Effect of Postsynthesis Purifications on Gold and Silver Nanoparticle

Article pubs.acs.org/JPCC

Effect of Postsynthesis Purifications on Gold and Silver Nanoparticle Ligand Coverage Siyam M. Ansar,† Fiaz S. Mohammed,† Gregory von White, II,† Maeve Budi,‡ Kristin Conrad Powell,† O. Thompson Mefford,‡ and Christopher L. Kitchens*,†,§ †

Department of Chemical and Biomolecular Engineering and ‡Department of Materials Science and Engineering, Clemson University, Clemson, South Carolina 29634, United States § Institute of Environmental Toxicology (CU-ENTOX), Clemson University, Pendleton, South Carolina 29670, United States S Supporting Information *

ABSTRACT: Wet chemical nanoparticle synthesis is commonly employed because of the ability to tailor and control nanoparticle size, shape, polydispersity, and surface chemistry; however, excess ligands or free surfactants in the colloidal dispersion can be detrimental for many nanoparticle applications. Postsynthesis purification using antisolvent precipitation is a widely employed method to remove the excess ligands or precursors; however, there has been little in-depth fundamental investigation of the dynamics between the nanoparticle, ligand, and dispersing media as well as the morphology and fate of the nanoparticle and ligands during nanoparticle processing. In this paper, we investigate the changes in ligand surface coverage for dodecanethiol-stabilized gold and silver nanoparticles in response to repetitive antisolvent precipitation and redispersion using gas chromatography, thermogravimetric analysis, and small-angle neutron scattering. These techniques were each used to determine percent surface coverage and equilibrium ligand partitioning between the nanoparticle surface and bulk solution, which was then modeled with the Langmuir isotherm to determine the binding free energy. The binding free energy for dodecanethiol on 4.2 nm diameter gold nanoparticles was found to be −23 kJ/mol by gas chromatography (GC) and −34 kJ/mol by small-angle neutron scattering (SANS). The binding free energy for dodecanethiol on 7.7 nm diameter silver nanoparticles was found to be −21 kJ/mol by TGA and −29 kJ/mol by SANS. While these numbers demonstrate variability based on the method, they are comparable to literature values. Other notable results from this work demonstrate the optimization of the purification process and avoidance of using excess antisolvent which can lead to coprecipitation of the excess ligand with the nanoparticles, hindering the purification. Finally, multiple techniques for determining ligand binding free energy are demonstrated as well as evaluation of the pros and cons of each method.

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INTRODUCTION Engineered nanomaterials have great potential for use in a wide array of fields including biomedical applications,1,2 catalysis,3,4 and sensory devices,5,6 which rely on the unique physical properties possessed at the nanoscale.7,8 More specifically, the advantageous size- and shape-dependent properties of gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) include surface plasmon resonances,9 catalytic activity,3 and photothermal activity.10 Aside from the inherent nanomaterial properties, the next most important characteristic is the surface chemistry. It is the stabilizing ligands surrounding the nanoparticle that are responsible for the interaction with the immediate surrounding environment and provide desired functionality to the nanomaterials.11 Throughout literature, a vast number of methods have been developed to tailor the surface chemistry of nanoparticles for different applications; however, solution-based methods are most often employed. Control of the nanoparticle size, shape, and aspect ratio is commonly done through variation in temperature, concentration, and the addition of other structure directing agents. In © 2016 American Chemical Society

these cases, gross excess surfactant or ligand is often employed in order to obtain increased yield; however, this practice can be detrimental to the end application without effective postsynthesis processing.12 A widely utilized technique for tailoring NP surface chemistry is a postsynthesis ligand exchange process, where a labile ligand molecule is replaced with a ligand containing a higher binding affinity to the metal surface.13 In the case of gold, thiol-containing molecules readily displace sacrificial ligands employed for synthesis (e.g., citrate) from the particle surface, altering the surface chemistry and physical properties.14,15 Despite the synthetic route or exchange process used, most applications require monodispersed nanoparticle populations that are free of excess surfactants or ligands used during synthesis. This is commonly achieved by postsynthesis processing/purificationa critical step in the removal of any Received: December 18, 2015 Revised: March 4, 2016 Published: March 7, 2016 6842

DOI: 10.1021/acs.jpcc.5b12423 J. Phys. Chem. C 2016, 120, 6842−6850

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The Journal of Physical Chemistry C

a highly accurate and sensitive analytical tool for quantitative analysis of volatile ligands as long as it is ensured that the bound and unbound ligands are discerned. SANS measurements indirectly provide the ligand surface coverage on the nanoparticle surfaces.18,19 However, this method is not universally available, is expensive, and requires relatively large amounts of sample. Each of these techniques was determined to be a viable method to quantify the ligand surface coverage on AuNPs and AgNPs and was further used to calculate the adsorption Gibbs free energy (ΔG°).

