Editorial Feature

Hydrogel Nanoparticles for Photonic Crystals

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At the Georgia Institute of Technology, scientists have created a range of hydrogel-based nanoparticles that can be used for developing photonic crystals. The optical characteristics of these crystals can be accurately tuned by thermally modifying the water content of the nanoparticles.

Process for Developing Photonic Crystals

The spherical and soft conformable particles can form the basis for a “photonic fluid.” This photonic fluid can be customized to create self-assembled periodic structures capable of transmitting certain wavelengths of light. Promising applications include new types of medical diagnostics, and optical limiting and optical switching in telecommunications.

We have a very simple processing method for taking one type of particle and creating a whole host of optical materials from it, as opposed to having to synthesise a new particle for each optical material you would like. We have a polymer solution that can be processed in normal ways that typically cannot be done with other types of colloidal photonic materials.

Andrew Lyon, Assistant Professor of Chemistry and Biochemistry, Georgia Institute of Technology

Poly-N-isopropylacrylamide lightly cross-linked with N,N′-methylenebis(acrylamide) is used to synthesize the nanoparticles. Following precipitation polymerization in aqueous media, the nanoparticles are isolated from the surrounding water through centrifugation. The gelatinous material, thus obtained, has a dim red, blue, or green hue.

The material is annealed by heating it above the volume-phase transition temperature of the component hydrogel particles, so as to impart it with desired optical properties. At this point, the photonic crystal loses its order and the nanoparticles start to lose water.

Once small amounts of water are removed, the material is allowed to cool down, re-absorb water, and finally, recrystallize. A thermal cycling process like this helps in packing the soft hydrogel particles into an organized 3D hexagonal array. This hexagonal array creates the periodic dielectric structure required for optical characteristics. The process is repeated up to 15 times until the ensuing crystalline structure has the required optical characteristics.

Role of Particle Hydration

By closely regulating the hydration of the nanoparticles, the scientists can adjust the colors by 1 nm steps across a wavelength range of over 200 nm. Upon heating the material above the volume-phase transition temperature, it turns into a liquid, which can be worked using typical polymer processing methods.

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