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 für 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 31P 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 surface binding environment.48-50 In this study, we utilize multinuclear NMR techniques to investigate the surface structure and ligand binding environment of PPh3-capped AuNPs and report the observation of a broad 31P resonance that is ascribed to nanoparticle-bound PPh3. In solution, the ligand exchange kinetic reactions were monitored by 1H, 2H, and 31P NMR to characterize the exchange process. Experimental Section Synthesis of Nanoparticles. PPh3-capped AuNPs were synthesized following a biphasic reaction procedure.51 Hydrogen tetrachloroaurate trihydrate (1.0 g, Strem Chemicals) and tetraoctylammonium bromide (1.6 g, Aldrich) were dissolved in a mixture of water and toluene (50 and 65 mL) under a continuous bubbling of nitrogen gas. PPh3 (2.3 g, Aldrich) was added to the solution and the resulting solution stirred vigorously until the toluene layer turned white. Sodium borohydride (1.4 g, Aldrich) dissolved in deionized (DI) water (10 mL) was rapidly poured into the solution, resulting in a dark-colored reaction mixture. After 3 h of stirring, the organic layer was separated, washed with 100 mL of DI water, and dried over nitrogen to produce a dark solid. A series of washes were performed with hexane, a methanol/water (3:2 volume ratio) mixture, and saturated aqueous sodium nitrite to remove biphasic catalyst and byproducts of the reducing agent. The resulting nanoparticles were dissolved in 10 mL of chloroform followed by filtration to remove residues. Excess pentane was added drop- by-drop to the solution to precipitate the PPh3-capped nano- particles, which were removed by filtration. The resulting AuNPs can be stored in their solid form without significant degradation for several months. Elemental analysis (Galbraith Laboratories) showed the following mass composition: Au, 67.2%; P, 2.65%; C, 20.69%; H, 1.45%; Cl, 1.47%. This corresponds to (Au)99(PPh3)24Cl12, which is in reasonable agreement with (Au)101(PPh3)21Cl5 reported by Weare et al.51 TEM Analysis. To determine the average particle size, we performed HRTEM (Philips CM-200 TEM/STEM instrument) characterization. A nanoparticle solution prepared in CH2Cl2 (0.5 mg/mL) was put on a carbon-coated copper grid (Ted Pella, Inc.) and allowed to dry under ambient conditions for 10 min before being placed in the sample holder. Nanoparticles were sampled randomly (N ) 214 particles) for constructing a histogram based on their size. 1H and 31P Solution NMR. Solution NMR samples were prepared in a Teflon-sleeved NMR tube (4.0 mm o.d., 3.2 mm i.d., Wilmad Glass) by dissolving nanoparticles (15.0 mg) in CD2Cl2 (0.3 mL, 0.5% (v/v) TMS, Cambridge Isotopes). It is known that solutions of PPh3-capped AuNPs decompose and deposit a gold film on glass surfaces over the course of several days,49 which we observed when similar solutions were run in standard glass NMR tubes. We have found that PPh3-capped AuNPs are stable in the Teflon-sleeved NMR tube for a couple of days with no visual evidence of gold deposition on the walls of the lined tube. All solution NMR samples were handled at ambient temperature and pressure. 