2. Thin water film confinement and evaporation concentrating 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.

#### 2.1. Strategy and model

#### 2.1.1. Strategy

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

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

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.

challenging.

128 Raman Spectroscopy

weakly interact with the noble metals [35, 36].

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

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

#### 2.1.2. Concentrating factor

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

$$\text{CF} = \frac{\text{C}\_m}{\text{C}\_0} \tag{1}$$

Figure 1. The schematic illustration for the water film confinement and evaporation-concentrating process. (a) A clean SERS substrate. (b) A liquid film formed on the substrate after dropping a certain amount of the solution on it. (c) The liquid film is decreased in thickness and solute is concentrated due to solvent's evaporation. (d) and (d') The liquid film is comparable in thickness to the field-enhanced space for the substrates with hydrophilic and hydrophobic surfaces, respectively [33].

Based on the above described strategy (or in Figure 1), if the substrate surface is hydrophilic, the concentrating factor can be approximately written as

$$CF = (1 - S) \cdot \frac{H\_0}{H\_m} \tag{2}$$

water and could be ignorable. For better understanding, here, let us semiquantitatively evaluate the CF value. If letting S≈0 and H0 = 1 mm, we can obtain CF = 105 when Hm = 10 nm, from Eq. (2). Such values should be credible in the order of magnitude. So, the solvent evaporation-

SERS-Based Sensitive Detection of Organophosphorus Nerve Agents

http://dx.doi.org/10.5772/intechopen.72630

131

In addition, if the water film is on the SERS substrate with hydrophobic surface and reduced to an enough thin thickness due to the evaporation, it could horizontally shrink and decrease the coverage area on the substrate, which would induce further concentrating. Therefore, the CF value would be higher in the local water films than that for the substrate with hydrophilic surface, as shown in Figure 1(d)'. In other words, the substrate with hydrophobic surface

Furthermore, according to Eq. (2), the strategy shown in Figure 1 should be more effective for the target molecules with lower volatility and/or when the target molecule concentration in the initial aqueous solution is very low due to the evaporation-induced concentrating effect.

In this strategy, the thin water film would function as follows: (1) Limitation of the target molecules to the small region above the SERS substrates. When the water film is becoming very thin in thickness, the molecules are localized in the field-enhanced space although they are not adsorbed on the substrate's surface; (2) Enrichment of the target molecules. Solvent's evaporation will induce the concentrating or enrichment of the target molecules in the water film and the increase of the target molecules' number in the field-enhanced space above the substrate; (3) Decrease of laser-induced thermal effect. The water film can protect the target molecules from laser-induced damage. So, the Raman intensities can also be enhanced by increasing the laser excitation power and (4) The evaporation-induced reorientation of the target molecules. During solvent evaporation, the target molecules could re-orientate [37]. This would generate a significant Raman signal enhancement due to the charge-coupling between

Here, the gold hierarchically micro-/nanostructured bowl-like array was chosen as the SERS substrate and the DMMP as the target molecules to demonstrate the validity of the above thin

The fabrication process of the SERS substrate was prepared by the electrodeposition on a preformed monolayer polystyrene (PS) colloidal crystal (2 μm in PS sphere-diameter) in the HAuCl4 aqueous solution at room temperature, as previously described. [38]. Figure 2(a) shows the typical morphology. The SERS substrate is the gold array consisting of the hexagonally arranged bowl-like pores with 2 μm in period. The skeleton among the pores in the array is built of nearly vertical quasi rod-shaped nanoparticles. The static contact angle of such substrate surface is about 105, exhibiting the slightly hydrophobic surface (see Figure 2b).

induced concentrating effect is quite significant.

should be of better concentrating effect.

2.1.3. Effects of the thin water film

the molecules and the metallic surface.

2.2. Application in SERS-based detection of DMMP

water film confinement and evaporation concentrating strategy.

