Fig. 1. A schematic depicting the various parameters influencing the SERS signal.

Fig. 2. The ideal requirements of SERS substrates are summarized in this schematic.

Fig. 3. Year wise publications on flexible SERS substrates obtained through a search in SCOPUS.

Fig. 4. Explosive trace detection using flexible SERS substrates detection of TNT, RDX, and PETN using self-assembly triangular nanoprisms on adhesive tape. Figure reproduced with permission from ref.⁵⁸, Royal Society of Chemistry.

Fig. 5. Various fabrication techniques used for paper-based SERS substrates. (a) Vapor deposition. (b) Inkjet printing. (c) Dipping. (d) Pen-on-paper. (e) Drop-casting on hydrophilic wells. (f) Self assembling. (g) In-situ reduction. Figure reproduced with permission from: (a) ref.⁷⁷, (b) ref.⁷², The Royal Society of Chemistry; ref.⁷¹, American Chemical Society; (d) ref.⁷³, John Wiley and Sons; (e) ref.⁸⁰, Springer Nature; (f) ref.⁸⁴, (g) ref.⁷⁹, American Chemical Society.

Fig. 6. Filter paper based SERS substrate by aggregated Ag/Au NPs for explosive molecule detection (Left side) (a) schematic of substrate preparation (b) and (c) FESEM images of bare filter and aggregated Ag NPs (Right side) SERS spectra of (a) PA (b) DNT (c) NTO using FP with optimized aggregated Ag NPs. Figure reproduced with permission from ref.⁹⁴, American Chemical Society.

Fig. 7. (a) A schematic of the synthesis of dual-functional PDMS-assisted paper-based SERS platform. (b) (i) The photograph of a sample collection from orange surface. (ii) A comparison of SERS spectra of CV with and without PDMS. (iii) SERS spectra of different concentrations of thiram (0.5−50 ppm). (iv) The peak intensity at 1380 cm⁻¹ of thiram in orange juice as a function of the spiked sample concentration. Figure reproduced with permission from ref.⁹⁵, Royal Society of Chemistry.

Fig. 8. Fabrication of flexible SERS substrates for Ag@T-A@SiO₂-Au nanofibrous substrates. Figure reproduced with permission from ref.¹⁰⁰, under a Creative Commons Attribution 4.0 International License.

Fig. 9. (a) Reflectance spectra of the ASFPAN nanofibrous membranes with Ag NPs; Photographs of three nanofibrous membranes (PAN, ASFPAN, and ASFPAN-Ag NPs) are shown in the inset. (b) SEM image and (c) TEM image of ASFPAN nanofibers (3 min). Inset in (c) shows the size distribution of Ag NPs.¹⁰⁷, American Chemical Society.

Fig. 10. (a) Schematic diagram representing the fabrication process of Au covered polymer nanostructure arrays using roll-to-roll ultraviolet nanoimprint lithography (R2R UV-NIL) technique (b) and (c) SERS spectra of R6G from 30 nm Au coating flexible substrate at different bending angles and bending cycles, respectively. Figure reproduced with permission from ref.¹¹⁴, under a Creative Commons Attribution 4.0 International License.

Fig. 11. (a) Schematic of flexible non-woven fabric based substrate and the (b) SERS spectra of carbyl on apples, oranges, and bananas surfaces. Figure reproduced with permission from ref.¹²⁹, under a Creative Commons Attribution 4.0 International License.

Table 1. Important review articles on various applications of SERS that have been reported in the last three-years (2019–2021).

Table 2. Summary of the recent flexible SERS substrates, their preparation methods, materials used, and the sensitivities achieved (2014-2021).

Table 3. A summary of the commercially available SERS substrates, their costs, sensitivities and their stability (non-exhaustive).