Surfactant-Free Vapor-Phase Synthesis of Single-Crystalline Gold

Article Cite This: Chem. Mater. 2017, 29, 8747-8756

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Surfactant-Free Vapor-Phase Synthesis of Single-Crystalline Gold Nanoplates for Optimally Bioactive Surfaces Youngdong Yoo,†,‡,# Hyoban Lee,†,# Hyunsoo Lee,§,∥ Miyeon Lee,† Siyeong Yang,† Ahreum Hwang,†,⊥ Si-in Kim,† Jeong Young Park,§,∥ Jaebum Choo,∇ Taejoon Kang,*,⊥,¶,○ and Bongsoo Kim*,† †

Department of Chemistry, KAIST, Daejeon 34141, Korea Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States § Center for Nanomaterials and Chemical Reactions, IBS, Daejeon 34141, Korea ∥ Graduate School of EEWS, KAIST, Daejeon 34141, Korea ⊥ Hazards Monitoring Bionano Research Center, KRIBB, Daejeon 34141, Korea ∇ Department of Bionano Engineering, Hanyang University, Ansan 15588, Korea ¶ BioNano Health Guard Research Center, KRIBB, Daejeon 34141, Korea ○ Deparment of Nanobiotechnology, KRIBB School of Biotechnology, UST, Daejeon 34113, Korea ‡

S Supporting Information *

ABSTRACT: We report the surfactant-free vapor-phase synthesis of atomically flat and ultraclean gold nanoplates. These gold nanoplates can offer optimally functional surfaces through the immobilization of molecules without unspecific adsorption and defect, which could be quite valuable for diverse applications including biomedical sensing, plasmonics, molecular electronics, electrochemistry, etc. The ultraflat, ultraclean, and single-crystalline nanostructures, including gold nanoparticles (NPs), gold nanowires (NWs), gold nanobelts, and gold nanoplates, are stereoepitaxially grown on a substrate with a specific orientation. Moreover, the nanostructures can be selectively synthesized by experimental conditions and locations of the substrate. The geometry and orientation of the nanostructures show strong correlations, suggesting a growth process of seed NPs → NWs → nanobelts → nanoplates. This synthetic process can be explained by the mechanism in which the height-to-width ratio of gold nanostructures is determined by the ratio of the atom-supply rate by direct impingement to the atom-supply rate by surface diffusion. We finely tuned the shapes (NPs, NWs, nanobelts, or nanoplates) and sizes (from several micrometers to hundreds of micrometers) of the gold nanostructures by adjusting the deposition flux. Crucially, the surfactant-free and atomically flat gold nanoplates could be optimally bioactive surfaces. We substantially decreased the nonspecific binding of avidin by immobilizing the biotinylated molecules onto the gold nanoplates. Compared with thermally deposited gold films, the single-crystalline gold nanoplates showed a 100 times lower detection limit in the recognition of the biotin−avidin interaction. We anticipate that the gold nanoplates will bring us one-step closer to the realization of ideal biomolecular sensors because the several bioactive gold surfaces can be prepared by immobilizing various biological molecules onto the gold nanoplates.

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elimination of defects remains difficult.34−36 Solution-phase synthesis of Au nanostructures has been widely explored, and several seed-mediated or seedless methods have been reported.37−56 These methods can produce Au nanostructures with atomically smooth surfaces; however, the use of toxic surfactants such as cetyltrimethylammonium bromide interrupts the formation of ultraclean Au surfaces.37−45,57 Recently, various synthetic strategies that do not involve surfactants have been developed;57 however, fine control of the morphology and size of the resulting Au nanostructures remains a challenge. For the preparation of ultraclean and ultraflat Au nanostructures, vapor-phase synthesis, rather than solution-

