From current synthesis methods, one approach to make them "greener" would be to change reactants and/or solvents to non-toxic, more environmental-bening or affordable compounds.
Then, the size, shape and properties of nanoparticles produced by the new method could be compared with the traditional method.
For instance, we could reproduce the synthesis of chitosan nanoparticles from water-in-oil microemulsions as soft templates, replacing traditional petroleum-based solvents by non-toxic bio-based solvents.
Article Water-in-oil microemulsions with greener oil solvents as sus...
From a physics point of view, during the synthesis of any nanoparticle, the most important stage is the nucleation. Given that the conditions such as temperature, pH, etc, are uniform throughout the solution, the size distribution of the nanoparticle boils down to the timescale in which all the nucleations happen. Green synthesis often relies on plant extracts, microbial secretions, or other biogenic materials, which inherently contain complex mixtures of bioactive compounds such as polyphenols, flavonoids, proteins, and enzymes. These components can vary significantly between sources and batches, leading to inconsistencies in nanoparticle properties. Using specific molecules rather than crude extracts will be ideal to ensure uniform properties for the nanoparticles.