We're not talking about a solution of viruses, this is measuring the electrochemistry of one individual virus particle, a virion. I'm not going to go into the details of the method, otherwise this post would be pages long, but the gist is that you put a virus onto a film of gold, locate the virus with AFM, then use the AFM tip and the gold film as electrodes and run AC current through it. You can get a unique signature by looking at the capacitance versus frequency. The results for five viruses: AV5, SV40, MVA, CPMV, and HSV1 are shown below.
You can see that each virus is distinguishable from one another by both the pattern of the line, as well as the magnitude of the capacitance; there's overlap in a couple of places, but you can still distinguish them from each other. The paper stated that the unique identification of these viruses is made possible through the difference in protein compositions, which have different dielectric constants.
They left out quite a few important aspects in this paper. I talked to a virologist to learn more about viruses, which showed me many things they omitted. It makes sense since their flaws lie mostly on the biology side, because they really only spent a couple sentences talking about viruses themselves.
Firstly, they make little to no mention of the viruses size. They state that they range from 30-300 nm and give their sizes, but don't discuss any possible contribution. If you look at the equation for a parallel plate capacitor, you see that the distance between the electrodes plays a role. We can't use that equation here, but the concept still remains.
Secondly, even less mention of the viruses shape is included. Four of the five viruses they studied are icosahedrally shaped, so they can be treated roughly as spheres, but one is more complex than that. That complex virus is MVA, and an image of it is shown above. Additionally, they don't mention that there is an entire helical class of viruses which could be studied. Based on their orientation on the film, would they give different results, since there would be a different distance between the electrodes? Would this cancel out since they're measuring more than one virus and averaging?
Lastly, not all viruses are stable outside of a host cell, so this can't measure all viruses, which is a pretty big limitation. There's a chance that they could measure an infected cell rather than a virus particle in order to identify the virus, but that seems unlikely given the large 'background noise' the cell would provide, and it would be hard to factor in all of the different stages of infection.
Overall, the paper is good - it's a pretty novel idea, but it definitely needs more work. I could go on a lot more about this, since I gave my 25 minute long Graduate Student Literature Seminar on this paper, but I won't.
MacCuspie, R., Nuraje, N., Lee, S., Runge, A., Matsui, H. (2008). Comparison of Electrical Properties of Viruses Studied by AC Capacitance Scanning Probe Microscopy. Journal of the American Chemical Society, 130(3), 887-891. DOI: 10.1021/ja075244z
Godspeed.
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