Article pubs.acs.org/Langmuir
Impact of As-Synthesized Ligands and Low-Oxygen Conditions on Silver Nanoparticle Surface Functionalization Kathryn A. Johnston, Ashley M. Smith, Lauren E. Marbella, and Jill E. Millstone* Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, United States S Supporting Information *
ABSTRACT: Here, we compare the ligand exchange behaviors of silver nanoparticles synthesized in the presence of two different surface capping agents: poly(vinylpyrrolidone) (MW = 10 or 40 kDa) or trisodium citrate, and under either ambient or low-oxygen conditions. In all cases, we find that the polymer capping agent exhibits features of a weakly bound ligand, producing better ligand exchange efficiencies with an incoming thiolated ligand compared to citrate. The polymer capping agent also generates nanoparticles that are more susceptible to reactions with oxygen during both synthesis and ligand exchange. The influence of the original ligand on the outcome of ligand exchange reactions with an incoming thiolated ligand highlights important aspects of silver nanoparticle surface chemistry, crucial for applications ranging from photocatalysis to antimicrobials.
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INTRODUCTION Silver nanoparticles (AgNPs) exhibit unique properties that are used in a wide range of applications, including antimicrobial coatings,1−3 photocatalysis,4,5 and sensing.6,7 Like all NPs, their surface chemistry can strongly influence their physical properties and subsequent behavior in these applications. AgNP surface chemistry encompasses both the crystallographic structure of the surface as well as the adsorbed ligands. The crystallographic structure depends on the size, shape, and composition of the particle and may also be influenced by surface oxidation processes. These oxidation processes are particularly important for Ag, since it is susceptible to oxidation when exposed to air.8−10 Silver ions released from Ag and oxidized Ag surfaces are important for antibacterial applications11 but are also a significant source of toxicity to organisms when released into the environment.12 In addition to the crystallographic composition of the particle, surface-adsorbed capping ligands also influence characteristics such as particle size, shape, and resistance to oxidation.13−15 Two common capping ligands used in the synthesis of AgNPs are trisodium citrate16−18 and poly(vinylpyrrolidone) (PVP).19−22 Both ligands have been used to generate AgNPs of various sizes and shapes, and they may also be displaced to provide a different particle functionality post-synthesis. The most commonly used ligands for this purpose contain terminal thiol groups because of their high affinity for Ag surfaces,23 and these molecules have been used to replace both citrate and PVP from the surface of AgNPs.7,24−30 Despite the significant research interest in AgNPs and their capping moieties, only a few studies have quantitatively examined their adsorbed ligands. Reported techniques include thermogravimetric analysis to © 2016 American Chemical Society
determine the footprint of 11-mercaptoundecanoic acid and 3mercaptopropionic acid31 and elemental analysis to measure the footprint of dodecanethiol.32 Although these methods can be used for a wide variety of systems to estimate the number of ligands, they cannot distinguish ligand identities. To address the issue of ligand identity, spectroscopic techniques such as surface-enhanced Raman spectroscopy (SERS) have been used,26 but SERS does not provide information on the absolute number of ligands adsorbed to AgNPs and requires a SERSactive substrate. Nuclear magnetic resonance (NMR) spectroscopy has also been used and can meet or exceed the current quantitative capabilities of the above methods as well as distinguish between multiple ligand identities. Indeed, NMRbased ligand quantification has now been demonstrated (using various protocols) for ligands appended to semiconductor,33,34 metal oxide,35 and gold36,37 NPs. Here, we use NMR in combination with other materials characterization techniques to address a commonly encountered issue in AgNP synthesis and application: the role of the initial capping ligand on subsequent NP surface chemistry. Under both ambient and low-oxygen conditions, we examined the properties of AgNPs capped with either citrate or PVP and then exchanged these ligands with poly(ethylene glycol) methyl ether thiol (PEGSH). Using 1H NMR spectroscopy, we obtained quantitative information about the resulting ligand shell and demonstrated that the original ligand plays a significant role in both the final ligand density and the surface Received: January 21, 2016 Revised: March 3, 2016 Published: April 14, 2016 3820
