Article pubs.acs.org/cm
Mechanism of Shape Evolution in Ag Nanoprisms Stabilized by ThiolTerminated Poly(ethylene glycol): An in Situ Kinetic Study Lijia Liu, Cheryl A. Burnyeat, Reegan S. Lepsenyi, Izuoma O. Nwabuko, and Timothy L. Kelly* Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5C9, Canada S Supporting Information *
ABSTRACT: Silver nanoprisms are found to undergo a shape transformation from triangular to disk-like upon functionalization with thiol-terminated poly(ethylene glycol) (PEG-SH). The shape transformation starts with a rounding of the nanoprism tips followed by a reduction in the overall particle size. The changes in nanoparticle morphology and surface chemistry during the PEGylation process are investigated by a combination of transmission electron microscopy, in situ UV− vis spectroscopy, and in situ solution-phase X-ray absorption near-edge structure at the Ag L3- and the S K-edge. We found that PEG-SH etches the Ag surface through a catalytic redox process, whereby oxidized surface atoms are first removed from the nanoprism tips (likely as the silver(I) thiolate complex) and then reduced back to elemental silver (in the form of small clusters) in the solution along with oxidation of the thiolate to the disulfide. The effect of PEG-SH concentration and molecular weight are investigated, along with the role of ambient oxygen in the etching process. The results have implications for the behavior of silver nanoprisms in various plasmonic applications (e.g., surface-enhanced Raman scattering and metal-enhanced fluorescence) and also provide insights into the degradation mechanism of silver nanomaterials in the environment. KEYWORDS: Ag nanoprisms, shape evolution, PEGylation, in situ kinetics
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INTRODUCTION Silver nanomaterials have been exploited in a wide variety of applications because of their unique size-dependent properties. As a result of their localized surface plasmon resonances (LSPRs), silver nanoparticles interact strongly with visible light; these interactions give rise to both amplified electric fields at the particle surface (near-field enhancement) and strong resonant scattering. Such properties enable the use of silver nanomaterials in a variety of plasmonic applications, including surface-enhanced Raman scattering,1,2 metal-enhanced fluorescence,3,4 plasmon-enhanced optoelectronic devices,5−7 plasmon-driven catalysis,8−10 superlenses,11,12 and biosensors.13−15 Silver nanoparticles have also proven to be excellent antimicrobial materials.16,17 They are increasingly used in a variety of commercial products, including cosmetics, textiles, water purification systems, and medical products such as bandages.18,19 Their slow oxidation to silver(I) and subsequent dissolution provides a steady supply of Ag+ ions in solution, which in turn bind to sulfur-containing proteins and enzymes, deactivating their function.20−23 Despite their widespread commercial application, the degradation mechanisms and ultimate fate of these nanomaterials in the environment are still poorly understood.24−26 The LSPR bands of a silver nanoparticle are highly sensitive to the particle shape.27−30 Compared to spherical nanoparticles, anisotropic nanoparticles show large enhancements of the electric field at their vertices, amplifying the strength of the electric-field enhancement by the lightning rod effect.27,31 As © 2013 American Chemical Society
such, synthetic routes have been developed for nanoparticles with a wide variety of anisotropic shapes, including cubes, rods, wires, plates, prisms, and octahedra.32,33 Among these, triangular silver nanoprisms (AgNPrs) are of particular interest because their LSPR bands are tunable across the entire visible spectrum and well into the near-infrared region by careful control of the particle aspect ratio.34 Additionally, particle shape has been shown to play a key role in the antimicrobial action of silver nanoparticles, and triangular prisms have been shown to be more effective at inhibiting the growth of Escherichia coli bacteria than spherical particles of equivalent size.35 Despite the advantageous properties imparted by the sharp nanoprism tips, their stability is moderate at best. Recent work has demonstrated that AgNPrs can be rapidly etched by halide ions (Cl−, Br−, and I−) down to disk-like nanoplates36−38 and that a similar restructuring process occurs at elevated temperatures.39 However, unlike halide ions, thiols are effective surface passivating agents and have been used to functionalize anisotropic Ag nanoparticles while preserving the particle shape.40−46 In particular, both Jiang et al.42 and Lee et al.43 have shown that the restructuring of AgNPrs from triangles to circular plates can effectively be arrested by the presence of alkanethiols. In addition, thiol-terminated DNA has been used to functionalize AgNPrs without affecting the particle shape, Received: June 14, 2013 Revised: August 8, 2013 Published: September 30, 2013 4206
