Article pubs.acs.org/Langmuir
Cite This: Langmuir XXXX, XXX, XXX-XXX
Ligand-Mediated Deposition of Noble Metals at Nanoparticle Plasmonic Hotspots Patrick J. Straney,†,‡ Nathan A. Diemler,†,‡ Ashley M. Smith,‡ Zachary E. Eddinger,‡ Matthew S. Gilliam,‡ and Jill E. Millstone*,‡,§,⊥ ‡
Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, United States Department of Chemical and Petroleum Engineering and ⊥Department of Mechanical Engineering and Materials Science, University of Pittsburgh, 3700 O’Hara Street, Pittsburgh, Pennsylvania 15261, United States
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S Supporting Information *
ABSTRACT: We report the use of gold nanoparticle surface chemistry as a tool for site-selective noble metal deposition onto colloidal gold nanoparticle substrates. Specifically, we demonstrate that partial passivation of the gold nanoparticle surface using thiolated ligands can induce a transition from linear palladium island deposition to growth of palladium selectively at plasmonic hotspots on the edges or vertices of the underlying particle substrate. Further, we demonstrate the broader applicability of this approach with respect to substrate morphology (e.g., prismatic and rod-shaped nanoparticles), secondary metal (e.g., palladium, gold, and platinum), and surface ligand (e.g., surfactant molecules and n-alkanethiols). Taken together, these results demonstrate the important role of metal−ligand surface chemistry and ligand packing density on the resulting modes of multimetallic nanoparticle growth, and in particular, the ability to direct that growth to particle regions of impact such as plasmonic hotspots.
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INTRODUCTION Multimetallic nanoparticles (NPs) are an emerging class of materials with the ability to combine or enhance the optoelectronic,1−3 magnetic,4−6 and/or catalytic7−10 properties of the elemental constituents. Of particular interest are plasmonic metals (e.g., Cu, Ag, and Au) combined with catalytically relevant transition metals (e.g., Cu, Pt, Pd), where the conversion of light into hot carriers can be used for subsequent applications via transfer of these carriers to neighboring molecules or materials.10−13 However, the efficacy of these processes is strongly influenced by the compositional architecture (i.e., alloy, core@shell, or Janus-type) of the final multimetallic NP construct.14−17 A widely adopted strategy for synthesizing multimetallic noble metal NPs involves the separation of NP nucleation and growth through the use of seed-mediated techniques.14 However, when “lossy” metals (e.g., Pt, Pd, or Rh) are introduced to the surface of a plasmonic NP substrate, there is damping and broadening of the surface plasmon.18,19 An attractive strategy to create hybrid particles while mitigating unfavorable changes in the localized surface plasmon resonance (LSPR) would be to confine metal deposition to where it is most effective for an application. For example, it would be advantageous in terms of both cost and resulting optoelectronic features to selectively deposit the second metal only at places of concentrated local electromagnetic field enhancement (termed “hotspots”) on the underlying plasmonic NP substrate.15,17 However, site-selective deposition of a secondary metal at these © XXXX American Chemical Society
positions is synthetically challenging and likely requires specific experimental conditions for each combination of deposited metal and NP substrate.14,20 In this report, we present a facile strategy to direct secondary metal deposition onto the plasmonic hotspots of Au nanoprisms and nanorods by understanding the relationship between substrate capping ligand, ligand packing density, and the resulting morphology of the deposited metal.
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EXPERIMENTAL SECTION
General Materials and Methods. Chloroplatinic acid (H2PtCl6, 8 wt % in H2O), hexadecyltrimethylammonium bromide (CTAB, 99%), palladium(II) chloride (PdCl2, 5 wt % in 10 wt % HCl), hydrogen tetrachloroaurate trihydrate (HAuCl4· 3H2O, 99.999%), Lascorbic acid (99%), sodium borohydride (NaBH4, 99.99%), sodium hydroxide (NaOH, 99.99%), sodium iodide (NaI, 99.999%), and sodium citrate tribasic dihydrate (99%) were obtained from SigmaAldrich (St. Louis, MO). 11-Amino-1-undecanethiol hydrochloride (AUT, 99.2%) was purchased from Dojindo Molecular Technologies, Inc. (Rockville, MD). Acetonitrile (ACN, 99.8%) was obtained from Fisher Scientific (Waltham, MA). 11-Mercaptoundecanoic acid (MUA, 98%) and 3-mercapto-2-methylpropanoic acid (MMPA) were Special Issue: Early Career Authors in Fundamental Colloid and Interface Science Received: September 20, 2017 Revised: November 7, 2017