1H and 31P solution NMR experiments were performed on 400 and 500 MHz Varian INOVA spectrometers equipped with a 5 mm Varian indirect- detection variable-temperature (VT) liquid probe, respectively. Tetramethylsilane (TMS) was used for 1H NMR internal and external chemical shift reference. Hole burning 1H NMR experiments were performed by irradiating a 1 s long radio frequency (rf) pulse with very low power (∼5 µW) on the broad resonance at 7.1 ppm that is associated with phenyl ring protons of PPh3-capped nanoparticles. The 31P solution spectra were obtained with 31P single-pulse excitation and 1H WALTZ decoupling52 during data acquisition. All spectra were collected after a π/2 pulse (22 µs) with a recycle delay of 5T1 (∼65 s) between each transient to ensure complete relaxation of the observed spins. The 31P solution spectra were referenced to 85% aqueous phosphoric acid. To determine whether all the 31P NMR signals associated with surface-bound PPh3 were detected, we performed quantitative 31P solution NMR of PPh3 and PPh3-capped AuNPs dissolved in CD2Cl2 separately. Spectra of both samples were collected under identical NMR experimental conditions with equal moles of PPh3 as established from TGA results shown in the Sup- porting Information (Figure S2), allowing for quantitative analysis of 31P NMR. Comparison of 31P NMR integrated areas ensured that all signals associated with PPh3-capped AuNPs were detected. Because the free PPh3 ligand has a relatively long T1 relaxation time, chromium(III) acetylacetonate was added to the pure solution of PPh3 to shorten the relaxation time of the observed spins so that experiments could be completed on a reasonable time scale. By adding 2.0 mg of chromium(III) acetylacetonate in solution, the delay time required between transients was lowered from 65 to 8 s (see Table 1 for T1 data). Solid-State MAS and CP-MAS 31P NMR. Solid-state magic angle spinning (MAS) and cross-polarization magic angle spinning (CP-MAS) NMR were performed on a 400 MHz Varian spectrometer equipped with a triple-resonance 3.2 mm MAS probe operating at a 31P Larmor frequency of 161.8 MHz and 1H decoupling frequency of 400.0 MHz. Approximately 30 mg of PPh3-capped AuNPs was packed in a 3.2 mm zirconia rotor. The solid-state 31P{1H} MAS spectra were collected by applying a 2.2 µs (π/2) pulse under 10 kHz MAS with a 100 kHz two-pulse phase-modulated (TPPM) 1H decoupling rf field TABLE 1: Chemical Shift Parameters, T1, T2′ Relaxation Times and NMR Line Width Values of 31P and 1H NMR Associated with PPh3-Capped 1.8 nm AuNPs. 31P and 1H Solution State NMR Was Performed on a 500 MHz NMR Instruments, while 31P Solid-State NMR Was Performed on a 300 MHz NMR Instrument. NMR experiment NMR spectral component δiso(ppm) T1 (s) T2′ (s) calculated fwhma (Hz) observed fwhmb (Hz) 31P solid (MAS) PPh3-capped AuNPs 56c 8 0.002 159 1500 Au(PPh3)Cl 33 ∼1000 1000 PPh3 (ligand) -8 ∼1000 37 31P solution PPh3-capped AuNPs 56c 1.5 0.012 27 1300 Au(PPh3)Cl 33.7 12 2.01 0.16 3 PPh3 (ligand) -5.4 13 3.75 0.08 3 1H solution PPh3-capped AuNPs 7.1c 1.4 0.04 8 550 Au(PPh3)Cl 7.5 4 2.1 0.15 3 PPh3 (ligand) 7.3 6 5.2 0.06 3 a Natural line width predicted from T2′ measurement. b Observed line width from the spectrum. c Denotes center of the broad peak. 16388 J. Phys. Chem. C, Vol. 113, No. 37, 2009 Sharma et al. during data acquisition. Cross-polarization was achieved using the -1 sideband in the Hartmann-Hahn profile with a contact time of 0.7 ms. A recycle delay of 5 s was used for all solid- state CP-MAS transient averaging. The 31P spectra were referenced with respect to 85% aqueous phosphoric acid by setting the 31P NMR peak of solid dihydrogen ammonium phosphate to 0.8 ppm.53 T1 and T2′ Relaxation Measurements. Spin-lattice (T1) and spin-spin (T2′) measurements for both 1H and 31P NMR were made by using inversion-recovery and spin-echo sequences, respectively.54-56 31P solid-state relaxation measurements (T1 and T2′) were made on a 300 MHz Chemagnetics spectrometer using a Varian triple-resonance 5 mm MAS probe spinning 10 kHz, while solution-state T1 and T2′ measurements were made on a 500 MHz Varian NMR spectrometer. Experimental data were fitted by using the functions Mτ ) Mo[1 - 2 exp(-τ/T1)] and M2τ ) Mo[exp(-2τ/T2′)] for obtaining T1 and T2′, respectively.54,55 From the T2′ times obtained, the natural line width of the resonances was estimated using the relation fwhm ) 1/πT2′ for a Lorentzian line shape.55 Kinetics of Ligand Exchange. 