Figure 2: LIQUISOR detection of Phenylalanine.

SERS spectrum of Phe 1 mM in PBS (red line) compared with solution phase Raman of Phe 100 mM in PBS (black line). Both spectra are taken in the same experimental conditions (wavelength 632.8 nm, objective 100X, power 6.7 mW), but for the integration times (10 s for SERS, 300 s for Raman). The spectra have been normalized to power and integration times, so the intensities can be directly compared. The Raman spectrum shows the most intense peaks of Phe at 1006, 1034, 1210, 1588, 1609 cm⁻¹ (see Supplementary Table S1 for modes assignment) just emerging from to the stronger vibrational fingerprints of PBS (modes at 880, 993, 1080 cm⁻¹), and of water (band at 1620 cm⁻¹). The highest SERS enhancement is experienced by the vibrational modes in the ranges around 1200, 1400–1500 and 1600 cm⁻¹ (see Supplementary Fig. S7). The inset is a zoom of the SERS (circles) in the 950–1030 cm⁻¹ range and the relative fit (blue line), highlighting the Phe ring breathing mode at 1005 cm⁻¹ emerging from the PBS band (here fitted by two Lorentzian line shapes, green lines). The SERS peaks are superimposed to a continuum background that has been subtracted for clarity.