Chemical Transformation of Nanorods to Nanowires: Reversible

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Chemical Transformation of Nanorods to Nanowires: Reversible Growth and Dissolution of Anisotropic Gold Nanostructures Bishnu P Khanal, and Eugene R. Zubarev ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b09203 • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 13, 2019

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Chemical

Transformation

of

Nanorods

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Nanowires: Reversible Growth and Dissolution of Anisotropic Gold Nanostructures Bishnu P. Khanal and Eugene R. Zubarev* Department of Chemistry, Rice University, 6100 Main Street, Houston, TX 77005, United States *Corresponding author: [email protected] ABSTRACT: This manuscript describes a reversible wet chemical process for the tip selective one-dimensional (1D) growth and dissolution of gold nanorods (AuNRs) and gold nanowires (AuNWs). Tip selective dissolution was achieved by oxidation of AuNRs with Au(III)/CTAB complex, whereas the growth of AuNRs was carried out by the reduction of Au(I) ions on AuNRs surface with a mild reducing agent, ascorbic acid (AA). Both the dissolution and growth processes are highly tip selective and proceed exclusively in one dimension. Decrease in the aspect ratio (AR) of AuNRs during the dissolution resulted in the blue shift in the longitudinal plasmon band (LPB) position and red shifts in LPB position were achieved by increasing the AR by 1D-growth of AuNRs. Both growth and dissolution processes are fully controllable and can be stopped and resumed at any given time when desired AR and/or LPB position is achieved. In addition, the tip selective 1D growth of AuNRs can be continued with the additional supply of Au(I)/CTAB/AA solution to produce extremely long gold nanowires (AuNWs). KEYWORDS: gold nanorods, aspect ratio, growth, dissolution, nanowires, reversible

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Gold nanorods (AuNRs) are receiving particular attention due to their very strong near infrared (NIR) LPB1-4 which makes them applicable in surface enhance Raman spectroscopy,4-12 sensing,13-15 diagnostics, imaging,16-21 and therapeutics.21-26 According to discrete dipole calculation,27 the longitudinal plasmon band (LPB) position of AuNRs depends on their aspect ratio (AR) ( max (nm) = 96AR + 418) which can be tuned from visible to infra-red by tuning their AR.1, 21, 24-25, 27-31 Among many synthetic methods, the seed-mediated growth method with28 and without29 the addition of silver nitrate (AgNO3) are the two most established procedures for the synthesis of AuNRs. The synthesis method with AgNO3 produces single crystalline AuNRs of AR (3±1), whereas the method without AgNO3 produces pentahedrally twinned high AR (up to 25±2) AuNRs. We have recently reported the synthesis of AuNRs by using hydroquinone32 and dopamine33 as a reducing agent as opposed to ascorbic acid (AA) which resulted in higher AR AuNRs with LPB ~1200 nm wavelength. All these procedures for the synthesis of AuNRs are limited to certain ARs, and, thus, to a corresponding certain LPB position. Thermal,34-36 laser-induced heating,37 dissolution with cyanide,38 and chemical39-45 reshaping have been applied to tune the plasmon position of nanoparticles. The intrinsic limitations of the reported procedures are that they can only decrease the LPB by decreasing the AR and have been applied only on the spherical nanoparticles and nanorods of AR 4 or less. Therefore, a procedure which enables the synthesis of AuNRs with full control over AR as well as LPB position is highly desirable. In addition, gold nanowires (AuNWs) are of great interest due to their excellent thermal and electrical conductivities45-47 which are important in the fabrication of optoelectronic devices48-50 and biological sensors.51 Several methods such as ultra-high vacuum transmission electron microscope (TEM) electron thinning technique,52 biological,53-55 template-assisted,56 and


