Solution Growth of Ultralong Gold Nanohelices Yong Wang,‡,§ Jiating He,§ Xiaoke Mu,∥ Di Wang,∥ Bowei Zhang,⊥ Youde Shen,# Ming Lin,§ Christian Kübel,∥ Yizhong Huang,⊥ and Hongyu Chen*,†,‡ †
Institute of Advanced Synthesis (IAS), School of Chemistry and Molecular Engineering, Jiangsu National Synergetic Innovation Centre for Advanced Materials, Nanjing Tech University, Nanjing 211816, P.R. China ‡ Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore § Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), #08-03, 2 Fusionopolis Way, Innovis, 138634 Singapore ∥ Institute of Nanotechnology and Karlsruhe Nano Micro Facility, Karlsruhe Institute of Technology, Karlsruhe 76021, Germany ⊥ School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore # Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore S Supporting Information *
ABSTRACT: Metallic nanohelices are extremely rare and, to date, have never been synthesized by a direct solution method. In this work, we report ultralong Au nanohelices grown in solution under ambient conditions. They are ultralong with several tens of micrometers in length, with extraordinary aspect ratio (length/diameter greater than 22 300) and the number of pitches (more than 22 000 pitches). The pitch and width are uniform within each helix but vary widely among the helices. Crystal analyses showed that the facets, twin boundaries, grain sizes, and orientations are aperiodic along the helices. The apparent smooth curving is only possible with a large number of surface steps, suggesting that these structural features are the mere consequence of the helix formation rather than the cause. We propose that the nanowires are formed by the active surface growth mechanism and that the helicity originates from the random and asymmetrical blocking of nuclei embedded within the floccules of ligand complexes, in the form of either asymmetric binding of ligands or asymmetric diffusion of growth materials through the floccules. The separate growth environment of these nuclei causes constant helicity within each helix but differing helicity among the individuals. The embedding also provides a robust environment for the sustained growth of the nanohelices, leading to their record length and consistency. KEYWORDS: solution growth, ambient condition, ultralong, asymmetric, gold, nanohelices
H
Preliminary measurements of simple setups have demonstrated the excellent properties of nano- or microhelices.1,2 Helical carbon3,4 and ZnO fibers5 were shown to be elastic; helices made of semiconductors, such as ZnO6 and InGaAs/ GaAs,7 have demonstrated abnormal nonlinear electronic transport behavior. In comparison, metallic nanohelices are expected to be more elastic with higher electric conductivity,1,2 which could lead to stronger circular dichroism8−12 and
elix is a unique structural feature that is asymmetric, elastic, and aesthetically attractive. The biopolymers in cells adopt helical or double helical structures in order to pack into compact segments without kinks. In mechanics, the helix is equally useful, as springs can evenly distribute the mechanical stress along the coiled wires. Screw propellers and helical gears can break symmetry in converting rotational motion into directional thrust. Stairs are often helical to avoid kinks for mechanical and aesthetic reasons. Finally, solenoids can smoothly alter the direction of electric current to create electromagnetism. It is conceivable that these general principles may be similarly exploited at the nanoscale. © 2017 American Chemical Society
Received: February 1, 2017 Accepted: June 5, 2017 Published: June 6, 2017 5538
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Figure 1. Representative TEM images showing the different types of the Au nanohelices: (a) stretched helices with large pitch/width ratio; (b) compact helices; inset shows an enlarged area; (c) helices with small pitch/width ratio; (d,e) folded double helices; (f) model helix with definitions; (g,h) intertwined double helices of equal (g) and different pitches (h); (i) triple-strand helices of different pitches. See Supporting Information for large-view images.
micro-13 and even nanosolenoids. However, metallic nanohelices are extremely rare.14,15 In terms of synthesis, helical nanostructures are limited in methods and applicable materials. Most of them were oxides and semiconductors, often synthesized by chemical vapor deposition (CVD). The chirality/helicity was proposed to arise from screw dislocation,16−19 asymmetric extrusion,20,21 strainmediated surface diffusion,22 or intrahelix electrostatic interactions induced by charge imbalance.5,23 In contrast, solution synthesis of inorganic nanohelices is rare, but it utilizes different types of materials, such as semiconductors,24 metals,14 and polymers.25−27 The origin of helicity is less studied as compared to the CVD method.1 It was proposed to arise from the tilted packing of matchstick-like molecules,28 the intrinsic atomic packing within a wire form,14,29−31 or the packing of polymers/wires within a confined space,25−27,32 often as a result of energy minimization to avoid kinks. These mechanisms found for nonmetallic helices are, in general, not compatible with metallic systems. Here, we present a synthesis of ultralong Au nanohelices by a solution method under ambient conditions. The pitch and width (as defined in Figure 1f) are extraordinarily uniform
within each helix but vary widely among the helices. The synthesis critically depends on the nature of the ligand, particularly its solubility. We propose that the Au nanohelices are formed by asymmetric growth: the helicity arises from the local environment of the nuclei embedded in the floccules of the ligand−Au complex. With the solvent−nucleus interface asymmetrically blocked, inequivalent growth rates at the circumference lead to a helix. This mechanism explains well the dependence on the ligand solubility and the structural variety among the individual helices.