1H NMR was used to assay the kinetics of ligand dissociation and exchange in PPh3-capped AuNPs. Nanoparticle solutions were made by dissolving dried solid AuNPs into neat CD2Cl2. The exchange kinetics were monitored via NMR spectra taken at various time intervals. All NMR spectra were acquired on a 400 MHz Varian Inova NMR spectrometer at room temperature (24 °C). The resulting data were plotted and fitted with exponential functions of the types Mo[1 - exp(-λt)] and Mo[exp(-λt)] for increasing and decreas- ing peaks as a function of time, respectively. To monitor ligand exchange between molecules bound to the surface of the Au nanoparticles and those free in solution, a perdeuterated sample of PPh3 was added to the nanoparticle solution. The rate constants predicted from curve fitting are reported and discussed in relation to the equilibrium constant for ligand dissociation in PPh3-capped AuNPs as depicted in Scheme 1. Synthesis of Perdeuterated PPh3 and Au(PPh3)Cl. Tri- (phenyl-d5)phosphine (d15-PPh3) was prepared in 70% yield by the reaction of (phenyl-d5)magnesium bromide with phosphorus trichloride.57,58 The synthesis product was purified by recrys- tallization in ethanol followed by vacuum distillation. The resulting material was a white crystalline powder. The melting point was found to be 79 °C, and the purity was found to be 99% by mass spectrometry and NMR. Perdeuterated gold(I) chloride triphenylphosphide complex (d15-Au(PPh3)Cl) was prepared by reacting hydrogen tetrachloroaurate trihydrate, HAuCl4 ·3H2O, with d15-PPh3 in a 96% ethanol solution. The product was retained by filtration using a glass frit and rinsed with 45 mL of diethyl ether. Recrystallization was performed by dissolving in methylene chloride and slow addition of pentane followed by cooling to -25 °C. The resulting material was filtered to give a white crystalline powder with 80% yield. A purity of >98% was confirmed by 31P and 1H NMR. 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 SCHEME 1: Ligand Exchange Model in PPh3-Capped AuNPs Showing Possible Pathways 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. Ligand Binding and Exchange Dynamics in AuNPs J. Phys. Chem. C, Vol. 113, No. 37, 2009 16389 isotropic chemical shifts, (ii) bulk magnetic susceptibility (BMS) effects,62 (iii) exchange broadening,63 (iv) reintroduction of residual 1H-1H dipole coupling64 due to restricted mobility at the interface, and (v) metal-ligand electron-nuclear interac- tions.42 The first two interactions (i, ii) result in inhomogeneous line broadening, while the last three (iii-v) lead to primarily homogeneously broadened lines. To ascertain whether the broad line width was homogeneous or inhomogeneous, a hole burning experiment55,65 was performed, and the result is shown in Figure 2. Under the conditions of this experiment, the observation of a hole in the broad resonance indicates an inhomogeneous line width. However, the breadth of the “hole” indicates that homogeneous broadening is also involved, but to a lesser extent. Figure 2B shows the evidence of inhomogeneous broadening of the broad phenyl proton resonance. This result is similar to that observed in alkanethiol-capped AuNPs using 1H-13C CP- MAS, also demonstrating inhomogeneous broadening with the conclusion that it is due to faceting of the nanoparticle surface, causing a distribution of chemical