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chemical57-61 have been reported for the synthesis of AuNWs. Nanowires produced by templateassisted methods have large width and very low overall yield, whereas assemblies of nanoparticles produce nanowires of poorly defined morphology. Moreover, the existing methods for the synthesis of AuNWs produce low yields and are usually limited to certain lengths. Here we report a very simple and effective chemical method for selective growth and dissolution of AuNRs from their tips with great control over their AR, consequently controlling the LPB position. We implemented a “bottom-up” approach in which short AuNRs were grown up to micrometers long AuNWs. Controlled reduction of Au(I) onto AuNRs resulted in a tip selective 1D growth of nanorods exclusively increasing their length while keeping the diameter virtually unchanged. On the other hand, controlled and selective dissolution of AuNWs was achieved by adding aqueous solution of HAuCl4.3H2O and CTAB (Au(III)/CTAB complex). The dissolution starts and propagates from the tips of AuNRs with no impact on their sides producing lower AR AuNRs. This method enables the synthesis of AuNRs of controlled AR with a specific LPB. Most importantly, 1D growth and dissolution of nanostructures was achieved via chemical reaction in completely reversible manner and the synthesis of extremely long AuNWs was accomplished starting from short AuNRs with full control to manipulate their length. The rate of 1D dissolution and growth of AuNRs was found to be proportional to the amount of oxidizing (Au(III)/CTAB) and reducing (Au(I)/CTAB/AA) agents supplied, therefore a precise control of the reacting agents is required to tune the AR of resulting nanostructures.


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RESULT AND DISCUSSION Dissolution of Single Crystalline AuNRs. Selective dissolution of single crystalline AuNRs from their tips was used to decrease their AR, leading to a controllable blue shift in LPB position. The starting AuNRs were prepared by the seed mediated growth method originally described El-Sayed et al.28 While performing other experiments, we observed that the addition of Au(III)/CTAB complex can fully dissolve AuNRs in solution. We decided to study this b

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1.9 1.4 0.9 0.4 -0.1 400

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Before Au(III) 1.67 µmol Au(III) 3.35 µmol Au(III) 6.67 µmol Au(III) 11.72 µmol Au(III) 16.75 µmol Au(III) 25.12 µmol Au(III) 33.5 µmol Au(III) 41.8 µmol Au(III) 50.25 µmol Au(III) 58.62 µmol Au(III) 67.0 µmol Au(III) 75.38 µmol Au(III) 83.75 µmol Au(III) 92.13 µmol Au(III) 100.00 µmol Au(III)

Figure 1. TEM images of single crystalline AuNRs before dissolution (a); and after the addition of 33.5 (b), 58.62 (c), 67.0 (d), 73.38 (e), and 100 µmol of Au(III)/CTAB complex (f). UV-Vis spectra of AuNRs during the dissolution upon the addition of Au(III)/CTAB complex (g). observation in a controlled fashion.

Therefore, a portion wise addition of Au(III)/CTAB

complex (prepared by missing HAuCl4.3H20 and CTAB) was carried out into the AuNRs solution upon vigorous stirring. After the complete consumption of Au(III)/CTAB (~2h), the AuNRs were examined under TEM and their LPB was monitored by UV-Vis spectroscopy. Figure 1 shows representative TEM images of AuNRs at various stages of dissolution. The length of AuNRs decreased while their diameter remains unchanged with the increase in the amount of Au(III)/CTAB solution added, which suggests that the dissolution starts and proceeds


