J. Phys. Chem. C 2009, 113, 16387–16393
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NMR Characterization of Ligand Binding and Exchange Dynamics in Triphenylphosphine-Capped Gold Nanoparticles Ramesh Sharma,† Gregory P. Holland,† Virgil C. Solomon,†,‡ Herbert Zimmermann,§ Steven Schiffenhaus,† Samrat A. Amin,† Daniel A. Buttry,† and Jeffery L. Yarger*,† Department of Chemistry and Biochemistry, Arizona State UniVersity, Tempe, Arizona 85287-1604, and Max-Planck-Institut fu¨r Medizinische Forschung, 69120 Heidelberg, Germany ReceiVed: June 1, 2009; ReVised Manuscript ReceiVed: July 19, 2009
Triphenylphosphine (PPh3)-capped 1.8 nm diameter gold nanoparticles (AuNPs) are characterized by a combination of 1H, 2H, and 31P solution- and solid-state NMR. The 31P{1H} NMR resonance associated with the surface-bound PPh3 is clearly identified and is present as a broad peak centered at 56 ppm. 31P and 1H hole burning NMR experiments show that the line broadening associated with the surface-bound PPh3 is primarily due to a variety of different chemical shift environments at the surface of the nanoparticles. The surface bound PPh3 can be displaced with either d15-PPh3 or Au(d15-PPh3)Cl in CD2Cl2 solution. In both cases, exchange results in loss of Au(PPh3)Cl from the nanoparticle surface, with no evidence for loss of the PPh3 ligand alone. Solution-state NMR was used to determine the room temperature rate constants for these exchange processes, with values of 0.17 and 0.20 min-1, respectively. Thus, essentially the same rate is observed for displacement of Au(PPh3)Cl from the surface with either d15-PPh3 or Au(d15-PPh3)Cl. The observed 31 P chemical shift of surface-bound PPh3 is consistent with mixed valence Au(0) and Au(I) at the nanoparticle surfaces, suggesting the presence of surface-bound Au complexes. Introduction Metal nanoparticles dissolved in solution or precipitated into the solid state are typically passivated with a protective layer of organic ligands to stabilize the particles against irreversible aggregation.1-5 Functionalized organic ligands having strong metal affinity are used to ensure selective adhesion with the nanoparticle surface. In the case of Au nanoparticles, soft ligands such as thiols and phosphines are often used to passivate the gold surface by creating ligand-capped gold nanoparticles (AuNPs).6-8 Since the discovery of fast solution-based synthetic processes for production of ligand-stabilized AuNPs in 1994,9 they have been found to contain many interesting electrical and optical properties10-12 and are promising materials for catalysis,5 photonics,13 sensors,14-17 fuel cell electrodes,18 contrasting agents for imaging,19 nanoelectronics,7,20 and drug delivery.21-23 Small Au nanoparticles (core diameter of 98% was confirmed by 31P and 1H NMR.
Figure 1. (A) HRTEM image of PPh3-capped AuNPs and (B) nanoparticle size distribution histogram. The core diameter is 1.8 ( 0.6 nm (N ) 214). A total of 72% of the investigated particles have a core diameter between 1 and 2 nm.
