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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers
Complementary Design for Pairing Between Two Types of Nanoparticles Mediated by a Bispecific Antibody: Bottomup Formation of Porous Materials from Nanoparticles Teppei Niide, Noriyoshi Manabe, Hikaru Nakazawa, Kazuto Akagi, Takamitsu Hattori, Izumi Kumagai, and Mitsuo Umetsu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03687 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019
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Complementary Design for Pairing Between Two
2
Types of Nanoparticles Mediated by a Bispecific
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Antibody: Bottom-up Formation of Porous Materials
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from Nanoparticles
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Teppei Niide, † Noriyoshi Manabe, †, ‡ Hikaru Nakazawa, † Kazuto Akagi, § Takamitsu Hattori, † ,¶
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Izumi Kumagai † and Mitsuo Umetsu *, †
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† Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku
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University, Aoba 6-6-11, Aramaki, Aoba-ku, Sendai 980-8579, Japan
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§ Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku,
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Sendai 980-8577, Japan
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KEYWORDS
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Antibody, Mesoporous material, Nanocomposite, Protein engineering, Thin film
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Recent advances in biotechnology have enabled the generation of antibodies with high affinity
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for the surfaces of specific inorganic materials. Herein, we report the synthesis of functional
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materials from multiple nanomaterials by using a small bispecific antibody recombinantly
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constructed from gold-binding and ZnO-binding antibody fragments. The bispecific antibody
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mediated spontaneous linkage of gold and ZnO nanoparticles to form a binary gold–ZnO
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nanoparticle composite membrane. The relatively low melting point of the gold nanoparticles
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and the solubility of ZnO in dilute acid solution then allowed for the bottom-up synthesis of a
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nanoporous gold membrane by means of a low-energy, low-environmental-load protocol. The
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nanoporous gold membrane showed high catalytic activity for the reduction of p-nitrophenol to
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p-aminophenol by sodium borohydride. Here, we show the potential utility of nanoparticle
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pairing mediated by bispecific antibodies for the bottom-up construction of nanostructured
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materials from multiple nanomaterials.
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INTRODUCTION
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Advances in nanotechnology have enabled the generation of nanomaterials with unique
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optical, magnetic, and electronic properties,1 and some of these nanomaterials can serve as
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nanoscale building blocks for the bottom-up fabrication of organized composite structures with
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functions that are difficult to achieve by means of top-down techniques.2–8 The functions of
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materials assembled from nanomaterials are profoundly affected by the structure of the final
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assembly. Nanomaterials with strictly controlled size and shape self-assemble into defined
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organized structures,9–12 and external factors such as physical force fields or chemical additives
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can be used to facilitate assembly into a desired dimensional geometry by controlling the
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interactions between the nanomaterials.13–16 However, using only the properties of the
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nanomaterials and external factors to control the organization of composite structures assembled
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from multiple nanomaterials has limited utility because specific bonding between pairs of
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nanomaterials, which is necessary for functionalization, is difficult to achieve.
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One method for achieving specific pairing between nanomaterials involves the use of
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biomolecules with high-affinity molecular-recognition ability, such as DNA17–25 and peptide
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aptamers.25–29 Single-strand DNA that is chemically immobilized on nanomaterials mediates
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reversible assembly and patterning via interaction with the complementary strand, and origami
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sequence design enables the creation of artificial nanostructured templates on which DNA-
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immobilized nanoparticles can be assembled.30–32 Peptide aptamers with affinity for material
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surfaces can also specifically bind pairs of materials, and, in theory, they do not need to be
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chemically immobilized on the materials to be bound. Peptide aptamers have been used for
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bottom-up patterning and assembly of proteins and nanomaterials;33,34 however, there are few
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reports on the use of peptide aptamers to link nanoparticles because the binding affinity of such
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peptides is usually insufficient to overcome the electrostatic repulsion between nanomaterials
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and the Brownian motion of nanoparticles.35,36
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Recently, we generated several antibody fragments with high affinity for a specific inorganic
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surface,37–39 and bispecific antibodies fabricated by fusing two different material-binding
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antibody fragments have been used to link two different nanomaterials,39 showing the potential
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utility of such antibody fragments for spontaneous assembly of nanomaterials. Building on our
