NIPAm-based soft colloidal photonic crystals with high color saturation

Nanogels can form colloidal crystals and exhibit structural color in aqueous solution. However, the strong scattering effect and limited arrangement of the nanogel leads to low color saturation. In this study, a series of monomers N-cyclohexyl maleamic acid (NCMA; refractive index (RI)= 1.523), N-phenyl maleanilic acid (NPMA; RI=1.634) and N-(1-naphthyl) maleamic acid (NNMA; RI=1.706) are copolymerized with N-isopropylacrylamide (NIPAm) to synthesize N-isopropylacrylamide-based (NIPAm-based) monodispersed nanogels. The conjugated molecules NPMA and NNMA in nanogels can greatly improve the color saturation levels of colloidal crystals relative to poly(N-isopropylacrylamide) (PNIPAm) and poly(NIPAm–NCMA) because of the noncovalent interactions and high refractive indices. The color saturation increases with increasing conjugated length and refractive index, and the bandgap can be adjusted by regulating the particle diameter or concentration. By introducing N-hydroxyethyl acrylamide and the crosslinker polyurethane acrylate (PUA) to construct the second polymer network, the embedded colloidal crystals possess reversible thermal responsiveness, and the wavelength of the reflection peak redshifts as drug release increases. These materials show great potential as indicators for drug monitoring and biosensing applications.