Silk is a high molecular weight natural polymer with a longstanding use in the textile industry and in biomedical applications owing to its excellent mechanical properties and biocompatibility(1). Structurally, silk consists mainly of a fibrous protein, fibroin, characterized by a high content of glycine, alanine and serine aminoacidic residues, and a coating glue-like protein, sericin, generally removed by degumming in most applications (2).Herein, we present electrospun fibrous mats of poly(ethylene oxide)/silk fibroin doped with spiropyran molecules that yield novel self-indicating smart textiles and biofunctional membranes for the recognition of metal cations in solution and for acidic gas sensing.
The combination of a nanostructured porous matrix with the acido- and metallochromic response of the SP dopants allows fast diffusion of the acid vapors/ ion solution within the material coupled with a fast optical and visual detection of the ongoing interactions between the composite and the analyte of interest. Specifically, the sensing mechanism is activated on demand upon UV irradiation through the reversible photoconversion of the neutral spiropyran (SP) species to its zwitterionic merocyanine (MC) isomer. The negatively charged oxygen on the phenolic moiety of MC can then undergo protonation upon exposure to acidic vapors of both inorganic and organic acids (3) as well as complexation in the presence of environmentally and biologically important cations, such as zinc and heavy metal ions. The subsequent generation of the proton/metal coordinated merocyanine produces a distinct variation in the emission spectrum and a chromic transition of the substrate. Moreover, the formation of such complexes is reversible and the pristine SP form can be restored upon depletion of the acid/ion or through visible irradiation, providing photochromic reusable sensing platforms that can be switched on and off repeatedly. Additionally, we demonstrate that the photochromism of SP can be further exploited to control the mechanical properties of the material through light activation since a significant difference in the elastic modulus is observed depending on the predominance of the SP or of the MC isomer in the composite.
1) F. Zhang, B. Zuo, Z. Fan, Z. Xie, Q. Lu, X. Zhang and D. L. Kaplan, Biomacromolecules, 2012, 13, 798–804.
2) S. Hofmann, C.T. Wong Po Foo , F. Rossetti, M. Textor, G. Vunjak-Novakovic, D. L. Kaplan, H.P. Merkle and L. Meinel, J. Control. Release, 2006, 111, 219-227.
3) M.E. Genovese, A. Athanassiou and D. Fragouli, J. Mater. Chem. A, 2015, 3, 22441–22447.