Fig. 1. SEM images and particle size distribution of Au NPs. (a)--(d) SEM images; (e)--(h) particle size distribution

Fig. 2. SERS performance test of Au NPs with different particle sizes. (a) SERS spectra under different Au NPs particle sizes and R6G concentrations; (b) SERS peak area at 1363 cm^-1 under different Au NPs particle sizes and R6G concentrations; (c) UV spectra of Au NPs with different particle sizes

Fig. 3. Uniformity and reproducibility of Raman signal of 45 nm Au NPs enhanced substrate. (a) SERS graphs of R6G with a concentration of 10^-6 mol/L tested at 20 different substrate locations; (b) calculated relative standard deviation (RSD) of SERS peak at 1363 cm^-1

Fig. 4. pH sensing using R6G molecules adsorbed on the surface of Au NPs. (a) SERS spectra of R6G at different pH levels; (b) SERS spectra within the range of stretching vibration spectra of C—N and C—C bonds; (c) mechanism of molecular structure transformation of R6G at different pH levels; (d) schematic of pH-dependent SERS measurement, pH-induced orientation change of R6G molecules and schematic of molecular protonation

Fig. 5. Peak area ratio of SERS spectrum of R6G at 1363 cm^-1 to 1314 cm^-1. (a) Fitted curve between peak area and pH; (b) change of peak area ratio between pH=3 and pH=7; (c) relationship between peak area and time

Fig. 6. Peak area ratio of R6G at different pH levels after adding common metal cations