Fig. 4. Solid-state Raman spectrum of PD 1 (top) as overview from 3500 to 500 cm⁻¹ and (bottom) with the relevant C C stretching region from 1800 to 1300 cm⁻¹ which was used for the subsequent SERS studies. It is important, that the signal at 1600 cm⁻¹ does not interfere with the solvent signals from the water/acetone mixture (Fig. S7 in ESI†).