Seed-Mediated Growth of Ultralong Gold Nanorods and Nanowires

Aug 7, 2013 - For exceptionally long gold nanowires reaching lengths up to 10 μm, ..... HNO3 concentration in the growth solution in flask C from 1 t...
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Seed-Mediated Growth of Ultralong Gold Nanorods and Nanowires with a Wide Range of Length Tunability Yu-Ning Wang, Wen-Tsing Wei, Chih-Wen Yang, and Michael H. Huang* Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan S Supporting Information *

ABSTRACT: This study reports a systematic approach to synthesize ultralong gold nanorods and nanowires using a seed-mediated growth approach. In the first series, the effect of growth solution pH on the lengths of nanorods prepared was investigated. Interestingly, although shorter rods (230−310 nm) were produced in a basic solution environment than in an acidic condition (330−410 nm), the nanorod yield is greatly improved with relatively few nanoplate byproducts formed. Nanorod growth proceeds quickly in a basic solution as evidenced by the fast solution color changes. By adjusting several experimental parameters with the aim to elongate the nanorod length in a tunable fashion, gold nanorods and nanowires with average lengths from 580 to 2850 nm can be synthesized by progressively increasing the HNO3 concentration in the final growth solution. Nanowire growth in a highly acidic solution is slower, and a substantially longer time is needed to reach long lengths. Further extension of the nanowire length can be achieved simply by reducing the volume of second growth solution transferred to the final growth solution. Nanorods and nanowires with lengths spanning from 700 nm to 4.5 μm were prepared in this series of experimental conditions. The longest nanowires can reach a length of up to 6 μm. The nanowires still maintain thin average diameters of 33−53 nm. The ability to make gold nanorods and nanowires over this exceptionally wide and useful length range is exciting because applications and demonstrations using ultralong gold nanorods and nanowires of most suitable lengths are now possible.

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INTRODUCTION Gold nanorods are a class of useful gold nanostructures because of the interesting properties derived from their strong lengthdependent surface plasmon resonance (SPR) absorption in the near-infrared region and one-dimensional morphology for nanoscale assembly applications.1−6 Their use as drug delivery vehicles has also been reviewed.7 For most studies on gold nanorods, relatively short rods with lengths of around 100 nm or less have been employed. More applications of gold nanorods can be considered if their lengths can be greatly elongated. To make longer gold nanorods with high aspect ratios and lengths beyond 600 nm, a seed-mediated and surfactant-directed synthesis approach has been developed by Murphy et al.8−10 Nitric acid was not used in their original procedures, so systematic tuning of the amounts of nitric acid used to promote nanorod growth was not investigated. We have shown that addition of nitric acid in the growth solution and use of cetyltrimethylammonium bromide (CTAB) surfactant in the preparation of seed particles can significantly improve the yield of gold nanorods with average lengths of 350−450 nm.11,12 However, for certain studies, such as the measurements of plasmon propagation lengths, nanocircuitry fabrication for conductivity and optical measurements, and plasmonic antenna fabrication, it is highly desirable to try highquality ultralong gold nanorods and nanowires if they are available.13−18 Frequently only one or a few nanowires are © 2013 American Chemical Society

needed in such measurements. For exceptionally long gold nanowires reaching lengths up to 10 μm, an acidic route (pH = 1) has been reported, but the solution may become so viscous that gelation can happen.19 To tune the lengths of long gold nanorods, variation of solution pH has been examined.20,21 In one study, tuning the solution pH and CTAB concentration can yield gold nanowires with lengths above 2 μm (pH = 2.5).20 In another study, the rod length can only reach around 600 nm at the lowest pH used (pH = 1.29).21 Although ultrathin gold nanowires have been reported with lengths reaching 2−4 μm, control of wire length was not demonstrated.22−24 Their ultrathin diameters of largely 1.6−3 nm and high mechanical flexibility make them a different class of onedimensional gold nanostructures from those synthesized by the seed-mediated growth approach. Few efforts have been devoted toward growth of gold nanorods and nanowires with lengths of 0.5−5 μm in recent years. The ability to prepare gold nanorods and nanowires over this extremely wide and broadly useful length range should greatly facilitate investigation of their plasmonic and other properties. In this study, we first examined the pH effect on the seedmediated growth of pentatwinned gold nanorods with average Received: March 18, 2013 Revised: July 25, 2013 Published: August 7, 2013 10491

