Research conducted at Tufts’ University predicts cooled-down plasma could be a safer way of sensing dangerous airborne nanoparticles.
Due to their small size, nanoparticles – present in a wide range of everyday objects ranging from vehicle exhaust to makeup and food packaging and even to clothing – can easily pass into the human body (specifically, through cell membranes) and can thus cause a variety of health issues. Because of this, scientists are invested in creating better and more reliable methods of nanoparticle detection that can safely catch the smallest and most dangerous of nanoparticles, which often go missed by current detection technology.
“Air quality reports tend to look at bigger particles like diesel soot or other things you can see,” Jeff Hopwood, engineering professor at Tufts and leader of the new research, said in a recent Tufts press release. “But detecting really small particles is much harder.”
However, existing nanoparticle detectors not only lack adequate sensitivity, but also adequate safety measures, as they use dangerous radioactive material to accomplish their tasks. In order to detect nanoparticles, the detectors emit radioactive fields that force nanoparticles to take on negative charges as they pass through. This allows for an electrical sensor to detect their presence.
But, as Hopwood points out, “The problem with this method is that the radioactive source is itself dangerous, and so it’s not a really practical method of looking for nanoparticles.”
With these concerns in mind, instead of using radioactive material, Hopwood suggests using a completely different, and arguably safer, substance – plasma.
Constituted by highly-energized free-moving electrons and ionized atoms, plasma – the fourth state of matter – readily conducts electricity, imparting an electrical field onto any object that enters it. This includes objects as small and elusive as nanoparticles. In this way, plasma could be extremely valuable in a nanoparticle detector.
However, Hopwood acknowledges there are obvious challenges to his ideas. Most notably, plasma, when occurring in the atmosphere, is notoriously difficult to manipulate and control. When heated, plasma’s free-moving electrons interact violently with gas molecules around them, rocketing air temperatures to thousands of degrees.
“At atmospheric pressure, you usually get a very hot, potentially destructive plasma,” Hopwood said. “Lightning, electrical sparks—those are all examples of hot plasmas.”
However, Hopwood suggests that there are ways to circumvent this problem – that is, by cooling plasma down and making it easier to manipulate.
In such mundane, everyday applications as neon lights and plasma TVs, plasma is cooled by being propagated within a vacuum chamber, removing air molecules that might generate dangerous high temperatures.
However, Hopwood wants to move beyond that – to cool plasma at standard atmospheric conditions. “If we can get plasma out of a glass or metal enclosure and keep it from heating up, that’s when we could do some really interesting stuff,” Hopwood said.
According to Hopwood, if cooled-down plasma could exist outside of a separate container, it could be used for a variety of important functions. Not only could it help detect nanoparticles, it could also help disinfect hospital surfaces by killing microbes that pass through its potent electrical field. Additionally, cooled plasma could potentially aid in lowering pollution, by destroying dangerous compounds such as toluene and benzene; furthermore, it could also help detect hazardous gases leaked at labs or industrial sites.
“You could essentially make something like a tricorder—that handheld sensor they use all the time on Star Trek,” Hopwood suggested. “It could tell you at a glance, ‘Is the air here safe to breathe?'”
However, in order to generate plasma cool enough to manipulate outside of a separate enclosure, scientists are restricted by size – namely, the plasma must be extremely small in order to be kept at such a low temperature under standard atmospheric conditions.
To accommodate this size restriction, Hopwood has worked in his lab to create what he calls “microplasmas” – miniscule balls of plasma that are less than a millimeter wide.
But even at a small size, plasma – such as a tiny “spark” caused by a circuit-board – can destroy sensitive electronics with its heat. In order to work against this, Hopwood is attempting to figure out how to “freeze” that spark at the instant of its inception. If done correctly, this process could produce a tiny, cool plasma.
“The spark naturally wants to become hot,” Hopwood explained. “Its natural progression is to go to a lightning-like hot plasma state. To create a cool plasma, you have to engineer something that says ‘stop, don’t do that.’ We’re basically creating cold lightning, in a sense.”
Hopwood has already begun to create “cold lightning” in his lab, using circuit boards configured with chips (previously used to power cell phone antennas) that stream microwave energy into an attached copper ring at a certain frequency. If it is the correct size, the copper ring will resonate like a tuning fork, causing electrical energy to blast across the gap, producing a tiny spark. The spark modifies the circuit’s electrical properties, reducing the spark’s potentially destructive power and tamping its temperature down. Repeated several billion times per second, the process will produce a cooled-down plasma in place of the hot spark.
“The trick is to add enough power to generate the early moments of a spark, but pull the power back before it gets too hot,” Hopwood said. “By repeating that process indefinitely, we effectively stop the spark in its tracks and create a tiny, cool plasma. You just have to find the right rhythm, or right frequency.”
However, Hopwood clarifies that his research is still very much unfinished; his “cold lightning” is still not yet ready to be used in nanoparticle detection. Right now, he and his colleagues are focused on improving their circuit board technology and developing even better microplasmas.
Of plasma, Hopwood asserts that, “The physics are really, really complicated. But I’ve always been driven by deep physical understanding. Okay, so you can do something—but how does it work? What’s the overlying principle? I guess in that way I’ve always kind of gone down the rabbit hole.”
-Melanie Abeygunawardana
