Synthesis and Optical Properties of Small Au Nanorods Using a

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Synthesis and Optical Properties of Small Au Nanorods using a Seedless Growth Technique Moustafa Ali, Brian Snyder, and Mostafa A. El-Sayed Langmuir, Just Accepted Manuscript • DOI: 10.1021/la301387p • Publication Date (Web): 23 May 2012 Downloaded from http://pubs.acs.org on May 31, 2012

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Synthesis and Optical Properties of Small Au Nanorods using a Seedless Growth Technique Moustafa R. K. Ali, Brian Snyder, Mostafa A. El-Sayed* Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0400 Key words (Small Nanorods, Synthesis, Seedless Scattering, Absorption, and Extinction)

Abstract

Gold nanoparticles have shown potential in photothermal cancer therapy and optoelectronic technology. In both applications, a call for small size nanorods is warranted. In the present work, a one-pot seedless synthetic technique has been developed to prepare relatively small monodisperse gold nanorods with average dimensions (length × width) of 18 × 4.5 nm, 25 × 5 nm, 15 × 4.5 nm, and 10 × 2.5 nm. In this method, the pH was found to play a crucial role in the monodispersity of the nanorods when the NaBH4 concentration of the growth solution was adjusted to control the reduction rate of the gold ions. At the optimized pH & NaBH4 concentrations, smaller gold nanorods were produced by adjusting the CTAB concentration in the growth solution. In addition, the concentration of silver ions in the growth solution was found to be pivotal in controlling the aspect ratio of the nanorods.

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The extinction coefficient values for the small gold nanorods synthesized with three different aspect ratios were estimated using the absorption spectra, size distributions, and the atomic spectroscopic analysis data. The previously accepted relationships between the extinction coefficient or the longitudinal band wavelength values and the nanorods’ aspect ratios found for the large nanorods do not extend to the small size domain reported in the present work. The failure of extending these relationships over larger sizes is a result of the interaction of light with the large rods giving an extinction band which results mostly from scattering processes while the extinction of the small nanorods results from absorption processes.

1. Introduction Gold nanoparticles have received much attention in recent years. They have unique plasmonic properties due to the interaction of light with conduction band electrons, which gives rise to their collective oscillation. This oscillation, known as localized surface plasmon resonance (LSPR) [1], is sensitive to the size and shape of the nanoparticles as well as the dielectric constant of the medium and metal. Gold nanoparticles of different shapes have varying and remarkable properties that have been widely exploited for use in many applications such as: high sensitivity labels in immune sensors [2], catalysis [3], photonics [4], information storage [5], optoelectronics [6], and surface-enhanced Raman spectroscopy (SERS) [7]. Gold nanorods are differentiated by inherently low toxicity [8–9] and the optical enhancing properties associated with their LSPR [10-13]. Gold nanorods also offer advantages of high biocompatibility and conjugation with a variety of bimolecular ligands, antibodies, and other targeting moieties. These properties have been exploited in areas of photothermal cancer medicine such as cancer imaging [14–18], diagnostics [19-21], and potential treatment [22–31].


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The size and shape are the primary factors that determine their catalytic, optical, surface activity, and photo-thermal properties of the nanoparticles [32]. In the synthesis of gold nanoparticles, control over these properties has been one of the most important and challenging tasks. This is especially important when the nanoparticles are rod shaped with different aspect ratios (length/width) [33]. In the presence of these nanoparticles, a strong SERS signal of adsorbed molecules has been observed, which is sensitive to the shape and size of the particles [34]. These rods have been used for in vitro [35] and in-vivo [36, 37] photo-thermal destruction of cancer cells. In the in-vivo studies in animal models, complete clearance of these nanoparticles was not accomplished and was retained especially in the liver and spleen [38]. It is expected that smaller nanorods would enhance nanoparticle clearance. Different methods are utilized to synthesize different shapes of gold nanoparticles. In general, for the synthesis of gold nanorods using surfactants, three approaches are used: electrochemical [41], ultraviolet irradiation photoreduction [42-44], and the seed mediated growth methods [40]. In the latter method, cetyltrimethylammonium bromide (CTAB) has proven to be an effective surfactant in gold nanoparticle preparation [42]. Gold nanorods have been produced with a width greater than 8 nm, and lengths of about 50 nm depending on the silver ion content [40]. A seedless growth technique was previously published which described the purpose in using their synthetic scheme to produce gold nanorods in a gram scale. Jana’s method produced gold nanorods that ranged in size from 4-50 nm [39]. The limitation of Jana’s method was the low monodispersity of gold nanorods and formation of a large concentration of spherical nanoparticles. This makes the use of the resulting small gold nanorods useless for biological work. In our one step seedless synthesis method, previous limitations have been overcome by adjusting the pH of the growth solution while utilizing NaBH4 as the reducing agent to produce