DOI: 10.1021/acs.langmuir.6b00232 Langmuir 2016, 32, 3820−3826
Article
Langmuir
Citrate-Capped AgNP Synthesis in Ambient Conditions. AgNPs capped with citrate were prepared using a previously reported procedure.17 To synthesize ∼25 nm AgNPs, a 100 mL aqueous reaction mixture containing citrate (5.00 mM) and tannic acid (0.40 mM) was prepared in a 3-neck round-bottom flask equipped with a reflux condenser. The molar ratio of tannic acid to citrate was 0.08:1. The reaction mixture was heated to reflux while stirring until a rapid drip rate was achieved (drip rate ∼1/s). Then, 1.00 mL of AgNO3 (25.00 mM) was injected. After addition, the reaction mixture quickly changed from a slight yellow color (due to the tannic acid) to a dark yellow within 1 min. This mixture was cooled to room temperature and transferred to a clean glass media bottle for refrigerated storage (∼4 °C). Citrate-Capped AgNP Synthesis in Low-Oxygen Conditions. A 100 mL aqueous reaction mixture containing citrate (5.00 mM) and tannic acid (0.40 mM) was prepared in a 3-neck round-bottom flask equipped with a reflux condenser. This mixture was bubbled with nitrogen for 30 min. Then, the reaction mixture was heated to reflux while stirring until a rapid drip rate was achieved (drip rate ∼1/s). Then, 1.00 mL of AgNO3 (25.00 mM), which had also been bubbled with nitrogen, was injected using a syringe. After addition, the reaction mixture changed colors as described above. Ligand Exchange of PVP-Capped AgNPs. Prior to use, 1.50 mL aliquots of as-synthesized PVP-capped AgNPs were centrifuged at 6000 rcf for 2 min in 1.50 mL centrifuge tubes to remove any large aggregates (Eppendorf 5424 centrifuge). The supernatant was removed, transferred to a new centrifuge tube, and used for the subsequent steps. The AgNPs were then concentrated by centrifuging the 1.50 mL aliquot at 20 000 rcf for 6 min. The supernatant was removed, and the particles were resuspended in 1.00 mL of water. The particles were then washed by centrifuging once more. The supernatant was again removed, and the particles were resuspended in 1.00 mL of water. Then, the PEGSH was added in two separate addition steps. In the first addition, 15 μL of PEGSH (12.90 mM) was added to each tube. This mixture was vortexed and then placed on a temperature-controlled mixer (Eppendorf R thermomixer) for 15 min at 800 rpm and 25 °C. After 15 min, the second addition of PEGSH was completed by adding PEGSH (12.90 mM) to each tube. The amount of PEGSH added to each tube was determined by calculating a PEGSH excess with respect to the total surface area of the AgNPs and the calculated minimum area that one PEGSH molecule would occupy on a Ag(111) surface (0.189 nm2, additional information about this approach is described in the Supporting Information and Figure S1). Scheme S1 illustrates an example of these calculations of PEGSH excess. PEGSH excesses ranging from 1 to 40 times were used in the reported experiments. The mixture of AgNPs and PEGSH was vortexed and then replaced on the temperature-controlled mixer for 24 h. Immediately after this mixing, the particles were centrifuged and washed twice with H2O and twice with D2O to remove excess PEGSH. After the last wash cycle, the supernatant was removed to yield the concentrated pellet of AgNPs used in subsequent ligand analyses (Scheme 1). Ligand Exchange of Citrate-Capped AgNPs. The citratecapped AgNPs were concentrated by centrifuging a 1.50 mL aliquot at 20 000 rcf for 6 min. The supernatant was removed, and the particles
oxidation state of the particles. We found that in all cases the polymer capping agent exhibited features of a weakly bound ligand, producing better ligand exchange efficiencies with an incoming thiolated ligand as well as being more susceptible to oxygen during both synthesis and ligand exchange compared to the citrate capping agent.
