Airborne Microchips Detect Chemicals

By Mike Howie

As our technological know-how advances, new devices simultaneously shrink in size and grow in power. We see this in the technology we use every day: laptops are continuously getting smaller and lighter, smartphones can easily accomplish tasks that once required a computer, and the latest smartwatches are packed with GPS, cellular connectivity and voice assistants. The simpler a device is, the smaller it can be. But many consumers may not realize how truly microscopic electronics are becoming.

Case in point: researchers at MIT have recently created sensors small enough to be sprayed in an aerosol mist. Detailed in a paper titled “Colloidal nanoelectronic state machines based on 2D materials for aerosolizable electronics” published in Nature Nanotechnology, the new chips are about as wide as a human hair — one micrometer thick and 100 micrometers across — and made by grafting 2D electronic materials onto colloidal particles. This results in tiny, functional electronic circuits, transistors, memory and sensors dubbed colloidal state machines (CSMs) that are capable of autonomously collecting, manipulating and storing information.

How do they work?

Each CSM has three parts: a photodiode that detects light to power the device, a chemiresistor that boosts conductance after binding with a specific analyte, and a memristor that turns on when the current surpasses a given threshold.

In use, the CSMs would be sprayed into an environment and given time to bind with the analyte of choice. They would then be collected and examined to see how many are “on” (meaning they detected the analyte) and how many remained “off” (meaning they did not detect the analyte). Using that information, the researcher could determine how much of the analyte is present in the tested environment.

While the researchers currently need to collect the CSMs and connect them to electrodes to determine whether or not they came in contact with certain particles, this method may not always be necessary. Future CSMs could illuminate when bound to target particles, and the team is researching ways to power the chips without ambient light and how to detect multiple analytes from a single chip.

Where could they be applied?

Most obviously, CSMs could be useful in confined environments that are too small for traditional probes, including everything from oil and gas conduits and chemical and biosynthetic reactors to the human digestive tract. To prove this, the researchers created a model pipeline, injected the chips, then filled the pipeline with gaseous ammonia — a highly toxic gas used in fertilizers and refrigerants, and one of the most dangerous compounds to transport through pipelines. After 30 minutes, the researchers shut off the ammonia and successfully retrieved and examined the CSMs.

Medical applications have yet to be tested, but the researchers predict that CSMs could be injected into a patient’s bloodstream to monitor chemical composition without needing to draw any blood, or taken as a pill or nasal spray to track digestive health.

The CSMs could also be used for environmental monitoring, detecting soot, bacteria, spores, volatile organic compounds and other particles over large areas. Soot, for example, is emitted from power plants and diesel engines, can travel upwards of 1,000 miles before settling and poses health, climate and environmental risks. It’s challenging to predict the distribution and impact of soot, and monitoring it over a wide area by traditional means is not economically viable.

Environmental monitoring is commonly conducted with satellites or unmanned aerial drones, which are costly and indirect, as well as sensors on the ground, which are labor intensive and slow to deploy. These aerosolizable electronics, on the other hand, can easily be dispersed as autonomously powered devices, reducing costs and labor and offering more rapid results.

How do you find them?

Collecting CSMs from a confined system like the model pipeline is relatively simple: the researchers simply stretched a piece of cheesecloth over the end of the pipe to collect the chips. But collecting CSMs dispersed over a large area would be much more difficult. To solve this issue, the researchers created a batch of chips designed as retroreflectors. These chips reflect low-intensity lasers back to the source and work from up to one kilometer away, even when the light hits them at a steep angle. With a custom laser-scanning system, the researchers can quickly detect the CSMs.

Though the chips are small and simple, they are mighty. They provide unprecedented access to confined environments and gather reliable data, making it easy for researchers, technicians and medical professionals across myriad industries to make more informed decisions in their critical work.