Mediated Growth of Zinc Chalcogen Shells on Gold Nanoparticles by

Jul 16, 2017 - Herein, we report a method that uses free-base amino acids to mediate the controlled hydrothermal growth of amorphous zinc oxide (a-ZnO...
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Mediated Growth of Zinc Chalcogen Shells on Gold Nanoparticles by Free-Base Amino Acids Matthew T. Klug,†,∥ Noémie-Manuelle Dorval Courchesne,∥,‡ Yoonkyung E. Lee,† Dong Soo Yun,∥ Jifa Qi,∥,§ Nimrod C. Heldman,∥,⊥ Paula T. Hammond,∥,‡ Nicholas X. Fang,† and Angela M. Belcher*,∥,§,⊥ †

Department of Mechanical Engineering, ‡Department of Chemical Engineering, §Department of Materials Science and Engineering, Koch Institute for Integrative Cancer Research, and ⊥Department of Biological Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States ∥

S Supporting Information *

ABSTRACT: Herein, we report a method that uses free-base amino acids to mediate the controlled hydrothermal growth of amorphous zinc oxide (a-ZnO) or nanocrystalline zinc sulfide (c-ZnS) shells on gold nanoparticles. By screening through a set of 13 candidate amino acids, we have identified four as being capable of mediating inorganic shell growth using an aqueous, low-temperature, one-pot process. In particular, unaggregated and monodisperse sols of exceptional quality are produced using L-histidine, which preserves colloidal stability and mediates the growth of continuous and remarkably uniform a-ZnO shells with a tunable thickness between 2 and 25 nm while avoiding the nucleation of free particles. By coupling spectral extinction measurements with generalized Mie theory calculations, we estimated the complex refractive index of the a-ZnO shell to be 1.47 + i0.09. It is expected not only that our Au@a-ZnO core−shell particles are suitable for both energy and biological applications but also that our process for growing inorganic shells could be extended to other nanocomposite systems comprised of different materials and geometries.

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as thin as possible, yet continuous, to maximize field enhancement in the near field and prevent photocurrent loss. Similarly, metal−semiconductor nanoparticles have also been used to enhance performance in catalytic systems.20−23 Although many shell materials have been synthesized on gold nanoparticles (AuNPs), including SiO2,24−29 TiO2,14 MnO2,30 Cu2O,31−33 Fe2O3,34 ZnO,35−37 CdS,16,38 CdSe,38 ZnS,38,39 and PbS,38 it is generally difficult to find procedures that are simple and reliable and provide precise control over the shell geometry. The most common shell materials, amorphous silica and titania, are synthesized in ethanolic solutions, which tend to induce particle agglomeration.27 Although procedures have been developed to stabilize the particles by priming the gold surface with aminopropyltrimethoxysilane,24 cetyltrimethylammonium bromide,40 or polyvinylpyrrolidone (PVP),29 an excess of such vitreophilic molecules will nucleate free oxide particles in solution, while a deficiency will allow some degree of particle aggregation to occur during shell growth.41 Furthermore, it is challenging to produce continuous silica shells thinner than ∼4 nm unless designer silica precursors such as diglyceroxysilane are used instead of the widely used tetraethyl orthosilicate.42

INTRODUCTION Because of the phenomenon of localized surface plasmon resonance (LSPR), noble metal nanoparticles demonstrate unique optical properties that are useful for biological and energy applications. They are commonly employed as optical dark-field imaging agents1,2 and nanoscale sensors3−5 because of their large extinction cross sections and the high sensitivity of the extinction peak to the refractive index of the surrounding environment. Their ability to concentrate light and heat in the near field can be utilized to improve light harvesting in solar energy technologies and allow the nanoparticles to serve as agents for triggered drug delivery6−8 or photothermal therapy.2,9,10 However, the practical implementation of noble metal nanoparticles in such applications generally requires the metal surface to be coated with a dielectric shell. For instance, metal oxides such as silica improve the stability and biocompatibility of metal nanoparticles,11 can prevent photoluminescence quenching by separating fluorophores from the metal surface,4,11 and can be functionalized with silane linkers.12 Likewise, the performance of solution-processed dye-sensitized,13−15 quantum dot,16 polymer,17 and perovskite18 solar cells has been enhanced by incorporating into the active layers noble metal nanoparticles with thin dielectric shells, which prevent charge trapping and exciton quenching from occurring at the bare metal surface.19 This role requires that the shell be © 2017 American Chemical Society

