3.2.1. Surface-modified SERS substrate

Au-coated Si nanocone array was used as the SERS substrate, which was prepared by sputtering deposition of gold on the Si nanocone array induced by PS colloidal monolayer and plasma etching strategy, as previously described in detail [42]. Figure 10 shows the Si nanocone array before and after sputtering deposition with a gold layer about 10 nm in thickness. Such array is of good uniformity in structure.

2-aminoethanethiol molecules were bound with the Au film on the array's surface by the thiol group, as shown in Figure 11. The peaks at 2881 and 2949 cm<sup>1</sup> are attributed to the symmetry stretching and asymmetry vibration of the CH2 in 2-aminoethanethiol [48], while the peak at 1600 cm<sup>1</sup> is ascribed to the in-plane bending vibration of the NH2 groups in 2-aminoethanethiol [48]. In addition, the peak at 1475 cm<sup>1</sup> originates from the shear vibration of the CH2 in 2-aminoethanethiol [48]. Furthermore, the peak at 769 cm<sup>1</sup> is assigned to swing plane vibration of CH2 chains, corresponding to the two methylene groups in 2-aminoethanethiol molecules [48]. It could thus be concluded that the modified array's surface was rich of 2-aminoethanethiol

Figure 10. The morphology of the as-prepared nanocone array. (a) The FESEM image of the Si nanocone array induced by plasma etching the PS colloidal monolayer on Si wafer. (b) The cross sectional image after sputtering deposition of gold on

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the Si nanocone array. The inset: A magnified image of a single Au-coated cone [42].

Figure 11. FTIR spectrum of the surface-modified au-coated Si cone array [42].

The modified substrate was then immersed into the ethanol solution of MPA in the presence of the coupling agent 1-ethyl-3-(3-(dimethylamino) propyl) carbodimide (or EDC for short) (2 mM) for 3 h which was long enough to reach the equilibrium adsorption of the MPA on

molecules.

3.2.2. Raman spectral measurements

Such Au-coated Si nanocone array was immersed into the ethanol solution of 2-aminoethanethiol (1 mM) for surface modification. FTIR spectral measurement has confirmed that the modified

Figure 10. The morphology of the as-prepared nanocone array. (a) The FESEM image of the Si nanocone array induced by plasma etching the PS colloidal monolayer on Si wafer. (b) The cross sectional image after sputtering deposition of gold on the Si nanocone array. The inset: A magnified image of a single Au-coated cone [42].

Figure 11. FTIR spectrum of the surface-modified au-coated Si cone array [42].

2-aminoethanethiol molecules were bound with the Au film on the array's surface by the thiol group, as shown in Figure 11. The peaks at 2881 and 2949 cm<sup>1</sup> are attributed to the symmetry stretching and asymmetry vibration of the CH2 in 2-aminoethanethiol [48], while the peak at 1600 cm<sup>1</sup> is ascribed to the in-plane bending vibration of the NH2 groups in 2-aminoethanethiol [48]. In addition, the peak at 1475 cm<sup>1</sup> originates from the shear vibration of the CH2 in 2-aminoethanethiol [48]. Furthermore, the peak at 769 cm<sup>1</sup> is assigned to swing plane vibration of CH2 chains, corresponding to the two methylene groups in 2-aminoethanethiol molecules [48]. It could thus be concluded that the modified array's surface was rich of 2-aminoethanethiol molecules.

#### 3.2.2. Raman spectral measurements

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

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

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

Au-coated Si nanocone array was used as the SERS substrate, which was prepared by sputtering deposition of gold on the Si nanocone array induced by PS colloidal monolayer and plasma etching strategy, as previously described in detail [42]. Figure 10 shows the Si nanocone array before and after sputtering deposition with a gold layer about 10 nm in thickness. Such array is

Such Au-coated Si nanocone array was immersed into the ethanol solution of 2-aminoethanethiol (1 mM) for surface modification. FTIR spectral measurement has confirmed that the modified

Figure 9. Schematic illustration for the interaction of the modifying agent (2-aminoethanethiol) with MPA molecules and

strongly interact with both the SERS substrate and MPA molecules.

sarin-simulated agent.

