Phase Transfer of Triangular Silver Nanoprisms from Aqueous to

May 17, 2013 - Department of Chemistry, University of Saskatchewan, 110 Science Place, ... with 16-mercaptohexadecanoic acid (MHA), and then primary o...
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Phase Transfer of Triangular Silver Nanoprisms from Aqueous to Organic Solvent by an Amide Coupling Reaction Lijia Liu and Timothy L. Kelly* Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan S7N 5C9, Canada S Supporting Information *

ABSTRACT: In this paper, we describe a procedure for the phase transfer of silver nanoprisms (AgNPrs) from aqueous solution to chloroform via an amide coupling reaction. AgNPrs are first modified with 16-mercaptohexadecanoic acid (MHA), and then primary or secondary amines are attached to the carboxylic acid end of the MHA ligand through a carbodiimide-mediated amide coupling step. Secondary amines, such as dicyclohexylamine and diphenylamine, are found to solubilize the nanoparticles in chloroform, whereas primary amines (e.g., butylamine and hexadecylamine) do not result in phase transfer. It is found that the AgNPrs functionalized with dicyclohexylamine show the highest stability and the least aggregation after undergoing phase transfer; in contrast, with a less nucleophilic amine, such as diphenylamine, the amide coupling reaction does not go to completion and the resultant AgNPrs are less stable and more prone to aggregation.

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INTRODUCTION Triangular silver nanoprisms (AgNPrs) have attracted extensive research interest because of the unique size- and shapedependent optical properties that arise as a result of their localized surface plasmon resonance (LSPR) modes.1−3 Because of their relatively large size and highly anisotropic shape and the high radius of curvature of the tips, triangular AgNPrs have a number of advantages over other plasmonactive nanoparticles. These include LSPR bands that are tunable throughout the visible and into the near-infrared (NIR) region of the spectrum and large increases in the strength of the electric field at the particle tips as a result of near-field enhancements.1−7 As a result of these highly advantageous properties, AgNPrs have a wide variety of applications, including their use as sensors,8 in surface-enhanced Raman spectroscopy (SERS)9,10 and metal-enhanced fluorescence (MEF) experiments,11,12 as contrast agents for photoacoustic imaging,13 and in plasmon-enhanced organic electronic devices.14,15 The synthetic routes to such particles, such as photochemical4,16,17 and chemical reduction methods,18−20 are wellestablished. Recent work has helped to elucidate the reaction mechanism of both approaches,6,21−23 and AgNPrs can now be produced with very good control over both edge length and thickness.2−5,20 However, despite this high level of control over particle shape and size, the preparation of AgNPrs is still almost exclusively carried out in aqueous media. While examples of the direct synthesis of AgNPrs in polar organic solvents are known, they offer more limited control over the particle shape,24 and to the best of our knowledge, this methodology has not been extended to nonpolar media. This limits the utility of the © 2013 American Chemical Society

AgNPrs. Many applications (e.g., SERS, MEF, and plasmonenhanced optoelectronic devices) require the metal nanoprisms to be blended or incorporated into host organic materials (e.g., polymers, dyes, or fluorophores). This often necessitates that both the nanoprism and the organic host are soluble in a common solvent. Therefore, to maximize the utility of this unique type of metal nanoparticle, the development of a simple, versatile strategy to transfer AgNPrs from aqueous to organic solvents is needed. Although a number of literature reports deal with the phase transfer of small, spherical, metal nanoparticles from aqueous to organic media through surface functionalization,25−27 few of these approaches are amenable to large (diameter > 20 nm), anisotropic nanoparticles. The van der Waals interaction energy between two spherical nanoparticles scales approximately linearly with the particle radius, and when coupled with the large Hamaker constants of gold and silver, this makes the stabilization of large noble metal nanoparticles a challenging task.28 One common approach to the phase transfer of larger particles involves using long-chain alkylamines to displace the initial surfactant (e.g., citrate) from the nanoparticle surface, rendering the nanoparticles hydrophobic. Using this approach, gold nanoparticles synthesized via the Turkevich method have been transferred into chloroform, diethyl ether, toluene, or hexanes by surface modification with hexadecylamine.29,30 Similarly, octadecylamine has been shown to solubilize large silver and gold nanoparticles in chloroform.31 However, in Received: February 13, 2013 Revised: May 16, 2013 Published: May 17, 2013 7052