reaction byproducts or free ligands in solution that may be detrimental to the end application. In a typical postsynthesis procedure, an antisolvent destabilization is coupled with centrifugal precipitation to purify and/or size fractionate the nanoparticles.16 The common solvent/antisolvent pairs used for nanoparticle size-selective precipitation and isolation include toluene/ethanol or hexane/ethanol for hydrophobic alkanethiol-modified AuNPs.17 Carbon dioxide has also been demonstrated as an effective antisolvent.18−20 Following precipitation and isolation by removal of the supernatant, the nanoparticles can be redispersed in neat solvent, free of excess surfactants and ligands that remain in the supernatanta process termed nanoparticle washing. Nanoparticles are usually “washed” multiple times with repeated centrifugation and redispersion in between each antisolvent addition, resulting in size-monodisperse nanoparticle dispersions that are “free” from excess ligands and surfactants. Despite the broad use of this nanoparticle processing, knowledge of the nanoparticle−ligand interactions and the effect of these purification procedures on the stabilizing ligand shell is commonly misconstrued. Following antisolvent precipitation and redispersion in neat solvent, the dispersion may very well be free of the ligand or surfactant; however, this is not in equilibrium, and the chemical potential driving force promotes ligand desorption from the nanoparticle surface into solution in order to establish equilibrium, which can occur quite rapidly. Several analytical methods have been used to quantify the ligand adsorbed onto the nanoparticle surfaces, such as nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FT-IR), surface-enhanced Raman spectroscopy (SERS), fluorescence techniques, and others.21−23 However, these techniques have major limitations in quantitative analysis of ligands on nanoparticle surfaces. Liu et al. studied the quantitative attachment of thiolated DNA onto AuNPs using fluorescence, which is a sensitive method to study the ligand molecules in solution and on the nanoparticle surfaces.23 The lacking general applicability is the major drawback of this method because the detecting ligand has to be a fluorophore or attached with a fluorophore. Recently, Zhang et al. introduced an isotope-encoded SERS internal reference method to quantify the organothiol adsorption onto AuNPs.15,21 Though, this method is accurate and simple to use, the method lacks general applicability as the ligand molecule has to be Raman active and needs an isotope substituted molecule of the same detecting ligand as an internal standard. NMR and FT-IR ligand quantification rely on the molecular peak intensity, which can be significantly affected by spectral crowding and interference from water can be problematic in reliable peak intensity measurement. Also, the NMR peak intensity can be effected by the size of the nanoparticles as well. For example, NMR peaks for ligands adsorbed onto large nanoparticles are significantly broadened, reducing accurate ligand quantification.24 Furthermore, each of these methods is challenged by the ability to discern between the bound ligand and ligand free in solution. This work explores the influence of multiple washing steps on the ligand surface coverage of AuNPs and AgNPs through thermogravametric analysis (TGA), gas chromatography (GC), and small-angle neutron scattering (SANS). TGA is generally a widely used analytical technique for quantitative analysis of ligand grafting density.25 However, TGA requires relatively large sample volumes and amount of solvent removal in order to provide sufficient quantity of dried sample for analysis. GC is

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EXPERIMENTAL SECTION Chemicals and Equipment. All chemicals were purchased from Sigma-Aldrich except toluene-d8 (99.5%) and AgNO3 which were purchased from Cambridge Isotope Laboratories and Amresco LLC, respectively. All chemicals were used without further purification. Water was deionized and filtered by a Milli-Q water system. UV−vis spectra were taken using a Varian Cary 50 UV−vis−NIR spectrophotometer. Centrifugal removal of supernatant was performed using a Sorvall Legend XTR centrifuge (Thermo Scientific). Citrate-Capped AgNP and AuNP Synthesis. For AuNP synthesis, 250 μL of 0.05 M citrate and 0.05 M HAuCl4 aqueous solutions was added to 50 mL of H2O while gently stirring. 500 μL of chilled 0.05 M NaBH4 in H2O was then added, and the resulting solution was stirred at 400 rpm for 15 min as the color of the solution changed from colorless to red. For AgNPs, the preceding procedure was repeated using 0.05 M AgNO3 as the metal salt precursor, and the formation of AgNPs afforded a yellow solution. Dodecanethiol-Capped AuNPs and AgNPs Synthesis via Ligand Exchange Method. To achieve ligand exchange, 50 mL of toluene was added to 50 mL of as-synthesized citratecapped AuNPs or AgNPs in a glass jar. To that, ∼600 μL of neat dodecanethiol (DDT) was added to the toluene phase, and the mixture was shaken vigorously and stirred until all nanoparticles were transferred to the organic phase. UV−vis spectroscopy, 1H NMR, and FT-IR all confirm the exchanging of citrate for DDT on the AuNPs (Supporting Information, Figure S1), transferring from the aqueous phase to the organic phase due to the change in surface chemistry. Time-resolved UV−vis spectroscopy was used to monitor the exchange and phase transfer. The wavelength at maximum absorbance of NPs in the aqueous layer red-shifted and increased in intensity after 10 min before slowly decreasing in intensity at all wavelengths as the particles phase-transferred to the toluene phase (Figure S1). The UV−vis signal disappeared after 25 min, confirming the ligand exchange reaction and complete phase transfer to the organic phase. 1 H NMR provided further proof of the purity of DDT ligand shell and ensured complete ligand exchange (Figure S1). The loss in resolution of the chemical shift peaks attributed to protons situated closest to the point of thiol attachment along with a broadening of the methylated protons confirms thiol attachment to the AuNP.24 Methylene groups close to the Au surface are confined, which affects their spin relaxations and results in the disappearance of the chemical shift peaks of protons in the 1H NMR spectra.26 We can also observe a weak chemical shift attributed to the free DDT ligand in solution. The 1H NMR data correlate very well to the proton NMR results of other AuNP systems involving thiolated ligands.24,27 However, this method is not conducive to dynamic timeresolved studies, and broadnening is not a conclusive method 6843


