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. 2021 Jun 10;12(1):3529.
doi: 10.1038/s41467-021-23813-6.

Controlled self-assembly of plant proteins into high-performance multifunctional nanostructured films

Ayaka Kamada #  1 , Marc Rodriguez-Garcia #  1   2 , Francesco Simone Ruggeri  1   3   4 , Yi Shen  1   5 , Aviad Levin  1 , Tuomas P J Knowles  6   7
Affiliations

Controlled self-assembly of plant proteins into high-performance multifunctional nanostructured films

Ayaka Kamada et al. Nat Commun. .

Abstract

The abundance of plant-derived proteins, as well as their biodegradability and low environmental impact make them attractive polymeric feedstocks for next-generation functional materials to replace current petroleum-based systems. However, efforts to generate functional materials from plant-based proteins in a scalable manner have been hampered by the lack of efficient methods to induce and control their micro and nanoscale structure, key requirements for achieving advantageous material properties and tailoring their functionality. Here, we demonstrate a scalable approach for generating mechanically robust plant-based films on a metre-scale through controlled nanometre-scale self-assembly of water-insoluble plant proteins. The films produced using this method exhibit high optical transmittance, as well as robust mechanical properties comparable to engineering plastics. Furthermore, we demonstrate the ability to impart nano- and microscale patterning into such films through templating, leading to the formation of hydrophobic surfaces as well as structural colour by controlling the size of the patterned features.

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Conflict of interest statement

The work described in this paper has been the subject of a patent application filed by Cambridge Enterprise (CE), a fully owned subsidiary of the University of Cambridge. T.P.J.K. is a member of the board of directors of Xampla, Ltd, a Cambridge spin-off company commercializing sustainable materials. The remaining authors declare no competing interests.

Figures

Fig. 1
Fig. 1. Solvation of plant proteins and generation of films through molecular self-assembly.
a A translucent aqueous soyprotein isolate (SPI) solution (10 w/v% SPI, 30 v/v% acetic acid) was obtained via ultrasonication treatment at elevated temperature (90 °C) for 30 min. The resultant SPI solution was cast on a pre-heated glass Petri dish. Upon cooling, a translucent hydrogel was formed. Following evaporation of the solvent, a free-standing film was obtained (scale bar 1 cm). b–d AFM images (b, c), and TEM image (d) of SPI fibrillar aggregates formed through the self-assembly process. Scale bars are 500 nm for b and d, and 100 nm for c. e SEM image of SPI hydrogel prepared through supercritical CO2 drying. Scale bar is 500 nm. f cryo-SEM image of SPI hydrogel. Scale bar is 500 nm. g TEM image of β-sheet nanocrystals in dried SPI film. Scale bars are 5 nm for the main image and 2 nm for the inset.
Fig. 2
Fig. 2. Secondary structure analysis of SPI self-assembly.
a, b ATR-FTIR spectra of SPI solution during cooling down from 90 to 20 °C (a) and their second derivatives (b). c Relative change in the secondary structure during cooling down from 90 to 20 °C. d, e ATR-FTIR spectra of original SPI powder and the dried self-assembled film (d) and their second derivatives (e). f Quantification of secondary structure content calculated from Amide I band of IR spectra for original SPI powder and the dried self-assembled film. The indicated error bars are the s.d. of the average of three different spectra, each one is co-average of 256 scans.
Fig. 3
Fig. 3. Schematic representation of a proposed mechanism for the self-assembly of SPI.
Through the ultrasonication treatment in acetic acid solution, the initially insoluble aggregates are solubilised and unfolded, making them avaialble to form new intermolecular interactions. Upon cooling down, the new intermolecular β-sheets structures are formed. The removal of solvent results in the formation of β-sheet nanocrystals within the film.
Fig. 4
Fig. 4. Mechanical properties of the self-assembled SPI film.
a Representative stress–strain curves for the dried nonstructured and the dried self-assembled films. b Mechanical properties of self-assembled SPI film in comparison to previously reported biomaterials, engineered materials,,, and plant-based materials (see Supplementary Table 2 for references). c Zoomed-in graph for self-assembled (orange), nonstructured (blue), and previously reported SPI films (green, the details of references are in Supplementary Table 3).
Fig. 5
Fig. 5. Optical appearance of self-assembled films.
a UV–vis spectra of nonstructured (blue) and self-assembled films (orange). b Optical image of nonstructured (right) and self-assembled (left) SPI films. c Photograph of a 30 × 40 cm film fabricated through large-scale processing. d Carrying bag generated by thermal welding.
Fig. 6
Fig. 6. Plant protein film for coating with barrier function.
a Schematic illustration of the coating process. A piece of paperboard was dipped in the SPI solution and pulled out slowly, leading to the formation of a gel coating its surface. The paperboard was allowed to dry at room temperature to achieve an anhydrous thin layer of coating. b Optical images of paperboards before and after SPI coating. c, d SEM images of paperboard without (c) and with (d) SPI coating. Scale bars represent 500 μm. e Water uptake of the treated and untreated paperboards measured over 30 min studied through gravimetry. The indicated error bars represent the s.d. of the average of three independent measurements. f, g Colour changes of CoCl2-stained paperboards before and after immersing in water for 5 s (Supplementary Movie 1). The paperboard was prepared without (f) and with (g) SPI coating, respectively.
Fig. 7
Fig. 7. Micro- and nanopatterning of plant protein film.
a Schematic illustration of the soft lithography process to pattern SPI film. b Optical image of the micro-patterned film. Scale bar represents 5 mm. c SEM image of the side view (top) and top view (bottom) of patterned micropillars. Scale bars represent 25 μm (top) and 100 μm (bottom). d Optical images of a water droplet on the film without (top) and with (bottom) micropillars. Scale bars represent 1 mm. e Water contact angles of the films, showing an increase of hydrophobicity in the patterned film. The indicated error bars are the s.d. of the average of three independent measurements. f, g Optical image of non-patterned (f) and nano-patterned photonic film (g). h, i SEM image of the nano-patterned film. Scale bars represent 20 μm (h) and 2 μm (i).
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