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exclusively from the tips of AuNRs. The average dimension (length x width) (l x w) of AuNRs before the dissolution was 60.4 ± 8.7 nm x 17.5 ± 2.7 nm (sample size (N) = 635 AuNRs) (see Fig. 2 and supporting information (SI) Fig. SI 1 for l x w size distribution histograms and the representative TEM images used for size calculations). Introduction of 33.5 µmol of Au (III)/CTAB complex reduced the average dimensions l x w to 51.3 ± 8.8 nm x 17.5 ± 2.08 nm (N = 722 AuNRs). Further additions of Au (III)/CTAB solutions in the amount of 58.62 µmol, 67.0 µmol, 73.38 µmol continued the tip selective dissolution resulting in 34.19 ± 5.4 nm x 18.1 ± 2.1 nm (N = 810 AuNRs), 28.49 ± 4.2 x 17.65 ± 1.8 nm (N = 776 AuNRs), 17.49 ± 2.7 x 16.67 ± 1.32 nm (N = 844 AuNRs) of AuNRs, respectively. Nearly isometric nanoparticles were observed after the addition of 73.38 µmol Au(III)CTAB solution. All nanoparticles were measured along their x and y axes which corresponded to their width and length, respectively. (See Fig. 2 and Figs. S1-5 for nanoparticles size distribution and additional representative TEM images used for size calculations). The dissolution continued until only spheres were present and after the addition of 100 µmol of Au(III)/CTAB solution all the AuNRs were completely dissolved (Fig. 1f) resulting in colorless solution. Note there is a significant reduction in the length of AuNRs, whereas the width remains virtually unchanged (Fig. 2 and Figs. SI 1-5). The high tip selectivity in dissolution is due to much higher surface curvature of the tips in comparison with the flat sides. As a result, they are poorly protected by the CTAB bilayer which favors extended (100) and (110) planes on the sides of the rods.62 The characteristic LPB position of as synthesized AuNRs was at 763 nm. As the AuNRs dissolved and their AR decreased, the LPB gradually blue shifted until there was no absorption peak (Fig. 1 g). Fig. SI 6 shows the change in color of the AuNRs solution during the 1D dissolution. The initial AuNRs


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solution color was brown and changed gradually to green, blue, purple, and finally colorless solution containing only Au(I) ions as a comproportionation product of Au(0) and Au(III) ions.

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Figure 2. Histograms showing the size distributions (length and width) of AuNRs during dissolution process. Change in length (panel a-e) and change in width (panel f-j), before dissolution, and after the addition of 33.5, 58.62, 67.0, 73.38 µmol of Au(III) solution respectively. As a control experiment, dissolution of AuNRs with the mixture of AuCl3 (as opposed HAuCl4.3H20) and CTAB under identical set of experimental condition were performed. Importantly, no difference in nature and kinetics of dissolution were observed when AuCl3 was used. This observation clearly suggests that the dissolution of AuNRs is not because of HCl present in HAuCl4.3H2O but due to the formation of Au(III)/CTAB complex. The most important outcome of this experiment is that one can control the AR and therefore, the corresponding LPB position of AuNRs at any point of reaction. The dissolution can be stopped and AuNRs can be isolated at any desired LPB position by centrifugation and subsequently dissolving nanorods in aqueous CTAB solution. The aforementioned procedure is very efficient at producing lower AR AuNRs. In strong correlation with what Mirkin and co-workers44 have reported, all attempts to induce 1D growth of these single crystalline nanorods along their pirnciple axis at various stages


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of dissolution were unsuccessful and instead resulted in a uniform amplification of the nanorods (Fig. SI 7). Dissolution of pentahedrally-twinned AuNRs: It is known that AuNRs prepared from citrate capped seeds without the addition of AgNO3 into the growth solution have a higher AR and a pentahedrally-twinned crystal structure.63 We hypothesized that because of the different crystal structure they might behave differently than the single-crystalline AuNRs during the dissolution and growth. Pentahedrally-twinned AuNRs were synthesized by the slight modification of the methods described by Murphy et al.22 In this method, pentehedrally twinned AuNRs form along with several side products consisting of spherical, triangular, pentagonal,

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hexagonal, trapezoidal nanoparticles, and some low aspect ratio nanorods. Pure pentahedrally twinned AuNRs were isolated from complex mixture by gravitational precipitation and partial

dissolution

technique.64

Figure 3a shows the TEM image of pure AuNRs before dissolution. The average dimensions (l x w) of these AuNRs were 281.39 ± 49.53 nm x 22.10 ± 1.5 nm (N = 493 AuNRs) (Fig. 6 shows the histograms for size