Received: June 20, 2017 Revised: July 8, 2017 Published: July 16, 2017 6993

DOI: 10.1021/acs.chemmater.7b02571 Chem. Mater. 2017, 29, 6993−7001

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Chemistry of Materials

Figure 1. Visual screening of reaction solutions identified four amino acids as being capable of stabilizing and mediating the growth of zincous shells on AuNPs from (a) a candidate pool of 13 free-base amino acids. (b) The initial solutions containing AuNPs and each of the corresponding amino acids are visually identical prior to the addition of ZnCl2. (c) After each reaction volume is heated at 80 °C for 9 days, only Cys, His, Trp, and Tyr yield stable colloidal solutions, whereas significant particle flocculation occurs with the other candidates. Transmission electron microscopy images of the core−shell particles generated by (d) Cys, (e) His, (f) Trp, and (g) Tyr mediation reveal the presence of a shell material coating the AuNPs in each case.

synthesizing shells on individual nanoparticles requires finding capping agents capable of stabilizing the particles by coordinating with both the core and shell materials and mediating the deposition of the shell material throughout the growth process. As a class of compact molecules, free-base amino acids meet these general requirements by (1) naturally displaying a wide array of functional groups, some of which might have affinity for both the core and shell materials, (2) displaying a carboxylic acid on the end opposite the functional group that can be deprotonated to provide the required negative charge for stabilizing the nanoparticles via electrostatic repulsion, and (3) potentially retaining their ability to promote the growth of inorganic materials. Zinc chalcogen materials were targeted for shell growth because several amino acids have been previously identified as possessing affinity for both zinc chalcogenides and gold43 and binding peptides can modify the growth and morphology of zinc oxide.44 Additionally, zinc sulfide has been successfully nucleated and grown on biomacromolecules under aqueous conditions at room temperature.45 By screening over a set of 13 candidates, we identified four free-base amino acids as being capable of stabilizing and mediating the growth of zinc chalcogen shells on gold nanoparticles. Of the four, one mediated the growth of nanocrystalline zinc sulfide (Au@cZnS) and three mediated the growth of amorphous zinc oxide

Cuprous oxide can be grown on AuNPs in aqueous solution; however, the resultant polycrystalline shells tend to be rough and cannot form continuous shells thinner than ∼10 nm.32 Conversely, non-epitaxial growth through solid-state cation exchange provides a means to form metal chalcogenide shells of precise thickness on AuNPs, but the procedure is a multistep process that requires transferring particles between organic and aqueous solvents.38 In contrast to these methods, we present an aqueous, one-step process that uses free-base amino acids to mediate the growth of continuous and uniform shells of either amorphous zinc oxide (a-ZnO) or nanocrystalline zinc sulfide (c-ZnS) on AuNPs. When L-histidine is used, the resulting core−shell colloids are monodisperse and unaggregated, and the shell thickness can be precisely tuned between 2 and 25 nm, thereby meeting the requirements of plasmonic particles with either ultrathin or customizable coatings necessary for many biological and energy applications. Our approach is inspired by the ability of nature to coordinate metal ions at the active sites of enzymes such as carbonic anhydrases and use proteins to direct the assembly of inorganic materials such as calcium carbonate in seashells and calcium phosphate in bones. Such in vivo interactions between biological and inorganic materials arise in part from the inherent functionality of amino acids, which we leverage here for the in vitro synthesis of core−shell particles. In a manner similar to protein-directed inorganic growth, successfully 6994