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Scheme 1. Synthesis of the Amide-Functionalized AgNPrs

Ag@SiO2 nanoparticles based on a triangular nanoprism core and, by functionalizing the surface with a long-chain alkylsilane, were able to solubilize the Ag@SiO2 particles in chloroform.32 While the core−shell particles are highly stable, most plasmonic effects (e.g., near-field enhancements) are highly distancedependent; with the encapsulation of the AgNPrs in a relatively thick silica shell, the magnitude of these plasmonic effects will be attenuated and the utility of the particles will be reduced. The effective phase transfer and stabilization of large AgNPrs into organic solvents therefore remains a largely unmet challenge. In this paper, we report a procedure for the phase transfer of AgNPrs into chloroform through an amide functionalization reaction. The surface of the AgNPrs was first functionalized with 16-mercaptohexadecanoic acid (MHA), which has been found to be an excellent stabilizing ligand for AgNPrs and can help prevent the etching of nanoprism tips.36 The free carboxylic acid is then coupled to various amines by means of N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide (EDC), which serves to activate the carboxylic acid to nucleophilic attack and amide bond formation. Nanoprisms functionalized with dicyclohexylamine underwent successful phase transfer into chloroform, displayed minimal aggregation, and were shown to have excellent long-term stability over a period of 1 week.

contrast to spherical nanoparticles, triangular AgNPrs have been shown to be susceptible to etching of the nanoprism tips by alkylamines, leading to the formation of disk-like particles with dramatically blue-shifted LSPR modes.31,32 This etching process prevents the direct use of alkylamines as phase-transfer agents. Other common approaches to phase transfer include the use of sodium oleate as a hydrophobic surface-passivating agent. This approach has been known for some time to solubilize large gold nanoparticles in hexanes.33 In 2009, Kulkarni et al. used a similar procedure to achieve the phase transfer of AgNPrs from sodium citrate solution into hexanes.34 The phase transfer was successfully accomplished without etching the nanoprisms; however, phase transfer to other nonpolar solvents, such as xylenes, toluene, benzene, and chloroform, was unsuccessful. Additionally, the oleate-capped AgNPrs displayed somewhat limited stability after phase transfer to hexanes. A blue shift in the LSPR peak over time indicated truncation and/or rounding of the nanoprism tips, and if the aqueous phase was removed, the nanoparticles rapidly aggregated. While large, spherical, gold nanoparticles have been successfully dispersed in organic solvents via several other routes (e.g., by incorporating dicyclohexylamine head groups onto a mercaptoacetic-acid-functionalized Au surface),35 to the best of our knowledge, there is only one other report of the phase transfer of AgNPrs. Xue et al. prepared core−shell 7053

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the same P3HT stock solution (0.5 mL) with pure chloroform (0.5 mL). Each of the solutions was spin-coated onto a pre-cleaned glass substrate at 1500 rpm for 1 min.

