Optical properties tuned by water

Researchers at the Georgia Institute of Technology have developed a family of hydrogel-based nanoparticles that can be used to form photonic crystals.

Researchers at the Georgia Institute of Technology have developed a family of hydrogel-based nanoparticles that can be used to form photonic crystals whose optical properties can be precisely tuned by thermally adjusting the particles’ water content.

‘We have a very simple and easy processing method for taking one type of particle and creating a whole host of optical materials from it,’ said Andrew Lyon, assistant professor of chemistry and biochemistry at Georgia Tech. ‘We have a polymer solution that can be processed in normal ways, which typically cannot be done with other types of colloidal photonic materials.’

Lyon and his colleagues are said to have fabricated nearly 100 different types of monodisperse hydrogel particles, in sizes ranging from 50 nanometers to 1 micron in diameter. The temperature at which the particles transition to a crystalline state can be controlled chemically during the synthesis process in a range from 10 degrees C to 60 degrees C.

The nanoparticles are synthesised from poly-N-isopropylacrylamide (pNIPAm) lightly cross-linked with N, N-methylenebis (acrylamide)(BIS). After precipitation polymerisation in aqueous media, the particles are separated from the surrounding water by simple centrifuging. The resulting glassy gelatinous material, which has a faint blue, green or red hue, is reportedly more viscous than honey.

To give it desirable optical properties, the material must be annealed by heating it past the volume phase transition temperature of the component hydrogel particles, at which the photonic crystal loses its order and the nanoparticles begin to give up water content.

After removing small amounts of water, the material is allowed to cool, re-absorb water and re-crystallise. This thermal cycling process serves to pack the soft hydrogel particles into an ordered 3-D hexagonal array, which produces the periodic dielectric structure needed for optical properties.

The annealing step is repeated as many as 15 times until the resulting crystalline structure has the desired optical properties. Crystals produced so far by Lyon and collaborators Justin Debord, SaetByul Debord and Clinton Jones reflect bright blue, green or red colours.

‘While the assembly of colloidal crystals from such particles has been reported previously, this represents the first report where the softness and thermoresponsivity of the component particles is used to create a colour-tuneable colloidal crystal via particle compression,’ Lyon said.

By closely controlling the hydration of the particles, the researchers can tune the colours by one-nanometer steps over a wavelength range of more than 200 nanometers.

When heated above the transition temperature, the material readily flows in a liquid form and can be cast, moulded or spin coated onto a surface using standard polymer processing techniques.

Though practical applications may be a long way off, the researchers envision uses in the telecommunications industry, where the precisely tuned photonic crystals could be used to extract information carried on optical fibres at specific wavelengths.