DNA-Guided Room-Temperature Synthesis of Single-Crystalline Gold Nanostructures on Graphdiyne Substrates.

Nobel metal nanoparticles with tunable morphologies are highly desirable due to their unique electronic, magnetic, optical, and/or catalytic features. Here we report the use of multilayered graphdyine (GD) as a substrate for the reductant-free, room-temperature synthesis of single-crystal Au nanostructures with tunable morphology. We find that the GD template rich in sp-carbon atoms possesses high affinity with Au atoms on the {111} facets, and that the intrinsic reductivity of GD facilitates the rapid growth of Au nanoplates. The introduction of single-stranded DNA strands further results in the synthesis of Au nanostructures with decreased anisotropy, i.e., polygons and flower-like nanoparticles. The DNA-guided tunable Au growth arises from the strong adsorption of DNA on the GD template that alters the uniformity of the interface, which provides a direct route to synthesize Au nanostructures with tailorable morphology and photonic properties.

Introduction

Noble metal nanostructures (e.g., Au, Ag) with anisotropic morphologies are an attractive class of materials due to their unique chemical and physical properties (e.g., electronic, magnetic, optical, and catalytic properties) that directly correlate with their structural parameters. Over the past decades, great progress has been made in synthesizing anisotropic Au/Ag nanostructures including nanorods,[1−3] nanorings,[4] nanowires,[5−7] polyhedrons,[8−10] and nanoplates/nanosheets.[11] To achieve morphologic anisotropy, template-guided growth is one of the most effective approaches. For example, surfactants like cetyltrimethylammonium bromide (CTAB),[11] polyvinylpyrrolidone (PVP),[12] and N,N-dimethylformamide (DMF)[13] are commonly used soft templates for anisotropic Au/Ag growth, which however raise concerns about toxicity and environmental risks. Alternatively, biomolecules (e.g., α-d-glucose,[14] plant extracts,[15−17] and DNA structures[18]) have been explored as “green” template agents. Recently, two-dimensional materials, especially graphene[19] or graphene oxide (GO),[20−22] have attracted much interest as hard templates for synthesis of surfactant-free anisotropic Au structures including nanobelts, nanowires, and nanosheets. However, these synthetic approaches usually require multistep processing, diverse reagents (e.g., seed crystals, stabilizers), and multifactor optimizations to generate different morphology/anisotropy. A facile yet general method to prepare Au nanostructures with tunable anisotropy is still lacking.

Graphdiyne (GD), as a new 2D carbon material,[23,24] is a 2D network with the connecting units consisting of a six-membered carbon ring in the center and six-carbon triple bonds attached to each of the ring carbon atoms. GD structure contains only sp- and sp2-hybridized carbon atoms with high π-conjunction. Compared to graphene, GD possesses higher reductivity and adsorption capability, and thus has been used as an excellent substrate for the synthesis of diverse metal structures[25−27] like zero valence single metal atoms,[28,29] Pd clusters,[30,31] and CdS/Graphdiyne heterojunctions.[32] In these cases, GD plays multiple roles, as nucleation sites, reduction agent, and structural support, thus avoiding addition reduction steps and simplifying the synthesis process. However, as far as we know, the resulting structures were mostly spherical/quasi-spherical. The morphology control of GD-mediated synthesis is less explored.

Herein, we report a one-step synthetic method of single-crystal Au nanoplates on GD with tunable anisotropy under ambient conditions. We find that the growth kinetics of the AuNSs on GD is significantly faster compared to that on GO. We attribute this feature to the high reductivity and adsorption capacity of GD. We show that the presence of oligonucleotides leads to tunable size and anisotropy of the Au nanostructures, which can be attributed to the altered uniformity of the GD interface.