EXPERIMENTAL SECTION

Materials. All chemicals were purchased from the suppliers indicated and were used as received without further purification. AgNO3 (99%) and NaBH4 (98%) were purchased from EMD Chemicals. Trisodium citrate dihydrate (TSC, 99%), polyvinylpyrrolidone (PVP, Mw = 40 kDa), MHA (90%), EDC (98%), dicyclohexylamine (99%), butylamine (99.5%), hexadecylamine (99%), dioctylamine (98%), bis(2-ethylhexyl)amine (99%), and poly(methyl methacrylate) (PMMA, Mw = 350 kDa) were purchased from Sigma-Aldrich. H2O2 (30%, w/w) and chloroform (99%) were purchased from Fisher, and diphenylamine (98%) was purchased from VWR. 1-Hexadecanethiol (97%) and 1-hexanethiol (96%) were purchased from Tokyo Chemical Industry Co., Ltd. Poly(3hexylthiophene-2,5-diyl) (P3HT, Mw = 50−70 kDa) was purchased from Rieke Metals, Inc. Characterization. Transmission electron microscopy (TEM) was carried out on a Philips CM10 microscope operating at 100 kV. Ultraviolet−visible−near-infrared (UV−vis−NIR) spectra were measured using a Cary 6000 spectrometer. Dynamic light scattering (DLS) experiments were performed using a Malvern Zetasizer. Fourier transform infrared (FTIR) spectroscopy was performed using a Bruker TENSOR 27 spectrometer. The AgNPr solution was air-dried to obtain the particles as a dry powder, which was ground with KBr and pressed into a thin disk. The thicknesses of thin film samples were measured using a profilometer (KLA Tencor, AlphaStep D-120). Raman spectroscopy was performed using a Renishaw InVia Reflex Raman microscope (1200 lines/nm grating) equipped with an argonion laser (Spectra-Physics model 153-M42-010) operating at 514.5 nm. The microscope was focused onto the sample with a 20× N PLAN objective lens (NA = 0.4), and Raman spectra were obtained using static mode with a detector exposure time of 10 s and 10 times accumulation. The incident laser power was 0.03 mW on the sample. The spectrometer was calibrated using an internal Si standard at 520 cm−1. Synthesis of AgNPrs. The synthesis was performed at room temperature under ambient conditions as reported in the literature.20 In brief, aqueous solutions of AgNO3 (50 mM, 100 μL), TSC (75 mM, 1 mL), H2O2 (30%, w/w, 120 μL), and PVP (17.5 mM, 0.2 mL) were added to 48 mL of Milli-Q water (18.2 MΩ cm). While the solution was stirred vigorously, freshly prepared aqueous NaBH4 (100 mM, 500 μL) was added rapidly. After 40 min, the color of the solution changed from light yellow to dark blue. The resulting AgNPrs were isolated by centrifugation at 13 000 rpm for 1.5 h and redispersed in 5 mM aqueous TSC (50 mL). Phase Transfer of AgNPrs. The AgNPrs were first functionalized with MHA. A solution of MHA (585 μL, 5 mM in ethanol) was added to the AgNPrs/citrate solution (48 mL), and the reaction mixture was stirred for 15 min. The as-formed MHA-functionalized AgNPrs (denoted as Ag-MHA) were purified and concentrated by centrifugation at 13 000 rpm for 1.5 h, followed by redispersion in ethanol (2 mL). A solution containing 50 mM EDC and 50 mM amine in ethanol (10 mL) was freshly prepared, to which was added the Ag− MHA solution (1 mL). The solution was stirred for 20 min, after which a blue precipitate had formed. The precipitate was isolated by centrifugation at 13 000 rpm for 45 min and redispersed in chloroform by sonication. Preparation of Ag−PMMA Thin Films. Dicyclohexylaminefunctionalized AgNPrs dissolved in chloroform (0.5 mL, ∼1 mM in Ag) were mixed with a chloroform solution of PMMA (20 mg/mL) to achieve a total volume of 1 mL. A 2 × 2 cm glass slide was cleaned by sonication in detergent (2% Elma Clean 65 concentrate in water) for 20 min, followed by rinsing with Milli-Q water and ethanol. The Ag− PMMA blend was then spin-coated on the glass substrate at 2000 rpm for 1 min. Preparation of Ag−P3HT Thin Films. A P3HT solution (1.5%, w/w) was prepared by dissolving P3HT in chloroform and stirring overnight. Dicyclohexylamine-functionalized AgNPrs dissolved in chloroform (0.5 mL, ∼1 mM in Ag) were then mixed with 0.5 mL of the P3HT solution. Reference samples were prepared by diluting