140 Raman Spectroscopy

3.1.2. Surface modification of SERS substrate

detection of MPA or sarin based on the SERS effect.

3.2. SERS measurements

3.2.1. Surface-modified SERS substrate

of good uniformity in structure.

SERS substrate (gold) [42].

The modified substrate was then immersed into the ethanol solution of MPA in the presence of the coupling agent 1-ethyl-3-(3-(dimethylamino) propyl) carbodimide (or EDC for short) (2 mM) for 3 h which was long enough to reach the equilibrium adsorption of the MPA on the SERS substrate. Here, the coupling agent EDC was employed to activate phosphonic groups in MPA molecules for coupling with the primary amines in the modifying agent. Finally, the soaked substrate was taken out and cleaned with deionized water and ethanol to remove any unbound molecules, and dried in the flow of N2 prior to the SERS spectral measurement under excitation at 785 nm and exposure time 10 s.

The Raman spectral pattern. Figure 12 demonstrates the Raman spectrum of the surfacemodified Au-coated Si nanocone array after immersion in the MPA solution (10�<sup>3</sup> M) with EDC. The Raman peaks at 643, 725, 976, 1025, 1212, 1412, and 1445 cm�<sup>1</sup> are clearly observed, as shown in curve (I) of Figure 12. On the contrary, for the surface-modified array without immersion or after immersion in the EDC solution without MPA or in the MPA solution without EDC, no Raman peak was detected, as illustrated in curves (II, III, IV) of Figure 12. So the Raman peaks in curve (I) of Figure 12 should be associated with the coexistence of MPA and EDC. By comparing with Figure 8(a), however, we can know that the Raman spectrum in curve (I) is completely different from that of the pure MPA. It means that the Raman peaks in Figure 12 are not attributed to MPA directly.

Concentration dependence. Further, the concentration-dependent Raman spectra were measured for the surface-modified SERS substrate (or Au-coated Si nanocone array) after soaking in the MPA solutions with different concentrations in the presence of EDC, as shown in Figure 13(a). The intensities of all Raman peaks increase with the rising MPA concentration in the solutions. The peak intensity I is approximately subject to the linear double logarithmic relation with the MPA concentration <sup>C</sup> from 10�<sup>8</sup> to 10�<sup>2</sup> M (or �1 to �1000 ppm), or

$$\text{LogI} = A\_0 + B\_0 \cdot \text{LogC} \tag{12}$$

The parameter B0 value is about 0.25 by fitting. We can thus rewrite Eq. (12) as a power

Figure 13. (a) The Raman spectra for the surface-modified Au-coated Si nanocone array after soaking in the MPA solutions with different concentrations in the presence of EDC. (b) Plot of the logarithmic peak intensity (I) at 976 cm�<sup>1</sup> versus the logarithmic MPA concentration (C) [the data are from (a)]. The straight line is the linear fitting results [42].

Regarding the Raman spectral origin and their evolution, it can be attributed to adsorption of MPA molecules and subsequent amidation reaction on the SERS substrate's surface, as sche-

After the coupling agent EDC was added to the ethanol solution of MPA, the coupling between them would activate the phosphonic groups in MPA [see Figure 14(a)] [47]. When the surface-modified SERS substrate was subsequently immersed into the MPA solution, because of the strong interaction between the amino groups in 2-aminoethanethiol and the phosphonic groups in the activated MPA molecules [47], the activated MPA molecules would

At this time, the adsorbed MPA molecules could react with the 2-aminoethanethiol molecules on the Au-coated Si cone array due to the amino groups in the 2-aminoethanethiol [47], or the

diffuse onto and be adsorbed on the substrate's surface, as illustrated in Figure 14(b).

where M is the constant independent of the concentration C.

