Effect of Growth Temperature on Tailoring the Size and Aspect Ratio of

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The Effect of Growth Temperature on Tailoring the Size and Aspect Ratio of Gold Nanorods xiaowei liu, yao jingwen, jiangjiang Luo, xiaoshuang duan, Yanbo Yao, and Tao Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01635 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 15, 2017

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The Effect of Growth Temperature on Tailoring the Size and Aspect Ratio of Gold Nanorods Xiaowei Liu, Jingwen Yao, Jiangjiang Luo, Xiaoshuang Duan, Yanbo Yao, Tao Liu* College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Soochow, P. R. China. Keywords: gold nanorods synthesis, temperature effect, size and aspect ratio control

Abstract

The influence of temperature on the gold nanorod synthesis process and its effect on tailoring the size and aspect ratio have not been fully investigated and understood. A comprehensive study, involving SEM and TEM microscopy, Vis-NIR spectroscopy, quantitative data analysis and theoretical simulation, is performed to understand the effect of growth temperature on size, aspect ratio, and shape uniformity of gold nanorods that are synthesized by a recently developed binarysurfactant seed-mediated AuNR synthesis method. It has been demonstrated that the temperature can be used as a simple processing parameter to viably tailor the size and aspect ratio of AuNRs as well as the corresponding surface plasmon resonance behavior. The temperature coefficients for length, diameter and aspect ratio have been respectively determined to be 3.5 nm/°C, 3.9 nm/°C and -0.18/°C. With a combination of controlling temperature and formulation, the binary surfactant

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seed-mediated AuNR synthesis method expects to be a convenient way for producing gold nanorods with a large range of size and aspect ratio suitable for different applications.

Introduction Owing to its small size and shape anisotropy, the conduction electrons in a gold nanorod (AuNR) manifest two different collective oscillation modes. One is parallel and another is perpendicular to the axial direction of the AuNR, which are respectively termed longitudinal and transverse surface plasmon resonance modes (LSPR and TSPR). 1 ,

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There are a variety of

fascinating optical properties associated with the surface plasmon resonance behavior of AuNRs, such as the enhanced scattering and absorption as well as the enhanced near-field intensities.3, 4 The unique optical properties of AuNRs makes them suitable for many different applications, ranging from photothermal therapies, drug delivering, medical imaging, disease diagnosis, plasmon enhanced spectroscopies and chemical sensing5 - 10 as well as photoresponsive micropillar actuator.11, 12 The TSPR wavelength of an AuNR is around 520 nm and shows little dependence on its size. In contrast, the wavelength of the LSPR mode strongly depends on the AuNR size, which is in general proportional to the aspect ratio – L/D of an AuNR.13, 14 To suit AuNRs for a specific application, one of the key issues is to tailor the LSPR mode to a desirable wavelength and to optimize the corresponding scattering and absorption efficiencies. As such, it is highly desirable to have a cost-effective and reproducible synthetic method capable of large-scale production of size-controlled AuNRs. Among different methods developed in the past decades, e.g., template method, 15 electrochemical method,16 photochemical method,17 the seed-mediated growth method, which was first invented by Murphy et al.,18 subsequently improved by El-Sayed and his co-workers19 and


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Murphy el al.,20 and later continuously optimized by different researchers,21 - 23 is simple, easy-tooperate and capable of high-yield producing AuNRs with controlled aspect ratio and uniform size.24, 25 Depending upon whether silver nitrate is used in the synthesis process, the seed-mediated AuNR synthesis method has been further classified as silver-assisted and without silver one. The seed-mediated method has now become the most important and been routinely practiced in the preparation of AuNRs for different applications. Most recently, the silver-assisted seed-mediated method was further augmented by a binary surfactant system discovered by Ye et al..26 With a mixture of hexadecyltrimethylammonium bromide (CTAB) and sodium oleate (NaOL) to replace the original single CTAB surfactant system, this novel approach is able to achieve highly uniform AuNRs with negligible shape impurities. Through adjusting the formulation of AuNR synthesis recipe, Ye et al.26 readily achieved AuNRs with a broad range of diameter between 15 – 50 nm and the corresponding LSPR wavelength between 650 to 1150 nm. On the basis of the comprehensive sets of data provided by Ye et al.,26 we are able to establish a multivariate regression model to correlate the length (L), diameter (D), and aspect ratio (L/D) of AuNR with respect to the components used in the recipe of the binary surfactant system. Figure 1 shows the regression results for L/D. Clearly, the L/D predicted by the regression model agrees reasonably well with those measured experimentally. This suggests that it is viable to achieve AuNR size and L/D control through manipulating the formulation of the synthesis recipe. Indeed, similar approaches have been adopted in the past by many different researchers to tailor the size and L/D of AuNRs, e.g., changing the amounts of AgNO3,19 inclusion of oxidizing agents H2O2, 27 ,overgrowth of AuNRs by using the binary surfactant system ,28 using fractional factorial design29 and response surface methodology experimental design methods.30