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small gold nanorods. From these optimized conditions, we changed the CTAB concentration resulting in the production of even smaller monodisperse gold nanorods with a width as small as 2.6 nm. The effects of AgNO3 ions on the growth mechanisms of nanorods were examined in order to develop a method to control the length of the nanorods to produce small rods with different aspect ratios. The preparation of gold nanospheres with different sizes using the seedless method was found to be dependent on the NaBH4 concentration used in the growth solution. Based on the results, it is apparent that the size and shape of the formed gold nanoparticles are dependent on the reduction rate of the gold ions in the solution. By using the seedless growth method, we have succeeded in the preparation of monodisperse gold nanorods with low dimensions and aspect ratios, which enables the nanorods to absorb in the near-infrared radiation for potential use in in-vivo photothermal therapeutic applications. The extinction coefficients of these small rods were also determined, which allows for easy concentration determination. 2. Materials and methods For in-vivo photo-thermal therapy using gold nanorods, near infrared light (600-800 nm) is used as it has effective penetration within the human body. The small nanorods prepared in this work absorb light at wavelengths around 700 – 800 nm. 2.1. Synthesis of gold nanorods (18.0 × 4.5 nm) using seedless growth method. The growth solution was prepared at 25-30 °C. HAuCl4 (5.0 mL; 1.0 mM) was added to 5.0 mL cetyltrimethylammonium bromide (CTAB; 0.2 M). Following this, AgNO3 (250 µL; 4.0 mM) was added and the solution was gently shaken. HCl (8.0 µL, 37%) was introduced to obtain a pH of 1-1.15. Then, 70 µL ascorbic acid (78.8 mM) was added to the solution with gentle shaking until the solution was clear. Immediately afterwards, Ice-cold NaBH4 (15 µL; 0.01 M)


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was injected to the unstirred growth solution and allowed to react for six hours. The resulting gold nanorods had dimensions of 18.0(±2.0) × 4.5(±0.5) nm, with an aspect ratio of approximately 4, and longitudinal plasmon absorption at ~ 770 nm (Figure 1). However, in a similar growth solution the silver ions content was changed to 270 µL AgNO3 and the resulting gold nanorods had dimensions of 25(±1) × 5 (±0.5) nm, aspect ratio of approximately 5, and longitudinal plasmon absorption ~ 800 nm (Figure 5). In another experiment, the AgNO3 volume increased to 290 µL, the resulting gold nanorods were found to have dimensions of 27 (± 1) × 5 .5 (± 0.5) nm, with an aspect ratio of approximately 5.3, and longitudinal plasmon absorption at ~ 810 nm (see Fig. 3 in Supporting Information). 2.2. Synthesis of gold nanorods (10.5 ± 1.5 x 2.8 ± 0.2 nm) using seedless growth method. The procedure discussed in Section 2.1 was followed under the following conditions: the volume of HAuCl4 was reduced from 5.0 to 2.5 µL, the volume of ascorbic acid was reduced from 70 to 35 µL and the volume of NaBH4 was reduced from 15 to 7.5 µL, the volume of AgNO3 was kept at 250 µL, The resulting gold nanorods were found to have dimensions of 10.5 (±1.5) x 2.8 (±0.2) nm, with an aspect ratio of approximately 3.7, and a longitudinal plasmon absorption band maximum at ~ 740 nm. 2.3. Synthesis of 14× 4.2 gold nanorods with low value of aspect ratio using seedless growth method. The procedure from Section 2.1 was used except that 10.0 mL CTAB (0.2 M) was used instead of 5.0 mL and the volume of AgNO3 was kept at 250 µL (4.0 mM). The resulting gold nanorods had an average dimension of 14 (± 1) × 4.2 (±0.3) nm, with an aspect ratio of approximately 3, and longitudinal plasmon absorption at ~ 700 nm. A summary of the variations of the reagents for the seedless techniques is given in Table 1.


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Table 1. Summary of Reagents used in Seedless Techniques.