RESULTS AND DISCUSSION Helical Au nanowires were discovered accidently when aged 4mercaptobenzoic acid (4-MBA) was used as the ligand. More specifically, the helical Au nanowires were grown by incubating HAuCl4 (final concentration: 1 mM), L-ascorbic acid (6 mM), NaOH (1 mM), and 4-MBA (1.4 mM, aged for 6 months) in an ethanol/water mixture (1 mL, v/v 7:3) at room temperature for 24 h. The products were isolated by centrifugation and purified in ethanol for several times. Transmission electron microscopy (TEM) revealed ultralong helices with a very low 5539
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As shown in Figures 1 and 2, the helical nanowires are ultralong with several tens of micrometers in length. Their extraordinary aspect ratio (length/diameter greater than 22 300, Figure S12) and the number of pitches (more than 22 000 pitches) are extremely rare in the literature. In many cases, they were too long to be measured. The remnants of the floccules often appear in TEM images as entangled organic fibers (Figures 1g, 5c, and Figures S15 and S29) or clumps (Figures 1b,c, 2b, and S12 and S13). During TEM observation, the electron beam irradiation turned the floccules into small nanoparticles, indicating the reduction of Au(I) species to Au(0) (Figure S27). Among the 1661 helices we have surveyed, 86.8% were single-strand helices (Figures 1a−c and S10−S13), and the rest were double- or multiple-strand helices (Figures 1d,e,g−i and S14−S17). The width and pitch within each helix were highly uniform over hundreds of pitches (e.g., Figures 1b and S12), but they varied widely among the different helices in the same sample. The diameter of the nanowires was uniform at about 7 nm. The width of the helices ranged from 15 to 150 nm, and the pitch length ranged from 8 to 250 nm (Figure S11). There was no obvious correlation between the width and pitch length: some helices have small width but long pitch, giving the appearance of stretched helices (Figure 1a, 85.8%); some helices have short pitch length, appearing as compact helices (Figure 1b, 1.0%), and in some helices, the width was much larger than the pitch length, so that they flattened out on the TEM grid as looped circles (Figure 1c, 2.2%). Among the double-strand helices, some of them appeared as folded (3.9%), where a U-turn end can be found and the neighboring strands were parallel without interlocking (Figures 1d and S14). In particular, a few examples were found where one strand was much shorter than the other (Figure 1e), clearly indicating that the folding occurred after the nanohelix formation. In addition, there were clusters of double helices where multiple U-turn ends can be found (6.1%, Figures 2f and S21). For the multiple loops extending from the same cluster, the width, pitch, and handedness were the same. Considering the interlocking of double helices, they cannot possibly be formed by folding. Shown in Figure 1g−i were the rare examples of nonlooped (not bound in a cluster) double and triple helices with the strands wound tightly around each other. The pitch of the opposing strands is sometimes the same (0.4%, Figures 1g and S15) and sometimes different (0.6%, Figure 1h,i). In Figure 1h,i, one of the strands is even straight, suggesting a different origin from the intertwined neighbor. The handedness of the helices can only be characterized by scanning electron microscopy (SEM), not from the 2D projections in TEM. However, the diameter of the nanowires is too small for SEM. Thus, we tried to grow the helices thicker by prolonging the reaction time (72 h). After overgrowth, the nanohelices became much thicker (from the initial 7 to about 20 nm). Helices with a small width now appeared almost like straight nanowires (Figure 2c and inset of Figure 2g), whereas those with width larger than 20 nm can still be recognized (Figure 2b). In SEM, the 3D features of the overgrown nanohelices are revealed by the clear contrast arising from the different heights and orientations, allowing the direct assignment of handedness. With a right choice of hand, when pointing four fingers to the coiling direction, the thumb should point to the vertical vector of the coiling motion (Figure 1f). In our observation, the handedness never changed within an individual helix, but there were both left- and right-handed ones in the mixture, with
