Giulia Guidetti

Flexible Photonic Cellulose Nanocrystal Films

The fabrication of self-assembled cellulose nanocrystal (CNC) films of tunable photonic and mechanical properties using a facile, green approach is demonstrated. The combination of tunable flexibility and iridescence can dramatically expand CNC coating and film barrier capabilities for paints and coating applications, sustainable consumer packaging products, as well as effective templates for photonic and optoelectronic materials and structures.

Hierarchical Self-Assembly of Cellulose Nanocrystals in a Confined Geometry

Complex hierarchical architectures are ubiquitous in nature. By designing and controlling the interaction between elementary building blocks, nature is able to optimize a large variety of materials with multiple functionalities. Such control is, however, extremely challenging in man-made materials, due to the difficulties in controlling their interaction at different length scales simultaneously. Here, hierarchical cholesteric architectures are obtained by the self-assembly of cellulose nanocrystals within shrinking, micron-sized aqueous droplets. This confined, spherical geometry drastically affects the colloidal self-assembly process, resulting in concentric ordering within the droplet, as confirmed by simulation. This provides a quantitative tool to study the interactions of cellulose nanocrystals beyond what has been achieved in a planar geometry. Our developed methodology allows us to fabricate truly hierarchical solid-state architectures from the nanometer to the macroscopic scale using a renewable and sustainable biopolymer.

Controlling the Photonic Properties of Cholesteric Cellulose Nanocrystal Films with Magnets

The self-assembly of cellulose nanocrystals is a powerful method for the fabrication of biosourced photonic films with a chiral optical response. While various techniques have been exploited to tune the optical properties of such systems, the presence of external fields has yet to be reported to significantly modify their optical properties. In this work, by using small commercial magnets (≈ 0.5-1.2 T) the orientation of the cholesteric domains is enabled to tune in suspension as they assemble into films. A detailed analysis of these films shows an unprecedented control of their angular response. This simple and yet powerful technique unlocks new possibilities in designing the visual appearance of such iridescent films, ranging from metallic to pixelated or matt textures, paving the way for the development of truly sustainable photonic pigments in coatings, cosmetics, and security labeling.

The Self-Assembly of Cellulose Nanocrystals: Hierarchical Design of Visual Appearance.

By controlling the interaction of biological building blocks at the nanoscale, natural photonic nanostructures have been optimized to produce intense coloration. Inspired by such biological nanostructures, the possibility to design the visual appearance of a material by guiding the hierarchical self-assembly of its constituent components, ideally using natural materials, is an attractive route for rationally designed, sustainable manufacturing. Within the large variety of biological building blocks, cellulose nanocrystals are one of the most promising biosourced materials, primarily for their abundance, biocompatibility, and ability to readily organize into photonic structures. Here, the mechanisms underlying the formation of iridescent, vividly colored materials from colloidal liquid crystal suspensions of cellulose nanocrystals are reviewed and recent advances in structural control over the hierarchical assembly process are reported as a toolbox for the design of sophisticated optical materials.

Unexpected stability of aqueous dispersions of raspberry-like colloids

Aqueous colloidal suspensions, both man-made and natural, are part of our everyday life. The applicability of colloidal suspensions, however, is limited by the range of conditions over which they are stable. Here we report a novel type of highly monodisperse raspberry-like colloids, which are prepared in a single-step synthesis that relies on simultaneous dispersion and emulsion polymerisation. The resulting raspberry colloids behave almost like hard spheres. In aqueous solutions, such prepared raspberries show unexpected stability against aggregation over large variations of added salt concentrations without addition of stabilisers. We present simple Derjaguin–Landau–Verwey–Overbeek (DLVO) calculations performed on raspberry-like and smooth colloids showing that this stability results from our raspberries’ unique morphology, which extends our understanding of colloidal stability against salting. Further, the raspberries’ stability facilitates the formation of superspheres and thin films in which the raspberry colloids self-assemble into hexagonally close-packed photonic crystals with exquisite reproducibility.The ability to stabilise colloidal suspensions in solution against salt-induced aggregation is critical to many industrial applications, but it remains challenging at high salt concentration. To overcome this problem, Lan et al. introduce a raspberry-like colloidal particle with controllable morphology.

