1. Introduction

Organophosphorus nerve agents (such as sarin, tabun, cyclosarin and soman, etc.) belong to the high-risk chemicals with strong poison [1]. When a person is exposed to such nerve agents, sarin for example, with 1.43 ppm, death would occur in few minutes if the agent is inhaled through his/her respiratory system. Even if the nerve agent enters the body through the skin or

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

through consumption, poisoning would occur in few hours [2]. It is thus vital to fast detect them in highly sensitive and portable way. There have existed some methods for the detection of these organophosphorus molecules, such as gas chromatography coupled to a mass spectrometer, ion mobility time-of-flight mass spectrometry, gas sensing and microcantilever, and so on [2–5]. But, these current techniques are of low sensitivity or of poor selectivity and are time-consuming. Both quick and trace detection of them have been expected and are most challenging.

2. Thin water film confinement and evaporation concentrating strategy

2.1. Strategy and model

2.1.2. Concentrating factor

2.1.1. Strategy

For the SERS-based detection of the organophosphorus nerve agents that can only weakly interact with the coin metal substrates, a new and effective route has been developed to capture the hydrosoluble organophosphorus molecules based on the thin water film confinement and evaporation concentrating strategy [33]. DMMP, which is very difficult to be captured by the noble metal substrates, was used as the target molecule, and the gold micro-/ nanostructured array was employed as the SERS substrate to demonstrate the validity of this strategy. It has been confirmed that by such strategy, the noble metal SERS substrate can effectively capture the DMMP molecules, realizing SERS-based trace detection of them.

SERS-Based Sensitive Detection of Organophosphorus Nerve Agents

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Normally, in the conventional detections based on the SERS effect, the SERS substrates are firstly soaked in the solution containing target molecules for a certain duration to make the molecules adsorbed on the substrate's surfaces, and then taken out and dried before spectral measurement. If the target molecules can only weakly interact with the substrates, however, such procedures would be of no avail because of no or too less molecules on the substrate's surface after drying. Here, a water film confinement and evaporation concentrating strategy

First, the aqueous solution containing target molecules is dropped onto a SERS-active substrate with hydrophilic surface (Figure 1a), which will then spread out and form a water film on the surface. The target molecules (or the solute molecules) are accordingly confined within the thin water film (see Figure 1b). The subsequent solvent (water) evaporation induces the thinner and thinner water film, and continuous concentrating of the solutes within the film, assuming that solute volatilization is insignificant or negligible compared with the water evaporation (see Figure 1c). When the water film decreases in thickness down to the nanometer level, all the target molecules confined in the water film are localized within the region above the substrate, within which the electromagnetic field can be enormously enhanced under external excitation (see Figure 1d). If Raman spectra are measured for this film at this moment, the Raman signal of the target molecules should be enhanced by both the concentrated solutes and the substrate surface. After complete drying, no target molecules will be left on the substrate due to their weak

For quantitative analysis, the evaporation-induced concentrating factor (CF) of the target molecules in the water film is defined as the ratio of solute concentration (Cm) in the water

> CF <sup>¼</sup> Cm C0

(1)

film at the moment of Raman spectral measurement to that at the initial stage (C0), or

could overcome the abovementioned problem, as schematically illustrated in Figure 1.

interaction with the substrate and hence we cannot obtain their Raman signals.

The detection based on surface-enhanced Raman scattering (SERS) effect, which has been extensively reported since its discovery in the 1970s [6–14], would be an appropriate and effective method for fast and ultrasensitive detection. The SERS effect originates from a significant enhancement in the effective Raman cross-section of the target molecules situated at or very near to the roughened noble metal surfaces or colloidal particles [15–25]. The detection based on the SERS effect has the characteristics of high sensitivity, fast response and fingerprint recognition with the ability to be close to a single molecule level [26–29]. The main contribution of amplification of Raman signal intensity arises from the local electromagnetic field enhancement due to the surface plasmon resonance (SPR) of the metal nanoparticles, which has been extensively reported [15–21]. In a conventional SERS detection technique, the target molecules need to stay on the "hot spots" or within the strong electromagnetic field enhancement areas above the SERS substrates [30, 31]. Traditionally, the SERS-based detection is limited to the target molecules, which have high affinity with the metal (gold or silver, etc.) surfaces. It is thus a prerequisite that the substrates can capture or adsorb the target molecules within the strong field-enhancement areas or on the hot spots for the SERS-based detection. However, the organophosphorus nerve agents (including sarin, cyclosarin and soman, etc.) have only very weak interaction (or even no affinity) with highly SERS-activated noble metal substrates and are hardly adsorbed on the substrates or have only very short residence time on them. Obviously, in this case, the normal SERS-based technique is difficult to realize the effective detection of such molecules [13, 32]. For instance, the organophosphorus molecule dimethyl methylphosphonate (DMMP, a typical sarin simulant agent), which is hardly adsorbed on the noble metal surfaces [33], is very difficult to be detected by the SERS-based technique with reproducible measurements and trace level [33, 34]. This is the reason why there have only been very limited reports on the SERS-based detection of the organophosphorus molecules that could only weakly interact with the noble metals [35, 36].

The SERS substrates after surface modification could selectively adsorb and enrich such molecules on their surfaces. However, it is generally difficult to obtain proper modifying agents for the given target molecules and to realize the reproducible measurements and quantitative detection of them without interfering effect. Besides, for the organic modifying agents, they may induce the complicacy of Raman spectral pattern and hence misidentification. Recently, there have been some new approaches developed for the SERS-based detection of the organophosphorus molecules. In this chapter, we introduce the progresses in this field, mainly including (1) the thin water film confinement and evaporation concentrating strategy and (2) the surface modification of the SERS substrates and amidation reaction. These works have provided 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.
