Fig. 1. Statistics and SEMs of different print layers. (a) Single layer; (b) three layers; (c) five layers; (d) seven layers; (e) nine layers

Fig. 2. Discussion on preparation parameters of inkjet printing. (a) Optical photos of silver sol with different multiples; (b) Raman test results of R6G with concentration of 10^-6 mol/L under silver sol with different concentration multiples; (c) relationship between intensity of 611 cm^-1 characteristic peak and concentration multiple; (d) optical photos with different printing layers; (e) Raman test results of R6G with concentration of 10^-6 mol/L under SERS substrate with different printing layers; (f) relationship between intensity of 611 cm^-1 characteristic peak and number of printed layers

Fig. 3. Raman properties of SERS substrate. (a) SERS spectra of R6G with different concentrations when printing 7 layers; (b) test results of large-area multipoint Raman spectra; (c) corresponding calculated RSD at 611 cm^-1 when printing seven layers; (d) RSD test results of different printing layers

Fig. 4. Discussion of paper hydrophobic treatment. (a) Optical photo before hydrophobic treatment; (b) optical photo after hydrophobic treatment; (c) mathematical model of cross-section before hydrophobic treatment; (d) mathematical model of cross-section after hydrophobic treatment; (e) SERS spectra of untreated paper for different R6G concentrations

Fig. 5. Flexible characteristic experiment. (a) Substrate folding Raman test; (b) substrate folding uniformity test; (c) schematic diagram of process of simulating actual testing of pollutants on apple surface; (d) Raman test of apple surface pollutants; (e) polymolecular test

Fig. 6. Electromagnetic field simulation under different numbers of silver nanoparticles. (a) Single silver nanoparticle; (b) two silver nanoparticles; (c) three silver nanoparticles; (d) five silver nanoparticles; (e) seven silver nanoparticles; (f) nine silver nanoparticles