It works like this. While the cancer cells take their sweet time to slowly kill you, they also manage to take in the sights and enjoy a meal or two, consuming and producing a rather specific set of volatile organic compounds, or VOCs (read: lung cancer BO). This chemical trace of lung cancer activity gets carried out of the body each time you exhale, somewhat akin to how Pepe le Pew’s pungent perfume wafts lazily behind him.
Now, it’s not enough to just have a smell. It has to be distinctive. Otherwise there’s no way to discriminate between the smell of cancer and that of the forgotten fish in the fridge. Enter scientists. The team identified 33 VOCs (over the mere 22 reported in earlier studies) common to healthy and lung cancer breath, but which are present in different concentrations and in different mixture compositions in lung cancer breath than in healthy breath. Not only that, but they also identified 9 uncommon VOCs which are present in the majority of lung cancer breath samples, but not in the majority of healthy breath samples. Thus, the lung cancer cells effuse a veritable olfactory fingerprint revealing their presence.Knowing what to look for is one thing. Actually finding it is another. Especially when what we’re looking for appear at levels in the parts per billion. Enter scientists again, this time with the real kicker: their design for the holy grail of noses, an electronic system which uses nanoparticles and the principle of chemiresistance to discriminate between breath from healthy people and breath from cancer patients.
Why use nanoparticles? Nanoparticles are, well, small. This makes them easy to integrate into many systems, great to use in portable device application, and perfect to functionalize (owing to their nice big surface to volume ratios). They are also relatively cheap and can be efficiently mass-produced. And if all that weren’t enough, here’s yet another reason why nano is the new black: materials at the nanoscale behave differently than they do at either the macro or atomic scale. Macroscopic materials’ properties don’t change, and atomic scale materials do all sorts of strange things not all yet understood. But at the nanoscale, material properties and behavior become very dependent on size and shape. Since we have the ability to precisely control the size and shape of nanomaterials, we have the ability to control all sorts of their chemical and physical properties. Nanoparticles, as it were, are the Goldilocks of materials.
In their design, the researchers exploited one controllable feature of nanoparticles in particular. That is, their conductivity. In macroscopic conductive materials, electrons flow easily from one point to another, yielding current. In a nanoparticle there obviously isn’t much room for electrons to run around. An isolated particle, then, is somewhat less than conductive. But, when many particles get close enough together, as if by magic, they begin to conduct! At this scale, electrons can basically hop from one nanoparticle to the next. In short, the material is conductive even when discontinuous, but its conductivity is inversely related to the size of discontinuity.
We’re almost there. I said that the researchers used nanoparticles as chemiresistors. Chemiresistors change their resistance in the presence of different chemical species. Since resistance is inversely related to conductivity, it should be directly related to the size of discontinuity e.g. interparticle distance. Therein lies the key. As different chemicals adsorb onto nanoparticle covered films, the films begin to bloat, and the nanoparticles begin to distance. If the nanoparticles were kids at a middle school dance, the adsorbing chemicals would be the chaperones. An increase in resistance is measured and the chemical species are detected! Now, I would be remiss if I didn’t mention that some adsorbed species actually decrease the resistance. In this case, the distancing effect is small compared to dielectric effects. Regardless, the point is the same: adsorbates change resistance.
So there it is. The researchers made a device consisting of an array of nine cross-reactive chemiresistors based on assemblies of 5 nm gold nanoparticles. The nanoparticle assemblies differed in what molecules covered, or functionalized, the gold nanoparticles, making each set more or less attractive to different chemical species. When the people blew into the devices, the VOCs in their breath were detected and if a combination of VOCs matching a lung cancer fingerprint was found, the patient was diagnosed. Detection made easy!This kind of technology is not limited to cancer detection. It can be used to detect all sorts of different chemicals. My friends already love blowing into breathalyzers to prove how drunk (read: baller) they are. Won’t they be thrilled when this kind of sensing technology is used to create supersensitive breathalyzers.