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prepared. First, two 25 mL flasks and one 50 mL conical flask were labeled A, B, and C. For samples a−f, or series I samples (see Table S1 in the Supporting Information), 1.9681 g of CTAB, deionized water (52.569 mL for sample a, 52.596 mL for samples b and e, 52.623 mL for samples c and d, and 52.542 mL for sample f), and 1.35 mL of 0.01 M HAuCl4 solution were mixed. Next, various amounts of NaOH or HNO3 solution were added to the mixture (81, 54, and 27 μL of 0.5 M NaOH solution for samples a, b, and c and 27, 54, and 108 μL of 0.5 M HNO3 solution for samples d, e, and f, respectively). At this point, the total solution volume is 54 mL. CTAB and HAuCl4 concentrations in the growth solution are, respectively, 0.1 M and 0.25 mM excluding the volume of ascorbic acid. The growth solution shows a light yellowish color. Then 4.5 mL of the growth solution was added to flasks A and B and 45 mL to flask C. Finally, 25, 25, and 250 μL of 0.1 M ascorbic acid were, respectively, introduced into flasks A, B, and C. The flasks were gently shaken until the solutions became colorless. For samples g−h (series II samples) and samples l−p (series III samples), the same procedure was adopted to make the growth solution. Here a mixture of 1.093 g of CTAB, 29.475 mL of deionized water, and 525 μL of 0.01 M HAuCl4 solution was made first. A volume of 2.5 mL of the solution was transferred to flasks A and B and 25 mL to flask C. CTAB and HAuCl4 concentrations in the growth solution are, respectively, 0.1 M and 0.175 mM excluding the volume of nitric acid and ascorbic acid. Subsequently, 0.5 M HNO3 solution was added to flask C only (0.0504, 0.252, 0.550, 1.1, and 3.0 mL of HNO3 solution for samples g, h, i, j, and k and 0.0504, 0.252, 1.1, 3.0, and 5.0 mL of HNO3 solution for samples l, m, n, o, and p, respectively). Table 1 provides the concentrations of nitric acid in the growth solution excluding the volume of ascorbic acid. Finally, 10, 10, and 100 μL of 0.1 M ascorbic acid solution was introduced into flasks A, B, and C, respectively. Again, the flasks were gently shaken until the solutions became colorless. This step would take a much longer time with increasing nitric acid amount because the solution becomes more viscous. Synthesis of Utralong Gold Nanorods and Nanowires. For series I, II, and III samples, 400, 200, and 200 μL of the gold seed solution was, respectively, added to flask A and shaken slightly for 10 s. Immediately 400 (series I) and 200 μL (series II and III) of the solution in flask A was transferred to flask B and shaken for 10 s. Finally, 4 mL (series I), 2 mL (series II), and 200 μL (series III) of the solution in flask B was transferred to flask C and shaken for 5 s. Flask C was left undisturbed for 12 h for completion of nanorod and nanowire growth. The upper solution which contained mostly spherical particles was gently removed. Gold nanorods (or nanowires) and some triangular plates were left at the bottom of the flask. The products were redispersed in 10 mL of deionized water. The nanostructures were concentrated by centrifugation at 2000 rpm for 20 min twice (Hermle Z323 centrifuge). Instrumentation. Transmission electron microscopy (TEM) characterization was performed on a JEOL JEM-2100 microscope with an operating voltage of 200 kV. Scanning electron microscopy (SEM) images of the samples were obtained using a JEOL JSM-7000F electron microscope. Powder X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD-6000 diffractometer with Cu Kα radiation. UV−vis−NIR extinction spectra were taken using a JASCO V-570 spectrophotometer.

lengths of 230−410 nm. Remarkably, a basic solution condition was found to give a much higher yield of nanorods. By mainly adjusting the concentration of nitric acid in the growth solution and changing the volumes of growth solution transferred, ultralong gold nanorods and nanowires with systematically tunable lengths from 500 nm to 6 μm can be synthesized. A much slower growth rate in acidic medium is critical to formation of long rods and wires. These long gold nanorods and nanowires with thin diameters of a few tens of nanometers are promising one-dimensional gold nanostructures for emerging applications. Such a wide and fine degree of length tunability offers selection of nanorods and nanowires most appropriate for a particular situation.