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The past research efforts in controlling the size and L/D of AuNRs mostly focused on formulation manipulation. To one’s surprise, the typically critical and relatively easy-to-control processing parameter - temperature, has not been fully studied for its effect on AuNR synthesis. In the open literatures, we have found only a few works touched upon this subject.31 - 33 The work in ref. 31 qualitatively investigated the temperature effect on the seed-mediated synthesis method. With varying the growth temperature at a few discrete values (3, 8, 24, and 42 oC), the authors observed that the aspect ratio of AuNR obtained by the single-surfactant seed-mediated method showed strong temperature dependence. It increases from ~1 at 42 oC to ~40 at 3 oC - an aspect ratio change of ~39 over a temperature span of 39 oC. The study presented by Zijlstra et al.32 covered a broad range of temperature from 25 oC to 97 oC to understand its effect on the seedless AuNR synthesis method. The quantitative results in ref. 32 revealed that the temperature coefficients for length-L and diameter-D are respectively -0.172 nm/°C and ~0 nm/°C. In contrast to ref. 31, the work in ref. 32 showed that only a moderate range of aspect ratio from 1.27 to 1.95 could be achieved over a temperature span of 72 oC. Zijlstra et al.’s study suggests that, for seedless AuNR synthesis method, simply by controlling the temperature is not an efficient approach to tailor the size and aspect ratio of AuNRs. This viewpoint has been challenged by a recent study performed by Semyonov et al.,33 in which a different reducing agent than the one in ref. 32 was used. Semyonov et al. found that when the temperature increases from 30 to 40 oC, the LSPR of AuNRs obtained by the seedless synthesis method blue shifts by ~179 nm. According to ElSayed,13 such a value of LSPR shift corresponds to an aspect ratio decrease by ~1.98, which indicates that the temperature could be a viable control parameter to tailor the size and aspect ratio of AuNRs. Unfortunately, the qualitative studies presented in ref. 33 did not provide sufficient


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quantitative data to allow for an establishment of a quantitative relationship between the growth temperature and the size and aspect ratio of AuNRs.

Figure 1. Multivariate regression model for AuNR synthesis by using the CTAB-NaOL binary surfactant system26 (the amounts of all components are based on a 500 ml growth solution). The brief review given above clearly indicates that the temperature is a critical parameter that could be used as a viable and simple approach to tailor the size and the aspect ratio of AuNRs to suit various applications. Moreover, a better understanding of the temperature effect on the AuNR synthesis process would be able to shed light on the anisotropic growth mechanism(s) of AuNRs.31, 32

Nevertheless, one also notes that the important role of temperature in AuNR synthesis has not

received sufficient attention in the past research efforts. In the present study, we focus on the newly developed binary-surfactant seed-mediated method26 to systematically investigate how the factor of temperature affect the length, the diameter, the aspect ratio and the corresponding statistical distributions as well as the LSPR modes of the resultant AuNRs. Quantitative relationship between the temperature and the concerned size and shape parameters (L, D, and L/D) will be established, which, in combination with the formulation multivariate model (Figure 1), expects to be able to


