Fig. 2
Colored tungsten oxide film as surface-enhanced Raman scattering (SERS) substrate. a Raman intensity of rhodamine 6G (R6G) (10−4 M) on the pristine uncolored film and colored substrates (scale bar: 1000 cps). b Statistical evolution of enhancement factor (EF) vs. optical absorbance for pristine and Al-intercalated substrates, indicating a roughly linear increase. A high coefficient of determination (R2) of 0.96 is obtained, demonstrating the linear dependence between EF and optical absorbance. Colored films obtained via Al intercalation at various negative potentials (−0.1, −0.2, −0.3, −0.4, −0.5, and −0.6 V) are denoted as Al-1, Al-2, Al-3, Al-4, Al-5, and Al-6, respectively. c Raman spectra of R6G collected for Al-6 substrate at different concentrations of 10−4, 10−5, and 10−6 M (scale bar: 500 cps). Inset: with narrowed y scale for 10−6 M (scale bar: 10 cps). d EF values for pristine and ion-intercalated substrates, which were prepared by discharging potentiostatically at −0.5 V for 180 s in 1 M aqueous analyte of each cation species. Error bars represent means ± SD of the EFs