DOI: 10.1021/acs.chemmater.7b02571 Chem. Mater. 2017, 29, 6993−7001

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Chemistry of Materials

Figure 2. (a) X-ray diffraction (XRD) spectra and (b) X-ray photoelectron spectroscopy (XPS) survey scan for His-mediated core−shell particles with several shell thicknesses. (c) A high-resolution transmission electron microscopy (HRTEM) image of His-mediated particles directly confirms the shell is amorphous. (d) XRD spectra and (e) XPS survey spectrum of Cys-mediated core−shell particles. (f) HRTEM image of a Cys-mediated core−shell particle confirms the shell is nanocrystalline with a lattice spacing that corresponds to the (002) planes for wurtzite ZnS. shell solutions were purified by centrifuging the particles down at 15000 rcf for 5 min, discarding as much of the supernatant as possible without disturbing the particles, resuspending the pellet in DI water, and repeating the spin−resuspension cycle again.

(Au@a-ZnO) shells, with L-histidine producing colloids of remarkable quality.

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EXPERIMENTAL METHODS

Chemicals. DL-Alanine (Ala, 99%), L-arginine (Arg, ≥98%), Lcysteine (Cys, ≥98%), L-glutamine (Gln, ≥98%), L-histidine (His), Lphenylalanine (Phe, 99%), L-proline (Pro, 99%), L-tryptophan (Trp, 99%), L-tyrosine (Tyr, 99%), zinc chloride (ZnCl2, anhydrous ≥98%), and tetrachloroaurate(III) hydrate (HAuCl4·3H2O) were purchased from Alfa Aesar. L-Lysine (Lys, >98%), L-methionine (Met, >98%), and L-serine (Ser, >99%) were purchased from Sigma-Aldrich. Sodium citrate dihydrate (ACS grade, 99% min) and glycine (Gly, Genar) were purchased from Mallinckrodt Chemicals. Ammonium hydroxide (NH4OH, 30 wt % in H2O) was purchased from VWR International, Inc. Citrate-capped gold nanorods [AuNRs, 10 nm diameter, 900 nm longitudinal mode LSPR peak, in deionized (DI) water] were purchased from NanoPartz, Inc. All water was deionized by an EMD Millipore Milli-Q system (18.2 MΩ). Synthesis of AuNPs. The Turkevich method46 was modified to produce 16 nm citrate-capped gold nanoparticles (∼3 × 1012 NPs/ mL). In brief, a solution of HAuCl4·3H2O (20 mM, 6.2 mL) was combined with 188 mL of deionized water in a round-bottom flask and brought to a boil while being stirred and refluxed. A solution of sodium citrate (34 mM, 12.5 mL) was quickly added while the mixture was being vigorously stirred, and boiling was continued for an additional 20 min before the flask was cooled to room temperature. Within a minute of injection, the solution turned black and gradually ripened into a deep ruby color. The AuNP sol was used without further purification. Synthesis of Zinc Chalcogen Shells on AuNPs. The total volume of the aqueous reaction solution was fixed at 20 mL. Before beginning, the threads of a clean 20 mL glass vial were wrapped with Teflon tape to better seal the vial during hydrothermal growth. In a typical reaction, the following items were sequentially combined in the vial: citrate-capped AuNPs (2 mL of the prepared solution), an appropriate amount of deionized water (17.08 to 16.10 mL), an aqueous solution of freshly prepared free-base amino acid (100 mM, 0.75 mL), and NH4OH (30 wt % in water, 0.15 mL). An aqueous stock solution of 50 mM ZnCl2 must be freshly prepared, and an aliquot (50−1000 μL) was added to the vial, which was then immediately capped and shaken. The cap was sealed with electrical tape and the vial placed inside an oven at 80 °C for 9 days in the dark without its contents being stirred. The as-synthesized colloidal core−

