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Many oceanic prey animals use transparent bodies to avoid detection. However, conspicuous eye pigments, required for vision, compromise the organisms’ ability to remain unseen. We report the discovery of a reflector overlying the eye pigments in larval decapod crustaceans and show how it is tuned to render the organisms inconspicuous against the background. The ultracompact reflector is constructed from a photonic glass of crystalline isoxanthopterin nanospheres. The nanospheres’ size and ordering are modulated to tune the reflectance from deep blue to yellow, enabling concealment in different habitats. The reflector may also function to enhance the acuity or sensitivity of the minute eyes by acting as an optical screen between photoreceptors. This multifunctional reflector offers inspiration for constructing tunable artificial photonic materials from biocompatible organic molecules.

Funding Information: We thank A. Upcher for assistance with transmission electron microscopy. We acknowledge the Swiss Light Source for providing synchrotron radiation facilities, and thank V. Olieric and T. Tomikazi for their assistance in using beamline PXI-X06SA. We thank D. Machlis, the BGU university photographer, for photographing the Lysmata amboinensis in the insert of Fig. . This work was performed using resources provided by the Cambridge Service for Data Driven Discovery (CSD3) operated by the University of Cambridge Research Computing Service ( www.csd3.cam.ac.uk ), provided by Dell EMC and Intel using Tier-2 funding from the Engineering and Physical Sciences Research Council (capital grant no. EP/T022159/1), and DiRAC funding from the Science and Technology Facilities Council ( www.dirac.ac.uk ). Funding was provided by an ERC Starting Grant (grant no. 852948, ‘CRYSTALEYES’) and an HFSP grant (grant no. RGP0037/2022) awarded to B.A.P. B.A.P. is the Nahum Guzik Presidential Recruit. B.A.P. is a recipient of the 2019 Azrieli Faculty Fellowship. Funding was also provided by the European Union’s Horizon 2020 research and innovation programme under a Marie Skłodowska-Curie grant agreement (no. 893136) and Academy of Finland grant (no. 347789) awarded to J.S.H. A Swiss National Science Foundation Grant (grant no. 40B1-0_198708) was awarded to L.S. Electron microscopy studies were supported by the Ilse Katz Institute for Nanoscale Science & Technology at Ben-Gurion University of the Negev. L.A. and D.O. acknowledge support from the Minerva Stiftung. | openaire: EC/H2020/893136/EU//MALDIP A fundamental question regarding light scattering is how whiteness, generated from multiple scattering, can be obtained from thin layers of materials. This challenge arises from the phenomenon of optical crowding, whereby, for scatterers packed with filling fractions higher than ~30%, reflectance is drastically reduced due to near-field coupling between the scatterers. Here we show that the extreme birefringence of isoxanthopterin nanospheres overcomes optical crowding effects, enabling multiple scattering and brilliant whiteness from ultra-thin chromatophore cells in shrimp. Strikingly, numerical simulations reveal that birefringence, originating from the spherulitic arrangement of isoxanthopterin molecules, enables intense broadband scattering almost up to the maximal packing for random spheres. This reduces the thickness of material required to produce brilliant whiteness, resulting in a photonic system that is more efficient than other biogenic or biomimetic white materials which operate in the lower refractive index medium of air. These results highlight the importance of birefringence as a structural variable to enhance the performance of such materials and could contribute to the design of biologically inspired replacements for artificial scatterers like titanium dioxide.

Funder: Emil Aaltosen Säätiö; Id: http://dx.doi.org/10.13039/501100004756, To unlock the widespread use of block copolymers as photonic pigments, there is an urgent need to consider their environmental impact (cf. microplastic pollution). Here we show how an inverse photonic glass architecture can enable the use of biocompatible bottlebrush block copolymers (BBCPs), which otherwise lack the refractive index contrast needed for a strong photonic response. A library of photonic pigments is produced from poly(norbornene‐graft‐polycaprolactone)‐block‐poly(norbornene‐graft‐polyethylene glycol), with the color tuned via either the BBCP molecular weight or the processing temperature upon microparticle fabrication. The structure–optic relationship between the 3D porous morphology of the microparticles and their complex optical response is revealed by both an analytical scattering model and 3D finite‐difference time domain (FDTD) simulations. Combined, this allows for strategies to enhance the color purity to be proposed and realized with our biocompatible BBCP system.