INTRODUCTION Atomically flat and ultraclean Au nanoplate is regarded as one of the best materials for bioactive surfaces because functional biomolecules can be immobilized on the Au nanoplate without unspecific adsorption and defect. The optimally bioactive Au surfaces can maximize the functionality of immobilized molecules and thus realize ideal high-performance devices for biomedical sensing,1−11 molecular electronics,12,13 plasmonics,14−22 electrochemistry,23−25 etc. Therefore, the synthesis of ultraclean and ultraflat Au nanostructures is a valuable area of research. Conventional Au nanostructures prepared by sputtering, electron-beam deposition, and electrodeposition26−29 have unavoidable surface defects.30−33 Although thermal annealing, UV-ozone cleaning, and hydroxyl-radical etching can substantially decrease the surface defects of Au structures, the complete © 2017 American Chemical Society

Received: July 12, 2017 Revised: August 30, 2017 Published: September 6, 2017 8747

DOI: 10.1021/acs.chemmater.7b02932 Chem. Mater. 2017, 29, 8747−8756

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Chemistry of Materials phase synthesis, is most suitable. Although vapor-phase synthesis is widely used for growing serveral nanostructures, only a few Au nanostructures have been synthesized in the vapor phase. Chen et al. have reported the vapor-phase preparation of Au nanocrystals by chloroauric acid pyrolysis.58 Ertorer et al. have immobilized Au nanoparticles (NPs) onto a glass surface by chemical vapor deposition.59 Cho et al. have deposited Au nanostructures onto reduced graphene oxide in the vapor phase.60 Griffiths et al. have synthesized Au nanoplates through thermal decomposition of precursors, which act as surfactants.61 Our group has also reported the synthesis of single-crystalline Au nanowires (NWs) in the vapor phase.62 These methods have successfully expanded the use of vapor-phase synthesis to Au nanostructures. Room exists, however, for the development of a morphology-controllable synthetic method to generate ultraclean and ultraflat Au nanostructures. Here, we report the surfactant-free vapor-phase synthesis of Au nanostructures to produce optimally bioactive surfaces. Perfect crystalline Au nanostructures are stereoepitaxially grown on a sapphire substrate by using an Au lump in an electric furnace. The resultant Au nanostructures are atomically flat and ultraclean because they are synthesized without any surfactants or surface-contaminating process. In addition, we were able to selectively synthesize Au NPs, NWs, nanobelts, and nanoplates by subtly tuning the atomic flux. More importantly, we demonstrate that an ultraclean and ultraflat Au nanoplate offers an optimally bioactive surface. Surfaceenhanced Raman scattering (SERS) sensors fabricated with Au nanoplates have achieved a 100 times lower detection limit than SERS sensors fabricated with conventional Au films through extreme reduction of nonspecific bindings. The optimally bioactive Au surfaces can be constructed by immobilizing functional biomolecules onto the gold nanoplates, enabling the realization of high-performance biomolecular sensors. Furthermore, the stereoarchitectured single-crystalline Au nanostructure arrays on the substrates would be highly attractive for applications in diverse fields such as nanoelectrochemistry, catalysis, and optoelectronics.

Figure 1. 45° tilted-view SEM images of Au nanoplate arrays on (a) acut, (b) m-cut, and (c) r-cut sapphire substrates. Au nanoplate arrays were epitaxially grown on the substrates with stereoaligned orientations.

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They also exhibit hexagonal, triangular, and parallelogrammic shapes, depending on the basal planes of the sapphire substrates. Interestingly, large Au nanoplates were synthesized on a-cut sapphire substrates while relatively small Au nanoplates were formed on m-cut and r-cut sapphire substrates. As the nanoplates grow, they become larger and thicker. Figure 1a shows that the Au nanoplates grew to well over 100 μm on the a-cut sapphire substrate. This size is much larger than the size of typical Au nanoplates prepared in a solution phase. In addition, some thin and small Au nanoplates could be obtained on r-cut sapphire substrates (Figure S1 in Supporting Information). The size of Au nanoplates can be controlled by the substrates and the reaction time. The surface topography of as-synthesized Au nanoplates was analyzed through high-resolution atomic force microscopy (AFM). For AFM measurements, the Au nanoplates were transferred onto a precleaned Si substrate by using a simple dry-transfer method.78 Figure 2a shows an AFM topographic image of an Au nanoplate. The surface-height variation of the Au nanoplate measured along the dotted cyan line in Figure 2a is less than 0.4 nm, with a 0.15 nm root-mean-square roughness (Rq) (Figure 2b). Given that the atomic radius of Au is approximately 0.14 nm, these experimental values are notable. The magnified topographic image of the Au nanoplate also shows the surface-height variation of 0.2 nm with 0.1 nm of Rq

