“…demonstrated a multilayer-nanoparticle-modified graphene oxide-coated MNF for sensing ethanol with a low detection limit (5.25 ppm) and a fast response (118 ms) [506] . In addition, many other functional films, such as black phosphorus [507] , WS2 layer [508] , and ternary cross-linked film (PVA-APTES-ICA) [509] , have been coated on MNFs for optical sensing.…”