Figure 3. TEM images of pentahedrally twinned AuNRs during the dissolution process. a) before Au (III) (b) after the addition of 0.75 µmol (c) 0.90 µmol (d) 1.1 µmol of Au(III).

distribution and Fig. SI 8 presents


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detailed statistical calculations and representative TEM imges used for size calculation) with theoretical UV-Vis absorbance ( max (nm) = 96AR + 418)27 at ~1635 nm which we have demonstrated before by transferring AuNRs to D2O.64 In this study ~350 nm and ~400 nm AuNRs synthesized during 1D growth (discussed below) were transferred to D2O to capture the long range UV-Vis-NIR absorbance (see Fig. SI 9). The theoretical LPB peak position for these AuNRs samples is ~2150 nm therefore only half of the peak can be observed up to 1840 nm range. In order to decrease the AR of pentahedrally twinned AuNRs, dissolution technique by using Au (III)/CTAB complex was applied as described above. After the introduction of 0.75 0.8

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Before Au(I) 0.50 µmol Au(I) 0.62 µmol Au(I) 0.70 µmol Au(I) 0.82 µmol Au(I) 1.00 µmol Au(I)

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Figure 4. UV-Vis-NIR spectra of pentahedrally twinned AuNRs a) during the dissolution with Au (III)/CTAB complex b) during the one-dimensional growth with Au(I)/CTAB/AA solution. The legends on the graphs correspond to the amount Au(III) and Au(I) ions added during the dissolution and growth of AuNRs. The UV-Vis-NIR cut off range is ~1400 nm due to strong absorbance from H2O molecule after 1400 nm range. Capturing UV-Vis beyond 1400 nm requires transferring AuNRs to D2O which was beyond the scope of this study. Few graphs have cut off at 1100 nm due to instruments’ limitation.


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µmol Au(III)/CTAB, the average length shrank to 198 ± 25.67 nm whereas the average width remained unchanged (22.03 ± 1.35 nm) (N = 674 AuNRs) (Fig. 3b). TEM analysis further showed that the dissolution exclusively started and proceeded from the tips of AuNRs and gradually reduced their AR (Fig. 3b and SI 10). Unsurprisingly, the nature and kinetics of dissolution in the case of pentahedrally twinned AuNRs was very similar to that of single crystalline AuNRs. Further addition of 0.15 µmol (total of 0.9 µmol) of Au(III)/CTAB complex dissolved the AuNRs up to the average dimension of 130 ± 30.17 nm x 21.62 ± 1.47 nm with the average AR 5.9 ± 1.35 (N = 709 AuNRs) (Fig. 3c and SI 11). The dissolution was continued with another portion of 0.2 µmol (total of 1.1 µmol) Au(III)/CTAB and resulted in AuNRs with the average dimensions of 65.34 ± 16.28 nm x 22.20 ± 1.09 nm (N = 1387 AuNRs) (Fig. 3d and SI 12). The shift in LPB with the change in AR of AuNRs was monitored by using UV-Vis spectroscopy. The gradual blue shifts in LPB was observed with the decrease in AR of AuNRs (Fig. 4a). The LPB of starting AuNRs was >1400 nm before the introduction of Au(III)/CTAB solution. After the addition of 0.90 µmol of Au(III), the LPB shifted to ~1117 nm. Further addition of 0.95 µmol, 1.00 µmol, 1.05 µmol and 1.1 µmol of Au(III) continuously blue shifted the LPB to 1035, 952, 887, and 777 nm, respectively. Further supply of Au(III)/CTAB complex dissolved the AuNRs completely resulting in colorless solution with no absorbance in UV-Vis range. (see Figs. SI 8, 10-12 for the statistical details for size calculation during the dissolution process and additional representative TEM used for size calculation). It should be noted that the length standard deviation (σ) of final AuNRs is significantly smaller than that of original AuNRs; the gradual decrease in length σ indicates that the AuNRs become more monodisperse during dissolution. The width of AuNRs and width σ remained unchanged, demonstrating the tip selectivity during the dissolution. The tip selective dissolution technique described for both


