Functionalized Au₂₂ clusters: synthesis, characterization, and patterning

Abstract

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.