Au Ag Core Shell Nanorods

PAPER www.rsc.org/nanoscale | Nanoscale

Uniform and controllable preparation of Au–Ag core–shell nanorods usinganisotropic silver shell formation on gold nanorods†

Yoshifumi Okuno, Koji Nishioka, Ayaka Kiya, Naotoshi Nakashima, Ayumu Ishibashi and Yasuro Niidome*

Received 16th February 2010, Accepted 13th April 2010

DOI: 10.1039/c0nr00130a

Anisotropic and controllable silver shell formation on gold nanorods was realized in a micellar solution

of hexadecytrimethylammonium chloride. Uniformity of the anisotropic Au–Ag core–shell particles

contributes separation of four extinction bands. The ability to manipulate the shapes and sizes of these

nanoparticles offers a wide-range control of the surface extinction from the visible to the near infrared

regions (450–800 nm).

Introduction

Metal shell formation on nanoparticles can optimize the elec-

tronic, catalytic, and optical properties of nanoparticles.1 Gold

and silver are frequently used for shell or core materials,1–11

because shell formation of these elements is a useful way to

control the spectroscopic properties of nanoparticles. The

distinct surface plasmon (SP) bands of gold and silver are

dependent on their sizes and shapes and contribute to enhancing

the optical properties of nanoparticles.12,13

The most typical metal nanoparticles that disperse in water are

probably gold nanoparticles.14,15 Gold nanoparticles are

frequently used as probe materials for analytical or bio-chemical

applications, due to their chemical stabilities, the SP bands

locating in visible and near infrared (IR) regions, and facile

surface modification using thiol compounds.16,17 There are many

publications reporting preparation of gold nanoparticles. The

preparation of spherical,18 thin triangular,19 cubic,20 wire-like,21

or rod-shaped22 gold nanoparticles have been reported. As

uniform gold nanoparticles can be synthesized in various shapes

and sizes, gold is a preferable material to prepare core nano-

particles.

Silver nanoparticles show large SP bands in ultra-violet (UV)

and visible regions.23–25 The distinct SP bands of silver nano-

particles contribute to effective enhancements for Raman scat-

tering,26 infrared absorption,27 and fluorescence28 of organic

molecules. Consequently, silver can be used as a functional

material because of its remarkable optical properties. Various

methods to obtain spherical, cubic, or octahedral silver nano-

particles have been reported;23,29,30 however, preparation of

anisotropic silver nanoparticles in a uniform shape has proven

challenging. Spectroscopic properties of the previously prepared

anisotropic silver nanoparticles indicated that the uniformities

observed for anisotropic nanoparticles were not as good as those

of anisotropic gold nanoparticles.31–40 To improve the uniformity

of anisotropic silver nanoparticles, a new approach to control the

growth of silver nanocrystals is required.

Department of Applied Chemistry, Faculty of Engineering, Motooka 744,Nishi-ku, Fukuoka, 819-0395, Japan. E-mail: [email protected]; Fax: +81 92 802 2843; Tel: +81 91 802 2841

† Electronic supplementary information (ESI) available: Fig. S1–S6. SeeDOI: 10.1039/c0nr00130a

This journal is ª The Royal Society of Chemistry 2010

In this work, gold nanorods were wrapped with silver shells to

obtain uniform anisotropic nanoparticles that showed the spec-

troscopic characteristics of silver. The gold nanorods are uniform

rod-shaped gold nanoparticles that show distinct optical char-

acteristics originating from their anisotropic shapes.41 Formation

of uniform silver shells on gold nanorods has been expected to be

a useful method to prepare uniform anisotropic silver nano-

particles. On the basis of this concept, several preparation

methods of Au–Ag core–shell nanorods have previously were

reported.42–45 However, in many cases, the resultant shells were

not uniform on gold nanorods. Due to the insufficient homoge-

neity of the silver shells, their SP bands showed large half-

bandwidths, and minor bands were found as shoulders to the

major peaks. Liu et al. reported that uniform silver shells could be

formed on gold nanorods in the presence of water-soluble poly-

mers.45 Their core–shell nanorods showed two extinction bands

that were assigned to longitudinal and transverse SP oscillations.

