以聚乙烯亚胺为模板合成银纳米簇及其性能与相关应用的研究
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摘要
仅有几个原子组成的金属纳米簇被认为是连接金属原子与纳米颗粒的桥梁。这些金属纳米簇具有与众不同的光、电以及化学性质,在近年来备受瞩目,其较小的尺寸、无毒性以及光稳定性等,都使其作为一种新型的荧光探针在化学检测及生物标记等领域具有广泛的应用前景。然而,这些尺寸微小的纳米簇极易团聚,使其在水相中合成十分困难。发展简便与环境友好的合成方法以获得具有特定组成及尺寸的金属纳米簇,对其实用化具有重要意义。与此同时,对于这一新型纳米材料的生长机制及其尺寸依赖的物理化学性质仍需进一步探索。本文对以聚乙烯亚胺(PEI)为模板的银纳米簇的合成、性能以及相关应用进行了研究与初步探索。
1.以PEI为模板合成银纳米簇
以PEI为模板、甲醛为还原剂,发展了一种简便的银纳米簇的水相合成方法,得到了稳定的、具有较高荧光强度的银纳米簇,其平均粒径约为1.8nm。实验中考察了Ag-配基的摩尔比、还原剂的种类及用量对合成银纳米簇的影响,以期得到更加稳定的银纳米簇。此外,实验还进一步利用荧光光谱、紫外可见吸收光谱、高分辨率透射电镜、X-射线粉末衍射以及热重分析等方法对所合成的银纳米簇的光学性能、尺寸以及形貌进行表征。以硫酸奎宁作为参照,PEI修饰的银纳米簇在乙醇中的量子产率约为3.8%。由于模板为高枝状聚合物,其三维网状结构可以对纳米簇起到有效的保护作用,因而以PEI为模板的银纳米簇的稳定性十分优异。
2.以PEI为模板的银纳米粒子在粒径转化过程中物理化学性质的研究
金属纳米簇具有明显的尺寸依赖的物理化学性质。实验中发现,当合成温度较高时,将首先形成粒径较大的银纳米颗粒,导致荧光猝灭；然而,随着放置时间的延长,PEI修饰的银纳米粒子出现了由纳米颗粒向纳米簇转变的现象,且伴随有明显的溶液颜色变化及荧光的恢复。实验中使用荧光光谱、紫外可见吸收光谱对这一过程进行详细的记录,并同时利用透射电镜、X-射线粉末衍射等方法证实了这一过程中银纳米粒子粒径的转化。当合成温度为90℃、加热时间为10min时,在室温放置过程中,银纳米粒子的平均粒径将由24h的5.3nm减小至168h的3.9nm,且粒径为1.0-2.0nm粒子所占比例大幅增加。与此同时,溶液颜色也由加热后的深红棕色逐渐变化至明亮的黄色,且其荧光显著增强。作为配基,PEI在这一转化过程中发挥了不可或缺的作用。同时,这一过程也清楚地显示了当由金属性的纳米颗粒向非金属性的纳米簇转化过程中,银纳米簇独特的尺寸依赖的物理化学性质的再现。
3.以PEI为模板的银纳米簇的溶剂化荧光现象及浓度依赖的荧光特性
目前,研究者对金属纳米簇的溶剂化效应仍然知之甚少。在本章工作中,首先在水溶液中合成了以PEI为模板的银纳米簇,然后将其分散到10种不同的有机溶剂中,包括甲醇、乙醇、乙二醇、正丙醇、异丙醇、N,N-二甲基甲酰胺、乙腈、四氢呋喃、二甲亚砜以及乙二醇甲醚,实验发现PEI修饰的银纳米簇的荧光颜色与强度存在明显的溶剂效应,而其溶液颜色并没有发生明显变化。例如,当PEI修饰的银纳米簇分散至四氢呋喃溶剂中时,其荧光颜色将由强烈的蓝色变化至明亮的黄色；而在乙二醇甲醚溶剂中,银纳米簇的荧光颜色将随放置时间的延长,从蓝色变化至绿色；然而,在这两种溶剂中,银纳米簇的紫外特征吸收峰仅表现出了吸光度的增强,而吸收峰位置并没有发生变化,即溶液颜色没有改变。这意味着溶剂诱导的由配体向金属核的电子转移的变化仅影响了银纳米簇的激发态而不是基态。因此,PEI修饰的银纳米簇仅表现出了明显的溶剂化荧光现象,却并不具有溶剂显色效应。此外,PEI修饰的银纳米簇还表现出了浓度依赖的荧光特性,其最大激发波长以及最大发射波长均随银纳米簇浓度的增大而发生明显的红移,因此,通过调节银纳米簇的浓度即可获得不同颜色的荧光。
4.以PEI为模板的银纳米簇：一种新型的高灵敏pH值传感器
金属纳米簇的荧光强度及发射波长对周围环境介质高度敏感,这就使其有希望发展成为一种多功能的传感器。而具有pH敏感的光学响应的纳米材料在环境、医药以及生物等领域的应用日益重要。在本章中,以PEI为模板的银纳米簇被用来作为一种高灵敏的pH值传感器,随着酸度的增加,其荧光逐渐猝灭,且溶液颜色也将由无色变至黄色,这就使得PEI修饰的银纳米簇还可以作为pH值显色剂进行应用。PEI修饰的银纳米簇的pH值的线性响应范围为5.02至7.96,在这一范围内,随着pH值的增加,其荧光可增强近10倍。此外,PEI修饰的银纳米簇在不同的缓冲溶液中也表现出了良好的pH响应特性。pH值诱导的PEI的构象变化及随后的银纳米簇的聚集导致了其荧光猝灭及溶液颜色的变化。以上这些均表明PEI修饰的银纳米簇作为pH值传感器将在生物、医学以及制药等领域具有广泛的应用前景。
