Novel opal hydrogels with tunable photonic band gap (PBG) were prepared, with sensitive responses to external stimuli in water. These self-assembled colloidal crystals display narrow stop bandwidth, consisting of polystyrene-co-poly(N,N-dimethylacrylamide) (PS-co-PDMAA) microspheres. The polymeric microspheres were engineered in water, exhibiting narrow size distribution; with numerous acrylamide groups on their surface, they are hydrophilic in nature (Figure 1 (a)). Interestingly, each microsphere has two distinguishable compositional regions. The inner and outer regions are rich in PS and PDMAA, respectively; thus, the inner region is hard and hydrophobic, and the outer region is soft and hydrophilic.Accordingly, the PS-co-PDMAA microspheres are ready to self-assemble, even via centrifugation, producing diffraction with high color purity. Promptly and sensitively, the centrifugation-induced three-dimensional (3D) photonic colloidal crystals (PCCs) responded to the amount of water presented. With an increase of the amount of water, the volume of the opal hydrogels increased, so did their lattice spacing (Figure 1 (b)-top); accordingly, their PBG shifted from the visible to near-infrared region of the electromagnetic spectrum. With narrow full width at half maximum (FWHM) in the range of 20 to 40 nm only, the diffraction could shift its peak position more than 500 nm (Figure 1 (c)). Remarkably, such shift was repeatable via the cycles of centrifugation and water addition. Furthermore, the water-tunable change in color is visible to the naked eye (Figure 1 (b)-bottom).The aforementioned observations could be reasonably attributed to the structure of the polymeric microspheres, the outer region of which is hydrophilic with a reproducible degree of hydration. Appreciably, the presence of thiocyanate (SCN-) ions shapes the hydration, because they break efficiently the hydrogen bonds between the repeat units of DMAA and water molecules. Thus, the diffraction of the opal hydrogels is affected delicately. With far-reaching implications, our PS-co-PDMAA PCCs have the potential in diffraction-based detections.Figure 1.  Schematic drawing of the interaction between water molecules and the repeat units of DMAA that are on the surface of our PS-co-PDMAA microspheres (a). The PBG of the PS-co-PDMAA PCCs is water-tunable, which could be repeatedly controlled by the cycles of centrifugation and water addition, and the resulting change in color of the PCCs is evident to the naked eye (b). The diffraction spectra collected from PS-co-PDMAA PCCs.

Water-tunable photonic colloidal crystals with narrow diffraction bandwidth

247th National Meeting (2014)

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