*2.2. Preparation of Flexible SERS Substrate*

Synthesis of β-CD@AuNPs: β-CD@AuNP sol was prepared using β-CD as a reducing agent and stabiliser, according to Zhao et al. [36]. In brief, 5 mL of 0.1 M phosphate buffer (PB), 1 mL of 0.01 M chlorauric acid solvent, 10 mL of 0.01 M β-CD solution were successively added to 35 mL ultrapure water and stirred vigorously until fully mixed. The mixture was heated to a set boiling temperature and maintained at this temperature for 60 min. The scanning electron microscopy (SEM) images (Figure S1 in the Supporting Information) of the prepared β-CD@AuNPs showed that the particle size was uniform at approximately 20–25 nm.

Preparation of β-CD@AuNP/PTFE: The detailed steps for assembling β-CD@AuNPs on a hydrophobic smooth PTFE film were as follows. First, the PTFE film was glued to a 5 × 2 cm glass slide with double-sided adhesive. The slide was then adsorbed on the machine and 0.45 mL of a perfluorinated liquid was dispersed by spin coating at a low speed of 600 rpm for 30 s and a high speed of 1500 rpm for 1 min. The coated film was heated for 30 min to obtain a spare film. Finally, the concentrated 10 uL β-CD@AuNP colloidal solution was poured onto a hydrophobic PTFE film. During the drying process, the contact line shrank because of the low surface friction of the PTFE film. Eventually, the initial droplet was concentrated in the cell domain with a diameter of 0.5–1 mm (Figure S2 in the Supporting Information).

Preparation of β-CD@AuNP/tape: A two-dimensional NP array of β-CD@AuNPs was obtained using a simple liquid–liquid interface self-assembly method [19] and was subsequently transferred onto silicon wafers. A piece of adhesive tape (clipped using scissors) was then used to cover the nanoarray of β-CD@AuNPs, being pressed firmly for 3–5 s. The tape was gently peeled off from the surface of the silicon wafer and β-CD@AuNP/tape was formed by transferring the β-CD@AuNP array onto the tape.

Preparation of β-CD@AuNP/PDMS: Similarly, flexible β-CD@AuNP/PDMS was prepared by transferring monolayer NP arrays of β-CD@AuNPs using PDMS films instead of silicon wafers and air-drying at 25–30 ◦C.
