Fig. 1. Schematic diagram of AgNPs@PMMA/P-Si SERS substrate.

Fig. 2. (a) SEM image of P-Si substrate; (b) SEM image of AgNPs@PMMA/P-Si substrates with AgNPs concentration of 0.75 g/mL in different magnification; (c) size distribution of AgNPs; (d) UV-vis spectra of AgNPs and AgNPs@PMMA solution.

Fig. 3. (a)−(e) SEM images of AgNPs@PMMA/ P-Si substrates with different concentrations of AgNPs in PMMA acetone solution: (a) 0.55 g/mL; (b) 0.65 g/mL; (c) 0.75 g/mL; (d) 0.85 g/mL; (e) 0.95 g/mL

Fig. 4. (a) SERS spectra of R6G (10^–7 mol/L) on the AgNPs@PMMA/P-Si substrates fabricated with different concentrations; (b) corresponding to the Raman peak at 614 and 774 cm^–1 for the R6G molecule in Fig. (a).

Fig. 5. (a) SERS spectra of R6G with concentrations from 10^–10 to 10^–14 mol/L on the AgNPs@PMMA/P-Si with AgNPs concentration of 0.75 g/mL substrate; (b) the intensity of R6G peak at 614 cm^–1 and 1365 cm^–1 changes as a function of R6G molecule concentration on the AgNPs@PMMA/P-Si substrates; (c) schematic illustration showing the Raman detection of SERS substrates; (d) comparison of the detection limit of the R6 G molecule on the 3D composite SERS substrate (10^–13 mol/L) and the detection limit (10^–4 mol/L) on the planar Si substrate.

Fig. 6. (a) Average SERS spectrum of the 10^–7 mol/L R6G molecules from 10 random positions in the base tower of AgNPs@ PMMA/P-Si with concentration of 0.75 g/mL substrate (red line is the average of 10 SERS spectrum lines); (b) statistical diagram of variation of Raman peak intensity at 614 cm^–1 is in the corresponding Fig. (a).

Fig. 7. (a) The Raman spectra of R6G molecule with 10^–7 mol/L on the AgNPs@PMMA/P-Si, AgNPs@PMMA/F-Si, and AgNPs/P-Si substrates; (b) the intensity of the signal at 614 cm^–1 of R6G (10^–7 mol/L) collected with 18 days every three days respectively on the AgNPs@PMMA/P-Si and AgNPs/PMMA/F-Si substrate.