Xia et al. reported uniform octahedral silver nanoparticles in

which gold nanorods were incorporated.46 These were Au@Ag

core–shell nanoparticles that showed one broad peak at�460 nm

and a shoulder peak at �350 nm. Previously, we have also

reported silver shell formation in hexadecyltrimethylammonium

chloride (CTAC) and hexadecyltrimethylammonium bromide

(CTAB) mixed micellar solutions.47 We showed that the silver

shell formation in a CTAC solution was much faster than that in

a CTAB solution. Fast nucleation and rapid growth of silver

metals on gold nanorods in a CTAC solution contributed to the

fast formation of the shell, but the fast reactions did not improve

the uniformity of the silver shells.47 Rapid nucleation is advan-

tageous for the formation of a uniform shell; however, if the

subsequent shell growth is competitively fast against the nucle-

ation, the formation of an inhomogeneous silver shell occurs. As

a possible strategy to obtain uniform Au–Ag core–shell nano-

rods, the growth rate of the shell should be suppressed. Two

types of experimental procedures that contained intrinsic

processes to suppress the chemical reactions of silver ions were

tried in this work.

Results and discussion

Silver chloride as a silver-ion source

The first procedure involved using silver chloride (AgCl) particles

suspended in water as a silver-ion source to obtain silver shells.

Nanoscale, 2010, 2, 1489–1493 | 1489

Fig. 1 Extinction spectra and TEM images of Au–Ag core–shell nano-

rods prepared by using AgCl particles at room temperature. (a) spectral

changes of a reaction solution after the addition of AgCl particles and

NaOH solutions (0, 10, 30, 60, 90, 120, 150, 180, 210, 240, 270 min). (b)

a TEM image of gold–silver core–shell nanorods (reaction time: 180 min).

The extinction spectra plotted against wavenumbers were shown in

Fig. S1 of the ESI.†

Fig. 2 Extinction spectra at 180 min after the addition of AgCl particles.

pH ¼ 4.1 (i), 4.5 (ii), 5.0 (iii), 5.4 (iv), 5.7 (v), 6.2 (vi).

The AgCl particles, the diameters of which were 600–900 nm,

were expected to feed AgCl2� ions gradually in the reaction

solutions containing concentrated CTAC molecules (80 mM).

An AgCl-suspended solution (10 mM, 0.25 mL) and an ascorbic

acid solution (0.1 M, 0.5 mL) were added to a CTAC solution (80

mM, 10 mL) containing a certain amount of gold nanorods at

room temperature. The reaction solution showed drastic spectral

changes when the pH of the reaction solutions was adjusted to

5.4 by the addition of a sodium hydrochloride (NaOH) solution.

The spectral changes are shown in Fig. 1(a). At 180 min after the

addition of the AgCl-suspended solution the spectral changes

had stopped, and subsequently the solution showed four bands

at 346, 400, 447 and 549 nm. During the shell formation, the

longitudinal SP bands of gold nanorods at around 900 nm moved

to the shorter wavelength regions. The two bands at around 400

and 447 nm also changed their peak positions and intensities.

The two bands moved to the longer wavelength regions, and

showed similar changes in the intensities with those of the

longitudinal SP bands in the longest wavelength regions. The

band at around 346 nm, on the other hand, did not show

remarkable peak shifts. The spectral changes of the four bands

have simultaneously stopped at 180 min after the addition of the

suspended AgCl solution.