environments for the bound ligands.42,66 While heterogeneous broadening dominates the observed line width, there could still be underlying homogeneous broadening that causes the burned hole to be broader than expected and contributes to increased relaxation rates in AuNPs. A series of NMR experiments were performed to explore whether BMS or chemical exchange affect the observed 1H phenyl resonance line broadening in PPh3-capped AuNPs. First, 1H NMR was performed on a dilute solution (1 mg/mL) because dilution decreases anisotropic BMS effects.62 Second, the 1H solution spectrum was collected under MAS NMR to eliminate any residual chemical shift anisotropy (CSA) or isotropic BMS broadening.62 Observing no difference in the line width of the two spectra compared to that observed in 1H solution NMR indicates that the broadening is not caused by BMS or CSA (see the Supporting Information, Figure S3). A sealed solution 1H NMR of PPh3-capped AuNPs measured in the temperature range of 298-193 K showed no difference in line width or position (data not shown). This result indicates that the ligand exchange rate between surface-bound PPh3 and Au(PPh3)Cl is slower than the 1H NMR time scale. Surface-bound ligands can experience both restricted mobility at the interface and strong metal-ligand interactions, potentially contributing to NMR line broadening. For example, the 13C NMR spectra of long-chain-alkanethiol-capped AuNPs show the broadest peaks for nuclei closest to the gold core and peaks that become substantially sharper as one approaches the terminal nuclei.36 The resonance line widths have been attributed to a combination of heterogeneous and homogeneous broadening. Specifically, the focus has been on changes in magnetic susceptibility at the metal-hydrocarbon interface and residual dipolar interactions. Recent studies do not completely agree with these interpretations, and therefore, it has been reported that the substantial broadening of 13C resonances in thiol-capped AuNPs is due to a strong interaction with the nanoparticle surface.41 For the line broadening observed in 1H NMR of PPh3- capped AuNPs, we attribute the primary contribution to a distribution of chemical environments. This is further confirmed by T2′ relaxation measurements. The full width at half-maximum (fwhm) values predicted from T2′ relaxation measurements for free PPh3 and surface-bound PPh3 are significantly different (see Table 1). The 1H resonance of the free PPh3 in solution has a natural line width of ∼0.06 Hz in contrast to 8 Hz for surface- bound PPh3. This difference in line width can be attributed to residual dipole coupling due to restricted mobility of the surface- bound ligands and matches the line width from NMR hole burning. Characterization of 31P NMR of PPh3-Capped Gold Nanoparticles. The stacked plot in Figure 3 shows a comparison of (A) 1H-31P CP-MAS, (B) hole burning direct excitation 31P MAS, and (C) solution 31P NMR spectra of PPh3-capped 1.8 nm AuNPs. All spectra were acquired under proton decoupling conditions. The resonance observed at 34 ppm is attributed to a phosphine-gold complex, Au(PPh3)Cl, an exchange product that is always present in solution-based PPh3-capped AuNPs.49 In both the solid and solution states, we observed a broad 31P resonance centered at 56 ppm that is ascribed to surface-bound 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 PPh3. In previous solution-state NMR of PPh3-capped Au nanoparticles, a surface-bound resonance was not observed.48 This is most likely due to the nuances of detecting broad resonances on a high-resolution liquid-state NMR instrument. The broad resonance observed in the 31P NMR spectra accounts for >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 solid- state 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 31P{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 borohy- dride 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 ( Thus, even though the exchange experiments discussed above suggest a dissociative mechanism, the behavior of pure solutions of the nanoparticles does not support a simple dissociative process. To further probe the kinetic processes at PPh3-capped AuNPs, we also examined the influence of d15- Au(PPh3)Cl in solution on exchange. Figure 4D shows the buildup of solution-phase Au(PPh3)Cl as monitored with 1H NMR when 20 mol equiv of d15-Au(PPh3)Cl was added to a CD2Cl2 solution of PPh3-capped AuNPs at room temperature. The results show buildup of a sharp 1H NMR peak having a chemical shift identical to that for Au(PPh3)Cl in solution. The position of this peak at 7.5 ppm shows that there is no free PPh3 exchanging with the complex in solution,49 implying that exchange of the complex does not require the presence of free PPh3. The appearance of the deuterated ligand, d15-PPh3, on the nanoparticles is verified by direct observation of a broad peak in the 2H NMR spectrum of d15-PPh3 from the nanoparticles isolated and recovered after this exchange experiment (Figure S5, Supporting Information). The rate of loss of Au(PPh3)Cl from the NP is calculated by fitting the data in Figure 4D, giving a value of 0.20 min-1. This is very similar to the values given above, showing that the displacement of Au(PPh3)Cl from the NP surface occurs at the same rate whether the incoming species is the PPh3 ligand or Au(PPh3)Cl complex. Taken together, these results suggest that Au(PPh3)Cl on the nanoparticle surface is exchangeable with d15-Au(PPh3)Cl in solution via a pathway that is most likely represented by path 2 in Scheme 1. Path 3 is ruled out because a lack of significant concentrations of free PPh3 in this experiment precludes that mechanism. The mechanism of ligand exchange in nanoparticles is important in understanding chemical kinetics and reaction rates. Previous studies of the ligand exchange mechanism in thiol- capped nanoparticles have demonstrated associative,77 dis- sociative,44,78-81 and associative interchange and dissociative interchange43 pathways. Ligand exchange rates and mechanisms of PPh3-capped AuNPs also have been indirectly observed in several studies through the exchange with thiol and disulfide ligands. For instance, the loss of PPh3 from 1.5 nm diameter PPh3-capped AuNPs with a diradical disulfide ligand in dichlo- romethane/toluene mixtures has a rate constant of 0.08 min-1 at room temperature.79 This is comparable to the rate constant we determine for loss of Au(PPh3)Cl caused by the presence of PPh3 in CD2Cl2 at room temperature and is 20-50 times faster than the typical exchange of thiol ligands on thiolated Au nanoparticle surfaces. However, just as is the case here, the mechanism of PPh3 exchange in the previous example was not a simple dissociative pathway. Rather, it was a complex process involving elements of dissociative processes but also different behavior at different stages of the exchange process.79 There are several factors that could complicate the kinetics for loss of the ligand and/or complex from nanoparticles of the type described here. For example, Murray and co-workers showed that exchange processes for thiolated AuNPs depend on the state of charge (i.e., the redox state) of the AuNP.33 Further, since the loss of Au(PPh3)Cl unavoidably leads to a change in the average oxidation state of the AuNP/cluster, that change would likely influence the exchange process. Finally, the exchange process may be mediated