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AgNO3 (50 mM), 1.0 mL of trisodium citrate (75 mM), 120 μL of H2O2 (30% w/w), and 200 μL of PVP (17.5 mM) were added to 48.1 mL of Milli-Q water (18.2 MΩ cm). The mixture was stirred vigorously as 500 μL of NaBH4 (100 mM) was injected. The solution immediately turned light yellow upon the addition of NaBH4. After ∼40 min, the solution turned dark blue, indicating the formation of AgNPrs. The as-prepared AgNPrs were centrifuged at 13 000 rpm (16 060g) for 1.5 h. The supernatant was removed, and the AgNPrs were redispersed in 5 mM aqueous trisodium citrate. PEGylation of AgNPrs. The PEGylation of AgNPrs was carried out in 5 mM aqueous trisodium citrate under ambient conditions. The AgNPr solution was adjusted to an optical density of 1 (∼0.15 mM Ag) prior to the reaction. The AgNPrs (2 mL) were then combined with a PEG-SH solution (1 mL) of the desired molecular weight and molar concentration. Characterization. Transmission electron microscopy (TEM, Philips CM10) was carried out using an operating voltage of 100 kV. UV−vis spectroscopy was performed using a Cary 6000 UV−vis spectrometer. For kinetic measurements, the AgNPrs and PEG-SH were combined, and spectra were acquired every 15 min for 20 h. For measurements carried out in an oxygen-free environment, both the AgNPr and PEG-SH solutions were degassed separately in sealed Schlenk flasks by sparging with N2 for 20 min. The solutions were then combined in a N2-filled cuvette using standard Schlenk techniques and sealed using a high-vacuum valve prior to measurement. XANES measurements were conducted at the Soft X-ray Microcharacterization Beamline (SXRMB) at the Canadian Light Source. The beamline is equipped with a double-crystal monochromator using either InSb (111) or Si (111) crystals.57 XANES spectra were obtained in the mode of X-ray fluorescence yield (FY) except where otherwise noted. The X-ray fluorescence was measured using a 4-element Si(Li) drift detector. For measurements on powders, the samples were spread on double-sided carbon tape and attached to the sample holder, which was placed at 45° toward the incident beam. Measurements were then done under vacuum. The solution samples were filled in polypropylene cells and sealed with X-ray transparent film (Ultralene film, 4 μm thick). The experimental layout of the endstation is shown in Figure 1.
and the resulting Ag-DNA complexes have been explored for a variety of biological applications.44,45 Given the stability and versatility of thiols in functionalizing silver surfaces,47 we attempted to use thiol-terminated poly(ethylene glycol) (PEGSH) as a ligand to modify the surface chemistry of triangular AgNPrs. During the reaction, we observed a shape transformation of the AgNPrs from triangular to disk-like. This observation was in marked contrast to previous reports of stable thiol-functionalized AgNPrs, including one previous example of AgNPrs derivatized with PEG-SH.46 Given the potential in vitro and in vivo applications of such nanoparticles,46,48,49 understanding their degradation mechanisms is crucial in preventing unwanted toxicity while at the same time maximizing their utility in biosensing, labeling, or antimicrobial applications. Better control over particle shape is also critical for the development of plasmonic applications, where the rounding of the particle vertices is expected to have deleterious effects on the near-field enhancement. In this article, we report a kinetic and mechanistic study on the PEGylation of AgNPrs using in situ UV−vis spectroscopy and in situ solution-phase X-ray absorption near-edge structure (XANES). A series of PEG-SH of varying molecular weight were reacted in various concentrations with triangular AgNPrs. Because the LSPR bands of the AgNPrs are highly sensitive to the particle shape, by monitoring the evolution of the UV−vis absorption features as a function of time the transformation of the particle shape can be followed in real time. However, although the UV and visible region of the spectrum can provide information about nanoparticle shape and size, they give no information whatsoever about spectroscopically silent components of the reaction mixture, such as Ag+, very small Ag0 clusters, and the PEG-SH itself. In previous studies,36−39 this has severely limited the ability to extract meaningful mechanistic information about the shape-transformation process. In contract, XANES is able to provide all of the necessary information on the Ag and PEG-SH local environments during the PEGylation reaction. XANES at the Ag L3edge monitors changes in the Ag absorption coefficient upon excitation of a 2p3/2 electron to unoccupied 4d and 5s states. The intensity of the absorption peak reflects the occupancy of the conduction band.50 As a result, the Ag L3-edge XANES is highly sensitive to the oxidation state of Ag.51,52 The S K-edge monitors the electronic transition of a S 1s electron to unoccupied 3p states, which has been used extensively to study the oxidation state and local environment of various Scontaining organic compounds as well as inorganic minerals.53−55 In our study, XANES at the Ag L3- and the S K-edge are measured before, during, and after the PEGylation reaction. The chemical speciation of Ag and S before and after PEGylation is elucidated, and a reaction mechanism for the shape-transformation process is proposed.