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EXPERIMENTAL SECTION

Chemicals. Hydrogen tetrachloroaurate (III) trihydrate (HAuCl4· 3H2O, 99.9%, Aldrich), cetyltrimethylammonium bromide (CTAB, 98%, Alfa Aesar), sodium borohydride (NaBH4, 98%, Aldrich), ascorbic acid (AA, 99.7%, Riedel-de-Haën), nitric acid (HNO3, 65%, Fluka), and sodium hydroxide (NaOH, Riedel-de-Haën) were used without further purification. Ultrapure distilled and deionized water was used for all solution preparations. Preparation of Gold Seed Solution. The procedure used to synthesize ultralong gold nanorods and nanowires begins with preparation of a seed particle solution. Scheme 1 offers a brief

Scheme 1. Illustration of the Procedure Used To Make Ultralong Gold Nanorods and Nanowires

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RESULTS AND DISCUSSION The first part of this study concerns the effect of solution pH on the seed-mediated synthesis of high aspect ratio gold nanorods. An important finding of this part of the work is that high-purity gold nanorods can actually be obtained in a basic solution condition. Growth solutions with different concentrations of NaOH or HNO3 were prepared and added to flasks A, B, and C. The growth solutions have pH values of 9.1 (sample a), 8.4 (sample b), 4.9 (sample c), 3.4 (sample d), 3.2 (sample e), and 3.0 (sample f). Figure 1 presents SEM images of the gold nanorods obtained from samples a−f. A large

summary of the synthesis procedure. A volume of 10 mL of aqueous solution containing 0.25 mM HAuCl4 and 0.10 M CTAB was prepared in a 20 mL vial. Concurrently, 10 mL of 0.02 M ice-cold NaBH4 was freshly made. Then 450 μL of the NaBH4 solution was introduced into the CTAB−HAuCl4 solution while stirring vigorously. The resulting solution turned brown immediately, indicating formation of gold particles. After stirring for 5 min, the solution was kept undisturbed for 2 h at 27 °C to decompose excess borohydride by water. Seed particles have sizes of a few nanometers. Preparation of Growth Solution. Depending on the lengths of gold nanorods to synthesize, slightly different growth solutions were 10492

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Table 1. List of the Concentrations of NaOH and HNO3 in the Growth Solution, Volumes of the Solution in Flask B Transferred to Flask C, and Dimensions of the Gold Nanorods and Nanowiresa sample NaOH or HNO3 in flasks A, B, and C amount transferred to flask C

a

a b c d e f sample

0.75 mMNaOH 4 mL 0.5 mMNaOH 0.25 mMNaOH 0.25 mM HNO3 0.5 mM HNO3 1 mM HNO3 HNO3 in flask C amount transferred to flask C

g h i j k sample

1 mM HNO3 5 mM HNO3 10 mM HNO3 20 mM HNO3 53 mM HNO3 HNO3 in flask C

l m n o p

1 mM HNO3 5 mM HNO3 20 mM HNO3 53 mM HNO3 83 mM HNO3

2 mL

amount transferred to flask C 200 μL

average length (nm)

average diameter (nm aspect ratio

max length (nm)

average length (nm)

± 31 ± 28 ± 39 ± 34 ± 27 ± 47 average diameter

580 ± 100 996 ± 360 1024 ± 185 1289 ± 390 2848 ± 654 average length (nm)

25.3 33.5 33.7 23.5 33.7 average diameter (nm)

22.9 29.7 30.3 54.8 84.5 aspect ratio

760 1730 1280 2390 4010 max length (nm)

± ± ± ± ±

38.2 40 42.1 53 52

18.3 38.6 39.7 65.3 85.5

950 2360 2660 4840 6340

234 261 313 334 389 412

700 1544 1669 3464 4450

129 354 432 726 1068

17.7 19 20.9 20.2 19.3 19.8 (nm)

13.2 13.7 15 16.5 20.1 20.8 aspect ratio

290 300 340 380 545 760 max length (nm)

Standard deviations of nanorod lengths are also provided.