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EXPERIMENTAL SECTION
General Considerations. Silver nitrate (AgNO3, 99.9999%), hydrogen tetrachloroaurate(III) trihydrate (HAuCl 4 ·3H 2 O, 99.999%), poly(vinylpyrrolidone) (PVP, average MW = 10 000 or 40 000 Da), L-ascorbic acid (reagent grade), tannic acid (puriss. grade), sodium citrate tribasic dihydrate (citrate, ≥99%), sodium borohydride (NaBH4, ≥99%), and nitric acid (HNO3, >99.999%, trace metal basis) were obtained from Sigma-Aldrich (St. Louis, MO). Poly(ethylene glycol) methyl ether thiol (PEGSH, average MW = 1000 Da) was obtained from Laysan Bio, Inc. (Arab, AL). Acetonitrile (ACN, 99.8%) and isopropanol (IPA, certified ACS plus) were obtained from Fisher Scientific (Waltham, MA). Deuterium oxide (D2O, 99.9%) was obtained from Cambridge Isotope Laboratories (Tewksbury, MA). All reagents were used as received unless otherwise indicated. NANOpure (Thermo Scientific, >18.2 MΩ·cm) water was used in the preparation of all solutions. Before use, all glassware and Teflon-coated stir bars were washed with aqua regia (3:1 ratio of concentrated HCl and HNO3 by volume) and rinsed thoroughly with water. Caution: aqua regia is highly toxic and corrosive and requires proper personal protective equipment. Aqua regia should be handled in a f ume hood only. PVP-Capped AgNP Synthesis in Ambient Conditions. AgNPs capped with PVP were synthesized using a modified literature procedure.20 Different concentrations and molecular weights of PVP were used in the synthesis: 5.00 mM 10 kDa, 5.00 mM 40 kDa, or 1.25 mM 40 kDa, where the concentration is calculated with respect to the number of PVP chains. These different molecular weights and concentrations were used to study the influence of polymer concentration and molecular weight on the replacement of PVP by PEGSH. For completeness, we examined and compared polymer concentrations with respect to both the total concentration of monomer and the total concentration of polymer chains. AgNPs were synthesized using 5.00 mM 10 kDa PVP and 5.00 mM 40 kDa PVP, where polymer concentrations were equivalent with respect to number of PVP chains. AgNPs were also synthesized using 5.00 mM 10 kDa PVP and 1.25 mM 40 kDa PVP, where polymer concentrations were equivalent with respect to the number of PVP monomers. To make the AgNPs, first, seeds were prepared by quickly injecting 0.60 mL of NaBH4 (0.10 M) into a solution containing 5.00 mL of PVP (at various concentrations, vide supra), 10 μL of HAuCl4 (0.25 M), and 5.00 mL of H2O. The seeds were then aged for 2 h. Next, to synthesize AgNPs that were ∼25 nm in diameter, 2.00 mL of PVP, 2.00 mL of acetonitrile, 200 μL of ascorbic acid (0.10 M), and 2.00 mL of H2O were mixed. The reaction mixture was placed in a cold water bath at 10 °C. Then, 150 μL of AgNO3 (0.10 M) was added. Finally, 10 μL of the seed solution was quickly injected while stirring. After addition, the reaction mixture slowly changed from clear, to yellow, to dark yellow, to dark yellowish-brown within 15 min. PVP-Capped AgNP Synthesis in Low-Oxygen Conditions. Seeds were prepared as described above. Next, 2.00 mL of PVP (at various concentrations, vide supra), 2.00 mL of acetonitrile, 200 μL of ascorbic acid (0.10 M), and 2.00 mL of H2O were mixed. The reaction mixture was placed in a cold water bath at 10 °C. Then, the reaction mixture was bubbled with nitrogen for 30 min. Separately, 150 μL of AgNO3 (0.10 M) and 10 μL of the seed solution were also bubbled with nitrogen for 30 min. After this time, the seed solution (which now included the AgNO3) was quickly injected to the reaction mixture using a nitrogen-purged syringe. After addition, the reaction mixture changed colors as described above. Particles prepared according to this procedure are referred to as being prepared in “low-oxygen conditions”.
