Fig. 7. Hydrophobic substrate characterization in SERS. A. Hydrophobic fibrous paper-based plasmonic substrate for SERS. (a) Photograph of paper substrate: as large as 13 × 30 cm² with high smoothness and uniformity. Inset: paper can be freestanding and flexible. (b) Contact angle measurement, and (c) low-magnification and (d) high-magnification SEM images of PLLA nanofibrous paper prepared via electrospinning. (e) TEM image and (f) absorption spectra of gold nanorods. (g) Contact angle measurements of PLLA nanofibrous paper after dropping GNRs–CTAB solution with increased CTAB concentration. CTAB = cetyl-trimethylammonium bromide. PLLA = poly(l-lactic acid). Reprinted with permission from Shao (2015) © American Chemical Society.²¹⁸ B. Chemically treated hydrophobic filter paper for SERS sensing in pesticide detection. (a) Surface roughness analysis of filter paper and calendered filter paper (Parker Print Surf instrument). (b) Contact angle of calendered filter paper and alkyl ketene dimer (AKD)-treated filter paper. Inset: photographs are water droplets on calendered filter paper and AKD-treated filter paper, respectively. (c) SERS spectra of AgNP spots on filter paper and AKD-treated filter paper treated by 5 μL of1 μM 4-aminothiophenol solution. Black line: SERS spectra of AgNP spots on filter paper. Red line: SERS spectra of AgNP spots on AKD-treated filter paper. Reprinted with permission from Lee (2018) © American Chemical Society.²¹⁹ C. Wetting behavior in different regimes on a hierarchical nanostructured surface (nm–μm range). (i) lotus, (ii) rose, (iii) Cassie and (iv) Wenzel. Further details of other regimes are given in Bhushan (2010).²⁰⁸ © M. Hardy 2021.