Article pubs.acs.org/JPCA
Size-Focusing Synthesis of Gold Nanoclusters with p‑Mercaptobenzoic Acid Laura M. Tvedte and Christopher J. Ackerson* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States S Supporting Information *
ABSTRACT: Etching or size-focusing methods are now widespread for preparation of atomically monodisperse thiolate-protected gold nanoparticles. Size-focusing methods are not widespread, however, in the production of water-soluble gold nanoparticles. Reported here is a new method for size-focusing of large gold nanoparticles utilizing pmercaptobenzoic acid. We observe preferential formation of three large gold nanoparticles with approximate masses of 23, 51, and 88 kDa. On the basis of the stability of these masses against further etching or growth, they appear to be especially stable sizes. These sizes are not prominent after etching challenges with organosoluble ligands, and the 51 and 88 kDa sizes appear to be novel stable thiolate-protected gold cluster sizes. The overall trend in particle size distribution over time is also unusual, with larger sizes dominating at longer time points.
1. INTRODUCTION Etching of polydisperse preparations of gold nanoparticles in excess thiol ligands can narrow the dispersity of a preparation. Often gold nanoclusters with exact molecular formula due to the notable stability associated with some sizes are produced.1−5 The sizes of these stable clusters are generally found to follow either geometric or electronic shell closures.6−8 This type of synthesis is most widely deployed with organothiolates, especially phenylethanethiol (PET). Certain atomically defined etch products, such as [Au25(PPh3)10(PET)5Cl2]2+, Au38(PET)24, Au144(PET)60, and Au 333 (PET) 79 , are now reproduced by many research groups.3,4,9−11 The most widespread methods of formation of all of these AuNCs entail etching with PET from a set of polydisperse AuNCs.12 Although this synthesis method is widespread for organosoluble ligands, there are so-far just two reports of etching or size-focusing synthesis of water-soluble AuNPs, both producing Au25(SR)18. Jin and co-workers report the use of (L)-glutathione (GSH) to etch a set of polydisperse Au@PPh3 clusters to water-soluble Au25(GS)18 in an organic environment.13 Kumar et al. use captopril (capt) as a sizefocusing agent in formation of Au25(capt)18.14 No reports of size-focusing syntheses that result in water-soluble clusters larger than Au25(SR)18 are yet made. Such water-soluble clusters may be useful in biological applications.15−17 In this manuscript, we investigate the use of p-mercaptobenzoic acid (pMBA) in a size-focusing synthesis performed in a completely aqueous environment. This is the first report of use of pMBA as a ligand for size focusing. We find that three products are formed through this method. These products do not appear to share identity with products formed by sizefocusing synthesis in organic solvents. © XXXX American Chemical Society
2. EXPERIMENTAL SECTION Full experimental methods can be found in the Supporting Information. 2.1. Materials. All chemicals were purchased from commercial sources and used as received without further purification. Water (18.2 MΩ·cm−1) was purified with a Thermo Scientific Barnstead Nanopure water purifier. 2.2. Synthesis. Au(I)-(SR) polymer was prepared by combining HAuCl4·3H2O with thiolate in basic aqueous solution (1:3.4 Au:SR) and stirring for 2 days. Thiolates used for polymer preparation were pMBA, m-mercaptobenzoic acid (mMBA), thiomalic acid (TM), GSH, (L)-methionine (MET), (L)-cysteine (CY), and 3-mercaptopropionic acid (MPA). Polydisperse AuNP starting material was prepared by reduction of the Au(I)-(SR) polymer with NaBH4. The two reduced polymers that were both water-soluble and sufficiently polydisperse contained pMBA or MPA. These reduced polymers were the only ones used as starting material for size-focusing. Concentration of this starting material was determined by UV−vis spectroscopy. Size-focusing of these polydisperse pMBA-protected AuNPs was performed through addition of excess ligand (pMBA, mMBA, TM, GSH, MET, CY, MPA, or 4-bromobenzenethiol, BBT) to the starting material (33.4 μM nanoparticles, 200 μM thiol). This solution was then heated to 76 °C for 10 days. The MPA-protected polydisperse AuNPs were etched with pMBA following the same procedure. Special Issue: A. W. Castleman, Jr. Festschrift Received: January 7, 2014
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Figure 1. Polyacrylamide gel electrophoresis showing the change in cluster sizes at 12 h increments over the course of the 10 day etch. * = 23, 51, and 88 kDa products as previously synthesized, ** = Au102(pMBA)44, Au144(pMBA)60, and Au∼288 standards. Lines across the image show the sizes of the 23, 51, and 88 kDa products for ease of visual analysis.
2.3. Characterization. Gold nanoparticles were initially characterized by analysis on a 7T/5C PAGE gel against Au102(pMBA)4418,19 and Au144(pMBA)6020 for determination of dispersity and size of particles. Separation of the three products formed by size-focusing with pMBA was performed using gel purification. Separated discrete particles of different sizes were characterized by determination of electrophoretic mobility,21 UV−vis spectroscopy (full spectra and determination of molar extinction coefficient at 510 nm), and transmission electron microscopy.
