How Chemical Sensing Devices Work

By: Meisa Salaita

smoke and carbon monoxide detector
A Nest smoke and carbon monoxide detector is seen on Jan. 16, 2014, in Provo, Utah. George Frey/Getty Images

Breathing in and out is something our bodies do without our having to think about it. In fact, we rarely even give a second thought to the air surrounding us until the quality is low somehow – maybe from the smoke of a nearby fire or smog enveloping our crowded cities.

But there can be dangerous gases that are not always visible. Carbon monoxide and radon are two examples of deadly air contaminants that are completely invisible. We don't realize we're being poisoned from the air we breathe until it is too late. Luckily, these gases aren't that prevalent, and with the help of a little chemical sensing technology, we can let carbon monoxide and radon detectors do the worrying for us.

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Chemical sensors are helpful far beyond just detecting deadly gases. These devices can be found in our homes, hospitals and in the military. There are many different types of sensors that detect different target molecules (also known as analytes). Although the sensors work in various ways, the gist is that a chemical interaction happens between the analyte and something in the sensor, and the device produces a measurable signal – a beeping or a color change to alert us to the presence of the target molecule.

Despite the differences in the construction of sensors, there are a few guiding principles that make any sensor good. The ideal one is inexpensive, foolproof and portable. Most importantly, any chemical sensor has two vital features: selectivity and sensitivity. There are more than 10 billion molecular substances in the world, so selectively detecting a single substance is no small feat [source: National Research Council]. Sensitivity is also incredibly important for detecting chemicals from a considerable distance or for trying to find very low concentrations of a target molecule. Other important sensor features are response time, packaging size and limit of detection — the lowest quantity of a substance that can be detected.

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So Sensitive! How Sensors Detect Target Molecules

carbon monoxide detector
A carbon monoxide detector can keep you from being poisoned by gas fumes. BanksPhotos/Getty Images

Scientists and engineers have developed a variety of sensors for different purposes, and as you can imagine, they all have their own ways of working. After all, a pregnancy test kit is not likely to have the same detection mechanism as a radon detector, right?

All chemical sensors target some sort of analyte, but what happens once the analyte is in the sensor is where the differences emerge. For example, the sensor can bind the analyte (think a lock-and-key type mechanism, but on the molecular level). Or, the sensor may be set up so that the analyte selectively passes through a thin film. Imagine the film being a chemical gatekeeper that only lets the target molecule through and stops everything else from going in. This type of sensor has the positive feature of being continually reusable. A third form of sensor uses up the analyte in a chemical reaction that generates a product that creates the readable signal [source: National Research Council]. These three very broad mechanisms cover the workings of most sensors, but there are still other types.

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For example, there are direct-read electrochemical sensors that use the diffusion of charged molecules to look for changes in current, conductivity or potential to see if a target analyte is present. Surface acoustic wave sensors employ acoustic waves sent from one electrode to another across a surface. The sensor is designed so that if the speed of the wave changes or if it loses intensity, it signals the presence of a target molecule bound to the surface. By taking measurements of these changes, the sensor may even be able to detect quantities of the material present [source: National Research Council].

Another cool innovation in chemical sensing technology moves toward detecting inherent properties of different chemical targets instead of using a molecular interaction to drive the detection. Different bonds in molecules each have signature vibration patterns that can be detected in the infrared region of the electromagnetic spectrum. By combining light sources, filters and detectors onto a single chip, scientists at Massachusetts Institute of Technology have been able to detect these molecular fingerprints in order to sense a whole host of molecules, from contaminants in water to electrolytes in the blood of newborn babies [source: Bender].

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How Chemical Sensors Help

No matter how they operate, chemical sensing devices are, without a doubt, working for you. Your home probably has at least a one detector for radon gas, smoke or carbon monoxide, depending on the laws in your state or country. Many radon sensors work by absorbing the radon itself or detecting the radioactive decay products of the lethal gas. Carbon monoxide, on the other hand, is not a radioactive material, so the detectors for this gas operate differently. One of the most common mechanisms for this chemical sensor is a riff on biology. These detectors mimic how carbon monoxide interacts with hemoglobin in blood in order to determine the presence of the gas. Another common detector in the home is a smoke detector. While some use radioactive materials to help sniff out smoke, most of the sensing in smoke detectors comes from the physical, not chemical, phenomenon of the smoke particles causing interference that is sensed by the detector.

Chemical sensing devices also have widespread use outside the home. One of the main places you'll see these devices in action is in search of biomolecules in medical settings. Biomolecule sensors are essentially specialized chemical sensors. Although they detect substances like hormones, these bodily substances are all molecules. After all, these sensors are made with many of the same guiding principles as other chemical sensors – selectivity, sensitivity and portability.

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Some of the most portable biomolecule sensors you may be aware of are associated with fertility measurements: pregnancy tests and ovulation tests. Both these chemical sensors detect the presence of certain hormones in urine. In the case of pregnancy tests, the sensor looks for the hormone human chorionic gonadotropin (hCG) in urine. The stick on which the woman urinates has antibodies that are coated with a chemical that bonds to hCG. If the biomolecule is present, the test reads positive [source: Parents Magazine]. Usually these chemical sensors have a colorimetric component so that when the analyte – in this case hCG – binds, it triggers a color change in the sensor, making the readout of the results pretty foolproof.