5.以PEI为模板的银纳米簇：一种新型的荧光及比色传感器高灵敏测定卤素离子
卤素离子是日常生活中常见的阴离子,其在工业、医疗及自然环境中均发挥着重要的作用。在本章中,PEI修饰的银纳米簇被用来作为一种新型的荧光及比色传感器高灵敏、高选择性地测定卤素离子(C1-、Br-以及I-)。由于卤素离子与银原子间的特征反应以及卤化银较低的溶度积常数,卤素离子诱导的银纳米簇的氧化腐蚀与聚集,导致了其荧光猝灭与溶液颜色的变化。与其他纳米尺度的卤化物传感器相比,PEI修饰的银纳米簇作为卤素离子传感器显示出更宽的线性范围与更低的检.出限,荧光法测定Cl-、Br-及Γ的线性范围依次为0.5-80μM、0.1-14μM及0.05-6μM；Cl-、Br-及r的最低检出限分别为200、65及40nM。特别是在较高离子强度的介质中,Br-或是Γ可以在与Cl-共存的情况下被选择性地识别,这在测定Br-与I-的实际样品中具有明显优势。同时,PEI修饰的银纳米簇还可用于比色法测定卤素离子,比色法具有简便、快速、测定成本低廉等优点。此外,PEI修饰的银纳米簇作为荧光探针也被成功用于自来水及矿泉水中Cl-含量的测定。
Noble metal nanoclusters, consisting of a small number of atoms with the size comparable to the Fermi wavelength of electrons, are considered to provide a valuable link between metal atoms and nanoparticles. Such nanoclusters have received considerable interests in recent years because their remarkable optical, electrical, and chemical properties are significantly different from the large noble metal nanoparticles. Because of the nanometer-size, nontoxicity, and photostability, silver nanoclusters are promising candidates as fluorescent probes for labeling and sensing applications. The synthesis of well-defined silver nanoclusters in aqueous solutions, however, is difficult due to the tendency of Ag nanoclusters to aggregate, so it is essential to develop simple and environmentally friendly synthetic methods to obtain silver nanoclusters with particular compositions and sizes. Meanwhile, some fundamental issues of these new emitters still remain unclear, such as the growth mechanism as well as the size-dependent physicochemical properties. In this dissertation, valuable explorations have been carried out on the synthetic method, property, and application of silver nanoclusters capped by hyperbranched polyethyleneimine (PEI).
