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Improved Chemical and Colloidal Stability of Gold Nanoparticles Through Dendron Capping Katherine C. Elbert, Jennifer D. Lee, Yaoting Wu, and Christopher B. Murray Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02960 • Publication Date (Web): 12 Oct 2018 Downloaded from http://pubs.acs.org on October 15, 2018
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Improved Chemical and Colloidal Stability of Gold Nanoparticles Through Dendron Capping Katherine C. Elbert,† Jennifer D. Lee,† Yaoting Wu,† Christopher B. Murray*,†,‡ † ‡
Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, United States Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia PA 19104, United States
Supporting Information Placeholder ABSTRACT: Nanoparticle (NP) stability is imperative
for commercialization of nanotechnology. In this study, we compare the stability of Au NPs with surfaces functionalized with oleylamine, dodecanethiol, and two dendritic ligands of different generations. The dendrimer ligands provide a significant increase in the chemical stability of the Au NPs when analyzed by cyanideinduced NP decomposition as well as an investigation into their colloidal stability at ambient conditions. These results were supported by absorption measurements, TEM, TGA, NMR, and SAXS, and show that dendrimers play a key role in improving the chemical and colloidal stability of NPs.
Keywords: Dendrimer, nanoparticle stability, colloidal stability, dendrimer-enhanced stability, shelf-life. Introduction Improvement of nanoparticle (NP) stability is of critical importance to nanotechnology as it enables processing, handling, and storage of NPs, providing opportunities for commercialization. However, many studies focus on NP stability for the length of a particular experiment, and the lifetime of the NP in solution is not considered. While factors that affect NP stability and ideal storage conditions may be known in research, there is relatively little published research in this area.1,2 Initial studies have investigated the lifetime and colloidal stability of magnetic NPs,3,4 gold NPs,5,6 and quantum dots.7 Many of the ligands used to provide increased stability are commercially available thiol molecules for applications in biological systems,8 as well as polymers as stabilizers.9,10 More specialized ligands, such as ionic species and carbenes have shown recently to provide stability in organic media.11,12 Currently, there has been limited investigation on the use of dendritic ligands, however dendrimers have a variety of applications in nanotechnology.13 Previous
efforts to improve stabilization and colloidal stability properties14 have particularly focused on applications in biological systems.6–8,15,16 However, given the examples of utilizing dendrimers to stabilize a variety of particles for catalytic applications,17–19 as well as gold particles20– 22 and quantum dots,23 we feel that these ligands could play a critical role in improving the stability and overall lifetime of NPs in solution. In this study, native ligands on the surface of NPs were replaced by dendrimeric ligands through a simple ligand exchange procedure.24 This approach is advantageous compared to strategies utilized in previous studies,22 as NP size distributions close to 5% can be achieved and morphology can be maintained, leading to any changes in size and morphology later more apparent. Additionally, the same batch of NPs can be used for all ligands studied, making direct comparisons more viable. This method, as well as the architecture of the dendritic ligands, sets our work apart from those previously studied. NPs with oleylamine (Olam) and dodecanethiol (DDT) were included in these studies, as these are two common, commercially available ligands for Au NP synthesis. The chemical stability of the dendrimerNPs was then evaluated using degradation studies, where solutions of the dendrimer-NPs were subject to oxidizing conditions, and the chemical stability of the NPs was followed by absorption measurements. Additionally, the dendrimer-NPs were suspended in a solution of hexanes and were allowed to sit at ambient conditions in a sealed vial filled with air for over one and a half years, where they remained in solution and minimal changes in NP size and morphology were observed. This study suggests that dendrimer coatings on NPs are particularly effective stabilizers, increasing the chemical and colloidal stability of the NPs compared with commercially available Olam and DDT. Experimental Materials. 3,5- dihydroxybenzoate (97 %), 2,6ditertbutyl 4-methyl pyridine (98 %), and oxalyl chloride