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single crystalline and pentahedrally twinned AuNRs enables the synthesis of AuNRs with LPB position ranging from ~524 to ~1635 nm. 1D Growth of Pentahedrally-twinned AuNRs: Most surprisingly, we were able to find a simple and reliable experimental technique to induce 1D growth in case of pentahedrally twinned AuNRs along their principal axis, which is in stark contrast to single crystalline AuNRs. In order to induce a 1D growth of AuNRs, mixture of Au(I) ions, CTAB, and ascorbic acid (Au(I)/CTAB/AA) was used. A slow addition of Au(I)/CTAB/AA resulted in selective deposition of metallic gold at the AuNRs tips. It is evident that there is a significant increase in length of AuNRs, whereas their width a

b

remained constant (~22 nm). Figure 5 shows TEM images of AuNRs during 1D tip selective growth process. The starting AuNRs (Fig. 5a) used for the growth

c

process were the resulting AuNRs after

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dissolution with the addition of 1.1 µmol Au(III)/CTAB nanorods with the average l x w of 65.34 ± 16.28 nm x 22.20 ± 1.09 nm (N = 1387 AuNRs) (Fig. 5a and SI continuous

12). The length of AuNRs increased up

transformation of pentahedrally twinned AuNRs

to 330.38 ± 61.18 nm after the addition of

Figure

5.

TEM

images

of

into AuNWs a) before Au(I), b) after 2.50 µmol c) 4.50 µmol d) 7.5 µmol addition of Au (I) and AA.

2.50 µmol Au(I)/CTAB/AA solution whereas the AuNRs width remained unchanged (22.79 ± 1.38 nm) (N = 617


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AuNRs) (Fig. 5b and SI 13), which is a noteable observation. As a result, the average AR increased to ~14.57 ± 2.9 and a red shift in the LPB ensued. Fig. 4b shows the continuous redshift in LPB position of AuNRs with the addition of Au(I)/CTAB/AA solution. The LPB of AuNRs before the introduction of Au(I) was at 777 nm. After the addition of 0.5, 0.62, 0.70, 0.82, and 1.0 µmol of Au(I) ions, the LPB peak gradually shifted to 966, 1092, 1173, 1302, and 1362 nm, respectively. Further addition of Au(I)/CTAB/AA solution (4.5 µmol and 7.5 µmol) continued the 1D growth of AuNRs resulting in 645.88 ± 108.28 nm x 24.30 ± 1.48 (N = 434 AuNRs) (Fig. 5 c) and 1024 ± 145.51 x 25.72 ± 3.0 nm AuNRs (Fig. 5d) (N = 525 AuNRs), respectively (see Fig. 6 and Figs. SI 13-15 for size distribution calculation and representative TEM iamges). The most interesting result of this experiment is that there is ~16 fold increase in length of AuNRs whereas the width remains the same without any appreciable change. However, the increase in length distribution suggests that when AuNRs reach greater than one micron length and essensially become nanowires (AuNWs), it becomes challenging to control their length distribution. The AuNRs kept growing one-dimensionally with the introduction of newer portion of Au(I)/CTAB/AA solution and the increase in their length was strictly proportional to the amount of Au(I)/CTAB/AA solution added. The growth stops when the reducing agent Au(I)/CTAB/AA is fully consumed. However, the growth of AuNRs can be fully resumed at any later point when a new portion of the gold precursor (Au(I)/CTAB/AA) is introduced. In principal, this technique is reminiscent of a living polymerization reaction, which offers a great control over the length of linear molecular structures. Most importantly, this continuous increase in length eventually leads to a critical transformation of AuNRs into gold nanowires (AuNWs) (Fig. 7) which is particularly important experimental observation for the controlled transformation of preformed