Results and Discussion

We first demonstrate the synthesis of Au nanoplates using multilayered GD as the template and HAuCl4 as the gold precursor (Figure A). For a typical synthesis reaction, the GD water suspension and HAuCl4 solution were mixed (detailed in Methods) and allowed to react for 5 h at room temperature, without any other reagent. The transmission electron microscopy (TEM) images (Figure A, and Figure S1A) show that high yields (∼60–80%) of nanoplates formed on the GD substrates, with triangular or hexagonal shape in agreement with previous studies.[33] The elemental mapping analysis (Figure A) and point-mode energy-dispersive X-ray (EDX) data (Figure S2) verified that these structures were indeed Au nanoplates on GD. The atomic force microscope (AFM) image (Figure C) shows that the typical thickness of the nanoplates on GD is ∼10 nm. The size of these Au nanoplates varies from ∼50 nm to ∼1.5 μm with increasing HAuCl4 concentration and growth time (Figures S3 and S4) (“size” here is defined as the normal distance from the base to the apex in the triangular Au nanoplates, and the normal distance between any two parallel edges in truncated hexagonal Au nanoplates).

Figure 1

GD-mediated synthesis of Au nanoplates. (A) Schematic of the synthesis of Au nanoplates on multilayer GD. (B) Representative TEM and elemental mapping images of an Au nanoplate on GD. (C) Representative AFM image and height profile of Au nanoplates on GD. (D) Representative TEM images of a triangular Au nanoplate. (E) SAED pattern acquired from the selected areas (red box) in D. The circled spot, boxed spot, and spot circumscribed by triangle correspond to allowed 1/3{422}, {220}, and {311}. Bragg reflections with lattice spacings of 2.5, 1.44, and 1.23 Å, respectively. (F) HRTEM images of the region within the red box in B. The inset is a Fourier transform of the image. The measured spacing within any two white parallel lines is ∼2.5 Å, which corresponds to the 1/3{422} interplanar spacing in face-centered-cubic (fcc) Au. Scale bars: (A,C) 500 nm; (B,D) 100 nm; (F), 2 nm.

GD-mediated synthesis of <span class="Chemical">Au nanoplates. (A) Schematic of the synthesis of Au nanoplates on multilayer GD. (B) Representative TEM and elemental mapping images of an Au nanoplate on GD. (C) Representative AFM image and height profile of Au nanoplates on GD. (D) Representative TEM images of a triangular Au nanoplate. (E) SAED pattern acquired from the selected areas (red box) in D. The circled spot, boxed spot, and spot circumscribed by triangle correspond to allowed 1/3{422}, {220}, and {311}. Bragg reflections with lattice spacings of 2.5, 1.44, and 1.23 Å, respectively. (F) HRTEM images of the region within the red box in B. The inset is a Fourier transform of the image. The measured spacing within any two white parallel lines is ∼2.5 Å, which corresponds to the 1/3{422} interplanar spacing in face-centered-cubic (fcc) Au. Scale bars: (A,C) 500 nm; (B,D) 100 nm; (F), 2 nm.

Next, we determined the crystallinity of the Au nanoplates using transmission electron microscopy (TEM) and X-ray diffraction (XRD). Figure D shows representative bright-field TEM images of a triangular <span class="Chemical">Au nanoplate. Selected area electron diffraction (SAED) patterns (Figure E) reveal three sets of sixfold symmetric spots. From the measurements of spot spacings and their relative orientations, we identify the circled spots and boxed spots by nanoplates as {311} and {220} Bragg reflections, respectively, in 111-oriented face-centered-cubic (fcc) Au crystal. We also observed the 1/3{422} reflections usually forbidden in the fcc structure, which indicate that the fcc crystal is ultrathin and the surface is atomically flat. All other reflections are absent in the SAED patterns, indicating that the nanoplates are single crystals. The high-resolution TEM (HRTEM) images (Figure F) of the nanoplates show a sixfold symmetric structure with an interplanar spacing of ∼2.50 Å, which is the same as that of 1/3{422} planes in fcc Au. The Fourier transform of the HRTEM image (Figure F inset) reveals sixfold symmetric spots, indicating the high degree of crystallinity of the Au structure. The truncated hexagonal nanoplates present the same crystal form (Figure S5). The XRD ω–2θ pattern of the GD-Au nanoplate powder sample (Figure S6) shows five peaks (2θ = 38.2°, 44.4°, 64.6°, 77.6°, and 81.8°) corresponding to 111, 200, 220, 311, and 222 reflections, respectively, due to face-centered cubic (fcc) structure of Au. The 111 peak has the highest intensities and the ratio of 200 and 111 peak intensities is 0.33 (<0.52).[34] These results together indicate that these Au nanoplates are typical fcc-structured 111-oriented single-crystals.