Dimensions (nm)

HAuCl4

CTAB

AgNO3 HCl

Ascorbic Acid NaBH4

18 (±2) × 4 .5 ( ± 0.5)

5.0 mL

5.0 mL

250 uL 8 uL

70 uL

15 uL

25 (± 1) × 5 ( ± 0.5)

5.0 mL

5.0 mL

270 uL 8 uL

70 uL

15 uL

27 (± 1) × 5 .5 (± 0.5)

5.0 mL

5.0 mL

290 uL 8 uL

70 uL

15 uL

10.5 (± 1.5)× 2.8 (± 0.2)

2.5 mL

5.0 mL

250 uL

8uL

35 uL

7.5 uL

14 (± 1) × 4.2 (± 0.3)

5.0 mL

10.0 mL 250 uL 8 uL

70 uL

15 uL

2.4. Using centrifugation to concentrate and prepare monodisperse nanorods. The solution was centrifuged for 15 min at 14,500 rpm and the supernatant was removed and the precipitate was redispersed in water and spun down at the same speed and time. 2.5. Synthesis of gold nanorods by seed-mediated growth. The growth solution was prepared using the methods of Nikoobakhat and El-Sayed [41]. Briefly, 5.0 mL HAuCl4 (1.0 mM) was added to 5.0 mL CTAB (0.2 M). Ice-cold NaBH4 (0.6 mL, 10.0 mM) was added to the stirred solution and allowed to react for 2-3 minutes, forming the gold seed solution. The growth solution was prepared by adding 5.0 mL HAuCl4 (1.0 mM) to 5.0 mL CTAB (0.2 M) and 270 µL of AgNO3 (4.0 mM). Then 70 µL ascorbic acid (78.8 mM) was added, followed by gentle mixing to form the growth solution. The seed solution (12.0 µL) was then added to the growth solution and allowed to react for six hours.


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Materials: HAuCl4, NaBH4, AgNO3, ascorbic acid, and cetyltrimethylammonium bromide (CTAB) were purchased from Sigma-Aldrich. Hydrochloric acid was purchased from VWR. Characterization was carried out using a Cary 500 UV-Vis spectrometer for the spectroscopic measurements while a JEOL 100 CX transmission electron microscope was used to image the samples. An Axial inductively coupled plasma (ICP) atomic emission spectrometer machine was used to determine the concentration of gold in the solutions. A minimum of three ICP measurements were taken for each nanorod sample.

3. Results and discussion The seeded growth method is commonly achieved through a two-step process. First, the seed solution is generated under conditions of chemical supersaturation, which leads to quick nucleation. Such conditions ensure a rapid reduction, thus obtaining small spheres. Secondly, the growth solution is used in a two-step reduction process. The first reduction (Au3+ →Au+) is completed using ascorbic acid. The second reduction in the seeded method (Au+→Au0) requires the presence of seeds because ascorbic acid is a weak reducing agent and cannot reduce the gold salt without seeds. The gold nanorods obtained through this process have a width of at least 10 nm. The role of the seeds is to activate the reduction process, resulting in gold nanorods in the presence of the Ag+ ions. In the seedless growth method, no seed preparation is required to produce gold nanorods with dimensions of 18 ± 2 × 4 .5 ± 0.5 because the nucleation and growth occur in the same solution. NaBH4 is used as a strong reducing agent in the growth solution in the absence of the seed in order to enhance the reduction of Au+ Au0. The resulting gold nanorods have a width of 4.5 nm in one setting and 2.5 nm in another.


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3.1. The influence of NaBH4 concentration on the size distribution of small sized gold nanorods at different pH. Different molar ratios of NaBH4 to HAuCl4 were used in the growth solution to replace seeds in the seedless growth methods. After the first reduction in which ascorbic acid was utilized in the seed mediated method, NaBH4 was added, which plays an essential role in growth. The seedless process depends on the direct growth of gold nanorods without any separation between the nuclei and the growing rods. The nucleation of the small nuclei and the growth of these nuclei happen under high chemical supersaturation. These conditions ensure generation and immediate growth of the gold nuclei. A UV-Vis spectrometer and a transmission electron microscope were used to track the rods’ evolution in shape and size. Four separate growth solutions with different molar ratios of NaBH4 to HAuCl4 [A (0.01), B (0.02), C (0.03), and D (0.04)] were prepared. The growth solutions were maintained at two different pHs. Figure 1A, maintained at pH ~2, shows two peaks for the gold plasmon band. The transverse plasmon band has an absorption peak of around 525 nm and is nearly identical for each solution. However the longitudinal plasmon band, which is attributed to the rod-shape of the particle, appears at a longer wavelength in each solution. By increasing the concentration of NaBH4 in four separate growth solutions, the longitudinal peaks red-shift. By comparing the absorbance spectra in sample B and C (Figure 1A), they have the same wavelength absorption. It has been observed that by increasing the NaBH4 on the growth solution the yield of nanorods increased which was seen from the increasing longitudinal surface plasmon (LSP) intensity as compared to the transverse surface plasmon (TSP) intensity. The lowest full width at half maximum (FWHM) for sample is a result of the highest concentration of NaBH4, sample D.