412 nm (sample f) as the solution pH decreases (see Table 1). The average nanorod diameters are in the range of 17−21 nm. The XRD pattern of the gold nanorods from sample a is available in Figure S3, Supporting Information. The (111) reflection peak gives an extremely high intensity relative to that of the (200) and (220) peaks. The (222) peak also has a higher intensity than that of the (200) peak. This diffraction pattern is typical for long gold nanorods, suggesting that the (111) lattice planes run parallel to the long axis of a nanorod.12 TEM analysis of the gold nanorods from sample a was also performed (see Figure S4, Supporting Information). Selected area diffraction (SAED) pattern taken over a single nanorod yields two sets of diffraction spots resulting from a superposition of square ⟨100⟩ and rectangular ⟨112⟩ zone patterns from an fcc structure.25 Such a SAED pattern is characteristic of nanorods with a pentatwinned structure bounded by 5 {100} side facets and 10 {111} end facets. The nanorod yield can be conveniently assessed by checking the UV−vis absorption spectra. Figure 2 gives UV−vis−NIR extinction spectra of samples a−f. The transverse absorption

Figure 1. Large-area SEM images of the gold nanorods obtained from samples (a) a, (b) b, (c) c, (d) d, (e) e, and (f) f. All scales bars represent 1 μm except in panels b and c, where the scale bars are equal to 100 nm.

amount of gold nanorods has been synthesized in each sample such that the rods readily form high-density packing arrangements on the substrate. SEM images suggest that the nanorod yield in these samples can be as high as 90% after removal of suspended spherical Au nanoparticles. The nanorods also appear fairly uniform in length (see Figure S1, Supporting Information, for their size distribution histograms). Figure S2, Supporting Information, gives small-area SEM images of the gold nanorods in samples a−f for close inspection of their dimensions and the nanoplate byproducts formed. The average nanorod lengths increase progressively from 234 (sample a) to

Figure 2. Normalized UV−vis−NIR extinction spectra of samples a−f. These spectra have been normalized by setting the lowest extinction points equal in the range of 600−640 nm for better comparison of spectral changes. 10493

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been tuned systematically by making one adjustment each time and finally combining these variables to arrive at the optimum condition. Figure 3 provides SEM images of the gold nanorods