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previous work, in the present study we developed a bottom-up means of constructing functional
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materials from multiple nanomaterials via material-binding antibodies. Specifically, we used a
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recombinant bispecific molecule constructed from two camel heavy-chain antibodies (VHHs),
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one with a high affinity for the surface of ZnO nanoparticles (4F2 VHH)38 and one with a high
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affinity for the surface of gold nanoparticles (E32 VHH)39, to synthesize plasmonic precipitates
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and nanoporous materials from ZnO and gold nanoparticles (Scheme 1). We chose to produce
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nanoporous gold-based materials because such materials have been used as unsupported
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heterogeneous catalysts and have been shown to have high selective catalytic activity for a wide
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variety of chemical reactions.40–42 Examples of the catalytic applications of nanoporous gold-
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based
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electrochemical oxidation of methanol,45 gas- and liquid-phase O2 reduction,46 and semi-
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hydrogenation of alkynes.47 In addition, these monolithic materials have attracted increased
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attention from the viewpoint of green chemistry because they work under mild conditions (low
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temperature and atmospheric pressure), have high durability and reusability, and are easily
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separated from reaction mixtures
materials
include
gas-phase
CO
oxidation,43
liquid-phase
glucose-oxidation,44
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Nanoporous gold materials are usually fabricated by alloying gold with other metals and then
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dissolving the added metal with concentrated nitric acid,48,49 which has the disadvantages of
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requiring a high energy input and leaving traces of the added metal in the resulting porous
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material. In contrast, the method we describe herein allowed us to generate a nanoporous gold
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membrane from relatively low-melting gold nanoparticles and soluble ZnO nanoparticles in
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dilute acids, which required low energy input and had a low environmental load. The membrane
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showed high catalytic activity for the reduction of p-nitrophenol to p-aminophenol by sodium
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borohydride. Here, we show the potential of using both bispecific antibodies to design
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complementary paired nanomaterials and binary nanomaterial complexes for building composite
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materials from multiple nanomaterials.
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EXPERIMENTAL PROCEDURES
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Expression and Purification of the 4F2-E32 VHH Dimer. To prepare the bispecific
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antibody, we used an expression vector encoding a dimer in which 4F2 VHH was fused to the N-
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terminus of E32 VHH via a llama IgG2 upper hinge-linker; the expression vector has previously
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been used to prepare bispecific antibodies with affinity for gold and ZnO.39 Escherichia coli
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BL21 (DE3) (Life Technologies, Carlsbad, CA, USA) were transformed with the vector, and the
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dimer was expressed and purified as described previously.39 Briefly, 2× YT medium containing
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ampicillin
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thiogalactopyranoside was added to induce the expression of antibody fragments. After
10
centrifugation, harvested cells were resuspended and sonicated to extract the expressed VHH
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dimer. The extracted dimer was purified by means of affinity purification with an anti-FLAG-tag
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resin column (Sigma-Aldrich, St. Louis, MO, USA) followed by size-exclusion chromatography
13
with a HiLoad Superdex 200-pg column 26/600 (GE Healthcare Bio-Sciences, Piscataway, NJ,
14
USA).
was
inoculated
with
transformed
E.
coli
cells,
and
isopropyl-β-d-
15 16
Use of the Bispecific Antibody for Preparation of Gold–ZnO Nanoparticles and
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Nanoparticle Composite Membranes. Gold nanoparticles (20, 50, and 100 nm, Sigma-Aldrich)
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and ZnO nanoparticles (100 nm, HakusuiTech Co., Osaka, Japan) were dispersed in separate 10
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mM aliquots of sodium phosphate solution (pH 7.5); the concentrations of the 20, 50, and 100
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nm gold nanoparticles were 500, 31.6, and 3.9 pM, respectively, and the ZnO nanoparticle
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concentration ranged from 0.56 to 56.0 µg/mL. After sonication, the gold and ZnO nanoparticle
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suspensions were mixed at various ratios in a vial, and then the 4F2-E32 VHH dimer was added
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at a final concentration of 1 µM. The resulting solutions were incubated at 4 °C for 1 day, and
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the precipitates that spontaneously formed in the presence of the VHH dimer were separated
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from the supernatant and dried in a vacuum. To measure the absorption spectra of the
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precipitates, we placed a glass plate at the bottom of a vial and poured the solution containing the
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gold and ZnO nanoparticles into the vial before adding the VHH dimer solution. The absorption
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spectra of the composite membranes that formed on the glass plates were measured by using
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UV–vis spectroscopy (U-3000, Hitachi Science & Technology, Tokyo, Japan). In addition, we
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used a field-emission scanning electron microscope (FE-SEM: S-4800, Hitachi Science &
8
Technology) operated at 15–25 kV to obtain images of the gold–ZnO nanoparticle composite
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membranes.
