Fluorescent silver nanoclusters

Abstract

Fluorescent metal nanoclusters (Ag2, Ag3, Ag4, and Cu7) were synthesized by reacting aqueous silver nitrate (AgNO3) or copper nitrate (Cu(NO3)2 solution with equivalent portions of aqueous sodium borohydride (NaBH4) solution in emulsion system at room temperature. Sodium bis-(2-ethyl hexyl) sulfosuccinate (AOT) was used to stabilize the microemulsion and 2, 2, 4-trimethyl pentane (isooctane) was the bulk (oil) phase. By confining the metal ions and reducing agents to the droplets of microemulsions, the number of atoms available to form metal clusters after reduction is controlled. This thesis is concerned with the synthesis of small, fluorescent metal clusters. Ordinarily, reduction of solutions metal salts in the presence of capping ligands leads to the formation of nanoparticles of diameter in the range of 10 – 100 nm. Such particles are already metallic, have no bandgap, and do not fluoresce, but exhibit plasmon resonances. Dynamic light scattering (DLS) measurements of all emulsions indicated a predominant droplet diameter of 100 nm and a smaller diameter peak in the distribution corresponding to reverse micelles at 5 nm. Rayleigh scattering measurements were fitted to theory with the droplet diameter (50 nm) as the sole free parameter. Two reaction concentrations consisting of 90 μM and 1 mM, with an average of 4 and 40 metal ions per droplet were investigated in detail. As well as chemical reduction, photochemical reduction of Ag (I) emulsions by UV light ( = 254 nm) was also studied. UV-Vis absorption spectra did not show plasmon resonance peaks in any of the microemulsion samples. In addition, all emulsion-synthesized NCs were fluorescent with an average emission intensity counts per second (cps) in the order of 3.0 x 105 cps. Generally, two emission bands were obtained at approximately  = 300 nm and  = 430 nm corresponding to optical gaps of 4.13 and 2.88 eV for Ag NCs. A third band at = 610 nm may be related to aggregation was also observed. Two emission bands were also observed for the Cu NCs at  = 350 nm and  = 401 nm corresponding to gaps of 3.54 and 3.09 eV. Confocal microscope images confirmed the luminescence of these MNCs, and together with the transmission electron microscopy (TEM), as well as, atomic force microscopy (AFM) demonstrated that small, roughly spherical NCs were synthesized. TEM and AFM results were in agreement for the NaBH4 and photoreduced Ag NCs with estimated diameters of 1.4 – 2.4 nm. The estimated diameter of the Cu NCs was ~1.0 – 2.4 nm. Electrospray ionisation mass spectrometry (ESI-MS) results provided more the Abstract Hector Henry Oyem ii molecular formulae of: [Ag2(H2O)H-]; [Ag4B3O5BH3.2H2O]-; [Ag3(H2O)n(OH)]- and [Ag4 (H2O)6 (OH)]- (photoreduced samples), and [Cu7B3O5.BH3.2H2O]– . Alternatively, fluorescent metal NCs were prepared by reducing metal ions bound to polyvalent single-stranded deoxyribonucleic acids (ssDNAs) ligands. The concept requires that the ligand binds both the metal ions and elemental metal atoms. Three single-stranded DNAs of length 22, 29, and 34-bases which had been previously reported to facilitate the synthesis fluorescent Ag NC were chosen for investigation: (1) 5'-TGACTAAAAACCCTTAATCCCC-3' (2) 5'-AGTCACCCCAACCTGCCCTACCACGGACT-3’ (3) 5’-GCAGGTTGGGGTGACTA AAAACCCTTAATCCCC-3'. Reduction of ssDNA-bound of Ag (I) ions disfavours aggregation in bulk solution and favours instead, the formation of small, fluorescent clusters. Two prominent emission bands were obtained with ssDNA1 in microemulsion, however one broad, but weak emission at 350 nm, in associated with a complex band comprising of four peaks with a maximum at 401 nm were observed for both the ssDNA2 and 3. However, emission spectra of the three ssDNA-Ag samples in 99 % deuterium oxide (D2O) solution was different from those of the emulsion samples, with a common band at 388 nm, followed by a very broad one at 590 and 630 nm for the DNA2 and 3 respectively. Apart from the 388 nm emission, which was generally ascribed to the ssDNA molecules, no other band was seen in the spectrum of the ssDNA1-Ag sample in D2O. The results of this study demonstrate that stable NCs of specific cluster sizes could be made by adapting the reaction conditions to confine the metal ions to small reaction volumes in microemulsions or on single-stranded DNA molecules.