In transmission electron microscopic (TEM) images

(Fig. 1(b)), the gold nanorods were observed in semi-transparent

layers that were consistent with those of previously observed

gold–silver core–shell nanoparticles.42–44 A high-resolution TEM

image and a energy dispersive X-ray spectrum are shown in

Fig. S2 of the ESI.† They indicate a lattice image of metal–silver

1490 | Nanoscale, 2010, 2, 1489–1493

shells and the presence of silver and gold in a nanorod, respec-

tively. These data reveal the formation of the Au–Ag core–shell

nanorods with uniform shapes. (Additional TEM images are

presented in Fig. S3 of the ESI.†) It should be noted that the

growth of the silver shells was anisotropic. As shown in the inset

of Fig. 1(b). The silver shells in the transverse direction were

thicker than those in the longitudinal direction. For palladium, it

is often the case that the metal palladium grows faster along the

[110] planes of gold than along the [100] planes.48 In a CTAC

solution, the surfaces of gold nanorods probably affected the

crystallography of the silver shells. In a CTAB solution, silver

shell formation frequently gave dumb-bell and symmetric (boat-

like) silver shells.42–44 It was shown that CTAB molecules formed

bilayers on gold nanorod surfaces.49 The same procedure as that

of Fig. 1 using a CTAB solution also gave dumb-bell or boat-like

silver shells (see Fig. S4 of the ESI†). Thus, the anisotropic silver

shell formation was also affected by the molecular assemblies on

gold nanorods. It was found that CTAC layers on gold nanorods

were advantageous for the formation of uniform core–shell

nanorods.

The TEM images showed no remarkable by-product in Fig. 1

and Fig. S3.† Thus, the spectroscopic properties in Fig. 1(a)

should be assigned to the optical responses of the core–shell

nanorods with anisotropic silver shells. The four bands in the

extinction spectra (Fig. 1(a)) were also observed in previous

research efforts;39,47 however, they were not clearly isolated from

each other; minor peaks were observed as shoulders of the major

peaks. (An absorption spectrum and a TEM image of Au–Ag

core–shell nanorods prepared by our previous method47 are

shown in Fig. S5 of the ESI.†) At the present stage of this work

we have not yet determined the origins of the four peaks;

however, the isolated four SP peaks in Fig. 1(a) indicate that the

new procedure using AgCl particles gives very uniform core–shell

nanorods.

Fig. 2 shows extinction spectra at 180 min after the addition of

AgCl particles at different pH conditions. At low pH conditions

(pH ¼ 4.1 (i) and 4.5 (ii)), longitudinal SP bands of gold nano-

rods shifted to shorter wavelength regions. Time dependent

spectral changes and TEM images are shown in Fig. S6 of the

ESI.† These spectral changes and TEM observations indicated

that the silver shell formation proceeded even at the lower pH

conditions, but the rate of the shell formation was very slow. At

pH ¼ 5.0 (iii) and 5.4 (iv), the shell formation has stopped by 180

min following the addition of the AgCl particles, and at that time

point, the four SP bands that are characteristic of the Au–Ag


core–shell nanorods that were observed. This pH range was

shown to be appropriate for preparing silver shells. In contrast,

at the higher pH conditions (pH ¼ 5.7 (v) and 6.2 (vi)), the

spectral changes stopped within 10 min and resulted in indistinct

profiles in the absorption spectra (Fig. S6 of the ESI†). This

indicates that the reduction of Ag ions at the higher pH is very

rapid and this reduction rate hindered the formation of uniform

Au–Ag core–shell nanorods.

Fig. 4 Extinction spectra, macroscopic photographs, and TEM images

Low pH reactions with silver nitrate

In the second procedure, silver nitrate was used as a silver ion

source for the formation of silver shells. This procedure has

previously been used in our paper,47 but the pH of the reaction

solutions is regulated to control the reduction of the silver ions.

