Manufacturing nanoparticles

There are four fundamental routes to making nano materials.

• Form in place
  These techniques incorporate lithography, vacuum coating and spray coating.
• Mechanical
  This is a ‘top-down’ method that reduces the size of particles by attrition, for example, ball milling or planetary grinding.
• Gas phase synthesis
  These include plasma vaporization, chemical vapor synthesis and laser ablation.
• Wet chemistry
  This is the range of techniques that are most applicable for characterization by light scattering techniques. These are fundamentally ‘bottom-up’ techniques, i.e. they start with ions or molecules and build these up into larger structures.
  
  These nanoparticle manufacturing techniques historically come under the title of ‘colloid chemistry’, and involve classical ‘sol-gel’ processes, or other aggregation processes.

These wet chemistry techniques currently offer the best quality nanoparticles from a number of points of view.

• They produce nanoparticles that are already in the form of a dispersion, hence high inter-particle forces can be designed in to prevent agglomeration.
  
• The formation of aggregates can be reduced or eliminated.
  
• The nanoparticles can be made to be very monodisperse, i.e. all the same size to within small tolerances.
  
• The chemical composition, and morphology can be closely controlled. This is especially important for research purposes where the quality of the material must be very high to ensure repeatable and meaningful results.

One of the most recent developments is the production in liquid carbon dioxide. This offers the promise of the controlled conditions of the ‘bottom-up’ wet chemistry approach, as well as the benefit of being able to remove the dispersant by simply reducing the pressure of the reaction container. This technique is currently used for removing the caffeine from tea and coffee, so the mechanics of handling the materials are well understood.