Sunday, July 15, 2018

Sample Preparation: Dilution, Dispersion and Surface Charge

Successful SEM imaging requires not only competent microscope operation but also expert sample preparation. In the case of imaging nanostructures, sample preparation is generally the most difficult task. In this example cellulose nanofibers were imaged at low energy due to their low density. In the first image we see an undifferentiated mass as the undiluted but dried fibers were imaged after being dried on a stub and coated with 4 nm of iridium to suppress charging.

The individual fibers are largely unobserved. This is due to the surface charge of the fibers causing them to aggregate to not only each other but also the stub itself. At this point successful imaging comes down to successful sample preparation.

The first plan of attack in any dispersion problem is dilution. In the second image the fibers were imaged after significant dilution. Dilution helps disperse nano-structures by simply keeping them from each other so that they can not aggregate. Dilutions of 1:100 to 1:1000 or more can be necessary.

It was found that dilution alone did not solve the aggregation problem. Dilution provided small patches of agglomerated fibers that resembled the first image. This was because cellulose fibers are generally tangled together and can not be separated upon dilution-- only individual clumps can be separated.

To mitigate surface charge the nanocellulose fibers were subject to washing in DI water and drying with a polar solvent to screen surface charge. Then the material was placed in a non-polar solvent to maintain space between the fibers. This mixture was then quickly dried so that free space between fibers was preserved.

In practice the surface potential of nanoparticles may need to be measured using an instrument that measures zeta-potential, and the surface potential neutralized using a solvent of an appropriate pH. this is often the case with such nanoparticle systems as aluminum oxide where surface potential is highly dependent upon synthesis and post synthesis handling.

The moral of the application note is to not give up when faced with useless images-- success is often a matter of a little experimentation with sample preparation.

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