Additionally, structural color is often dynamic, as PBG properties can be adjusted by external physical or chemical stimuli through manipulation of refractive index contrast and lattice constant in photonic crystal structures [28,67]. This review focuses on recent progresses in application of bio-inspired photonic materials with variable structural colors as colorimetric sensors.2.?Coherent Scattering of LightThe colorful appearances of the PCs materials can be ascribed to interference and reflection, which can be described by Bragg’s and Snell’s laws [7,64] as shown in Figure 1. The law is given by:��=2D(neff2?cos2��)1/2(1)where �� is the wavelength of the reflected light, neff is the average refractive index of the constituent photonic materials, D is the distance of diffracting plane spacing, and �� is the Bragg angle of incidence of the light falling on the nanostructures.

Based on the equation, there are several methods for tuning structural color, such as changing the diffracting plane spacing D, the average refractive index neff, Bragg glancing angle ��, and changing the neff and D simultaneously. The dependence of �� on PCs material characteristics can be employed in the application of sensors. The use of photonic crystals as colorimetric sensors is the focus here. Colorimetric photonic-crystal sensors are based on structural colors tuned by external physical or chemical stimuli through the manipulation of refractive index and lattice constant.Figure 1.Incident light with a wavelength predicted by a modified Bragg-Snell equation (Equation (1)) undergoes diffraction when propagating through a PC.

The wavelength of light that is coherently scattered is centered on ��, and can be estimated by the …3.?Structure Colors from the Natural Photonic Crystals3.1. Natural Photonic Nanostructures that Can Form Structural ColorsOver millions years AV-951 of evolution, living organisms have created an amazing variety of photonic structures to produce a colorful natural world. The structural colors generated by the photonic architecture in organisms have attracted a great amount of interest over time. These organisms have the ability to control the transportation of light using periodical photonic nanostructure units located on the surface of their bodies. In general, the bright structural colors of natural creatures play an important role in sexual attraction, social behavior and environmental camouflage [7].

According to variations of refractive index and period in space, natural PCs can be classified as 1D, 2D, and 3D frameworks, respectively, as shown in Figure 2.Figure 2.Typical naturally occurring photonic structures with various structural colors. (A) 1D periodicity in the form of multilayers existing in green and purple neck feathers of domestic pigeons [10]. (B) Some discrete 1D periodicity found in Morpho butterflies …