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The spatial arrangement of nanoparticles in the cross-section of a gold–ZnO nanoparticle
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composite was observed by FE-SEM and transmission electron microscopy (TEM: JEM-2100F,
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JEOL, Tokyo, Japan). To make a cross-sectional sample with ~100 nm thickness, two pieces of
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substrate (3 × 5 mm2 each) were adhered together with epoxy resin (AR-R30, Nichiban Co.,
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Tokyo, Japan) and incubated at room temperature for 24 h until the epoxy resin had cured. The
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assembled substrate was then polished by using an ion milling system (PIPS Model 691, Gatan,
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Pleasanton, CA).
17 18
Preparation of nanoporous gold membranes. Nanoporous gold membranes were prepared
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from the gold–ZnO nanoparticle composite membranes obtained by using the VHH dimer.
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Specifically, gold–ZnO nanoparticle membranes deposited on a glass plate were sintered at
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various temperatures ranging from 200 to 400 °C for 36 h and were then soaked in a 100 mM
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glycine–HCl solution (pH 3.5) at room temperature for 10 h to remove the Zn. The plates were
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then washed three times with water and dried in a vacuum. The membranes that formed on the
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plates were analyzed by means of UV–vis spectroscopy and FE-SEM. To prepare materials for
2
comparison, we concentrated a gold nanoparticle suspension 10-fold (to 5 nM) by means of
3
centrifugation and then dried the suspension on a glass plate in the presence or absence of ZnO
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nanoparticles (5.6 µg/mL).
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Catalytic reduction of p-nitrophenol in the presence of nanoporous gold membrane.
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Phosphate buffer solutions of 20 nm gold nanoparticles and ZnO nanoparticles were mixed in a
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vial at a ZnO/gold volume ratio of 1/1, and then the 4F2-E32 VHH dimer (1 µM in the same
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buffer) was added to the mixture to precipitate the gold–ZnO nanoparticles. After the supernatant
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was removed, the precipitates were dried in a vacuum and then subjected to sintering and
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treatment with the glycine–HCl buffer in the vial, as described above. To prepare materials for
12
comparison, we concentrated a gold nanoparticle suspension 10-fold (to 5 nM) by means of
13
centrifugation and then dried the suspension in a vial in the presence or absence of ZnO
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nanoparticles (5.6 µg/mL). The dried precipitates were also sintered and treated in the vial with
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the glycine–HCl buffer. To each of the vials containing dried precipitates, we added 2.5 mL of a
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solution containing 100 mM NaBH4 and 1 mM p-nitrophenol. Every 10 min, 10 µL of the
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reaction solution was removed and diluted by 1/10, and the absorbance of p-nitrophenol at 400
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nm was measured by means of UV–vis spectroscopy (U-3000).
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RESULTS
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Spontaneous assembly of gold–ZnO nanoparticles. When phosphate buffer solutions of 20
3
nm gold nanoparticles and approximately 100 nm ZnO nanoparticles were mixed at an equal
4
volume ratio and allowed to incubate for 1 day at 4 °C, no precipitate formed (Figure 1a-I) and
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the supernatant showed plasmon absorption (Figure 1b-I). However, when the 4F2-E32 VHH
6
dimer, which was constructed from a ZnO-binding VHH fused to the N-terminus of a gold-
7
binding VHH via a rigid peptide linker, was added to the mixture of the two types of
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nanoparticles, a red precipitate spontaneously formed (Figure 1a-II) and the supernatant showed
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no plasmon absorption (Figure 1b-II). Aggregation of gold nanoparticles generally leads to a red-
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shift of the plasmon absorption to purple and gray. However, the precipitate that formed in the
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presence of the 4F2-E32 VHH dimer had red plasmon absorption, implying that the dimer linked
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the gold and ZnO nanoparticles and that the linkages inhibited aggregation of the gold
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nanoparticles.
