表面增強拉曼光譜技術在反恐物質快速篩選上的應用

Abstract

研究以表面增強拉曼光譜法 (SERS)，成功的發展了碳疽桿菌主成分DPA (dipicolinic acid) 及火炸藥主分TNT (2,4,6-trinitrotoluene) 之快速篩選方式。一般未添加奈米銀的情況下，難以觀測到DPA或TNT的拉曼光譜。添加奈米銀雖有助於增加拉曼散射的強度，但是以傳統方式配製的奈米銀，經常在過程當中加入氯化鈉等含氯離子的鹽類作為聚合劑。這類的聚合劑對DPA的拉曼散射強度不但沒有幫助，反而會有抑制的現象。本實驗開發三種不同還原方式，包括檸檬酸鈉還原法、鹽酸羥胺還原法與硼氫化鈉還原法，配製出適合吸附DPA與TNT的具特殊性質奈米銀。尤其以檸檬酸鈉還原法配製的奈米銀，搭配硫酸鈉或是硼酸鈉對增強DPA的拉曼散射效果最好。其中波數1010 cm-1的振動峰（symmetric ring breathing mode），具有明顯特徵性，可供定性與定量分析之用。此等奈米銀溶液呈黃綠色，粒徑分布落在30 ~ 80 nm，UV吸收最大值為440 nm，半高寬為130 nm，對DPA的偵測極限約為100 ppm。本實驗發現，當添加硫酸鈉作為聚合劑時，還能更加提高DPA的拉曼散射強度。這是因為硫酸鈉會破壞奈米銀的電雙層結構，使得大量的DPA能吸附在奈米銀的表面上。在這樣的情況下，DPA的偵測極限甚至能達到0.5 ppm。為了瞭解DPA與奈米銀的吸附方式，本研究以理論計算結果與實驗值進行比對。結果發現C-H out-of-plane 的訊號並沒有增強，但是844 cm-1(C-COO bend)則有明顯增強。因此，推測DPA是以羧基接觸奈米銀表面，使DPA分子垂直吸附到奈米銀顆粒上。此外，本研究也發現檸檬酸鈉還原法配製的奈米銀，對於TNT的拉曼散射有明顯的增強效果。TNT的偵測極限可達到10 µgL-1。在特徵譜峰上，931 cm-1、952 cm-1 (C-H (ring ) out of plane bend vibrational mode) 與1372 cm-1 (NO2 symmetric, C-N stretching mode)的訊號，更有明顯被增強。由被增強的訊號位置，本研究推論TNT是以苯環平躺在奈米銀表面的方式吸附。In this study, a rapid screening method bases on surface-enhanced Raman Spectroscopy (SERS) is successfully developed to detect TNT (2,4,6-trinitrotoluene) and DPA (dipicolinic acid), which is the main component of explosives and the excellent marker compound in Bacillus anthracis (anthrax). It is very difficult to obtain the Raman signals of DPA and TNT without adding nano-silver in the sample. Although the Raman scattering intensity can be improved by adding nano-silver made by traditional method to the sample, aggregation reagent like sodium chloride is always added to enhance the SERS signal, too. It has been found that when sodium chloride is treated as aggregation reagent, the SERS signal of DPA would be suppressed. In this research, three different nano-silver colloids are made by three different methods, which are named sodium citrate method, hydroxylamine method and borohydride method, respectively. The nano-silver colloids synthesized by different reductants have different particle radius and surface charge characters, and the colloid reduced by sodium citrate is the best colloid to enhance the SERS signal of DPA and TNT. When this colloid is added to DPA, the signal at 1010cm-1 (symmetric ring breathing mode) is obvious characteristic, then DPA can be quantitatively and qualitatively analyzed by monitoring this signal. The colloid reduced by sodium citrate has a yellow-green looking, and the particle size is from 30 to 80 nm, and the FWHM is 130 nm. When mixing the colloid with DPA, the detection limit is about 100 ppm. In this research finds that when sodium sulfate is treated as aggregation reagent, the SERS signals of DPA can be enhanced even more. This is because the double layer of nano-silver will be destroyed by the sodium sulfate , making a large number of DPA can be effectively adsorbed on the surface of nano-silver. Under such circumstances, the detection limit of DPA can down to 0.5 ppm. To understand how DPA adsorbs to the nano-silver, herein compared the results of the assignments of Raman vibrational mode to the SERS peaks. The results showed that the signal of C-H out-of-plane vibrational mode was not enhanced, but the signal at 844 cm-1(C-COO bend) was enhanced obviously. Confirming that the adsorption of DPA molecule occurs perpendicular to the surface of the silver particle. Besides, this study also found that the colloid reduced by sodium citrate has great enhance ability to the SERS signal of TNT, and the detection limit is 10µgL-1. Comparing the SERS peaks of TNT with the theoretic assignments, the signals at 931 cm-1、952 cm-1 (C-H (ring ) out of plane bend vibrational mode) and at 1372 cm-1 (NO2 symmetric, C-N stretching mode) are enhanced obviously. The relatively strong enhancement of these modes suggest that TNT molecules on nano-silver particle are oriented parallel to the surface of the particle.