One of the key ingredients to every scientific discipline is the ability to detect stuff. This should be an obvious statement, but it's amazing how many scientific advances have come about not because people were searching for anything in particular, but because a new instrument allowed them to see further, see smaller things, or detect smaller amounts. So I am always excited to see new sensor developments, even if they only have industrial applications or ultimately come to nothing.
Being an optics guy, I think the best way to detect something is optically. Combine this with the joys of plasmonics, and it becomes a little difficult to distract me. Add to that the joys of something called impedance matching, and I am in my own little version of heaven. This is exactly what a team of researchers have done, using it to create a hydrogen sensor.
Why hydrogen? It's dangerous. Hydrogen is the smallest of molecules, and it will find the tiniest of holes in any reaction chamber, making the risk of leaks a constant. To make matters worse, a hydrogen-oxygen atmosphere becomes explosive with just four percent of hydrogen present and remains explosive until you get past 77 percent hydrogen. To be safe, you need to detect hydrogen levels well below four percent.
This may sound a bit esoteric, but hydrogen is used in a lot of industrial processes. Not to mention that, to power hydrogen fuel cells—one of the proposed replacements for gasoline and diesel—you will have a tank full of a highly volatile and explosive gas in the back of your car. I think a sensitive, reliable, and inexpensive leak detector might be a valuable addition to such a vehicle, don't you?
Doing it with optics
So we know why we want to detect hydrogen. And doing it without devices that might initiate an explosion seems like a good idea, which is why an optical approach is appealing. However, hydrogen is quite difficult to detect optically. Most molecules can be detected through absorption measurements: if you illuminate the gas with the right color of light, some will be absorbed, and this can be detected. Sensitivities in the parts per trillion range have been demonstrated.
Unfortunately, because of the symmetry of the hydrogen molecule, this doesn't work. Quite simply, the rules of quantum mechanics preclude direct absorption measurement.
With direct detection out, the researchers turned to an indirect method: the effect of hydrogen on plasmons, light fields that propagate across the surface of a metal. However, the normal approach to plasmonics doesn't work with hydrogen. You see, for biology applications—detecting comparatively huge DNA and protein molecules—the presence of the molecules changes the refractive index of the material at a metallic surface. This change in refractive index changes the efficiency with which plasmons are generated on those surfaces, and this can be detected by looking for changes in the reflectivity of the surface (if you generate a plasmon, less light is reflected).
But hydrogen is tiny and has basically no effect on the refractive index of the material at its surface. The researchers turned, instead, to the metallic substrate in which the plasmon is generated. They noted that when hydrogen encounters palladium, it creates a chemical bond. Indeed, it becomes incorporated into the crystalline structure of the palladium, expanding the spacing between palladium atoms. Surely, these dramatic changes must effect the propagation of the plasmon?
This turns out to be a difficult question to answer, because palladium, by itself, is not very good at supporting plasmons anyway. Even when you choose everything optimally, most incident light is reflected, and only a small fraction is turned into a plasmon. In terms of a sensor, this means you are detecting small changes on top of a large background signal—not desirable.
This is where the power of looking at light propagation through the lens of circuit design really pays off. By using the concept of impedance matching (see side bar), the researchers could ensure that, under normal circumstances, light would be absorbed to generate plasmons, while in the presence of hydrogen it would not.
Although impedance matching has been the domain of electrical engineering, it is slowly becoming an explicit tool in optics—it has always been implicitly used. In this case, the researchers placed a series of palladium wires on top of a gold substrate with a glass spacer in between. The structure and spacing of the wires make it very easy to couple light with a certain color into a surface plasmon on the wires.
The free electrons floating around in the gold feel these plasmons and start oscillating as well. These two oscillations are exactly out of phase with each other. If the two plasmons were to radiate, their fields would be out of phase—meaning destructive interference and no light—so, instead, all of that energy is dissipated as heat within the metals.
Ideally, this implies that there is no light reflected. But in physics, as in life, nothing is quite perfect, and the researchers get a minimum of between 0.5 and 0.1 percent reflectance, depending on the width and spacing of the wires (which also means that the color of light required changes as well). Once hydrogen is added, the optical properties of the palladium start to change, and the nearly perfect balance between the two plasmons is disrupted. As a result, light begins to radiate from the structure more efficiently.
The researchers showed that they could easily detect 0.5 percent hydrogen with what appears to be a lot of room to spare. Interestingly, they did not find out how low a percentage of hydrogen they could detect, which may be due to their gas control system. Nevertheless, 0.5 percent is a good start. And certainly four percent is very easy to sense with this system.
It also has the potential to be relatively cheap. It doesn't take much gold or palladium, while the light source could be as simple as a light emitting diode.
Nano Letters, 2011, DOI: 10.1021/nl202489g
Listing image by Photograph by The College of Optics and Photonics
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