案例——水中草甘膦的光学探测

背景：

草甘膦(N-磷酸甘氨酸)用于全球范围内不同的除草剂的商业配方。作为一种用途广泛的和系统性的除草剂，草甘磷被成功地用于控制文化和花园的杂草。抗除草剂转基因作物的发展使的草甘磷在农业上广泛应用。

讨论了人类的健康风险，考虑到食物中除草剂的残留和水污染的后果。一些研究的侧重于草甘磷的致癌性，而另一些研究表明，在地下水中积累的金属草甘磷化合物增加了水的硬度，这可能是导致肾脏疾病的原因。然而，由于其广泛的应用和常规分析检测的复杂性，仍然需要对其检测的高度敏感、快速和成本有效的方法进行开发。表面增强的拉曼光谱(SERS)已经成为一种用于检测水中污染物和食物中除草剂痕迹的可选择的技术。SERS克服了基于拉曼散射的审讯技术的一个最重要的局限性:小截面的低强度的信号和分析低浓度分子。电磁和电荷转移的两种机制，是由拉曼散射在金属纳米结构附近的加剧造成的。金属纳米结构中的光入射可能会产生一种电子的相干震荡在表面产生一种局部的表面等离子共振(LSPR)。当分析物接近纳米结构时，LSPR的强电磁场可能产生SERS。分析物也可能被吸附在金属纳米粒子的表面，与这个表面交换电荷，产生放大的振荡和Raman散射。一些人已经提出了一种方法用来检测固定基底的草甘磷以及其他物质的纳米结构的胶体溶液。草甘磷的Raman光谱结果不仅来自于与基质的相互作用，而且也与使用的溶剂有关。因此，用固体和液体基质获得不同的光谱。利用水作为溶剂，观察到由氢键产生的依赖于溶液的pH值扩。

实验：

仪器：卤钨灯(LS-1, Ocean Optics)，光纤(Ocean Optics P200-2-UV-VIS)，光谱仪(Ocean Optics, HR 4000, 200 - 1100 nm composite grating HC-1)、632.8 He-Ne 激光器(Uniphase, 11335P, 9 mW power on the sample position)、滤波器(Thorlabs, FL05632.8-1)、光纤(Thorlabs, M37L02)、探测器(Horiba, iHR550, 1200 g/mm grating blazed at 600 nm)、CCD探测器(Synapse, 1024 x 256 elements)、电子显微镜(Zeiss, EVO-MA15)

实验结果：

不同胶体样本的实际外观见图2。从左到右：水+草甘膦、新鲜的AgNPs、添加66草甘膦的AgNPs、添加100μL新鲜草甘膦的AgNPs和两周后的添加100μL新鲜草甘膦的AgNPs。

随着草甘膦含量的增加，胶体的颜色因为离子团从黄色到深绿色。尽管这个不稳定、几天后出现的明确的沉淀（右边样品）。这些草甘膦的浓度，溶液PH值接近5。

图3(a)显示胶体的光谱：新鲜的AgNPs（蓝线）、添加66μL草甘膦的AgNPs（红线）、添加100μL草甘膦的AgNPs（黑线）。第二共振带的显示，AgNPs LSPR带在400nm,相对原始的LSPR band带有一个相对红移。

结论：

我们演示了一种能定量分析水中的草甘磷的光谱光纤传感器。该传感方案依赖于激光剥蚀在柠檬酸钠溶液中所产生的银纳米粒子间的相互作用，在测量时间范围内形成稳定的胶体和分析物。消光和两种不同浓度的样品的Raman光谱验证了这一概念，导致了在水中探测极限为1.7mg/L的草甘磷。

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文章来源：R. E. de Góes等在2017年发表的文章“Optical Detection of Glyphosate in Water”