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RESULTS AND DISCUSSION The synthesis of the amide-functionalized AgNPrs is depicted in Scheme 1. The as-prepared AgNPrs are initially stabilized by citrate anions tightly adsorbed to the surface. This electrostatic stabilization renders the particles highly stable in aqueous solution but completely insoluble in the majority of nonpolar organic solvents. As a first step, the AgNPrs are functionalized with a monolayer of MHA. MHA has been shown to passivate the surface of AgNPrs, resulting in a more stable nanoparticle surface with improved resistance to etching by both amines and halides.36 The MHA group serves two roles in the phasetransfer process: it not only provides the carboxylic acid functionality required for the subsequent amide coupling reaction but also protects the underlying AgNPr from etching by the amine. The MHA-capped AgNPrs are centrifuged and redispersed in ethanol to remove the excess unreacted thiol from solution. The acid-terminated nanoprisms are then added to a phase-transfer solution consisting of an equimolar solution of EDC and amine dissolved in ethanol. The terminal carboxylic acid is activated by the EDC, forming an Oacylisourea intermediate, which can be displaced by either primary or secondary amines to form the corresponding amides. After the reaction is complete, the AgNP surface is terminated by long alkyl chains bearing hydrophobic amide head groups. The nanoprisms readily precipitate from ethanol and can be redispersed in chloroform. Figure 1 shows a photograph of the AgNPrs at different stages of the synthesis. Each vial contains a biphasic mixture of

Figure 1. Photograph of AgNPrs at different stages of the phasetransfer process. The top layer is water, and the bottom layer is chloroform. (a) As-prepared AgNPrs, (b) MHA-functionalized AgNPrs, (c) MHA-functionalized AgNPrs after activation by EDC, (d) dicyclohexylamine-functionalized AgNPrs, and (e) diphenylaminefunctionalized AgNPrs.

water and chloroform to better illustrate the phase-transfer process. The as-prepared AgNPrs (Figure 1a) exhibit a characteristic blue color as a result of their LSPR absorption bands in the red−NIR region of the spectrum. The citratestabilized nanoprisms clearly display no solubility in chloroform, as indicated by the colorless organic phase. After functionalization with MHA, the nanoprisms are still confined to the aqueous phase; however, upon reaction of the terminal carboxylic acid with EDC and either dicyclohexyl- or diphenylamine, the AgNPrs are rendered hydrophobic and 7054

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readily dissolve in chloroform. Interestingly, when MHAcapped AgNPrs were reacted with EDC in the absence of amine, the O-acylisourea-functionalized nanoparticles were also found to be soluble in chloroform; however, over a period of ca. 12 h, the particles aggregated and precipitated from solution. In contrast, the amide-terminated AgNPrs remained in solution and showed no signs of precipitation, presumably because of a combination of the improved hydrophobic character (relative to the O-acylisourea) and increased steric stabilization of the bulky dicyclohexyl and diphenyl head groups. Functionalization with shorter α,ω-thiol/carboxylate linkers (e.g., mercaptoacetic acid), followed by EDC-mediated coupling to dicyclohexylamine, was also attempted, but the phase transfer was not successful; the AgNPrs remain as a precipitate in chloroform. The reaction of the MHA-capped AgNPrs with other secondary amines was also investigated. AgNPrs functionalized with either dioctylamine or bis(2-ethylhexyl)amine display moderate solubility in chloroform after the phase-transfer process (see Figure S1 of the Supporting Information) but then precipitate from solution after several hours. The use of primary alkylamines with either short (butyl) or long (hexadecyl) alkane chains results in a blue precipitate that is either insoluble (butylamine) or very sparingly soluble (hexadecylamine) in chloroform (see Figure S1 of the Supporting Information). This is in agreement with attempts to directly functionalize the nanoprisms with long-chain alkanethiols (e.g., 1-hexanethiol and 1-hexadecanethiol). The use of 1-hexanethiol results in either oxidation or particle coalescence, as indicated by the formation of a gray, insoluble precipitate and a complete loss of the plasmon band. The AgNPrs modified with 1-hexadecanethiol show better stability and some solubility in chloroform, but the phase-transfer efficiency is low. The branched structure provided by the tertiary amide structure clearly plays a role in stabilizing and solubilizing the AgNPrs. The UV−vis−NIR spectra of the AgNPrs were measured both before and after amine functionalization (Figure 2). Because of the poor solubility of the AgNPrs functionalized