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set a strong foundation for repeatable and reproducible synthesis of AuNRs with desirable and broader range of size and aspect ratios. Last but not least, the quantitative relationship thus established would provide insights for a better understanding of the anisotropic growth mechanism(s) of AuNRs obtained by the binary-surfactant seed-mediated method. Experimental Materials Hexadecyltrimethylammonium bromide (CTAB, ≥ 99.0%), L-ascorbic Acid (AA, BioUltra grade ≥ 99.7%) and sodium borohydride (NaBH4, ≥ 96.0%) were all purchased from China National Pharmaceutical Group Corporation (SINOPHARM). Sodium oleate (NaOL, purity greater than 97.0%) was purchased from Tokyo Chemical Industry Co., Ltd.. Hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O, concentration of Au ≥ 49 wt. %) was purchased from Sigma-Aldrich Co, LLC.. Silver nitrate (AgNO3, purity ≥ 99.8%) was purchased from Forever China Chemical Technology Co., Ltd. (Jiangsu, China). Hydrochloric acid (HCl, 36~38 wt. % in water) was purchased from China Sun Specialty Products Co., Ltd.. All chemicals were used as-received without further purification. A GWA-UN1-F10 Ultrapure Water Generator (Beijing Purkinje General Instrument Co., Ltd) was used to supply 18 MΩ·cm deionization (DI) water for aqueous solution preparation. Synthetic procedures and methods The procedure described in ref. 26 was followed for synthesizing AuNRs at different temperatures. The protocol includes two steps. One is seed preparation and the other is growth solution preparation. For all samples, the volume of the growth solution was fixed at 40 ml, and the synthesis recipe was also remained unchanged: A – CTAB (0.56 g), B – NaOL (0.1234 g), C – Seed (0.0907 ml), D – AgNO3 (4 mM, 1.1397 ml), and E – HCl (12.1M, 0.196 ml). The seed solution was prepared by first mixing and shaking 2.5 ml 0.5


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mM HAuCl4 with 2.5 ml 0.2M CTAB in a glass vial at 30 oC to obtain a transparent yellowish solution. Subsequently, to this mixture, a freshly prepared 0.5 ml ice-cooled 6 mM NaBH4 solution was injected. After the addition of NaBH4, the seed solution turns into tea-like color. Upon 2 mins further stirring, the seed solution was kept quiescently in a water bath at 25 oC for 30 minutes before its mixing with the growth solution for AuNR synthesis. The growth solution was prepared by first dissolving 0.56 g CTAB and 0.1234 g NaOL in 20 ml deionized water in an Erlenmeyer flask. After dissolution of the binary surfactants, 1.1397 mL AgNO3 (4 mM) was added and the mixed solution was kept undisturbed at 30 oC for 15 minutes. Subsequently, to this mixture, 20 ml 1 mM HAuCl4 solution was added, and the solution was stirred mildly (magnetic stirrer, 700 rpm) for 90 mins. During this process, the color of the mixed solution changed from golden yellow to colorless. After 90 minutes, the stirring speed was reduced to 400 rpm and 0.196 ml HCl was added to adjust the solution pH value. Upon additional 15 mins stirring, 0.1 ml L-Ascorbic acid (0.064 M) was injected and further vigorous stirring (1200 rpm) was applied for 30 secs. Subsequent to this step, 0.0907 ml seed solution was added to the growth solution with a vigorous stirring for another 30 secs. The finally obtained AuNR growth solution was kept quiescently in a water bath (HH·S11-2-S, Shanghai CIMO Medical Instrument Manufacturing Co., LTD, temperature accuracy ± 0.5 °C) for 12 hrs at a pre-determined temperature. According to our observations, the formulation used in this study would have CTAB precipitation issue if the temperature is below 21 °C; and there is no AuNR other than Au nanospheres being produced if the temperature is above 40 °C. For this reason, we mainly focused on a temperature range from 21 °C to 35 °C to study the temperature effect on AuNR growth. After 12 hrs growth, the optical absorption spectra of the AuNR solution was recorded with DI water as the reference by a Vis-NIR high resolution fiber-optic spectrometer (Aurora4000,