Results and Discussion TEM Analysis of PPh3-Capped Gold Nanoparticles. The TEM characterization in Figure 1 shows an average diameter of 1.8 nm with a standard deviation of 0.6 nm (N ) 214 particles). Of the 214 particles analyzed, 72% have a core diameter between 1 and 2 nm. The lack of a significant plasmon band in the ultraviolet-visible (UV-vis) absorption spectrum is a further indication of an average particle diameter smaller than 2 nm (see the Supporting Information, Figure S1).59 It was apparent from HRTEM images that nanoparticles in solution form aggregates over the course of several days. For this reason, all the solution NMR experiments presented below were performed within 48 h of sample preparation. The synthesis described above is similar to the procedures used to produce Au55 or Au101 PPh3-capped nanoparticles, often called 1.5 nm AuNPs.46 While specific cluster sizes (e.g., Au55 or Au101) are often given in the literature, the typical as-synthesized materials do not produce a monodisperse material.25,26,60,61 We obtain a size distribution similar to those of previous reports and will refer to our PPh3-capped gold nanoparticles as 1.8 nm AuNPs. Peak Assignment of 1H Solution NMR. The 1H solution NMR spectrum of PPh3-capped 1.8 nm AuNPs dissolved in CD2Cl2 is shown in Figure 2A. The most significant feature of the spectrum is the broad resonance centered at 7.1 ppm, which is assigned to the phenyl ring protons of surface-bound PPh3.51 The small peaks observed at 5.3, 1.5, and 0 ppm are due to the solvent (CH2Cl2), residual water (in CD2Cl2), and 0.5% (v/v) TMS present in the solvent as an internal chemical shift reference, respectively. A complex of gold, Au(PPh3)Cl, is observed around 7.5 ppm as a sharp component on top of the broad resonance.46,51 A small percentage (3-15%) of the Au(PPh3)Cl complex is known to be present as an impurity in PPh3-capped nanoparticles even after multiple purification steps, which is due to slow dissociation of the complex from the nanoparticle surface when dissolved in solution.46,51 Line Broadening in 1H Solution NMR of PPh3-Capped Gold Nanoparticles. There are several factors that can cause line broadening in 1H solution NMR of AuNPs, including (i) ligand environment heterogeneity, causing a distribution of
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Figure 2. 1H NMR spectra (A) of PPh3-capped 1.8 nm AuNPs dissolved in CD2Cl2 and (B) with spectral hole burning of the broad phenyl resonance at 7.1 ppm prior to NMR detection. The arrow above the phenyl resonance in spectrum B indicates the rf frequency for hole burning. Also, the rf frequency distribution for hole burning is 90% of the total signal when quantitative measurements are performed. Our proton-decoupled 1H-31P CP-MAS NMR spectrum of PPh3-capped AuNPs (∼Au99) is similar to the solidstate MAS NMR spectrum observed for PPh3-capped Au55 nanoparticles by Kolbert et al.67 The broad 31P MAS NMR line width is primarily inhomogeneously broadened as shown by the hole burning experiment in Figure 3B. This is further supported by the T2′ measurements, which predict a natural line width for the broad 56 ppm 31P resonance to be an order of magnitude smaller than the apparent spectral line width (see Table 1). The results from the 31P hole burning experiment and the T2′ measurements independently provide evidence for an inhomogeneously broadened line width that is due to the presence of a heterogeneous environment at the surface of the nanoparticles. The liquid-state 31P{1H} NMR spectrum of PPh3-capped AuNPs reveals fine structure on top of the broad resonance between 45 and 65 ppm, which is not evident in the solid-state 31 P{1H} MAS or CP-MAS NMR spectra. We speculate that each sharp resonance is due to a specific phosphine-ligated gold cluster (e.g., Au6-Au11) formed in the nanoparticle solution during or after synthesis on the basis of the line width and the chemical shift range.68-74 The chemicals used in this synthesis with slight variation in composition and reaction procedure are known to produce phosphine-ligated gold cluster complexes that have a chemical shift and line width in the range observed here. For example, the reduction of Au(PPh3)Cl by sodium borohydride results in gold cluster compounds such as Au11(PPh3)8Cl3,74,75 which has a 31P chemical shift of 51.9 ppm.71,76 Overall, the narrow peaks account for a small fraction of the sample. Further, there are no easy techniques to separate these compounds from each other because they have similar solubility and low stability in solution. Nanoparticles stored in solution over an extended period of time decompose to form phosphine-gold complexes, which is shown by 31P solution NMR collected after three weeks on the original solution sample (see the Supporting Information, Figure S4). As a result, there is complete disappearance of the broad resonance at 56 ppm and reappearance of spurious peaks having different chemical shifts. The major byproducts from the decomposition are Au(PPh3)Cl and metallic gold. The solid-state 31P{1H} MAS NMR spectrum of PPh3-capped AuNPs does not contain the fine structure observed in the liquid -state spectrum, Figure 3C. Previous NMR reports show that the 31P MAS NMR spectra of PPh3-ligated crystalline gold cluster compounds show estimated homogeneous line widths of >400 Hz, which is broad compared to the NMR line width of organic compounds encountered routinely.69 In contrast, these compounds have a sharp 31P NMR resonance (
