Self-assembly of lignin nanoparticles to photonic crystal Jinrong Liu , Mathias Nero, Kjell Jansson, Tom Willhammar, Mika H. Sipponen* Department of Materials and Environmental Chemistry, Stockholm University, Sweden Biomass resources are needed to develop green and sustainable functional materials. Among them, lignin is the second most abundant component of wood and the main by-product of the pulping industry 1 . In this project, I develop lignin nanoparticles (LNPs) as photonic crystals. Photonic crystals are novel optical materials because of the periodic arrangement of dielectric materials, which is one type of structural coloration. The periodic structure allows specific electromagnetic waves to propagate and specific ones to be forbidden, which gives rise to a photonic band gap (PBG) 2 . When the bandgap corresponds to some wavelength in visible range (400-800 nm), optical properties occur in the crystal. One advantage of LNP is its surface potential (-37 mV) which stabilizes the colloidal dispersion, which therefore permits avoiding the use of a surfactant. However, the strict requirement of particle size distribution (<5%) 3 is one main challenge for achieving close-packed structures. Especially for preparing LNPs it is difficult to achieve monodispersity by solvent shifting method because of challenges of separating and controlling nucleation and growth rate of particles. In this project, I first used solvent extraction method to extract ethanol-insoluble and acetone-soluble fraction of soda lignin. The LNP prepared from the acetone-soluble fraction showed narrower size distribution than the original soda lignin. Then I developed a centrifugation method to facilitate assembly of the particles into photonic crystals. The centrifugation assistance is important during self-assembly, because it applies force to LNP and overcomes the Brownian motion of particles. Therefore, particles arrange during the deposition process. Transmission electron microscopy (TEM) images show that LNPs are semi-hexagonally close-packed. The assembly of particles is likely driven by minimizing total interaction energy. After assembly of LNP, red, orange, yellow, green, blue, indigo, and violet colours were observed in different layers of the LNP crystals. The iridescent structural colours come from the coherent scattering of ordered photonic structures. Reflectance spectra of the crystals corresponded to the observed colours’ wavelength ranges. Atom force microscope (AFM) images of the surface of LNP crystal show hexagonal close-packed particles, which can potentially serve as waveguide and reflective surfaces according to Bragg’s law. References 1. Wang J, Zhang D, Chu F. Wood-Derived Functional Polymeric Materials. Adv Mater . 2001135 , 1-21 (2020). 2. Cai Z, Li Z, Ravaine S, et al. From colloidal particles to photonic crystals: Advances in self-assembly and their emerging applications. Chem Soc Rev . 50 , 5898-5951 (2021). 3. Jiang P, Bertone JF, Colvin VL. A lost-wax approach to monodisperse colloids and their crystals. Science 291 , 453-457 (2001).
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