Quantised Charging Of Gold Nanoclusters Film

Nanoscale science and technology, as an important research area in modern studies, has entered a new stage of rapid development over the past two decades. At the intermediate level between atom/molecule and bulk materials, nanomaterials display new physicochemical properties, including mechanical,, electrical and optical response, thermal conductivity, elasticity and so on. Extensive and intensive investigations have shown that very novel properties and more complex and effective applications can be achieved through rational design of nanomaterials. Among nanomaterials, monolayer-protected gold nanoclusters (MPCs) have received much attention due to their specific quantised charging property. Based on the above background, this thesis focuses on the redox charging of MPCs film and its applications in the chemical sensing area. The whole thesis consists of three Parts.The first part is Chapter 1, which gives a brief introduction to the characteristics and functions of monodisperse nanomaterials. It also incudes a comprehensive summary of the recent development on the synthesis, crystalline structural evolution, electronic energy quantization and quantised charging of MPCs.The second part includes Chapter 2 and Chapter 3, which deals with the quantised charging of MPCs in diverse solvents and electrolytes. Multilayer films consisting of MPCs coated on electrode surfaces and immersed in aqueous media was investigated in Chapter 2. On the use of water-soluble electrolytes featuring a lipophilic cation, discrete reductive charging of MPC films has been unambiguously observed for the first time, which has been verified by the electrochemical quartz crystal microbalance measurements. The results have shown that the reductive charging of MPC films is tightly concomitant with the cation transfer at the film/solution interface. A further study was then carried out on the reductive electron transfer dynamics. As determined by potential step chronoamperometry, electrons are transported within the film phenomenologically via the diffusion-like hopping (self-exchange) between the localized MPC sites with an average first order rate constant of 104s-1. This value is comparable with those for the similar kind of films immersed in organic electrolytes, but much smaller than those for the all-solid or dry films. It thus turns out that the solvent swelling imposes negligible effect on the electron hopping dynamics and that the electron transfer is most likely limited by the diffusive redistribution of the counterions in order to keep the local electroneutrality. Meanwhile, it was observed that Cl-inevitably results in the destruction of electrochemical charging of MPC films. Therefore, the reactivity of halide anion (Cl-) with MPCs was conducted by detailed analyzations through cyclic voltammetry, electrochemical quartz crystal microbalance and X-ray photoelectron spectroscopy measurements. Experimental findings indicate that the destruction is most likely resulted from the strong affinity of Cl-for the surface of MPCs, leading to a significant variation of the surface structure and thereby quenching the electrochemical charging property. In the end, a comparison between the behaviors of MPC monolayer film and that of MPCs mutlilayer films in aqueous electrolyte solutions was made. The electrochemical charging response has been observed with relatively small counterions but not large ones. This effect can be explained by the proximity of counterions to the MPC surface, which determines whether the electronic charges on MPCs can be effectively compensated by the formation of ion pairs. Only small counterions are permitted to approach the MPC surface by permeating the alkanethiolate layer to achieve a sufficient charge compensation. In contrast, big ions are sterically excluded and thus disfavor the electrochemical charging.Multilayer films consisting of MPCs coated on electrode surfaces and immersed in nonaqueous media was further investigated in Chapter 3. When immersed in imidazolium based ionic liquids (ILs), the anion-dependent oxidative charging of MPC films features the absence of reductive charging, which is similar to that of anion-rectified/limited oxidative charging previously observed in aqueous media. This is simply due to the inadequate hydrophobicity of the imidazolium cation. If replacing the imidazolium cation with more hydrophobic tetrahexylammonium, the reductive charging current will be displaced. The ionic dependence manifests that the charging process is an ion-coupled electron transfer event with the oxidative/reductive charging of MPCs at the electrode/film interface concomitant with the ionic partition at the film/IL interface. Given that ILs are "designed solvents" with structures and functions easily tuned by ionic components (both anion and cation), this system provides a diverse possibility of regulating the electronic properties of nanoparticle thin films by ionic/solvent functions. In addition, this chapter also examines the counterion-dependent redox charging properties of gold nanocluster films in polar organic solvents. Different from those demonstrated in polar aqueous and weakly polar/nonpolar organic media, the charging response is typically characteristic of a significant voltage separation between the first oxidation and reduction, dependent on the lipophilicity of electrolyte counterions. Thus, the overall process is proposed to be an ion transfer coupled electron transfer reaction, which is limited thermodynamically by the counterion migration at the film/solution interface and kinetically by the diffusion-like electron hopping among MPCs within the film.In the third part of the thesis, namely Chapter 4, the MPCs modified ITO electrode was employed as the chemical sensor for the detection of trolox. Trolox is an analogous of a-tocopherol, a naturally occurring lipid-soluble antioxidant that composes the most active component of viatamin E. The modification of MPCs film on the ITO electrode highly improves the hydrophobility of the electrode surface, thus increasing the affinity of trolox. As a result, the overpotential of trolox oxidation at the ITO electrode was significantly reduced and the current is obviously enhanced simultaneously. A tea real sample detection was carried out, showing the excellent anti-interference ability and performance of the MPCs modified electrode.