Synthesis, Property And Application Of Silver Nanoclusters Capped By Hyperbranched Polyethyleneimine

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 SolutionHighly 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 NanoclustersIn 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 FluorescenceThe 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 NanoclustersSilver 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 StrengthAg 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.