Photochromic spiropyran-doped silk fibroin poly(ethylene oxide) nanofibers which combine the attractive properties and biocompatibility of silk with the photocontrollable and reversible optical, mechanical, and chemical response of the spiropyran dopants are herein presented. As proved, the reversible variation of the absorption and emission signals of the mats and of their Young's modulus upon alternate UV and visible light irradiation is ascribed to the reversible photoconversion of the spiropyran form to its polar merocyanine counterpart. Most importantly, the interactions of the merocyanine molecules with acidic vapors as well as with heavy metal ions dispersed in solution produce analyte-specific spectral changes in the emission profile of the composite, accompanied by a characteristic chromic variation. Because of the high surface-to-volume ratio of the nanofibrous network, such interactions are fast, thus enabling both an optical and a visual detection in a 30-60 s time scale. The sensing platform can be easily regenerated for more than 20 and 3 cycles upon acid or ion depletion, respectively. Overall, the photocontrolled properties of the silk composites combined with a straightforward preparation method render them suitable as porous materials and scaffolds with tunable compliance and reusable nanoprobes for real time optical detection in biomedical, environmental, and industrial applications.
Keywords: nanofibers; optical sensing; photochromism; pollutants; silk; spiropyrans.