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Figure 1. Layout of the synchrotron beamline endstation. The inset shows a solution of AgNPrs in the sample cell used for the measurements.
EXPERIMENTAL SECTION
Materials. All chemicals were purchased from commercial suppliers and used without further purification. AgNO3 (99%) and NaBH4 (98%) were purchased from EMD Chemicals. Trisodium citrate dihydrate (99%), polyvinylpyrrolidone (PVP, 40 kDa), poly(ethylene glycol) methyl ether thiol (PEG-SH, 1 kDa), Ag2S (99.9%), sodium dodecyl sulfate (99%), sodium 1-dodecanesulfonate (99%), and trans-4,5-dihydroxyl-1,2-dithiane (99%) were purchased from Sigma-Aldrich. H2O2 (30% w/w) was purchased from Fisher. PEG-SH (2, 5, and 10 kDa) were purchased from Laysan Bio Inc. Synthesis of Silver Nanoprisms. AgNPrs were synthesized following procedures reported in the literature.56 In brief, 100 μL of
The sample cell is placed on the sample holder that faces the incident beam at 45° in an aluminum box. During the measurement, the box is purged with helium to provide an air-free environment. To obtain spectra with a good signal-to-noise ratio, the AgNPrs solutions were concentrated by centrifugation to achieve a molar concentration on the order of 5 mM in Ag. The concentrated AgNPrs solution and a PEG-SH solution of 50 mM were mixed in a 1:1 volume ratio. For ex situ measurements, spectra were taken 20 h after the beginning of the reaction. For in situ measurements, AgNPrs and 1 4207
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kDa PEG-SH were mixed and sealed in the sample cell, and the reaction was conducted under magnetic stirring throughout the measurement.
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RESULTS AND DISCUSSION The morphology and UV−vis spectrum of the as-prepared citrate-protected AgNPrs are shown in Figure 2. The synthetic
Figure 2. (a) TEM image, (b) size distribution, and (c) UV−vis spectrum of the as-prepared AgNPrs.
procedure produces AgNPrs that are clearly triangular in shape, with an average edge length of 35 nm. The nanoprism solution shows a characteristic UV−vis absorption spectrum (Figure 2c): the main LSPR in-plane dipole mode is visible as an intense band centered at 690 nm, whereas the in-plane quadrupole mode appears as a shoulder at 471 nm, and a sharp peak at 331 nm is due to the out-of-plane quadrupole mode.28 These LSPR bands are highly sensitive to the shape and size of the nanoparticles as well as the dielectric constant of the environment.28,58 We first discuss the effect of PEG-SH concentration on the underlying AgNPrs morphology. PEGylation of the AgNPrs was carried out using 1 kDa molecular weight PEG-SH of 0.1 mM and 1 mM total concentration (on the basis of the mass of the repeat unit), and the evolution of the AgNPr UV−vis spectra are shown as contour plots in Figure 3. It can be seen that the main in-plane dipole and out-of-plane quadrupole LSPR bands exhibit pronounced blue shifts as a function of reaction time, indicating substantial changes in the AgNPrs size and shape over the course of the reaction. Immediately after the reaction with 0.1 mM PEG-SH, the inplane dipole band of the AgNPrs undergoes a 27 nm red shift, as can be seen from an expanded view (the first 150 min) of the 2D plot (Figure 3c). The band then gradually undergoes a pronounced blue shift along with a concomitant