2.2.1. Surface morphology and wettability of the SERS active substrate

in which

$$S = \frac{M\_0 - M\_m}{M\_0} \quad \text{and} \quad H\_m \ge d \tag{3}$$

where H0 is the initial thickness of water film after the water droplet is spreading out on the substrate, Hm is the corresponding value when the Raman spectral measurement is performed, d is the distance (in nanometer scale) from the substrate surface and within which the electromagnetic field can be significantly enhanced during laser excitation, S is the loss-ratio of the target molecules in the water film due to the possible volatilization during solvent evaporation and M0 and Mm are the mole numbers of the target molecules in the water film at the initial and the spectral measurement stages, respectively.

Generally, compared with the evaporation of the solvent, volatilization of the molecules like DMMP in solutions is significantly slower due to the much heavier molecular weight than water and could be ignorable. For better understanding, here, let us semiquantitatively evaluate the CF value. If letting S≈0 and H0 = 1 mm, we can obtain CF = 105 when Hm = 10 nm, from Eq. (2). Such values should be credible in the order of magnitude. So, the solvent evaporationinduced concentrating effect is quite significant.

In addition, if the water film is on the SERS substrate with hydrophobic surface and reduced to an enough thin thickness due to the evaporation, it could horizontally shrink and decrease the coverage area on the substrate, which would induce further concentrating. Therefore, the CF value would be higher in the local water films than that for the substrate with hydrophilic surface, as shown in Figure 1(d)'. In other words, the substrate with hydrophobic surface should be of better concentrating effect.

Furthermore, according to Eq. (2), the strategy shown in Figure 1 should be more effective for the target molecules with lower volatility and/or when the target molecule concentration in the initial aqueous solution is very low due to the evaporation-induced concentrating effect.

### 2.1.3. Effects of the thin water film

Based on the above described strategy (or in Figure 1), if the substrate surface is hydrophilic,

Figure 1. The schematic illustration for the water film confinement and evaporation-concentrating process. (a) A clean SERS substrate. (b) A liquid film formed on the substrate after dropping a certain amount of the solution on it. (c) The liquid film is decreased in thickness and solute is concentrated due to solvent's evaporation. (d) and (d') The liquid film is comparable in thickness to the field-enhanced space for the substrates with hydrophilic and hydrophobic surfaces, respectively [33].

CF <sup>¼</sup> ð Þ� <sup>1</sup> � <sup>S</sup> <sup>H</sup><sup>0</sup>

where H0 is the initial thickness of water film after the water droplet is spreading out on the substrate, Hm is the corresponding value when the Raman spectral measurement is performed, d is the distance (in nanometer scale) from the substrate surface and within which the electromagnetic field can be significantly enhanced during laser excitation, S is the loss-ratio of the target molecules in the water film due to the possible volatilization during solvent evaporation and M0 and Mm are the mole numbers of the target molecules in the water film at the initial

Generally, compared with the evaporation of the solvent, volatilization of the molecules like DMMP in solutions is significantly slower due to the much heavier molecular weight than

<sup>S</sup> <sup>¼</sup> <sup>M</sup><sup>0</sup> � Mm M<sup>0</sup>

Hm

and Hm ≥ d (3)

(2)

the concentrating factor can be approximately written as

and the spectral measurement stages, respectively.

in which

130 Raman Spectroscopy

In this strategy, the thin water film would function as follows: (1) Limitation of the target molecules to the small region above the SERS substrates. When the water film is becoming very thin in thickness, the molecules are localized in the field-enhanced space although they are not adsorbed on the substrate's surface; (2) Enrichment of the target molecules. Solvent's evaporation will induce the concentrating or enrichment of the target molecules in the water film and the increase of the target molecules' number in the field-enhanced space above the substrate; (3) Decrease of laser-induced thermal effect. The water film can protect the target molecules from laser-induced damage. So, the Raman intensities can also be enhanced by increasing the laser excitation power and (4) The evaporation-induced reorientation of the target molecules. During solvent evaporation, the target molecules could re-orientate [37]. This would generate a significant Raman signal enhancement due to the charge-coupling between the molecules and the metallic surface.
