激光诱导液面自组装法制备光纤SERS探针及其农药残留检测应用 
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1 引言
农药在保障农作物产量方面发挥着重要作用,广泛应用于蔬菜、水果等的各个种植生长阶段。然而,农药的不合理使用会导致农产品中农药残留超标,长期食用可造成人体肝脏、肾脏等器官的不可逆损伤甚至诱发癌变,严重威胁着人类身体健康[1]。因此,对农残高灵敏度快速识别与检测至关重要。目前,农残检测方法主要有气体层析质谱法、高效液相色谱法、免疫分析法、酶抑制法等[2-5]。其中,质谱法、色谱法检测灵敏度高,但存在样品预处理复杂、检测时间长等缺点;免疫分析法、酶抑制法可实现快速检测,但灵敏度较低,识别性相对较差。
近年来,表面增强拉曼散射(SERS)光谱因具有样品预处理简单、检测灵敏度高、检测时间短、分子指纹特性突出等显著优点[6-8],在农残检测中具有重要应用前景,受到人们的普遍关注。例如,在SERS基底制备方面,设计并制备出栗子形、三角形、爆米花形等[9-11]不同形貌的纳米颗粒,提高了农残检测灵敏度;在检测方式方面,发展了诸如溶胶混合、浸泡吸附和滴定干燥等方法[12-14],实现了对硫丹、福美双和甲基对硫磷等农残的SERS检测,并已开始尝试果蔬的SERS检测实际应用[15-16]。然而,迄今为止农残SERS检测大多是基于基片型SERS基底(即在硅或二氧化硅基片上制备贵金属纳米结构),由大型显微拉曼光谱仪实现,而基片型SERS基底在制备时难以实现贵金属纳米颗粒在宏观尺度上的均匀分布,进而影响检测重复性;同时,大型拉曼光谱仪价格昂贵,也不利于现场农残检测应用。
高灵敏度光纤SERS探针[17-18]为实现农残高灵敏度、高重复性、快速检测提供了一种新思路。光纤SERS探针通过提高SERS相互作用面积来提高检测灵敏度,且收集到的拉曼信号为相互作用面积上的整体积分,降低了对纳米颗粒分布均匀性要求,从而可有效提高检测重复性。不仅如此,光纤SERS探针还易于与便携式拉曼光谱仪联用,构建出便携式农残SERS光谱快检仪以满足现场检测需求。但是,基于光纤SERS探针的农残检测研究报道极少。本实验室在光纤SERS探针方面已开展了系列研究工作,发展了激光诱导化学沉积法[19-21]、静电自组装法[22]、激光诱导液面自组装法[23]等多种方法制备光纤SERS探针。本文将在前期激光诱导液面自组装的一步检测法[23]基础上,针对现场检测应用场景,进一步发展出两步检测法,并成功用于农残的快速检测。一步检测法是指通过将贵金属纳米颗粒溶胶与待测物溶液均匀混合,并将光纤置于其与混合溶胶形成的弯液面处,在合适的诱导激光下同时实现光纤SERS探针的制备和SERS光谱的检测,即“边制备边检测”。一步检测法中为了实现高的检测灵敏度,对实验条件提出了较为苛刻的要求[23],如光纤在弯液面的位置需精确到0.1 mm以内,溶胶初始温度为(25±1) ℃,环境湿度为30%~60%等。这在实验室条件下容易满足,但是在外场的实际检测环境中却难以保证,限制其实际现场检测应用。因此,本文提出两步检测法的“先制备后检测”策略,即先利用激光诱导液面自组装法在纳米颗粒溶胶中批量制备光纤SERS探针,然后将其用于待测溶液的SERS光谱检测。实验结果表明,在实验室条件下,利用激光诱导液面自组装法能够实现高灵敏度金纳米棒光纤SERS探针的可控、重复制备,结合自主研制的便携式拉曼光谱仪,其对福美双、甲基对硫磷等农残的SERS检测灵敏度分别达到10-7 mol/L和5×10-7 mol/L,检测重复性相对标准差(RSD)小于6%。这种基于激光诱导液面自组装的两步检测法在农残的现场快检中具有潜在应用前景。
2 激光诱导液面自组装法制备光纤SERS探针
激光诱导液面自组装法中[23],在诱导激光辐照下,光纤与纳米颗粒溶胶界面处形成弯液面,该弯液面内的贵金属纳米颗粒吸收光能量转换为热量,引起弯液面附近温度局部升高;该局域热效应与液体表面张力共同作用下,弯液面处纳米颗粒的浓度显著增加且较高的局域温度会加剧纳米颗粒的布朗运动,实现纳米颗粒在光纤端面的沉积。前期工作中,我们将银纳米立方体/乙醇溶胶与待测物(
激光诱导液面自组装法制备光纤SERS探针的实验装置如
图 1. 激光诱导液面自组装法制备光纤SERS探针示意图
Fig. 1. Experimental setup for preparing fiber SERS probes with laser-induced self-assembly method in a meniscus
图 2. (a)平端面光纤探针的SERS光谱检测实验装置; (b)激光诱导功率为70 mW时不同时间下制备的光纤SERS探针测试结果;(c)最强峰1376 cm-1处SERS信号强度与激光诱导时间的关系;(d)~(f)激光诱导时间为3,7,9 min时端面处的SEM图像
Fig. 2. (a) Homemade instrument for SERS detection using fiber SERS probes; (b) measured SERS spectra of fiber SERS probes fabricated with different laser radiation times when laser irradiation has a power of 70 mW; (c) relationship between SERS intensity and laser radiation time at Raman peak of 1376 cm-1; (d)-(f) SEM images of fiber probes with different laser radiation times at 3, 7, and 9 min, respectively