decrease in the peak intensity. After ∼120 min, the peak intensity decreases more slowly, whereas the peak continues undergoing a slow, steady blue shift throughout the remainder of the reaction (Figure 3a). When the PEG-SH concentration is increased to 1 mM, the in-plane dipole LSPR band of the AgNPrs still undergoes an initial red shift, but this is followed by a much more pronounced blue shift (Figure 3d). The maximum peak intensity also drastically decreases during the first 40 min of the reaction. After 3 h, the blue shift stops, and the center of the LSPR band remains constant at 545 nm (Figure 3b); however, the peak intensity keeps decreasing throughout the course of the entire reaction. The in-plane dipole LSPR band of the PEG-SH functionalized AgNPrs exhibits the same trend of an initial red shift followed by a blue shift regardless of the PEG-SH
Figure 3. UV−vis contour plots of the reaction mixture of AgNPrs and 1 kDa PEG-SH: (a) 0.1 mM PEG-SH, 0−1200 min reaction time; (b) 1 mM PEG-SH, 0−1200 min reaction time; (c) 0.1 mM PEG-SH, 0− 150 min reaction time; and, (d) 1 mM PEG-SH, 0−150 min reaction time. Magnified view (300−360 nm region) of the UV−vis contour plots for the PEGylation process using (e) 0.1 mM PEG-SH and (f) 1 mM PEG-SH. The color-coding of the optical density is the same for graphs in the same row.
concentration; however, the magnitude and rate of the blue shift appears to be dependent on the PEG-SH concentration. The red shift can be easily explained because of the increase in the local dielectric environment upon thiol coordination to the Ag surface. Blue shifts of the plasmon band are generally due to a decrease in the nanoparticle aspect ratio, which could be due to either a truncation of the nanoprism tips or a decrease in particle edge length. The out-of-plane quadrupole LSPR band (at 331 nm), however, exhibits a red shift upon PEGylation, which is an indication of an increase of the AgNPr thickness. Because the length of the AgNPrs is shortened, some of the Ag atoms that are detached from the nanoprism tips are redeposited on the flat surface (i.e., the {111} planes) of the AgNPrs. The red shift of the out-of-plane quadrupole LSPR band happens simultaneously with the blue shift of the main in-plane dipole LSPR band; as can be seen in Figure 3e,f, the peak shift persists throughout the reaction when using 0.1 mM PEG-SH, whereas the similar shift only takes place in the first 3 h in the presence of 1 mM PEG-SH. The morphology of the AgNPrs during the course of the PEGylation reaction was therefore examined (Figure 4). Because the most noticeable changes in the UV−vis spectra occur within the first 2 h, TEM images were also taken of the particles after 2 h of reaction time as well as of the final product. It can be clearly seen that in the presence of 0.1 mM PEGSH, the AgNPrs undergo an evolution of morphology from triangular to disk-like plates. The average size of the nanoparticles decreases slightly over time. In the first 2 h, the tips of the nanoparticles are no longer sharp compared to the as-prepared AgNPrs. The size distribution (Figure S1) shows 4208
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Figure 4. TEM images of AgNPrs after reaction with 1 kDa PEG-SH: (a) 0.1 mM PEG-SH, 2 h; (b) 0.1 mM PEG-SH, 20 h; (c) 1 mM PEGSH, 2 h; and (d) 1 mM PEG-SH, 20 h. The scale bars in all graphs are 50 nm.
