Surface coating of freshwater cultured pearls with silk fibroin solution

Abstract

Silk fibroin solutions are prepared using alkaline solution (LiBr, NaOH and KOH) by dissolving silk fibers from waste fabric industry or local community silk industry. From the experiment, it was found that the optimal silk fibroin solution can be prepared from 2 g (2% w/v) of silk fibroin fiber dissolved in 0.35 M NaOH and adjusted the pH of solution to neutral pH for the coating. The price of NaOH is 7.5 times cheaper than that of LiBr and KOH. The concentration of NaOH used was also lower. These helped reduced costs, chemicals and waste from the preparation, the time for the dissolving and the time of the dialysis method. After that, the silk fibroin solution was coated on the surface of low quality freshwater cultured pearl followed by methanol treatment, then the coated objects were analyzed by ATR FT-IR and Raman microspectroscopy (both techniques are non- destructive). The results elucidated the stick of silk fibroin on pearl surface by observing characteristic peaks of N-H stretching, Amide I and the shoulder peaks of Amide II and Amide III. Methanol extracts water from molecular silk fibroin which causes the changing of the amorphous structure to crystalline. The silk fibroin became water insoluble so that the effectiveness of silk fibroin adhesion on pearl surface could be unproved. Moreover, silk fibroin film on pearl surface protect surface of pearl from heat, solvent and detergents. So, silk fibroin increases the luster of pearl surface.