Having established the GD-mediated Au nanoplate synthesis method, we next investigated the growth kinetics of the Au nanoplates. Figure A,B (and Figure S7) shows representative TEM images and size distributions of the products derived from 10, 20, 30, or 60 min growth time. We find that with a 10 min growth, there have been nanoparticles (<100 nm) (Figure S7A) with a transitional morphology between the sphere and the obtuse triangular plate. Then, the anisotropy, as well as the size of the structures, increases along with the growth time, resulting in sharp triangular nanoplates of over 200 nm (Figure S7D) in 1 h. We also monitored the change of visible absorption spectra in real time during the growth process (Figure C). In the first 10 min, an absorption peak appeared at 526 nm (characteristic of smaller-sized plasmonic nanostructures), suggesting that the kinetics of GD-mediated Au nucleation and growth is fast even at room temperature. Along with the growth time, the absorption intensity rose while the peak shifted toward the longer wavelengths, reaching 542 nm after 60 min, suggesting the formation of larger-sized Au structures. These results agree with the observations under TEM, indicating that the size of the Au nanostructures grew fast. As a control, we tested Au growth mediated by graphene oxide (GO) under the same condition. The result shows that after a 1 h reaction, there were few nanoplate structures under TEM, and no obvious plasmonic peak appeared in the adsorption spectrum (Figure S8). These results together indicate that the GD enables Au growth with fast kinetics, allowing the synthesis of Au nanoplates at room temperature within an hour.

Figure 2

Kinetics of GD-mediated Au growth. (A) Typical TEM images (left) and size distribution histograms of the Au nanoplates (right) from different growth time (10, 20, 30, and 60 min). Scale bar, 50 nm. (B) Average size of the Au nanoplates plotted as a function of growth time (N = 100). (C) Kinetics of the visible absorption spectra (400–800 nm wavelength, 10–60 min, with 10 min interval).

As we know, 2D Au nanocrystals with high surface energy are intrinsically not favored by thermodynamics. Au nanocrystals under thermodynamic control tend to adopt quasi-spherical shapes. According to this principle, speeding up (e.g., via raising the reaction temperature) the atomic addition may decrease the yield of Au nanoplates. However, in this study, we found that the increased temperatures (55 and 95 °C for 5 h) led to similar high yields (∼72% and ∼80%, respectively) of Au nanoplates with much larger sizes (Figure S9) compared to room-temperature products, suggesting that the GD can offer strong dimensional constrictions on the Au growth, which overwhelmed the tendency of nonoriented aggregation of Au atoms with high reducing speed. Thus, the morphology of the Au nanoplates on GD is controlled in a thermodynamically favored way.

To study the mechanism behind this fast growth kinetics, we carried out density functional theory (DFT) calculations to analyze the interactions between GD and Au atoms. A 3 × 3-supercell Au{111} surface model (with four layers of Au atoms) was used to interact with a GD sheet model of equal size. This interaction (Figure ) results in binding energy (ΔEbinding) of ∼6 eV. Carbon atoms at different positions of the GD show different adsorption energies compared to the Au atom. Among them, the sp C atoms adjacent to the benzene ring show the highest adsorption energy (Eadsorption = −1.22 eV, Figure A), indicating that the alkynyl plays an important role in Au–GD interaction. The Au binding site between the sp C atoms (S1, ΔEbinding = −0.7 eV, Figure B) is more energetically favorable than the site in the center of the benzene ring (S2, ΔEbinding = −0.58 eV), suggesting that S1 may serve as the starting point of Au growth.[28] The triangular alkynyl bridge ring on GD matches the lattice of Au{111} (Figure C), which may also favor the formation of {111}-oriented triangular/hexagonal Au nanoplates. In the simulated Au–GD structure, the Au–C distance is less than 3 Å. In comparison, a graphene sheet of the same size, although containing more C atoms (32 C atoms in the graphene sheet versus 16 C atoms in the GD sheet), exhibits weaker binding energy (ΔEbinding = ∼4.5 eV) with the Au surface. The Au–graphene binding resulted in a larger Au–C distance (max distance >4 Å) compared to the Au–GD binding. These results suggest that the fast growth of planar Au structures on GD may be attributed to the high binding energy and the proximity between Au and carbon atoms on GD, which exert spatial and dimensional constrictions on the Au growth in two dimensions.

