Bioinspired bright noniridescent photonic melanin supraballs

Abstract

Structural colors enable the creation of a spectrum of nonfading colors without pigments, potentially replacing toxic metal oxides and conjugated organic pigments. However, significant challenges remain to achieve the contrast needed for a complete gamut of colors and a scalable process for industrial application. We demonstrate a feasible solution for producing structural colors inspired by bird feathers. We have designed core-shell nanoparticles using high-refractive index (RI) (~1.74) melanin cores and low-RI (~1.45) silica shells. The design of these nanoparticles was guided by finite-difference time-domain simulations. These nanoparticles were self-assembled using a one-pot reverse emulsion process, which resulted in bright and noniridescent supraballs. With the combination of only two ingredients, synthetic melanin and silica, we can generate a full spectrum of colors. These supraballs could be directly added to paints, plastics, and coatings and also used as ultraviolet-resistant inks or cosmetics.

Fig. 1. Natural inspirations and the optical model.

(A) Two biological examples to enhance color brightness: a green-winged teal (Anas crecca) wing feather and a cross-sectional transmission electron microscopy (TEM) image of a single barbule (left) and an iridescent wild turkey (M. gallopavo) wing feather and a cross-sectional TEM image of a single barbule (right). Scale bars, 500 nm. The photos of teal (credit to F. Pestana) and turkey (credit to T. Llovet) are from flickr.com under license numbers CCBY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/) and CCBY 2.0 (https://creativecommons.org/licenses/by/2.0/). (B) Normal reflectance spectra from the (111) plane of FCC lattices made of core-shell nanoparticles and homogeneous nanoparticles with similar sizes and equivalent refractive indices: high-RI core/low-RI shell nanoparticles (core: RI, 1.74; diameter, 200 nm; shell: RI, 1.45; thickness, 50 nm), equivalent homogeneous nanoparticles (RI, 1.54; diameter, 300 nm), and low-RI core/high-RI shell nanoparticles (core: RI, 1.45; diameter, 267 nm; shell: RI, 1.74; thickness, 16.5 nm). (C) The reflectance intensity ratio between core-shell and homogeneous structures changes as we vary the ratio of core radius to core-shell nanoparticle total radius.

Fig. 2. CS-SMNP synthesis and self-assembly.

(A) A scheme describing the method of synthesizing silica-coated melanin nanoparticles. (B) TEM images of CS-SMNPs: 160/0, 160/36, and 160/66 nm, respectively. The red dashed circles represent the boundary of the core and shell. Scale bars, 100 nm. (C) A scheme showing the self-assembly of supraball structures via a reverse emulsion process. (D) An image of rainbow-like flowers, painted with supraball inks made of five different sizes of CS-SMNPs: navy blue, 123/36 nm; blue-green, 123/43 nm; olive, 160/36 nm; orange, 160/50 nm; red, 160/66 nm.

Fig. 3. Characterization of supraballs.

(A) Optical images of supraballs made of four types of nanoparticles: 224-nm pure silica nanoparticles and 160/0-, 160/36-, and 160/66-nm CS-SMNPs. Scale bars, 0.5 mm. (B) Reflectance spectra and optical images for individual supraballs consisting of 224-nm pure silica nanoparticles (cyan curve, cyan supraball), 160/0-nm CS-SMNPs (purple curve, purple supraball), 160/36-nm CS-SMNPs (olive curve, olive supraball), and 160/66-nm CS-SMNPs (red curve, red supraball). The shaded area indicates the SD from 12 samples, plotted using pavo package in R (32). Each black box in the insets represents the size of the area probed by the optical measurements (4 × 4 μm). (C) Angle-resolved spectra for olive inks, as shown in Fig. 2D. The inset scheme shows the setup for angle-resolved backscattering measurements where we fixed α = 15° and varied angle θ between the source and the sample from 40° to 90°. (D) FDTD simulations of normal reflectance spectra from supraballs consisting of three different sizes of CS-SMNPs, where absorption of melanin was considered.

Fig. 4. Microstructures of supraballs.

Each column represents supraballs made of different sizes of CS-SMNPs. (A) SEM images of whole supraball morphologies. (B) High-resolution SEM images of top surfaces of supraballs. (C) Cross-sectional TEM images of the inner structure of supraballs. Scale bars, 2 μm (A), 500 nm (B), and 500 nm (C).

Fig. 5. Supraballs from binary CS-SMNPs.

(A to C) Optical images, SEM images of top surface of supraballs, and cross-sectional TEM images for supraballs consisting of 160/0- and 160/36-nm CS-SMNPs (A), 160/0- and 160/66-nm CS-SMNPs (B), and 160/36- and 160/66-nm CS-SMNPs (C). The mixing ratio was 1:1 by mass. Top: Real images of supraballs made of mixed CS-SMNPs and sketches of supraballs, illustrating the organization of CS-SMNPs. Scale bars, 500 nm. (D) Optical images of supraballs prepared using different mass ratios of 160/36- and 160/66-nm CS-SMNPs. Each black box represents the size of the area probed by the optical measurements (4 × 4 μm).