Synthesis and Applications of Fluorescent Noble Metal Nanoclusters
Date Issued
2014
Date
2014
Author(s)
Chen, Wei-Yu
Abstract
Fluorescent noble metal nanoclusters (NCs) and nanodots (NDs) are interesting materials and widely employed in the biosensing and bioimaging, mainly because of their unique optical and catalytic properties, including strong fluorescence, size-dependent emission wavelengths, magnetism, and high photostability. This thesis focuses on the preparation, characterization, and application of water-soluble fluorescent noble metallic nanoclusters/nanodots (NCs/NDs). First, we have employed cytosine-rich oligonucleotides to prepare strongly fluorescent and highly photostable DNA-templated gold/silver nanoclusters (DNA–Au/Ag NCs) through the NaBH4-mediated reduction method. Electrospray ionization-mass spectrometry (ESI-MS) and inductively coupled plasma mass spectrometry (ICP-MS) were employed to characterize the DNA–Au/Ag NCs, revealing that each DNA–Au/Ag NCs contain two Au atoms and one Ag atom. Based on that fact that sulfide (S2−) ion-induced fluorescence quenching of DNA–Au/Ag NCs, we further developed a fluorescence turn-off assay for the high sensitive detection of S2− ions at concentrations as low as 0.83 nM. For preparation the functional Au NDs, we combined the biocompatible liposomes (Lip) and fluorescent 11-mercaptoundecanoic acid–gold nanodots (11-MUA–Au NDs) to prepare the 11-MUA–Au ND/Lip hybrids by incorporation of gold nanoparticles (∼3 nm) and 11-MUA molecules in hydrophobic phospholipid membranes that self-assemble to form small unilamellar vesicles. A simple and homogeneous fluorescence assay for phospholipase C (PLC) was developed on the basis of the fluorescence quenching of 11-MUA–Au ND/Lip hybrids in aqueous solution. The fluorescence of the 11-MUA–Au ND/Lip hybrids is quenched by oxygen (O2) molecules in solution, and quenching is reduced in the presence of PLC. PLC catalyzes the hydrolysis of phosphatidylcholine units from Lip to yield diacylglycerol (DAG) and phosphocholine (PC) products, leading to the decomposition of Lip. The diacylglycerol further interacts with 11-MUA–Au NDs via hydrophobic interactions, leading to inhibition of O2 quenching. The 11-MUA–Au ND/Lip probe provides a limit of detection of 0.21 nM for PLC, with high selectivity over other proteins, enzymes, and phospholipases. For preparation of self-assembly Au NDs, hybridized ligands were used to etching and stabilization of gold nanoparticles (~3 nm). These NDs were employed to detect nitrite based on analyte-induced photoluminescence (PL) quenching. 11-Mercaptoundecanol (11-MU) and its complexes with amphiphilic ligands (ALs) etch Au nanoparticles through hydrophobic interactions and form a densely packed ligand shell on the surface of each core in the as-formed Au NDs. We tested such ALs as three fatty acids and three quaternary ammonium surfactants with alkyl chain lengths of 10–16 carbons. The results show that chain length, ligand density, and functional group (charge) of ALs play important roles in determining the optical properties of Au NDs. Tetradecanoic acid (TA)/11-MU–Au NDs are highly dispersible in aqueous solution and allow detection of nitrite down to 40 nM with selectivities (>100-fold) greater than that for common ions present in natural (lake and sea) water samples. We further prepared antimicrobial Au NDs which surfaces were co-immobilized with antibacterial peptide (surfactin; SFT) and 1-dodecanethiol (DT). SFT, a cyclic lipopeptide, has been credited with antibacterial, antiviral, antifungal, anti-mycoplasma and hemolytic activities. The hybrid SFT/DT-capped Au NDs (SFT/DT-Au NDs) were prepared through the self-assembly of antimicrobial peptides (SFT) on DT-anchored Au NDs by the nonspecific hydrophobic interactions between the alkyl chains of the SFT and the DT molecules. Relative to SFT and DT-Au NDs, SFT/DT-Au NDs possessed superior antimicrobial activity toward non-multi-drug resistant (non-MDR) Escherichia coli (E. coli), Proteus vulgaris (P. vulgaris), Proteus vulgaris (P. vulgaris), Salmonella enterica serovar Enteritidis (S. enteritidis), and Staphylococcus aureus (S. aureus) bacteria as well as the multi-drug resistant (MDR) bacteria, methicillin-resistant S. aureus (MRSA). We demonstrated the water solubility, PL as well as antibacterial activity of Au NDs were highly dependent on the ligand ratio of SFT/DT on Au NDs. In vitro haemolysis and cytotoxicity analyses of SFT/DT-Au NDs have revealed their insignificant haemolysis in red blood cells (RBCs) and low toxicity in selected cell lines.
Subjects
nanoclusters
nanodots
liposomes
sulfide ions
phospholipase C
amphiphilic ligands
nitrite ions
surfactin
antibacterial peptides
multidrug-resistant bacteria
Type
thesis
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