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AuNPs was examined with TEM. Nanoparticles were deposited on Cu grids covered with a Formvar carbon film and imaged with an accelerating voltage of 120 and 300 kV. TEM images were obtained using a Hitachi 7600 and Hitachi 9500 for atomic resolution. Gas Chromatography (GC) Analysis. An aliquot of the supernatant (10 μL) was taken from each sample after each wash and injected into an Agilent 7695A GC using the Agilent 7683B automatic liquid sampler. The inlet temperature was set at 280 °C, the oven at 50 °C, heating rate of 10 °C/min to 250 °C. The FID was set at 300 °C. A calibration curve for DDT was created correlating the GC peak area with known ligand concentrations to determine concentrations of ligands at various stages of washing before and after ligand exchanges (Figure S2). The moles of ligand present on the nanoparticle surface was determined using the calculated concentration, nanoparticle concentration, and amount of solvent added after each wash. Small-Angle Neutron Scattering (SANS). SANS experiments were performed on the CG-2 General SANS instrument at the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratories (ORNL, Oak Ridge, TN). All samples were prepared to be 1% by volume and considered dilute. Each sample was loaded into a 2 mm path length banjo cell and measured at 25, 37, and 50 °C. Three sample-to-detector distances were used (0.3, 6, and 14.5 m) to obtain a q-range from 0.001 to 0.67 Å−1 with a neutron wavelength of λ = 6 Å for the 0.3 and 6 m distances and λ = 18 Å for the 14.5 m distance. The neutron resolutions, Δλ/λ, were equal to 12% (fwhm). Empty beam background, empty cell background, solvent background, detector sensitivity, sample transmission, and sample thickness were considered during raw data reduction. The solvent and empty cell background measurements were used to normalize all SANS data. The reduced scattering intensities, I(q), were fit to a core−shell model as a function of the scattering vector, q(θ). Here, q(θ) = 4π sin(θ)/ λ, and θ is defined as the scattering angle. All SANS fitting was performed using Igor Pro 6.03 software and models provided by NIST.29

for determing surface structure. Other techniques are more suitable for determining structure on AuNPs, but they each possess their own dificulties.28 FT-IR spectoscopy further confirms the presence of DDT on the AuNP surface, however unquantitatively (Figure S1). Purification of Ligand-Exchanged AgNPs and AuNPs. The DDT-capped NP dispersions obtained via ligand exchanges were concentrated to a 2.5 mL sample using a rotary evaporator and then transferred to a centrifuge tube for purification. Ethanol was added to each tube, making the volume equivalent to the starting volume of the NP synthesis, and then centrifuged at 14 000 rpm for 10 min to precipitate the particles. The supernatants were decanted and analyzed via GC while the residual particles are redispersed in minimal toluene. This process is considered one complete wash which was repeated up to three times. Direct Synthesis of Dodecanethiol-Capped AgNPs. These nanoparticles were synthesized per the “two-phase arrested precipitation method” developed by Brust et al.16 A typical synthesis of AgNPs requires a solution of 0.19 g of AgNO3 in 36 mL of H2O mixed with 24.5 mL of chloroform solution consisting of 2.7 g of tetraoctylammonium bromide (TOAB), the phase-transfer catalyst. This mixture was stirred for 60 min, upon which the aqueous phase was clear due to transfer into the organic phase. This aqueous phase was then pipetted out, and 240 μL of neat DDT was added to remaining chloroform mixture. This was allowed to stir for 5−10 min while a solution of 0.5 g of NaBH4 in 30 mL of H2O was freshly prepared and then added as a reducing agent. The dispersion was further stirred for 4−12 h. The dark brown organic dispersion contained DDT-stabilized AgNPs. Purification of the AgNPs Synthesized by the Direct Synthesis Method. Antisolvent (ethanol) was added to 0.4 mL of particles initially synthesized above in a centrifuge tube. The centrifuge tube was shaken vigorously for 1−2 s and then allowed to sit for 15 min so that the mixture could come to equilibrium. The volume ratio of solvent to ethanol was varied between 1:100, 1:50, 1:25, and 1:10 (i.e., either 40, 20, 10, or 4 mL of ethanol was added to the centrifuge tube as antisolvent, respectively). The dispersion was then centrifuged at 14 500 rpm for 10 min precipitating the particles and allowing the supernatant (containing excess ligands and by products) to be decanted. The tubes were inverted and allowed to air-dry. This represents a single washing step that was repeated up to four times. Each additional wash required 0.4 mL of toluene added to the centrifuge tube followed by 2−3 min of water bath sonication to redisperse the nanoparticles. Once redispersed, the antisolvent is introduced and the centrifugation precipitation is repeated. At the end of the washing procedure 0.15 mL of n-hexane was added to the centrifuge tube after air drying followed by sonication for 2−3 min to redisperse the nanoparticles before further characterization. The hexane allows for quicker TGA sampling as it evaporates more quickly leaving behind little to no residual solvent. Thermogravimetric (TGA) Analysis. TGA measurements were performed on a TA Instruments SDT Q600. The precipitated particles were redispersed in 10−20 μL of hexane and deposited into a clean alumina TGA pan. The temperature was ramped to 100 °C at 2 °C/min and held for 10 min to remove all solvent. The temperature was then ramped to 350 °C at 5 °C/min under a N2 purge of 20 mL/min. Transmission Electron Microscopy (TEM) Analysis. The morphological structure of DDT capped AgNPs and