Block Copolymer Micelles for Photonic Fluids and Crystals

Block copolymer micelles (BCMs) are self-assembled nanoparticles in solution with a collapsed core and a brush-like stabilizing corona typically in the size range of tens of nanometers. Despite being widely studied in various fields of science and technology, their ability to form structural colors at visible wavelength has not received attention, mainly due to the stringent length requirements of photonic lattices. Here, we describe the precision assembly of BCMs with superstretched corona, yet with narrow size distribution to qualify as building blocks for tunable and reversible micellar photonic fluids (MPFs) and micellar photonic crystals (MPCs). The BCMs form free-flowing MPFs with an average interparticle distance of 150-300 nm as defined by electrosteric repulsion arising from the highly charged and stretched corona. Under quiescent conditions, millimeter-sized MPCs with classical FCC lattice grow within the photonic fluid-medium upon refinement of the positional order of the BCMs. We discuss the generic properties of MPCs with special emphasis on surprisingly narrow reflected wavelengths with full width at half-maximum (fwhm) as small as 1 nm. We expect this concept to open a generic and facile way for self-assembled tunable micellar photonic structures.

Research data supporting "Controlling the Photonic Properties of Cholesteric Cellulose Nanocrystal Films with Magnets"

The data are organized and grouped in dedicated .zip files for each Figure they contribute to. All figures (ToC, Figures 1-4 and SI_1-17) are present in high resolution in each sub-folder. Software for file extensions: .mi (Gwyddion), .mat, .m and .fig (MATLAB), .blend (Blender). The following Info is also available in README_OpenData.pdf: *** ToC. Original photographs (.jpg) Figure 1. Original photographs (.jpg) Figure 2. Original graphics 2A-F (.png, .fig) spectrometer settings (.mat) Original photographs (Figure 2K) Figure 3. Original microscopy photographs (Figures 3A-L) (.png) and scale bar (.png) Bertrand lens and k-space calibration with grating (.png, .xlsx, xls) Figure 4. Original SEM photographs (.tif) Figure S1. AFM (.tiff, .mi) Figure S2. Titration file (.xlsx, xls, .png) Figure S3. Original photographs (.jpg) Sample preparation and Phase diagram (.xlsx, xls) Figure S4. Original photographs (.jgp) evaporation rate (.xlsx, xls) Figure S5. Original photographs (.jgp) Figure S6. Original photographs (.jgp) Figure S7. Original photographs (.jgp) Figure S8. Original photographs and schematics (.png) Figure S9. Original photographs and schematics (.jpg, .png) Figure S10. Original SEM images (.tif) Figure S11. Original SEM images (.tif) Figure S12. Original SEM images (.tif) Figure S13. Original SEM images (.tif) Figure S14. Experimental magnetic field mapped of the tilted field geometry (.xlsx, .xls) Visualization of the magnetic field for tilted field geometries (.m, .png, .fig) 3D schematic of the two magnets with the iron plate (.png, .blend) Visualization of the magnetic field for simple geometries (.pdf, .png) Figure S15. Computing of the magnetic field for the tilted field geometry (magnetic_field_calculated_mapping.xlsx, .xls) Computed magnetic field for the tilted field geometry (TwoMagVertAndTwoMagUpsideDown.xlsx, .xls) Script in MATLAB to create the figures (.m) Original graphic files (.fig, .png) Figure S16. Original figures (.fig, .png) Script in MATLAB illustrating the formula used to create the fits (.m) fit datapoints (.txt) Figure S17. Original photography (.jpg)

Research data supporting "Shape Memory Cellulose-based Photonic Reflectors"

