The fluorescence of the microsensors is switched off when they encounter nitro explosive molecules, such as the trinitrophenol molecules represented here. Copyright Elsevier
The fluorescence of the microsensors is switched off when they encounter nitro explosive molecules, such as the trinitrophenol molecules represented here. Copyright Elsevier

Self-propelled fluorescent and magnetic microsensors can detect nitrogen-containing explosives. The sensors offer wider possibilities in environmental monitoring applications, in addition to the more obvious security and forensics uses.

“Our system can detect trace amounts of nitro explosives within a few minutes,” says Hong Wang of the research group based at China University of Mining and Technology. The development and testing of the technology is reported in the journal Applied Materials Today.

The sensors are based on a structure known as a covalent organic framework (COF). This consists of chains of carbon-based (organic) molecules linked together into a highly porous crystalline arrangement.

The COFs are naturally fluorescent, but this fluorescence is turned off by chemical interactions that occur when they meet up with nitro explosive molecules. That forms the basis of the explosive-sensing abilities, but there were other challenges to overcome.

A major problem is that the COFs are hydrophobic – literally water-hating – structures, yet the explosives the researchers want to detect are commonly found in water-based “aqueous” solutions. To force the COF microspheres to move through aqueous solutions, the researchers created a built-in micromotor composed of a supply of hydrogen peroxide and a catalyst – manganese dioxide – that can steadily degrade the hydrogen peroxide into water and oxygen. The resulting stream of oxygen bubbles directed out of the microspheres becomes a source of propulsion.

Finally, a guidance system was required. This came in the form of magnetic iron oxide nanoparticles built into the microspheres. An external magnet could then be used to guide the microspheres to move in any desired direction, powered by their stream of bubbles.

Each microsystem is encased in a sphere of biodegradable polymer, creating the self-propelled sensors that could be guided to mix thoroughly with aqueous solutions, seeking out nitro explosive molecules and indicating their presence by the quenching of their fluorescence.

The creative system proved exceptionally effective in tests, detecting even trace quantities of nitro-explosives in minutes.

“I have been focusing on the development of micro- and nanomotors since 2013,” says Wang, explaining that this latest success follows on from a long trail of earlier research.

The next steps planned by Wang and her colleagues are to work on properly quantifying the response, to establish a precise indication of the level of nitro explosives in samples. And she emphasises that the environmental monitoring opportunities could be at least as significant as applications in security and forensics testing. Some nitro explosive molecules are commonly used in fireworks, and also for non-explosive applications in the dye, fabric and textile industries.

“This unavoidably leads to release of these well-established toxic pollutants into the environment,” says Wang. She points out that they have been linked with severe respiratory disorders, skin and eye irritation, liver and kidney damage, and potentially cancer-causing mutagenic effects.

The basic system demonstrated in this research paper might also be adapted to seek out and report on the presence of many other compounds of interest.


Article details:

Wang, H. et al: “Fluorescent self-propelled covalent organic framework as a microsensor for nitro explosive detection,” Applied Materials Today (2020)

Click here to read the article in the journal.