RESULTS AND DISCUSSION Surfactant-free and atomically flat Au nanoplates are free of the side effects caused by surfactants in in vitro and in vivo biomedical applications.63−67 Additionally, ultraclean Au nanoplates enable the long-distance propagation of surface plasmons, patterned growth of graphene nanoribbons, and formation of well-defined oxide/metal interfaces and thus may be utilized for nanoantennas, graphene-based nanodevices, and inverse oxide/metal catalysts.68−77 Therefore, Au nanoplates have been a desirable synthetic target. We synthesized single-crystalline Au nanoplates on sapphire substrates through a simple vapor-transport method using an Au lump as a precursor. A horizontal-hot-wall single-zone furnace system was used, and the system was purged with Ar gas for 30 min before the reaction to maintain an inert atmosphere. The Au lump, placed in an alumina boat at the center of the heating zone, was heated to 1200 °C, and the Au vapor was transported to the sapphire substrate, where Au nanoplates were grown. The temperature of the substrates was maintained at 1000 °C during the reaction. Figure 1 shows scanning electron microscopy (SEM) images of Au nanoplates on a-cut, m-cut, and r-cut sapphire substrates. The Au nanoplates are epitaxially aligned and have well-defined facets. 8748


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Chemistry of Materials

synthetic method may be an excellent alternative to existing solution-phase methods for the preparation of ultraclean, ultraflat, and single-crystalline Au nanoplates. Because our vapor-transport method uses only an Au lump as a precursor, it allows Au nanoplates to be synthesized without any surfactants or other contamination from solution and it substantially reduces the surface roughness (Rq = 0.15 nm). In addition, the large-scale ultraflatness of the Au nanoplates was measured over 13 μm, a notably long distance. Intriguingly, we found that the shape of the Au nanostructures depended on the location of the substrate. Figure 3a shows the distribution of Au nanostructures on a single m-cut sapphire substrate. Au nanoplates possessing side lengths of 2−2.5 μm and thickness of 50−150 nm are dominantly formed on the front part of the substrate. Au nanobelts, NWs, and NPs are sequentially formed on the rear part of the substrate. Figure 3b−e shows SEM images of Au nanoplates, nanobelts, NWs, and NPs on an m-cut substrate. Four types of Au nanostructure array were simultaneously grown on a single substrate, and the shape of the nanostructures was determined on the basis of their location on the substrate. In these experiments, Au vapor started to condense and deposit onto the front part of the substrate. Therefore, the amount of Au vapor contained in the Ar gas gradually decreased as the Ar gas flowed over the substrate. This finding led us to hypothesize that the Au-nanostructure growth strongly depends on the deposition flux. To further confirm the deposition flux-dependent growth behavior, we controlled the deposition flux of Au atoms on the substrate independently by changing the precursor temperature while maintaining a substrate temperature of 1000 °C. The Au-deposition flux increased approximately 2.7-fold when the temperature was increased by 50 °C, as estimated from the Au vapor pressure.80 Figure S2 shows that NWs, nanobelts, and nanoplates were mainly formed on the substrate at Au source temperatures of 1100 °C, 1150 °C, and 1200 °C, respectively. Together, these results demonstrate that the shape of the Au nanostructure can be controlled by the deposition flux of Au atoms. How can changes in the deposition flux lead to the selective production of NWs, nanobelts, or nanoplates? We answered this question by elucidating the growth mechanism of Au nanostructures, as described below. For the investigation of the growth mechanism of Au nanostructures, we analyzed the morphologies and crystal structures of Au nanostructures in detail. Figure 4 shows SEM images of Au NPs, NWs, nanobelts, and nanoplates on an mcut sapphire substrate. As shown in Figure 4a,f,k, Au NPs have two mirror-symmetric orientations. Given that m-cut sapphire has 2-fold symmetry, the two orientations of Au NPs are reasonable. Figure 4b,g,i shows SEM images of slightly grown Au NPs. These NPs also have two mirror-symmetric orientations. Interestingly, we observed that Au NWs had the same two mirror-symmetric orientations (Figure 4c,h,m), thus suggesting that Au NWs were grown from seed NPs. Figures 4d,i,n and 4e,j,o present SEM images of Au nanobelts and nanoplates, respectively. More interestingly, the orientations of the Au nanobelts and nanoplates are exactly the same as the orientations of the NPs and NWs. The strongly correlated geometries and orientations of Au nanostructures indicate that Au nanostructures grow via the process of seed NPs → NWs → nanobelts → nanoplates. In addition, some Au NPs are observed in the vicinity of Au NWs, nanobelts, and nanoplates