with primary amines, spectra are only reported for nanoprisms derivatized with secondary amines. It can be seen that the asprepared AgNPrs exhibit three major absorption bands, located at 695, 472, and 329 nm. These three bands are characteristic of triangular nanoprisms and are attributed to the in-plane dipole, in-plane quadrupole, and out-of-plane quadrupole LSPR modes, respectively.1 After reaction with MHA, the profile of the extinction spectrum remains the same but all three LSPR bands shift slightly to a longer wavelength. This is consistent with thiol coordination to the silver surface and the resulting increase of the local dielectric environment of the nanoprisms. Upon reaction with EDC in the absence of any amine, the nanoprisms are soluble in chloroform for only a short period of time before precipitating. This aggregation is reflected in the UV−vis−NIR spectrum. In comparison to the spectra of either the as-prepared or MHA-capped AgNPrs, there is a large red shift of the in-plane dipole LSPR band and a significant increase in the band full width at half maximum (fwhm) (Table 1). Table 1. Comparison of the Position and fwhm of the inPlane Dipole LSPR Band before and after Phase Transfer sample AgNPrsa Ag−MHAb AgNPrs with AgNPrs with AgNPrs with AgNPrs with AgNPrs with a

dicyclohexylaminec diphenylaminec bis(2-ethylhexyl)aminec dioctylaminec EDC onlyc

λmax (nm)

fwhm (eV)

693 708 714 729 734 754 747

0.65 0.60 0.78 0.83 0.87 0.88 0.93

In 5 mM aqueous citrate solution. bIn ethanol. cIn chloroform.

While the shift in peak position is undoubtedly due in part to the change in solvent refractive index upon replacing ethanol (n = 1.36) with chloroform (n = 1.45),37 the substantial peak broadening (fwhm = 0.93 eV) is an indication of particle aggregation, which would also be expected to result in a red shift of the plasmon band. The similarity of the overall band shape to the original AgNPr sample suggests that there is little change in the particle size or shape after the phase-transfer process. Similar red shifts and spectral broadening are observed for the amine-functionalized AgNPrs. The AgNPrs functionalized with dicyclohexylamine and diphenylamine display the lowest levels of peak broadening (0.78 and 0.83 eV fwhm) and the smallest red shifts (6 and 21 nm) of any of the samples. Increased broadening (0.87 and 0.88 eV fwhm) is observed in the AgNPrs coupled to bis(2-ethylhexyl)amine and dioctylamine. These data are consistent with the observation of particle aggregation in solutions of the bis(2-ethylhexyl)amineand dioctylamine-functionalized nanoprisms (see Figure S1 of the Supporting Information). The cyclohexyl and phenyl substituents appear better able to prevent aggregation and to solubilize the nanoprisms in chloroform than their acyclic counterparts. Following a similar strategy, we attempted to transfer the functionalized AgNPrs into other organic solvents. Aside from chloroform, the dicyclohexylamine-functionalized AgNPrs show good solubility in dichloromethane and are sparingly soluble in chlorobenzene, but the phase transfer of AgNPrs into dichlorobenzene, hexanes, and toluene was not successful (see Figure S2 of the Supporting Information). The following

Figure 2. UV−vis−NIR spectra of AgNPrs before and after amine coupling. The spectra are normalized to the LSPR in-plane dipole peak and vertically offset for clarity. From the bottom to the top, (a) assynthesized AgNPrs, (b) MHA-capped AgNPrs, and AgNPrs functionalized with (c) dicyclohexylamine, (d) diphenylamine, (e) bis(2-ethylhexyl)amine, (f) dioctylamine, and (g) EDC only. Spectra were measured in (a) 5 mM aqueous citrate solution, (b) ethanol, and (c−g) chloroform. 7055