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RESULTS AND DISCUSSION For this work, the primary challenge is to quantifiably discern the difference between ligand that is bound to the nanoparticle surface and ligand that is free in solution. The equilibrium partitioning of ligands is driven by chemical potential and the binding energy of the ligand to the nanoparticle surface. By effectively measuring the equilibrium partitioning, the strength of binding can be calculated. Furthermore, extensive washing of NPs can be detrimental to colloidal stability due to continual removal from the NP surface; however, the extent of this is also determined by the strength of binding. The DDT functionalized AuNPs and AgNPs in this work are colloidally stable and do not precipitate under centrifugal forces alone during the NP purification process. To facilitate particle precipitation, the addition of ethanol antisolvent reduces the dispersion stability of the DDT ligands by altering the polarity of the solvent environment. Concentration of the NPs by rotary evaporation and ethanol addition allows for effective precipitation during the purification steps and significantly reduces the amount of antisolvent usage.27 Characterization of Nanoparticle Washing by Gas Chromatography. Gas chromatography (GC) was effectively used to determine the DDT concentration in the postprecipi6844


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Figure 1. (A) Measured concentration of DDT remaining in supernatant after AuNP precipitation using GC for each wash cycle. (B) UV−vis absorption spectra of dodecanethiol-capped AuNPs before wash and after second and third washes. (C, D) The TEM images of dodecanethiolcapped AuNPs taken before and after third wash. The solvent:antisolvent ratio of washing solvent is ∼1:20. The nominal concentration of dodecanethiol before washing is 47.7 mM, and AuNPs size is ∼4.2 ± 0.9 nm. The insets show the histogram of distribution of sizes of particles. The scale bar of each TEM image is 50 nm.

adsorbed on NP surface.11 Coprecipitation is more pronounced with higher NP and ligand concentrations. This is also observed with AgNPs synthesized via the direct method and is discussed below. However, the DDT surface coverage after the third wash is smaller than the reported DDT monolayer packing on AuNPs (10.2 molecules/nm2).30 Comparing the reported DDT monolayer packing density on AuNPs with the DDT packing density after the third wash indicates that the DDT on AgNPs is submonolayer only after the third wash. Thus, the amount of DDT bound to the AuNPs and free DDT in supernatant only after the third wash were modeled with the Langmuir adsorption isotherm model (eq 1) to determine the DDT binding constant (K) to AuNPs. θ is the fraction of AuNP surface coverage that is calculated with the amount of adsorbed DDT determined using GC data and reported DDT monolayer packing on AuNPs (10.2 molecules/nm2).30

tation supernatant with consecutive wash cycles. This is an indirect method of determining the DDT concentration free in solution after ligand exchange onto the NP surfaces and with each wash. For this study, the DDT ligand exchanged AuNPs were employed (Figure S1). With each wash, the excess DDT is removed, which results in the desorption of bound ligands from the NP surface. The equilibrium concentration of ligands in solution is a balance between ligands that are particle-bound and free in solution, even after multiple washing cycles (Figure 1A). A calibration curve correlating GC peak area and their DDT concentration was established for GC analysis (Figure S2). The free DDT in the supernatant after the first, second, and third washes were calculated from GC analysis to be 45.6, 1.9, and 0.2 mM, respectively. The significant decrease in ligand concentration in the supernatant between the first and second washes is attributed to a large excess of ligand used during synthesis and phase transfer. In the first wash, the difference in original DDT concentration and that in the supernatant is attributed to what is bound to the NP surface. By analyzing the amount of thiol in the supernatants by GC, the total number of moles of thiol that are separated with the nanoparticles during precipitation was calculated. TEM indicated a AuNP average core diameter of 4 nm (Figure 1C,D). The NPs were modeled as spheres and assuming a size distribution with low polydispersity. Using the total number of moles of DDT transferred to the particles, surface coverage per square nanometer was calculated (see Supporting Information). The calculated DDT surface coverage on AuNPs after the first, second, and third washes is 333, 38.8, and 6.84 molecules/nm2, respectively. DDT surface coverage after the first and second washes are significantly higher than the reported value for DDT monolayer packing on AuNPs (10.2 molecules/nm2).30 Compared to the reported value, our data indicate that DDT is in gross excess, resulting in an unreasonable estimated surface coverage. This is likely the result of coprecipitation of the free ligand with the NPs during the first wash, which is driven by the hydrophobic interaction between the free ligand and ligand

θ=

KC 1 + KC

(1)