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By repeating the above experiment at a lower pH, the effect of pH on the growth solution was studied. Four separate growth solutions with different molar ratios of NaBH4: HAuCl4 were used in the growth solution, as described above, but at pH ~1. Figure 1B shows the UV-Vis-NIR absorption spectra with different molar ratios at pH ~1. It was determined the FWHM for the longitudinal band of the samples prepared at the two pH values. It was observed that samples prepared at lower pH have smaller FWHM than that prepared at the higher pH .The smallest FWHM (Figure 1B) was found for the sample prepared by adding 15 µL NaBH4 (sample D) at pH~ 1. The pH of the solution plays a crucial role in both the reduction of Au3+ Au+ as well as Au+ Au0. When the pH of the solution is decreased, [43] the reducing power of ascorbic acid and NaBH4 also decreases [44-46]. The samples that were prepared at lower pH are more homogeneous, as seen from the increasing of the ratio of the longitudinal surface plasmon (LSP) band intensity to that of the transverse surface plasmon (TSP) band. The growth time for sample D at pH ~2 (Figure 1A) was 20-60 minutes, compared to 4-6 hours for solutions that were maintained at pH ~1 (Figure 1B). It was concluded that by decreasing the pH of the growth solutions, the homogeneity of the small gold nanorods increased. The pH of the synthesis medium was controlled to maximize the reducing ability of NaBH4 which is responsible for nucleation and optimize the synthesis of gold nanorods. Based on the TEM image (Figure 2B) of the sample prepared at pH ~1, the gold nanorods were small and the sample was monodisperse as shown in the size distribution histogram. The homogeneity for the produced nanorods increased at the lower pH. The nanorod width for sample D is 4.2 ± 0.8 nm and the aspect ratio is approximately 4.2 ± 0.8 (Supporting Information Figure 2). However, the TEM image (Figure 2A) shows the size and shape of the nanorods with the highest molar ratio of NaBH4 to HAuCl4


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(0.04) prepared at pH ~2 was small but not monodisperse. The size distribution histogram shows the average width is 6.0 ± 2.5 nm and aspect ratio is 3.6 ± 2 (Supporting Information Figure 1). The reduction of Au3+ Au0 occurs in two steps, and it was found that the second reduction step (Au+ Au0) depends on the pH of the growth solution. The pH plays a crucial role in the rods’ growth process [46-48]. The reduction time increased with decreasing the pH. This suggests that the reduction ability of the NaBH4 decreases with decreasing pH. This slows down the formation of the gold atoms (and thus of the nanorods), which should produce a more monodisperse sample.

A

A B C D

B 1.0

A b so rb a n ce (a .u .)

1

A B C D

0.8

2

Absorbance

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0.6

0.4

0.2

0.0 0 400

600

800

1000

400

500

Wavelength (nm)

600

700

800

900

1000

Wavelength (nm)

Figure 1. The UV-Vis-NIR absorption spectra with different additions of different volumes of 0.01 M NaBH4. The four additions are 2 µL (A), 5 µL (B), 10 µL (C), and 15 µL (D). Figure 1A is at pH ~2 and Figure 1B is at pH ~1.


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Figure 2. The effect of pH on the small gold NRs distributions using the seedless method. The two sample are prepared at the molar ratios of NaBH4: Au. (0.04), but the synthesis media was maintained at pH ~2 (A) & pH ~1 (B). Scale bars are 20 nm. 3.2 Synthesis of relatively small gold nanorods at high CTAB concentration in seedless growth In the seedless method, increasing the CTAB concentration on the growth solution at the optimized conditions of NaBH4 and pH mentioned above (Section 2.2). It was noticed that the gold nanorods with a new smaller size were produced 2.5 ± 0.4 nm, (Supporting Information Figure 2). The average aspect ratio for these rods is 3.6 ± 0.5. The higher concentration of CTAB in the growth solution protects the {110} facets. This allows growth in the longitudinal direction to take place parallel to the {001} planes, stabilizing initial single crystalline nuclei more and decreasing the growth of rods more than usual so the gold nanorods are smaller compared to those prepared at a lower CTAB concentration.


