Simultaneous TG―FTIR studies show high thermal resilience in core organic matrix of molluscan gastropod shell of N. josephinia

Abstract

Nacre—a biological composite of alternating layers of aragonite platelets and organic matrices, derived from biomaterials like molluscan shells has recently attracted a lot of interest as a potential substitute for ceramic material in orthopedic tissue engineering. It is formed as a result of interesting biomineralization processes. Molluscs use a striking variety of structural motifs to build their shells composed of calcium carbonate and a small percentage of proteins. These integral proteins determine the structural organization and properties of the mineralized composite. However, the composition and thermal stability of these proteinous matrices remains a mystery. Until now it was presumed that the organic matrix in a shell degrades completely up to 500 °C (stage―I). Stage―II comprises degradation of CaCO3 to CaO spanning about 600 °C to 850 °C. However, in case of a gastropod mollusc shell of Neverita josephinia, stage―III was also observed, in which majority weight loss of core organic proteinous matrix (OPM) exceeding 6 wt%, starts after about 1000 °C of thermal heating. It is observed that the core organic matrix (OM) has varied composition throughout the shell, stronger bonding, complex topology and has very high thermal resilience even up to 1400 °C. Simultaneous analysis of evolved gases by FTIR spectrometer coupled with thermogravimetric analyzer shows the composition of non-mineral species is more than 8 wt%. The results are supported by room temperature as well as gas phase by FTIR spectroscopy, XRD, SEM, CHNS analyses. The XRD spectra of the pristine shell powder and that calcined at 550 °C reveal aragonite to calcite phase transformation in calcium carbonate. The SEM shows the cross-lamellar morphology of the gastropod mollusk shell. There are two different sets of organic matrices: one with complex topology and a very tight binding to the mineral components (intra-crystalline), and the other with a loose binding to the mineral (inter-platelet area). Elemental analysis (CHNS) data, also supports this. This study identifies that the organic matrix, comprising proteins, peptides, lipids, and carbohydrates, exhibits complex topology and strong mineral bonding, contributing to the shell's notable mechanical strength and resistance to thermal degradation. The implications of this high thermal resilience for the shell's mechanical properties and potential biomineralization processes are discussed, suggesting a reevaluation of the organic matrix's role in molluscan shells.