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Changchun New Industries Optoelectronics Technology Co., Ltd.). The AuNR samples used for SEM and TEM imaging were prepared by first centrifuging the as-prepared AuNR solution at an effective centrifugal force of 17,000 rcf for 30 mins. The precipitates were then collected and washed/re-dispersed with DI water multiple times. The re-dispersed AuNR suspension was deposited and dried on silicon wafer and TEM grid respectively for obtaining the samples for SEM and TEM imaging. The SEM imaging was performed with a Hitachi SU8010 ultra-high resolution scanning electron microscopy at an operating voltage of 15 kV; and the TEM imaging was carried out with a Hitachi HT 7700 transmission electron microscopy at an operating voltage of 120 kV. Results and discussion Size and shape uniformity of AuNRs The AuNRs obtained at different growth temperatures were imaged by both SEM and TEM to determine their size and the corresponding statistical distribution. Figure 2a and 2b respectively show the representative SEM and TEM images of the AuNRs synthesized at 21, 22, 23, 24, 25, 27, 29, 30, 31, 33 and 35 °C. According to the SEM and TEM results, we observe that the gold nanoparticles obtained at all temperatures are predominantly rod-shape (AuNRs). The number percentage of other than rod-shape particles has been estimated for all samples. Within the temperature range being investigated here, the averaged number percentage of non-rod shape particles is 7.5 ± 5.2%. For the best case at 25 °C, we do observe the percentage of non-rod by-products only accounts for 0.7%, which corroborates Ye et al.’s results26 that the binary-surfactant seed-mediated AuNR synthesis method is indeed capable of producing AuNRs with excellent shape uniformity.


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21oC

22oC

23oC

24oC

25oC

27oC

29oC

30oC

31oC

33oC

35oC

Figure 2. SEM (top) and TEM (bottom) images of the AuNRs synthesized at different temperatures. The scale bar in all images is 100 nm.

Table 1. Length, diameter, and aspect ratio of AuNRs and the corresponding normal distribution fitting parameters.

Temperature T (oC) 21 22 22 (rpt) 23 24 25 27 29 30 31 33 35

Length - L (nm) Normal Experimental Distribution Fit 79.0 ± 11.8 78.8 ± 12.0 73.9 ± 8.1 73.9 ± 8.1 72.6 ± 13.0 72.6 ± 13.0 79.8 ± 9.9 79.9 ± 9.7 83.6 ± 11.7 83.7 ± 12.3 87.9 ± 8.5 87.3 ± 8.7 83.5 ± 7.4 83.3 ± 7.5 103.2 ± 9.6 102.1 ± 11.2 120.9 ± 12.3 120.5 ± 12.1 121.8 ± 12.7 121.8 ± 12.7 109.1 ± 11.8 109.1 ± 11.8 115.1 ± 15.1 115.3 ± 14.7

Diameter - D (nm) Normal Experimental Distribution Fit 19.7 ± 4.5 19.6 ± 4.3 18.9 ± 2.6 18.9 ± 2.6 17.5 ± 4.2 17.5 ± 4.2 21.9 ± 3.7 22.0 ± 3.7 25.2 ± 5.5 25.4 ± 5.8 27.1 ± 3.0 26.8 ± 3.1 31.6 ± 4.0 31.7 ± 4.1 43.5 ± 4.8 42.8 ± 5.6 61.3 ± 8.9 61.1 ± 9.1 60.7 ± 8.3 60.7 ± 8.3 54.8 ± 7.6 54.8 ± 7.6 69.9 ± 10.7 69.4 ± 10.5

Aspect Ratio - L/D Normal Experimental Distribution Fit 4.13 ± 0.69 4.12 ± 0.68 3.97 ± 0.59 3.97 ± 0.59 4.26± 0.71 4.26± 0.71 3.71 ± 0.56 3.70 ± 0.55 3.43 ± 0.64 3.41 ± 0.67 3.29 ± 0.48 3.30 ± 0.49 2.69 ± 0.42 2.67 ± 0.42 2.40 ± 0.32 2.42 ± 0.39 2.01 ± 0.30 2.02 ± 0.49 2.04 ± 0.30 2.04 ± 0.30 2.02 ± 0.30 2.02 ± 0.30 1.67 ± 0.22 1.68 ± 0.22

