Trimeric clusters of silver in aqueous AgNO3 solutions and their role as nuclei in forming triangular nanoplates of silver.

The extensive use of silver nanostructures as optical labels, substrates for surface-enhanced Raman scattering (SERS), near-field optical probes, and contrast agents for biomedical imaging has led to a steadily growing interest in the chemical synthesis of such species. More importantly, the optical properties of silver nanostructures can be tailored with great versatility by controlling their shapes during synthesis. A remarkable example is that of triangular nanoplates of silver, a class of nanostructures with two-dimensional anisotropy. The nanoplates exhibit fascinating optical properties, such as intense quadrupole resonance peaks that are absent in small nanospheres and have found use in chemical and biological sensing. Since the first publication on this subject by Mirkin and co-workers in 2001, a number of different synthetic routes have been demonstrated, including those based on photoor thermally induced transformation and on direct chemical reduction. All of these methods rely on the slow generation of neutral silver atoms to enable kinetic control. Although kinetic control has also been used in the synthesis of platelike nanostructures from other noble metals, it remains largely unresolved how this process works. 8] It has been proposed that light of proper wavelengths or that certain capping ligands, such as citrate, are responsible for the formation of silver nanoplates. 6e] However, our most recent work demonstrated that the platelike morphology could also be obtained in the absence of both light and citrate. Herein, we elucidate the mechanism of nanoplate formation by focusing on the silver clusters that dominate nucleation. Despite the technological importance of nanocrystals and the extensive efforts that have been devoted to studying them, attempts to synthetically and systematically control their shapes and properties have met with limited success. One barrier to success is the fact that very little is known about the details of nucleation involved in the formation of nanocrystals. In the case of a metal, it is still unclear how a precursor salt is reduced into neutral atoms that then aggregate and evolve into nanoscale crystals. The g-radiation-based synthesis developed by Henglein has shed some light on the nucleation process by controlling the generation of zero-valent atoms and thus their agglomeration into small clusters. Both UV/Vis spectroscopic and scanning tunneling microscopic studies of these clusters suggested that Ag4 2+ and Ag8 4+ were the most abundant species involved in the nucleation stage. Growth of these clusters into nanocrystals likely occurred through a combination of aggregation and atomic addition. Herein we demonstrate, for the first time, that there exists a smaller cluster, Ag3 + or Ag3, in the nucleation stage of a solution-phase synthesis that employs AgNO3 as a precursor to silver. These trimeric clusters can serve as nuclei for the addition of newly formed silver atoms and eventually lead to the formation of triangular nanoplates. Mass spectrometry provides a tool for simple identification and characterization of silver clusters possibly contained in aqueous AgNO3 solution. Since a mass spectrometer can separate and detect ions of different masses, it allows the different isotopes of a given element to be easily distinguished. It is also feasible to quickly identify clusters of different sizes by analyzing the isotope patterns. Natural silver comprises a nearly 1:1 mixture of two isotopes with atomic masses of 106.9 and 108.9 amu. A trimeric cluster of silver contains three silver atoms that may come in four different combinations: three Ag atoms, two Ag atoms plus one Ag atom, one Ag atom plus two Ag atoms, or three Ag atoms. As a result, Ag ions are expected to appear in the mass spectrum as a doublet (with two peaks located at m/z 106.9 and 108.9), while Ag3 + clusters give rise to a quadruplet (with four peaks located at m/z 320.7, 322.7, 324.7, and 326.7 with a ratio of 1:3:3:1). Figure 1a shows a positive-mode mass spectrum taken from an aqueous solution of AgNO3 immediately after its preparation. In the m/z range from 80 to 1100, there are four sets of peaks with distinct isotope patterns. According to the isotope patterns and their corresponding m/z ratios, the peaks can be assigned to Ag, [Ag2NO3] , Ag3 , and [Ag3(NO3)2] . The insets show the doublet and quadruplet patterns characteristic of Ag and Ag3 . The positive charge on Ag3 + might be intrinsic to the trimeric cluster, or it might be caused by oxidation during the electrospray ionization process. Therefore, the trimeric clusters of silver in aqueous AgNO3 can be either positively charged (Ag3 ) or neutral (Ag3). The negative-mode mass spectrum (Figure S1 in the Supporting Information) indicates that there are also a number of negatively charged complexes in the aqueous AgNO3 solution, with notable examples including [Ag(NO3)2] , [Ag2(NO3)3] , and [Ag3(NO3)4] . As established in previous work, [*] Dr. Y. Xiong, I. Washio, Dr. J. Chen, Dr. M. Sadilek, Prof. Y. Xia Department of Chemistry University of Washington Seattle, WA 98195 (USA) Fax: (+1)206-685-8665 E-mail: xia@chem.washington.edu