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A

1.2

1.0

0.8

Absorbance

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0.6

0.4

0.2

0.0 400

600

800

1000

Wavelength (nm)

Figure 3. UV-Vis-NIR absorption spectra (A) and TEM (B) obtained from solutions sampled at a higher concentration of CTAB and at ~ pH 1. Scale bar is 20 nm. 3.3. Effect of silver ions and CTAB concentration in the seedless and seeded growth methods. The growth kinetics of the gold nanorods length can be controlled mainly by modifying the silver ions in the reaction mixture. The role of silver ions in producing different aspect ratios of gold nanorods with width around 11 nm, in the seeded method, the silver monolayer over Au {110} acts as a strongly binding surfactant to protect the facet from further growth in the presence of Ag (I) allow the gold atoms to be deposited at the most energy favorable place [40]. Thus, studying the effect of AgNO3 ions concentration in the seedless growth technique on the length of the gold nanorods produced by another comparing between the seeded and seedless growth solutions contains the same amount of silver ions. It is found that decreasing the silver ions concentration in the seedless growth solution to 150 µL rods of lower aspect ratio are produced, with a longitudinal plasmon of 700 nm, but the width increases to ~ 7 nm (Supporting information Figure 6). The best way to adjust the aspect ratio in the seedless growth and keep


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the width < 5 nm is to increase the CTAB in the solution (Section 2.3). As shown in Figure 4, the plasmon of gold nanorods prepared by the seed-mediated method is nearly the same for gold nanorods that was prepared from the seedless method as mentioned in the experimental method growth technique. As shown in the TEM images (Figure 5) the gold nanorods obtained using the seeded method showed particles with an average width of 11.0 ± 2.3 nm. The gold nanorods prepared by seedless method have an average rod width of 4.5 ± 0.7 nm (Supporting Information Figure 5). The aspect ratio for both sets nanorods is nearly the same (3.0 ± 0.5) (Supporting Information Figure 4). After the addition of silver ions to CTAB, the silver ions are adsorbed at the gold nanoparticle surface in the form of AgBr. This restricts the growth and stabilizes the nanorod surface. By increasing CTAB in the growth solution, more AgBr is deposited preferentially on the (100) facets over the (001) facets, so the particles will be thinner with a slight change in length. [4950].

A

B

0.6

1.0

0.5

Absorbance

0.8

Absorbance (a.u.)

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0.6

0.4

0.3

0.4

0.2

0.2

0.0 400

0.1

500

600

700

800

900

1000

0.0 400

Wavelength (nm)

600

800

1000

Wavelength (nm)

Figure 4. UV-Vis-NIR absorption spectra of gold nanorods prepared by the seeded (A) and seedless (B) methods.


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Figure 5. TEM images of gold nanorods obtained by the seeded methods (A) and seedless methods (B) using 250 µL of silver nitrate. The aspect ratio of the two samples is nearly the same ~2.8. Scale bar is 20 nm.

3.4. Effect of NaBH4 concentration on the gold nanospheres in the absence of silver ions. NaBH4 and AgNO3 in the growth solutions play an important role in the growth of gold nanorods. However, it is promising to study the effect of NaBH4 on the gold nanospheres produced in the absence of AgNO3. In the absence of silver ions, the growth occurs at the same rate, thus making gold nanospheres. As shown in Fig. 6, the UV-Vis spectra peak of growth solution sample C is the narrowest, followed by B, then A. It is noted that growth solution C gave the smallest gold nanospheres and most homogeneity, as shown in the TEM (Figure 7.) The produced particles had sizes of 27.0 ± 4.0, 11.0 ± 2.0, and 9.0 ± 2.0 nm (Supporting Information Figure 7). By using the NaBH4 for Figures 10 A, B, and C, respectively, it was observed that the solution became dark fastest in growth solution C, followed by B, then A. When NaBH4 is increased the reduction growth time decreased, the number of initial nucleation sites increased, and smaller spheres are synthesized. This took place using higher NaBH4 concentration where a


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larger number of nucleation sites, and thus the share of each site from the constant total gold ions is less, resulting in smaller particles. 2.5

A B C

2.0

Absorbance (a.u.)

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1.5

1.0

0.5

0.0 400

600

800

1000

Wavelength (nm)

Figure 6. UV-Vis spectra taken for different amounts of NaBH4 (A, 15 µL), (B, 50 µL) and (C, 600 µL).

Figure 7. TEM images of Au nanospheres at different concentrations of NaBH4 : 15 µL (A), 50 µL (B), and 600 µL (C). Scale bar is 20 nm.