in some way by the presence or availability of surface sites for the incoming species (either the ligand or the complex). The occupancy of such sites and how that occupancy depends on the identity and/or concentration of the incoming species in solution may influence the observed kinetic behavior. For example, saturation of such sites in an associative interchange mechanism may mask the expected concentration dependence expected for the incoming species. In such a case, associative interchange and dissociative mech- anisms may be difficult to discern. The present results can be considered in the context of the emerging view of thiolated AuNPs as Au cores surrounded or capped by Au complexes. For example, recent crystal structures described a Au102 cluster comprised of a Au49 Marks decahedral core capped with bridging Au3(SR)2 “staples” 24 and a Au25 cluster comprised of a Au13 icosahedral core capped by bridging Au4(SR)3 “semirings”.31 In the present case, the displacement of Au(PPh3)Cl rather than PPh3 from the nanoparticles is consistent with a similar view for PPh3-capped AuNPs, namely, that they are comprised of a Au core surrounded by Au complexes containing both PPh3 and Cl- ligands. This is further supported by the fact that the chemical shift value of surface- bound PPh3 is similar to that for PPh3-ligated cationic gold cluster compounds that have both Au(0) and Au(I) atoms.68,70,72 Assuming a structure for PPh3-capped AuNPs similar to that for the thiolated AuNPs discussed above, it is perhaps not surprising that exchange would lead to displacement of Au complexes rather than free PPh3 ligands. Conclusions A series of solid-state and solution NMR studies of PPh3- capped AuNPs by 1H, 2H, and 31P NMR showed broad res- onances associated with surface-bound phosphine ligands. The broadened resonances are due to the chemical shift heterogeneity as indicated by a combination of solid-state, solution, and relaxation NMR results. The 31P chemical shift of surface-bound PPh3 is consistent with those found in PPh3-ligated gold cluster compounds, suggesting the presence of both gold(0) and gold(I) at the nanoparticle surfaces. A series of kinetic exchange experiments showed that PPh3-capped AuNPs undergo exchange with loss of Au(PPh3)Cl from the NP surface. The results are consistent with a structural model for the PPh3-capped AuNPs that is similar to those recently described for thiolated Au102 and Au25 clusters. Acknowledgment. This research was supported by the U.S. Department of Energy, Basic Energy Sciences (Grants DE- FG02-05ER46235 and DE-FG36-06G01G016029), National Science Foundation (Chemistry Division Grant NSF 0313661), W.M. Keck Foundation, and Department of Defense. We thank Prof. R. Marzke for use of the 300 MHz Chemagnetics-Varian NMR spectrometer. Supporting Information Available: (Figure S1) UV-vis, (Figure S2) TGA, and (Figure S3) 1H solution-state and MAS NMR of 1.8 nm PPh3-capped AuNPs, (Figure S4) 31P solution and MAS NMR of AuNP samples stored for different time periods, (Figure S5) 2H solution-state NMR of d15-PPh3- and d15-PPh3-capped nanoparticles, (Figure S6) 1H NMR of d15-PPh3- exchanged nanoparticles, (Figure S7) 1H NMR of PPh3-capped particles in CD2Cl2 solution immediately after dissolving, (Figure S8) 1H NMR of 1-octanethiol exchange of 1.8 nm PPh3- capped AuNPs, and (Figure S9) quantitative 31P NMR of AuNPs in solution. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Biju, V.; Itoh, T.; Anas, A.; Sujith, A.; Ishikawa, M. Anal. Bioanal. Chem. 2008, 391, 2469. (2) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edwards, P. P. Chem. Soc. ReV. 2000, 29, 27. 