3.1.2. Surface modification of SERS substrate

Generally, the Raman signals could be detected only when the MPA molecules are adsorbed on the SERS substrates. However, the MPA molecules can hardly be adsorbed on the noble metals, due to the weak interaction between them. The surface modification strategy was used to overcome such problem. A surface modifier should be chosen in such a way that it can strongly interact with both the SERS substrate and MPA molecules.

The 2-aminoethanethiol molecule contains two-head groups such as amino and thiol groups. It is well known that there is a strong covalent bond interaction between thiol and gold according to the theory of hard and soft (Lewis) acids and bases [45, 46]. As for the amino group, it can react with phosphonic group to generate phsophonamidate in the presence of coupling agents [such as dicyclohexylcarbodiimide, N, N-diisopropylcarbodiimide, 1-ethyl-3-(3-(dimethylamino) propyl) carbodimide] [47]. Therefore, the thiol groups in 2-aminoethanethiol molecules would tend to be bound with gold substrate to form AudS covalent bonds, and the amino groups would selectively capture phosphonic groups in MPA molecules in the solution, as schematically shown in Figure 9. 2-Aminoethanethiol could thus be a suitable modifying agent of the SERS substrate. It is expected that the surface-modified SERS substrate would selectively capture the organophosphorus molecules (such as sarin, MPA), as demonstrated in Figure 9. In this case, we could realize detection of MPA or sarin based on the SERS effect.

#### 3.2. SERS measurements
