Natural structural color materials, especially those that can undergo reversible changes, are attracting increasing interest in a wide variety of research fields. Inspired by the natural creatures, many elaborately nanostructured photonic materials with variable structural colors were developed. These materials have found important applications in switches, display devices, sensors, and so on. In this critical review, we will provide up-to-date research concerning the natural and bio-inspired photonic materials with variable structural colors. After introducing the variable structural colors in natural creatures, we will focus on the studies of artificial variable structural color photonic materials, including their bio-inspired designs, fabrications and applications. The prospects for the future development of these fantastic variable structural color materials will also be presented. We believe this review will promote the communications among biology, bionics, chemistry, optical physics, and material science (196 references).

Barcode particles have a demonstrated value for multiplexed high-throughput bioassays. Attempts to develop this technology tend to focus on the generation of featured barcodes both with a large number of identifications to increase the throughput and with novel functions to improve the assays. Here, we report a new class of barcodes that are composed of multiple photonic crystal or magnetic-tagged ethoxylated trimethylolpropane triacrylate (ETPTA) cores and polyethylene glycol (PEG) hydrogel shells. These barcodes are prepared by polymerizing microfluidic multiple double emulsions. As the photonic crystal cores possess distinct reflection peaks, our barcodes allow for a substantial number of coding levels for multiplexing applications. The hydrogel shells surrounding the barcodes enable the creation of three-dimensional scaffolds for immobilizing probes. Moreover, the presence of magnetism in the barcodes confers their controllable movement under magnetic fields, which can be used to significantly increase the sensitivity of the bioassays and to simplify the processing. These features make photonic crystal barcodes ideal for biomedical applications.