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3.5. The extinction coefficient of the small gold nanorods: Determination and properties. 3.5.1 Determination of Extinction Coefficient. Extinction coefficients are used to relate concentration and absorbance. From Beer’s Law [46]:

A=εlc

(1)

A is absorbance (unitless), ε is the extinction coefficient (M-1cm-1), l is the path length (cm), and c is the nanoparticles’ concentration (M). The extinction coefficient (ε) had been determined previously for gold nanorods prepared using the seeded method. The width of these nanorods was approximately 10 nm [53]. The seedless growth method described here results in small gold nanorods with width ≤ 5 nm. The average volume of gold nanorods was determined using TEM. This was used to calculate the average number of gold atoms per rod, as the gold density is 59 atoms/nm3 [53, 54]. Inductively coupled plasma (ICP) was used to determine the number of gold atoms/mL for the same solution. Dividing the total concentration of gold atoms by the average number of atoms per rod, one can calculate the number of rods per mL (n) of the nanorod solution. The nanorod concentration, c, of the nanorods in moles/liter is thus = (n/N) × 1000, where N is Avogadro’s number. The absorbance was measured with a UV-Vis spectrophotometer of the same solution and entered into equation (1), from which the extinction coefficient (ε) is calculated. 3.5.2 Properties and comparison with the optical properties of the larger rods: For large rods, relationships have been established between the longitudinal surface plasmon (LSP) band wavelength as well as its extinction coefficient and their aspect ratios. Can these relationships be extrapolated to the small size rods prepared in the present work?


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The TEM images of three different small gold nanorods samples, A, B, and C with three different average values of, aspect ratios and extinction coefficients, are shown in (Figure 2B, Figure 10B, and Supporting Information Figure 4) The samples were prepared using the seedless growth method (Section 2.1) with slightly more silver ions in the growth solutions for samples B and C to obtain larger rods. Through the use of the known gold nanorod concentrations for the three samples (obtained using the ICP measurement [55]) the average volumes (sizes) of the nanorods, then the molar extinction coefficients for gold nanorods were calculated. Figure 8 compares some optical relationships for the larger gold nanorods obtained from reference material [53] with those for the small nanorods, determined experimentally. Figure 8A shows the relation between the extinction coefficient and the LSP band wavelength. Figure 8B shows the relationship between the extinction coefficient and the aspect ratio, and Figure 8C shows the relationship between the LSP band wavelength and the aspect ratio. From these figures, it is clear that there is a comparable relationship between the wavelength and aspect ratio for both size regimes. However, the relationship between the extinction coefficient and either the aspect ratio or the wavelength observed for the large rods disappears for the small rods. The extinction coefficient is determined by different type of interaction of the nanorods with light for the two size regimes. The failure of extending the relationship involving the extinction in the two size regimes, could be explained by the fact that while the value of the extinction coefficient is dominated by the cross section of the scattering processes for the large rods , they are dominated by the cross section of the absorption processes for the small ones [56]. Our observation in Fig. 8 A & B further suggests that scattering processes (operating in the large size regime) are more sensitive to the nanorod size than absorption processes (operating in the small size). This is indeed expected.


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B6 5

-1

5

9

-1 9

Reference (Big Rods) Small Rods

-1

Extinction Coefficient *10 (M cm )


6

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Extinction Coefficient *10 (M cm )

A

4

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0 1

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Aspect Ratio


800

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750

700

650

600

1

2

3

4

5

6

Aspect Ratio

Figure 8. The relationship between ε & aspect ratio (A), ε & LSP wavelength (B), and the aspect ratio & LSP wavelength (C) for large gold nanorods synthesized using the seed mediated method and smaller nanorods synthesized using the seedless technique.


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3.6. Comparison between gold nanorods of the same aspect ratio prepared by seedless and seeded methods. Different parameters can influence small gold nanorods growth such as silver ions content on the growth solution. By adding 270 µL of 4.0 mM AgNO3 to the seeded and seedless growth solutions nanorods with the same aspect ratios, but with different sizes, were obtained. The longitudinal surface plasmon absorbance of the nanorods that was prepared from the two methods is nearly 800 nm (Figure 9). As shown in the TEM images, the gold nanorods obtained using the seeded method showed average particle dimensions of 54.0 × 11.2 nm. The gold nanorods prepared by seedless method have average particle dimensions of 25.0 × 5.2 nm (Figure 10). The aspect ratio for both sets of nanorods is 4.8 ± 0.6. Table 2 shows a comparison between the extinction coefficients for the two gold nanorods. The extinction coefficient is smaller for the gold nanorods prepared using the seedless method than those prepared using the seeded technique [53]. The ratio of the extinction coefficients (seeded/seedless) having the same aspect ratio (4.8) is ~26 and a volume ratio of ~10. Obviously the two ratios are not the same. It is possible that as the nanoparticle gets smaller, its surface is more defective giving rise to surface scattering of the coherent motion of the conduction band electrons. This leads to weaker plasmonic fields and thus smaller extinction coefficient (the seedless particles).

