20 Mar 2008
A new imaging technique that detects tiny particles using a common optical microscope could offer an alternative to fluorescent labelling.
Researchers in the US and Italy are using a conventional optical microscope to detect nanoparticles that are much smaller than the instrument's normal resolution. Eann Patterson of Michigan State University and Maurice Whelan of the European Commission DG Joint Research Centre came up with the idea while taking images of cells.
The two-man partnership noticed that its high-resolution images were interrupted with caustics, bright envelopes of light surrounding a dark shadow. Light interacting with nano-sized vesicles was producing patterns many times larger than the intracellular structures, elegantly capturing the nanoscale on a conventional inverted optical microscope.
"Using the technique, we believe that it will be possible to track particles in three-dimensions and in real time, allowing interactions to be studied non-invasively. Examples include self-assembly processes, or the binding of nanoparticles to functionalized surfaces," Patterson told nanotechweb.org. "The procedure offers major advantages over fluorescent labelling of particles, which typically requires long exposure times and suffers from photo-bleaching."
So how is it done? "Set-up your microscope for normal viewing with Kohler illumination and then close all of the diaphragm apertures to a minimum," he explained. "Some caustics may be immediately obvious, if not move slightly out of focus to observe them."
Patterson and Whelan have yet to find the lower size limit for their procedure. In fact, the major restriction so far has been the lack of suitably sized and authenticated test samples.
Initial work suggests that the caustics produce a magnification of between a hundred and a thousand times the particle size.
To develop potential applications, the researchers are now modelling the behaviour of light.