Letter pubs.acs.org/Langmuir
Synthesis of Toroidal Gold Nanoparticles Assisted by Soft Templates Yong Yan, Pramod Padmanabha Pillai, Jaakko V. I. Timonen, Fateme S. Emami, Amir Vahid, and Bartosz A. Grzybowski* Department of Chemistry and Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States S Supporting Information *
ABSTRACT: A three-component system comprising surfactant molecules and molecularly cross-linked metal centers assembles into nanoring structures. The thickness of the nanorings is determined by the dimensions of the surfactant bilayer while the dimensions of the ring opening depend on and can be regulated by the concentrations of the participating species. Once formed, these organic−inorganic hybrids can be transformed, by air plasma treatment, into all-metal nanorings exhibiting strong adsorption in the near IR.
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large aggregates of CTAB−gold salt complexes (Figure S1) since the ratio of CTAB to gold salt was significantly lower than that commonly used in the synthesis of gold nanoparticle. (Normally, when the ratio is high enough, such as in the hundreds, the solution is clear with no signs of turbidity.22) Upon addition of 1.0 μL of dithiol (99%, 1,3propanedithiol) the mixture gradually became clear and the color changed from yellow to light white, indicating that the reduction of AuCl−4 was taking place (probably from Au3+ to Au+).23 Once the solution became clear (∼300 s), 1 mL of sodium borohydride (NaBH4, 50 mM) was added to completely reduce all of the gold salt to gold nanoparticles.
INTRODUCTION Among a multitude of nanostructures synthesized to date, “toroidal/ring” particles have recently received considerable attention as potential mimics of biological transmembrane channels,1−3 elements useful in information storage,4 electronics,5 medicine,6,7 and as constructs with which to investigate several fundamental properties of matter on the nanoscale.8,9 In particular, metallic nanorings exhibiting unique plasmonic properties10 have been considered in the context of confining and manipulating light,11 telecommunication waveguides,12 optical data storage,13 quantum information processing,14 optical antennas,15 and chemical and biological sensors16 as in medical applications including photothermal ablation of cancer cells and tumors.17 However, the synthesis of these nanorings in large quantities remains challenging; approaches such as imprinting lithography,18 colloidal lithography,19 and electron beam lithography20 are relatively expensive and lowyielding, whereas solution-based methods are only in the proofof-concept stages.21 In this letter, we describe a straightforward and large-scale method for synthesizing gold nanorings from three-component toroidal templates comprising cationic surfactants, gold salt, and dithiol linkers. These components first assemble in solution into nanorings of discrete nanoparticles which then, upon electron beam irradiation or air plasma treatment, fuse into continuous structures. The thickness of the nanorings thus formed is constant at ∼20 nm; at the same time, the pore size depends on and can be regulated by the ratio of dithiol and gold salt concentrations, [dithiol]/[HAuCl4].
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RESULTS AND DISCUSSION As shown in the SEM images (Figure 1a), nanoring structures then formed in high yield and were relatively uniform in size. The TEM image in Figure 1b shows that these rings incorporated ∼4 nm nanoparticles which, under high-resolution TEM, showed lattice fringes of 0.23 and 0.20 nm, corresponding to gold {111} and {200} plane sets. Both poly- and single-crystalline particles were present (Figures 1c and S2).24 The critical role of dithiol in the synthesis of gold nanorings was confirmed by a set of control experiments. In one of them, 1-propanethiol was used instead of a 1,3-propanedithiol despite its similar length, the thiol could not act as a bridging agent; consequently, no nanorings but only gold nanoparticles were formed (SEM in Figure S3). Second, only relatively large (∼40 nm) nanoparticles were formed when no thiols (neither dithiol nor monothiol) were added to the reaction mixture (Figure S4). In another control experiment, the amount of dithiol added was small ([dithiol]/[HAuCl4] ≈ 0.5 and below); under these conditions, the rings were ill-defined, their size
EXPERIMENTAL SECTION
To prepare the nanorings, 1 mL of cationic surfactant, cetyltrimethylammonium bromide (CTAB, 10 mM), 1 mL of gold salt (HAuCl4, 10 mM), and 8 mL of deionized water were first mixed to give a turbid aqueous solution (at room temperature and with continuous stirring with a stir bar). This turbidity could be ascribed to the formation of © XXXX American Chemical Society
Received: May 30, 2014 Revised: August 4, 2014
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Figure 1. (a) SEM image of gold nanorings. (b) TEM image of a nanoring comprising discrete AuNPs. (c) High resolution TEM image of gold nanoparticles from (b). These nanorings were synthesized using 1 mM CTAB, 1 mM HAuCl4, 1 mM dithiol, and 5 mM NaBH4. The reducing agent was injected after 300 s of dithiol addition.