Table 2. Properties of seedless and seeded nanorods with an absorbance at 800 nm. Seeded Au Nanorods

Seedless Au Nanorods

Dimensions

54 × 11 nm

25 × 5 nm

Volume (nm3)

5129

490

Ratio (Seeded/Seedless)

10


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4.9 × 109

Extinction coefficient

1.9 × 108

26

ε (M-1 cm-1)

B

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0.6

A bso rba nce

A A bsorbance (a.u.)

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0.4

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0.2 0.2

0.0

400

500

600

700

800

900

1000

0.0 400

Wavelength (nm)

600

800

1000

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Figure 9. UV-Vis-NIR absorption spectra of gold nanorods prepared by the seeded (A) & seedless (B) method using 270 µL AgNO3. The concentration of rods is 0.2 nM (A) and 5 nM (B).


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Figure 10. TEM images for rods prepared by the seeded method (A) and seedless method (B). The aspect ratio of the two samples is nearly the same ~4.8. Scale bar is 20 nm for A and 100 nm for B. Conclusions: 1- Relatively small gold nanorods with widths ranging from 2.5 – 3 nm in one setting, and 4 5.5 nm in another setting with different aspect ratios were synthesized using a one-step seedless growth procedure. Seeds are necessary for large gold nanorods, but NaBH4 was used as a substitute in the seedless growth process. 2- The pH of the solution also plays an important role in the synthesis of nanorods. Decreasing the pH decreases the rate of the reduction, which increases the growth time, (as long as the concentration of NaBH4 has been optimized in the growth solution). This ensures a more monodisperse solution. 3-

CTAB is an important contributor in controlling the gold nanorods dimension when using a seedless growth technique. Increasing the CTAB concentration produces rods with smaller size. The growth kinetics of the length and the width of gold nanorods can be controlled by changing the CTAB as well as the silver ion concentrations in the reaction mixture.

4- By increasing the amount of NaBH4 added to the solution in the absence of silver ions, smaller gold nanospheres were synthesized. 5- While the extinction coefficient is found to depend linearly on either the aspect ratio or the wavelength of the longitudinal plasmonic band for the large nanorods, it is found to be


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independent for the small nanorods. This was explained by the fact that the extinction band of the plasmonic nanoparticle is dominated by scattering loss processes while for small nanoparticles, they are determined by absorption processes. Each of these properties has different dependences on the size of the nanoparticle. 6- From conclusion 5 and the fact that the extinction coefficient of the large rods are more sensitive to size than the small rods, one concludes that the scattering processes are more sensitive to size changes than absorption, as one expects. Acknowledgements This work was funded by the chemistry division of NSF (DMR-0906822). The authors also thank Lauren A Austin for confirming the reproducibility of the seedless technique and Megan Mackey as well as Lauren for proofreading the manuscript. References [1] El-Sayed, M. A. Some Interesting Properties of Metals Confined in Time and Nanometer Space of Different Shapes. Acc. Chem. Res. 2001, 34, 257-264. [2] Kim, S.-N.; Rusling, J. F.; Papadimitrakopolous, F. Carbon nanotubes for electronic and electrochemical detection of biomolecules. Adv. Mater. 2007, 19, 3214. [3] Formo, E.; Lee, E.; Campbell, D.; Xia, Y. Functionalization of electrospun TiO2 nanofibers with Pt nanoparticles and nanowires for catalytic applications. Nano Lett. 2008, 2, 668. [4] Shevchenko, E. V.; Ringler, M.; Schwemer, A.; Talapin, D. V.; Klar, T. A.; Rogach, A. L.; Feldmann, J.; Alivisatos, A. P. Self-assembled binary superlattices of CdSe and Au nanocrystals and their fluorescence properties. J. Am. Chem. Soc. 2008, 130, 3274.