Summary of available data Original or unprocessed data is provided in support of the article “Shape Memory Cellulose-based Photonic Reflectors”. The data is structured into two folders, each correlating to a specific data type presented in the published article. Folder 1: Polarised optical microscopy (POM) The polarised optical microscopy images (.png) used in Figures 1 and in Figure 2 of the Manuscript are provided into their corresponding folders. The scale bar is included in each folder for reference together with an image of the fiber used to collect the spectra. Both folder contain also the reflectivity spectra (.mat) used to plot the graph in Figure 1 and in Figure 2. The films were imaged under with polarizing filters, as described in the Experimental section of the Manuscript. Folder 2: Scanning electron microscopy (SEM) Micrographs cellulose films cross-sections showing the chiral nematic arrangement of cellulose nanocrystals retained after the shape memory polymer infiltration (.tif). The folder ‘Figure 3’ contains high and low magnification images, used for Figure 3 of the Manuscript, the folder ‘Figure S2’ contains the low magnification images, used for Figure S2 of the Supplementary Information. The folder ‘Table 2’ contains the low and high micrographs used to extrapolate the data presented in Table 2 of the Manuscript.

Research data supporting “Flexible photonic cellulose nanocrystal films”

Original or unprocessed data is provided in support of the article “Flexible photonic cellulose nanocrystal films”. The data is structured into eight folders, each correlating to a specific data type presented in the published article. Folder 1: Fourier Transformed Infra-Red Spectroscopy (FT-IR) The data used to plot Figure 1b in the manuscript are provided (.xlsx and .pdf). They correspond to the FT-IR spectra of pure cellulose nanocrystals and cellulose nanocrystals with different surfactant loading, as per description. Folder 2: Scanning electron microscopy (SEM) Micrographs cellulose films cross-sections showing the chiral nematic arrangement of cellulose nanocrystals retained for all surfactant loadings are provided (.tif). The folder ‘Figure 2’ contains high magnification images, used for Figure 2 of the manuscript, the folder ‘Figure S6’ contains the images, used for Figure S6 of the supplementary information. Folder 3: Mechanical Tests The data used to plot the stress versus strain behaviour for different surfactant loading, Figure 4a in the manuscript, and different counter-ions, Figure 4b in the manuscript, are provided (.xlsx and .pdf). The folder contains also the original figures used for the insets, respectively with high surfactant loading, Figure 4a, and intermediate surfactant loading, Figure 4b. Folder 4: Polarised optical microscopy (POM) The polarised optical microscopy images (.png) used in Figures 3 of the manuscript and in Figure S5 of the supplementary information are provided into their corresponding folders. The scalebar is included in each folder for reference. The folder ‘Figure 3 contains also the reflectivity spectra (.mat) used to plot the graph in Figure 3b. The films were imaged under with polarizing filters, as described in the Methods section of the manuscript. Folder 5: Nuclear Magnetic Resonance (NMR) The NRM spectra used for Figure S1 of the supplementary information are reported with their corresponding labels (.jpg and .docx). Folder 6: Powder X-Ray Diffraction (PXRD) The data used to plot the graph in Figure S2 of the supplementary information are provided (.xlsx and .pdf). Folder 7: Suspension Stability The original photographs (.jpg) of vials containing cellulose nanocrystal suspensions in different solvents and corresponding polarity indexes (.txt and .docx) used for Figure S3 of the supplementary information are provided. Folder 8: Atomic force microscopy (AFM) This data used for Figure S4 of the supplementary information are provided. The micrograph reported in Figure S4 is provided (.mi and .png). Micrographs of the individual cellulose nanocrystals (.mi and .png) and the histograms that illustrate the distribution of lengths and heights determined from this sample (.png) Original .mi data that can be opened with the free software Gwyddion are provided in the subfolder ‘size distirbution’. The length and height size distributions were extracted from the .mi figures using Matlab.