Using the binding affinity (1.0 × 104 M−1) calculated form the Langmuir adsorption isotherm model (see Supporting Information) and Gibbs free energy equation, the free energy of DDT adsorption to AuNPs in toluene/EtOH mixture is −23 kJ/mol. This value is significantly smaller than the literature reported for organothiols binding to AuNPs in water (−38 kJ/ mol); however, the fact that a single data point was used for determination leads to a low degree of accuracy.15,21 Regardless, this demonstrates experimental feasibility and the ability to obtain a reasonable binding energy. The ∼95% reduction of DDT concentration after first wash does not affect the UV−vis absorption spectra of the AuNPs (Figure 1). The wavelength of maximum UV−vis absorption of AuNPs, ∼515 nm, remains constant over the three washes, indicating that NPs are not clustering or increasing in size during the washing (Figure 1B). Transmission electron microscopy (TEM) provides further evidence of NP size and shape consistency with washing. The average TEM particle size 6845


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Figure 2. TEM image of dodecanethiol-capped silver nanoparticles (A) before washed with ethanol, after third wash at a solvent:ethanol volume ratio of (B) 1:10, (C) 1:50, and (D) 1:100. The insets show the histogram of distribution of sizes of particles obtained using ImageJ. The scale bar of each image is 20 nm.

of AuNPs before and after third wash is 4.2 ± 0.9 and 4.0 ± 1.0 nm, respectively, and the particle size and shape of AuNPs (Figure 1D) were unchanged, even after the third wash. Although, there is a noticeable decrease in the ease of NP precipitation after wash number 2, requiring increased centrifugation times required for complete precipitation. We hypothesize that the higher DDT ligand concentration in solution leads to coprecipitation of the nanoparticles with excess ligand during the first wash. A similar effect was observed by Zhang et al. during the adsorption of a heterocyclic mercaptobenzimadazole onto 13 nm AuNP surfaces which also reported an increase in centrifugation time by 25 min once the ligand concentration was less than 1.25 μM.15,21 Nanoparticle concentration can also influence precipitation where more dilute dispersions require additional antisolvent and/or increased centrifuge time. Excessive antisolvent can promote ligand coprecipitation. Characterization of Nanoparticle Washing Using Thermogravimetric Analysis (TGA). For this part of the study, TGA was used to measure the amount of ligand precipitated with the NPs, rather than what remained in solution following precipitation. The direct synthesis (Brust method) AgNPs were employed, and the amount of antisolvent added during the purification and washing process was investigated. The amount of antisolvent addition or solvent to antisolvent ratio has a significant effect on the NP precipitation efficiency, where too little results in incomplete precipitation and excess results in ligand coprecipitation. TEM analysis demonstrated that AgNPs core sizes were not significantly affected by either increases in solvent ratio (Figure 2) or by increases in washing number (Figure S3). The measured average particle size of AgNP after three washes at solvent:antisolvent ratios of 1:10, 1:25, 1:50, and 1:100 were 7.6 ± 1.6, 7.2 ± 1.9, 7.6 ± 1.6, and 6.8 ± 1.1 nm, respectively, which are not significantly different from the average particle size of AgNP before washing (7.7 ± 2.1 nm). This was further evidenced by UV−vis absorption spectra maxima over the

washing process as all volume ratio’s exhibited absorption maxima ∼430 nm, with relatively small deviation and no clear trends as wash number increases (Figure S4). As with the citrate−DDT exchanged particles, the wavelength of maximum UV−vis absorption (∼430 nm) and the diameter of the NP core determined from TEM remained constant for the direct synthesized AgNPs during washing. TGA can be used to quantify the number of DDT ligands on the surface from the percent weight loss resulting from ligand decomposition (Figure S5). The evolution of percent weight lost attributed to DDT versus wash number at all solvent:antisolvent ratios is displayed in Table 1. In all cases, Table 1. Average Total Percentage of Weight Lost during TGA for the AgNPs Purified at Solvent:Ethanol Volume Ratios of 1:10, 1:25, 1:50, and 1:100a wash number solvent:ethanol volume ratios 1:10 1:25 1:50 1:100 a

1 57 29 72 61

± ± ± ±

2 20 16 20 28

22 25 60 55

± ± ± ±

3 5 11 8 18

14 15 58 57

± ± ± ±

4 3 4 5 7

9 26 55 36

± ± ± ±

2 12 17 10

Error bars represent the standard deviation of three runs.

a reduction in percentage weight loss is observed with wash number; however, the most significant results were the lower DDT weight fractions obtained at ratios of 1:25 and 1:10. As with the ligand exchanged particles, AgNP precipitation became increasingly difficult with each successive wash, requiring more time and energy for particle precipitation once two washes had been completed, further supporting our claim that two washes was enough to remove the majority of the unbound free ligand. With increasing ethanol fractions from 1:10 to 1:100, we believe that the increased solvent polarity promotes the nonpolar alkanethiols to associate more strongly with the 6846