16392 J. Phys. Chem. C, Vol. 113, No. 37, 2009 Sharma et al. (3) Murray, R. W. Chem. ReV. 2008, 108, 2688. (4) Shon, Y. S.; Choo, H. C. R. Chim. 2003, 6, 1009. (5) Schmid, G. Chem. ReV. 1992, 92, 1709. (6) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (7) Schmid, G.; Corain, B. Eur. J. Inorg. Chem. 2003, 2003, 3081. (8) Schenhar, R.; Rotello, V. M. Acc. Chem. Res. 2003, 36, 549. (9) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (10) Fauth, K.; Kreibig, U.; Schmid, G. Z. Phys. D 1989, 12, 515. (11) Wilcoxon, J. P.; Martin, J. E.; Provencio, P. J. Chem. Phys. 2001, 115, 998. (12) Zhang, H.; Schmid, G.; Hartmann, U. Nano Lett. 2003, 3, 305. (13) Zhang, X.; Sun, B.; Friend, R. H.; Guo, H.; Nau, D.; Giessen, H. Nano Lett. 2006, 6, 651. (14) Raguse, B.; Chow, E.; Barton, C. S.; Wieczorek, L. Anal. Chem. 2007, 79, 7333. (15) Huang, C. C.; Chang, H. T. Anal. Chem. 2006, 78, 8332. (16) Liu, T.; Tang, J.; Zhao, H.; Deng, Y.; Jiang, L. Langmuir 2002, 18, 5624. (17) Ipe, B. I.; Yoosaf, K.; Thomas, K. G. J. Am. Chem. Soc. 2006, 128, 1907. (18) El-Deab, S. M.; Ohsaka, T. Electrochem. Commun. 2002, 4, 288. (19) Debouttière, P.-J.; Roux, S.; Vocanson, F.; Billotey, C.; Beuf, O.; Favre-Réguillon, A.; Lin, Y.; Pellet-Rostaing, S.; Lamartine, R.; Perriat, P.; Tillement, O. AdV. Funct. Mater. 2006, 16, 2330. (20) Feldheim, D. L.; Grabar, K. C.; Natan, M. J.; Mallouk, T. E. J. Am. Chem. Soc. 1996, 118, 7640. (21) Gibson, J. D.; Khanal, B. P.; Zubarev, E. R. J. Am. Chem. Soc. 2007, 129, 11653. (22) Chithrani, B. D.; Chan, W. C. W. Nano Lett. 2007, 7, 1542. (23) Hong, R.; Han, G.; Fernandez, J. M.; Kim, B. j.; Forbes, N. S.; Rotello, V. M. J. Am. Chem. Soc. 2006, 128, 1078. (24) Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. Science 2007, 318, 430. (25) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1997, 119, 12384. (26) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 882. (27) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys. Chem. 1995, 99, 7036. (28) Teo, B. K.; Shi, X.; Zhang, H. J. Am. Chem. Soc. 1992, 114, 2743. (29) Murray, R. W. Chem. ReV. 2008, 108, 2688. (30) Akola, J.; Walter, M.; Whetten, R. L.; Hakkinen, H.; Gronbeck, H. J. Am. Chem. Soc. 2008, 130, 3756. (31) Heaven, M. W.; Dass, A.; White, P. S.; Holt, K. M.; Murray, R. W. J. Am. Chem. Soc. 2008, 130, 3754. (32) Whetten, R. L.; Price, R. C. Science 2007, 318, 407. (33) Song, Y.; Harper, A. S.; Murray, R. W. Langmuir 2005, 21, 5492. (34) Lica, G. C.; Zelakiewicz, B. S.; Tong, Y. Y. J. Electroanal. Chem. 2003, 554-555, 127. (35) Mayer, C. Annu. Rep. NMR Spectrosc. 2005, 55, 205. (36) Terrill, R.; Postlethwaite, T.; Chen, C.; Poon, C.; Terzis, A.; Chen, A.; Hutchison, J.; Clark, M.; Wignall, G.; Londono, J.; Superfine, R.; Falvo, M.; Johnson, C.; Samulski, E.; Murray, R. J. Am. Chem. Soc. 1995, 117, 12537. (37) Hostetler, M. J.; Wingate, J. E.; Zhong, C. J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17. (38) Zelakiewicz, B. S.; de Dios, A. C.; Tong, Y. Y. J. Am. Chem. Soc. 2003, 125, 18. (39) Kohlmann, O.; Steinmetz, W. E.; Mao, X. A.; Wuelfing, W. P.; Templeton, A. C.; Murray, R. W.; Johnson, C. S. J. Phys. Chem. B 2001, 105, 8801. (40) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 2001, 105, 8785. (41) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Reven, L. Langmuir 1996, 12, 1262. (42) Badia, A.; Demers, L.; Dickinson, L.; Morin, F. G.; Lennox, R. B.; Reven, L. J. Am. Chem. Soc. 1997, 119, 11104. (43) Song, Y.; Huang, T.; Murray, R. W. J. Am. Chem. Soc. 2003, 125, 11694. (44) Donkers, R. L.; Song, Y.; Murray, R. W. Langmuir 2004, 20, 4703. (45) Caragheorgheopol, A.; Chechik, V. Phys. Chem. Chem. Phys. 2008, 10, 5029. (46) Woehrle, G. H.; Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 2005, 127, 2172. (47) Conte, M.; Wilson, K.; Chechik, V. Org. Biomol. Chem. 2009, 7, 1361. (48) Petroski, J.; Chou, M. H.; Creutz, C. Inorg. Chem. 2004, 43, 1597. (49) Schmid, G. Development in Transition Metal Cluster Chemisry. Structure and Bonding; Springer-Verlag: Berlin, 1985; Vol. 62, p 52. (50) Wang, W.; Murray, R. W. Langmuir 2005, 21, 7015. (51) Weare, W. W.; Reed, S. M.; Warner, M. G.; Hutchison, J. E. J. Am. Chem. Soc. 2000, 122, 12890. (52) Shaka, A. J.; Keeler, J.; Frenkiel, T.; Freeman, R. J. Magn. Reson. 