Sensitive and Rapid Extraction-Free Detection of Furfural in Mineral and Natural Ester Oils by 4-ATP Modified SERS Substrate

Furfural serves as a significant indicator of the aging process of oil-paper insulated transformer. However, the commonly used liquid chromatography (LC) method is cumbersome, time-consuming, and can only be analyzed in a laboratory. Surface-enhanced Raman scattering (SERS) technology can realize nondestructive testing, but it is still limited to low sensitivity, long adsorption time, and standard mineral insulating oil in the laboratory. In this article, the pyramid-shaped Si–Au–Ag SERS substrate was coupled with 4-aminothiophene (4-ATP) to prepare a substrate that can specifically trap furfural molecules in insulating oil. In addition, a portable tilting injector using acrylic material was designed, which combined with portable Raman spectrometer that can provide the reaction conditions for the rapid extraction-free (no laboratory pretreatment) and highly sensitive detection of furfural molecules in mineral and natural ester oil. Compared with ordinary SERS substrates, the detection limit is enhanced about 40 times, reaching 0.05 and 0.01 mg/L for furfural in mineral and rapeseed-based natural ester insulating oil, respectively, and the adsorption equilibrium time is down to 120 s. After building the quantitative analysis curve, this method successfully realizes the analysis of accelerated aging oil samples and field oil samples for the first time. Compared with the conventional LC method, the absolute deviation and relative deviation are less than 0.078 mg/L and 5.534%, respectively. This detection technology has strong applicability to the evaluation of oil-paper insulation aging state.