Temperature-Induced Gelation Enhances Fluorescence in Nanogel Photonic Crystals

Colloidal photonic crystals (CPCs) with unique optical properties can enhance the fluorescence intensity and have attracted considerable attention. However, to the best of our knowledge, there has been no study on enhancing the fluorescence intensity of CPCs self-assembled from thermosensitive N-isopropylacrylamide (NIPAM)-based nanogels. NIPAM-based nanogels shrink rapidly, and CPCs disappear above the phase transition temperature (Tp), decreasing their fluorescence intensity. Herein, N-isopropylacrylamide (NIPAM), N-acryloyl-l-phenylalanine (Aphe), and a derivative (NI3) of fluorescent naphthalimide were polymerized to synthesize P(NIPAM-Aphe-NI3) nanogels that assembled into CPCs and greatly improved the fluorescence intensity below and above Tp. The Tp of nanogels was determined via ultraviolet–visible spectroscopy and dynamic light scattering, while the phase transition behavior was studied via variable-temperature Fourier-transform infrared spectroscopy. The rheological results demonstrated that CPCs underwent a remarkable sol–gel transition with a stable structural color of CPCs above the Tp. The synergistic effect of the water-absorbing carboxylic group and the large steric hindrance of the phenyl group maintained the structural color of the CPCs above Tp. Notably, the fluorescence emission of different CPC structures was enhanced by adjusting the nanogel size. As CPCs assembled with a nanogel size of 304 nm, their reflection peak wavelength matched the emission peak wavelength of fluorescent NI3, resulting in a maximum enhancement of 4-fold that of nanogels without structural color below Tp. The maximum fluorescence intensity of CPCs with structural color was 8-fold higher at temperatures up to 49 °C above Tp than that of the nanogels without structural color. The remarkable increase in the fluorescence intensity of CPCs above Tp is reported for the first time and can greatly advance the development of fluorescence science.