Ro(a)d to New Functional Materials: Sustainable Isolation of High-Aspect-Ratio β-Chitin Microrods from Marine Algae

Abstract

High-aspect-ratio rod-shaped chitins such as chitin whiskers or chitin nano- and microfibers are particularly promising for a wide range of applications, including electrorheological suspensions, lightweight reinforcement material for biocomposites, biomedical scaffolds, and food packaging. Here, we report the first mild water-based mechanical extraction protocol to isolate β-chitin microrods from the marine algal species Thalassiosira rotula while preserving their structural integrity throughout the process. The resulting microrods could be distributed into two populations based on the fultoportulae from which they are extruded. The rods exhibit typical dimensions of 12.6 ± 4.0 µm in length and 75 ± 21 nm in diameter (outer fultoportulae) or 17.5 ± 4.7 µm in length and 170 ± 39 nm in diameter (central fultoportulae), yielding high aspect ratios of ~168 and ~103 on average, respectively. Due to this environmentally friendly extraction, the high purity of the synthesized chitin, and the renewable algal source, this work introduces a sustainable route to produce pure biogenic β-chitin microrods.

Keywords: HAADF-STEM; Thalassiosira rotula; chitin microrods; high aspect ratio; sustainable biogenic nanomaterials; β-chitin.

Figure 1

Thalassiosira rotula cells and chitin microrods. (a) Light microscopy images of Thalassiosira rotula cells connected by extracellular chitin rod bundles (black arrow). (b) SEM image of a Thalassiosira rotula cell showing the central chitin rod formation apparatus (white arrow). (c) Magnification of the area around the central chitin rod formation apparatus consisting of multiple central fultoportulae, as well as the outer fultoportulae distributed on the biosilica valve surface. (d) Higher magnification of one of the Thalassiosira rotula composite chitin fibers consisting of bundles of individual rods originating from central fultoportulae, highlighting the hierarchical properties of diatom chitin fibers.

Figure 2

Electron micrograph of chitin rods from Thalassiosira rotula isolated with the water-based mechanical extraction method and statistical evaluation of extracted chitin rods. (a) Exemplary SEM image of a rod sample. White arrows point to instances where rods bend, and the red arrows point to locations where the rods unwind, showing that they constitute multiple fibrils. (b) Scatterplot of length and diameter of n = 100 individual rods measured from multiple SEM images. Gaussian mixture modeling (mclust) was used to classify rods into two populations corresponding to synthesis from outer (blue) and central (red) fultoportulae (FP). (c) Bimodal distribution of chitin rod lengths. Classification by Gaussian mixture modeling (mclust) resulting in two populations corresponding to shorter (purple) and longer (green) chitin rods. Mean values and standard deviation are provided in the legend. The dotted line represents the cutoff of 16.95 µm. (d) Bimodal distribution of chitin rod diameters separated into outer (blue) and central (red) fultoportula populations. Solid curves represent Gaussian fits calculated with the mclust package in R. The dotted line represents the cutoff of 107 nm between the two populations determined by Gaussian mixture modeling. Mean values and standard deviation are provided in the legend. (e) Rod length distributions divided into FP populations. Mean lengths of the two populations are provided beneath the boxplots. p-values were calculated using a two-sample t-test in R (**** = 3.1 × 10⁻⁶).

Figure 3

HAADF-STEM images of the chitin rods isolated from Thalassiosira rotula using the water-based extraction procedure, showing a nanofibrillar structure in the chitin rods. (a) Two chitin rods crossing. (b) Magnification of (a). The white arrow points to a secondary nanofibril protruding from the main rod.