1. Synthesis of Highly Stable Fluorescent Ag Nanoclusters Capped by Hyperbranched Polyethyleneimine in Aqueous Solution
Highly fluorescent, stable, and water-soluble Ag nanoclusters have been successfully prepared by using PEI as a capping agent. The optical and fluorescent properties of Ag nanoclusters can be primarily controlled by varying the Ag-to-ligand molar ratios, the different types of reducing agents, and the volumes of formaldehyde used. The fluorescence spectra, UV-vis spectra, high resolution transmission electron microscopy (HRTEM), X-ray powder diffraction (XRD), and thermogravimetry analysis (TGA) were carried out to characterize the optical properties and morphologies of PEI-capped Ag nanoclusters. The as-prepared PEI-capped Ag nanoclusters exhibit a quantum yield of3.8%in ethanol calculated by using quinine sulfate as a reference. Because of the protective structure of three-dimensional network of the polymer template, the PEI-capped Ag nanoclusters also show high stability.
2. Transition from Nanoparticles to Nanoclusters:Microscopic and Spectroscopic Investigation of Size-Dependent Physicochemical Properties of Polyamine-Functionalized Silver Nanoclusters
In this paper, an interesting process is described in the synthesis of silver nanoclusters capped by PEI:a transition from nanoparticles to nanoclusters takes place spontaneously over time accompanied with the reappearance of size-dependent physicochemical properties of silver nanoclusters. The TEM images, XRD patterns, fluorescence spectra, and UV-vis spectra accurately record this process. As PEI-capped Ag particles were prepared by a heating process at90℃for10min and then a cooling process to ambient temperature, the average diameter changed from5.3nm after24h to3.9nm168h later and the percentage of the particles of1.0-2.0nm increased significantly over168h. Meanwhile, the color of Ag colloid solutions changed from deep reddish brown to bright yellow with an obvious enhancement of fluorescence intensity. The "smart" ligand shell of PEI plays a key role in the size transition. It could tailor the nanoparticles to just the right size of nanoclusters, resulting in the color change and fluorescence recovery. Therefore, this work clearly exhibits the reappearance of size-dependent physicochemical properties of silver nanoclusters with size reduction in the range where the transition from metallic to molecular behavior takes place.
3. Solvatofluorochromism of Polyethyleneimine-Encapsulated Ag Nanoclusters and Their Concentration-Dependent Fluorescence
The solvatochromism of metal nanoclusters is still an argument topic. In this work, on the basis of PEI-encapsulated Ag nanoclusters. we present some interesting results on the chemical-environment-responsive fluorescence of Ag nanoclusters in11different solvents, which may shed some light on this issue. In water and alcohols, the nanoclusters emit intense blue fluorescence; as they are dispersed in water-tetrahydrofuran (THF) mixtures, the fluorescent color changes from intense blue (in pure water) to intense yellow (in pure THF); while they are dispersed in ethylene glycol monomethyl ether (EGME), the fluorescence changes from blue to green color with time. However. accompanying with the changes of fluorescence spectra, there is no obvious shift of the absorption features of Ag nanoclusters in the solvents mentioned above. It means that the solvent-induced the changes of electron transfer from ligands to metal core actually influence the excited state rather than the ground state of Ag nanoclusters. Therefore, the PEI-capped Ag nanoclusters show obvious solvatofluorochromic but not solvatochromic properties. Furthermore, the concentration-dependent fluorescence of PEI-capped Ag nanoclusters has also been studied in this work. It is found that the emission could be tuned from blue to yellow by changing the concentration of Ag nanoclusters.