RESULTS AND DISCUSSION Amino Acid Screening Study. Visual screening was performed to assess the ability of several candidate free-base amino acids (Figure 1a) to stabilize the particles before and throughout the growth process. An aqueous solution of each amino acid candidate was first added to a colloid of citratecapped AuNPs (AuNPs-Cit) to provide it the opportunity to displace the citrate capping agent (Figure 1b), and NH4OH and ZnCl2 were added thereafter. The ability of each amino acid to stabilize the AuNPs in the presence of salt was assessed by color. The same plasmonic properties that make AuNPs useful in optical, biological, and energy applications provide a means of extracting information about colloidal stability through simple colorimetric assessment. Solutions that retain the red color characteristic of well-dispersed AuNP colloids indicate that the amino acid has sufficient affinity to displace citrate from the gold surface and stabilize the particles in the presence of the shell precursor ions, whereas solutions that turn purple indicate the aggregation of AuNPs and that the associated amino acid cannot stabilize the particle surface. In time, the AuNPs will completely flocculate from such solutions to form a fine sediment at the bottom of these vials. As shown in Figure S1 and summarized in Table S1, of the amino acids considered, only Cys, His, Lys, Met, Trp, and Tyr sufficiently stabilize AuNPs in the presence of the shell precursor salts, whereas all others behaved like AuNPs-Cit in the absence of any amino acid and began to flocculate prior to being heated. These observations agree with previously reported molecular dynamics computations, which identified Tyr, Phe, His, Trp, Met, Cys, Arg, and Lys as having the strongest affinities for a gold surface in decreasing order.47 The strong interaction between aromatic amino acids and gold has been attributed to π-electrons, whereas those containing sulfur atoms are known to form dative bonds.48 Unlike the computations, our assay 6995

DOI: 10.1021/acs.chemmater.7b02571 Chem. Mater. 2017, 29, 6993−7001

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Figure 3. Energy dispersive X-ray spectroscopy (EDS) characterization of His-mediated core−shell particles through (a) a survey spectrum and (b) a one-dimensional line scan with the corresponding path indicated on the high-angle annular dark-field TEM image in the inset. (c) TEM micrograph of His-mediated core−shell particles and corresponding two-dimensional energy-filtered TEM (EFTEM) elemental maps of (d) Zn, (e) O, and (f) both.

for ZnO but cannot do so for Arg and Gly because neither molecule could stabilize the AuNPs in the reaction solution prior to shell growth. Characterization of Materials. A series of characterization techniques was performed to identify each shell material. In the case of His-mediated shells, the X-ray diffraction (XRD) spectra of Figure 2a reveal that the only crystalline component is the gold core. The high-resolution TEM (HRTEM) image presented in Figure 2c clearly shows the lattice fringes of the AuNP core but no crystallinity in the shell, which directly confirms the shell is indeed amorphous. Elemental analyses of these core−shell particles through both X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDS), shown in Figures 2b and 3a, respectively, indicate the core−shell particles contain Au, Zn, and O (note that the Cu and C EDS signals arise from the carbon-coated copper TEM grid). In contrast, the XRD spectrum plotted in Figure 2d and the HRTEM images in Figure 2f reveal that both the gold core and shell material are nanocrystalline in the Cys-mediated core− shell particles. The measured lattice spacings of 0.31 nm in the HRTEM image (Figure 2f) are consistent with the (002) planes of wurtzite ZnS, which is confirmed by the agreement of the XRD spectral peaks with its reference spectrum. The XPS survey scan shown in Figure 2e indicates Zn, S, and Au are present in the particles. Spatial elemental analysis was performed using both EDS and energy-filtered TEM (EFTEM) to directly determine the elements comprising the His-mediated shell material. An EDS line scan through the center of two adjacent core−shell particles in Figure 3b indicates that the shell material is zincous and the core material is gold; however, the observed signal for oxygen is weak because of the poor sensitivity of EDS to lighter elements (Z < 10). However, EFTEM elemental mapping of His-mediated particles in Figure 3c−f clearly shows that the shell material is comprised of both oxygen and zinc. Highresolution chemical XPS scans of the O 1s and Zn 2p3/2 peaks shown in Figure S5 reveal that the binding energies are consistent with ZnO rather than Zn(OH)2.54 Thus, the