band of gold nanorods centers at 500 nm, while the longitudinal band is expected to appear far beyond the spectral range into the near-infrared region with positions dependent on the aspect ratios of the nanorods.12 The longitudinal band is much stronger than the transverse band, so a rising trend in extinction toward the near-infrared region is observed in all spectra. The most important part of the spectra giving the most obvious changes is the surface plasmon resonance (SPR) absorption band from the nanoplate byproducts with a band position centered at 820−860 nm.26,27 The nanoplates have sizes of around 100 nm (see Figure S2, Supporting Information). Gold nanoplates also exhibit another band in the near-infrared region (∼1150 nm in this study). The SPR absorption band for the nanoplates decreases significantly and steadily from sample f to sample a, meaning that nanoplate formation is drastically reduced with increasing solution pH. Thus, despite the relatively short lengths of the nanorods synthesized in a basic solution condition with average lengths of 230−260 nm, nanorods can be obtained in very high yields with production of few nanoplates. In-situ UV−vis−NIR spectra were also taken for sample f (see Figure S5, Supporting Information). A gold SPR absorption band located at 540 nm and a shoulder band at 630 nm continued to grow in intensity from the first moment of reaction to around 15 min. Then the spectral profile stabilized even after 2 h. A gradual decrease in overall spectral absorbance was recorded after 13 h since reaction started due to settlement of the particles. These SPR bands are associated with formation of faceted particles suspended in the solution, showing that a large quantity of such particles has been produced. Nanorods settled to the bottom of the flask, and so they were not recorded in the in situ spectra. By collecting the product formed at the bottom of the reaction flask after the reaction, the UV−vis spectrum gives absorption features of nanorods and nanoplates similar to that shown in Figure 2, revealing that nanorods are mainly found at the bottom of the flask even with constant stirring of the solution. Previously we have shown that use of nitric acid in the seedmediated growth of gold nanorods can greatly improve the nanorod yield.11,12 When the same series I procedure was employed but replacing HNO3 with HCl of the same concentrations in the growth solution (0.25 and 1.25 mM), gold nanorods with similar lengths can be prepared (see Figure S6 and Table S2, Supporting Information). The results show that nitrate ions are not important to growth of long gold nanorods. A similar observation has been reported before.19 Previous consideration of nitrate ions facilitating the organization of CTAB into elongated micellar structures is incorrect.12 With the attempt to synthesize ultralong gold nanorods reaching lengths well beyond 500 nm and hopefully beyond 1 μm, several parameters in the growth process have been modified. For series II preparations to make ultralong gold nanorods and nanowires, major changes involve lowering HAuCl4 concentration in the growth solution from 0.25 to 0.175 mM, addition of nitric acid only to flask C instead of adding the acid to all three flasks of growth solution, lowering the volume of ascorbic acid introduced, reducing the total growth solution volumes in all flasks, and lowering the amount of solution transferred from one solution to the next. These changes were made on the basis of experimental observations to effectively extend nanorod length in a tunable fashion while limiting production of nanoplates. Reaction conditions have

Figure 3. SEM images of the gold nanorods and nanowires obtained from samples (a) g, (b) h, (c) i, (d) j, (e, f) and k. All scales bars represent 1 μm except in panel c, where the scale bar is equal to 100 nm.

and nanowires obtained from samples g−k. By progressively increasing the concentration of HNO3 in the growth solution in flask C from 1 to 53 mM, average nanorod lengths increase from 580 to 2850 nm. Nanorods in samples j and k have lengths significantly longer than 1 μm and are considered as nanowires. Figure S1, Supporting Information, shows the nanowire length distribution can be quite large. This is reasonable considering their extremely long lengths. Slight differences in the growth rate over a long reaction time can result in a large length distribution. Although appreciably more nanoplates have been observed, large quantities of nanorods and nanowires have been synthesized in these samples such that they can still form high-density side-by-side assembly. The nanowire yield can be more than 15%. Their diameters are still nicely controlled to a range between 25 and 34 nm, giving these 1-dimensional nanostructures high aspect ratios of 23−85. To understand how changes in the experimental conditions can lead to such a significant increase in the lengths or aspect ratios of gold nanorods and nanowires synthesized, a few points can be considered. First, it is obvious that increasing the amount of nitric acid introduced is the key because a slower rate of growth under acidic condition allows the rods to grow longer. Another major change is that the volumes of seed and growth solutions transferred were reduced from series I to series II. The idea here is that the presence of fewer seed particles should favor growth of longer wires, because each seed particle has more supply of gold atom source available to it for growth. This strategy has been used to vary the size of polyhedral gold nanocrystals synthesized by a seed-mediated growth appraoch.28,29 Since the volumes of the growth solution were reduced, the volumes of ascorbic acid added were also decreased. Other experimental changes were made to lower the yield of nanoplate byproducts, including addition of nitric acid to only flask C. 10494

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Figure 4. (a) TEM image of the gold nanowires obtained from sample k with a solution pH of 1.0. (b, c) Magnified TEM image of the square region at the end of the nanowire shown in panel a and its corresponding SAED pattern. (d, e) Magnified TEM image of the square region near the middle section of the nanowire shown in panel a and its corresponding SAED pattern.

of 450 nm was recorded. Nanowires with lengths of 1.9−2.2 μm were observed after 5 h of reaction. The results clearly reveal that nanowires are synthesized via a long growth process; several hours are needed to fully grow the nanowires to their long lengths. Such a drastic difference in the gold nanorod growth rate can be understood by considering how solution pH influences equilibrium of the redox reaction. First, in the presence of CTAB surfactant, ligand exchange can occur to form AuBr4− due to the stronger affinity of bromide ions to bind to gold atoms than chloride ions do as seen for palladium.30 In such case, it is AuBr4− instead of AuCl4− being reduced to form Au atoms. The following redox reaction should occur to form Au.

