PETTIBONE AND HUDGENS VOL. 5 ’ NO. 4 ’ 2989–3002 ’ 2011 www.acsnano.org 2989 March 07, 2011 This article not subject to U.S. Copyright. Published 2011 by the American Chemical Society Gold Cluster Formation with Phosphine Ligands: Etching as a Size-Selective Synthetic Pathway for Small Clusters? John M. Pettibone and Jeffrey W. Hudgens * Chemical and Biochemical Reference Data Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States T he development of synthetic tech- niques for monolayer protected clus- ters (MPCs) continues to be the topic of ongoing research because MPCs exhibit unique nuclearity-selective properties that differ from their bulk metal counterparts. In general, relatively monodisperse MPC synth- eses have been developed with protecting ligands such as thiols, amines, and poly- meric ligands, which have multiple roles as stabilizers, place holders, or etching agents. 1-7 Understanding the specific role of the ligands is necessary for maximizing their potential for material development. Phosphine-protected Au nanoclusters have been examined for catalysis, imaging, drug delivery platforms, and targeting agents. 8-11 These MPCs can also be building blocks for larger, higher order structures. The protect- ing ligands impact the physicochemical properties of the monolayer protected clus- ters including solubility, reactivity, optical properties, and aggregation. Triphenylphos- phine (PPh 3 ) and other phosphorus ligands are currently described as place holders or surfactants in cluster formation because they can be readily replaced through ligand exchange reactions; 7,12-14 however, pre- vious synthetic approaches that form spe- cific MPCs or ligated nanoparticles of different sizes describe PPh 3 as a useful protecting ligand for selectivity, 15-21 sug- gesting that PPh 3 can play substantial roles during cluster formation. Syntheses of solution-phase, phosphine- protected gold clusters commonly involve the reduction of an oxidized Au precursor in the presence of PPh 3 , resulting in a distribu- tion of nascent clusters. To narrow the product distribution, synthesis procedures may include the adjustment of synthetic parameters controlling specific reaction rates, for example, reduction, through changes in stir rate, temperature, reducing agent, etc. Additional separation steps may be needed to isolate desired products, increasing the overall time and cost of MPC syntheses. The reduction kinetics of oxidized Au precursors are partially controlled by the reaction be- tween the primary reducing reagent and solvent (or solvent mixture), and these reac- tion rates have been shown to strongly affect cluster size. 22-24 For example, substi- tution of methanol for acetone during the synthesis of thiol-protected Au 38 clusters was demonstrated to increase the nuclear- ity of nascent clusters; however, the govern- ing mechanism was not discussed. 25 Different alcohols have reportedly different reaction rates with NaBH 4 , 22,24,26 which can also directly affect the nascent cluster formation. 27 The reduction of AuClPPh 3 by NaBH 4 in methanol systems is relatively * Address correspondence to hudgens@nist.gov. Received for review January 5, 2011 and accepted March 7, 2011. Published online 10.1021/nn200053b ABSTRACT Triphenylphosphine (PPh 3 ) is commonly used during syntheses of stable, closed-shell monolayer protected clusters (MPCs). Models of transition metal (TM) cluster and nanoparticle syntheses commonly assign PPh 3 a passive role as a chemical placeholder, electron balancing species, or surfactant. This study provides the first direct evidence that PPh 3 is a proactive etching agent that promotes the formation of specific closed-shell cluster sizes. To observe this effect, we developed a colorimetric tool that simultaneously monitors size distribution and population of PPh 3 - protected clusters as a function of time. The distribution of the clusters is assigned to different bin sizes by chemical conversion with L 3 (L 3 = 1,3-bis(diphenylphosphino)propane): (i) total conversion of PPh 3 -protected Au 8 and Au 9 clusters into [Au 6 L 3 4 ] 2þ and (ii) ligand exchange with [Au x (PPh 3 ) y ] zþ (10 e x e 13) clusters to form L 3 -protected Au 10 and Au 11 clusters. Evolution of the nascent cluster distribution in ethanol and methanol solvent systems was monitored by the colorimetric assay, which revealed a cyclic process of growth and etching reactions around the most stable cluster species to form nearly monodisperse product distributions. We formally define the population growth of specific clusters through cyclic processing of the Au MPCs as “size selective” processing. The current study highlights the need for incorporating bidirectional processing, including relative rate information, into TM kinetic models for ligands with growth and etching efficacy. KEYWORDS: colorimetric assay . growth mechanism . nucleation . ligand-protected clusters . nanoparticles . triphenylphosphine ARTICLE