does not identify Arg and Phe as being capable of stabilizing the gold colloid under the synthesis conditions. While Arg has been used to stabilize27 and even reduce49 AuNPs, there has been speculation that the ionic strength of our solution is too high for successful stabilization to occur during ligand exchange. Although Phe is indeed aromatic like His, Trp, and Tyr, it may have difficulty displacing the citrate capping agent on the AuNPs because of the hydrophobic nature of the benzyl group. Hence, Phe might be unable to reach the gold surface, whereas the imidazole, indole, and phenol groups of His, Trp, and Tyr, respectively, are sufficiently polar to do so. The ability of each amino acid to mediate the growth of a shell and stabilize the shell material was also assessed by visual examination after sealing the reaction solution in a vial and heating it at 80 °C for 9 days. On the basis of the shell growth curve presented in Figure S2 and the corresponding transmission electron microscopy (TEM) images in Figure S3, 9 days was selected as an appropriate growth time because longer times resulted in only marginally thicker shells. As shown in Figure 1c and summarized in Table S1, stable sols were obtained at the end of the screening test with Cys, His, Trp, and Tyr. A representative extinction spectrum for each of these four mediated core−shell sols is presented in Figure S4a. As confirmed via TEM in Figure 1d−g, each of these molecules does indeed mediate the growth of a shell around the AuNPs; however, those synthesized from His and Cys are remarkably monodisperse and unaggregated. The striking difference between the two cases is the shell morphology; the Hismediated shells are smooth and uniform, whereas the Cysmediated shells are rough and nanocrystalline. Our findings are consistent with prior related reports. Freebase L-histidine50 and L-cysteine51 have both been used to cap quantum dots coated with a ZnS shell, while Au-ZnS nanocomposites have been formed using Cys as a sulfur source.52 Likewise, a previous yeast display study demonstrated that Cys, His, and Trp have a strong affinity for Au and ZnS,43 whereas a previous peptide display study with Escherichia coli identified Arg, Trp, and Gly as residues that enriched binder proteins for ZnO.53 Our results can confirm the affinity of Trp 6996

DOI: 10.1021/acs.chemmater.7b02571 Chem. Mater. 2017, 29, 6993−7001

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Chemistry of Materials

Figure 4. Proposed synthetic pathway for the hydrothermal growth of zinc chalcogen shells on AuNPs in aqueous solution through amino acid mediation. The original citrate capping agent is displaced by an amino acid displaying a functional group with a stronger affinity for the gold surface. The addition of NH4OH ensures the carboxylic acid groups on the amino acid are deprotonated and coordinates Zn2+ into tetraammine complexes, which are electrostatically attracted to the negatively charged AuNPs. Upon heating at 80 °C, the amino acids mediate the hydrothermal growth of the zinc chalcogen shell material on the AuNP surface. The amino acids cap the shell surface and keep the core−shell particles electrostatically stabilized throughout the growth process. His promotes the growth of smooth a-ZnO shells, whereas Cys participates in the growth of rougher c-ZnS shells.

c-ZnS on the AuNPs. When the new material is formed, it is capped by the excess amino acid molecules in solution. Thus, the amino acids mediate the growth process by allowing new material to gradually deposit while stabilizing the particles in solution. Whereas Cys must decompose to provide sulfur atoms for the formation of c-ZnS, precisely how His, Trp, and Tyr mediate the growth of a-ZnO beyond capping the surface cannot be inferred. This proposed pathway is supported by control experiments in which each of the ingredients in the reaction fluid was systematically omitted (see Figures S7−S9 for details). It must be noted that no shells are formed when amino acids are omitted from the reaction solution, which reveals the amino acid molecules are critical for directing shell growth (Figure S7). It is unlikely that any byproducts from the Turkevich method used to synthesize the AuNPs have a role in the growth process as we have successfully formed a-ZnO shells using this process on commercially available citrate-capped gold nanorods as is shown in Figure S10 (see the Supporting Information for details). a-ZnO Shell Thickness Tuning. Unlike the previously reported method35 for growing a-ZnO on AuNPs in aqueous solution, our His-mediated growth process minimizes the synthesis of free a-ZnO particles and does not produce visibly porous shells. TEM imaging suggests that our shells are generally solid and continuous (Figures S2, S3, and S11b−g), which likely results from His gradually mediating the growth of the shells over several days rather than a few hours. A further advantage is our ability to precisely tune the thickness of the aZnO shells by adjusting the concentration of ZnCl2 in the reaction solution. Figure 5 presents the average shell thickness

