Biomineral photonic materials with a gradient structure: The ligament of the pearl oyster Pinctada margaritifera

Abstract

The bivalve fibrous ligaments (FLs) are unusual biomineralized materials evolved to open two shell valves as needed. For a long time, the inside beautiful structural colors of the FLs have received little attention. Here, using optical microscopy, SEM, and fiber optic spectrometry combined with the image processing method and pair correlation function (Pcf) analysis, we investigate the structure and optical property of the FL of the pearl oyster Pinctada margaritifera. In this FL, we find that: (1) the aragonite fibers are uniform in diameter with an average of 98 ± 10 nm; (2) they are spatially organized with short-range order; and (3) they vary gradiently in their nearest-neighbor distance (d) from 148 nm to 103 nm along the lateral-inner direction of the FL. Therefore, this FL possesses a 2D amorphous gradient photonic structure, which results in that the FL displays extremely variable structural colors spanning from blue to yellow. In short, this work provides a new model of biological photonic structure, which may inspire the design and synthesis of novel photonic materials.