Fig. 1. Schematic diagram of substrate size

Fig. 2. Schematic diagram of sample titration position

Fig. 3. Electrochemical deposition device

Fig. 4. Fabricated SERS substrates.（a）Substrate 1；（b）Substrate 2；（c）Substrate 3.

Fig. 5. SEM characterization of the substrate.（a）Substrate 1；（b）Substrate 2；（c）Substrate 3.

Fig. 6. Dilution instruments and reagents

Fig. 7. Raman spectra of spike proteins and SERS substrates

Fig. 8. Raman spectra of protein solution at a concentration of 8×10^-6 mg/mL on substrate 1

Fig. 9. Spectral intensity of different substrates at 1 380 cm^-1.（a）Substrate 1；（b）Substrate 2；（c）Substrate 3.

Fig. 10. Average spectral intensity at 1 380 cm^-1 for different substrates

Fig. 11. Raman spectra of S proteins with different concentrations at Substrate 1.（a）1×10^-5 mg/mL；（b）8×10^-6 mg/mL；（c）6×10^-6 mg/mL；（d）4×10^-6 mg/mL；（e）2×10^-6 mg/mL；（f）1×10^-6 mg/mL.

Fig. 12. Spectral intensity of different substrates as a function of S protein concentration

Fig. 13. Relationship between spectral intensity and protein concentration of different substrates S proteins

Fig. 14. Quantification of S protein concentration.（a）Quantitative variation curves of S protein concentration with Raman spectral intensity；（b）Detection error.