yield (18 MΩ·cm−1) was used in all reactions. Copper specimen grids (300 meshes) with Formvar/carbon support film (referred to as TEM grids in the text) were purchased from Beijing XXBR Technology Co. Characterization. TEM images were collected from a JEM-1400 transmission electron microscope (JEOL) operated at 100 kV. Highresolution TEM was performed on an FEI Titan 80-300 equipped with a Gatan US1000 slow scan CCD camera operated at 300 kV. ACOMTEM analysis was performed using a FEI Tecnai F20 ST operated at 200 kV in microprobe (μp) STEM mode with spot size of 8, camera length of 100 mm, condenser aperture of 30 μm, gun lens of 6, and extraction voltage of 4.5 kV, resulting in a spot size of about 1 nm and a semi-convergence angle of 1.4 mrad. An ASTAR system (Nanomegas) was used for ACOM diffraction data acquisition. SEM images were collected from a JEOL-6700F SEM operated at 10/15 kV. All NMR spectra were acquired on Bruker AVIII 400 MHz NMR spectrometers. 1H NMR (400 MHz) chemical shifts were recorded relative to SiMe4 (δ 0.00). Multiplicities were given as s (singlet), d (doublet), t (triplet), q (quartet), and m (multiplet). The number of protons (n) for a given resonance was indicated by nH. Coupling constants were reported as a J value in Hz. ESI/MS analysis was conducted on a Thermo Scientific LCQ Fleet mass spectrometer. Preparation of TEM Samples. TEM grids were treated with oxygen plasma in a Harrick plasma cleaner/sterilizer for 1 min to improve the surface hydrophilicity. The grid was placed face-down on a droplet of as-synthesized sample laid on a plastic Petri dish. A filter paper was used to wick off the excess solution on the TEM grid, which was then dried in air for 10 min. Preparation of 4,4′-Dithiobisbenzoic Acid Diethyl Ester (1). The 4,4′-dithiobisbenzoic acid (2) was synthesized according to the literature method.44 Typically, 4-mercaptobenzoic acid (1.54 g, 10 mmol) and iodine (1.29 g, 5 mmol) were mixed in 40 mL of absolute ethanol, followed by the addition of triethylamine (6 mL). Finally, the solution was gently stirred for 16 h. The excess iodine was removed by the addition of sodium thiosulfate (10%, aq) after the reaction was completed. The cloudy solution was concentrated by rotary evaporator under reduced pressure and adjusted to pH 2 with hydrochloric acid (0.02 M). The white precipitate was collected and dried under vacuum. The product was directly used for the synthesis of 4,4′dithiobisbenzoic acid diethyl ester (1) without purification. 4,4′-Dithiobisbenzoic acid diethyl ester (1) was synthesized according to the literature procedure.34 Concentrated sulfuric acid (0.1 mL) was slowly added into a suspension of 2 (0.55 g, 1.8 mmol) in ethanol (99.8%, 15 mL). The mixture was refluxed for 18 h. After being cooled to room temperature, the solvent was evaporated under reduced pressure. The crude was suspended in dichloromethane (25 mL) and washed with a saturated NaHCO3 aqueous solution (3 × 20 mL) and water (3 × 20 mL). The organic layer was dried over MgSO4 and evaporated to yield 0.58 g (90%) of compound 1 as a colorless solid. 1H NMR (400 MHz, CDCl3): δ 7.981−7.959 (d, 3J(H,H) = 8.4 Hz, 4H), 7.532−7.511 (d, 3J(H,H) = 8.4 Hz, 4H), 4.365−4.347 (q, 3 J(H,H) = 7.1 Hz, 4H), 1.371 (t, 3J(H,H) = 7.1 Hz, 6H); [M + Na]+ 385. Preparation of Au Nanohelices. 4,4′-Dithiobisbenzoic acid diethyl ester (1) (120 μL, 1 mM in ethanol) was added into 580 μL of ethanol, followed by the addition of 120 μL of ascorbic acid (aq, 50 mM), 50 μL of HAuCl4 (aq, 20 mM), 20 μL of NaOH (aq, 50 mM), and 110 μL of H2O. After vortex mixing, the solution was left undisturbed at room temperature for 24 h. The ultralong gold nanohelices were precipitated and washed by ethanol once to remove the residue ligand and reactants. The nanohelices were then redispersed in 50 μL of ethanol and stored.
