Studies On Formation And Biosensing Of Fluorescent Silver Nanoclusters Templated By DNA

Noble metal nanoclusters composed of several to tens of atoms behave like a molecule with discrete energy band structures due to their confined electron movement in a very small space. The optical properties of noble metal nanoclusters are dramatically affected by their sizes. Recently, DNA-templated silver nanoclusters (Ag NCs) have become emerging sets of fiuorophores for wide applications such as biolabeling and biosensing/chemical sensing. Among them, the interaction between DNA and silver nanoclusters as well as emission modulation of fluorescent silver nanoclusters by DNA sequence are now becoming one of the hottest topics. In this thesis, many methods have been employed to investigate DNA single-nucleotide polymorphism (SNP) detection by DNA-templated fluorescent silver nanoclusters; emission modulation of DNA-templated fluorescent silver nanoclusters by divalent magnesium ion; and the effect of electronic dipole moment induced by DNA base stacking on the properties of silver nanoclusters. The main contents of the thesis are summarized as follows:1. The abasic site-containing DNAs (AP-DNAs) were employed as capping scaffolds for the formation of fluorescent Ag NCs using NaBH4as reductant to investigate the application of fluorescent silver nanoclusters for DNA single-nucleotide polymorphism (SNP) detection by the methods of fluorescence spectroscopy, UV-Vis spectroscopy and so on. The results show that the formation of fluorescent Ag NCs in the AP site-containing DNA duplex is highly selective for cytosine facing the AP site and guanines flanking such site and can be in situ employed as readout for SNP detection. Furthermore, the fluorescent signal-on sensing for SNP based on this inorganic fluorophore is substantially advantageous over the previously reported signal-off responses using low-molecular- weight organic ligands. We expect that this approach will be employed to develop a practical SNP detection method.2. Single-stranded DNAs (ss-DNAs) and double-stranded DNAs (ds-DNAs) were used as capping scaffolds for creation of Ag NCs using NaBH4as reductant. Emission modulation of DNA-templated fluorescent silver nanoclusters by divalent magnesium ion and the brief mechanism were investigated by the methods of fluorescence spectroscopy, fluorescence lifetime, UV-Vis, TEM, Tm and so forth. The results show that divalent Mg2+can modulate the emission of fluorescent Ag NCs during or after the clusters’creation. Tuning the emitting intensities and band positions can be realized by Mg2+addition for the examined ss-DNA templates, which is dependent on the addition moment of Mg2+, while only intensity modulation can be achieved for the used ds-DNAs. All these results suggest that efficient screening of the negative charges of DNA backbone upon addition of the divalent ion is responsible for the modulation by adaptively accommodating the formed Ag NCs.3. A series of abasic site containing DNAs (AP-DNAs) were employed as capping scaffolds for the formation of Ag NCs using NaBH4as reductant. The effect of electronic dipole moment induced by DNA base stacking on the properties of silver nanoclusters were investigated by changing the sequences located by one base away from the AP site (also as growth site for Ag NCs) using fluorescence spectroscopy, fluorescence lifetime, Tm, Job’s plot. The theoretical mechanism was also discussed briefly. The results demonstrate that when the microenvironment of Ag NCs is permanent, although the sequences located by one base away from the AP site are changed, it doesn’t influence the fluorescent Ag NCs’size, and the size of the AP site-born Ag NCs isn’t affected by increasing Ag+concentrations. Ag2NCs should be the species responsible for the observed emissions convinced by Job’s plot analysis. Moreover, the formed Ag NCs’emissions are gradually red shifted by these sequences changed from thymine (T), cytosine (C), adenine (A) to guanine (G). And this shift in the emissions is strongly dependent on the base stacking direction. The excited state of the Ag NCs can be stabilized to much more degree by the electronic dipole moment of the Gs that is the most oxidable base, which lead to the red shift.