3b) further confirm the toroidal shape of the forming structures. We can therefore conclude that the rings comprising CTAB, gold, and dithiols are present before the addition of NaBH4 and before the formation of discrete AuNPs upon borohydride reduction. Remarkably, the experimentally observed ∼20 nm thickness of the nanorings is very close to the 20 nm thickness of a modeled bilayer structure of CTAB-Au-dithiol (Figure 3e). This correspondence suggests that the following mechanism is plausible. In the initial stage, CTAB can form spherical micelles having hydrophobic cores, surrounded by a “shell” of gold salt associating via electrostatic interactions. Upon addition of dithiols, the spherical micelles coalesce and reassemble into toroidal/ring structures to maximally reduce the contact between hydrophobic alkyl chains and water molecules; this arrangement is energetically more favorable than fibers exposing some alkyl chains at the ends and actually sometimes observed in experiments (red dashed circles in Figure 3b). We note that the transformation of micelles into tori is quite common and was proposed in some other amphiphilic molecular or block copolymer systems.3,27 The ultimate goal of our research has been to produce continuous-metal rather than disjointed-nanoparticle rings. We
uniformity was very poor, and the sample contained excess gold nanoparticles (Figure S5). The dimensions of the rings could be controlled by adjusting the [dithiol]/[HAuCl4] concentration ratio. Figure 2 illustrates how upon the increase in this ratio from 1 to 4 the pore diameter of nanorings (averaged over the long and short axes) gradually decreased from ∼70 to 0 nm (solid nanoparticles; additional large-area SEM images are provided in Figures S6− S9). Interestingly, the (wall) thickness of the nanorings remained constant at ∼20 nm irrespective of [dithiol]/ [HAuCl4]. With these experimental trends established, we wished to understand, at least qualitatively, the mechanism that governs the formation of the nanorings. The first observation we make is that CTAB is well known to form micellar structures at a concentration of 1 mM and above. The positively charged micelles can bind to the negatively charged gold salt to form CTAB-AuCl4− complexes.25 Our SEM studies of this complex evidence the formation of micrometer-sized “flakes” (Figure S1); it is only upon the addition of a dithiol that these flakes transform exclusively into nanorings (Figure 3a, SEM image). We emphasize that this effect cannot be attributed to any drying or substrate effects since the cryo-TEM studies (Figure B
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Figure 2. (a−d) SEM images of gold nanorings synthesized with [dithiol]/[HAuCl4] ratios equal to 1, 2, 3, and 4, respectively. Concentrations used were 1 mM CTAB, 1 mM HAuCl4, and 50 mM NaBH4 (1 mL), and the stirring time before the addition of NaBH4 was 300 s. (e) As the [dithiol]/ [HAuCl4] concentration ratio increases, the pore size decreases. Error bars are based on the measurement of at least 300 nanorings for each condition.
Figure 3. (a) SEM and (b) cryo-TEM images of ring structures formed by CTAB, gold, and dithiol prior to borohybride reduction. Concentrations used were 1 mM CTAB, 1 mM HAuCl4, and 1 mM dithiol, all stirred for 300 s. The inset in (a) gives the statistics of the wall thickness of the nanorings based on the inspection of 331 particles. The average thickness is ∼20 nm, which is close to a two-layer CTAB-Au-dithiol nanoring structure. (c) The powder XRD spectrum of nanorings features two broad peaks, which, based on our computer modeling, can be assigned to the interplane spacing at smaller (2.7 Å) and larger (3.8 Å) curvature regions of the elliptical toroidal nanorings illustrated in (d). (d) The molecular cartoon and (e) cross-section of a CTAB-Au-dithiol assembly. Images were generated in Visual Molecular Dynamics (VMD) software using a Tcl script. Molecular structures of motifs were generated and loaded into the graphical interface and then rotated and translated to form the toroidal structure.26
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These structural changes were accompanied by pronounced changes in the UV−vis−NIR spectra. Whereas the original/ untreated nanorings absorbed mostly in the UV, they gradually became strongly visible (due to the plasmon resonance of Au nanoparticles30) near-IR absorbers as the structures metallized (Figure 4g). This strong NIR absorption heralds potential uses of our nanorings in optics and medicine.10,17 In conclusion, our procedure enables the synthesis of goldnanoring precursors and all-metal gold nanorings in large quantities (almost pure, Figures 1a and 3a) and with precise structural control. The unique feature of the underlying mechanism is that the thickness of the rings is dictated by the thickness of the “templating” surfactant bilayer. Thus, in addition to the control of the ring’s pore sizes we demonstrated here, it should be possible to adjust the ring thickness by using surfactants of different lengths. The precisely crafted metallic nanorings should find multifarious uses in optics and sensing.11,16,19
expected that such continuous ring structures would have optical properties significantly different from thoses of aggregates of disjointed AuNPs held together by organic linkers. To achieve this goal, we pursued two approaches. In the first one, we irradiated the ring structures (CTAB/Au/dithiol before borohydride reduction) with a TEM electron beam; such irradiation is known to produce structural transformations both through heating the sample28 and by sputtering away atoms.29 Here, the irradiation induced the growth and coalescence of polycrystalline AuNPs (Figure S10) whose dimensions depended on the irradiation intensity and time (Figure 4a−c). Still, this method did not yield continuous metal rings.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental procedures and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Non-Equilibrium Energy Research Center (NERC), which is an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award DE-SC0000989.
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