溶胶中匀速向上提拉形成的弯液面最大高度为0.27~0.3 mm,由于该弯液面高度对探针制备有影响[23],经反复实验,优化后的弯液面高度为(0.2±0.03) mm。
激光诱导功率和诱导时间是激光诱导液面自组装方法制备光纤SERS探针的关键。首先,将诱导激光功率固定为70 mW,根据激光诱导时间的不同(3~9 min),制备5根不同的光纤SERS探针。需要指出的是,利用光纤SERS探针对待测物进行SERS光谱检测时,由于纳米材料表面分子脱附困难,探针通常只能单次使用。将5根不同的光纤SERS探针分别插入10-5 mol/L福美双溶液中,利用实验室自行研制的光纤探针增强型便携式拉曼光谱仪[
其次,改变诱导激光功率至60 mW和85 mW,分析激光诱导功率对探针制备的影响。考虑到不同诱导功率下最优诱导时间可能不同,在每个功率下,改变诱导时间进行探针制备。
图 3. (a) 60 mW和(b) 85 mW下不同激光诱导时间制备的探针测试结果;(c) 60,70,85 mW下1376 cm-1处SERS信号强度与激光诱导时间的关系
Fig. 3. SERS results for the fiber probes at different laser powers of (a) 60 mW and (b) 85 mW; (c) relationship between SERS intensity and laser radiation time under different laser powers of 60, 70, and 85 mW for the Raman peak of 1376 cm-1
3 光纤SERS探针在农残检测中的应用
在上述优化实验条件(70 mW,7 min)下,重复制备16根光纤SERS探针,以备后续使用。以10-5 mol/L福美双溶液为例对光纤SERS探针测试重复性进行评估。
图 4. 优化条件下制备的光纤SERS探针对10-5 mol/L福美双的(a)检测重复性和(b)检测灵敏度
Fig. 4. SERS results of 10-5 mol/L thiram by the optimized fiber SERS probes. (a) Repeatability; (b) sensitivity
以甲基对硫磷为例,将该光纤SERS探针成功应用于有机磷农药残留的检测。将上述剩余的4根预先制备好的光纤SERS探针分别插入不同浓度的甲基对硫磷溶液中,利用自行研制的便携式拉曼光谱仪[
图 5. 不同浓度下甲基对硫磷的检测灵敏度
Fig. 5. SERS detection sensitivity of methyl parathion by optimized fiber SERS probes
4 结论
发展了基于激光诱导液面自组装的两步检测法,在金纳米棒溶胶中实现了高灵敏度光纤SERS探针的重复制备,进而实现氨基甲酸酯类农药残留福美双和有机磷类农药残留甲基对硫磷等的痕量、快速检测。实验中将光纤端面置于弯液面合适位置[距溶胶液面(0.2±0.03) mm],并研究激光诱导功率和诱导时间对探针制备的影响,得到优化的激光功率和诱导时间分别为70 mW 和7 min。优化条件下制备的光纤SERS探针与实验室自行研制的光纤探针增强型便携式拉曼光谱仪相结合,实现了两种典型农残的高灵敏度、高重复性检测:对福美双和甲基对硫磷的检测灵敏度分别达到10-7 mol/L和5×10-7 mol/L,检测重复性RSD小于6%。该激光诱导液面自组装两步检测法中,一方面通过在实验室环境中严格控制探针制备条件,能够实现高灵敏度光纤SERS探针的重复、批量制备,利于促进光纤SERS探针的实用化进程;另一方面,预先制备好的光纤SERS探针可以满足不同现场环境下的SERS检测需求,且其与便携式拉曼光谱仪相结合,为构建小型化农残快检仪提供一种新思路,具有潜在的应用前景。
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Article Outline
董子豪, 刘晔, 秦琰琰, 毛庆和. 激光诱导液面自组装法制备光纤SERS探针及其农药残留检测应用[J]. 中国激光, 2018, 45(8): 0804009. Dong Zihao, Liu Ye, Qin Yanyan, Mao Qinghe. Fabrication of Fiber SERS Probes by Laser-Induced Self-Assembly Method in a Meniscus and Its Applications in Trace Detection of Pesticide Residues[J]. Chinese Journal of Lasers, 2018, 45(8): 0804009.
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