Origin of the common UV absorption feature in cultured pearls and shells

UV–Vis reflectance spectroscopy combined with field emission scanning electron microscopy (FE-SEM) was used to characterize the nacres of seawater and freshwater cultured pearls and shells. While distinct spectral features were observed for different types of pearls or shells, a characteristic absorption band at about 275 nm was identified in the UV–Vis spectra for the nacres of pearls and shells regardless of their growing environments or physical appearance. The UV absorption peak was no longer observable after the nacres were coated with Pt, indicating that the laminated structure of the nacre’s surface, which remains intact after coating, was not responsible for producing the peak. In addition, the peak was observed in different parts of the nacre where the thickness of the individual aragonite platelets varied significantly, indicating that the inner multilayered microstructure of nacre did not contribute to the formation of this characteristic absorption. It is therefore here suggested that the characteristic UV band of the nacres of pearls and shells originates from the organic matrix in nacre. Given the established key role of the organic matrix in facilitating the unique brick-and-mortar architectures of nacre, which is known for its superior mechanical properties, the present study could potentially provide the basis for designing advanced optical and biomedical materials.