In the clinical setting, two of the most common methods for chemical-based biomolecule detection are ELISA (enzyme-linked immunoabsorbent assay) and the Western blot. Depending on the size and type of the biomolecule in question and the information they want about the molecule, scientists and clinicians will often turn to one of these chemical sensing techniques to identify different analytes in mixtures of biomolecules [sources: ThermoFisher Scientific, Mahmood and Yang].

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Sci-fi Detection Becoming a Reality

A glass head holds a neural interface sensor developed by DARPA, which reads signals from the brain and stimulates neurons to combat memory loss. DARPA is investing in chemical sensing research. Stacey Rupolo/Chicago Tribune/TNS via Getty Images
A glass head holds a neural interface sensor developed by DARPA, which reads signals from the brain and stimulates neurons to combat memory loss. DARPA is investing in chemical sensing research. Stacey Rupolo/Chicago Tribune/TNS via Getty Images

A big driver for the future directions of chemical sensing is the military. Funding agencies like the U.S. Defense Advanced Research Projects Agency (DARPA) have big dreams for what chemical sensing might be able to do for its armed forces. Imagine being able to put a sensor on the top of a tent in a war zone that can detect explosives or toxic gases 4 miles (6 kilometers) away.

Or picture a sensor that is so tiny, it can be embedded into fabrics. When the sensor detects a toxic agent, the color of the fabric changes, alerting soldiers to the presence of chemical toxins in the air. What about a sensor that could to alert soldiers to their dehydration levels? The implications for such technology would not only be the upper hand in battle, but also save potentially thousands of lives.

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Chemicals also could be placed in airports to help stop terrorism by detecting minute concentrations of explosives without us having to undergo the extensive hassles of the security lines. In fact, scientists have already unveiled a sensor that can rapidly detect drugs and explosives close to 100 feet (30 meters) away with airports in mind [source: Engineering360]. These sensors could be installed at airport entrances and other places with large groups of people. Some types of chemical sensing devices can even be attached to cell phones to scan larger areas as law enforcement officials move around [source: Hsu].

Of course, chemical sensors don't only need to be used to divert nefarious activities. Detecting metals contaminants in water and possibly toxic chemicals in the air via sensors in security badges are just some of the ways that chemical sensing devices are changing our world.

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Lots More Information

Author's Note: How Chemical Sensing Devices Work

Nanoparticle sensors sewn into fabric that could tell me more about myself or the air around me? This is seriously the stuff of sci-fi. Except it's not. Maybe the military is the one pushing the research forward on things like this, but soon after soldiers start using it, it'll be commonplace for the rest of us. Gives wearable technology a whole new meaning.

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More Great Links

  • Bender, Eric. "Chemical sensing on a chip." MIT News. April 8, 2016. (Nov. 2, 2016) http://news.mit.edu/2016/chemical-sensing-chip-anuradha-agarwal-0408
  • Black, Angela. "How Carbon Monoxide Detectors Work." HowStuffWorks.com. Dec. 30, 2008. (Nov. 2, 2016) https://home.howstuffworks.com/home-improvement/household-safety/tips/carbon-monoxide-detector.htm
  • Brain, Marshall. "How Smoke Detectors Work." HowStuffWorks.com. April 1, 2000. (Nov. 2, 2016) https://home.howstuffworks.com/home-improvement/household-safety/fire/smoke.htm
  • Brain, Marshall and Freudenrich, Craig. "How Radon Works." HowStuffWorks.com. Oct. 4, 2000. (Nov. 2, 2016) https://home.howstuffworks.com/home-improvement/household-safety/tips/radon.htm
  • Engineering360. "High-Speed Chemical Sensor Could Aid Airport Security." Aug. 29, 2016. (Nov. 4, 2016) http://insights.globalspec.com/article/3146/high-speed-chemical-sensor-could-aid-airport-security
  • Hsu, Jeremy. "Smartphones Could Form Chemical Detection Networks." LiveScience. Nov. 27, 2009. (Nov. 4, 2016) http://www.livescience.com/5915-smartphones-form-chemical-detection-networks.html
  • Mahmood, Tahrin and Yang, Ping-Chang. "Western Blot: Technique, Theory, and Trouble Shooting." North American Journal of Medical Sciences. Vol. 4. No. 9. 429-434. Sept. 2012. (Nov. 4, 2016) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3456489/
  • Millstone, Jill. associate professor of chemistry, University of Pittsburgh. Personal Interview. Oct. 25, 2016.
  • National Research Council. "Expanding the Vision of Sensor Materials." The National Academies Press. 1995. https://www.nap.edu/read/4782/chapter/1
  • Parents Magazine. "How do pregnancy tests work?" 2009. (Nov. 3, 2016) http://www.parents.com/advice/pregnancy-birth/getting-pregnant/how-do-pregnancy-tests-work/
  • ThermoFisher Scientific. "Overview of ELISA." (Nov. 4, 2016) https://www.thermofisher.com/us/en/home/life-science/protein-biology/protein-biology-learning-center/protein-biology-resource-library/pierce-protein-methods/overview-elisa.html