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Next, to study the influence of the ZnO/gold volume ratio on plasmon adsorption, we varied
15
the concentration of ZnO nanoparticles in the mixture. After the mixtures were dried on glass
16
plates, the resulting precipitates showed plasmon adsorption regardless of the ZnO/gold ratio
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(Figure 1c-row1 and 1d; i–v). However, even when there was no ZnO in the mixture, a gold
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nanoparticle precipitate formed in the presence of the 4F2-E32 VHH dimer (Figure 1c-row1 and
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1d; 0), suggesting that 4F2 VHH (ZnO-binding VHH) in the dimer bound non-specifically to the
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bare gold nanoparticle surface. This phenomenon is most likely the result of non-specific
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interactions between bare nanoparticles and proteins, leading to the formation of a protein corona
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on the surface of the gold nanoparticles.50,51 Although non-specific linking between the gold
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nanoparticles was induced by the 4F2-E32 VHH dimer, the precipitate color and UV–vis spectra
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varied depending on the ZnO/gold ratio. The extinction spectra derived from the gold
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nanoparticles sharpened and were blue-shifted with increasing ZnO/gold ratio up to a ratio of 1/1
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(Figure 1d: 0–iii). Whereas, further increases in the ratio broadened the band again, as well as
4
increasing the height of the baseline; however, the extinction spectra was observed around 520
5
nm (Figure 1d: iv, v). Considering that increasing the amount of ZnO likely led to interruption of
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the transmission of incident light due to light scattering, it is possible that the plasmon absorption
7
band was also similarly obscured. According to these results, the ZnO might have prevented the
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aggregation of the gold nanoparticles in the precipitates by specifically linking with gold
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nanoparticle via the 4F2-E32 VHH dimer. When a 10 times concentrated (5 nM) gold
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nanoparticle suspension without 4F2-E32 VHH dimer, which had obvious plasmon absorption
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(Figure S1 in the Supporting Information), was dried on a glass plate, an uneven precipitate with
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no plasmon absorption formed (Figure 1c-row1 and 1d; vi); loss of plasmon absorption was also
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observed upon drying of a precipitate prepared from a concentrated gold nanoparticle suspension
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that contained ZnO nanoparticles (Figure 1c-row 1 and 1d; vii). These results indicated that the
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4F2-E32 VHH dimer induced assembly of gold–ZnO complexes with little loss of plasmon
16
resonance.
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Gold–ZnO nanoparticle precipitates were also prepared by using gold nanoparticles with
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diameters of 50 and 100 nm. Again, precipitates formed spontaneously in the presence of the
19
VHH dimer (Figure 1c-row 2 and -row 3; i–v); and the precipitates (Figure 1e and 1f), but not
20
the supernatants (Figure S2a and S2b in the Supporting Information), showed plasmon
21
absorption. The plasmon absorption bands of the precipitates on glass plates were sharper when
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the ZnO/gold ratio was 0.5/1 and 1/1 than when other ratios were used (Figure 1e and 1f; ii and
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iii), which was consistent with the results obtained when 20 nm gold nanoparticles were used.
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Thus, controlling the ZnO/gold ratio was crucial for maintaining plasmon resonance in the gold–
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ZnO nanoparticle precipitates.
3 4
Distribution of gold nanoparticles in the gold–ZnO nanoparticle composite membranes.
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Scanning electron microscopy (SEM) was used to investigate the distribution of gold
6
nanoparticles within the nanoparticle precipitates that formed in the presence of the VHH dimer
7
(Figure 2). At ZnO/gold ratios of 0.5/1 and 1/1, the 20 nm gold nanoparticles were
8
homogenously dispersed throughout the precipitates (Figure 2-row1; ii and iii). These results are
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consistent with the UV–vis spectra results shown in Figure 1d. Although in many parts of the
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SEM images, the ZnO nanoparticles in the precipitate could not be observed clearly because of
11
the low contrast, the nanoparticles were visible in some of the more translucent areas of the
12
precipitate (Figure 2-row1; iv and v). We also found that 50 and 100 nm gold nanoparticles were
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homogenously dispersed in the precipitates at a ZnO/gold ratio of 1/1 (Figure 2-row2 and -row3;
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iii). However, at ZnO/gold ratios exceeding 1/1, no plasmon resonance was observed for the
15
precipitates (Figure 1e and 1f), even though the gold nanoparticles were not aggregated, as seen
16
in the SEM images (Figure 2-row2 and -row3; iv and v). Considering that increasing the amount
17
of ZnO likely led to interruption of the transmission of incident light due to light scattering,52–54
18
it is possible that the plasmon absorption band was also similarly obscured (Figure 1e and f; iv
19
and v).
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To examine the interior of the nanoparticle precipitate, we prepared a partial cross-section
21
sample (thickness, ~100 nm) of gold–ZnO nanoparticle precipitate prepared with 20 nm gold
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nanoparticles at a ZnO/gold ratio of 1/1, which was then observed by means of SEM,
23
transmission electron microscopy (TEM), and energy-dispersive X-ray (EDX) mapping (Figure
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3). The cross-section SEM image showed that the ZnO and gold nanoparticles were widely
2
distributed throughout the precipitate (Figure 3a). The cross-section TEM image and EDX
3
mapping image showed that the precipitate contained both gold and ZnO nanoparticles and that
4
the two types of nanoparticles were bound in a complex (Figure 3b–d). Thus, we confirmed the
5
formation of a composite membrane derived from ZnO and gold nanoparticles.