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[5] Murray, C. B.; Sun, S.; Doyle, H.; Betley, T. Monodisperse 3d transition-metal (Co, Ni, Fe) nanoparticles and their assembly into nanoparticle superlattices Mater. Res. Soc. Bull. 2001, 26, 985. [6] Kamat, P. V. Photophysical, photochemical and photocatalytic aspects of metal nanoparticles. J. Phys. Chem. B. 2002, 106, 7729–7724 [7] Rycenga, M.; McLellan, J. M.; Xia, Y. Controlling the Assembly of Silver Nanocubes through Selective Functionalization of Their Faces. Adv. Mater. 2008, 20, 2416 [8] Connor, E. E.; Mwamuka, J.; Gole, A.; Murphy, C. J.; Wyatt, M.D.; Gold nanoparticles are taken up by human cells but do not cause acute cytotoxicity Small 2005, 1, 325–327. [9] Khan, J.A.; Pillai, B.; T.K. Das, Y. Singh, S. Maiti.; Molecular effects of uptake of gold nanoparticles in HeLa cells Chembiochem 2007, 8, 1237–1240. [10] Shukla, R.; Bansal, V.; Chaudhary. M.; Basu, A.; Bhonde, R. R.; Sastry, M. Biocompatibility of gold nanoparticles and their endocytotic fate inside the cellular compartment: A microscopic overview. Langmuir 2005, 21, 10644–10654. [11] Link, S.; El-Sayed, M. A.; Simulation of the optical absorption spectra of gold nanorods as a function of their aspect ratio and the effect of the medium dielectric constant. J. Phys. Chem. B. 2005. 109, 10531–10532. [12] Link, S.; El-Sayed, M. A.; Shape and size dependence of radiative, non-radiative and photothermal properties of gold nanocrystals. Int. Rev. Phys. Chem. 2000, 19, 409–453. [13] Mie, G. Articles on the optical characteristics of turbid tubes, especially colloidal metal solutions. Ann. Phys. 1908, 25, 377–445. [14] Katz, E.; Willner, I.; Integrated nanoparticle-biomolecule hybrid systems: Synthesis, properties, and applications Angew. Chem., Int. Ed. 2004, 43, 6042–6108. [15] Huh, Y. M.; Y.W. Jun.; Song, H. T.; Kim, S.; Choi, J. S.; Lee,J. H.; Yoon, S.; Kim, K. S.; Shin, J. S.; Suh, J. S.; Cheon, J.; In vivo magnetic resonance detection of cancer by using multifunctional magnetic nanocrystals J. Amer. Chem. Soc. 2005, 127, 12387–12391.


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[16] Durr, N. J.; Larson, T.; Smith, D. K.; Korgel, B. A.; Sokolov, K.; Ben-Yakar, A. Twophoton luminescence imaging of cancer cells using molecularly targeted gold nanorods Nano Lett. 2007, 7, 941–945. [17] Sokolov, K.; Follen, M.; Aaron, J.; Pavlova, I.; Malpica, A.; Lotan, R.; Richards-Kortum, R. Real-time vital optical imaging of precancer using anti-epidermal growth factor receptor antibodies conjugated to gold nanoparticles. Cancer Res. 2003, 63, 1999–2004.

[18] Wang, H.; Huff, T. B.; Zweifel, D. A.; He, W.; Low, P. S.; Wei, A.; Cheng, J. X. In vitro and in vivo two-photon luminescence imaging of single gold nanorods. Proc. Natl. Acad. Sci. USA. 2005, 102, 15752–15756. [19] Rosi, N.L.; Mirkin, C. A.; Nanostructures in biodiagnostics Chem. Rev. 2005, 105, 1547– 1562. [20] El-Sayed, I. H.; Huang, X.; El-Sayed, M. A. Surface plasmon resonance scattering and absorption of anti-EGFR antibody conjugated gold nanoparticles in cancer diagnostics: Applications in oral cancer Nano Lett. 2005, 5, 829–834.

[21] Loo, C.; Lowery, A.; Halas, N. J.; West, J. L.; Drezek, R. Immunotargeted Nanoshells for Integrated Cancer Imaging and Therapy. Nano Lett. 2005, 5, 709–711. [22] Johannsen, M.; Gneveckow, U.; Eckelt, L.; Feussner, A.; Waldofner, N.; Scholz, R.; Deger, S.; Wust, P.; Loening, S. A.; Jordan, A.; Int. J. Hyperthermia. 2005, 12, 637–647. [23] Kam, N. W . S.; O’connell, M.; Wisdom, J. A.; Dai, H. J.; Carbon nanotubes as multifunctional biological transporters and near-infrared agents for selective cancer cell destruction. Proc. Natl. Acad. Sci. USA. 2005, 102, 11600–11605. [24] Chen, J.; Wang, D.; Xi, J.; Au, L.; Siekkinen, A.; Warsen, A.; Li, Z.Y.; Zhang, H.;Xia, Y.; Li, X.; Immuno gold nanocages with tailored optical properties for targeted photothermal destruction of cancer cells Nano Lett. 2007, 7, 1318–1322. [25] El-Sayed, I. H.; Huang, X.; El-Sayed, M. A. Selective laser photo-thermal therapy of epithelial carcinoma using anti-EGFR antibody conjugated gold nanoparticles.Cancer Lett. 2006, 239, 129–135.


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Synthesis and Optical Properties of Small Au Nanorods Using a

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