<sup>I</sup> <sup>¼</sup> <sup>M</sup> � <sup>C</sup><sup>0</sup>:<sup>25</sup> (13)

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ð14Þ

function, or

3.3. Amidation reaction

matically illustrated in Figure 14.

amidation reaction between them

3.3.1. Amidation reaction-induced Raman spectra

where Ao and B0 are the constants independent of the concentration. Figure 13(b) shows the typical result corresponding to the main peak at 976 cm�<sup>1</sup> , exhibiting a good linear relation.

Figure 12. The Raman spectra for the Au-coated Si nanocone array after soaking in different solutions for 3 h. Curve (I): After soaking in the MPA solution (10�<sup>3</sup> M) with the EDC. Curve (II): The array without immersion; curve (III): After soaking in the EDC solution without MPA; curve (IV): After soaking in the MPA solution without EDC [42].

Figure 13. (a) The Raman spectra for the surface-modified Au-coated Si nanocone array after soaking in the MPA solutions with different concentrations in the presence of EDC. (b) Plot of the logarithmic peak intensity (I) at 976 cm�<sup>1</sup> versus the logarithmic MPA concentration (C) [the data are from (a)]. The straight line is the linear fitting results [42].

The parameter B0 value is about 0.25 by fitting. We can thus rewrite Eq. (12) as a power function, or

$$I = M \cdot \mathbb{C}^{0.25} \tag{13}$$

where M is the constant independent of the concentration C.

#### 3.3. Amidation reaction

the SERS substrate. Here, the coupling agent EDC was employed to activate phosphonic groups in MPA molecules for coupling with the primary amines in the modifying agent. Finally, the soaked substrate was taken out and cleaned with deionized water and ethanol to remove any unbound molecules, and dried in the flow of N2 prior to the SERS spectral

The Raman spectral pattern. Figure 12 demonstrates the Raman spectrum of the surfacemodified Au-coated Si nanocone array after immersion in the MPA solution (10�<sup>3</sup> M) with EDC. The Raman peaks at 643, 725, 976, 1025, 1212, 1412, and 1445 cm�<sup>1</sup> are clearly observed, as shown in curve (I) of Figure 12. On the contrary, for the surface-modified array without immersion or after immersion in the EDC solution without MPA or in the MPA solution without EDC, no Raman peak was detected, as illustrated in curves (II, III, IV) of Figure 12. So the Raman peaks in curve (I) of Figure 12 should be associated with the coexistence of MPA and EDC. By comparing with Figure 8(a), however, we can know that the Raman spectrum in curve (I) is completely different from that of the pure MPA. It means that the Raman peaks in

Concentration dependence. Further, the concentration-dependent Raman spectra were measured for the surface-modified SERS substrate (or Au-coated Si nanocone array) after soaking in the MPA solutions with different concentrations in the presence of EDC, as shown in Figure 13(a). The intensities of all Raman peaks increase with the rising MPA concentration in the solutions. The peak intensity I is approximately subject to the linear double logarithmic

where Ao and B0 are the constants independent of the concentration. Figure 13(b) shows the

Figure 12. The Raman spectra for the Au-coated Si nanocone array after soaking in different solutions for 3 h. Curve (I): After soaking in the MPA solution (10�<sup>3</sup> M) with the EDC. Curve (II): The array without immersion; curve (III): After

soaking in the EDC solution without MPA; curve (IV): After soaking in the MPA solution without EDC [42].

LogI ¼ A<sup>0</sup> þ B<sup>0</sup> � LogC (12)

, exhibiting a good linear relation.

relation with the MPA concentration <sup>C</sup> from 10�<sup>8</sup> to 10�<sup>2</sup> M (or �1 to �1000 ppm), or

measurement under excitation at 785 nm and exposure time 10 s.

Figure 12 are not attributed to MPA directly.

142 Raman Spectroscopy

typical result corresponding to the main peak at 976 cm�<sup>1</sup>

Regarding the Raman spectral origin and their evolution, it can be attributed to adsorption of MPA molecules and subsequent amidation reaction on the SERS substrate's surface, as schematically illustrated in Figure 14.