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of a smaller size are also present, which is commonly seen during chemical reduction synthesis.2,3 Figure 3b shows the morphology of the Ag−MHA nanoprisms after the reaction with EDC and subsequent redispersion in chloroform. The size of the nanoprisms does not substantially change after the reaction with EDC or upon changing the solvent; however, some small prisms show a certain degree of tip rounding. A broader size distribution can also be seen from the TEM image, which is consistent with the broadening of the UV−vis−NIR spectrum (Figure 2). If dicyclohexylamine or diphenylamine are included in the reaction mixture alongside the EDC, the AgNPrs retain their original shape, with little to no truncation or rounding of the nanoprism tips, as shown in panels c and d of Figure 3. To better evaluate the effect of the phase-transfer procedure on the aggregation state and colloidal stability of the nanoprisms in solution, dynamic light scattering (DLS) measurements were performed on the AgNPrs at each step of the process. Figure 4 shows the hydrodynamic size distribution, in terms of particle diameter, for dilute solutions of AgNPrs. Each plot is the average of three consecutive measurements. The AgNPrs in citrate solution have hydrodynamic diameters between 6 and 26 nm, with a median diameter of 12 nm. This is substantially smaller than the average edge length of the nanoprisms, as measured by TEM (30 nm). This inconsistency can be understood when it is considered that the particle size derived from DLS data is based on the Brownian motion of small particles, which is modeled using the Stokes−Einstein equation. An inherent assumption in the use of the Stokes−Einstein equation is that all particles are perfectly spherical, which is clearly not the case for the AgNPrs. The nanoprisms simply have a diffusion coefficient equal to what would be expected for a 12 nm diameter sphere. After functionalization with MHA, the average nanoprism diameter increases to 22 nm. This is consistent with the addition of a long-chain carboxylic acid to the particle surface. It can also be

discussion therefore focuses on the use of chloroform as a solvent. The morphologies of the AgNPrs were characterized using TEM. A micrograph of the as-prepared AgNPrs is shown in Figure S3 of the Supporting Information; the nanoprisms are clearly triangular in shape, with an average edge length of 30 ± 12 nm and thickness of 5 ± 1 nm. After functionalization with MHA, the nanoprisms retain their overall shape and approximate size distribution (Figure 3a). Some nanoprisms

Figure 3. TEM images of AgNPrs before and after amine functionalization: (a) MHA-functionalized AgNPrs, (b) MHAfunctionalized AgNPrs after activation by EDC, (c) AgNPrs functionalized with dicyclohexylamine, and (d) AgNPrs functionalized with diphenylamine.

Figure 4. DLS measurement of the size distributions of as-prepared and functionalized AgNPrs: as-prepared AgNPrs in 5 mM aqueous citrate solution (purple line), MHA-functionalized AgNPrs in ethanol (blue line), dicylohexylamine-functionalized AgNPrs in chloroform (green line), diphenylamine-functionalized AgNPrs in chloroform (dark yellow line), and EDC-activated AgNPrs in chloroform (dark red line). 7056

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exhibit good stability in chloroform. After 1 week of storage, there was no change in the position of any of the LSPR bands. The main LSPR band shows a gradual decrease in intensity over the course of 1 week, which is likely due to a slight amount of precipitation; however, the magnitude of the difference (less than a 10% change in optical density over 1 week) indicates that the functionalized AgNPrs have generally good colloidal stability. The triangular shape also remains unchanged after 4 days of storage, with only a slight degree of tip rounding observed after 7 days. In contrast, the diphenylaminefunctionalized AgNPrs display an increased propensity for precipitation, as evidenced by the more pronounced decrease in the intensity of the LSPR band. This is entirely consistent with the increased degree of aggregation observed in the DLS data (Figure 4). Overall, in comparison to dicyclohexylamine, the diphenylamine-functionalized AgNPrs are less stable in chloroform. To better understand the differences in solubility and stability between the various amine-functionalized AgNPrs, FTIR spectroscopy was used to examine the extent of amide formation in each reaction. The carbonyl stretching band is used to distinguish the carboxylic acid (i.e., MHA) from the amide. Figure 6 shows the FTIR spectra (in the range of 2000−

seen that there is a second peak in the size distribution at very small diameters (∼1 nm). For anisotropic particles with high aspect ratios, translational and rotational motions can separately contribute to the decay of the autocorrelation function; theoretical calculations and light scattering experiments on Au nanorods have demonstrated that similar bimodal size distributions are actually due to separate contributions from translational and rotational diffusion.38,39 While the appearance of a small secondary population of nanoparticles or instrumental artifacts cannot be ruled out, the second peak in the size distribution may simply be due to the rotational motion of the nanoprisms. The size of the nanoprisms increases further after the amine coupling step. Comparing the dicyclohexylamine-functionalized AgNPrs to those functionalized with diphenylamine, the average size of the latter is much larger. The EDC-activated AgNPrs also show an average particle size comparable to the diphenylamine-functionalized AgNPrs. These results are in good agreement with the UV−vis−NIR results (Figure 2), which suggest some aggregation for all samples but especially for the EDC- and diphenylaminefunctionalized particles. To examine the long-term stability of the amide-functionalized AgNPrs in chloroform, the absorption spectra of the dicyclohexylamine- and diphenylamine-functionalized AgNPrs were measured immediately after dissolution in chloroform and remeasured after the solutions were stored at 4 °C for 1, 4, and 7 days (Figure 5). TEM images of the nanoprisms stored after 4 and 7 days are shown in Figure S4 of the Supporting Information. The dicyclohexylamine-functionalized AgNPrs