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unbound (free) and bound thiol, respectively. After 1 wash, there is ∼57% decrease in weight and only one degradation transition at 230 °C observed from the first derivative spectra. As the wash number increases, so does the onset temperature for the degradation transitions. This trend is attributed to the increase in the amount of energy required to break the thiol− AuNP bond, as removal of the excess free ligands and weakly bound chains results is progressive in order of strengths of association. As shown earlier (Table 2), after the first wash with 1:10 solvent:antisolvent ratios, the DDT surface coverage is below the 16.9 molecules/nm2 maximum monolayer surface coverage reported for AgNPs.30 Therefore, the percentage weight loss data of DDT bound to the AgNPs for 1:10 solvent:antisolvent samples after the first wash were fit with the Langmuir adsorption isotherm model (Figure 4) to determine the

ligand-stabilized AgNPs, resulting in coprecipitation and improved phase separation with centrifugation. This leads to excess unbound ligands in the nanoparticle precipitate, explaining the higher weight percent loss for the 1:50 and 1:100 ratios compared to a 1:10 and 1:25 solvent:antisolvent volume ratios. Considering the standard deviations of all wash measurements, we can conclude that the higher amounts of antisolvent results in less efficient washing, due to persisting free ligand coprecipitation. The DDT surface coverage was approximated assuming 8 nm diameter AgNPs spheres (Figure 2) with the measured weight fraction of DDT (Table 1). The surface coverages reported in Table 2 illustrate the variability in sample sets as well as the Table 2. Calculated Surface Coverage (molecules/nm2) for AgNPs Precipitated at Solvent:Ethanol Volume Ratios of 1:10, 1:25, 1:50, and 1:100a wash number solvent:ethanol volume ratios 1:10 1:25 1:50 1:100

1 55 20 155 103

± ± ± ±

2 20 16 117 90

12 15 65 60

± ± ± ±

3 7 8 20 39

7 8 57 57

± ± ± ±

4 3 3 10 17

4 16 60 24

± ± ± ±

2 10 34 10

a

Reported error is the standard deviation of three runs. Sizes of particles were approximated to be 8 nm for all the processes.

effect of solvent:antisolvent ratios on the number of molecules per nm2. It must be noted that the 1:10 and 1:25 ratio samples are the only particles that yield packing density values (12 and 15 molecules/nm2 after second wash) close to other experimentally reported DDT monolayer packing density on AgNPs.30 Recently, Ansar et al. reported the DDT packing density on AgNP using ICP optical emission spectroscopy is 2.8 nmol/cm2 (16.9 molecules/nm2).30 In summary, poor choice of the solvent−antisolvent pair or excessive use of antisolvent can often result in ineffective nanoparticle washing and purification. Building on these findings, the 1:10 solvent:antisolvent volume ratio used to purify the AgNPs was further analyzed. An important feature observed from the TGA curves of the freshly prepared particles in Figure 3 is the presence of two weight loss (48% and 42%) transitions. These separate processes are more clearly defined by analyzing the first derivative of the TGA data showing onsets at 190 and 230 °C (Figure 3b). The first (48%) and second (42%) weight loss transitions are attributed to

Figure 4. Adsorption isotherm of DDT onto AgNPs. The dots represent the experimental results for AgNP-DDT washed with a solvent:antisolvent volume ratio of 1:10. Solid line represents the Langmuir linear regression fitting of the experimental data.

maximum DDT adsorption (Γmax) and binding constant (K). The linear form of the Langmuir adsorption equation is shown in eq 2. Γ = Γmax

KC 1 + KC

(2)

Γ is the adsorbed DDT, and C is the free DDT in the supernatant of washing solution that was calculated by taking the deference between the consecutive surface coverages (Table 2). Using the maximum DDT adsorption (Γmax) from the adsorption isotherm and the surface area of the AgNPs in the

Figure 3. (A) TGA curves and (B) first derivative of the TGA curves for AgNPs with 0−5 washes with a solvent:antisolvent volume ratio of 1:10. 6847


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Å on the 3 nm diameter AuNPs and 14.0 Å on the 6 nm AgNPs in toluene, which are consistent with values reported in our previous works.18,19 The shell thicknesses are shorter than reported (∼15 Å) for DDT extending from a flat gold surface determined by ellipsometry, which is attributed to the nanoparticle curvature.34,35 The larger DDT thickness on AgNPs compared to AuNPs after the first wash can be attributed to nanoparticle curvature difference;18 however, the structures of DDT on AgNPs and AuNPs do have differences. Vibrational spectroscopy is a well-established method for probing the ordering (crystallinity) of alkanethiols on coinage metal surfaces.36−38 Recently Ansar et al. reported that DDT forms well-ordered crystal-like structures on AgNPs and disordered structure on AuNPs.30 The differences in DDT conformation are reflected by the sharp difference between the SERS spectra and can influence partitioning and washing losses from the NP surface. The decreasing ligand shell thickness with washing likely results from the decreased surface coverage and increased steric freedom for the ligand to fold in on the NP surface, a phenomena similar to the decreased thickness observed with increasing curvature. This is supported by the surface coverage decline with washing, determined independently from the ligand shell scattering length density. Both quantify a loss in ligand from the NP surface with successive washing.19,39,40 After the first wash, the DDT surface coverages on AuNPs and AgNPs are 77% and 67% (Figure 5), respectively. These values are consistent with reported DDT surface coverage on AgNPs and AuNPs in the range of 5−7 nm in size by Anand et al.39 As the number of washes increases, the surface coverage decreases to a point below 50% on AgNP after the third wash and on AuNPs after the fifth wash. UV−vis and TEM measurements indicate no significant effect on the morphology or size of the AgNPs and AuNPs even after a 50% loss of surface stabilizing ligand by ethanol washing (data not shown here). The percent surface coverage data for AgNPs and AuNPs in Figure 5 was also fit with the Langmuir adsorption isotherm model to determine the maximum DDT adsorption and binding constants (Figure S7). Using the maximum DDT adsorption from the adsorption isotherm and the surface area of the NPs in the ligand binding solution, it is estimated that the maximum packing density of DDT on AgNPs and AuNPs in toluene/EtOH mixture is 3.4 and 4.6 molecules/nm2, respectively. Using the binding affinity calculated from the adsorption isotherm fitting and Gibbs free energy equation, it is calculated that the free energy of DDT adsorption to AgNPs and AuNPs in toluene/EtOH mixture is −29 and −34 kJ/mol, respectively. The DDT packing density on AgNPs estimated by TGA is 3.8 times higher than that of SANS. The estimated DDT packing density on AuNPs (4.6 molecules/nm2) is smaller than that of DDT packing density on AuNPs estimated by GC. Also, SANS DDT packing density value is on the low end of values reported for alkanethiol packing density on AuNPs (4−10.2 molecules/nm2).30,41 The DDT binding energy to AgNPs estimated by SANS (−29 kJ/mol) is similar to that calculated by TGA reported above (−21 kJ/mol), and the value estimated by SANS for AuNPs (−34 kJ mol−1) is also consistent with the reported value for organothiols binding to AuNPs (−38 kJ/mol).15,21 The DDT adsorption free energy on AuNPs estimated using SANS data is higher than the value estimated from GC data; however, more confidence is placed in the SANS number based on the collected data. Organothiol packing density on AuNPs and AgNPs and the adsorption free

