Aqueous Synthesis And Room-Temperature Surface Ligand Chemistry Of Colloidal Nanocrystals

The most attractive feature of colloidal nanocryatals is their dimension-dependent properties. Controlled synthesis of monodisperse nanocrystals is critical for studying and exploiting those unique properties. In the recent 10 years, great progress has been made in synthesis of colloidal nanocrystals in non-aqueous solution mostly because of the improved understanding of the mechanisms for controlling their shape, size, and properties. In comparison to the synthesis in non-aqueous solution, aqueous-based synthesis is not well developed, although many outstanding scientists have paid a significant amount of attention to this subject. One of the apparent reasons is lack of a systematically and quantitatively studied model system for aqueous-based system.Surface organic ligands are well recognized as a key factor in synthesis and manipulation of colloidal nanocrystals. Their integral role in the function of nanocrystals-ligands complex is also well known. However, the bonding nature of organic liangds to the surface atoms of nanocrystals and the related bonding processes have been known very little. Challenges along this direction include preparation of quality surfaces, experimental detection methods, and mathematic models for quantitatively analyzing the experimental results.The above two aspects of nanocrystals chemistry are the focus of my dissertation. Aqueous-based synthesis was explored using noble metal colloidal nanocrystals, and ligand chemistry was studied using CdSe semiconductor nanocrystals as the model system. In addition, applications of the resulting nanocrystals were also briefly explored. The main results are outlined as followings. 1. Citrate reduction of dot-shaped gold nanorystals in aqueous solution: mechanisms and synthesis.Synthesis of gold nanocrystals by citrate reduction, reported by Turkevich et al in 1951, is the most studied aqueous system for synthesis of colloidal nanocrystals. The size variation of the resulting gold nanocrystals can be readily realized by simply changing the ratio of citrate and auric acid. Although the reaction system is usually quite simple in terms of starting materials, namely auric acid, sodium citrate, and water, the chemical reactions involved could be very complex. Typically, sodium citrate is a known weak base, which should change the solution pH in a certain extent if its concentration varies. In addition, the reactivity of the gold complexes and the molecular forces between citrate protection groups and gold surfaces were reported to be also strongly dependent on pH. These thoughts suggest that the role of sodium citrate as a pH mediator should not be completed ignored. Herein, we systematically and quantitatively studied the growth process of gold nanocrystals in citrate reduction. The results confirmed the hypothesis, that citrate plays a determining role as the pH mediator in the reaction system. With this understanding, nearly monodisperse gold nanocrystals in the size range from about 20nm to 40nm were synthesized by simply varying the solution pH.2. pH-mediated growth of branched gold nanocrystals without surfactant template.Following the success of the above chapter, this chapter shall demonstrate that, without using a surfactant template, branching of gold nanocrystals can be achieved by simply changing the pH in the growth solution using dot-shaped gold seeds. Experimental results indicate that the effect of pH in growth solution to the Au3+ precursor—but not the Au+ monomers as suggested in the literature—must be taken into account. Likely, this effect was the dominating factor for controlling the branching of the nanocrystals. The branching facet was identified as {110}. Preliminary results imply that the branched gold nanocrystals may offer stronger enhancement of the surface Raman signals in comparison to the dot-shaped gold nanocrystals.3. Preparation and formation mechanism of Au/Ag Core/Shell Nanoparticles.Gold and silver colloids are extensively used SERS substrates due to their excellent enhancement ability, easiness to make, and convenience for usage. SERS enhancement factors for Ag are substantially higher than that for Au,. However, Ag colloidal nanocrystals can not be easily controlled in their sizes and are not long-time stable. Core/shell Au/Ag nanoparticles were prepared by deposition of Ag through chemical reduction onto the surface of the preformed 13 nm Au nanoparticles. By changing the molar ratio of Ag to Au, the thickness of the shell and thus the size of bimetallic particles could be controlled in a convenient way. The formation mechanism of the"core/shell"type nanoparticles was studied by UV-Vis spectra, transmission electron microscopy (TEM), etc.4 Ligand bonding and dynamics on colloidal nanocrystals at room temperature.Ligand chemistry of colloidal nanocrystals, similar to synthesis of high quality nanocrystals, can be better studied with a good model system. CdSe semiconductor nanocrystals are probably the best choice at present because of their well-developed synthetic chemistry and growth mechanisms. In addition, their strongly surface-sensitive photoluminescence (PL) can be used as convenient probe for quantifying surface bonding events. Using CdSe-octylamine nanocrystals-ligand complex as the model system, the PL and NMR results in this chapter shall confirm the linear relationship between surface lignad coverage and PL brightness. Our experimental methods further allowed distinction between bonded and free ligands and monitor temporal evolution of the ligand bonding on the surface of the nanocrystals. The results were treated with a simple mathematic model and kinetic and thermodynamic constants for the nanocrystals-ligand adsorption and desorption processes were thus obtained.5. Immunoassay Using the Probe-labeled Au/Ag Core/shell Nanoparticles Based on Surface-Enhanced Raman ScatteringAn immunoassay method using the probe-labeled Au/Ag core/shell nanoparticles based on Surface-Enhanced Raman Scattering (SERS) was developed. Mouse polyclonal antibodies against hepatitis B surface antigen (mouse Anti-HBsAg PAb) were covalently immobilized onto the silicon chips with a self-assembled monolayer of (3-amino-propyl) trimethoxysilane by glutaraldehyde (GA) activation. Au/Ag core/shell nanoparticles were labeled with the mouse monoclonal antibody against hepatitis B surface antigen (mouse Anti-HBsAg MAb) and the Raman-probe molecules, p-mercaptobenzoic acid (MBA). SERS technique was applied to detect the immunoreactions between immunoAu/Ag core/shell nanoparticles with the probe and the corresponding antigens captured by the Anti-HBsAg PAb assembled on the silicon chips. AFM and XPS were utilized to evidence the immobilization of Anti-HBsAg Pab and the related interaction processes.