On the basis of the multiple SEM and TEM images obtained for a given sample, the size L and D were measured and the corresponding L/D was calculated. The corresponding sample mean and standard deviation for length - L

∆L, diameter - D

∆D, and aspect ratio - L/D

∆ are

listed in Table 1. It should be mentioned that, with a t-test at the significance level of 0.05 or 95% confidence level, there is no statistical difference between the SEM and the TEM results for the same quantity (L, D, or L/D) of the same sample. For this reason, we combined the SEM and TEM


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results to create a larger sample size (> 400) for a meaningful statistical analysis. The size uniformity of the AuNRs has been examined by creating the histogram for length-L, diameter-D and aspect ratio-L/D of the sample at each temperature. Figure 3 shows the representative results for the samples obtained at 21, 25 and 35 °C. Firstly, we found that, for all samples, the length-L, diameter-D and aspect ratio-L/D all can be nicely fitted by a normal distribution function 1/ √2

exp

, where µ and σ are respectively the population mean and the standard

deviation. The results of µ and σ for L, D and L/D of all samples are listed in Table 1, which, as expected, are comparable with the corresponding sample mean and sample standard deviation. With the normal distribution fitted results, we can quantify the uniformity of L, D, and L/D by the corresponding quantity σ/µ. Averaging over all samples, the uniformity parameter σ/µ for L, D, and L/D has been respectively estimated to be 11.6% ± 2.0%, 15.5% ± 3.6%, and 16.4% ± 3.0%. The size uniformity of our samples is comparable to though not as good as the ones reported by Ye et al.,26 of which the sample mean and sample standard deviation are: L D

∆D = 5.1% ± 1.1% and - L/D

∆L = 5.5% ± 1.0%,

∆L/D = 10.6% ± 1.8%. The temperature change has not

introduced any systematic trend for the variation of the size (L, D) and shape (L/D) uniformity parameters. To estimate the batch-to-batch variation, we replicated the synthesis protocol at 22 oC, and the reproducible L, D, and L/D results are also listed in Table 1. The reasonably good batchto-batch reproducibility once again suggests the robustness of the newly developed binary surfactant AuNR synthesis method. In Ye et al.’s work,26 the synthesis formulation was varied but the temperature was fixed. In our study, the synthesis formulation was fixed, but the AuNR growth temperature was varied. Given these two different scenarios, our study and theirs both give comparable results regarding


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the AuNR size and shape uniformity. This indeed suggests the robustness of the newly developed binary-surfactant seed-mediated AuNRs synthesis method.

Figure 3. Histograms for L, D, and L/D of the AuNRs synthesized at 21, 25 and 35 oC. The solid lines are the corresponding normal distribution fitting results. Temperature dependence of L, D, L/D and surface plasmon resonance of AuNRs As shown by the SEM and TEM images in Figure 2a and 2b, one can clearly tell that the length and diameter of AuNRs increase with increasing the growth temperature; but the aspect ratio shows opposite trend. Figure 4 makes this qualitative microscopic observation clearer, in which the temperature dependent L, D and L/D are respectively shown in Figure 4a, 4b and 4c. Within experimental errors, the temperature dependence of L, D and L/D all obey a linear relationship. The temperature coefficients for L, D and L/D are respectively 3.5 nm/°C, 3.9 nm/°C and -0.18/°C. In terms of the trend of aspect ratio – L/D on temperature, our results qualitatively agree with the previous reports. 31 - 33

That is L/D decreases with increasing temperature. Given this similarity, however, we would


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like to comment on the different nature of the aspect ratio decrease with temperature observed in our study as compared to those previously reported. In our work, both length and diameter increase with temperature, but with the latter a higher temperature coefficient. As such, the temperature increase causes a decrease of the aspect ratio. In ref. 32, the length of AuNRs decreases with increasing temperature, which is the main reason to cause the observed decreasing trend of the aspect ratio with temperature. The different nature of the aspect ratio dependence on temperature observed in our study and that in ref. 32 implies that the seedless AuNR growth method might have a different mechanism from the seed-mediated one. We speculate that this conclusion also applies to ref. 33 where the AuNRs were synthesized by seedless method and to ref 31 where the AuNRs were synthesized by using a single CTAB surfactant through seed-mediated method. Unfortunately, such speculation cannot be further confirmed due to a lack of sufficient quantitative data in ref. 31 and 33.