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(98 %) were purchased from Acros Organics. All other reagents were purchased form Sigma Aldrich and used without further purification. All solvents were ACS grade or higher, purchased from Fisher Scientific, and used as received. Au NP Synthesis. The Au NPs were synthesized following reported procedures.25,26 To synthesize the 4.4 nm gold NPs, 200 mg HAuCl4·3H2O was dissolved in 10 mL of oleylamine (Olam) and 10 mL of tetralin under a flow nitrogen with magnetic stirring. Then, a mixture containing 90 mg of borane tert-butylamine (TBAB) dissolved in 1 mL of Olam and 1mL of tetralin was injected into the HAuCl4 solution, upon which the solution turned deep red immediately. The reaction was kept at room temperature while stirring for 2 h, followed by the addition of acetone (120 mL) and centrifugation at 8000 rpm for 3 minutes. The resulting NPs were redispersed hexane, precipitated with ethanol, and centrifuged at 8000 rpm for 3 min to isolate the precipitate. This process was repeated once more, and the particles were stored in hexane for further use. The synthesis of additional sizes of Au NPs have been previously described.27 Ligand Exchange. All ligand exchanges were performed using a previously reported generalized procedure,24,27 where 10 mg of the desired ligand was dissolved in 3 mL of choloroform, which was sequentially added to 1 mL of a 10 mg/mL solution of Au@Olam in hexanes. The resulting mixture was stirred at room temperature for 1 hour before being quenching by addition of methanol. Upon centrifugation (6000 rpm, 5 minutes), the resulting precipitate was collected, and the supernatant liquid was discarded. The remaining solid was redispersed in hexanes and upon addition of methanol/ethanol precipitated out of solution and isolated by centrifugation. To ensure removal of any excess ligands, this last step was repeated at least twice. Characterization Methods. 1H NMR (500 MHz) spectra were recorded on Bruker UNI500 or BIODRX500 NMR spectrometer. All spectra were referenced using solvent residual signals (CDCl3: 1H, δ 7.27 ppm). Small-angle transmission X-ray scattering (SAXS) was performed on a Multi-angle X-ray Scattering Facility equipped with a Bruker Nonius FR591 40 kV rotating anode generator operated at 65 mA, Osmic Max-Flux optics, 2D Hi-Star Wire detector, and pinhole collimation, with an evacuated beam path. Samples were prepared in capillary tubes and collected for 1 hour. For optical extinction spectra, Solution-phase measurements were collected on an Analytical Spectral Devices QSP
350-2000 UV-VIS-NIR spectrometer. TEM micrographs were collected using a JEOL 1400 microscope operated at 120 kV. The TEM was calibrated using a MAG*I*CAL® TEM calibration standard. Thermogravimetric analysis (TGA) was carried out using a TA Instruments TGA Q600 apparatus in the temperature range of 25 oC to 500 oC under N2 flow at a heating rate of 10 o C/min. Thermal transitions were determined on a TA Instruments Q2000 differential scanning calorimeter (DSC) equipped with a liquid nitrogen cooling system with 10 oC/min heating and cooling rates. Results and Discussion To study the influence of dendritic ligands on NP colloidal stability in organic solvents, we utilized dendrimers previously shown to be effective at dispersing Au NPs in a variety of organic solvents.27 The structure of each dendrimer, G1C12 and G2C12, is shown in Figure 1. The two dendrimers selected have identical surface anchoring units, disulfide-based functional group, as well as the same hydrophobic moieties at the terminus of the molecules, as shown in Scheme S1. While the complete synthetic details and characterization have been previously reported,27 briefly, methyl 3,4,5trihydroxybenzoate was reacted with dodecylbromide using Williamson etherification. The resulting methyl ester was either hydrolyzed or reduced and subsequently chlorinated to yield intermediates for the first generation dendrimer or to build higher generation dendrimers, respectively. The higher generation dendron was synthesized by reacting the chlorinated intermediate with 3,5dihydroxybenzoate via Williamson etherification, followed by hydrolysis to yield a carboxylic acid. Both generation acids can proceed in a similar manner by esterification reactions between the acid and our disulfidebearing binding group moiety. A simple ligand exchange procedure was used to graft the dendritic ligands onto the surface of the Au NPs,24 where the NPs with various ligands will be denoted Au@ligand. The successful ligand exchange was confirmed with NMR, shown in Figure S1 and 2, as well as increased interparticle spacing, highlighted in Figure S3, and Thermogravametric Analysis (TGA) (Figure S5). From Transmission Electron Microscope (TEM) analysis, it was confirmed that this ligand exchange procedure did not alter the inorganic NP’s size or
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Figure 1. Absorption curves of various Au NPs (0.1 mg/mL in THF) after adding of 1.5 mM NaCN in H2O, captured over 15 minutes: (a) Au@Olam, (b) Au@DDT, (c) Au@G1C12, (d) Au@G2C12. (e) The decay of the Au NPs, with the intensities from a-d extracted from 400 nm and 523 nm (multiple runs for each).
