苋菜红(Amaranth)作为一种人工合成食品添加剂, 常被添加于汽水、 山楂和糖果等当中, 但苋菜红是由煤焦油中分离出来的苯胺染料为原料制成, 过量食用会导致基因突变甚至致癌, 严重危害身体健康。 因此, 对其检测至关重要。 表面增强拉曼光谱技术具有样品前处理简单、 分析速度快和准确性高等优点, 已在化学、 生物和医学领域越来越显示出巨大的潜能。 目前, 用拉曼光谱技术对苋菜红检测的理论与实验研究还未见报道。 而对拉曼光谱及表面增强理论机理的研究可以为在食品中检测及鉴定苋菜红提供可靠的科学依据。 所以利用密度泛函理论全面探究苋菜红的表面增强拉曼机理并与实验结果进行对比, 对食品中的苋菜红检测研究有很好的预测及指导意义。 一方面, 利用共聚焦显微拉曼光谱仪对苋菜红粉末进行拉曼光谱检测, 得到其拉曼光谱; 另一方面, 搭建苋菜红分子结构, 并基于密度泛函理论对分子结构进行优化处理, 从前线轨道、 静电势、 极化率及自然键轨道布局分析四个角度进行计算分析, 得出偶氮基团处(-N15N16-)是苋菜红分子与Ag原子配位的最佳位置。 在此基础上, 使用B3LYP/6-31++G(d, p))基组(C, H, O, N, S, Na)和B3LYP/Sdd基组(Ag)对苋菜红分子与1个Ag原子及3个Ag原子团簇的复合物(Amaranth-Ag1, Amaranth-Ag3)进行结构优化和表面增强拉曼光谱计算。 将苋菜红分子的实验与理论拉曼光谱进行比较, 发现二者吻合较好, 且在1 228, 1 329, 1 467和1 529 cm-1处苋菜红分子的拉曼活性很明显。 另外, 苋菜红与Ag的复合物有明显的拉曼增强效应, 增强效果随着Ag原子个数的增多而愈加明显, 不仅拉曼光谱的峰值个数有增多, 而且其对应光谱峰位强度也有增强。 进一步通过振动模式的归属, 得到鉴定和识别苋菜红的拉曼特征峰。 该研究为利用表面增强拉曼光谱技术检测食品中的苋菜红提供了一定的实验参考和理论依据。

Amaranth is a synthetic food additive. It is often added to soda, hawthorn and candy, etc. However, Amaranth is made from the coal tar isolated by the benzene dye as raw material, and excessive consumption will lead to genetic mutations and even cancer, which is harmful to health. Therefore, the detection of Amaranth is important and necessary. Surface-enhanced Raman spectroscopy has the advantages of simple pre-processing, rapid analysis and high accuracy for samples, which has been increasingly shown great potential in the fields of chemistry, biology and medicine. At present, the theoretical and experimental research on Amaranth with Raman spectroscopy technology has not been reported. The study of Raman spectroscopy and the theoretical mechanism of surface enhancement can provide a reliable scientific basis for the detection and identification of Amaranth in food. Therefore, the study on the surface-enhanced Raman mechanism of Amaranth and compare it with the experimental results using the density functional theory is necessary. It has a good prediction and guiding significance for the detection of Amaranth in food. On the one hand, Amaranth was detected by using micro-confocal Raman spectrometer. On the other hand, the structure of Amaranth was built, and Density Functional Theory was used to optimize the molecular structure and calculate the front-line orbit, electrostatic potential, polarization and natural bond orbital analysis. And the azo group (-N15N16-) is the best position for Amaranth to coordinate with Ag atom. The optimum structure and surface-enhanced Raman spectra of Amaranth-Ag1, Amaranth-Ag3 complexes molecules were calculated by using B3LYP function with 6-31++G (d, p) (C, H, O, N, S, Na) base set and Sdd (Ag) base set, receptively. The results showed that the experimental results at 1 228, 1 329, 1 467 and 1 529 cm-1 were in good agreement with the theoretical results and the Raman activity of Amaranth molecule was obvious. Raman enhancement effect of Amaranth-Ag complex was significant and the Raman enhancement effect increases with the number of Ag atoms. It can be found that not only the numbers of Raman peaks increased, but also the corresponding intensity increased. Furthermore, the vibrational mode was assigned and the characteristic peaks for detection and inspection of Amaranth can be found. The study provides an experimental reference and theoretical basis for the detection of Amaranth by using surface-enhanced Raman spectroscopy.