4. Highly Sensitive Fluorescent and Colorimetric pH Sensor Based on Polyethyleneimine-Capped Silver Nanoclusters
Silver nanoclusters capped by PEI have been developed as a highly sensitive fluorescent and colorimetric pH sensor. The probe responds rapidly to pH fluctuations and has such absorption characteristics that the color changes from the colorless or a nearly colorless state to a colored state with increasing acidity, so PEI-capped Ag nanoclusters could also be used as a color indicator for colorimetric pH detection. Quantitatively, the fluorescence intensity of PEI-capped Ag nanoclusters exhibits a linear fashion over the pH range of5.02to7.96, and increases by around ten-fold approximately with greater fluorescence at higher pH values. The repulsion development and conformational change of PEI with decreasing pH induce the aggregation of Ag nanoclusters, which is considered to produce an obvious color change and fluorescence quenching of Ag nanoclusters at low pH values. As expected, this pH probe is also sensitive to different buffer solutions, except for those containing some anions that could react with Ag nanoclusters. Besides, the ionic strength of the buffers has a little influence on the pH responsive behavior. Our pH sensor with nanoscaled physical dimensions would be a promising candidate in the applications in biological, medical, and pharmaceutical fields.
5. Polyethyleneimine-Templated Ag Nanoclusters:A New Fluorescent and Colorimetric Platform for Sensitive and Selective Sensing Halide Ions and High Disturbance-Tolerant Recognitions of Iodide and Bromide in Coexistence with Chloride under Condition of High Ionic Strength
Ag nanoclusters functioned by PEI have been developed as a new fluorescent and colorimetric platform for sensitive and selective recognition of halide ions (e.g.. Cl-. Br-and Γ). The recognition mechanism is based on the unique reactions between halide ions and the silver atoms. In particular, halide-induced oxidative etching and aggregation can produce a strong fluorescence quenching of Ag nanoclusters. This sensing system exhibits a remarkably high selectivity toward halide ions over most of anions and cations, and shows good linear ranges and lower detection limits:the linear ranges are0.5to80μM for Cl-,0.1to14μM for Br-, and0.05to6μM for I-, respectively; the limits of detection for Cl-, Br-, and Γ, at a signal-to-noise ratio of3, are estimated to be200,65, and40nM, respectively. Specifically, Br-and Γ could be recognized selectively in the coexistence with Cl-under the condition of higher ionic strength, which is a significant advantage in the detection of Br-and Γ in real samples. In addition, the recognition of halide could be performed by colorimetric method, which is also attractive and promising because of its simplicity, rapidity, and low cost. Furthermore, this sensing system has been applied successfully to the detection of Cl-in real water samples.
