Nonclassical Crystallization of Goethite Nanorods in Limpet Teeth by Self-Assembly of Silica-Rich Nanoparticles Reveals Structure–Mechanical Property Relations

Sat, 04 May 2024 09:29:32 +0800

In May 2024, Dr. Zhaoyong Zou and colleagues from Wuhan University of Technology published a research paper titled “Nonclassical Crystallization of Goethite Nanorods in Limpet Teeth by Self-Assembly of Silica-Rich Nanoparticles Reveals Structure–Mechanical Property Relations” in the Journal of Colloid and Interface Science.

1. Research Background

Through billions of years of evolution, organisms have developed remarkable biomaterials with intricate structures and superior mechanical properties through biological processes. Unlike traditional ceramic materials that require extreme conditions such as high temperature and high pressure for synthesis, these biological materials form under relatively mild physiological conditions. Understanding the formation processes of these biomaterials and their structure-property relationships provides valuable inspiration for the development of advanced materials.

Biomineralization is a common strategy employed by organisms to form hard tissues with ordered hierarchical structures and excellent mechanical properties. In addition to widely existing calcium-based and silicon-based minerals, iron-based minerals are also commonly found in aquatic and terrestrial organisms, such as magnetotactic bacteria, chitons, and limpets. The teeth of limpets consist of chitin fibers (α-chitin) that form a mineralization template, with goethite (α-FeOOH) crystals nucleating and crystallizing on these fibers, along with amorphous silica (SiO₂·nH₂O). Limpet teeth typically wear out within 12-48 hours, with new teeth rapidly replacing them. Teeth at different developmental stages coexist on the same radula, facilitating comparative studies. The tensile strength of mature teeth ranges from 3.0 to 6.5 GPa, far exceeding that of spider silk, considered the strongest natural material. This exceptional mechanical performance stems from the complex composition and structure of goethite crystals within the teeth.

While Fe(III) rarely forms goethite spontaneously under normal conditions unless exposed to extreme high temperatures and pH levels, biological synthesis of goethite occurs under remarkably mild conditions. The mechanisms of goethite nucleation and crystallization in limpet teeth, as well as the factors regulating crystal growth, remain poorly understood. Therefore, understanding the crystallization mechanism of goethite in limpet teeth is crucial for developing new material manufacturing techniques and advanced materials.

2. Research Content

In this study, the research team selected the Chinese coastal species Cellana grata as the model organism. Based on optical imaging, the radula was divided into four distinct developmental stages: Stage I (unmineralized teeth), Stage II (early mineralization with increasing mineral content), Stage III (advanced mineralization), and Stage IV (fully mature teeth). Special attention was given to Stage I and II teeth after removing the brown outer membrane, with Stage II further subdivided into three critical sub-stages (Stage II-1 to II-3) to examine mineralization differences.

Figure 1. Radula and morphological differences of teeth at different developmental stages in limpets.

The mature teeth were divided into leading part (LP) and trailing part (TP), with LP development preceding TP. Mature teeth primarily consist of goethite and amorphous silica, with silicon enriched in LP and iron concentrated in TP. At the junction, elemental distribution is minimal. TP features large nanorods aligned axially with uniform orientation, while LP contains smaller nanorods arranged radially and axially, embedded within a silica matrix. Raman spectroscopy revealed greater orientation differences between LP and TP, with higher crystallinity or fewer impurities in LP goethite crystals.

Figure 2. Morphological and compositional comparisons of different regions in mature limpet teeth (Stage IV).

Mechanical testing showed that LP exhibits higher hardness and elastic modulus than TP, with the junction region showing the lowest mechanical properties. The maximum hardness and elastic modulus of LP reached 5.22 GPa and 54.4 GPa, respectively. These findings indicate that the enhanced mechanical properties correlate positively with silica enrichment, increased crystallinity of goethite crystals, and denser arrangement of nanorods.

Figure 3. Micromechanical properties of different regions in mature limpet teeth (Stage IV).

Further investigation of crystallization and orientation across developmental stages revealed that Stage I teeth consist of non-mineralized α-chitin. In Stage II-1, goethite diffraction peaks gradually emerge above the broad chitin peaks. As mineralization progresses through Stages II-2, II-3, and III, peak intensities increase while the width of the goethite (111) peak narrows, indicating improved crystallinity. By Stage III, WAXS data closely resembles Stage IV, suggesting mineralization is nearly complete. Azimuthal intensity distributions showed that the chitin (013) reflection aligns with the goethite (110) reflection, indicating chitin fibers play a critical role in mediating goethite crystal growth.

Figure 4. Crystallization and orientation studies of goethite crystals in limpet teeth at different developmental stages.

Mineral deposition analysis revealed that Stage I teeth contain both mineral particles and nanorods on the chitin template. During Stage II sub-stages, LP develops first with numerous nanoparticles attached to nanorods. Subsequently, nanorod size increases, with TP nanorods rapidly forming as LP mineralization nears completion. This indicates Stage II represents the most intense mineralization phase, with nanoparticles serving as critical components in nanorod crystallization.

Figure 5. Morphological and structural comparisons of Stage I and II limpet teeth.

TEM observations showed that early mineral structures consist of fine nanorods formed by nanoparticle assembly. Subsequent attachment of amorphous nanoparticles to nanorods creates crystalline domains with clear lattice fringes. Initial nanorod precursors contain uniformly distributed iron, silicon, and oxygen, indicating composite materials of amorphous iron oxide/hydroxide and silica. As crystallinity increases, mature nanorods become predominantly composed of iron and oxygen, with silicon expelled to the surface. These results demonstrate the crucial role of amorphous silica in goethite nanorod formation.

Figure 6. Crystallization growth changes of goethite nanorods in Stage II limpet teeth LP.

3. Conclusion and Outlook

This work investigated the crystallization pathway of goethite crystals in limpet teeth and their structure-mechanical property relationships. Results show distinct compositional, structural, and mechanical differences between LP and TP regions of mature teeth. Although LP contains smaller goethite nanorods than TP, their denser arrangement and higher silica content result in superior hardness and modulus. Importantly, goethite crystal growth follows a nonclassical crystallization mechanism where amorphous nanoparticles first aggregate to form nanorods, which then gradually transform into single crystals. Silica plays a crucial role in this process by stabilizing initial amorphous nanoparticles and inhibiting goethite nanorod growth.

These findings provide significant insights into the formation process of limpet teeth and the nonclassical crystallization mechanisms of iron-based biominerals. They offer new understanding of silica-mediated structure-mechanical property relationships and valuable references for developing biomimetic materials with superior mechanical properties.

This research was supported by the National Natural Science Foundation of China (Grant No. 52172287) and the Key Special Project for “Transformative Technologies” (Project No. 2021YFA0715700).

Yan Lu, a 2020 master’s student at Wuhan University of Technology, is the first author of the paper, with Dr. Zhaoyong Zou as the corresponding author.

Paper Information: Yan Lu, Luyao Yi, Zeyao Fu, Jingjing Xie, Qunfeng Cheng, Zhengyi Fu, Zhaoyong Zou*, Nonclassical crystallization of goethite nanorods in limpet teeth by self-assembly of silica-rich nanoparticles reveals structure–mechanical property relations. Journal of Colloid and Interface Science, 2024, 669: 64-74.

Limpet Teeth | Bioprocess Inspired Fabrication

Nonclassical Crystallization of Goethite Nanorods in Limpet Teeth by Self-Assembly of Silica-Rich Nanoparticles Reveals Structure–Mechanical Property Relations

1. Research Background

2. Research Content

3. Conclusion and Outlook