Figure 6. FTIR spectra of the AgNPrs after formation of the EDCactivated intermediate and upon functionalization with different amines.

1400 cm−1) of the AgNPrs upon functionalization with different amide head groups. The full range of all FTIR spectra can be found in Figure S5 of the Supporting Information. The EDC-activated AgNPrs show a carbonyl stretching band at 1695 cm−1, which is very similar in position to the bands observed in both MHA and EDC (see Figure S5 of the Supporting Information). It is therefore impossible to distinguish the relative contributions of the carboxylic acid and the O-acylisourea intermediate to the overall band intensity. Upon the addition of diphenylamine, an additional band appears at 1654 cm−1, which corresponds to the CO stretching band of an amide. The continued presence of the band at 1695 cm−1 indicates that there are still either unreacted carboxylic acid groups or unreacted O-acylisourea intermediates present. Despite the fact that the reaction did not go to completion, the amide bonds that did form do impart a certain

Figure 5. UV−vis−NIR spectra of (a) dicyclohexylamine-functionalized AgNPrs and (b) diphenylamine-functionalized AgNPrs. Measurements were performed immediately after phase transfer to chloroform (black line) and after 1 day (red line), 4 days (blue line), and 7 days (pink line) of storage at 4 °C. 7057

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on the blue coloration of the AgNPrs and the characteristic AgNPr plasmon band is clearly evident in the UV−vis−NIR spectrum. Previous approaches to the deposition of AgNPrs have typically involved the surface functionalization of glass with 3-aminopropyltrialkoxysilane, followed by adhesion of the AgNPrs;14,40 however, these approaches are time-consuming, difficult to control, and result in essentially monolayer coverage of the substrate. Given the strong distance dependence of the near-field enhancement, having the nanoprisms more evenly distributed throughout a polymer host is expected to result in a more even distribution of the plasmonic effects as well. To demonstrate just such an application, the SERS effect of the AgNPrs was examined on thin films made by blending dicyclohexylamine-functionalized AgNPrs with poly(3-hexylthiophene). The absorption spectrum of the resultant film is shown in Figure 8a, and although the intensity is weak

degree of stability to the AgNPrs, as evidenced by their improved colloidal stability relative to the EDC-activated AgNPrs. In the case of the reaction with dicyclohexylamine, the carbonyl band disappears and only the amide band at 1652 cm−1 is seen in the spectrum. Thus, under the same experimental conditions, the reaction of dicyclohexylamine with the acid-terminated nanoprisms is more efficient, which explains the lower levels of aggregation and the improved stability of the particles in chloroform. This is likely due to the difference in the nucleophilicity of the two amines. In diphenylamine, the lone pair of nitrogen is partially delocalized onto the adjacent phenyl substituents; dicyclohexylamine can undergo no such delocalization and is therefore a much better nucleophile. As for the other two secondary amines, although the functionalized AgNPrs exhibit similar stability in chloroform, the FTIR spectra show that dioctylamine can be coupled to the carboxylic end of the MHA chain, while no noticeable amount of bis(2-ethylhexyl)amine is successfully coupled to the MHA. The IR spectrum when the latter is used shows only one peak at 1693 cm−1, which is diagnostic of either the carboxylic acid or O-acylisourea intermediate. There is no evidence of an amide band at 1650 cm−1. This may be due in part to the increased steric hindrance of the 2-ethylhexyl substituent, which leads to reduced reaction rates compared to the less hindered dioctylamine. In the case of butylamine, the coupling reaction goes to completion (as indicated by the absence of a peak at 1696 cm−1 and the presence of a new amide band at 1620 cm−1), while in the reaction with hexadecylamine, there is a small residual peak at 1697 cm−1, indicative of a small amount of unreacted starting material. This suggests that the poor solubility of the AgNPrs functionalized with primary amines is not due to any issue with the reactivity of the amine but that the branched structure created by the reaction with a secondary amine is necessary to provide the required level of solubility in chloroform. To evaluate the solution processability of the dicyclohexylamine-functionalized AgNPrs, a blend of PMMA and AgNPrs in chloroform was prepared. Figure 7 shows the UV−vis−NIR spectrum of a thin film (460 ± 70 nm) of AgNPrs/PMMA prepared by spin-coating. As seen from the inset, the film takes