1.以PEI为模板合成银纳米簇
以PEI为模板、甲醛为还原剂,发展了一种简便的银纳米簇的水相合成方法,得到了稳定的、具有较高荧光强度的银纳米簇,其平均粒径约为1.8nm。实验中考察了Ag-配基的摩尔比、还原剂的种类及用量对合成银纳米簇的影响,以期得到更加稳定的银纳米簇。此外,实验还进一步利用荧光光谱、紫外可见吸收光谱、高分辨率透射电镜、X-射线粉末衍射以及热重分析等方法对所合成的银纳米簇的光学性能、尺寸以及形貌进行表征。以硫酸奎宁作为参照,PEI修饰的银纳米簇在乙醇中的量子产率约为3.8%。由于模板为高枝状聚合物,其三维网状结构可以对纳米簇起到有效的保护作用,因而以PEI为模板的银纳米簇的稳定性十分优异。
2.以PEI为模板的银纳米粒子在粒径转化过程中物理化学性质的研究
金属纳米簇具有明显的尺寸依赖的物理化学性质。实验中发现,当合成温度较高时,将首先形成粒径较大的银纳米颗粒,导致荧光猝灭；然而,随着放置时间的延长,PEI修饰的银纳米粒子出现了由纳米颗粒向纳米簇转变的现象,且伴随有明显的溶液颜色变化及荧光的恢复。实验中使用荧光光谱、紫外可见吸收光谱对这一过程进行详细的记录,并同时利用透射电镜、X-射线粉末衍射等方法证实了这一过程中银纳米粒子粒径的转化。当合成温度为90℃、加热时间为10min时,在室温放置过程中,银纳米粒子的平均粒径将由24h的5.3nm减小至168h的3.9nm,且粒径为1.0-2.0nm粒子所占比例大幅增加。与此同时,溶液颜色也由加热后的深红棕色逐渐变化至明亮的黄色,且其荧光显著增强。作为配基,PEI在这一转化过程中发挥了不可或缺的作用。同时,这一过程也清楚地显示了当由金属性的纳米颗粒向非金属性的纳米簇转化过程中,银纳米簇独特的尺寸依赖的物理化学性质的再现。
3.以PEI为模板的银纳米簇的溶剂化荧光现象及浓度依赖的荧光特性
目前,研究者对金属纳米簇的溶剂化效应仍然知之甚少。在本章工作中,首先在水溶液中合成了以PEI为模板的银纳米簇,然后将其分散到10种不同的有机溶剂中,包括甲醇、乙醇、乙二醇、正丙醇、异丙醇、N,N-二甲基甲酰胺、乙腈、四氢呋喃、二甲亚砜以及乙二醇甲醚,实验发现PEI修饰的银纳米簇的荧光颜色与强度存在明显的溶剂效应,而其溶液颜色并没有发生明显变化。例如,当PEI修饰的银纳米簇分散至四氢呋喃溶剂中时,其荧光颜色将由强烈的蓝色变化至明亮的黄色；而在乙二醇甲醚溶剂中,银纳米簇的荧光颜色将随放置时间的延长,从蓝色变化至绿色；然而,在这两种溶剂中,银纳米簇的紫外特征吸收峰仅表现出了吸光度的增强,而吸收峰位置并没有发生变化,即溶液颜色没有改变。这意味着溶剂诱导的由配体向金属核的电子转移的变化仅影响了银纳米簇的激发态而不是基态。因此,PEI修饰的银纳米簇仅表现出了明显的溶剂化荧光现象,却并不具有溶剂显色效应。此外,PEI修饰的银纳米簇还表现出了浓度依赖的荧光特性,其最大激发波长以及最大发射波长均随银纳米簇浓度的增大而发生明显的红移,因此,通过调节银纳米簇的浓度即可获得不同颜色的荧光。
4.以PEI为模板的银纳米簇：一种新型的高灵敏pH值传感器
金属纳米簇的荧光强度及发射波长对周围环境介质高度敏感,这就使其有希望发展成为一种多功能的传感器。而具有pH敏感的光学响应的纳米材料在环境、医药以及生物等领域的应用日益重要。在本章中,以PEI为模板的银纳米簇被用来作为一种高灵敏的pH值传感器,随着酸度的增加,其荧光逐渐猝灭,且溶液颜色也将由无色变至黄色,这就使得PEI修饰的银纳米簇还可以作为pH值显色剂进行应用。PEI修饰的银纳米簇的pH值的线性响应范围为5.02至7.96,在这一范围内,随着pH值的增加,其荧光可增强近10倍。此外,PEI修饰的银纳米簇在不同的缓冲溶液中也表现出了良好的pH响应特性。pH值诱导的PEI的构象变化及随后的银纳米簇的聚集导致了其荧光猝灭及溶液颜色的变化。以上这些均表明PEI修饰的银纳米簇作为pH值传感器将在生物、医学以及制药等领域具有广泛的应用前景。
5.以PEI为模板的银纳米簇：一种新型的荧光及比色传感器高灵敏测定卤素离子
卤素离子是日常生活中常见的阴离子,其在工业、医疗及自然环境中均发挥着重要的作用。在本章中,PEI修饰的银纳米簇被用来作为一种新型的荧光及比色传感器高灵敏、高选择性地测定卤素离子(C1-、Br-以及I-)。由于卤素离子与银原子间的特征反应以及卤化银较低的溶度积常数,卤素离子诱导的银纳米簇的氧化腐蚀与聚集,导致了其荧光猝灭与溶液颜色的变化。与其他纳米尺度的卤化物传感器相比,PEI修饰的银纳米簇作为卤素离子传感器显示出更宽的线性范围与更低的检.出限,荧光法测定Cl-、Br-及Γ的线性范围依次为0.5-80μM、0.1-14μM及0.05-6μM；Cl-、Br-及r的最低检出限分别为200、65及40nM。特别是在较高离子强度的介质中,Br-或是Γ可以在与Cl-共存的情况下被选择性地识别,这在测定Br-与I-的实际样品中具有明显优势。同时,PEI修饰的银纳米簇还可用于比色法测定卤素离子,比色法具有简便、快速、测定成本低廉等优点。此外,PEI修饰的银纳米簇作为荧光探针也被成功用于自来水及矿泉水中Cl-含量的测定。
Noble metal nanoclusters, consisting of a small number of atoms with the size comparable to the Fermi wavelength of electrons, are considered to provide a valuable link between metal atoms and nanoparticles. Such nanoclusters have received considerable interests in recent years because their remarkable optical, electrical, and chemical properties are significantly different from the large noble metal nanoparticles. Because of the nanometer-size, nontoxicity, and photostability, silver nanoclusters are promising candidates as fluorescent probes for labeling and sensing applications. The synthesis of well-defined silver nanoclusters in aqueous solutions, however, is difficult due to the tendency of Ag nanoclusters to aggregate, so it is