Figure 2. AFM analysis of ultraclean and ultraflat Au nanoplate. (a) AFM topographic image of Au nanoplate. (b) Sectional view along the dotted cyan line (13 μm) in (a). The surface-height variation is less than 0.4 nm with 0.15 nm of Rq. (c) The surface-height distribution of Au nanoplate measured in the pink square of (a). Sq is only 0.15 nm. (d) Fourier band-pass-filtered atomic stick−slip image showing a welldefined Au (111) surface.

(Figure S2 in the Supporting Information). Figure 2c is the surface-height distribution measured in the pink square of Figure 2a. The root-mean-square roughness (Sq) was only 0.15 nm, thus confirming the atomically flat surface of the Au nanoplate. Figure 2d shows a high-resolution AFM image of an Au nanoplate in which the atomic stick−slip events are clearly resolved.79 A Fourier band-pass-filtered image of the Au nanoplate shows an obvious 3-fold symmetric atomic stick− slip pattern with a periodicity of 0.27 ± 0.02 nm, a value the is nearly consistent with the interatomic spacing (0.288 nm) on the Au (111) surface. This lattice-resolved AFM image clearly verified that the surfaces of Au nanoplates were well-ordered and atomically flat. The large-area formation of atomically flat surfaces is crucial to allow state-of-the-art nanomaterials to establish effective interfaces with various systems. The proposed vapor-phase 8749


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Figure 3. (a) Schematic of the shape distribution of Au nanostructures along a single m-cut sapphire substrate. (b−e) 45° tilted-view SEM images of Au NPs, NWs, nanobelts, and nanoplates on a single m-cut sapphire substrate. The shape of Au nanostructures is controllable by their location on the substrate.

On the basis of our detailed investigation of the Au nanostructures, we suggest that the growth process proceeds as seed NPs → NWs → nanobelts → nanoplates. Previously, we have reported that the growth of single-crystalline Au NWs can initiate from single-crystalline Au seed crystals.62 In this work, we explain the two-dimensional crystal-growth process from seed NPs to nanobelts and nanoplates. Initially, Au atoms can move on the sapphire substrate and nucleate, thereby forming a seed NP that is fitted onto the surface crystal structure of the substrate. Then, Au atoms can be supplied to the seed NP either by direct impingement from the vapor or by surface diffusion. The anisotropic growth of Au nanostructures proceeds kinetically,81 and the directional growth rate is strongly affected by the directional supply rate of Au atoms to the seed NP. Because the direct-impingement direction of Au atoms is mostly vertical, impinging Au atoms primarily contribute to the vertical growth of the seed NP. However, Au atoms approaching the seed NP by surface diffusion primarily contribute to the horizontal growth. Notably, the lattice mismatch influences the horizontal growth. When an Au atom arrives at the seed NP through surface diffusion, the lattice mismatch between the seed NP and substrate can affect whether the Au atom will attach to the seed NP. The lattice mismatch between the seed NP and m-cut sapphire substrate is only 0.1% along the Au < 110> direction, thus enabling the horizontal growth of the seed NP along the Au ⟨110⟩ direction. Consequently, the height-to-width ratio of Au nanostructures can be determined from the ratio of the vertical growth rate to the horizontal growth rate, which, in turn, is proportional to the ratio of the supply rate of Au atoms by direct impingement to that by surface diffusion. We showed that the shape of an Au nanostructure can be controlled by the deposition flux of Au atoms (Figures 3 and S4 in Supporting Information). Under a low deposition-flux condition, Au atoms arriving on a substrate can form only small Au clusters, which desorb easily. Thus, the Au atom supply by surface diffusion is very small, whereas that by direct impingement would be dominant. This supply by direct

