Fig. 1. (a) XRD spectra of TiN, TiN-Au, TiN-Ag and TiN-Au-Ag composite substrate; (b) Peak fitting graph of TiN-Au-Ag composite substrate; (c) Absorption spectra of different SERS substrate

Fig. 2. (a)—(d) FE-SEM image of substrate; (e)—(f) TEM image of Ag colloid solution; (g)—(i) EDS image of TiN-Au-Ag composite substrate

Fig. 3. HOMO-LUMO orbital and HOMO-LUMO gap of nicotinic acid

Fig. 4. 2D-FDTD simulation of the substrate
(a): Ag colloid solution; (b): TiN-Ag substrate;(c): TiN-Au substrate; (d): TiN-Au-Ag composite substrate

Fig. 5. (a) SERS spectra of nicotinic acid on different substrate; (b) Histogram of nicotinic acid on different substrate

Fig. 6. (a) SERS spectra of nicotinic acid at different pH value; (b) The structure charge of nicotinic acid with variation of pH; (c) SERS histogram of nicotinic acid at different pH value

Fig. 7. (a) SERS spectra of diffesent concentration of nicotinic acid adsorbed on the substrate; (b) Relationship between the logarithm of nicotinic acid concentration and logarithm of Raman intensity at 1 033 cm^-1

Fig. 8. (a) SERS spectra of different concentration of nicotinic acid in gypenoside solution; (b) Relationship between the logarithm of nicotinic acid concentration and logarithm of Raman intensity at 1 033 cm^-1