6 7
Formation of nanoporous gold membranes. Next, we used gold–ZnO nanoparticle
8
composite membranes prepared with 20, 50, or 100 nm gold nanoparticles at a ZnO/gold ratio of
9
1/1 to produce nanoporous gold membranes by sintering the gold nanoparticles and dissolving
10
the ZnO with dilute acid. The melting point of bulk gold is 1064 °C, but nanoscale gold has a
11
lower melting point;55,56 and ZnO materials can be dissolved in dilute acid solution.57 First, we
12
heated the gold–ZnO membranes at various temperatures ranging from 200 to 400 °C to sinter
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the gold nanoparticles, and we then dissolved the ZnO component in a glycine–HCl buffer
14
solution (pH 3.5). Figure 4 shows scanning electron micrographs and the elemental compositions
15
of the resulting materials.
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For the membranes prepared with 20 nm gold nanoparticles, welding of the gold nanoparticles
17
was observed when the temperature exceeded 250 °C (Figure 4a-row1). Therefore, heating at
18
250 °C led to the formation of a nanoporous structure with a pore size of approximately 10 nm
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(Figure 4b-i), but heating at higher temperatures led to the formation of larger vacancies (Figure
20
4a-row1). However, when the gold precipitate that formed at a ZnO/gold ratio of 0/1 in the
21
presence of 4F2-E32 VHH dimer was heated at 250 °C, SEM analysis revealed a heterogeneous
22
three-dimensional nanoporous structure (Figure S3a in the Supporting Information) that was
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present even before sintering (Figure S3b in the Supporting Information), suggesting that 4F2-
2
E32 VHH dimer prevented excessive welding between gold nanoparticles in the precipitate.
3
For the membranes prepared with 50 and 100 nm gold nanoparticles, welding of the gold
4
nanoparticles was observed when the temperature exceeded 300 °C; in both cases, larger
5
vacancies were formed compared with when 20 nm gold nanoparticles were used (Figure 4a-
6
row2 and -row3). These results indicate that the pore size of the nanoporous structure can be
7
regulated by altering the size of the gold nanoparticles and the sintering temperature. That is, to
8
produce a porous network structure with a high surface area, small gold nanoparticles and low-
9
temperature sintering should be used. To examine the elemental composition of the nanoporous
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gold membrane, we used EDX spectrometry (Figure 4b-ii). The spectrum of the membrane
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formed at 250 °C from the 20 nm gold nanoparticles showed no ZnO signal, indicating that the
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material was composed entirely of gold.
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Catalytic performance of the nanoporous gold membranes. To evaluate the catalytic
15
activity of the nanoporous gold membrane, we measured the kinetics of the reduction of p-
16
nitrophenol to p-aminophenol by sodium borohydride (NaBH4). p-Nitrophenol has an intense
17
absorbance at 400 nm, and reduction to p-aminophenol leads to a blue-shift of the absorbance (to
18
300 nm), along with a decrease in its intensity. As control materials, we prepared a suspension
19
containing the same amount of 20 nm gold nanoparticles as was used for producing the
20
nanoporous membrane. The suspension was then dried with or without ZnO nanoparticles in the
21
absence of the VHH dimer, and the resulting residues were sintered at 250 °C and treated with
22
glycine–HCl buffer.
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We analyzed the time dependence of the absorption spectrum of p-nitrophenol in the presence
2
of NaBH4 and the nanoporous gold membrane or one of the control materials (Figure 5). The
3
control material prepared from gold nanoparticles alone and that prepared from gold and ZnO
4
nanoparticles without the VHH dimer produced apparent reaction rates (kapp) of 0.080 × 10−3 and
5
0.227 × 10−3 s−1, respectively (Figure 5a, 5b, 5d-3, and 5d-4). In contrast, the nanoporous gold
6
membrane formed from the gold and ZnO nanoparticles in the presence of the VHH dimer
7
markedly enhanced the reaction rate (Figure 5c and 5d-6); specifically, kapp was 9 times that for
8
the material prepared from gold nanoparticles alone and 3 times that for the material prepared
9
from gold and ZnO nanoparticles without the VHH dimer (0.743 × 10−3 s−1). SEM showed that
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the control material prepared from gold nanoparticles alone and that prepared from ZnO and gold
11
nanoparticles without the VHH dimer had no nanopores and relatively large nanopores,
12
respectively (Figure S4a and S4b in the Supporting Information), implying that the nanoporous
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gold membrane formed in the presence of the VHH dimer had a larger surface area than did the
14
control materials.
15
We also measured the catalytic activity of a gold material prepared by simply drying gold
16
nanoparticles without subsequent sintering or treatment with glycine–HCl buffer, and we found
17
that the apparent reaction rate (0.245 × 10−3 s−1) was comparable with that for the control
18
material prepared from ZnO and gold nanoparticles without VHH dimer (Figure 5d-2 and 5d-4).