CONCLUSIONS In summary, we report a facile method to prepare ultralong gold nanohelices with up to 20 000 pitches. Out of many possible problems, we identify three core mechanistic questions: (1) What is the active chemical ingredient in the aged 4-MBA that is responsible for the helical growth? (2) How does one reconcile the highly consistent growth of individual helices and the structural variety among the individuals? (3) Is the crystal structure the cause or consequence of the helix formation? We have to propose a coherent mechanism that can resolve these puzzles and be consistent with all experimental observations. More specifically, we find that ligand 1 is the key ingredient and propose that the nuclei inside ligand floccules are asymmetrically blocked, leading to helical growth of the nanowires. Embedded within the floccules, the nuclei are well protected and able to maintain their growth environment for a prolonged period, leading to great length with highly consistent helicity. At the same time, different nuclei would experience different asymmetric environment, leading to structural variety. This variety of helices and the random occurrence of grain orientation and surface steps provide a strong support to the active surface growth mechanism, where crystal structure is the mere consequence of the growth. Moreover, the suspended seeds are more accessible to the growth materials compared to the substrate-bound seeds of the previous system, allowing extensive growth. Such a growth environment can be possibly exploited in the future for advanced synthesis of helical nanostructures. 5544
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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/acsnano.7b00710. Details of the discussion about the floccules, large-area TEM images of the Au helices (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] or
[email protected]. ORCID
Yizhong Huang: 0000-0003-2644-856X Hongyu Chen: 0000-0002-5325-9249 Author Contributions
Y.W. and H.C. designed the project. Y.W. performed the experiments and most of the characterizations. J.H., X.M., D.W., B.Z., M.L., C.K., and Y.H. performed the HRTEM characterizations. Y.S. helped with the AFM characterizations. Y.W. and H.C. wrote the paper. All authors discussed the results and commented on the manuscript. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the MOE (RG 14/13 and RG 5/ 16) of Singapore, the National Natural Science Foundation of China (21673117), and the financial support from Nanjing Tech University and SICAM Fellowship from Jiangsu National Synergetic Innovation Center for Advanced Materials. The authors thank the Karlsruhe Nano Micro Facility (KNMF), a Helmholtz large scale research infrastructure operated at Karlsruhe Institute of Technology (KIT). REFERENCES (1) Wang, Y.; Xu, J.; Wang, Y.; Chen, H. Emerging Chirality in Nanoscience. Chem. Soc. Rev. 2013, 42, 2930−2962. (2) Ren, Z.; Gao, P. X. A Review of Helical Nanostructures: Growth Theories, Synthesis Strategies and Properties. Nanoscale 2014, 6, 9366−9400. (3) Lau, K. T.; Lu, M.; Hui, D. Coiled Carbon Nanotubes: Synthesis and Their Potential Applications in Advanced Composite Structures. Composites, Part B 2006, 37, 437−448. (4) Chen, X.; Zhang, S.; Dikin, D. A.; Ding, W.; Ruoff, R. S.; Pan, L.; Nakayama, Y. Mechanics of a Carbon Nanocoil. Nano Lett. 2003, 3, 1299−1304. (5) Gao, P. X.; Ding, Y.; Mai, W.; Hughes, W. L.; Lao, C.; Wang, Z. L. Conversion of Zinc Oxide Nanobelts into Superlattice-Structured Nanohelices. Science 2005, 309, 1700−1704. (6) Gao, P. X.; Ding, Y.; Wang, Z. L. Electronic Transport in Superlattice-Structured ZnO Nanohelix. Nano Lett. 2009, 9, 137−143. (7) Hwang, G.; Hashimoto, H.; Bell, D. J.; Dong, L.; Nelson, B. J.; Schö n, S. Piezoresistive InGaAs/GaAs Nanosprings with Metal Connectors. Nano Lett. 2009, 9, 554−561. (8) Gansel, J. K.; Thiel, M.; Rill, M. S.; Decker, M.; Bade, K.; Saile, V.; von Freymann, G.; Linden, S.; Wegener, M. Gold Helix Photonic Metamaterial as Broadband Circular Polarizer. Science 2009, 325, 1513−1515. (9) Kuzyk, A.; Schreiber, R.; Fan, Z.; Pardatscher, G.; Roller, E.-M.; Hogele, A.; Simmel, F. C.; Govorov, A. O.; Liedl, T. DNA-Based SelfAssembly of Chiral Plasmonic Nanostructures with Tailored Optical Response. Nature 2012, 483, 311−314. (10) Esposito, M.; Tasco, V.; Todisco, F.; Benedetti, A.; Tarantini, I.; Cuscuna, M.; Dominici, L.; De Giorgi, M.; Passaseo, A. Tailoring 5545
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