ligand binding solution, it is estimated that the maximum packing density of DDT on AgNPs in toluene/EtOH mixture is 12.9 molecules/nm2. The literature reported alkanethiol packing density was found to vary widely: as small as ∼2.2 molecules/nm2 for 3-mercaptopropanesulfonate on AgNPs and 16.9 molecules/nm2 for DDT on AgNPs.30,31 Using the binding affinity (4.3 × 103 M−1) calculated from the adsorption isotherm fitting and Gibbs free energy equation, it is calculated that free energy of DDT adsorption to AgNPs in toluene/ EtOH mixture is −21 kJ/mol. This value is significantly smaller than the literature reported free energy of adsorption (∼−105 kJ/mol) for decanethiol in methanol to Ag(111) surface using electrochemical methods32 and also smaller than, but more consistent with the value (−31 kJ/mol) reported for 3mercaptopropanesulfonate adsorption on the surface of citratestabilized AgNPs in water.33 The effective DDT desorption from the AgNP surface in each step of washing reflects the smaller free energy of adsorption of DDT on the AgNP surface reported here. Characterization of Nanoparticle Washing by SmallAngle Neutron Scattering (SANS). Small-angle neutron scattering was a third technique employed for both AuNPs and AgNPs. SANS is unique in that the NP ligands can be characterized in situ, without significant manipulation. Specifically, the use of a deuterated solvent and hydrogenated ligand enables measurement of the ligand shell thickness and chemical composition via the ligand shell scattering length density.18,19 In our prior work, we showed a gradual decrease in DDT ligand shell thickness or collapse on AuNPs in response to increased ethanol antisolvent concentration leading to eventual destabilization.19 In this study, we investigate the effect of washing on the ligand shell thickness, solvation, and percent surface coverage of DDT ligands on AuNPs (3.0 ± 0.7 nm) and AgNPs (6.0 ± 1.1 nm) synthesized by the direct method (Figure S6). Figure 5 shows that with increasing wash number

Figure 5. Dodecanethiol ligand (left) shell thickness and (right) percent surface coverage plotted as a function of number of washing for AuNPs (red ■) and AgNPs (●) dispersed in toluene-d8. The average particle diameters of AgNPs and AuNPs are 6.0 ± 1.1 and 3.0 ± 0.7 nm, respectively. The NPs washed with a solvent:antisolvent volume ratio of 1:10.

the DDT shell thickness and surface coverage decreased for both AgNPs and AuNPs. For these measurements, the ligand shell thickness is determined by fitting the scattering data with a core−shell model. The surface coverage is determined by fitting the scattering length density for the ligand shell, which can yield the composition of ligand and solvent in the shell, determined by law of averages for a mixture.18,19 A distinct advantage of SANS is that only the ligand shell is probed and the free ligand dispersed in solution does not confound the results. After the first wash, the DDT shell thicknesses are 12.0 6848


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Table 3. Summery of the Thiol Packing Density on AuNPs and AgNPs and Gibbs Free Energy of Thiol Binding to AuNPs and AgNPs Determined Using Different Methods packing density (molecules/nm2) method used

thiol lignad

AuNP

GC* TGA* SANS* ICP-OES30 SHS42 ICP-OES43 ICP-MS41 XPS44 SERS21 SHS33

dodecanethiol dodecanethiol dodecanethiol dodecanethiol 1,2-benzenedithiol 3-mercaptopropionic acid 3-mercaptopropionic acid 11-mercaptoundecanoic acid 2-mercaptobenzimidazole 3-mercaptopropanesulfonate

6.8 4.6 10.2

12.9 3.4 16.9 7.0

AuNP

AgNP

−34

−21 - 29

−38 −31

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ACKNOWLEDGMENTS This work was sponsored by the National Science Foundation Grant No. CBET-1057633. We acknowledge the Instrument Scientists, Yuri B. Melnichenko and Volker S. Urban, at the High Flux Isotope Reactor (HFIR) at Oak Ridge National Lab (ORNL) for their assistance in conducting the SANS experiments.

