Noncontact Metal–Spiropyran–Metal Nanostructured Substrates with Ag and Au@SiO₂ Nanoparticles Deposited in Nanohole Arrays for Surface-Enhanced Fluorescence and Trace Detection of Metal Ions

The fluorescence intensity of the ring-opened merocyanine (MC) form of spiropyran can be enhanced by metal nanostructures. However, direct contact between spiropyran and the metal results in fluorescence quenching, which will reduce the enhancement amplitude of MC fluorescence. Here, Ag nanoparticles (NPs), a TiO2 dielectric layer, a poly(methyl methacrylate)-spiropyran (PMMA-SP) layer, and Au shell-isolated NPs with SiO2 shells (Au@SiO2) are successively assembled on a Si wafer with micropyramids and nanoholes (Siph). The hierarchical structure of this substrate is similar to insects’ compound eyes with nano- and microstructures, which can reduce the reflection of incident light. Meanwhile, the Au/Ag-coupled plasmonic nanocavities can enhance the intensity of a local electromagnetic (EM) field. In addition, TiO2 and SiO2 layers can avoid the nonradiative energy transfer caused by the direct contact between spiropyan and the metal. Based on the above advantages, the fluorescence enhancement factor (EF) of this substrate is as high as 195, which is the maximum reported so far. Zn2+, Cu2+, and Sn2+ can also be detected on Siph/TiO2/PMMA-SP/Au@SiO2, whose detection limits are 0.054, 0.038, and 0.049 μM, respectively. Furthermore, this substrate can be reused to detect metal ions by taking advantage of the reversible isomerization of spiropyran. The bioinspired substrate has positive significance in improving the fluorescence intensity of MC, and its lack of specific recognition ability for metal ions is expected to be further improved by molecular modification of spiropyran.