Visualized optical sensors based on two/three-dimensional photonic crystals for biochemicals

Responsive photonic crystals (RPCs) constructed by periodic two/three-dimensional (2D/3D) photonic crystals (PCs) and responsive-material hosts, are important visualized optical sensors. Their optical diffraction color can be tuned reversibly by external stimuli, such as pH, metal ions, biomolecules, vapors and solvents, hence leading to wide applications as visualized sensors. This review introduces the recent progress of RPCs based on 2D/3D PCs for visual detection of chemical and biological analytes, including the preparation of 2D PCs, 3D PCs films, 3D PCs microbeads and their applications as visualized sensors. The different cases of detecting various chemical and biological analytes by naked eyes are presented. Emphasis is given to the description of their respective sensing mechanisms with the different systems for chemical and biological analytes. Compared with 3D RPCs sensors, 2D RPCs sensors have shorter response time, better stabilization and higher production efficiency, however, the diffraction intensity of 2D RPCs based on monolayered 2D polystyrene (PS) microsphere array are weak. 2D RPCs sensors based on 2D Au nanosphere can significantly improve the diffraction intensity compared with traditional 2D RPCs sensors based on monolayered PS microsphere array. The much higher scattering cross section of Au nanosphere leads to 2D Au nanosphere array with ultrahigh optical diffraction intensity, which are highly helpful for their practical application as visual sensors and further quantitative detection by monitoring the diffraction peak position and intensity.