(Figure 3c,d,e). These NPs are the seed NPs that did not grow up during the reaction. Figure 5a shows a cross-sectional transmission electron microscopy (TEM) image of Au NPs grown upward approximately 30 nm. This NP was cut along the yellow dotted line in Figure 4a. Figure 5b shows a high-resolution TEM (HRTEM) image, and fast Fourier transform (FFT) patterns of Au NP were obtained from the pink square in Figure 5a. The TEM analysis results showed that the Au NP is single-crystalline without twins and is enclosed by {111} top and side facets and a {113} bottom facet on a sapphire (1010̅ ) plane. The schematic of the atomic planes at the epitaxial interface between the Au (113) and sapphire (101̅0) planes shows that the interfacial energy between these two planes is highly favorable (Figure S3 in Supporting Information). The lattice spacing of the Au (110) plane is 1.44 Å and that of the sapphire (0001) plane is 6.49 Å, thus suggesting that the lattice mismatch between the two planes is 18.3%. However, 9 layers of Au and 2 layers of sapphire have 0.1% domain-matched misfit along the Au ⟨110⟩//sapphire ⟨0001⟩ direction. This small lattice mismatch enables the epitaxial formation of Au seed crystals on the substrate, thereby leading to the growth of Au nanostructures. Figure 5c,e,g shows cross-sectional TEM images of an Au NW, nanobelt, and nanoplate, respectively. Cross-sectional specimens were prepared using a focused-ion beam (FIB) technique after Au nanostructures were transferred onto Si substrates. The NW and nanobelt were cut along the plane perpendicular to their growth direction, and the nanoplate was cut along the plane perpendicular to its side. HRTEM images and selected-area electron diffraction (SAED) patterns of the NW, nanobelt, and nanoplate show that these Au nanostructures are all enclosed by {111} planes (Figure 5d,f,h). The data also indicate that the NW and nanobelt grow along the ⟨110⟩ direction and that the nanoplate has a ⟨110⟩ edge direction. Most importantly, all of the Au nanostructures (NP, NW, nanobelt, and nanoplate) shown here are perfectly singlecrystalline, without twins. 8750