#### 3.3.1. Amidation reaction-induced Raman spectra

After the coupling agent EDC was added to the ethanol solution of MPA, the coupling between them would activate the phosphonic groups in MPA [see Figure 14(a)] [47]. When the surface-modified SERS substrate was subsequently immersed into the MPA solution, because of the strong interaction between the amino groups in 2-aminoethanethiol and the phosphonic groups in the activated MPA molecules [47], the activated MPA molecules would diffuse onto and be adsorbed on the substrate's surface, as illustrated in Figure 14(b).

At this time, the adsorbed MPA molecules could react with the 2-aminoethanethiol molecules on the Au-coated Si cone array due to the amino groups in the 2-aminoethanethiol [47], or the amidation reaction between them

$$\begin{array}{ccccc} \text{\huge{0} - \harrow 0 \text{\huge{0} + \harrow \text{ss} - \harrow - \harrow - \harrow - \harrow \text{res} - \harrow \text{res} - \harrow \text{res} - \harrow \text{res} - \harrow \text{res} - \harrow \text{res} - \harrow \text{res} & \text{res} \\ \cline{\huge{0} - \harrow \text{res} - \harrow \text{res} - \harrow \text{res} - \harrow \text{res} - \harrow \text{res} & \text{res} - \harrow \text{res} - \harrow \text{res} \end{array}$$

Figure 14. The schematic illustration for the MPA molecules' adsorption and amidation reaction on the Au-coated Si nanocone array. (a) Ethanol solution of the EDC-activated MPA. (b) The activated MPA molecules diffuse into and are adsorbed on the surface-modified array. (c) The amidation reaction on the substrate is finished. (d) The reaction products (organic phosphorus amide molecules) are bound to the substrate [42].

would occur on the surface of the array. The reaction products N-(2-mercaptoethyl)-Pmethylphosphonamidic acid C3H10O2NPS should be formed and bound or anchor on the surface of the array [see Figure 14(c)]. Besides, the byproduct N-acylurea was also produced in the solution. It is water-soluble and removable before Raman spectral measurement [see Figure 14(d)]. So, the Raman spectrum shown in curve (I) of Figure 12 could be ascribed to the product C3H10O2NPS on the substrate.

Evidently, the higher MPA content in the solution would induce the more activated MPA molecules adsorbed on the modified Au-coated Si nanocone array, and the more reaction products C3H10O2NPS bound on the array. This would result in higher Raman peak intensity, showing increase of the Raman peak intensity with the rising MPA content in the solutions, as illustrated in Figure 13(a).

Quantitatively, as mentioned earlier, the concentration-dependent Raman intensity can be described by a power function [see Eq. (13)]. It should be associated with the adsorption behavior of the MPA molecules on the modified substrate. According to the Freundlich theory [49], the adsorption of molecules on a heterogeneous surface could be described by:

$$
\boldsymbol{\sigma}\_e = \boldsymbol{K}\_F \cdot \boldsymbol{\mathsf{C}}^1 \tag{15}
$$

intensity I of a Raman peak should be proportional to the number density of the molecules adsorbed on the substrate within the area of a laser spot or show a linear relation with the

where K<sup>0</sup> is the constant. By combining Eqs. (15) and (16), we have the relationship between

I ¼ K � C

where K ¼ K<sup>0</sup> � KF. Eq. (17) is in complete agreement with Eq. (13), which has also confirmed the Freundlich-typed adsorption of the MPA molecules on the substrate. By combining Eqs. (13) and (17), the value of MPA adsorption parameter (n) can thus be estimated to be n = 4. This also presents a simple way to measure the adsorption parameters, which are normally