Figure 8. (a) UV−vis−NIR and (b) Raman spectra of P3HT thin films with and without AgNPrs.

compared to the P3HT absorption band, the presence of the AgNPrs is confirmed by their characteristic LSPR band at 750 nm. The thickness of the P3HT thin film is 91 ± 2 nm, and the thickness of the film containing both P3HT and AgNPrs is 85 ± 5 nm. Under 514 nm laser excitation (resonant with both the P3HT absorption band and the AgNPr LSPR band), an enhancement of the Raman scattering intensity can be observed for the sample containing the AgNPrs (Figure 8b). The magnitude of the SERS enhancement observed in the 1456 cm−1 P3HT band is comparable to previous reports of AgNPr/ P3HT blends excited at the same wavelength;41 however, in this case, the film under measurement is 3 times as thick. Because the AgNPrs are no longer confined to the glass/ polymer interface, the SERS enhancements should be observed throughout the entire volume of the polymer film. We believe that this increased solution processability will be of fundamental significance in a variety of plasmonic applications.

Figure 7. UV−vis−NIR spectrum of the AgNPr/PMMA film on glass. The inset shows a photograph comparing AgNPr/PMMA-coated glass (right) to bare glass (left). Note that the top right corner of the glass substrate was covered with tape during the spin-coating process, leaving that area of the substrate bare. 7058

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CONCLUSION In summary, AgNPrs were successfully transferred from aqueous solution to chloroform upon amide functionalization. AgNPrs with dicyclohexylamide head groups exhibit the highest stability and show no significant change in shape over the course of the phase-transfer process. The diphenylaminemodified AgNPrs show lower stability, which is due to an incomplete coupling reaction. The branched structure created by the coupling of secondary amines to the carboxylic-acidterminated nanoprisms is shown to be necessary for the phasetransfer process, because similar reactions with primary amines resulted in the formation of insoluble precipitates. Our results demonstrate that this dicyclohexylamine-coupling protocol is an easy, efficient method for the phase transfer of large AgNPrs. Preliminary results show that this methodology can also be used for the phase transfer of other nanoparticle shapes (see Figure S6 of the Supporting Information) and that this solution processability enables the direct incorporation of silver nanomaterials into thin films of a variety of organic materials (e.g., PMMA and P3HT). This is expected to have important consequences in a wide variety of plasmonic applications.

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ASSOCIATED CONTENT

* Supporting Information S

TEM image of the as-prepared AgNPrs, photograph of AgNPrs in chloroform upon functionalization with other primary and secondary amines, photograph of dicyclohexylamine-functionalized AgNPrs redispersed in various organic solvents, TEM images of dicyclohexylamine- and diphenylamine-functionalized AgNPrs after 4 and 7 days of storage, full-range FTIR spectra of MHA-, EDC-, and amine-functionalized AgNPrs, and photograph of Ag nanospheres before and after phase transfer into chloroform (Figures S1−S7). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Natural Sciences and Engineering Research Council of Canada (NSERC) and the University of Saskatchewan are acknowledged for financial support. Timothy L. Kelly is a Canada Research Chair in Photovoltaics. This research was undertaken, in part, thanks to funding from the Canada Research Chairs Program. We thank Dr. Pia Winnek for her help on the FTIR instrument, Dr. Guosheng Liu for help with TEM imaging, and Waleed Saeid Mohammed for his assistance with the DLS measurements. Raman spectroscopy was conducted at the Saskatchewan Structural Sciences Centre, which is supported by the University of Saskatchewan. Jason Maley is greatly appreciated for the technical support on the Raman instrumentation.

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