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DOI: 10.1021/acs.langmuir.7b03309 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir purchased from Santa Cruz Biotechnologies, Inc. (Dallas, TX). Deuterium oxide (D2O, 99.9%) was obtained from Cambridge Isotope Laboratories, Inc. (Tewksbury, MA). All reagents were used as received unless otherwise indicated. NANOpure water (Thermo Scientific, > 18.2 MΩ•cm) was used for all washing, synthesis, and purification protocols as well as in the preparation of all solutions. All stock solutions were aqueous and prepared fresh before each reaction, unless otherwise noted. All glassware was 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 personal protective equipment. Aqua regia should be handled in a f ume hood only. Synthesis of Au Nanoprisms. Au nanoprisms were synthesized according to literature protocols.21,22 Briefly, Au seeds were prepared by adding 0.25 mL of 0.1 M NaBH4 to a rapidly stirring solution containing 9.0 mL of H2O, 0.25 mL of 0.01 M HAuCl4, and 0.25 mL of 0.01 M trisodium citrate. The solution was stirred for 30 s and was allowed to rest undisturbed at room temperature for 2 h to allow degradation of remaining NaBH4. After the aging period, three growth solutions were prepared (referred to as A, B, and C). Here, A was prepared by adding 2.5 mL of 0.01 M HAuCl4, 0.5 mL of 0.1 M NaOH, and 0.5 mL of 0.1 M ascorbic acid to 90.0 mL of 0.05 M CTAB solution that was also 50 μM in NaI. The solution was mixed by hand after addition of each reagent and was optically transparent after all reagents were added. Solutions B and C were prepared in an identical manner, except that the volume of all reagents was decreased 10-fold (for example, the volume of 0.05 M CTAB/0.05 mM NaI solution was decreased from 90.0 to 9.0 mL). Au nanoprisms were synthesized using an iterative seed addition protocol, where growth was initiated by adding 1.0 mL of the seed solution to C. Immediately after seed addition, C was mixed by hand for 2 s (as measured by standard lab timer) and a 1.0 mL aliquot was quickly removed and added to B. After mixing B for 2 s, the entire contents of B was added to A, which was then mixed by hand for 10 s and allowed to react for ∼2 h until nanoprisms growth was complete. Purification of Au Nanoprisms. After addition of the seed solution to the growth solution (∼2 h), the nanoprisms were purified according to previous literature protocols.22 Briefly, the reaction mixture was heated in a water bath to 37 °C for 1 min to dissolve any CTAB that may have recrystallized during the growth period, which can interfere with purification by centrifugation. To purify the prisms from pseudospherical impurities and excess reagents, 90 mL of the reaction mixture was divided into 15 mL conical tubes and centrifuged at 800 rcf for 15 min (Eppendorf centrifuge 5804 with swing bucket rotor A-4−44). After centrifugation, the nanoprisms deposit as a thin film on the walls of the conical tube, so both the supernatant and pellet were removed. The nanoprism film was resuspended in 1.0 mL of water, and this solution was then vortexed (Analogue Vortex Mixer, 120 V, 50/60 Hz, Fisher Scientific) to yield a slightly green, translucent colloid. The prisms were then transferred to 1.5 mL conical vials and washed again by centrifuging at 2200 rcf for 5 min using a Spectrum mini-centrifuge (SC1006 R) to yield a pellet. After removal of the supernatant, the nanoprism pellets were resuspended in 1.0 mL of water and recombined in a 15 mL centrifuge tube. The concentration of nanoprisms in the purified stock solution was determined by UV−vis−NIR spectroscopy, where concentration was measured as the optical density (O.D., a.u.) at the maximum absorption wavelength (λmax of approximately 1260 nm, see the Supporting Information for details pertaining to UV−vis−NIR measurements) of the in-plane dipole LSPR. The solution of purified nanoprisms was then diluted with water to an O.D. of 1.0 a.u. at λmax and were used the same day. Pd Island Deposition on Au Nanoprisms. In order to deposit Pd islands on Au nanoprisms, 1.0 mL of Au nanoprisms (O.D. at λmax = 1.0 a.u.) was added to a 1.5 mL conical vial. Then, 30 μL of 10 mM ascorbic acid was added (for final PdCl2:ascorbic acid molar ratio of 1:5), and the solution was briefly mixed by vortexing for 5 s. Next, 30 μL of 2 mM PdCl2 was added, and the solution was mixed again by vortexing. After allowing 1 h for completion of NP growth, the reaction mixture was purified from excess reagents by centrifugation (5
min at 2200 rcf using a Spectrum mini-centrifuge). The supernatant was removed and the particles were resuspended in 1.0 mL of H2O by brief sonication (∼10 s). Disrupting Pd Nanoisland Linearity by Decreasing CTAB Concentration. In order to obtain disordered Pd nanoisland deposition, Au nanoprisms were purified as described above, except an additional washing step was performed (5 min at 2200 rcf using a Spectrum mini-centrifuge, Scheme S1). The supernatant was removed, and the colloid concentration was adjusted to an O.D. of 1.0 a.u. at λmax. The concentration of CTAB was qualitatively monitored throughout the washing procedure by measuring the volume of the pellet remaining after each washing step; for a pellet volume of 10 μL, each wash constitutes a 1:100 dilution from the original CTAB concentration of 50 mM. After two washes and subsequent dilution to 1.0 O.D. at λmax, the CTAB concentration was estimated to be 5 μM based on dilution values. Immediately after purification, Pd was deposited as described above to yield Au nanoprisms decorated with randomly organized Pd nanoislands. Restoring Pd Nanoisland Linearity by Reintroducing CTAB. Au nanoprisms were synthesized and purified by centrifugation two times as described above (yielding an approximate CTAB concentration