For confirmation of reaction (9) occurring on surface of the substrate, the amidation reaction experiment was carried out, according to Vijay et al.'s method [50], by preparing the ethanol solution with EDC, MPA and 2-aminoethanethiol and continuously stirring it at room temperature for 15 h, as previously described in detail [42]. The pure amidation compound was thus acquired. The FTIR measurement was conducted for this compound, as shown in Figure 15. All peaks can be ascribed to the vibrations of N-(2-mercaptoethyl)-P-methylphosphonamidic acid

vibrations of the carbon chains (CH2)2 in C3H10O2NPS; the peak at 894 cm�<sup>1</sup> is assigned to the stretching vibration of (PdCH3) + (PdO); and the peaks at 1041 and 1064 cm�<sup>1</sup> are from the stretching vibrations of (CdN) [48]. These indicated that pure amide C3H10O2NPS was

1

the intensity of Raman signal and the MPA concentration in the soaking solution:

acquired by the time-consuming measurement of the adsorption isotherms.

(C3H10O2NPS) [51]. For instance, the peaks at 731, 768 and 812 cm�<sup>1</sup>

Figure 15. FTIR spectrum of the products after amidation reaction [42].

3.3.2. Confirmation of the amidation reaction

I ¼ K<sup>0</sup> � q (16)

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<sup>n</sup> (17)

correspond to twisting

adsorption amount q:

obtained.

where qe is the equilibrium adsorption amount, the parameters KF and n are the parameters reflecting the adsorption capacity and adsorption intensity, respectively. Obviously, the intensity I of a Raman peak should be proportional to the number density of the molecules adsorbed on the substrate within the area of a laser spot or show a linear relation with the adsorption amount q:

$$I = \mathbb{K}\_0 \cdot \boldsymbol{\eta} \tag{16}$$

where K<sup>0</sup> is the constant. By combining Eqs. (15) and (16), we have the relationship between the intensity of Raman signal and the MPA concentration in the soaking solution:

$$I = \mathbf{K} \cdot \mathbf{C}^{\mathsf{L}} \tag{17}$$

where K ¼ K<sup>0</sup> � KF. Eq. (17) is in complete agreement with Eq. (13), which has also confirmed the Freundlich-typed adsorption of the MPA molecules on the substrate. By combining Eqs. (13) and (17), the value of MPA adsorption parameter (n) can thus be estimated to be n = 4. This also presents a simple way to measure the adsorption parameters, which are normally acquired by the time-consuming measurement of the adsorption isotherms.

#### 3.3.2. Confirmation of the amidation reaction

would occur on the surface of the array. The reaction products N-(2-mercaptoethyl)-Pmethylphosphonamidic acid C3H10O2NPS should be formed and bound or anchor on the surface of the array [see Figure 14(c)]. Besides, the byproduct N-acylurea was also produced in the solution. It is water-soluble and removable before Raman spectral measurement [see Figure 14(d)]. So, the Raman spectrum shown in curve (I) of Figure 12 could be ascribed to the

Figure 14. The schematic illustration for the MPA molecules' adsorption and amidation reaction on the Au-coated Si nanocone array. (a) Ethanol solution of the EDC-activated MPA. (b) The activated MPA molecules diffuse into and are adsorbed on the surface-modified array. (c) The amidation reaction on the substrate is finished. (d) The reaction products

Evidently, the higher MPA content in the solution would induce the more activated MPA molecules adsorbed on the modified Au-coated Si nanocone array, and the more reaction products C3H10O2NPS bound on the array. This would result in higher Raman peak intensity, showing increase of the Raman peak intensity with the rising MPA content in the solutions, as

Quantitatively, as mentioned earlier, the concentration-dependent Raman intensity can be described by a power function [see Eq. (13)]. It should be associated with the adsorption behavior of the MPA molecules on the modified substrate. According to the Freundlich theory

qe ¼ KF � C

where qe is the equilibrium adsorption amount, the parameters KF and n are the parameters reflecting the adsorption capacity and adsorption intensity, respectively. Obviously, the

1

<sup>n</sup> (15)

[49], the adsorption of molecules on a heterogeneous surface could be described by:

product C3H10O2NPS on the substrate.

