3. The surface modification and amidation reaction

In addition to the abovementioned thin water film confinement strategy, here, we introduce another approach to the SERS-based ultrasensitive detection of metal weakly interacted organophosphorus nerve agent sarin based on surface modification of the SERS substrates and amidation reaction [42]. The methanephosphonic acid (MPA) was chosen as the sarin simulation agent (or the target molecule). The Au-coated Si nanocone array was surface-modified with 2-aminoethanethiol molecules and used as the SERS-substrate for detection of MPA. It has been demonstrated that the modified substrate can selectively capture MPA in the solution under the existence of the coupling agent, and hence realize the SERS-based detection of the MPA in the solution with good selectivity and high sensitivity.

#### 3.1. Surface modification-based SERS detection strategy

#### 3.1.1. Choice of sarin-simulated agent

thermal effect, leading to the low Raman signal instead. However, the case here is an exception. The thin water film could protect the target molecules from the laser-induced damage because the water film can remarkably reduce the laser-induced thermal effect, as demonstrated in Figure 7(a), corresponding to the Raman spectra of DMMP in the water film on the substrate, excited with different laser powers under the same evaporation duration. We could use the maximal power (Pmax = 17 mW) of the equipment in this case, while in the conventional measurement only 5 mW or less is usually used. It has been shown that the intensity of the peak at 2936 cm<sup>1</sup> has a good linear relation with the power in whole power range, as indicated in Figure 7(b). The straight line passes through the origin. So, for this thin water film confinement and evaporation concentrating strategy, one can use enough high laser excitation power (>17 mW) to further increase Raman scattering intensity, exhibiting the stronger CERS

Based on the abovementioned text, using the thin water film confinement and evaporation concentrating strategy, one can effectively capture the hydrosoluble and weak affinity molecules within the strong electromagnetic field enhanced space above the SERS substrate and realize the SERS-based detection of them. The thin water film not only confines the target molecules within

It should be mentioned that the hydrophobic substrate surface, slower evaporation and stronger excitation power can further increase CERS effect. Especially, the slow and controlled evaporation in the anaphase would lead to several orders of magnitude in higher CERS effect. The strategy given here is an effective route to the SERS-based detection of the soluble molecules, which are of small Raman scattering cross-section and hardly adsorbed on the SERS substrates, by choosing proper solvents, but not suitable for the volatile soluble molecules as

Figure 7. (a) The Raman spectra of DMMP aqueous solution droplet on the substrate, under the excitation with different laser powers (P), after the evaporation for the same duration. The maximal excitation power of the equipment Pmax = 17 mW. (b) The plot of the intensity of the peak at 2936 cm<sup>1</sup> versus the laser excitation power [the data are from

a limited space but also protects the target molecules from laser-induced damage.

effect.

138 Raman Spectroscopy

2.4. Suitability of the strategy

the liquid film cannot confine these molecules.

(a)]. The solid line is the linear fitting results [33].

For convenient study of SERS-based detection of sarin, its simulation agent should be chosen. Such simulation agent should be of less or moderate toxicity but the chemical properties and especially the Raman spectrum should be similar to sarin. It has been found that methanephosphonic acid (MPA) is also a suitable simulation agent for sarin, in addition to the commonly used DMMP. The molecular formula of sarin and MPA are (CH3)2CHOOPF(CH3) and CH5O3P, respectively. Both have the C-P bonds and the similar bond length, chemically belonging to the organophosphorus group. Both MPA and sarin can produce amidation reaction with amino compounds [43]. It is expected that these similarities in chemical structures could have similar Raman spectral pattern to each other.

The Raman spectra of sarin and MPA were simulated based on density functional theory (DFT) by means of the Gaussian 09 software [44]. Figure 8(a) is the measured Raman spectrum for the pure MPA. The simulated Raman spectrum is very similar in the primary and minor peaks except the small difference in the peak positions, demonstrating the validity of the spectral simulations. Figure 8(b) shows the simulated Raman spectrum of sarin. Correspondingly, the

Figure 8. The Raman spectra for MPA (a) and sarin (b). (a) The measured Raman spectrum of pure MPA (excited by 785 nm laser). (b) The simulated Raman spectrum of sarin based on DFT calculations [42].

vibrational peaks can be assigned according to the DFT calculations [42]. The Raman spectral bands are mostly similar in wavenumbers for sarin and MPA. The MPA can thus be used as a sarin-simulated agent.