Scheme 1. General Ligand Exchange Procedure for AgNPs with PEGSH
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DOI: 10.1021/acs.langmuir.6b00232 Langmuir 2016, 32, 3820−3826
Article
Langmuir were resuspended in another 1.50 mL aliquot of AgNPs and centrifuged again. The supernatant was removed, and the particles were resuspended in 1.00 mL of water. The particles were then washed by centrifuging once more. The supernatant was again removed, and the particles were resuspended in 1.00 mL of water. Then, the PEGSH was added in two separate addition steps. In the first addition, 15 μL of PEGSH (12.90 mM) was added to each tube. This mixture was then placed on a temperature-controlled mixer for 15 min at 800 rpm and 25 °C. After 15 min, the second addition of PEGSH was completed by adding PEGSH (12.90 mM) to each tube. The amount of PEGSH added to each tube was determined as described above and in Scheme S1. PEGSH excesses ranging from 1 to 40 times excess were used. The mixture was then replaced on the temperature-controlled mixer for 24 h. Immediately after this mixing time, the particles were washed twice with H2O and twice with D2O to remove excess PEGSH. After the last wash cycle, the supernatant was removed to yield the concentrated pellet of AgNPs used in subsequent ligand analyses (Scheme 1). 1 H NMR Analysis. All NMR measurements were performed on a Bruker 400 Ultrashield magnet with AVANCE III 400 Console or a Bruker 600 Ultrashield magnet with AVANCE III 600 Console (Bruker Biospin, Billerica, MA) at 298 K. NMR samples were prepared as discussed above by concentrating the AgNPs, followed by digestion with 1 drop (∼5 μL) of concentrated HNO3. These samples were allowed to digest overnight before dilution with D2O to a volume of 500 μL. An IPA reference was used for the determination of unknown ligand concentrations. To each sample, 5 μL of dilute IPA (0.24% v/v; 5 μL of IPA in 2.00 mL of D2O) was added. The unknown ligand concentrations were determined by comparison to a 5-point standard curve with a range of 1.00−0.01 mM ligand (1.00, 0.50, 0.10, 0.05, and 0.01 mM, prepared in D2O). For each standard, the integral of a specific ligand peak was divided by the integral of the IPA peak and plotted against concentration of ligand (Figures S2 and S3). Following an internal standard approach for the unknown concentrations of ligand on the AgNP,38 the ligand peak was integrated and similarly divided by the known integrated IPA peak to yield the concentration upon comparison with the calibration curve. The T1 of PEGSH was measured to be 781 ms, and the T1 of IPA was measured to be 3.1 s. For all experiments, a minimum recycle delay of 3× the longest T1 was used. For all quantitative analyses, a minimum signal-to-noise ratio of 25 was used. ICP-MS Analysis. Inductively coupled plasma mass spectrometry (ICP-MS) analysis was performed using an argon flow with a NexION spectrometer (PerkinElmer, Inc.). A nitric acid (Sigma-Aldrich, >99.999% trace metal basis) solution was diluted with water for a 5 vol % nitric acid matrix. AgNP samples were taken from the digested and diluted NMR samples as described in the Experimental Section. From this solution, 1 μL was further diluted to 15 mL using 5% nitric acid matrix. Unknown Ag concentrations were determined by comparison to a 5-point standard curve with a range of 1−30 ppb (1, 5, 10, 20, and 30 ppb prepared by volume) from a Ag standard for ICP (Fluka, TraceCERT 1000 ± 2 mg/L Ag in HNO3) diluted in the 5% nitric acid matrix. All standards were measured 5 times and averaged, while all unknown samples were measured in triplicate and averaged. A 5 min flush time with 5% nitric acid matrix was used between all runs, and a blank was analyzed before each unknown sample to confirm removal of all residual metals from the instrument. XPS Analysis. XPS spectra were obtained using an ESCALAB 250XI XPS with a monochromated, microfocused Al Kα X-ray source (spot size = 200 μm). Survey and high-resolution spectra were collected with a pass energy of 150 and 50 eV, respectively. Spectra were charge referenced to adventitious carbon (284.8 eV). NPs were drop-cast onto silicon wafers (University Wafer, Boston, MA) cleaned for ultra-high vacuum analysis.39
Figure 1. TEM images for representative AgNPs functionalized with citrate (25.2 ± 3.3 nm) (A), 5 mM 10 kDa PVP (25.4 ± 2.1 nm) (B), and 5 mM 40 kDa PVP (27.9 ± 3.1 nm) (C), along with corresponding absorption spectra (D). Insets are the size distribution histograms generated from sizing at least 200 particles. Scale bars are 200 nm.
synthesized pseudospherical AgNPs capped with either citrate (Figure 1A) or PVP (Figure 1B, C). Each type of AgNP was relatively monodisperse (standard deviation
