The Power of Modern Microscopes

Today’s entry is a little different than ones I have entered before – it’s not so much about a discovery as it is about a technology. This technology, which has advanced rapidly over the years I have been in science, is the microscope. I use microscopes in my research, as do most life scientists. Most of the ones I use are relatively low on the technology scale. But there is a vast range of microscopes available, for everything from low resolution light microscopy to extremely sensitive atomic resolution electron microscopy, and they are making it possible for us to look more closely at the inner workings of tissues, cells, and molecules than we could ever have dreamed possible.

Conventional compound light microscopes use a system of glass lenses to bend light and magnify an image. The magnification ability of a light microscope is limited, therefore, by both the size and shape of glass lenses and the physical properties of visible light. Standard compound microscopes usually have a magnification range of 40 to 1000-fold. However, where compound microscopes typically run into problems is with resolution. Resolution is the ability to distinguish 2 very small and closely spaced entities as being distinct. If you blow up a standard digital picture from 4x6 to something much larger, like poster size, you will get a very magnified, fuzzy picture. That’s because you have poor resolution – and no matter how much more you blow it up, you can’t make it any clearer. With microscopes, the story is the same - even if you have enormous magnifications, poor resolution just means you have a bigger picture of something fuzzy.

What if you want to look at something more closely than a light microscope will allow? One way is to use a Scanning Electron Microscope. Scanning Electron Microscopes (SEMs, for short), have a magnification ability up to 200,000-fold. The resolution limit of SEMs is very good; some of them can resolve objects down to 5 nanometers. (That’s 5 x 10-7 centimeters, roughly a quarter of the size of the thinnest known spider web!) It achieves this resolution because it does not use visible light to magnify the image; instead, it uses electrons. The sample to be examined must be able to conduct electricity; to do that, it is coated with a conductive material like gold. The sample is placed in front of a beam of electrons, which is sent through a series of magnetic lenses designed to focus them very tightly in one spot. The spot of electrons is focused back and forth across the specimen, row by row. As it scans, it knocks electrons off the surface of the sample, which are detected by the microscope. An amplifier condenses all of this information into a final image, built up from the number of electrons emitted from each spot on the sample. SEMs are commonly used to look at very small surfaces; for example, the compound lens of a fly eye looks quite beautiful when visualized by SEM.

SEMs are extremely powerful, but they are not the ultimate in microscopy. The most powerful microscope available in the world is the atomic force microscope, or AFM. AFM has, simply put, the most magnification and resolution power of any microscopy system on the planet. The principles of AFM are fairly straightforward. An atomically sharp tip is created on the end of a flexible cantilever that bends in response to force between the tip and the sample (kind of like a diving board). A laser sends a beam of light towards the tip, which is reflected at a certain angle. The tip is then moved across the sample of interest. As the tip moves in response to the sample, the angle at which the laser beam is reflected changes. This change is detected by the microscope, which is then turned into a 3-dimensional picture of the item being scanned. The resolution limit of AFMs is extremely high; they can reach a lateral (side-to-side) resolution of 1 nanometer (1 x 10-7 centimeters) and a height resolution of 1 angstrom (1 x 10-8 centimeters). That’s powerful enough to look at the structure of individual proteins!

Most scientists do not need to look at an object in that great of a detail. However, for those that do, these advances have made possible looking at things that would have been unimaginable 20 years ago. It kind of makes me wonder – 20 years from now, what kind of microscopes will we have? And what currently unimaginable things will we be able to see? Individual atoms? Subatomic particles? Now that’s what I call small!