**3. Detection of PCBs at trace amount by SERS**

**Figure 10.** The intensity of the two characteristic peaks of R6G on hexagonal lattices of Ag nanorods: (a) 612 cm-1 and

To gain a better understating of the latticing effect on the Raman scattering, we plot the in‐ tensity of the two characteristic peaks of R6G molecules, i.e. at ~ 612 cm-1 and ~ 1508 cm-1, as a function of the separation distance of the lattice patterns, by comparing with the unpat‐ terned Ag film (the two red dash line represent the sum and difference between the average and uncertainty of signal strength). Fig.10(a) and 10(b) shows respectively the intensity of the two peaks versus the separation distance of the lattice patterns. It is seen from the fig‐ ures that for the two peaks, they demonstrated very similar dependence behavior on the separation distance. In comparison with signals from the unpartterned area, we notice that

(b) 1508 cm-1.

44 Organic Pollutants - Monitoring, Risk and Treatment

With the highly sensitive SERS substrates described before, one can detect trace amount or‐ ganic molecules by the SERS method.

SERS is extremely sensitive in water solutions, for water does not have any Raman peaks. When detecting organic pollutants in nonaqueous systems, we use volatile organic solvents, such as acetone, to dilute pollutants. As the organic solvent shows high Raman background, we need to make the solvent volatilizated completely before SERS measurement.

The powders of 2, 3, 3′, 4, 4′-pentachlorinated biphenyl used in this study were commercial‐ ly available fromthe AccuStandard Company. Since there is no Raman data of 2, 3, 3′, 4, 4′ pentachlorinated biphenyl reported, we first measured its Raman spectrum and that of acetone, for comparison, see Fig 11(a). To clearly show most characteristic peaks of 2, 3, 3′, 4, 4′-pentachlorinated biphenyl, the Raman spectrum was plotted in two regions of 300 to 1000 cm-1 and 1000 to 1700 cm-1 respectively, see Figs 11(b) and 14(c). From the Figs one sees that the strongest peaks are located at 342, 395, 436, 465, 495, 507, 517, 598, 679, 731, 833, 891, 1032, 1136, 1179, 1254, 1294, 1573, and 1591 cm-1, respectively; while for acetone the charac‐ teristic peaks are at 530, 786, 1065, 1220, 1428 and 1709 cm-1, respectively. It is suggested that 2, 3, 3′, 4, 4′-pentachlorinated biphenyl is distinguishable from acetone and that acetone can be used as the solvent for the SERS measurements, as 2, 3, 3′, 4, 4′-pentachlorinated biphen‐ yl is not soluble in water.

Because the SERS sensitivity is also dependent on the sample treatment, we employed in this study a very simple method to prepare SERS samples, i.e. dropping a small volume (~ 0.5 uL) of solutions of 2, 3, 3′, 4, 4′-pentachlorinated biphenyl in acetone on Ag nanorods using a top single channel pipettor and then blowing away the acetone with a continuous, gentle nitrogen blow. Fig 12(a) shows the Raman spectra of 2, 3, 3′, 4, 4′-pentachlorinated biphenyl dissolved in acetone at concentrations of 10-4 to 10-10mol/L, respectively. The accu‐ mulation time of each Raman spectrum was 50 seconds and we used only 1% laser power to avoid changing of pentachlorinated biphenyl. When the small volume (~ 0.5 uL) of solutions of 2, 3, 3′, 4, 4′-pentachlorinated biphenyl was dropped on Ag nanorods, it became a circular spot with diameter of about 4 mm. The Raman spectrum was accumulated from a 2 um di‐ ameter circular area on the substrates. Therefore, for the solution at concentration of 10-10mol/L, only about ten 2, 3, 3′, 4, 4′-pentachlorinated biphenyl molecules (2\*10-23mol) would be accumulated in SERS; if that was at concentration of 10-8mol/L, about 1000 mole‐ cules (2\*10-21mol) would be accumulated and so on.

(a) (b)

The Detection of Organic Pollutants at Trace Level by Variable Kinds of Silver Film with Novel Morphology

http://dx.doi.org/10.5772/53602

47

**Figure 12.** a) SERS spectra of PCB-5 dissolved in acetone with various concentrations; (b) SERS spectrum of PCB-5 in

One sees that the Raman peaks of 10-4 mol/L PCB-5 solution located at 342, 495, 598, 679, 1032, 1136, 1179, 1254, 1294, 1573 and 1591 cm-1 march the Raman peaks of powder PCB-5 very well, this is quite different from the characteristic peaks of acetone. The peak around 1390 cm-1 represents disordered and amorphous carbon on the substrates. Fig 12(b) shows the SERS spectra of 10-8 mol/L PCB-5. Peaks located at 495, 1032, 1294, 1573 and 1591 cm-1 can march the Raman peaks of powder PCB-5. It indicates that the peaks shown in Fig 12(a) and (b) are the characteristic peaks of dissolved PCB-5, and PCB-5 with a concentration of

Large scale arrays of aligned and well separated single crystalline Ag nanorods on planar silicon substrate can be fabricated by GLAD method and these Ag films can be used as SERS substrates. With these substrates 2, 3, 3', 4, 4'- PCB-5 molecules were detected even at a con‐ centration of 10-8mol/L by the SERS method, which indicates that trace amount of PCBs can

10-8mol/L can be detected by the SERS method in the authors' work.

be detected by the SERS method with Ag nanorods as SERS substrates [44].

acetone at a concentration of 10-8mol/L.

**Figure 11.** a) Comparison of Raman spectra of PCB-5 powders and acetone; (b) and (c) show details of the Raman spectrum of PCB-5 powders.

The Detection of Organic Pollutants at Trace Level by Variable Kinds of Silver Film with Novel Morphology http://dx.doi.org/10.5772/53602 47

**Figure 12.** a) SERS spectra of PCB-5 dissolved in acetone with various concentrations; (b) SERS spectrum of PCB-5 in acetone at a concentration of 10-8mol/L.

One sees that the Raman peaks of 10-4 mol/L PCB-5 solution located at 342, 495, 598, 679, 1032, 1136, 1179, 1254, 1294, 1573 and 1591 cm-1 march the Raman peaks of powder PCB-5 very well, this is quite different from the characteristic peaks of acetone. The peak around 1390 cm-1 represents disordered and amorphous carbon on the substrates. Fig 12(b) shows the SERS spectra of 10-8 mol/L PCB-5. Peaks located at 495, 1032, 1294, 1573 and 1591 cm-1 can march the Raman peaks of powder PCB-5. It indicates that the peaks shown in Fig 12(a) and (b) are the characteristic peaks of dissolved PCB-5, and PCB-5 with a concentration of 10-8mol/L can be detected by the SERS method in the authors' work.

Large scale arrays of aligned and well separated single crystalline Ag nanorods on planar silicon substrate can be fabricated by GLAD method and these Ag films can be used as SERS substrates. With these substrates 2, 3, 3', 4, 4'- PCB-5 molecules were detected even at a con‐ centration of 10-8mol/L by the SERS method, which indicates that trace amount of PCBs can be detected by the SERS method with Ag nanorods as SERS substrates [44].

**Figure 11.** a) Comparison of Raman spectra of PCB-5 powders and acetone; (b) and (c) show details of the Raman

spectrum of PCB-5 powders.

46 Organic Pollutants - Monitoring, Risk and Treatment
