| Metal nanoparticles have been widely used in sensing, catalysis, imaging, biomedicine, optical and electronic devices, and information storage due to their unique properties. Owing to the unique localized surface plasmon resonance (LSPR) properties, metal nanoparticles can act as excellent optical probes in analytical chemistry (e.g., optical sensing and imaging). The LSPR properties of metal nanoparticles as well as their performances in various areas are closely related to the element, morphology, size, assembly mode, and other parameters. However, the applications of metal nanoparticles in analytical chemistry on the basis of their LSPR are still challenging. For example, researchers are usually hard to obtain metal nanoparticles with high quality according to the requirements for applications; the biocompatibility of the nanoparticles and their stability in biological medium is usually bad; the mechanisms of the growth and assembly of metal nanoparticles with a specific structure are not very clear; it is difficult to tune the LSPR properties of the metal nanoparticles for the applications in analytical chemistry; the reproducibility of some nanoparticle-based analytical methods is not satisfied. To address these issues, we have systematically studied the synthesis and assembly of metal nanoparticles as well as their applications in photoanalytical chemistry. This thesis includes the following two parts:Part I Synthesis and assembly of metal nanoparticles, including the following three parts:A simple, one-pot, and "green" method was developed for the simultaneous synthesis and self-assembly of Au nanoparticles, using a biocompatible polysaccharide derivative, chitosan-ninhydrin (CHIT-NH) conjugate, as both a reducing agent and a stabilizer. Firstly, the side chain of chitosan was modified with ninhydrin through covalent bond, and a novel CHIT-NH conjugate could be obtained. Then, we achieved the synthesis and assembly of quasi-spherical Au nanoparticles at physiological temperature through the interaction between CHIT-NH conjugate and Au3+. This new macromolecule could also act as a stabilizer and thus adsorb on the surfaces of the formed Au nanoparticles. Due to its uneven distribution on the surfaces, some facets of the Au nanoparticles were exposed and thus induced the self-assembly of Au nanoparticles into nanochains via dipole-dipole interaction. Compared to the dispersed Au nanoparticles, the LSPR extinction spectrum of the anisotropic Au nanochains red shifts to a longer wavelength.Then, we further studied the universality of the above strategy for the simultaneous growth and self-assembly of Au nanoparticles, as well as systematically studied the LSPR properties of the anisotropic assemblies and explored the mechanism involved in such synthesis. Using another polysaccharide, dextran, as a template, we prepared a new polyaldehyde dextran (PAD) through a redox reaction between dextran and a strong oxidant. With PAD as both a reducing agent and a stabilizer, we also achieved the biomimetic synthesis and assembly of Au nanoparticles via a one-pot approach. The morphology of Au nanochains could be controlled through adjusting the reaction conditions such as the concentration of reagents, reaction time and temperature. Mechanism investigations further suggest that dipole-dipole interaction between nanoparticles and the intermolecular hydrogen bonding of stabilizers are the main driving forces for the assembly of Au nanoparticles.Except for the one-pot method, we also developed a two-step and versatile approach for the end-to-end assembly of Au nanorods by means of the specific molecular recognition between thymine (T)-rich oligonucleotides and mercury (Ⅱ). Moreover, the relationship between the near-field plasmon coupling of Au nanorods and their distances or assembly modes were also discussed. For the assembly of Au nanorods, the T-rich DNA was firstly conjugated to the ends of Au nanorods through thiol-Au covalent. Then, the T-T base pairs could strongly bind up and readily form a structure of T-Hg2+-T configuration in the presence of Hg2+ions, inducing the assembly of Au nanorods in an end-to-end mode. By designing the DNA sequence, we could precisely control the distance between the nanorods and thus manipulate their plasmon coupling. This strategy for the assembly of nano-scaled materials, which relies on the receptor-ligand molecular recognition, can be extended to the fabrication of other nanomaterial assemblies and devices.Part II The applications of metal nanoparticles in photoanalytical chemistry, including the following three parts: Firstly, we developed a simple approach to the preparation of dextran-capped Au nanoparticles at room temperature, and further used them as optical probes for the colorimetric detection of an antihypertensive drug (dihydralazine sulfate) on the basis of their unique LSPR properties. The as-obtained Au nanoparticles were uniform in size, biocompatible, and very stable even if in a medium of high ionic strength. The hydrazine groups of dihydralazine sulfate are able to react with the aldehydes on the surface of Au nanoparticles to form hydrazone, resulting in the decrease of the spacing of nanoparticles. Based on the change of LSPR properties of the Au nanoparticles, we could quantificationally determine dihydralazine sulfate in uric samples with the the recovery in the range of98.3-102.5%.Metal nanoparticles can also act as a signal reporter for optical imaging based on their unique LSPR light scattering. We proposed a noncovalent strategy to fabricate metal nanoparticle/graphene oxide (MNP/GO) hybrids and achieved the direct illumination of graphene in dark-field microscopic system. DNA-founctionalized Au/Ag nanoparticles could anchored on the surfaces of GO through π-π interaction between DNA bases and GO. Owing to the strong LSPR scattering of MNPs, the profiles of graphene could be clearly observed using an ordinary optical microscope. This graphene-involved composite which has collective properties can be promising candidates in a variety of applications such as bioimaging, drug delivery, and cancer therapy.Finally, we used the LSPR scattering properties of metal nanoparticles for real-time and in situ monitoring of the chemical reactions. With the aid of an ordinary optical microscope, we achieved the real-time observation of the oxidative etching of an individual Ag nanocube. The optical information and morphology of the Ag nanoparticles at different stages of the etching process could both be obtained, which clearly elucidated the mechanism of the oxidative etching on metal nanoparticles. In addition, the results from theoretical simulation also confirmed the mechanism that the oxidative etching of an Ag nanocube tends to start from its corners due to the relatively high energy at these sites relative to{100} facet. This strategy will enable us better understand the behaviors of nanoparticles in a variety of chemical reactions and biological processes.In conclusion, we have developed simple and efficient methods for the synthesis and assembly of metal nanoparticles. Meanwhile, the key parameters affecting the LSPR propertise of metal nanoparticles have also been systematically investigated. Then, the well-defined metal nanoparticles were used as probes for optical sensing and imaging. Therefore, the main contribution of the thesis is that it has developed new strategies for the optical sensing and imaging based on the LSPR properties of metal nanoparticles. This thesis will provide sufficient experimental evidences and new insights for the synthesis and assembly of metal nanomaterials. Moreover, it expands the applications of metal nanoparticles in analytical chemistry and optical imaging. |
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