Figure 4. Temperature dependence of (a) Length – L, (b) Diameter – D, and (c) aspect ratio – L/D of AuNRs. Note the error bar is a reflection of the broadness of the distribution in L, D, and L/D for a given sample, the numerical value of which is the sample/population standard deviation listed in Table 1.


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Our results have shown that over a moderate temperature span of 14 °C, the AuNR aspect ratio has changed by ~2.5. This suggests that the temperature can be indeed utilized as a viable and simple factor to tailor the aspect ratio and therefore the wavelength of the LSPR of AuNRs to suit various applications. This point can be further strengthened by the temperature dependent surface plasmon resonance behavior of AuNRs. The results are shown in Figure 5. In accordance with the temperature dependence of the aspect ratio, the LSPR blue shifts to smaller wavelength with increasing temperature. In contrast, the TSPR mode slightly red shifts to longer wavelength. The peak positions of the LSPR and TSPR modes for all samples are summarized in Table 2 and are plotted against the aspect ratio in Figure 5b and 5c respectively. In Figure 5b, we can identify a linear relationship between the LSPR and the aspect ratio, which is a well-known result that have been identified previously.13 Typically, the TSPR of AuNRs shows negligible aspect ratio dependence. Nevertheless, as shown in Figure 5c, the peak position of the TSPR evidently decreases with increasing the aspect ratio; and the extent of change gradually diminishes with increasing the aspect ratio. That the TSPR of our AuNR samples has a dependence on the aspect ratio is due to their larger diameter. This is especially true for the AuNRs synthesized at higher temperatures (Table 1). To corroborate this point, we used the size information listed in Table 1 and performed a numerical simulation with the software package MNPBEM developed by Hohenester et al.34 to calculate the extinction spectra of AuNRs randomly dispersed in water. The theoretically determined LSPR and TSPR results are listed in Table 2 and shown in Figure 5b and 5c. Evidently, for both LSPR and TSPR, the experimental results agrees nicely with those determined by theoretical calculations, which indeed suggests that the aspect ratio dependence of the TSPR on temperature as observed in Figure 5c is not an artefact.


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Figure 5. Temperature dependence of (a) extinction spectra of AuNRs obtained at different temperatures; (b) relationship between the LSPR and aspect ratio of AuNRs; (c) relationship between the TSPR and aspect ratio of AuNRs. Blue square: experimental data; Red circle: theoretical calculation by MNPBEM.34 Table 2. Experimental and theoretical (Boundary Element Method - BEM)Error! Bookmark not defined. results on LSPR and TSPR of the AuNRs obtained at different temperatures. Temperature - T (oC) 21 22 23 24 25 27 29 30 31 33 35

LSPR (nm) Experimental BEM 817 848 806 824 793 806 755 783 783 774 711 708 702 709 690 705 692 708 678 676 664 659

TSPR (nm) Experimental BEM 513 510 510 510 512 512 515 514 510 514 522 517 519 520 536 533 536 532 532 526 550 544


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Mechanism of temperature dependence of L and D of AuNRs The mechanism(s) of the temperature effect on controlling the length, diameter and aspect ratio of AuNRs can be understood within the current mechanistic framework of AuNR growth. The past research has recognized that the formation of CTAB bilayer on the gold surface is one of the key factors for the anisotropic growth of AuNRs.24, 25, 35 In silver assisted seed-mediated method, the CTAB works along with the silver containing species (halides, CTAB complexes, element silver) to selectively bind on certain crystal facets of the gold particle ({110} and {100}) to block the widthwise growth. As a consequence, the particle favorably grows on the exposed {111} facets on the ends to direct the formation of AuNRs. 36 - 39 Recently, Vaia and his co-workers 40 relied on an arresting-growth approach to carry out an in-depth microscopy and spectroscopy combined studies to elaborate the silver-assisted seed-mediated AuNR growth mechanism. It was found that the AuNR growth process can be divided into five stages. In stage I, the seed particles grow rapidly in an isotopic fashion. In stage II, due to the binding and blocking action of the CTAB and/or silver containing species on {100}/{110} facets, the particles experience a rapid anisotropic growth; the lengthwise growth rate is much greater than that of widthwise. During stage II, micelle adsorption and CTAB bilayer reorganization will also occur at the ends of AuNR. This then creates an energy barrier for the Au atoms to deposit onto the ends and therefore slows down the lengthwise growth rate. In stage III, the adatom addition at the boundary between {111} and {100}/{110} and its subsequent migration to the side of the AuNR to make the particle grow widthwise. In stage IV and V, the growth rate is significantly reduced, and the AuNRs mainly experience the shape smoothing and reconstruction of the particle surface crystal facets. Within the frame work established by Vaia et al.,40 the finally achieved size and the aspect ratio of the AuNRs are mainly determined by Stage


