Artist's conception of a thought experiment demonstrating thermal-noise imaging. A single fluorescent particle bounces around a room, but high-speed imaging of its movements over time reveals the location of a chair. [Image: Jenna Luecke, University of Texas at Austin]
Imagine standing in a dark room in which a single, glowing ball is bouncing around. The ball avoids a region in the center of the room. Over time, you might conclude that an object sits in that region, even though you cannot see it.
That thought experiment illustrates thermal-noise imaging, a technique that a team at a U.S. university has applied to nanoscale imaging of soft biological structures (Nat. Commun., doi: 10.1038/ncomms12729). The technique could lead to new means for studying live cells and other tiny biological structures.
Follow the bouncing ball
Thermal-noise imaging was invented about a decade ago, but until the new study at the University of Texas at Austin, no one had succeeded at applying the method to nanoscale biological materials. Other super-resolution imaging methods would fail to combat the blurring effects of Brownian motion in liquids surrounding the nanostructures.
The team, led by Ernst-Ludwig Florin, confined a single fluorescent polystyrene nanoparticle with optical tweezers inside a custom-built photonic force microscope, and recorded its movements with a high-bandwidth position detector based on forward-scattered laser light. The particle—with a radius of roughly 95 nm, plus a coating about 5 nm thick—moved around the trapping potential, which was subdivided into voxels (“volume pixels”) about 10 nm on a side. The researchers then drew up a histogram of the particle's measured locations, with a correction for the influence of the trapping potential.
The big reveal
When no obstacle was present, the particle was most often in the center of the trap, as Boltzmann statistics predict. In the presence of an object such as a collagen filament, though, the excluded volume of the object was revealed on the histogram. To map a comparatively thick and optically dense filament, the researchers moved the center position of the optical trap around on a grid and combined the images to make a larger picture.
The technique achieves a resolution of better than 10 nm, according to the Texas scientists. The resolution depends in part on the radius of the fluorescent particle, although smaller particles would yield images with higher background noise. Multiple particles could be used if future researchers could correct for crosstalk.