水环境中Hg(Ⅱ)的污染对生态环境和人类健康危害极大, 目前Hg(Ⅱ)的检测主要有原子光谱/质谱和电化学等方法, 但存在检测仪器昂贵、 操作繁琐及前处理复杂等缺点, 难以在日常水环境中微量Hg(Ⅱ)现场检测的应用。 因此, 建立一种灵敏、 准确、 快捷和经济的水中Hg(Ⅱ)检测方法具有重要意义。 试纸法是将普通的化学反应从玻璃仪器转移到试纸上进行的一种快速检测方法, 利用试剂与目标物之间产生的化学反应, 通过颜色的变化可对目标物进行定性或半定量检测, 具有操作简便、 快速等优点。 碳量子点是一类粒径小于10 nm的碳基纳米材料, 具有优异的荧光性能、 较低的毒性和较高的化学稳定性。 利用Hg(Ⅱ)对碳量子点的荧光具有灵敏和高效的猝灭作用, 构建了一种双色比率荧光试纸片用于快速检测水中微量Hg(Ⅱ)的含量。 其中, 采用氮掺杂水溶性碳量子点(NCDs)作为荧光响应信号、 罗丹明B(RhB)作为荧光内标信号, 在单一波长(355 nm)激发下产生位于440和580 nm的双色荧光发射峰。 当体系加入不同浓度Hg(Ⅱ)后, NCDs表面官能团与Hg(Ⅱ)之间的静电作用和金属配位协同作用使荧光发生猝灭, 而RhB的荧光信号保持不变, 利用440和580 nm双色荧光信号或其强度的比值(F440/F580), 可实现对微量Hg(Ⅱ)的快速检测。 实验对检测条件进行了优化, 结果表明在HAc-NaAc缓冲液浓度为1 mmol·L-1、 pH为7的条件下, F440/F580值与Hg(Ⅱ)浓度(0～3 μmol·L-1)呈现良好的线性关系, 线性方程为F440/F580=-0.785 2ｃHg(Ⅱ)+3.103 8, 相关系数r＞0.99, 以3倍标准偏差计算的检出限为2.7 nmol·L-1(n=9)。 对湖水与自来水中Hg(Ⅱ)进行加标回收实验, 其加标回收率在91.9%～117.9%之间, 说明该方法灵敏、 准确, 能用于水中Hg(Ⅱ)的检测。 同时, 将NCDs和RhB浸渍于尼龙片上构建了双色比率荧光检测试纸片, 在紫外灯(365nm)照射下可观测到试纸发射淡蓝紫色荧光。 而随着Hg(Ⅱ)浓度的增加, 荧光颜色从淡蓝紫色到橙色发生变化, 每次检测时间只需3分钟, 裸眼可检出Hg(Ⅱ)浓度低至10 nmol·L-1, 实现了对水中微量Hg(Ⅱ)的灵敏、 快速检测。 此外, 该方法对Hg(Ⅱ)的检测表现出良好的特异性。 因此, 基于碳量子点和罗丹明B构建的双色比率荧光试纸片具有携带方便、 操作简单, 以及灵敏和快速等优点, 为水中微量Hg(Ⅱ)的快速检测提供了新的方法和思路。

The pollution of Hg(Ⅱ) in water environment is extremely harmful to the ecological environment and human health. The current methods for Hg(Ⅱ) detection mainly include atomic spectrometry, mass spectrometry, electrochemistry and so on. But the traditional detection methods need expensive equipments, complicated operation processes, and complex sample preparation, which limit the applications of these methods for Hg(Ⅱ) detection in real samples. It is still a great challenge to develop a sensitive, rapid, simple and cost-effective method for trace Hg (Ⅱ) detection in aqueous solutions. Test paper method is a rapid detection method which transfers the chemical reactions from the glass instruments to test paper. Based on the chemical reaction between the reagents and the targets, the test paper can qualitatively or semi-quantitatively detect the targets through color changes. Carbon dots (CDs), which are the carbon-based nanomaterials with particle sizes less than 10 nm, have many excellent fluorescence properties including low toxicity and high chemical stability. Inspired by the test paper method, a two-color fluorescent test strip for Hg(Ⅱ) detection in water was constructed based on the fact that the fluorescent of CDs could be effectively quenched by Hg(Ⅱ). The sensor, comprised of nitrogen doped carbon dots (NCDs) and rhodamine B (RhB), exhibited dual color emissions at 437 and 575 nm respectively under a single excitation wavelength of 350 nm. When the detection system is added with different concentrations of Hg(Ⅱ), the photoluminescence of the NCDs can be quenched by Hg(Ⅱ) due to synergetic strong electrostatic interaction and metal-ligand coordination between the surface functional group of NCDs and Hg(Ⅱ), while the fluorescence of RhB remains unchanged, and Hg(Ⅱ) can be quantitatively detected based on the ratios of the dual fluorescence emissions (F440/F580). Under the optimized detection conditions of 1 mmol·L-1 HAc-NaAc buffer solution at pH 7, the ratios of F440/F580 were linearly corresponded to Hg(Ⅱ) at the concentrations ranging from 0 to 3 μmol·L-1 with a linear equation of F440/F580=-0.785 2cHg (Ⅱ)+3.103 8 (r>0.99). The detection limit was 2. 7 nmol·L-1 (n=9) based on three standard deviations. The adding standard recoveries of Hg(Ⅱ) detection in lake water and tap water ranged from 91.9% to 117.9%. A visualized two-color fluorescent test strip was prepared by a simple soaking method of NCDs and RhB under optimal conditions. Upon the addition of different concentrations of Hg(Ⅱ), the color of test paper changed from light purple to orange accordingly under a UV lamp (365 nm), in which each of the detection time took only 3 minutes, and Hg(Ⅱ) could be detected as low as 10 nmol·L-1 by naked eyes. In addition, the detection method presented excellent specificity. Therefore, the established method has the advantages of high sensitivity and accuracy, easy operation and portability, and can be used to rapidly detect Hg(Ⅱ) on-site in water environment.