19
Because the gold nanoparticles retained their original shape in the material formed by simply
20
drying the nanoparticles (Figure S4c in the Supporting Information), the material showed high
21
catalytic activity. The reasons why the material prepared from gold nanoparticles alone did not
22
show the highest catalytic activity include the possible influence of molecules present on the
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gold surface58–60 as well as differences in the crystal planes61 and surface area compared with the
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other materials.
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We also evaluated the catalytic activity of a nanoporous gold material prepared from the
4
precipitate that formed without ZnO nanoparticles in the presence of 4F2-E32 VHH dimer and
5
found that this material had a kapp value of 0.487 × 10−3 s−1 (Figure 5d-5), indicating that it had a
6
larger surface area and catalytic interface than the other control materials. The kapp value,
7
however, was approximately 2/3 times that for the nanoporous gold prepared from gold and ZnO
8
nanoparticles with the VHH dimer (0.743 × 10−3 s−1). This finding is likely the result of the gold
9
nanoparticles remaining separated from each other by ZnO nanoparticles during sintering, which
10
prevented the formation of large aggregates of the gold and increased the number of catalytically
11
active sites on the material surface. Thus we concluded that the nanoporous gold prepared from
12
the gold and ZnO nanoparticles in the presence of the VHH dimer offered high surface area led
13
to showing the high catalytic activity.
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DISCUSSION
2
Pairing of nanomaterials by use of an interface molecule designed from complementary
3
antibody fragments. The bottom-up formation of superstructures from nanomaterials results in
4
functions that are difficult to obtain by means of top-down fabrication techniques. Van der Waals
5
and coulomb forces between nanomaterials are critical driving forces for their assembly, and
6
homogeneity in their size, shape, and surface properties leads to uniform interactions between
7
the materials and thus to the formation of well-ordered assemblies. However, only a limited
8
number of homogeneous nanomaterials have been synthesized, and intentional linking of two
9
specific nanomaterials has rarely been achieved, even with nanomaterials having strictly
10
controlled structures. Conjugation of organic molecules to the surface of nanomaterials can
11
homogenize their surface properties to induce uniform interactions between nanomaterials, and
12
conjugation between nanomaterials and molecules with recognition ability, such as DNA 17–24,62–
13
64
14
for molecules with recognition ability to be covalently conjugated to the nanomaterials, as well
15
as the conjugation process itself, can restrict the types of nanomaterials that can be used.
and proteins,65–67 can lead to interactions between specific nanomaterials. However, the need
16
In the present study, we used a small bispecific antibody comprising two material-binding
17
antibody fragments, E32 VHH and 4F2 VHH, which strongly bind to gold and ZnO surfaces,
18
respectively, without the need for surface modification. The strong affinity of the VHHs for their
19
respective nanomaterials resulted in spontaneous assembly and precipitation of gold–ZnO
20
nanoparticles, which could be deposited as thin films that exhibited plasmon absorption. A wide
21
variety of peptides with binding affinity for a specific material surface have been identified
22
through large-scale selection based on phage and cell surface display techniques, and material-
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binding antibodies with high affinity can be generated from the peptides by means of our
2
CAnIGET method.38
3
Although the mechanisms of the binding of these antibodies to their respective material
4
surfaces are not clearly understood, the van’t Hoff plots in our previous reports suggest that
5
enthalpy change, not entropy change, is the major underlying factor in this binding.37,38 Indeed,
6
an important factor in the adsorption of proteins to material surfaces is usually the entropy
7
change that results from dehydration and desorption of the capping agents on the material surface
8
and the structural rearrangement of the adsorbed proteins.68,69 Therefore, we hypothesized that
9
the antibodies examined in the present study were held on the nanoparticle surface via selective
10
attachment of their complementary-determining region loop. When we used denaturing reagents
11
to detach the VHHs from the nanoparticles, 10 mM glycine–HCl buffer (pH 2.0) and 50 mM
12
NaOH caused complete dissolution of the ZnO–gold precipitates, resulting in a nanoparticle
13
dispersion with an extinction spectra at 520 nm (Figure S5 in the Supporting Information). In
14
contrast, adding distilled water or 1 M NaCl resulted in no dissolution of the precipitates. This
15
re-dispersion of the gold nanoparticles suggests that glycine–HCl buffer and NaOH solution
16
denatured the VHHs, which supports the idea that the ZnO and gold nanoparticles were linked
17
via 4F2-E32 VHH dimer.
