Optical characterization of silk protein films.
Bucciarelli A., Maniglio D., Mulloni V., Motta A., Quaranta A.
Silk proteins are versatile biopolymers that had been used since their introduction in biological applications. Due to its high transmittance in the visible spectrum, the silk fibroin protein has been used to develop optical sensors and optical fibers compatible with the biological environment. In recent years the necessity of a methodology to obtain a precise patterning on this protein has addressed the research on fibroin photocrosslinkable photoresists (FPP) and sericin photocrosslinkable photoresist (SPP) two modified proteins in which, using a conjunction reaction, methacrylate groups were added as side group on the protein chain. Films made of these materials can be patterned via contact photolithographic procedures, to realize ordered structures with micrometric resolution opening to the use of these proteins for applications in bio-optics. While for pure fibroin an extensive optical characterization has been done, FPP SPP and sericin have never been studied in detail despite their use to develop micro-optical components. In particular, an investigation on the refractive index has never been conducted. In this work we present the optical characterization of thin films produced by spin coating of low-concentration formic-acid solutions of sericin, FPP, and SPP on a silicon substrate. In particular, using a spectroscopic ellipsometer we were able to detect a variation in the refractive index due to the crosslinking process, and the influence of the presence of different radical initiators (IRGACURE 2959 and lithium phenyl(2, 4, 6-trimethylbenzoyl)phosphinate). Using pure fibroin and sericin films as a reference, we report an increase in the refractive index due to the presence of radical initiators and a further small increase due to the crosslinking process.