Figure 3

DFT calculations of interactions between GD carbon atoms and Au atoms. (A) Adsorption energy (Eadsorption) mapping of carbon atoms on GD toward Au atoms. (B) Possible initiating binding sites (with different binding energy Ebinding) of Au atoms on GD. (C) Distance between Au atoms and C atoms on GD. Upper, side view; lower, top view.

Having established that the properties of the GD surface play a critical role in the formation of anisotropic Au nanostructures, we next asked whether we can tune this anisotropy by modulating the GD interface. It has been proved that single-stranded (ss) DNAs can effectively adsorb on GD sheets.[35,36] Thus, we employed ssDNAs to react with GD, and investigate the effects on the morphology of Au structures. We found that along with the rise of ssDNA concentration, the proportion of Au nanoplates in the products decreases, while the low-anisotropy (or quasi-spherical) Au nanoparticles increase (Figure and Figure S10). These nanoparticles include single crystals, twin crystals, and polycrystals (decahedrons, icosahedrons, etc.) (Figure D and Figure S11A). Interestingly, when the concentration of A20 reaches 3 μM (unbound A20 strands were removed by centrifuging), the majority of the products on GD become flower-like nanoparticles (size <50 nm) with multiple tips (Figure E and Figure S11B).

Figure 4

DNA-tailored Au growth on GD. (A) Schematic illustration. (B–E) Representative TEM images showing dependency of the size and anisotropy of Au nanostructures on the concentration of oligonucleotide (A20) concentration from 10 nM to 3 μM. Scale bars: upper row (magnified view), 50 nm; lower row (wide view), 250 nm.

We reason that this morphologic tuneability of the Au nanostructures can be attributed to the morphology of the 2D interface for Au growth being modulated by the adsorption of ssDNA on the GD surface. Previous studies have shown that the GD can adsorb ssDNA effectively via van der Waals force and π–π stacking interaction between nucleobases and GD.[35,36] Therefore, ssDNAs adsorbed on GD might break the flatness and the continuity of the interface templating Au growth, leading to uneven distribution of the nucleation sites, and irregular growth directions, thus resulting in Au nanoparticles with decreased sizes and more irregular morphologies. Although DNA strands have long been utilized to modulate the morphology of seeded Au growth products in solution,[18,19,37−40] here we demonstrate that Au growth on the 2D GD interface can be regulated well by DNA via tailoring the surface morphology, which may have implications in interface synthesis of tailorable nanostructures.

Conclusions

This GD-based synthesis method shows several advantages. First, this strategy possesses methodological simplicity. Previous studies typically employed two-step seed-mediated growth approaches to synthesize asymmetric <span class="Chemical">Au nanostructures.[41] Here in this study, the DNA-GD-mediated growth allows one-step, seedless synthesis of Au nanostructures with different morphologies. This tailorable Au growth at room temperature enables rapid generation of plasmonic signals in response to nucleic acids or other molecules. Given that the interactions between nucleic acids and 2D materials have been exploited for molecular sensing,[35,42] our system may have potential in developing plasmonic biosensing strategies which can work under ambient conditions. Moreover, the resulting Au nanostructures are surfactant-free, which facilitate functionalization (especially DNA functionalization) and biomedical applications that require high water-solubility and low toxicity.

Taken together, we report a one-step synthesis method of single-crystal Au nanosheets on GD with tunable morphology under ambient conditions. We find that the growth kinetics of the Au nanostructures on GD is significantly faster compared to that on GO. This feature can be attributed to the high reductivity and high adsorption energy of GD. We show that the adsorption of oligonucleotides on GD can regulate the interface of Au growth, leading to tunable size and morphology of the resulting Au nanostructures. In future studies, we will investigate the effects of GD’s structural properties (e.g., size and layer number) on Au growth, which may help improve the structural uniformity of resulting Au nanostructures. We envision that the Au–GD heterostructures with tunable electronic/optical properties may find broad applications in nanoelectronics, nanophotonics, and plasmonic biosensing.