18
With the setup used in the present study, we were able to harvest 4F2-E32 VHH dimer at an
19
average yield of 1.5 mg/L-culture of BL21 (DE3) strain, and 33.3 µg of dimer in 1 mL of
20
reaction suspension was needed to make a ZnO–gold membrane. Therefore, 4F2-E32 VHH
21
dimer harvested from 1 L of culture can be used to process 45 mL of reaction suspension.
22
Although our approach using bispecific material-binding antibodies is expensive in terms of
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labor and time compared with conventional chemical and physical approaches, the antibody
2
fragments produced allow for the production of a wide array of superstructures.
3 4
Bottom-up formation of nanoporous gold materials. Porous materials with a large surface
5
area are critically important for various applications, including catalysis, filtration, trapping, and
6
sensing. Such materials can be formed by assembling atoms or nanomaterials into a desired
7
porous structure, or by supporting functional materials on a porous scaffold substrate. Dealloying
8
is a useful process for synthesizing highly functional porous gold materials.70 Specifically, gold
9
materials are alloyed with Ag, Al, or Cu metal, and the alloys are then soaked in concentrated
10
nitric acid to remove the added metal.48,49 However, because high temperature (>1000 °C) is
11
required for alloy formation, this is a high-energy process; in addition, traces of the metal
12
additive remain in the resulting porous materials. Gold nanoparticles have also been used for
13
synthesizing porous materials by means of a process involving deposition of the nanoparticles on
14
templates consisting of an organic colloidal crystal assembly.71,72 Heating at a relatively low
15
temperature (~300 °C) removes the organic template and sinters the gold nanoparticles to form a
16
macroporous structure. In the present study, we used gold nanoparticles, which can be melted at
17
a relatively low temperature (250–300 °C), and ZnO nanoparticles, which can be dissociated by
18
dilute acid; therefore, neither high-energy alloy formation nor the use of a concentrated acidic
19
solution was required. Furthermore, sintering of the gold nanoparticles and removal of the ZnO
20
led to the formation of a membrane with nanopores as small as those in the finest porous
21
structures reported previously; consequently, the membrane had high catalytic activity. We
22
expect that the nanoporous gold prepared in the present study can catalyze other chemical
23
reactions. For example, Wen et al. recently reported a gold aerogel prepared via dopamine-
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induced destabilization of beta-cyclodextrin-functionalized gold nanoparticles and showed that
2
the aerogel catalyzed electrochemical oxidation of glucose and ethanol.73
3
Nanoporous gold-based materials have been extensively studied because of their catalytic
4
activities and high surface-to-volume ratios. Our present study contributes information useful for
5
tuning of the thickness and morphologies of nanoporous gold films without the introduction of
6
impurities. Compared with nanoporous gold without impurities, nanoporous gold prepared by
7
dealloying usually has higher chemical reactivity because metal atoms remaining from the
8
dealloying process accelerate catalytic reaction rates by suppressing surface reconstruction
9
dynamics and providing active sites for the reaction.61 Therefore, to maximize the catalytic
10
activity of nanoporous gold without impurities, further reduction of nanoparticle size and the
11
introduction of high-index facets should be explored.
12
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CONCLUSIONS
2
In summary, we demonstrated pairing between two different types of nanoparticles mediated
3
by a bispecific material-binding antibody to form binary nanoparticles without the need for
4
surface modification. The binary nanoparticles showed plasmon resonance when assembled into
5
nanoparticle composite membranes. Sintering of the membranes and subsequent treatment with
6
glycine–HCl buffer led to the formation of a nanoporous gold membrane with high catalytic
7
activity for the reduction of p-nitrophenol by sodium hydride. Our results illustrate the utility of
8
high-affinity material-binding antibodies for the design of tools for mediating the pairing of
9
nanomaterials and subsequent formation of nanostructured hybrid materials.
10 11 12
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Scheme 1. Synthesis of a nanoporous gold membrane from gold–ZnO nanoparticles via a
3
bispecific metal surface-binding antibody.
4
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Figure 1. Precipitation of gold nanoparticles. (a) Photographs of phosphate buffer solutions of
3
gold nanoparticles (20 nm, 500 pM) and ZnO nanoparticles (~100 nm, 5.6 µg/mL) at a ZnO/gold
4
volume ratio of 1/1 after incubation for 1 day at 4 °C in the (I) absence or (II) presence of 1 µM
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4F2-E32 VHH dimer. (b) Absorption spectra of (I) the gold and ZnO suspension after incubation
2
in the absence of the dimer and (II) the supernatant obtained after incubation in the presence of
3
the dimer. (c) Photographs of gold–ZnO precipitates formed on glass plates from gold
4
nanoparticles with diameters of 20, 50, and 100 nm and ZnO/gold volume ratios ranging from
5
0.1/1 to 10/1. (d–f) Absorption spectra of gold–ZnO precipitates formed from gold nanoparticles
6
with diameters of 20 nm (d), 50 nm (e), and 100 nm (f).