aggregate of material characterization data indicates that His mediates the growth of amorphous zinc oxide shells on AuNPs. The XPS spectra (Figure 2b) for samples with both thin (∼2 nm) and thick (∼23 nm) shells each display a nitrogen peak, which we attribute to the His molecules. The fact that a 23 nm shell is too thick for the Au 4p and 4d XPS peaks to be detected (Figure 2b) suggests that the His molecules are indeed capping the shell surface as opposed to only sitting at the AuNP−shell interface. Similar measurements shown in Figure S6 reveal that Trp and Tyr also mediate a-ZnO shells on AuNPs and display similar XRD and XPS spectra. Proposed Synthesis Pathway. On the basis of the aggregate of our data, we propose the synthesis pathway illustrated in Figure 4 to explain how free-base amino acids mediate the formation of zinc chalcogen shells on AuNPs while preserving colloidal stability. First, the functional group on the amino acid displaces the citrate capping agent on the AuNP and binds to the gold surface. Adding NH4OH increases the pH and ensures the particle is electrostatically stabilized by deprotonating the outer carboxylic acid group. Upon addition of aqueous ZnCl2, Zn2+ ions are coordinated by ammonia to form tetraamminezinc(II) complexes, [Zn(NH3)4]2+. This is evidenced by the initial formation of a characteristic white precipitate upon dispensing ZnCl2 into the reaction solution, which quickly redissolves upon mixing. The positively charged complexes are attracted to the negatively charged AuNPs; however, the bulky size of the complex prevents them from packing densely enough around the nanoparticles to completely screen out the surface charge. Upon being heated, the complexes decompose and hydrothermally nucleate a-ZnO or 6997

DOI: 10.1021/acs.chemmater.7b02571 Chem. Mater. 2017, 29, 6993−7001

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Chemistry of Materials

Figure 5. Shell thickness of His-mediated Au@a-ZnO core−shell particles can be precisely tuned by varying the concentration of ZnCl2 in the reaction solution. (a) Representative TEM images of individual core−shell particles and (b) corresponding ZnCl2 concentrations required to synthesize each sample. The average shell thickness for each reaction condition (circles), as obtained by processing TEM images of each sample, is compared against the predicted shell thickness (···) in the event of complete consumption of the ZnCl2 reagent. The filled circles correspond to data for samples for which nearly every particle contains a single AuNP in the core, whereas the empty circles indicate conditions where a significant portion of the population has multiple AuNPs in the core (see Figure S11). The error bars correspond to the standard deviation.

storage conditions of the salt are likely to impact the reaction kinetics of a-ZnO formation. Determination of the a-ZnO Shell Complex Refractive Index. The appearance of the reaction solutions before and after His-mediated hydrothermal shell growth is shown in Figure S13, and the corresponding extinction spectrum for each is shown in Figure 6. When the ingredients are combined

for each reaction condition, as determined by processing TEM images, as well as an image of an individual representative core−shell particle. This series of TEM images in Figure 5a demonstrates that uniform and continuous shells with thicknesses between 2 and 40 nm can be grown on individual AuNPs. The measured thickness for each ZnCl2 concentration is compared against a predicted thickness, which assumes complete conversion of ZnCl2 to the a-ZnO shell material (see the Supporting Information for details). The measured shell thicknesses are in good agreement with the predicted values for intermediate shell thicknesses but deviate at both high and low ZnCl2 concentrations. TEM images shown in Figure S11 reveal that for concentrations of 2.0 mM, populations of core−shell particles emerge with multiple AuNPs in the core, and hence, the shells become thicker because the material is no longer distributed between individual AuNPs but rather across small AuNP aggregates. This observation suggests that 2.0 mM ZnCl2 is the threshold for instability where at some point during the growth of the shells, the surface charge is sufficiently screened by counterions to destabilize a significant population of the individual particles and promote the formation of clusters. Both the population of multiple-core core−shell particles and the average number of AuNPs comprising the core increase with an increase in ZnCl2 concentration (Figure S11a). The observation that the a-ZnO shell is thinner than expected at ZnCl2 concentrations of