4. Conclusions and outlook

We have introduced some recent progresses in the SERS-based detection of the organophosphorus nerve agents, including the thin water film confinement evaporation concentrating strategy and the SERS substrates' surface modification/amidation reaction. For the former, when the solution containing target molecules is dropped on the SERS substrate and forms a thin water film on it, the target molecules are limited within the film. Subsequent water evaporation leads to the enrichment or concentrating of the target molecules within the region of strongly enhanced electromagnetic field above the substrate, and hence significantly enhances the Raman signal or induces the CERS effect. The validity of this strategy has been demonstrated by taking the sarin simulant DMMP as the target molecule, which are hardly adsorbed on the gold substrates, exhibiting significant CERS effect during water film evaporation and showing a good linear relation between the reciprocal intensity for the Raman characteristic peak and the evaporation interval, which is in agreement with quantitative description of evaporation-induced solute concentrating. The thin water film not only confines the target molecules within a limited space but also protects the target molecules from laser-induced damage. This approach should also be suitable for the other soluble molecules with low volatility. For the latter, because the 2 aminoethanethiol molecules possess two-head groups: amino and thiol groups: one can be bound with gold film and the other can capture the phosphonic groups in sarin simulation agent MPA in presence of the coupling agent EDC, the 2-aminoethanethiol-modified SERS substrate could selectively capture MPA molecules in the solution, which thus induces the amidation reaction on the substrate's surface. The reaction products or C3H10O2NPS molecules are still bound on the substrate's surface. Correspondingly, we could obtain the Raman spectra of amide C3H10O2NPS, which correspond to the MPA molecules adsorbed on the substrate. The Raman peak intensity shows a good linear double logarithmic relation with the MPA concentration in a large range, which could be attributed to Freundlich adsorption behavior of MPA on the surface-modified SERS substrate. The minimum detection level of MPA is down to 1 ppb. We can thus quantitatively detect MPA or sarin in solutions based on the SERS effect. This route could also be suitable for the other organophosphorus nerve agents and some other molecules weakly interacted with the coin metal substrates by choosing appropriate modifiers. In a word, the abovementioned progresses provide new ways for highly efficient SERS-based detection of the organophosphorus nerve agents and some other target molecules that weakly interact with the coin metal substrates.