TEM analysis on a long gold nanowire formed in sample k was performed (see Figure 4). The nanowire appears quite uniform in diamater throughout its length. SAED patterns taken over the end and central regions of the nanowire give the same diffraction pattern comprised of a superposition of square and rectangular zone patterns. This analysis shows that the gold nanowire still maintains the same pentatwinned structure as that for the shorter rods. In the synthesis of gold nanowires, it is obvious to realize that they need a much longer time to form as evidenced by the slow solution color changes. Figure S7, Supporting Information, shows photographs of flask C for sample a as a function of reaction time. Under the most basic solution pH used (pH = 9.1), a light pinkish color develops within 30 s after the transfer of growth solution from flask B to flask C. The pinkish color darkens continuously to look purplish in 3 min, and the dark purplish solution color stops changing within 10 min of reaction. This visual observation indicates a rapid nanorod growth rate under a basic solution environment. This should explain their uniform but relatively short lengths, because nanorod growth stops possibly within 10 min of reaction to prevent further extension of their lengths. The distribution of nanorod diameter is also narrower with a shorter reaction time. Figure S8, Supporting Information, presents photographs showing the solution color changes in flask C of sample k for growth of gold nanowires. The solution barely turns a light pinkish color after 2 h of reaction. The solution color darkens and appears to be not changing after 5 h of reaction. Clearly the nanorod and nanowire growth rate is much slower under acidic solution condition, and so a much longer time is required to form long nanowires. TEM examination of some intermediate products formed in flask C of sample k was carried out to confirm the slow growth involved in formation of gold nanowires (see Figure S9, Supporting Information). A drop of the reaction solution was withdrawn from the flask and added directly to the TEM grid. After 1 h and 40 min of reaction, a nanorod with a length of 75 nm and a width of 35 nm was found. Other rods also displayed similar lengths. After 3.5 h of reaction, a nanorod with a length

2AuBr4 − + ascorbic acid + H 2O ⇌ 2Au + dehydroascorbic acid + 2H+ + 4Br −

Addition of NaOH shifts the equilibrium to the right, favoring greater supply of gold atoms for particle growth. Sodium ascorbate, formed by mixing ascorbic acid and NaOH, has also been reported to be an effective reducing agent for formation of gold nanorods.10 Furthermore, hydroxide ions can act as a reducing agent. By mixing NaOH and HAuCl4 to form a solution with 0.75 mM NaOH and 0.25 mM HAuCl4, the solution quickly turns from yellowish to colorless by forming Au(I) species, indicating the reducing ability of NaOH. With addition of CTAB surfactant, the solution gradually turns purplish within a day as a result of gold nanoparticle formation. Let us consider the acidic solution condition next. AuBr4− has a similar standard reduction potential (0.805 V) to that of HNO3 (0.955 V), so the reduction of AuBr4− to form metallic Au can be slower in the presence of nitric acid. Most importantly, it has been found that solution viscosity increases dramatically by simply mixing CTAB surfactant with a HNO3 or NaNO3 solution but not with a HCl solution, suggesting the role of nitrate ions in causing this effect. The increase in solution viscosity upon adding a NaNO3 solution to a CTAB solution has been found to cause a drastic structural transformation of spherical micelles into long and flexible wormlike micelles, and 10495

