Simple synthesis of three primary colour nanoparticle inks of Prussian blue and its analogues

Historic Prussian blue (PB) pigment is easily obtained as an insoluble precipitate in quantitative yield from an aqueous mixture of Fe3+ and [FeII(CN)6]4− (Fe2+ and [FeIII(CN)6]3−). It has been found that the PB pigment is inherently an agglomerate of 10–20 nm nanoparticles, based on powder x-ray diffraction (XRD) line broadenings and transmission electron microscopy (TEM) images. The PB pigment has been revived as both organic-solvent-soluble and water-soluble nanoparticle inks. Through crystal surface modification with aliphatic amines, the nanoparticles are stably dispersed from the insoluble agglomerate into usual organic solvents to afford a transparent blue solution. Identical modification with [Fe(CN)6]4− yields water-soluble PB nanoparticles. A similar ink preparation is applicable to Ni-PBA and Co-PBA (nickel and cobalt hexacyanoferrates). The PB (blue), Ni-PBA (yellow), and Co-PBA (red) nanoparticles function as three primary colour inks.