■

CONCLUSIONS An optimized purification procedure, for the removal of all excess DDT, was developed and explored for the purification of AgNPs and AuNPs. We demonstrated that the solvent:antisolvent ratio is a key component in the successful removal of excess ligands and phase transfer agents. As the amount of ethanol antisolvent is increased, the excess ligands or surfactants will coprecipitate with the nanoparticles, thus maintaining excess ligand in solution throughout the washing procedure. At the optimum solvent:antisolvent ratio, each subsequent wash will decrease the surface coverage on AgNPs and AuNPs below the monolayer DDT surface coverage. The significance of this work is enabling researchers to fully understand the effect of antisolvent washing on the stabilizing ligand surface coverage and thus colloidal stability for given applications. Furthermore, knowing the adsorption free energy, the free ligand concentration can be approximated, which can influence nanoparticle applications. The scope of this study also highlights the use of GC, TGA, and SANS as effective means of quantifying surface coverage on NPs, which may be more readily available to researchers than other more specialized techniques.

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REFERENCES

(1) Kang, B.; Mackey, M. A.; El-Sayed, M. A. Nuclear Targeting of Gold Nanoparticles in Cancer Cells Induces DNA Damage, Causing Cytokinesis Arrest and Apoptosis. J. Am. Chem. Soc. 2010, 132, 1517− 1519. (2) Scarberry, K. E.; Dickerson, E. B.; McDonald, J. F.; Zhang, Z. J. Magnetic Nanoparticle−Peptide Conjugates for in Vitro and in Vivo Targeting and Extraction of Cancer Cells. J. Am. Chem. Soc. 2008, 130, 10258−10262. (3) Zhou, X.; Xu, W.; Liu, G.; Panda, D.; Chen, P. Size-Dependent Catalytic Activity and Dynamics of Gold Nanoparticles at the SingleMolecule Level. J. Am. Chem. Soc. 2010, 132, 138−146. (4) Ansar, S. M.; Perera, G. S.; Ameer, F. S.; Zou, S.; Pittman, C. U.; Zhang, D. Desulfurization of Mercaptobenzimidazole and Thioguanine on Gold Nanoparticles Using Sodium Borohydride in Water at Room Temperature. J. Phys. Chem. C 2013, 117, 13722−13729. (5) Guo, C.; Boullanger, P.; Jiang, L.; Liu, T. Highly sensitive gold nanoparticles biosensor chips modified with a self-assembled bilayer for detection of Con A. Biosens. Bioelectron. 2007, 22, 1830−1834. (6) Wang, J.; Wang, L.; Liu, X.; Liang, Z.; Song, S.; Li, W.; Li, G.; Fan, C. A Gold Nanoparticle-Based Aptamer Target Binding Readout for ATP Assay. Adv. Mater. 2007, 19, 3943−3946. (7) Qi, W. H.; Wang, M. P. Size and shape dependent melting temperature of metallic nanoparticles. Mater. Chem. Phys. 2004, 88, 280−284. (8) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. Anisotropic Metal Nanoparticles: Synthesis, Assembly, and Optical Applications. J. Phys. Chem. B 2005, 109, 13857−13870. (9) Ghosh, S. K.; Pal, T. Interparticle Coupling Effect on the Surface Plasmon Resonance of Gold Nanoparticles: From Theory to Applications. Chem. Rev. 2007, 107, 4797−4862. (10) Hogan, N. J.; Urban, A. S.; Ayala-Orozco, C.; Pimpinelli, A.; Nordlander, P.; Halas, N. J. Nanoparticles Heat Through Light Localization. Nano Lett. 2014, 14, 4640−4645. (11) Caragheorgheopol, A.; Chechik, V. Mechanistic Aspects of Ligand Exchange in Au Nanoparticles. Phys. Chem. Chem. Phys. 2008, 10, 5029−5041. (12) Khajeh, M.; Laurent, S.; Dastafkan, K. Nanoadsorbents: Classification, Preparation, and Applications (with Emphasis on Aqueous Media). Chem. Rev. 2013, 113, 7728−7768.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b12423. FTIR and NMR spectra of the DDT functionalized AuNPs, GC calibration curve, TEM images for DDT capped AgNPs and AuNPs, DDT packing density calculation, UV−vis spectra and TGA curves of DDT functionalized AgNPs, and adsorption isotherms of DDT onto AgNPs and AuNPs (PDF)

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free energy of adsorption (kJ/mol) −23

7.8 6.3 4.9 3.4

energy of thiol binding to AuNPs and AgNPs determined in this study (∗) and referenced literature using different analytical methods are summarized in Table 3.

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AgNP

AUTHOR INFORMATION

Corresponding Author

*Tel +1 864 656 2131; e-mail [email protected] (C.L.K.). Notes

The authors declare no competing financial interest. 6849


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Effect of Postsynthesis Purifications on Gold and Silver Nanoparticle

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