is made a computer program matches that smell print with molecules in the breath associated with lung cancer. This allowed doctors to predict who had lung cancer and who did not.
Watch the video here:
Crime Prevention and Security
- The ability to detect odourless chemicals makes it ideal for use in the police force, such as the ability to detect drug odours despite other airborne odours capable of confusing police dogs.
- It may also be used as a bomb detection method in airports.
- For identification of volatile organic compounds in air, water and soil samples.
- For environmental protection.
Building an Electronic-Nose
Watch the video to find out how the e-nose works http://www.sciencefriday.com/videos/watch/10128