Fig. 1. Quantitative SERS detection of metabolites on a non-uniform plasmonic chip using a ratiometric Raman probe. (a) Schematic illustration of the detection principle: metabolite molecules are catalysed by the corresponding oxidase to generate H₂O₂ which specifically oxidizes the probe 3-MPBA to 3-HTP, yielding a ratiometric SERS response. (b) Field-emission scanning electron microscopy (FESEM) image of a SERS chip (insets: transmission photograph of a SERS chip and the extinction spectrum). (c) SERS spectra of 3-MPBA adsorbed on a gold chip collected at different wavelengths. (d) SERS spectra of gold chips modified with 3-HTP (blue curve) and 3-MPBA before (black curve) and after (red curve) reaction with H₂O₂. (e) SERS intensity ratio (I₈₈₂/I₉₉₈) of the probe 3-MPBA as a function of H₂O₂ concentration (inset: linear response obtained for H₂O₂ concentrations between 50 and 250 nM). Error bars represent the standard error of the mean of triplicate measurements.