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II and III. As such, the temperature variation induced size and aspect ratio change of AuNRs observed in our experiments is believed to be operative mainly in these two stages. On the basis of Vaia et al.’s work, we define the activation energy for the Au atoms to deposit onto the ends in stage II and III as EL, which dictates the lengthwise growth of AuNRs. Take the energy barrier for the adatoms at the ends to migrate into sides in stage III as Em. Then the activation energy for diameter growth would be ED = Em + EL, since, for widthwise growth process, the Au atom needs to deposit first on the ends then migrate to the side. According to the Arrhenius rate equation, we have the lengthwise and widthwise growth rate kL and kD respectively as:

Eq. (1)

Eq. (2)

where AL and AD are the pre-exponential factor and have no temperature dependence; R is the molar gas constant; and T is the temperature. It should be emphasized here that, in constructing Eq. (1) and (2), we have not considered the deposition and migration details of Au atoms and how the deposition and migration are related to the changes of L and D. Due to such a complexity, e.g., the growth rates kL and kD are not constants, which have a somewhat complicated dependence on growth time and therefore the AuNR size,28 Eq. (1) and (2) should be considered at best the first order approximation. With the rate equations given above, it is clear that, when the temperature increases, both lengthwise and widthwise growth rate increases. Considering a fixed growing period (roughly the time for the completion of stage II and stage III), we may qualitatively say that both the length and diameter should increase with increasing temperature. This agrees with the experimental results shown in Figure 4a and 4b. Furthermore, since ED is greater than EL, a higher


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temperature favours the diameter growth more than the length. Therefore, there occurs a decreasing trend of the aspect ratio with temperature (Figure 4c). Conclusions On the basis of a recently developed binary-surfactant seed-mediated AuNR synthesis method, we investigated the effect of growth temperature on tailoring the shape and size of AuNRs. The comprehensive SEM and TEM microscopic measurements, Vis-NIR spectroscopic results and the subsequent quantitative analysis establish the robustness of the new method for producing AuNRs with good size and shape uniformity. Moreover, it has been demonstrated that the size and aspect ratio of AuNRs and the corresponding LSPR can be easily tailored by controlling the temperature in a narrow range (~15 °C). The temperature coefficients for L, D and L/D have been quantitatively determined to be 3.5 nm/°C, 3.9 nm/°C and -0.18/°C. A better understanding of the temperature dependence of the size and aspect ratio of AuNRs is achieved within the framework of the 5-stage AuNR growth mechanism. The temperature control in conjunction with formulation manipulation would expect to provide a convenient way for producing gold nanorods with a large range of size and aspect ratio suitable for different applications.

Corresponding Author *Fax: +86 512 65877952; E-mail address: [email protected] (T. Liu). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENT We acknowledge the research funding received from the National Natural Science Foundation of China (NSFC51673140) and the startup funds provided by Soochow University (Q410900116) for supporting this work.

ABBREVIATIONS AuNR - gold nanorod; LSPR, longitudinal surface plasmon resonance; TSPR, transverse surface plasmon resonance; CTAB, hexadecyltrimethylammonium bromide; NaOL, sodium oleate; L, length; D, diameter; L/D, aspect ratio; AA, L-ascorbic Acid.

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