Materials and Methods

Chemicals and Materials

Multilayer graphdiyne (GD) flake was prepared according to the method reported in our previous work,[24] which was carried out with a cross-coupling reaction using hexaethynylbenzene as a precursor on the surface of copper. Hydrogen tetrachloroaurate (III) hydrate (HAuCl4·3H2O, 99.999%; Sigma-Aldrich), oligonucleotides (synthesized by Sangon Biotech Co., China, diluted to 100 μM in Milli-Q water). No unexpected or unusually high safety hazards were encountered in this work.

Preparation of Multilayer GD Flake Suspension

All of our aqueous GD flake suspensions were prepared by ultrasonication of 20 mg of GD powder in 50 mL of deionized water. The ultrasonication was performed in water for 10 min.

Synthesis of Au Nanoplates on GD

The reaction solution was composed of 200 μg mL–1 of the as-prepared multilayer GD flake suspension and different concentrations of HAuCl4. After added HAuCl4, the solution was mixed and reacted at room temperature for different times. The final product was collected by centrifuge at 8000 rpm (or 6800 g, CT15RE, Hitachi, Japan) for 10 min, washed thoroughly with deionized water three times. The precipitate was resuspended in deionized water.

DNA-Guided Synthesis of Au Nanostructures on GD

500 μL of 200 μg mL–1 GD flake suspension solution was first incubated with 10 nM to 3 μM of DNA (A20) for 1 h to allow DNA adsorb onto the GD surface. The mixture was centrifuged at 14,000 rpm (or 20,817 g) for 30 min and the supernatant was removed. The precipitate was resuspended in the deionized water, and this procedure was repeated three additional times. Then, the precipitate was resuspended in 480 μL of the deionized water. 20 μL of 100 mM HAuCl4 was introduced to the mixture to initiate the reduction reaction. After different reaction times, the final product was collected by centrifuging at 8000 rpm for 10 min, washing with deionized water three times and resuspending in deionized water.

Transmission Electron Microscopy (TEM) Imaging

For TEM imaging, 10 μL sample solution was deposited on a copper grid or for 10 min, after which the excess solution was wicked away with filter paper. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) and high-resolution TEM (HRTEM) images and selected area electron diffraction (SAED) patterns were obtained with a Tecnai TF20 microscope operated at an acceleration voltage of 200 kV.

Atomic Force Microscope (AFM) Imaging

Samples (5 μL each) were deposited onto freshly cleaved mica and left to adsorption for 3 min the mica was then washed three times with Milli-Q water (Millipore) and the sample was scanned under ScanAsyst mode with Sharp Nitride Lever tips (Bruker, USA). The image of the Au nanoplates is analyzed by the nanoscope analysis software.

Kinetic UV–vis Absorption Spectra of Nanoplates

Time-dependent evolution of UV–vis spectra of nanoplate growth solution without DNA was obtained by UV–vis spectroscopy (U-3010 UV–vis spectroscopy, Hitachi, Japan). The absorbance spectrum of the growth solution was collected after initiation of the reaction for 10, 20, 30, 40, 50, and 60 min, respectively.

DFT Calculations of Interactions between GD Carbon Atoms and Au Atoms

All the geometry optimizations and energies were obtained by employing the density functional theory (DFT) calculations using the Vienna ab initio simulation package (VASP).[43−46] The Perdew–Burke–Ernzerhof generalized gradient approximation (known as GGA-PBE) was used to describe the exchange-correlation energy. The Blöchl’s all-electron-like projector augmented wave (PAW) method with a kinetic cutoff energy up to 400 eV was used to describe the interactions between valence electrons and ion cores.[47,48] To construct the compound structure of GD and Au, a 3 × 3 supercell with four layers of Au{111} surface model was used when combined with GD unit cell. Geometries were optimized until the energy was converged to 1.0 × 10–6 eV/atom and the force was converged to 0.01 eV/Å. A vacuum layer as large as 15 Å was used along the c direction normal to the surface to avoid periodic interactions.

The binding energy, ΔEbinding, is defined as followswhere EGD/Au(111) is the total energy of the compound structure, EAu(111) and EGD are the energies of the isolated Au(111) surface and GD, respectively.