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Figure 6. Histograms showing length and width distribution of AuNRs during the dissolution and growth process and the corresponding TEM images. Length distribution (a-g), width distribution (o-u) and corresponding TEM images (h-n) before dissolution, and after the addition of 0.75 µmol, 0.90 µmol, & 1.1 µmol of Au (III) and 2.50 µmol, 4.50 µmol, 7.5 µmol of Au(I) during the growth and dissolution processes, respectively. The scale bar in all TEM images is 200 nm. AuNRs to AuNWs. The growth and dissolution process on a single batch of AuNRs was repeated three times and no morphological changes in AuNRs were observed, which strongly suggests that the process is fully reversible multiple times. After three rounds of growth and dissolution, the AuNRs were carefully analyzed by high resolution TEM (HRTEM) to understand if any morphological changes occurred on the surface of AuNRs. HRTEM revealed no crystallographic changes in AuNRs (see Fig. SI16). Each nanorod has five equivalent flat (100) side facets and five single crystal subunits connected by twinned (111) facets as was previously observed by Mann et al.63 The exact reason and mechanism of both 1D dissolution and growth on pentahedrally twinned AuNRs as opposed to only 1D dissolution, but no growth on single crystalline AuNRs, is not fully understood yet. We hypothesize that the density of CTAB coverage on the (111) tips


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of pentahydrally twinned AuNRs is much lower due to their higher surface curvature when compared to (100) tips of single crystalline AuNRs. In addition, the rate of gold atoms migration from (111) tip to higher energy (100) side facets is likely to be low, which may explain the a

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Figure 7. Representative TEM images of AuNWs synthesized by 1D growth of small AuNRs: a) 1.5 -2 µm b) 2-2.5 µm c) and d) 2.5 -3.5 µm in length. constant diameter of the nanorods and nanowires during their growth. In contrast, the surface diffusion of gold atoms from the tips to the side facets of single crystalline AuNRs can be


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significant because the tips contain not only (111), but also (110) facet. In fact, it has been recently demostrated by Chon et al.65 that the rate of surface diffusion strongly increases with the aspect ratio of single crystalline AuNRs, and that may be the reason why a uniform amplification is obseved when one attempts to increase their length alone. The AuNRs growth process was continued by the introduction of newer portion of Au(I)/CTAB/AA solution which resulted in the transformation of small AuNRs to long gold nanowires (AuNWs) measuring up to 4.6 µm in length and 180 in AR, which constitutes nearly ~5000% increase in length (Figs. 7 and SI 17). The most critical step for the 1D growth was the time and concentration of Au(I)/CTAB/AA. Maintaining fixed and low concentration of Au(I)/CTAB/AA controls the supply of Au(I) ions which slows down the kinetics of Au(I) disproportionation to Au(0) and Au(III) resulting in the selective deposition on the more active tips of AuNRs compared to CTAB-blocked (100) side facets. However, if the concentration of AA or Au(I) is set high, the fast reduction occurs resulting in the deposition on both tips and sides of AuNRs. The one-dimensional growth of AuNRs can be stopped at any length and LPB position and AuNWs can be isolated with centrifugation and re-dispersion in CTAB aqueous solution. This experimental data suggests that one can have complete control over the length of AuNRs/AuNWs, which can be increased and/or decreased at will in a fully reversible fashion upon the addition of Au(I)/CTAB/AA and Au(III)/CTAB solutions, respectively.