Table 1. Comparison of Number of Gold Atoms As Determined by Two Methods (PAGE and TEM) and Number of p-Mercaptobenzoic Acid Ligands As Determined by PAGE Electrophoretic Mobility Calculations no. of pMBA
no. of Au
3. RESULTS AND DISCUSSION Size-focusing of polydisperse pMBA-protected nanoparticles was attempted by incubation at 76 °C for 10 days in the presence of an empirically determined (see Supporting Information for method) large molar excess of each of the following thiolate ligands: pMBA, mMBA, BBT, TM, GSH, MET, and CY. For mMBA, BBT, TM, GSH, MET, and CY, no narrowing in dispersity of the original polydisperse material is observed, as determined by inspection of polyacrylamide gel electrophoresis (PAGE) gels of the product of each attempted reaction (Figure S1, Supporting Information). For pMBA, the size-focusing protocol resulted in three discrete products. PAGE analysis (Figure 1, S2, Supporting Information) shows the generation and evolution of these products over time. Initially, the pMBA products become notably more monodisperse than the starting material (Figure 1: 0−72 h). After this initial size-focusing, the product distribution favors larger and larger sizes over time. The electrophoretic mobility of the three stable products is constant over the course of ∼3 days. However, the distribution of products favors the larger discrete products or bulk gold at longer times. This is in marked contrast to the usual observation in size-focusing synthesis, where the size trends are from larger polydisperse material to smaller monodisperse products.13 Notably, though, the three products we observe here do appear to possess remarkable stability to the chemical challenge of excess thiolate ligand, suggesting special stability. These products appear to also be stable in storage in 1 mg/mL aqueous solution at 22 and 4 °C for at least 4 months, as determined by PAGE analysis (Figure S3, Supporting Information). Analysis of electrophoretic mobility21 of the three sizefocused products on a set of polyacrylamide gels (Figure S4, Supporting Information) suggests the following molecular formulas: Au115(pMBA)49, Au250(pMBA)98, and Au459(pMBA)170 (Table 1). Electrophoretic mobility (Figure
product
PAGE
TEM
PAGE
23 kDa 51 kDa 88 kDa
115 ± 8 250 ± 36 459 ± 59
145 ± 30 270 ± 20 434 ± 15
49 ± 3 98 ± 13 170 ± 20
S5, Supporting Information) was analyzed using Orange G as a solvent front dye and Au102(pMBA)44 and Au144(pMBA)60 as standards.18−20 Using the number of gold atoms in these formulas, an approximate diameter can be calculated for each particle (see Supporting Information for calculation information). Each particle preparation was also analyzed by transmission electron microscopy (TEM, Figure 2). Using TEM images of the nanoparticles, ImageJ software22 was used to measure diameter (Figure 2). In this analysis, diameters of particles were drawn into the image using the software and measured. 1173, 1578, and 1651 particles were measured for the small, medium, and large product sizes, respectively. The diameter of the smallest product was determined to be 1.67 ± 0.05 nm; the medium-size product, 2.06 ± 0.08 nm; and the largest product, 2.41 ± 0.15 nm (Table 2). Table 2 compares the assignment of each product by TEM and quantitative electrophoresis. The agreement between the two methods used to assign these clusters improves as product size increases. Notably, TEM measurements are more error prone for smaller particles, as it is difficult to determine the edges of the particles against the background of the carbon support film. Thus, where the measurements disagree for the smallest particles, the PAGE measurements may be considered more accurate. We attempted both ESI and MALDI based mass spectrometry (MS) on these products, but this approach was not successfulno mass spectra have been obtained for this set of products. MS of water-soluble clusters is apparently more technically challenging than for organosoluble clusters. Due to relatively large uncertainties compared to MS in the methods we used to characterize these three products, there is some uncertainty in their assignments. The smallest 23 kDa compound is similar in mass to Au102(pMBA)44.18 The gel mobility of the 23 kDa compound from this synthesis, however, is always slightly less than that of B
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Au102(SR)44 in side-by-side comparison. This suggests that the synthetic product characterized here may be either a Au102-like product that is always synthesized in tandem with Au102(SR)4419 or possibly another one of the products identified by Dass in what may be an island of special stability for a set of products near 21 kDa in mass.23 This is similar to the apparent island of stability for the 8 kDa product,24,25 where Au36, Au38, and Au40 are all now confirmed by mass spectroscopy, column purification, and single crystal X-ray structures.26−29 The intermediate sized 51 kDa compound is similar in mass to the 59 kDa compound isolated in direct-synthesis with pMBA-protected AuNPs.20 Data in both cases is not of sufficient resolution to make an assignment of formula, but the similar sizes and ligand shells must be noted. The largest product has an 88 kDa core mass and appears as a completely novel product, intermediate in size between the previously reported Au333(SR)79 (66 kDa) and Au∼530(SR)∼100 (104 kDa) compounds.4,5 All of these larger products were the result of size-focusing syntheses. Optical spectra obtained for each of the three compounds provides some insight into the size-dependent evolution of surface plasmon resonance (SPR, Figure 3). There is no
Figure 3. UV−vis spectra of three clusters formed through the presented size-focusing synthesis. The maximum of the traces in the region 475−600 nm occurs at 512 nm for the 51 and 88 kDa products. There are no distinct peaks in the corresponding region for the 23 kDa product.