1983, 52, 335. (53) S. Reinhard, J. B. Magn. Reson. Chem. 2003, 41, 406. (54) Mackenzie, K. J. D.; Smith, M. E. Multinuclear Solid-State NMR of Inorganic Materials; Elsevier Science: Amsterdam, 2002; Vol. 6. (55) Fukushima, E.; Roeder, S. B. Experimental Pulse NMR: A Nuts and Bolts Approach; Addison-Wesley Publishing Co., Inc.: Reading, MA, 1981. (56) Farrar, T. C. Pulse Nuclear Magnetic Resonance Spectroscopy; The Farragut Press Chicago: Madison, WI, 1989. (57) Dodonow, J.; Medox, H. Ber. Dtsch. Chem. Ges. 1928, 61, 901. (58) Bioanco, V. D.; Doronzo, S. Inorg. Synth. 1976, 16, 164. (59) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (60) Dahl, J. A.; Maddux, B. L. S.; Hutchison, J. E. Chem. ReV. 2007, 107, 2228. (61) Warner, M. G.; Reed, S. M.; Hutchison, J. E. Chem. Mater. 2000, 12, 3316. (62) VenderHart, D. L.; Earl, W. L.; Garroway, A. N. J. Magn. Reson. 1981, 44, 361. (63) Bain, A. D. Prog. NMR Spectrosc. 2003, 43, 63. (64) Lipsitz, R. S.; Tjandra, N. Annu. ReV. Biophys. Biomol. Struct. 2004, 33, 387. (65) Bloembergen, N.; Purcell, E. M.; Pound, R. V. Phys. ReV. 1948, 73, 679. (66) Holland, G. P.; Sharma, R.; Agola, J. O.; Amin, S.; Solomon, V. C.; Singh, P.; Buttry, D. A.; Yarger, J. L. Chem. Mater. 2007, 19, 2519. (67) Kolbert, A. C.; De Groot, H. J. M.; Van der Putten, D.; Brom, H. B.; De Jongh, L. J.; Schmid, G.; Krautscheid, H.; Fenske, D. Z. Phys. D 1993, 26, 24. (68) Van der Velden, J. W. A.; Bour, J. J.; Bosman, W. P.; Noordik, J. H. Inorg. Chem. 1983, 22, 1913. (69) Van der Velden, J. W. A.; Beurskens, P. T.; Bour, J. J.; Bosman, W. P.; Noordik, J. H.; Kolenbrander, M.; Buskes, J. A. K. M. Inorg. Chem. 1984, 23, 146. (70) Van der Velden, J. W. A.; Bour, J. J.; Steggerda, J. J.; Beurskens, P. T.; Roseboom, M.; Noordik, J. H. Inorg. Chem. 1982, 21, 4321. (71) Vollenbroek, F. A.; Van den Berg, J. P.; Van der Velden, J. W. A.; Bour, J. J. Inorg. Chem. 1980, 19, 2685. (72) Kappen, T. G. M. M.; Schlebos, P. P. J.; Bour, J. J.; Bosman, W. P.; Beurskens, G.; Smits, J. M. M.; Beurskens, P. T.; Steggerda, J. J. Inorg. Chem. 1995, 34, 2121. (73) Kappen, T. G. M. M.; Schlebos, P. P. J.; Bour, J. J.; Bosman, W. P.; Smits, J. M. M.; Beurskens, P. T.; Steggerda, J. J. Inorg. Chem. 1995, 34, 2133. (74) Bartlett, P. A.; Bauer, B.; Singer, S. J. J. Am. Chem. Soc. 1978, 100, 5085. (75) Woehrle, G. H.; Warner, M. G.; Hutchison, J. E. J. Phys. Chem. B 2002, 106, 9979. (76) Vollenbroek, F. A.; Bour, J. J.; Trooster, J. M.; Van der Velden, J. W. A. J. Chem. Soc., Chem. Commun. 1978, 907. (77) Montalti, M.; Prodi, L.; Zaccheroni, N.; Baxter, R.; Teobaldi, G.; Zerbetto, F. Langmuir 2003, 19, 5172. (78) Song, Y.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 7096. (79) Ionita, P.; Caragheorgheopol, A.; Gilbert, B. C.; Chechik, V. Langmuir 2004, 20, 11536. (80) Ionita, P.; Caragheorgheopol, A.; Gilbert, B. C.; Chechik, V. J. Am. Chem. Soc. 2002, 124, 9048. (81) Kassam, A.; Bremner, G.; Clark, B.; Ulibarri, G.; Lennox, R. B. J. Am. Chem. Soc. 2006, 128, 3476. JP905141H Ligand Binding and Exchange Dynamics in AuNPs J. Phys. Chem. C, Vol. 113, No. 37, 2009 16393
Comments
Report "NMR Characterization of Ligand Binding and Exchange Dynamics in Triphenylphosphine-Capped Gold Nanoparticles"