morphology, which is shown by the histograms in Figure S4, and additionally by the absorption measurements, Figure S6. Previous studies have shown tracking cyanideinduced decomposition with absorbance measurements can be an effective probe for evaluating the efficacy of an organic monolayer in protecting the surface of Au NPs, which provides information on the surface accessibility and stability of the Au NP inorganic core.22,28–30 Solutions of the various Au NPs were prepared (0.1 mg/mL in THF), and rapidly mixed with a sodium cyanide (NaCN) solution (1.5 mM in H2O). The change in absorbance was monitored using UV-vis spectroscopy, where the curves before addition of NaCN are shown in Figure S6. Absorbance decay curves were tracked over time, and the resulting normalized data is shown in Figure 1. From a direct comparison, it is clear that Au@Olam are the most susceptible to degradation with NaCN compared with Au@DDT, Au@G1C12, or Au@G2C12. Further, the peak from the plasmon resonance of the Au NPs retains its position for the case of Au@G1C12 and Au@G2C12, whereas for Au@Olam and Au@DDT, the plasmon peak is not only decreased, but a red shift is observed. These observations are quantitatively apparent when the change in absorbance at 400 nm and 523 nm was compared across the decay time, as shown in Figure 1e, where the error bars were derived from multiple runs of each sample. Wavelengths of 400 nm and 523 nm were used for this comparison, as above 400 nm the localized plasmon resonance of the Au NP dominates the spectra,
while it has been shown that the absorbance at 400 nm directly correlates to the concentration of Au(0) in the solution.31 However, previous studies have used the maxima from the plasmon resonance for comparison,6,28 so these values were incorporated into the error as well. From Figure 1, it is clear that the dendritic ligands provide increased stability for the Au NPs compared with Olam or DDT. For the dendritic ligands and Olam, most of the decay occurs within the initial minutes after addition of NaCN. Interestingly, when DDT is on the surface of the Au NPs, the decay continues over the time of the study. A previous study by Shon et. al analyzes a similar dendrimer architecture but different end group moieties on the periphery found that larger generation dendrimer ligands provide more protection against chemical degradation.22 In this study, the maximum dendrimer coverage of each particle was about 60 % of the total ligands, and the NPs were synthesized using a strategy that yielded a size distribution of 35%. In contrast, our study was designed to maximize the amount of each ligand on the surface of the NPs through a generalized ligand exchange procedure, where first the Au NPs were synthesized using an approach that yields size distributions closer to 5%, making it easier to extract trends from the data. We found that the smaller dendrimer was a more effective ligand at providing chemical stability. The contradictory results could be attributed to the difference in synthetic strategy, as maximum coverage of a smaller generation dendrimer is known to be distinct from that of a larger generation, as will be discussed further below. Furthermore, directly comparing the chemical stability