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Figure 2. SEM images of gold–ZnO nanoparticle precipitates formed from gold nanoparticles
3
with diameters of 20, 50, and 100 nm at ZnO/gold volume ratios ranging from 0.1/1 to 10/1. The
4
images were obtained at acceleration voltages of 15–25 kV. Scale bars are 500 nm.
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Figure 3. Cross-section micrographs of a gold–ZnO precipitate formed from 20 nm gold
3
nanoparticles. (a) SEM image, (b) TEM image, (c) magnified TEM image of the red-dashed
4
square in (b), and (d) EDX mapping image of the area shown in (b).
5
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1 2
Figure 4. Structural analysis of the gold–ZnO membranes after heating and dissolving
3
treatments. (a) Scanning electron micrographs of membranes obtained after sintering the gold–
4
ZnO nanoparticle precipitates (prepared at a ZnO/gold ratio of 1/1) at various temperatures from
5
200 to 400 °C and then dissolving the ZnO in glycine–HCl buffer. All scale bars are 1 µm. (b)
6
Scanning electron micrographs (i) and EDX spectrum (ii) of the gold–ZnO membrane obtained
7
after sintering gold–ZnO precipitate fabricated with 20 nm gold nanoparticles at 250 °C and
8
dissolving the ZnO.
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Figure 5. Temporal dependence of absorption spectra of a reaction mixture containing p-
3
nitrophenol, NaBH4, and (a) a control material prepared from gold nanoparticles, (b) a control
4
material prepared from gold and ZnO nanoparticles, and (c) a nanoporous gold membrane
5
formed from gold and ZnO nanoparticles in the presence of the 4F2-E32 VHH dimer; spectra are
6
shown at 10 min intervals. (d) Reaction time dependence of the log of absorbance at 400 nm: in
7
the absence of gold (1, black) and in the presence of a gold material prepared from gold
8
nanoparticles without sintering or acid treatment (2, orange), a gold material prepared from gold
9
nanoparticles with sintering and acid treatment (3, blue), a gold material prepared from gold and
10
ZnO nanoparticles (4, green), a gold material prepared with sintering and acid treatment from the
11
precipitate produced from gold nanoparticles in the presence of 4F2-E32 VHH dimer (5, purple),
12
and a nanoporous gold membrane prepared with sintering and acid treatment from gold and ZnO
13
nanoparticles in the presence of the 4F2-E32 VHH dimer (6, red). The catalytic reduction
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reactions were carried out with 100 mM NaBH4, 1 mM p-nitrophenol, and ~122 µg of gold
2
catalyst (5 mL, prepared from 500 pmol/L 20 nm gold nanoparticles). The reaction solutions
3
were diluted by 1/10 prior to measurement of the absorption spectra.
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ASSOCIATED CONTENT
2
Photographs and UV-vis spectra of phosphate buffer solutions containing gold nanoparticles
3
(diameters: 50 nm and 100 nm) and ZnO in the presence of 4F2-E32 VHH dimer; Photographs
4
of a concentrated colloidal suspension of 20 nm gold nanoparticles before and after drying; SEM
5
images of gold structures formed in the absence of 4F2-E32 VHH dimer
6
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1
AUTHOR INFORMATION
2
Corresponding Author
3
*E-mail:
[email protected] 4
Present Addresses
5
‡ Faculty of Pharmaceutical Sciences, Tohoku Medical and Pharmaceutical University, 4-4-1
6
Komatsushima, Aoba-ku, Sendai 981-8558, Japan
7
¶ Department of Biochemistry and Molecular Pharmacology, New York University School of
8
Medicine, New York, NY 10016, United States
9
Notes
10
Page 30 of 43
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
2
The authors thank Mr. Shunsuke Kayamori and Dr. Takamichi Miyazaki for his excellent
3
technical support for the preparation of the samples used to produce the cross-section
4
micrographs and in recording the electron micrographs. This work was supported by a Scientific
5
Research Grant from the Ministry of Education, Culture, Sports, Science and Technology of
6
Japan (to I.K.; 24000011, M.U.; 16H04570, 16K14483, H.N.; 16K18296, and T.N.; 18K14059)
7
and by the Precursory Research for Embryonic Science and Technology program of the Japan
8
Science and Technology Agency (to M.U.).
9
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