Research ArticlePLANT SCIENCES

Photonic paper: Multiscale assembly of reflective cellulose sheets in Lunaria annua

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Science Advances  01 Jul 2020:
Vol. 6, no. 27, eaba8966
DOI: 10.1126/sciadv.aba8966
  • Fig. 1 L. annua seedpod macroscopic appearance and hierarchical morphology.

    (A) Macroscopic photographs of an L. annua seedpod showing the matte two external valves surrounding the internal silvery-white septum. The seeds attached to the valves developed from the septum’s funicles. (B) Macroscopic photograph of the highly reflective septum with average size of ~3.1 × 4.6 cm and (C) closeup photograph of the septum showing thin-film interference colors and a texture reminiscent of arrays of colorful parallel elongated cells. (D) Top view SEM image of the septum showing multidomains made by 1D arrays of cylindrical cells oriented in parallel fashion inside each domain. (E) Cross-sectional SEM image of the septum hollow cell bilayer arrays. (F) High-magnification cross-sectional SEM image of an individual hollow cell with visible thin-film assembly in the cell wall cladding and alignment of the nanofibrils along the cell circumference. Photo credit: F.G.O., Tufts University.

  • Fig. 2 Optical characterization of L. annua septum.

    (A) Bright-field reflection micrograph of the septum surface observed in top view showing thin-film interference–like colors spanning the entire visible spectrum. (B) Reflectance spectra collected within single-colored regions, reflecting magenta, blue, and green, and corresponding optical micrographs (white circle indicates the fiber collection spot, Ø ~4 μm, normalized to a white diffuser). (C) Average reflectance spectrum of the septa collected over large areas (collection spot Ø ~60 μm, normalized to a white diffuser) is indicated by the solid line and SD is the shaded areas. (D to I) Multispectral bright-field reflection analysis of the septum: The broadband reflectance RGB image (D) can be unmixed in individual false-color spectral maps, each defined by the wavelength λi, that identifies the spatial distribution of each selected spectral response corresponding to the spectral bands whose maximum reflectance peaks are, respectively, λ3 ~485 nm (E), λ5 ~564 nm (F), λ7 ~621 nm (G), and λ8 ~740 nm (H), as verified by the reflectance spectra shown in the insets. a.u., arbitrary units. (I) The individual spectral maps, defined by the wavelengths λ18, can be recombined to generate a false-color composite spectral map, which emphasizes the multicolored response of the septum.

  • Fig. 3 Mechanical characterization of L. annua septum.

    SEM cross-sectional images and corresponding schematics (insets) of the observed cell array relative orientation in the septum: respectively, (A) parallel, (B) normal, and (C) at an angle. (D) Macroscopic picture of a bent Lunaria septum demonstrating high flexibility. (E) Transmission diffraction grating patterns with schematic representations of the layer relative orientation observed in cross section (insets). (F) The angular resolved reflectance of the septum, with intensity normalized against a Lambertian diffuser, demonstrates the overall reflectance capacity of the septum. (G) Representative weight-normalized cumulative volume of intruded mercury as a function of the applied pressure. Kernel density plots of (H) Young modulus, (I) ultimate tensile strength, and (J) elongation of the tested septa strips. The horizontal red lines divide the Kernel plots in the 25th (bottom), 50th (intermediate), and 75th percentile (top). Photo credit: G.G., Tufts University.

  • Table 1 Mechanical properties of L. annua in comparison with other cellulose-based films.

    Young modulus (E), ultimate tensile strength (σ), percentage elongation (ε), and corresponding density normalized values, respectively, (E/ρ), (σ/ρ), and (ε/ρ) for the Lunaria septa, cellulose microfibril films, and shear-aligned cellulose nanocrystal films. The minimum and maximum values are reported for the L. annua and cellulose microfibril films (46), whereas the values in axial and perpendicular direction to the cellulose nanocrystal alignment are reported for the cellulose nanocrystal films (45). The density-normalized values are calculated assuming a density of 0.26, 1.5, and 1.55 g/cm3 for the Lunaria septa, the cellulose microfibril films, and the cellulose nanocrystal films, respectively.

    E (GPa)E/ρ (GPa)/(g cm−3)σ (MPa)σ/ρ (MPa)/(g cm−3)ε (%)ε/ρ (%)/(g cm−3)
    Lunaria septa0.5 to 0.81.9 to 3.22.2 to 26.48.4 to 1030.3 to 3.31 to 12.7
    Cellulose microfibril films (46)1 to 17.50.7 to 11.730 to 15520 to 103.32.5 to 11.51.7 to 7.7
    Cellulose nanocrystal films (45)6.7 to 29.74.3 to 19.248 to 7731 to 49.60.3 to 0.90.2 to 0.6

Supplementary Materials

  • Supplementary Materials

    Photonic paper: Multiscale assembly of reflective cellulose sheets in Lunaria annua

    G. Guidetti, H. Sun, B. Marelli, F. G. Omenetto

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