The last thing you would expect to find among droplets in the air are minuscule sensors capturing and storing information about their environment. Writing in Nature Nanotechnology, Volodymyr Koman and colleagues report polymer particles with a network of 2D materials grafted onto their surfaces, which can be dispersed as a fluid and act as airborne sensors. The sub-millimetre particles are so light that they can be sprayed and can travel through the air, reaching places inaccessible to traditional electronic sensors.
Koman and colleagues assemble electronic circuits comprising 2D materials on the surface of the polymer using top-down fabrication methods. The sensing component in the circuit — a chemiresistor — is made from a monolayer of MoS2 and the information is stored by a memristor formed from MoS2 flakes sandwiched between gold and silver electrodes. The chemiresistor and memristor are powered by a photodiode in the form of a p–n heterojunction of MoS2 and WSe2 monolayers. The components in the electronic circuit are stacked sequentially on the polymer substrate and connected using gold and silver contacts. The polymer and electronic circuits are then dispersed in a water-based solution and sprayed using a nebulizer.
Conventional electronics often cannot withstand harsh environments because of problems with mechanical and chemical stability. In addition, electronics usually require voltages of several volts and milliwatts of power and such energy sources are too large to be used with sub-millimetre particles. “To bypass stability and power limitations, we used 2D materials to make devices that survive aerosolization and function using sub-volt voltages,” says Koman. The lightweight circuits are 1.24 μm thick, with an areal density of ~1.4 g m−2 and only require 30 nW to operate. Although the mechanical stability of 2D materials is generally low, the use of a polymeric substrate endows the materials with higher mechanical stability and thus enables their use as airborne sensors.
“To bypass stability and power limitations, we used 2D materials to make devices that survive aerosolization and function using sub-volt voltages”
When sprayed in the air, the electronic circuits can sense ammonia gas or soot nanoparticles in the environment. More specifically, the photodiode converts light into current, which turns on the internal state of the memristor only if the chemiresistor detects the analyte. Detection occurs because the gas or soot particles change the conductance of MoS2 in the chemiresistor. In this way, the information about the presence of the analyte is sensed and stored in the memristor until the sensors are retrieved and read out.
In the present sensing system, the information can be stored in the memristor for two hours, which is “sufficient for certain applications, like sensing leaks and currents in industrial pipelines, mapping synthetic reactors and in geological explorations,” says Koman. However, for other applications, such as extended transport pipelines, the information would need to be stored for days, or even weeks.
In the future, these droplet-sized sensors may be used in biosensors and large-area sensors for bacteria, spores, smoke particles or dust components. The aerosol-based delivery of the sensors may also make them attractive for use in confined or harsh environments, for example, in oil and gas conduits and even in space. “We are expanding the library of low-power electrical components, so that the circuits can be used for more complex tasks, be cognitive of their environment, interact with each other and send out the collected information wirelessly,” says Koman.
Koman V. B. et al. Colloidal, nanoelectronic state machines based on 2D materials for aerosolizable electronics. Nat. Nanotechnol. https://doi.org/10.1038/s41565-018-0194-z (2018)