Acknowledgements

For the Raman spectral measurements, the pure amide (C3H10O2NPS) was diluted, with ethanol, to a given concentration. The Au-coated Si nanocone array was then immersed into the C3H10O2- NPS-contained ethanol solution before the Raman spectral measurements. Figure 16 shows the results corresponding to the solution with 1.0 <sup>10</sup><sup>3</sup> M in C3H10O2NPS concentration. The spectral pattern is in good agreement with that shown in curve (I) of Figure 12. So, the Raman spectrum in curve (I) of Figure 12 should be attributed to the amidation compound C3H10O2NPS. These results have confirmed that the amidation reaction occurred on the surface of the modified Au-coated Si nanocone array during its immersion in the MPA solutions with EDC, and that the reaction products C3H10O2NPS molecules were formed on and bound with

Figure 16. The Raman spectrum for the Au-coated Si nanocone array after immersion in the ethanol solution with

As mentioned earlier, the Au-coated Si nanocone array modified with 2-aminoethanethiol can capture selectively MPA in the solution in the presence of EDC via diffusion and adsorption, leading to the amidation reaction and the formation of C3H10O2NPS molecules which were still bound on the array's surface. The bound C3H10O2NPS molecules were corresponding to the MPA molecules adsorbed on the SERS substrate. Therefore, by using the 2-aminoethanethiol-modified Au-coated Si nanocone array, we can realize the SERS-based ultrasensitive and quantitative detection of MPA in the solution. The obtained Raman spectra are from the C3H10O2NPS molecules but corresponding to the MPA, which exhibits a linear double logarithmic relation between the Raman peak intensity and the MPA concentration, as described in Eq. (4). Since there exist similarities between MPA and sarin in chemical properties and Raman spectral pattern, as mentioned in Section 3.1.1, it is thus expected that the abovementioned method is also suitable for sarin detection. Finally, it should be mentioned that the method introduced here relies on the activation of phosphonic groups by the coupling agent EDC which creates reactive phosphonamide. Both EDC and the by-product N-acylurea can be removed by subsequent substrate cleaning before

the array's surface.

1.0 <sup>10</sup><sup>3</sup> M C3H10O2NPS [42].

146 Raman Spectroscopy

Raman spectral measurement.

3.4. Quantitative SERS-based detection of MPA

This work is financially supported by the National Key Research and Development Program of China《Fundamental Research on nano sensing materials and high performance sensors focused on pollutants detection》(Grant No. 2017YFA0207101), Natural Science Foundation of China (Grant No. 51531006, 11574313, 11374300 and 51571188) and the CAS/SAF International Partnership Program for Creative Research Teams.