essential to develop simple and environmentally friendly synthetic methods to obtain silver nanoclusters with particular compositions and sizes. Meanwhile, some fundamental issues of these new emitters still remain unclear, such as the growth mechanism as well as the size-dependent physicochemical properties. In this dissertation, valuable explorations have been carried out on the synthetic method, property, and application of silver nanoclusters capped by hyperbranched polyethyleneimine (PEI).
1. Synthesis of Highly Stable Fluorescent Ag Nanoclusters Capped by Hyperbranched Polyethyleneimine in Aqueous Solution
Highly fluorescent, stable, and water-soluble Ag nanoclusters have been successfully prepared by using PEI as a capping agent. The optical and fluorescent properties of Ag nanoclusters can be primarily controlled by varying the Ag-to-ligand molar ratios, the different types of reducing agents, and the volumes of formaldehyde used. The fluorescence spectra, UV-vis spectra, high resolution transmission electron microscopy (HRTEM), X-ray powder diffraction (XRD), and thermogravimetry analysis (TGA) were carried out to characterize the optical properties and morphologies of PEI-capped Ag nanoclusters. The as-prepared PEI-capped Ag nanoclusters exhibit a quantum yield of3.8%in ethanol calculated by using quinine sulfate as a reference. Because of the protective structure of three-dimensional network of the polymer template, the PEI-capped Ag nanoclusters also show high stability.
2. Transition from Nanoparticles to Nanoclusters:Microscopic and Spectroscopic Investigation of Size-Dependent Physicochemical Properties of Polyamine-Functionalized Silver Nanoclusters
In this paper, an interesting process is described in the synthesis of silver nanoclusters capped by PEI:a transition from nanoparticles to nanoclusters takes place spontaneously over time accompanied with the reappearance of size-dependent physicochemical properties of silver nanoclusters. The TEM images, XRD patterns, fluorescence spectra, and UV-vis spectra accurately record this process. As PEI-capped Ag particles were prepared by a heating process at90℃for10min and then a cooling process to ambient temperature, the average diameter changed from5.3nm after24h to3.9nm168h later and the percentage of the particles of1.0-2.0nm increased significantly over168h. Meanwhile, the color of Ag colloid solutions changed from deep reddish brown to bright yellow with an obvious enhancement of fluorescence intensity. The "smart" ligand shell of PEI plays a key role in the size transition. It could tailor the nanoparticles to just the right size of nanoclusters, resulting in the color change and fluorescence recovery. Therefore, this work clearly exhibits the reappearance of size-dependent physicochemical properties of silver nanoclusters with size reduction in the range where the transition from metallic to molecular behavior takes place.