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of NPs with dendrimers on their surface to those with commercially available, common ligands is an important benchmark for comparison. The stability that the dendrimeric ligands provide is possibility from multiple sources. It is well known that the thiol-gold bond is strong compared to the amine bond formed from Olam, however Au@DDT decomposed at a faster rate than Au@G1C12 and Au@G2C12. Comparing Au@G1C12 and Au@G2C12, the higher generation dendrimer increases the lifetime compared to the native ligand, Olam, but less so compared with the G1C12 ligand. This suggests that the shape of the dendritic molecule plays a large role stabilizing NPs, as their typical wedge-like shape can create a dense monolayer around a spherical particle,32,33 however the higher generation ligands with increased branching create a less even organic shell around the NPs, due to their steric bulk.6,34 This is most likely due to the packing of the ligand on the surface of the NP, as increased branching leads to a larger cone angle of the ligand itself,35,36 which in turn results in a decrease in the density of ligands on the surface of the NP.24 This decrease in ligand density and coverage of the NPs could result in a more accessible Au core. It has also been suggested that the chemical structure of the ligand plays an important role in NP stability, again due to packing of the ligand on the NP surface, and the ability for certain chemical compositions to create more complete organic shells around the NP. 6,34 To further study the potential shelf-life of Au NPs with our series of dendritic ligands grafted on their surface, solutions of Au NPs in hexanes were left at ambient conditions. Only the dendrimer-Au NPs and Au@Olam were used in this study as the dendrimer-Au NPs performed best in the previous study. Impressively, the Au NPs with dendrimers on their surface stayed in solution for over 1.5 years, and there was minimal change in the size or morphology of the inorganic core of the NPs. This is highlighted by Figures 2 and 3, where Figure 2 shows the TEM images of the Au NPs (originally 7.6 ± 0.5 nm in diameter) directly after ligand exchange as well as 1.5 years later. The Au NPs are the same size and morphology as after the initial ligand exchange, which is emphasized by Figure S7, where the histograms from these TEM images show a similar size range when the dendritic ligands are on the surface of Au NPs compared to when Olam is on the surface. The Au@Olam NPs are observed to be 10.7 ± 4.2 nm in diameter, whereas the Au@G1C12 are 8.3 ± 0.4 nm and Au@G2C12 are 7.9 ± 0.5 nm. This data was collected from TEM images where some NPs were dropcast, while others were allowed to assemble at a liquid-air interface, to ensure that any distribution in size of the NPs was accounted for, as NPs of varying sizes may be ejected from a film. However, the assembly properties of the NPs are maintained, as shown in Figure S9. Addi-
tionally, NMR spectra in Figure S2 confirms that the dendritic ligand is still present on the surface of the Au NPs. An additional, larger size of Au NPs was subject to the same conditions, and as with the smaller NPs, those with dendritic ligands on their surfaces retained their size and morphology, shown in Figures S9 and S10.
Figure 2. TEM images of Au NPs (originally 7.6 ± 0.5 nm in diameter): (a) Au@G1C12 directly after ligand exchange and (b) Au@G1C12 1.5 years after ligand exchange (now 8.3 ± 0.4 nm), (c) Au@G2C12 directly after ligand exchange and (d) Au@G2C12 1.5 years after ligand exchange (now 7.9 ± 0.5 nm), and (e) Au@Olam directly after synthesis and (f) Au@Olam 1.5 years after synthesis in solution containing excess Olam (now 10.7 ± 4.2 nm). Scale bars are 100 nm.
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Figure 3. Au NPs (originally 7.6 ± 0.5 nm in diameter) left at ambient conditions for 1.5 years with various ligands on their surfaces: (a) SAXS of Au@Olam (now 13.0 ± 5.6), Au@G1C12 (now 8.8 ± 0.8 nm), and Au@G2C12 (now 9.0 ± 1.1 nm), where the black curves are simulations of the data, and (b) absorption measurements of these NPs as well as the Au@Olam NPs directly after synthesis.