CONCLUSION In conclusion, a method for fine tuning the dimensions and optical properties of 1D gold nanostructures ranging from small single crystalline and pentahedrally twinned AuNRs to micrometers-long AuNWs have been developed. This is achieved by reversible length


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manipulation of pre-synthesized AuNRs, which themselves can be used a 1D seed particles. The 1D dissolution of AuNRs was carried out with Au(III)/CTAB complex and the growth was carried out with Au(I)/CTAB/AA solution. This method enables the synthesis of AuNRs of desired aspect ratio and LPB peak as the growth and dissolution can be stopped at any given point of reaction and AuNRs/AuNWs can be isolated from the reaction mixture via quick centrifugation and re-dispersion in CTAB aqueous solution. This study also demonstrates that the reversible growth and dissolution is possible only in the case of pentahedrally twinned AuNRs. However, only the tip selective dissolution to reduce the length and AR is possible for single crystalline AuNRs. The different behavior during 1D growth between single crystalline and pentahderal AuNRs is possibly due to the difference in their tip‘s surface curvature and the density of CTAB binding at their tips. The reversible growth and dissolution technique described in this report is a very efficient and simple technique for the synthesis of AuNRs ranging from ~20 nm to 4.6 µm in length with the aspect ratio ranging ~3 to 180. The continuous 1D growth of AuNRs essentially transforms the small AuNRs to microns-long AuNWs. It is worth noting that the presented technique brings a number of advantages over existing methods of nanowires synthesis. The use of rod-shaped seeds (as opposed to conventional spherical seed particles)66,67 ensures the purity of the resulting NWs and the absence of random numcleations that are known to lead to the formation of various morphologies and insaparable mixtures. In addition, the synthesis is not only reversible, but can be repeated multiple times without any increase in the diameter of the nanostructures. Therefore, the methods presents a rare opportunity for direct transformation of nanorods to nanowires.


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METHODS Material and Characterization: Unless otherwise stated all the chemicals were purchased from commercial suppliers and used without further purification. Cetyltrimethylammonium bromide (CTAB) was purchased from Acros Organics Inc. Hydrogen tetrachloroaurate trihydrate (HAuCl4.3H2O), AuCl3, ascorbic acid, sodium borohydride, silver nitrate, and sodium citrate were purchased from Sigma-Aldrich. De-ionized (DI) water was used for all the experiments. TEM images were obtained on a JEOL 1200EX transmission electron microscope operating at 100 kV accelerating voltage, and JEOL 2010 TEM microscope using carbon-coated copper grid (Electron Microscopy Sciences). For the preparation of TEM samples 1.5 mL of nanorods solution was centrifuged (13000 rpm for smaller size and 5000 rpm for bigger size AuNRs) for 10 min followed by removal of the supernatant containing excess CTAB. The precipitate was redispersed in 100 µL of pure DI water upon brief sonication for 10-15 sec. The TEM samples were prepared by dip coating method followed by drying in ambient conditions. AuNRs size calculation: ImageJ software was used for AuNRs size calculation. The images used for size calculation were randomly selected from different locations of TEM grids in order to identify the most representative images. From each image used, 10 nanorods were randomly selected and their size (l x w) was carefully measured by using photoshop ruler tool. Next, the representative TEM images were imported to ImageJ for size calculation. The size of nanorods from ImageJ output was verified and compared with that of photoshop manual measurements. The ImageJ size calculation algorithms were adjusted until their output size matched with that of manual measurements. All the partial nanorods from the edge of an image were excluded from size calculations. The data output from ImageJ was analysed by using stastistical software JMP from SAS.


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Synthesis of single crystalline gold nanorods: Single crystalline AuNRs were synthesized by using seed mediated growth method described by El-Sayed et al.28 Table 1. Volume of Au(III)/CTAB solution used for the dissolution of regular AuNRs in the period of 2 hrs. Entry

Volume of Au(III)/CTAB

Time (hrs.)

solution (mL) 1.

1.00

0

2.

1.00

2

3.

2.00

4

4.

3.00

6

5.

3.00

8

6.

5.00

10

7.

5.00

12

8.

5.00

14

9.

5.00

16

10.

5.00

18

11.

5.00

20

12.

5.00

22

13.

