**Application of Self-Assembled Raman Spectrum-Enhanced Substrate in Detection of Dissolved Furfural in Insulating Oil**

**Haiyang Shi 1,\*, Weigen Chen 1, Fu Wan 1,\*, Lingling Du 2, Shuhua Zhang 1, Weiran Zhou 1, Jiayi Zhang 1, Yingzhou Huang 3 and Chengzhi Zhu 4**



**\*** Correspondence: shihaiyang@cqu.edu.cn (H.S.); fuwan@cqu.edu.cn (F.W.); Tel.: +86-651-117-95 (H.S. & F.W.); Fax: +86-651-024-42 (H.S. & F.W.)

Received: 21 November 2018; Accepted: 19 December 2018; Published: 23 December 2018

**Abstract:** Accurate detection of dissolved aging features in transformer oil is the key to judging the aging degree of oil-paper insulation. In this work, in order to realize in situ detection of furfural dissolved in transformer oil, silver nanoparticles were self-assembled on the surface of gold film with P-aminophenylthiophenol (PATP) as a coupling agent. Rhodamine-6G (R6G) was used as the probe molecule to test the enhancement effect. By optimizing the molecular concentration, molecular deposition time, and silver sol deposition time of PATP, the nanoparticles were made more uniform and compact, and an enhanced substrate with rich hot spots was obtained. The optimum substrate was developed, and surface-enhanced Raman spectroscopy (SERS) detection of trace furfural dissolved in transformer oil was realized. The results showed that the substrate prepared under the conditions of 0.1 mol/L PATP, 5 h deposition in PATP and 12 h immersion in silver sol, had the best reinforcement effect (that is, uniform and compact particle arrangemen<sup>t</sup> and no particle clusters). By use of this substrate, the minimum detectable concentration of furfural in transformer oil was about 1.06 mg/L, which provides a new method for fast and nondestructive detection of transformer aging diagnosis.

**Keywords:** surface-enhanced Raman spectroscopy; transformer aging; concentration detection