3. Solvatofluorochromism of Polyethyleneimine-Encapsulated Ag Nanoclusters and Their Concentration-Dependent Fluorescence
The solvatochromism of metal nanoclusters is still an argument topic. In this work, on the basis of PEI-encapsulated Ag nanoclusters. we present some interesting results on the chemical-environment-responsive fluorescence of Ag nanoclusters in11different solvents, which may shed some light on this issue. In water and alcohols, the nanoclusters emit intense blue fluorescence; as they are dispersed in water-tetrahydrofuran (THF) mixtures, the fluorescent color changes from intense blue (in pure water) to intense yellow (in pure THF); while they are dispersed in ethylene glycol monomethyl ether (EGME), the fluorescence changes from blue to green color with time. However. accompanying with the changes of fluorescence spectra, there is no obvious shift of the absorption features of Ag nanoclusters in the solvents mentioned above. It means that the solvent-induced the changes of electron transfer from ligands to metal core actually influence the excited state rather than the ground state of Ag nanoclusters. Therefore, the PEI-capped Ag nanoclusters show obvious solvatofluorochromic but not solvatochromic properties. Furthermore, the concentration-dependent fluorescence of PEI-capped Ag nanoclusters has also been studied in this work. It is found that the emission could be tuned from blue to yellow by changing the concentration of Ag nanoclusters.
4. Highly Sensitive Fluorescent and Colorimetric pH Sensor Based on Polyethyleneimine-Capped Silver Nanoclusters
Silver nanoclusters capped by PEI have been developed as a highly sensitive fluorescent and colorimetric pH sensor. The probe responds rapidly to pH fluctuations and has such absorption characteristics that the color changes from the colorless or a nearly colorless state to a colored state with increasing acidity, so PEI-capped Ag nanoclusters could also be used as a color indicator for colorimetric pH detection. Quantitatively, the fluorescence intensity of PEI-capped Ag nanoclusters exhibits a linear fashion over the pH range of5.02to7.96, and increases by around ten-fold approximately with greater fluorescence at higher pH values. The repulsion development and conformational change of PEI with decreasing pH induce the aggregation of Ag nanoclusters, which is considered to produce an obvious color change and fluorescence quenching of Ag nanoclusters at low pH values. As expected, this pH probe is also sensitive to different buffer solutions, except for those containing some anions that could react with Ag nanoclusters. Besides, the ionic strength of the buffers has a little influence on the pH responsive behavior. Our pH sensor with nanoscaled physical dimensions would be a promising candidate in the applications in biological, medical, and pharmaceutical fields.
5. Polyethyleneimine-Templated Ag Nanoclusters:A New Fluorescent and Colorimetric Platform for Sensitive and Selective Sensing Halide Ions and High Disturbance-Tolerant Recognitions of Iodide and Bromide in Coexistence with Chloride under Condition of High Ionic Strength
Ag nanoclusters functioned by PEI have been developed as a new fluorescent and colorimetric platform for sensitive and selective recognition of halide ions (e.g.. Cl-. Br-and Γ). The recognition mechanism is based on the unique reactions between halide ions and the silver atoms. In particular, halide-induced oxidative etching and aggregation can produce a strong fluorescence quenching of Ag nanoclusters. This sensing system exhibits a remarkably high selectivity toward halide ions over most of anions and cations, and shows good linear ranges and lower detection limits:the linear ranges are0.5to80μM for Cl-,0.1to14μM for Br-, and0.05to6μM for I-, respectively; the limits of detection for Cl-, Br-, and Γ, at a signal-to-noise ratio of3, are estimated to be200,65, and40nM, respectively. Specifically, Br-and Γ could be recognized selectively in the coexistence with Cl-under the condition of higher ionic strength, which is a significant advantage in the detection of Br-and Γ in real samples. In addition, the recognition of halide could be performed by colorimetric method, which is also attractive and promising because of its simplicity, rapidity, and low cost. Furthermore, this sensing system has been applied successfully to the detection of Cl-in real water samples.
引文
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