Nanoparticles, by definition, are really small. So small, in fact, that they are essentially impossible to see. How do you see something that isn’t visible without an extremely powerful microscope (like an electron microscope)?
There are many methods that allow you to “see” nanoparticles. Actually, a more adequate description would be “detecting” nanoparticles since most of the methods used don’t actually take an image of them.
One of the methods for detecting nanoparticles is called atomic force microscopy, or AFM. This is a really cool method whereby a tiny lever arm with a protruding point at the end, called a cantilever, is either dragged right over a sample, taps the sample, or is oscillated near it’s resonance frequency just above the sample. As you can imagine, dragging the tip right over a sample will result in the cantilever moving up and down, giving you a topographic “map” of he material. The oscillation technique takes advantage of the fact that the resonance frequency and other measurable properties from the ocillating cantilever will change due to interactions with the sample. These changes in measurements can be back tracked to get a map of the sample.
In AFM the point on the cantilever is extremely small, the tip being below the nanoscale itself. This technique is great for finding if nanoparticles are present and their basic structure, but it is an invasive technique. The sample is essentially destroyed after measurement; so, are any better methods available?
Dynamic Light Scattering, or DLS, is a noninvasive method for characterizing nanoparticles. This method involves shooting a laser beam into a liquid sample and measuring light scatter. By correlating the scatter through Brownian motion and statistical analyses, the size of particles can be determined. After measurement, the sample can be removed and still be in it’s original and unaltered state.
What are the downsides to DLS? Since it’s partially a statistical method, it’s not entirely based on physical phenomena, and therefore gives you the probable sizes of nanoparticles in a sample. Therefore, some nanoparticles may be completely looked over. For example, say a solution contains five times as many 600 nm diameter particles as 50 nm diameter particles. There’s a good chance that the algorithm would never report the presence of the 50 nm particles. Also, this method only gives you a distribution of particle sizes (essentially a graph of intensity, an arbitrary % value, as a function of particle diameter) and provides no picture or map of the particle structure.
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