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These particles were originally 10.2 ± 0.6 nm in diameter, and after 1.5 years at ambient conditions Au@G1C12 became 10.8 ± 0.6 and Au@G2C12 10.9 ± 0.7. However, direct comparisons with Au@Olam particles of the same size were not possible, as these particles aggregated completely, which can be seen in Figure S12. This finding is further supported by small-angle transmission X-ray scattering (SAXS), shown in Figures 3 and S11, where the size distribution only slightly increases over time when a dendritic ligand is on the surface of the NP. Gaussian calculations from SAXS show a slight increase in both size and size distribution (standard deviation represented with ±), 8.8 ± 0.8 nm for Au@G1C12 and 9.0 ± 1.1 nm for Au@G2C12, both for NPs that were originally 7.6 ± 0.5 nm. SAXS data from the larger Au NPs, shown in Figure S11 show a similar result, where NPs that were originally 10.2 ± 0.6 nm became 11.1 ± 1.2 nm for Au@G1C12, and 12.5 ±1.9 nm for Au@G2C12. That the Au@G2C12 has a slightly larger size and size distribution is consistent with our findings from the decay studies, as the larger generation dendritic ligand does not stabilize the inorganic core as effectively as the G1C12 dendrimer. Comparatively, for Au@Olam, even with additional, free Olam added into the solution, the size distribution of the NPs increases for the NPs that stay in solution, to be 12.3 ± 2.4 nm and 13.0 ± 5.6 nm for two solutions that were kept separately. This finding of different sizes and distributions is consistent with results from TEM, where for Au@Olam, the shelf-life is less predictable, as it varies from case to case. Absorption measurements shown in Figure 3 provide additional support for this finding. For Au@G1C12, the absorption curve taken after 1.5 years in solution overlaps completely with the measurement taken directly after synthesis (Au@Olam). When G2C12 is on the surface, the maximum remains unchanged over time, however there is an increase in the width of the curve, indicating an increase in the size distribution of the NPs, which is consistent with the SAXS measurements. Comparatively, the presence of these two ligands on the surface of the NPs results in retention of the absorption features, as both a red shift and line broadening is observed over time for the Au@Olam case. Figure S12 highlights examples of NPs that have aggregated to varying extents during the same timeframe, which points to a lack of predictability for Au NP lifetimes in solution with Olam as the surface ligand. Conclusions We show that NP stability in organic solvents can be greatly improved when NPs are functionalized with dendritic ligands. This was confirmed using a cyanideinduced decay study, where Au NPs with dendritic ligands on their surface show significant stability im-
provements compared to Au@Olam and Au@DDT. A direct investigation into the shelf-life of the Au NPs further confirmed the improved retention of size and morphology exhibited by NPs functionalized with our dendritic ligands. These findings add to a growing library of surface modification strategies for improving colloidal stability of NPs. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. 1 H NMR spectra for all ligands, 1H NMR spectra of each ligand on Au NPs, TEM images of Au NPs used for degradation studies and their histograms, TGA data for ligands and Au NPs functionalized with various ligands, additional absorption spectra, additional TEM images and histograms of Au NPs of two different sizes for shelf-life study, and accompanying SAXS.
AUTHOR INFORMATION Corresponding Author
*Email:
[email protected] Funding Sources
No competing financial interests have been declared.
ACKNOWLEDGMENT K.C.E. acknowledges NSF Grant No. 1709827 and support from NSF Graduate Research Fellowship Program under Grant No. DGE-1321851. J.D.L. acknowledges support from the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award no. DE-SC0001004. Y. W. acknowledges CNRS-UPENN-SOLVAY through the Complex Assemblies of Soft Matter Laboratory (COMPASS), in partnership with the University of Pennsylvania’s NSF MRSEC under award no. DMR-112090. C.B.M. acknowledges the Richard Perry University Professorship at the University of Pennsylvania.
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