(organic phosphorus amide molecules) are bound to the substrate [42].

illustrated in Figure 13(a).

144 Raman Spectroscopy

For confirmation of reaction (9) occurring on surface of the substrate, the amidation reaction experiment was carried out, according to Vijay et al.'s method [50], by preparing the ethanol solution with EDC, MPA and 2-aminoethanethiol and continuously stirring it at room temperature for 15 h, as previously described in detail [42]. The pure amidation compound was thus acquired. The FTIR measurement was conducted for this compound, as shown in Figure 15. All peaks can be ascribed to the vibrations of N-(2-mercaptoethyl)-P-methylphosphonamidic acid (C3H10O2NPS) [51]. For instance, the peaks at 731, 768 and 812 cm�<sup>1</sup> correspond to twisting vibrations of the carbon chains (CH2)2 in C3H10O2NPS; the peak at 894 cm�<sup>1</sup> is assigned to the stretching vibration of (PdCH3) + (PdO); and the peaks at 1041 and 1064 cm�<sup>1</sup> are from the stretching vibrations of (CdN) [48]. These indicated that pure amide C3H10O2NPS was obtained.

Figure 15. FTIR spectrum of the products after amidation reaction [42].

4. Conclusions and outlook

the coin metal substrates.

Acknowledgements

tional Partnership Program for Creative Research Teams.

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

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This work is financially supported by the National Key Research and Development Program of China《Fundamental Research on nano sensing materials and high performance sensors focused on pollutants detection》(Grant No. 2017YFA0207101), Natural Science Foundation of China (Grant No. 51531006, 11574313, 11374300 and 51571188) and the CAS/SAF Interna-

Figure 16. The Raman spectrum for the Au-coated Si nanocone array after immersion in the ethanol solution with 1.0 <sup>10</sup><sup>3</sup> M C3H10O2NPS [42].

For the Raman spectral measurements, the pure amide (C3H10O2NPS) was diluted, with ethanol, to a given concentration. The Au-coated Si nanocone array was then immersed into the C3H10O2- NPS-contained ethanol solution before the Raman spectral measurements. Figure 16 shows the results corresponding to the solution with 1.0 <sup>10</sup><sup>3</sup> M in C3H10O2NPS concentration. The spectral pattern is in good agreement with that shown in curve (I) of Figure 12. So, the Raman spectrum in curve (I) of Figure 12 should be attributed to the amidation compound C3H10O2NPS. These results have confirmed that the amidation reaction occurred on the surface of the modified Au-coated Si nanocone array during its immersion in the MPA solutions with EDC, and that the reaction products C3H10O2NPS molecules were formed on and bound with the array's surface.

#### 3.4. Quantitative SERS-based detection of MPA

As mentioned earlier, the Au-coated Si nanocone array modified with 2-aminoethanethiol can capture selectively MPA in the solution in the presence of EDC via diffusion and adsorption, leading to the amidation reaction and the formation of C3H10O2NPS molecules which were still bound on the array's surface. The bound C3H10O2NPS molecules were corresponding to the MPA molecules adsorbed on the SERS substrate. Therefore, by using the 2-aminoethanethiol-modified Au-coated Si nanocone array, we can realize the SERS-based ultrasensitive and quantitative detection of MPA in the solution. The obtained Raman spectra are from the C3H10O2NPS molecules but corresponding to the MPA, which exhibits a linear double logarithmic relation between the Raman peak intensity and the MPA concentration, as described in Eq. (4). Since there exist similarities between MPA and sarin in chemical properties and Raman spectral pattern, as mentioned in Section 3.1.1, it is thus expected that the abovementioned method is also suitable for sarin detection.

Finally, it should be mentioned that the method introduced here relies on the activation of phosphonic groups by the coupling agent EDC which creates reactive phosphonamide. Both EDC and the by-product N-acylurea can be removed by subsequent substrate cleaning before Raman spectral measurement.