The brightest colours in nature often originate from the interaction of light with materials structured at the nanoscale. Different organisms produce such coloration with a wide variety of materials and architectures. In the case of bacterial colonies, structural colours stem for the periodic organization of the cells within the colony, and while considerable efforts have been spent on elucidating the mechanisms responsible for such coloration, the biochemical processes determining the development of this effect have not been explored. Here, we study the influence of nutrients on the organization of cells from the structurally coloured bacteria Flavobacterium strain IR1. By analysing the optical properties of the colonies grown with and without specific polysaccharides, we found that the highly ordered organization of the cells can be altered by the presence of fucoidans. Additionally, by comparing the organization of the wild-type strain with mutants grown in different nutrient conditions, we deduced that this regulation of cell ordering is linked to a specific region of the IR1 chromosome. This region encodes a mechanism for the uptake and metabolism of polysaccharides, including a polysaccharide utilization locus (PUL operon) that appears specific to fucoidan, providing new insight into the biochemical pathways regulating structural colour in bacteria., This work was funded by by EU’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 722842 (ITN Plant-inspired Materials and Surfaces–PlaMatSu), the Swiss National Science Foundation under project P2ZHP2_183998, the Biotechnology and Biological Sciences Research Council (BBSRC) iCASE fellowship (2110570), the BBSRC David Phillips fellowship (BB/K014617/1), the BBSRC grant BB/V00364X/1, the ERC (ERC-St-Grant014H2020 639088), the ERC BiTe (101001637) and the Engineering, the Physical Sciences Research Council (ESPRC) NanoDTC grant (EP/L015978/1) and the EPSRC Underpinning Multi-User Equipment Call (EP/P030467/1). CI would like to thank the iLAB, Utrecht, for facilities.

Passive radiative cooling (RC) enables the cooling of objects below ambient temperature during daytime without consuming energy, promising to be a game changer in terms of energy savings and CO reduction. However, so far most RC surfaces are obtained by energy-intensive nanofabrication processes or make use of unsustainable materials. These limitations are overcome by developing cellulose films with unprecedentedly low absorption of solar irradiance and strong mid-infrared (mid-IR) emittance. In particular, a cellulose-derivative (cellulose acetate) is exploited to produce porous scattering films of two different thicknesses, L ≈ 30 µm (thin) and L ≈ 300 µm (thick), making them adaptable to above and below-ambient cooling applications. The thin and thick films absorb only (Formula presented.) of the solar irradiance, which represents a net cooling power gain of at least 17 W m, compared to state-of-the-art cellulose-based radiative-cooling materials. Field tests show that the films can reach up to ≈5 °C below ambient temperature, when solar absorption and conductive/convective losses are minimized. Under dryer conditions (water column = 1 mm), it is estimated that the films can reach average minimum temperatures of ≈7–8 °C below the ambient. The work presents an alternative cellulose-based material for efficient radiative cooling that is simple to fabricate, cost-efficient and avoids the use of polluting materials., This work was supported by the BBSRC David Phillips fellowship (BB/K014617/1) and the Horizon 2020 Framework Programme Marie Curie Individual Fellowship (793643-MFCPF), ERC SeSaME ERC2014STG H2020 639088, the PoC 963872, Cellunan, and the Isaac Newton Trust (SNSF3) and the Philip Leverhulme Prize (PLP-2019-271). The European Union Horizon 2020 Research and Innovation Program partly funded this research (Marie Sklodoswa-Curie Actions Grant No. 665919 that supported J.J.F and the project FLEXPOL Grant. No. 721062 that supported G.L.W). Moreover, the research received funds from the Spanish Minister of Science, Innovation and Universities via the Severo Ochoa Program (Grant No. SEV-2017-0706) that supports ICN2, and the projects PGC2018-101743-B-I00 (SIP) and RTI2018-093921-A-C44 (SMOOTH), as well as by the CERCA Program/Generalitat de Catalunya. L.S. acknowledges the support of the Swiss National Science Foundation under project 40B1-0 198708. P.D.G gratefully acknowledges the support the Ramon y Cajal fellowship (RYC-2015-18124).

Living organisms have developed a wide range of appearances from iridescent to matte textures. Interestingly, angular independent structural colors, where isotropy in the scattering structure is present, only produce coloration in the blue wavelength region of the visible spectrum. One might, therefore, wonder if such observation is a limitation of the architecture of the palette of materials available in nature. Here, by exploiting numerical modeling, we discuss the origin of isotropic structural colors without restriction to a specific light scattering regime. We show that high color purity and color saturation cannot be reached in isotropic short-range order structures for red hues. This conclusion holds even in the case of advanced scatterer morphologies, such as core-shell particles or inverse photonic glasses — explaining recent experimental findings reporting very poor performances of visual appearance for such systems.

Vivid colours found in living organisms are often the result of scattering from hierarchical nanostructures, where the interplay between order and disorder in their packing defines visual appearance. In the case of Flavobacterium IR1, the complex arrangement of the cells in polycrystalline three-dimensional lattices is found to be a distinctive fingerprint of colony organization. By combining analytical analysis of the angle-resolved scattering response of in vivo bacterial colonies with numerical modelling, we show that we can assess the inter-cell distance and cell diameter with a resolution below 10 nm, far better than what can be achieved with conventional electron microscopy, suffering from preparation artefacts. Retrieving the role of disorder at different length scales from the salient features in the scattering response enables a precise understanding of the structural organization of the bacteria. © 2020 The Author(s) Published by the Royal Society. All rights reserved.