American Chemical Society
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Functionalized Au22 Clusters: Synthesis, Characterization, and Patterning

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journal contribution
posted on 2009-10-28, 00:00 authored by E. S. Shibu, B. Radha, P. K. Verma, P. Bhyrappa, G. U. Kulkarni, S. K. Pal, T. Pradeep
We synthesized fluorescent, porphyrin-anchored, Au22 clusters in a single step, starting from well-characterized Au25 clusters protected with glutathione (−SG) by a combined core reduction/ligand exchange protocol, at a liquid−liquid interface. The prepared cluster was characterized by UV/vis, photoluminescence, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, elemental analysis, and matrix-assisted laser desorption ionization mass spectrometry. The absence of a 672 nm intraband transition of Au25 and the simultaneous emergence of new characteristic peaks at 520 and 635 nm indicate the formation of the Au22 core. An increase in the binding energy of 0.4 eV in Au 4f core-level peaks confirmed the presence of a reduced core size. Quantitative XPS confirmed the Au/S ratio. The presence of a free base, tetraphenylporphyrin (H2TPPOAS−), on the Au22 core was confirmed by fluorimetric titrations with Cu2+ and Zn2+ ions. From all of these, the composition of the cluster was determined to be Au22[(−SG)15(−SAOPPTH2)2], which was supported by mass spectrometry and elemental analysis. We utilized the fluorescence nature of these water-soluble clusters for the fabrication of fluorescent patterns by soft lithography. The patterns were studied using tapping-mode atomic force microscopy and confocal fluorescence imaging.