apparent SPR for the smallest product, which is consistent with other studies suggesting that the smallest size cluster in which a plasmon is present is Au144(SR)60, and in this case the plasmon is localized not to the surface but to the core.9,30,31 The intermediate, 51 kDa product does show spectral features consistent with surface plasmon resonance. Notably, this is smaller than the Au333(SR)79 65 kDa compound, which is the smallest compound widely reported with classically observed SPR.4 The comparative spectral properties of all three compounds are otherwise consistent with increasing surface plasmon resonance features as the compounds become larger. The optical spectra were normalized using empirically determined molar extinction coefficients. These are shown for each compound presented (Table 3). These extinction coefficients follow the general trend seen in other gold nanoparticles such as Au25(PET)18, Au102(pMBA)44, and Au144(pMBA)60 (Table 3). The results of these size-focusing experiments are surprising in three ways. First, three discrete sizes of nanoparticles are always obtained in this size-focusing synthesis. This appears to
Figure 2. Transmission electron microscopy imaging of the 88 kDa (2.41 nm, top panel), 51 kDa (2.06 nm, middle panel), and 23 kDa (1.67 nm, bottom panel) products.
Table 2. Comparison of Gold Nanoparticle Diameter and Number of Gold Atoms from Electrophoretic Mobility Calculations (eph) and TEM Images (TEM) product
Deph (nm)
23 kDa 51 kDa 88 kDa product
1.55 2.01 2.46 no. of Aueph
23 kDa 51 kDa 88 kDa
115 250 459
DTEM (nm) 1.67 ± 0.05 2.06 ± 0.08 2.41 ± 0.15 no. of AuTEM 159 278 462
absolute diff (nm) 0.12 0.05 0.05 absolute diff 44 28 .
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ε510 (M−1 cm−1)
88 kDa 51 kDa 23 kDa Au144(pMBA)60 Au102(pMBA)44 Au25(PET)18
(5.0 ± 0.2) × 105 (1.89 ± 0.04) × 105 (5.3 ± 0.2) × 104 4.34 × 105 1.75 × 105 3.4 × 103
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REFERENCES
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4. CONCLUSIONS Three large p-mercaptobenzoic acid-protected gold nanoparticles are identified in a size-focusing synthesis. These products have been separated via gel purification and characterized for approximate mass. The products of this sizefocusing synthesis are in the size range ∼1.5 to 2.5 nm, over the size range where development of surface plasmon resonance occurs. Further investigation of these apparently especially stable products may produce insights into why these products possess special stability. ASSOCIATED CONTENT
S Supporting Information *
Detailed experimental procedures and calculation of the number of gold atoms from TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.
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ACKNOWLEDGMENTS
The authors thank Colorado State University for supporting this work. This research was conducted while C.J.A. was a New Investigator in Alzheimer’s Disease Grant recipient from the American Federation for Aging Research. The authors thank Dr. O. Andrea Wong for performing transmission electron microscopy and Scott Compel for useful discussions.
be universal, with a dependence only on the initial concentration of nanoparticles and ligand. There does not appear to be any “scale-up” effect on the synthesis: performing the size-focusing at 2× and 3× the normal volumes still results in the formation of the same three products. Second, it appears that there are two competing phenomena occurring within the 10-day incubation. In the early phase of this process, it appears that excess ligand is narrowing the dispersity of the products present, presumably in the same process that operates when excess organosoluble ligands are present.13 In this etching process,12 nanoparticles only become apparently smaller. At later times, etching processes appear to compete with growth processes. Presumably, growth is occurring because of a ligand dissociation and particle fusion process. What is unusual about the growth observed here, and consistent with a simultaneous etching process, is that the same three sizes of nanoparticles are observed throughout, with only a shifting in the distribution toward larger particles. Usually, nanoparticle growth, such as Ostwald ripening or sizedefocusing, results in increasing polydispersity. The trend shown by this size-focusing synthesis may be indicative of potential future work in the synthesis of large gold nanoparticles, particularly in large aqueous preparations. The third way in which these results are surprising is that the stable nanoparticles that come out of this synthesis do not appear to be analogous to the sizes that result from size focusing or etching in organic solvents. Whether these especially stable clusters are geometrically or electronically stabilized will be the subject of future study.
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AUTHOR INFORMATION
Corresponding Author
*C. J. Ackerson: e-mail,
[email protected]. Notes
The authors declare no competing financial interest. D
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