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avidin system. Because avidin can bind biotinylated molecules (i.e., antibodies, inhibitors, DNA) while having minimal impact on their biological activity, the detection of avidin provides a convenient and practical pathway for extending the analyte accessibility of a biosensor.82−86 The surfaces of Au nanoplates were modified in a stock solution of EZ-Link biotin HPDP and rinsed with an excess of solvents, thus immobilizing biotinylated molecules onto the Au nanoplates. After the biotinylation of an Au nanoplate, we investigated the topography of the nanoplate. Figure 6a shows an AFM topographic image of a biotinylated Au nanoplate. The AFM image (upper panel of Figure 6a) and the sectional view along the dotted cyan line (bottom panel of Figure 6a) indicate that biotinylated molecules are well ordered, with a height of ∼2 nm on the Au nanoplate. The magnified 3-dimensional (3D) topographic image of the cyan square further confirms the presence of well-coated biotinylated molecules on the Au nanoplate. For comparison, we prepared thermally evaporated Au films and modified the surfaces of the films in the same manner. The AFM topographic image of the biotinylated Au film (upper panel of Figure 6c), the sectional view along the dotted cyan line (bottom panel of Figure 6c), and the magnified 3D topographic image of the cyan square (Figure 6d) illustrate that the molecules are irregularly aligned on the Au film. The SERS enhancement can be obtained from the nanogap structure, which produces an intense electric field (hot spots) under optical illumination.3,4,26,78,86 For the detection of avidin, we constructed Au NPs on a Au nanoplate nanogap structure through the biotin−avidin interaction and measured SERS signals from the nanogap structure. Briefly, the biotinylated Au nanoplate was incubated in an avidin solution and immersed in a biotinylated Au-NP solution, leading to the fabrication of Au NPs on a nanoplate structure through the biotin−avidin interaction (Figures 6e and S6 in the Supporting Information). After the nanoplate was washed to remove nonspecifically bound NPs, SERS measurements were carried out. The upper panel of Figure 6f is a plot of the SERS intensity (1005 cm−1) band as a function of avidin concentration. Full SERS spectra of Figure 6f and corresponding SEM images are displayed in Figure S7 and S8. The SERS intensity increased linearly with avidin concentration from 0 M to 1 nM. The detection limit is the lowest concentration of avidin that provides SERS signals distinguishable from the signal intensity at 0 M of avidin (zeroconctration signal). We were able to distinguish the SERS signals of 0 M and 10 pM, thus suggesting that Au NPs on a nanoplate structure can detect avidin at a low concentration of 10 pM. We also attempted to detect avidin using a biotinylated Au film. Au NPs on a film structure were constructed in the same manner, and SERS signals were measured. A plot of the SERS intensity (1005 cm −1 ) band as a function of avidin concentration (bottom panel of Figure 6f) showed a detection limit of 1 nM. The ultraflat and ultraclean Au nanoplate provided highly improved sensing ability compared with that of the Au film. This improved performance appears to originate from the significantly decreased nonspecific binding of NPs and corresponding very weak zero-concentration signal. Since the Au nanoplate can offer an optimally bioactive surface through the uniform immobilization of biotin (Figure 6a,b), the nonspecific binding of Au NPs is strongly suppressed (Figure S8a in the Supporting Information) and a very weak zeroconcentration signal is obtained. On the other hand, Au film

Figure 4. Top-view SEM images of (a) Au NPs, (b) slightly grown NPs, (c) NWs, (d) nanobelts, and (e) nanoplates on m-cut sapphire substrate. Pink and yellow circles and ellipses indicate that Au nanostructures have two mirror-symmetric orientations. Magnified 45° tilted-view SEM images of (g) slightly grown Au NP, (h) NW, (i) nanobelt, and (j) nanoplate grown from (f) seed NP of type A. Magnified 45° tilted-view SEM images of (l) slightly grown Au NP, (m) NW, (n) nanobelt, and (o) nanoplate grown from (k) seed NP of type B.

impingement causes the seed NPs to grow upward along the ⟨110⟩ direction, at an angle of 58.52° with respect to the substrate surface, and to then become NWs. Notably, the NWs have the most thermodynamically favorable {111} top and side facets when the seed NPs grow along the ⟨110⟩ direction. Under a high deposition-flux condition, Au atoms form large clusters composed of tens to hundreds of atoms on the substrate, which do not desorb easily. The number of Au atoms diffusing on the substrate increases nonlinearly, and the Au atom supply rate by surface diffusion increases more rapidly than that by direct impingement. This change in supply rates leads to the growth of nanobelts and nanoplates. When we provided very high deposition flux by using Au powder, which has a very large surface area, instead of an Au lump, horizontal Au NWs grew at a precursor temperature of 1200 °C (Figure S5 in Supporting Information). Under a very high depositionflux condition, the Au atoms supplied by surface diffusion overwhelmed those supplied by direct impingement, thus leading to the growth of horizontal NWs instead of nanoplates. Finally, we examined whether the surfactant-free and atomically flat Au nanoplate was able to act as an optimally bioactive surface. To test the biomolecular-recognition reaction by using an Au nanoplate, we used the well-studied biotin− 8751