that most particles are within the size range of 15−27 nm after 2 h but that the distribution is very broad. It should be noted that because the AgNPrs are no longer perfect triangles, the particle size is calculated as the diameter of a circular disk with equivalent surface area. After 20 h, almost all particles have been transformed to a disk-like shape with diameters in the range of 15−20 nm. As depicted in Figure S4, if a triangular nanoprism with an edge length of 35 nm is etched down to a disk-like plate, the resulting nanoparticle should have a diameter of 20 nm, in good agreement with the observed size distributions. There is also an increased fraction of small particles with sizes of ∼5 nm in the solution, and the number of large particles decreases significantly. The peak width of the AgNPr plasmon band after 20 h of reaction time is narrower than it was after only 2 h (Figure S1c), which is consistent with a narrowing of the particle-size distribution. After reaction with 1 mM PEG-SH for 2 h, the AgNPrs exhibit a fairly broad size distribution. The larger particles with diameters over 25 nm remain roughly triangular in shape with slightly rounded tips, whereas the smaller particles are more disk-like. After 20 h of reaction time, only the disk-like particles remain in the solution along with particles of a diameter smaller than 2 nm. The decrease in the overall LSPR intensity over the course of the PEGylation reaction (Figure S2c) is therefore at least in part because of the decrease in extinction cross section, which is proportional to the particle size.28 To examine further the role of PEG-SH in sculpturing the AgNPr morphology, UV−vis spectroscopy was performed by using PEG-SH of different molecular weights and concentrations. Figure 5 lists the in situ UV−vis results of AgNPr PEGylation using 1, 2, 5, and 10 kDa PEG-SH arranged as a function of both PEG-SH concentration and molecular weight. A complete data set of UV−vis spectra, including higher concentrations of PEG-SH, are shown in Figures S5−S7. Because solutions of constant polymer concentration but variable molecular weight would have varying concentrations
Figure 5. UV−vis spectra of AgNPrs during PEGylation with PEGSHs of different concentration (in S content) and molecular weight. Each contour plot represents a series of color-coded UV−vis spectra in the range of 300−1000 nm (x axis) over the course of 0−1200 min reaction time (y axis).
of −SH end groups, a series of reactions were carried out at both constant polymer concentration and at constant −SH concentration. For example, the 0.1 mM, 1 kDa PEG-SH is 4.4 μM in −SH, whereas the 0.1 mM, 2 kDa PEG-SH contains 2.2 μM of −SH. In other words, plots along the same diagonal line were carried out at the same polymer concentration, whereas plots along the same horizontal line indicate a constant −SH concentration. For a fixed molecular weight, increasing the PEG-SH concentration generally results in a faster tip-rounding and nanoplate-thickening process, as evidenced by a more rapid blue shift of the in-plane dipole band and red shift of the out-ofplane quadrupole band. For a fixed polymer concentration (along the diagonal direction), PEG-SH of higher molecular weights have a weaker sculpturing effect on the particles and appear to better preserve the original particle shape. The LSPR bands remain nearly constant when the AgNPrs are reacted with 0.1 mM (0.44 μM by −SH) PEG-SH of 10 kDa. TEM shows that after PEGylation the triangular shape of the AgNPrs is preserved (Figure S3). For a constant −SH concentration (horizontal direction), higher molecular weight polymers (i.e., 10 kDa) better protect the shape of the AgNPrs. However, this is not always the case. In particular, PEGylation using 2 kDa PEG-SH instead of 1 kDa PEG-SH leads to a nearly complete quench of the nanoprisms LSPR bands. As a result, we need to consider the function of both the PEG unit and the −SH unit in the PEGylation process of the nanoprisms. 4209
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slight increase of white-line intensity; this suggests that the AgNPrs remain metallic, with a slight oxidation of the surface. When the same reaction is carried out with the 1 kDa PEG-SH, the increase in the white-line intensity is more prominent, and the fine structure of the postedge features is lost. The increase in the white-line intensity suggests that a small amount of Ag is oxidized during the PEGylation process, but the absence of a sharp edge peak in the spectrum (as is seen for AgNO3 or Ag2S) indicates that the degree of oxidation is low. The oxidized Ag could be in the form of silver thiolates,60 which form as the result of the oxidative addition of a thiol to the silver surface. The substantial broadening of the postedge peaks could be due to a decrease in the crystallinity of the AgNPrs. The postedge oscillations originate from multiple scattering by neighboring atoms.61 When the local environment of Ag becomes more disordered (amorphous), such fine spectral features get broadened. This may be an indication of the formation of ultrasmall Ag clusters in which the Ag atoms lack long-range order. This is consistent with the observation of very small nanoparticles in the TEM images (Figure 4d). There is a decrease in the number of nanoparticles with sizes above 15 nm after PEGylation and an increase in the number of nanoparticles with sizes below 2 nm as well as a decrease in the overall intensity of the plasmon band (Figure S2). From these data, it is clear that material is being lost from the tips of the nanoprisms but still remains as Ag0 in the form of small clusters in solution. The very small diameter of these clusters (
