## 3 Results and Discussion
### Detection of Fungicides on a SERS-Based Sensing Platform  
In this section, AuNR-coated glass was used as a SERS-based sensing platform for detection of thiophanate methyl. No Raman signals of thiophanate methyl can be observed for glass or AuNR-coated glass, indicating thiophanate methyl-free surfaces (Figure 6d,e). However, after deposition of 100 mg/L of the fungicide, all of its Raman characteristic peaks can be observed at 613 cm\({}^{-1}\), 726 cm\({}^{-1}\), 779 cm\({}^{-1}\), 898 cm\({}^{-1}\), 959 cm\({}^{-1}\), 1039 cm\({}^{-1}\), 1154 cm\({}^{-1}\), 1267 cm\({}^{-1}\), 1298 cm\({}^{-1}\), 1538 cm\({}^{-1}\), 1601 cm\({}^{-1}\), 1708 cm\({}^{-1}\), which are ascribed to -N-C=S deformation, N-H wagging of -(C=O)-NH-(C=S)-, C=S stretching, C-S stretching, C=S, =C-H deformation, N-C-N asymmetric stretching, C-O, C=C, C-N, C=O and  
Figure 4: Transmission electron microscopy (TEM) images of (**a**) Au nanosheets and (**b**) AuNRs. The insets: High resolution transmission electron microscopy (HRTEM) images of (**a**) Au nanoseeds and (**b**) AuNRs. Particle size distribution of (**c**) Au nanoseeds, and (**d**,**e**) AuNRs. (**f**) Aspect ratio of AuNRs.  
Figure 5: (**a**) X-ray diffraction (XRD) spectra of AuNRs, (**b**) Energy-dispersive X-ray (EDX) spectra of AuNR-coated glass.  
N-H, respectively (Figure 6a) [40]. It should be noted that even in the presence of thiophanate methyl, those signals cannot be observed as the concentration of thiophanate methyl is as low as 5 mg/L (Figure 6c). Interestingly, at the same concentration of thiophanate methyl, most of the characteristic Raman peaks of thiophanate methyl were clearly observed after deposition of thiophanate methyl onto the AuNR-coated glass substrate (Figure 6b). These results indicated that AuNR-coated glass can be used as a SERS-based sensing platform for detection of fungicides.  
The sensitivity of the method was further evaluated through the corresponding Raman spectra of thiophanate methyl deposited onto AuNR-coated glass substrates with concentrations ranging from 1 mg/L to 7 mg/L. It can be seen that the characteristic peaks of thiophanate methyl could be clearly identified in all Raman spectra (Figure 7). Typically, the SERS intensity of thiophanate methyl gradually decreases with the decreasing concentration. The limit of detection (LOD) was determined as low as 5 mg/L (S/N = 3 for the peak 779 cm\({}^{-1}\)). The limit of quantification (LOQ), therefore, was estimated as \(\sim\)17 mg/L.  
In order to prove the reproducibility of the measurement, SERS spectra of 5 mg/L of thiophanate methyl were obtained at 10 randomly selected locations on the platform. No significant difference  
Figure 6: Raman spectra of (**a**) 100 mg/L of thiophanate methyl deposited glass, (**b**) 5 mg/L of thiophanate methyl deposited onto AuNR-coated glass substrate, (**c**) 5 mg/L of thiophanate methyl deposited glass, (**d**) AuNR-coated glass, and (**e**) glass substrate.  
Figure 7: Raman spectra of different concentrations of thiophanate methyl: (**a**) 7 mg/L, (**b**) 5 mg/L, (**c**) 3 mg/L, and (**d**) 1 mg/L recorded on AuNR-coated glass platform.  
was observed between those positions (Figure 8). The relative standard deviation (RSD) was found to be 5.7%, which indicates that the platform is able to be used for routine SERS analysis with a good reproducibility.