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Figure 5. (a) Cross-sectional TEM image of Au NP on m-cut sapphire substrate. (b) HRTEM image and FFT patterns of Au NP obtained from the pink square of (a). The epitaxial relationship between the seed NP and substrate is Au (113)//sapphire (101̅0) planes. All facets except the bottom of seed NP are {111} planes. (c, e, g) Cross-sectional TEM images of (c) Au NW, (e) nanobelt, and (g) nanoplate. Cross-sectional specimens were prepared using a focused ion-beam technique after the Au nanostructures were transferred onto Si substrates. (d, f, h) HRTEM images and SAED patterns of (d) Au NW, (f) nanobelt, and (h) nanoplate. All side facets of Au nanostructures are {111} planes. TEM analysis results confirm that Au nanostructures are perfectly single-crystalline without twins. horizontal hot-wall single-zone furnace with a 1-in.-inner-diameter quartz tube. The setup was equipped with pressure and mass-flow controllers. Ar gas was flowed at a rate of 100 sccm, maintaining the chamber pressure at 5−15 Torr. The system was purged with Ar gas for 30 min before the start of each reaction to maintain an inert atmosphere. The Au lump, placed in an alumina boat at the center of the heating zone, was heated, and the vapor was transported to the lower temperature region by the carrier gas, where Au nanostructures were grown on substrates. The distances from the center of the heating zone to the center of the substrate were 5.3, 6.1, 6.9, and 7.7 cm at precursor temperatures of 1100 °C, 1150 °C, 1200 °C, and 1250 °C, respectively. The temperature of the substrates was maintained at 1000 °C for all experiments. The size of the substrate was 5 × 5 mm2, and the reaction times ranged from 60 to 90 min. Construction of Au NPs on a Nanoplate Structure through the Biotin−Avidin Interaction. As-synthesized Au nanoplates were transferred onto a precleaned Si substrate by using a simple drytransfer method. The Au nanoplates on the Si substrate were incubated in a 160 nM stock solution of EZ-Link Biotin HPDP (Pierce) for 24 h and then rinsed with an excess of phosphate-buffered saline (PBS) solution and water. Au NPs (Sigma−Aldrich) with a diameter of 10 nm were functionalized by the addition of 2 mL of 160 nM stock solution of EZ-Link Biotin HPDP. The mixture was shaken and then allowed to stand for 24 h. The unbound molecules were removed by two rounds of centrifugation. The biotinylated Au nanoplates on the Si substrate were immersed in a PBS solution of avidin (Sigma−Aldrich) for 3 h and washed with a PBS solution. These Au nanoplates were incubated in a biotinylated Au-NP solution for 3 h, and Au NPs on a nanoplate architecture were formed by biotin−avidin interactions. The Au NPs on the film structure were also prepared in the same manner. The Au film was prepared on a precleaned silicon substrate by a thermal-evaporation method assisted by deposition of 10 nm of Cr followed by 300 nm of Au. Characterization. Field-emission SEM images were collected with a Phillips XL30S. To avoid charging effects during the SEM observations, samples were coated with gold. TEM images, HRTEM images, and SAED patterns were recorded on a TECNAI F30 TEM operated at 300 kV and on a JEOL JEM-2100F operated at 200 kV.

prepares irregularly immobilized biotin as shown in Figure 6d. This leads to nonspecific adsorption of Au NPs onto a Au film (Figure S8e in Supporting Information) and thus the relatively high zero-concentration signal. For the realization of highly sensitive sensors that can detect specific protein markers, it is challenging to decrease the high background-noise level caused by nonspecific binding in clinical samples such as serum.87,88 Surface charges scattered at a rough surface edge limit the sensitivity and specificity of the sensors by inducing nonspecific adsorption of proteins through van der Waals and electrostatic forces.89 The proposed SERS sensor with atomically smooth single-crystal Au nanoplates would avoid these problems because Au nanoplates offer optimally bioactive surfaces.

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CONCLUSIONS We reported the surfactant-free vapor-phase synthesis of morphology-tuned Au nanostructures and elucidated their detailed growth mechanism. The geometric shape (NPs, NWs, nanobelts, and nanoplates) and size of the Au nanostructures can be finely controlled through adjusting the deposition flux of Au atoms. In-depth understanding of the growth mechanism of epitaxial Au nanostructures on crystalline substrates may be applied to the epitaxial growth of other new materials. Furthermore, Au nanoplates play an important role in the fabrication of highly sensitive biomolecular sensors because their perfectly flat and clean surfaces can greatly decrease the nonspecific bindings and zero-signal intensity. We anticipate that coherently linking functional molecules and/or nanomaterials onto the Au nanoarchitectures will provide superb platforms for diverse novel applications.

