MICRO

There are many ways to image organisms and objects to discover their structure and form.

The naked eye can only resolve down to a certain level of detail before magnifiers are required. A simple magnifying lens then an optical stereo microscope and finally a compound monocular light microscope allow us to employ light to image things to a certain magnification before we can no longer resolve detail.

Scanning Electron Microscopy (SEM) utilises electrons to image things and allows us to image much smaller areas than is possible with light. A beam of electrons of known energy and size is scanned across a small area, Topographical Contrast Imaging producing a photomicrograph of the area scanned.

Live Telepresence Microscopy for the Queensland Education Department. Students from as far away as West Papua explore creatures in a Scanning Electron Microscope at The University of Queensland’s Centre for Microscopy and Microanalysis. Credit: Dr John Hunt

In order for the images to be crystal clear the imaging needs to be done in a vacuum and the return signal amplified by coating the area in Gold, a high atomic number material. Scanning an electron beam over carbon yields few electrons from the surface but if you coat the area in an extremely thin layer of Gold, Atomic Number 79, large numbers of electrons are released when excited by the electron beam.

Images are instantly recognisable despite the fact that the topographical contrast imaging our brain is processing is opposite to what it expects in a similar light illuminated situation. A basketball in sunshine has a shiny flat surface perpendicular to view and sides which are dull due to light scattering. With SEM the surfaces parallel to the beam give off many more electrons than a flat area at right angles to the beam. Edges are bright whereas the faces are dull.

Surfaces parallel to the scanning electron beam return the brightest signal as more area is stimulated. Surfaces perpendicular to the beam appear much duller as the area stimulated is at a minimum.

SEM has it’s resolution limits too and we need to change to another method to realise focussed images of areas at much higher magnification, called Transmission Electron Microscopy (TEM). Typically the object to be imaged is mounted in a resin substrate and very thin slices are then cut, stained to improve electron density and imaged in a vacuum using electrons instead of light in Transmission Microscopy. Atomic resolution is possible with this method.

Rows of atoms in three distinct orientations are revealed in this Transmission Electron Micrograph of a Magnetosome. Credit: Dr Tony Taylor.

Atomic Force Microscopy (AFM) and other methods allow us to image individual atoms at high resolution and even manipulate them if desired. This work was beyond my scope at The University of Queensland though instrumentation was available within the Centre for Microscopy and Microanalysis.