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this hinders the flow of solution.31 Ionic diffusion is more difficult under this condition, so nanorod growth is much slower when more nitric acid is added. It is highly desirable to further elongate the nanowire length in a tunable fashion. Remarkably, this can be done by simply reducing the volume of growth solution transferred from flask B to flask C from 2 mL in series II to 0.2 mL in series III while keeping all other conditions unchanged (see Scheme 1). This modification is based on the idea that if fewer nucleating rods are introduced to flask C, more gold precursor sources are available for particle growth and so longer nanowires may be obtained. Figure 5 shows SEM images of gold nanorods and

considered as 1-dimensional nanostructures and are single crystalline (although they are pentatwinned) throughout the length of the nanowires. These features make nanoelectronic measurements more reliable. Nanowires which are formed from fusion of shorter segments contain many junctions.32 This can be undesirable for nanoelectronic measurements because of the greater electrical resistance present at the junctions and inconsistent results from one wire to another. With substantial dimensional differences between the ultralong nanowires and nanoplates, it should be possible to develop simple separation strategies to obtain high-purity nanowires for their use in many situations, such as the use of more precise centrifugation steps and proper filter papers or membranes.

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CONCLUSIONS In this study, we examined the effect of growth solution pH on the length and purity of high aspect ratio gold nanorods made by a seed-mediated synthesis approach. Although nanorods show shorter lengths when prepared under a basic solution condition than when they are synthesized in an acidic solution, the nanorod yield is actually enhanced with relatively few nanoplate byproducts formed. By adjusting several reaction conditions with the aim to extend the nanorod length, ultralong gold nanorods and nanowires with tunable lengths have been synthesized from 500 nm to 4.5 μm by progressively increasing HNO3 concentration in the growth solution. The longest nanowires can reach a length of 6 μm. These nanowires still maintain a pentatwinned structure typically present in gold nanorods. A long growth time is needed to make these nanowires in an acidic solution environment. The ability to systematically tune gold nanorod and nanowire lengths over such a wide and useful range should broadly facilitate their applications. They also possess desirable features including thin diameters and mechanical rigidity. New research ideas can be considered utilizing these readily accessible 1-dimensional nanostructures.

Figure 5. SEM images of the gold nanowires obtained from samples (a) l, (b) m, (c) n, (d, e) o, (f) and p. All scales bars represent 1 μm except in panel a, where the scale bar is equal to 100 nm.

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ASSOCIATED CONTENT

S Supporting Information *

nanowires synthesized in samples l−p by progressively increasing HNO3 concentration in the growth solution in flask C from 1 to 83 mM. Although the solution can become more viscous with increasing HNO3 concentration, sample p still remains fluidic. With the exception of sample l (average length of 700 nm), long nanowires were produced in all other samples. Again, a large quantity of nanowires have been formed in each sample that they also form ordered side-by-side assembly. The nanowire yield is likely less than 15%. Although many nanoplates are still present, the nanowires can reach longer lengths in a tunable fashion from average lengths of 1540 (sample m) to 3460 (sample o) and 4450 nm (sample p). Nanowire diameters are still nicely controlled in the range of 40−53 nm (see Table 1). Thus, gold nanorods and nanowires with approximate lengths of 0.5, 0.7, 1, 1.5, 3, 3.5, and 4.5 μm have been synthesized in this study. This wide range of length tunability has not been demonstrated before. Sample p actually contains many wires with lengths in the range of 5−6 μm, and the longest gold nanowires synthesized have lengths beyond 6 μm (see Figure S10, Supporting Information). The ability to make such ultralong gold nanowires should greatly facilitate nanocircuit fabrication and electrical conductivity measurements using gold nanowires. Use of these individual nanowires for nanoelectronics can be straightforward if the byproducts can be removed, because the nanowires are sufficiently thin to be

Table listing the reagent amounts used in the preparation of growth solution, length distribution histograms, photographs showing the solution color changes, additional SEM images, XRD pattern, and TEM images of nanorods and nanowires. 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 We thank the National Science Council of Taiwan for support of this work (Grant NSC 98-2113-M-007-005-MY3). We also thank Lian-Ming Lyu for assistance in TEM analysis of our samples.

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REFERENCES

(1) Huang, X.; Neretina, S.; El-Sayed, M. A. Gold Nanorods: From Synthesis and Properties to Biological and Biomedical Applications. Adv. Mater. 2009, 21, 4880−4910.

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dx.doi.org/10.1021/la400985n | Langmuir 2013, 29, 10491−10497