Fig. 1. The molecular structure of furfural

Fig. 2. Schematic diagram of laboratory confocal laser Raman spectroscopy experimental platform

Fig. 3. Changes of TEM copper mesh before and after chemical reaction

Fig. 4. Labelled detection sites

Fig. 5. Electron microscope images of micro-nano SERS substrate
(a): 1 μm; (b): 200 nm

Fig. 6. SERS Raman spectra of transformer oil mixed with furfural (a), substrate (b), transformer oil (c) and furfural (d)

Fig. 7. Comparison of Raman spectra of furfural transformer oil samples with different concentrations

Fig. 8. The relationship between the intensity ratio of the Raman characteristic peak of furfural in transformer oil at 1 702 cm^-1 to the internal standard and the content of furfural in transformer oil

Fig. 9. Raman spectra of obtained from site 1 to site 15

Fig. 10. Intensities of Raman characteristic peak at 1 702 cm^-1 measured from site 1 to site 15