5.00

24

14.

5.00

26

15.

5.00

28

Dissolution of single crystalline AuNRs: In a 200 mL Erlenmeyer flask, 3.64 g of CTAB was dissolved in 50 mL of water upon gentle heating with a heat gun.

Separately, 66 mg of

HAuCl4.3H2O was dissolved in 50 mL of water and mixed with CTAB solution. The resulting mixture was used for dissolution of AuNRs (15 mg in 500 mL solution) as shown in a table 1.


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Synthesis of pentahedrally-twinned AuNRs: Pentahedrally twinned high aspect ratio AuNRs were synthesized by using seed mediated approach29 and their purification and isolation was done by selective dissolution and gravitational sedimentation technique as described in our earlier report.64 Dissolution of pentahedrally-twinned AuNRs: Dissolution of high aspect ratio AuNRs was carried out with a solution containing 0.1 M CTAB and 5x10-4 M HAuCl4.3H2O. The time of addition and volume of solution are shown in the table 2. Table 2. Volume of Au(III)/CTAB used for the dissolution of pentahedrallytwinned AuNRs in the period of 14 hrs. Entry Volume of Au (III)/CTAB

Time (hrs.)

solution (mL) 1.

0.5

0

2.

0.5

14

3.

0.5

28

4.

0.5

42

5.

0.5

56

6.

0.5

70

7.

0.5

84

8.

0.5

98

9.

0.5

112

10.

0.5

126

11.

0.3

140

12.

0.2

154

13.

0.2

168


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ACS Nano

14.

0.2

172

15.

0.1

184

16.

0.1

196

17.

0.1

210

One-dimensional growth of pentahedrally-twinned AuNRs: For the selective growth of pentahedrally-twinned AuNRs from their tips, a fresh growth solution was prepared each time by dissolving 364 mg of CTAB and 0.98 mg of HAuCl4.3H2O in 10 mL of DI water. To this solution, 45 μL of 0.0788 M ascorbic acid was introduced. The resulting mixture was added to the AuNRs solution at the rate of 10 µL per 5 seconds with vigorous stirring and left undisturbed. The time of addition and the volume of solution used are given in the table 3.

Table 3. Volume of Au(I)/CTAB/AA added for the one-dimensional growth of pentahedrally-twinned AuNRs in 14 hrs. interval Entry Volume of Au(I)/CTAB/AA solution

Time (hrs.)

(mL) 1.

0.5

0

2.

0.5

14

3.

0.5

28

4.

0.5

42

5.

0.5

56

6.

0.5

70

7.

0.5

84

8.

0.5

98


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9.

1.00

112

10.

2.00

126

11.

3.00

140

12.

3.00

154

13.

3.00

168

14.

3.00

182

15.

3.00

196

16.

3.00

210

17.

3.00

224

18.

3.00

238

19.

3.00

252

20.

3.00

266

21.

3.00

280

22.

3.00

294

ASSOCIATED CONTENT The Supporting Information (SI) is available free of charge on the ACS publication website at http://pubs.acs.org. Fig. SI 1-5, 8, 10-15 contain the detailed statistical calculation for size distribution during the growth and dissolution process and additional representative TEM images of AuNRs used for size measurement. Fig. SI 6 shows the photographs of AuNR solution showing the change in color during dissolution process. Fig. SI 7 shows the TEM images of uniform amplification of single crystalline AuNRs when attempted the 1D growth. HRTEM image of pentahedrally twinned AuNRs is shown in Fig. SI 16. Additional TEM images of long AuNWs are shown in Fig. SI 17.


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ACS Nano

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Eugene R. Zubarev: 0000-0002-6401-3287

ACKNOWLEDGMENT Financial support provided by NSF (DMR-1105878) and Welch Foundation (C-1703) is gratefully acknowledged by the authors.

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Table of Content (TOC) figure:


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Chemical Transformation of Nanorods to Nanowires: Reversible

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