Stability of the DMF-Protected Au Nanoclusters: Photochemical, Dispersion, and Thermal Properties

Abstract

We have reported the synthesis of dimethylformamide (DMF)-protected gold nanoclusters using a surfactant-free DMF reduction method. DMF-protected gold nanoclusters (Au NCs) are obtained without the formation of gold nanoparticles and bulk metals as byproducts using a hot injection process for the homogeneous reduction. The as-prepared DMF-protected Au NCs were a mixture of various-sized Au NCs with a cluster number of less than 20 including at least Au(8) and Au(13). The photoluminescence emission from Au(8) and Au(13) was confirmed in the photoluminescence spectra. The Au NCs are stabilized with DMF molecules through the interaction of amide groups of DMF with Au NCs. DMF-protected Au NCs in solution were found to have high thermal stability, high dispersion stability in various solvents, and high photochemical stability. The DMF-protected Au NCs dispersed well for at least a month in various solvents such as water, acid (pH 2), alkali (pH 12) and 0.5 M NaCl aqueous solution, and methanol without further surface modification. The thermal stability of DMF-protected Au NCs was approximately 150 degrees C, which was comparable to that of thiolate-protected Au NCs. The photobleaching of Au NCs in water gradually occurred under UV light irradiation (356 nm, 1.3 mW/cm(2)) because of the photoinduced oxidation of Au NCs. After 8 h irradiation, the fluorescence intensity slowly decreased to approximately 50% of the maximum and to approximately 20% after 96 h under the present condition, compared to the photobleaching of CdSe semiconductor quantum dots. We also found that the fluorescence intensity remained to be about 30% of the maximum even in the presence of concentrated 30% H(2)O(2). These findings demonstrate that the photobleaching process under the UV irradiation is effectively suppressed for DMF-protected Au NCs.