Fig. 3. Chip-based ratiometric SERS analysis of metabolites. (a) Cholesterol concentration-dependent SERS spectra. (b) Plot of relative Raman intensity (I₈₈₂/I₉₉₈) as a function of cholesterol concentration from 5 μM to 5 mM. The inset shows the linear response for cholesterol (5 μM to 100 μM). Plots of relative Raman intensity (I₈₈₂/I₉₉₈) as a function of (c) glucose and (d) lactate concentration. The insets show the linear curves for (c) glucose (10 μM to 500 μM) and (d) lactate (18 μM to 72 μM).