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EXPERIMENTAL SECTION

Surfactant-Free Vapor-Phase Synthesis of Au Nanostructures. Single-crystalline Au nanostructures were synthesized in a 8752


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were recorded by a thermoelectrically cooled electron-multiplying charge-coupled device (Andor) mounted on a spectrometer with a 1200 groove/mm grating. The acquisition time was 100 s.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02932. Thin and small Au nanoplates grown on an r-cut sapphire substrate, magnified AFM topographic image of a Au nanoplate, epitaxial relationship at the interface between Au seed NP and m-cut sapphire substrate, selective synthesis of Au nanostructures by deposition flux, synthesis of horizontal Au NWs in very high deposition flux condition, Au NPs on a nanoplate structure through the biotin−avidin interaction, full SERS spectra corresponding to Figure 6f, and SEM images of Au NPs on a Au nanoplate and Au film (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.K.). *E-mail: [email protected] (B.K.). ORCID

Jeong Young Park: 0000-0002-8132-3076 Jaebum Choo: 0000-0003-3864-6459 Bongsoo Kim: 0000-0001-5245-4715 Author Contributions #

Y.Y. and H.L. contributed equally.

Notes

The authors declare no competing financial interest.

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Figure 6. (a) AFM topographic image of biotinylated Au nanoplate (upper panel) and sectional view along the dotted cyan line (bottom panel). Biotinylated molecules are well-ordered with ∼2 nm height on the Au nanoplate. (b) Magnified 3D topographic image of cyan square in (a). (c) AFM topographic image of biotinylated Au film (upper panel) and sectional view along the dotted cyan line (bottom panel). Biotinylated molecules are irregularly aligned on the Au film. (d) Magnified 3D topographic image of cyan square in (c). (e) Schematic illustration of Au NPs on a nanoplate structure through the biotin− avidin interaction. (f) Plot of SERS intensity (1005 cm−1) band as a function of avidin concentration obtained from Au NPs on a nanoplate structure (upper panel) and Au NPs on a film structure (bottom panel).

ACKNOWLEDGMENTS This research was supported by grant (2017R1A2B4010073) through the National Research Foundation (NRF) funded by Ministry of Science and ICT (MSIT) of Korea, Global Frontier Project (H-GUARD_2014M3A6B2060489) through the Center for BioNano Health-Guard funded by MSIT of Korea, Public Welfare & Safety Research Program (NRF2012M3A2A1051682) through the NRF funded by MSIT of Korea, Institute for Basic Science (IBS-R004-G4-2014-a00), and KRIBB initiative Research Program. TEM analysis was performed at KBSI in Daejeon.

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After Au nanostructures were dispersed in ethanol, a drop of the solution was placed a holey carbon-coated copper grid for the preparation of TEM specimens. Cross-sectional TEM specimens were prepared with a dual-beam FIB (FEI Nova 600 NanoLab) equipped with a nanomanipulator (Kleindick MM3A). Contact and noncontact AFM (Agilent 5500) were used to acquire topographic and friction images by using silicon tips (PPP-CONT and PPP-NCH, Nanosensors), which had resonant frequencies of 13 and 330 kHz, force constants of 0.2 and 42 N/m, and tip radii of less than 10 nm at ambient pressure and room temperature. To investigate stick−slip behaviors at the Au nanoplates, low normal forces of less than 1 nN were used to prevent the disappearance of regular stick−slip phenomena because of sudden increases in friction and adhesion between the tip and the sample surface.90 SERS spectra were obtained using a home-built micro-Raman system based on an Olympus BX41 microscope.26 The 632.8 nm radiation of a He−Ne laser (Melles Griot) was used as an excitation source and was focused on samples through a 100× objective (Mitutoyo, NA = 0.7). The SERS signals

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