Fig. 4. PCA scores plot discriminating between each of the 14 chalcogenopyrylium dyes and grouping them according to their structures and SERS spectra. The red cluster contains the trimethine dyes 9–14 which produce the best SERS signals, blue cluster highlights the monomethine dyes (1–3,5,7,8) which work well as reporters for SERS and the green clustering contains the two dyes which produce only weak signals with HGNs (dyes 4 and 6).