Fig. 3(a) shows extinction spectra at 180 min after the addition of

the silver nitrate solutions at different pH conditions. Under

acidic conditions (pH ¼ 3.0 (i), 3.8 (ii), and 4.4 (iii)), the changes

in the spectra indicate that the shell formations are very slow. At

higher pH conditions (pH ¼ 4.6 (iv), 4.9 (v) and 5.4 (vi)), silver

ions appear to be quickly reduced, but large SP bands are

observed at �410 nm. This band is assigned to colloidal silver

nanoparticles. Thus, at the higher pH conditions, the formation

of the colloidal silver nanoparticles competes against the

formation of the silver shells on gold nanorod surfaces.

In acidic conditions, reduction of silver ions by the ascorbic

acid is very slow because ascorbate is active for the reduction.8

The dissociation constant (pKa) of ascorbic acid is pKa¼ 4.2, and

at this pH, the reaction speeds and the resultant morphologies of

the silver shells are drastically changed.

Heating of the reaction solutions accelerates the silver shell

formation at lower pH conditions (pH ¼ 3) using AgNO3.

Fig. 3(b) shows extinction spectra at after 90 min at 60 (vii), 70

(viii), and 80 (ix) �C. At 60 �C (vii), the spectrum shows the four

distinct SP bands that are assigned to the formation of the silver

Fig. 3 Extinction spectra of Au–Ag core–shell nanorods prepared by

using AgNO3 solutions. (a) Extinction spectra at 180 min after the

addition of AgNO3 solutions at room temperature. pH ¼ 3.0 (i), 3.8 (ii),

4.4 (iii), 4.6 (iv), 4.9 (v), 5.4 (vi). (b) Extinction spectra at 180 min after the

addition of AgNO3 solutions at 60, 70, 80 �C.


shells. TEM observations indicated that silver shells were formed

on gold nanorods (Fig. S7 of the ESI†). At higher temperatures

(70 (vii) and 80 (ix) �C), the reaction rates of the formation of the

silver shells were faster than those at 60 �C. Moreover, the

profiles of the SP bands and shapes of silver shells formed at

these two high temperatures were not affected. These results

indicated that the reaction temperatures could accelerate the

shell growth processes without degrading the uniformity of the

core–shell nanorods. The control of the shell growth processes

was found to be an important factor in obtaining a silver shell.

Characteristics of the shell formation

The thickness of the silver shells can be controlled by the relative

amounts of silver ions and gold nanorods. Fig. 4 shows the

extinction spectra, photographs of the reaction solutions, and

TEM images of the core–shell particles that were prepared by

using AgCl particles at pH ¼ 5.4 at room temperature. The

results of shell formation using AgNO3 solutions (pH ¼ 3.0, 70�C) at the different Ag/Au ratios (7.1–57) are shown in Fig. 5.

The spectra in Fig. 4 and 5 indicated that good reproducibility of

the two developed synthesis procedures was possible (intensities

of the longitudinal SP bands were not normalized) and the

of Au–Ag core–shell nanorods prepared by using different amount of

AgCl particles. (a) Extinction spectra at 180 min after the addition of

AgCl particles at room temperature. Molar ratios of Ag/Au were 57 (i),

28.5 (ii), 19 (iii), 14.2 (iv), 9.5 (v), and 7.1 (vi). Concentration of silver ions

was constant (0.25 mM). (b) Macroscopic photographs of reaction

solutions. (c) TEM images of the core–shell nanorods. The scale bars

indicate 50 nm.

Fig. 5 Extinction spectra, macroscopic photographs, and TEM images

of Au–Ag core–shell nanorods prepared by using different amount of

AgNO3 solutions. (a) Extinction spectra at 180 min after the addition of

AgNO3 solutions at 60 �C. Molar ratios of Ag/Au were 57 (i), 28.5 (ii), 19

(iii), 14.2 (iv), 9.5 (v), and 7.1 (vi). Concentration of silver ions was

constant (0.25 mM). (b) Macroscopic photographs of reaction solutions.

(c) TEM images of the core–shell nanorods. The scale bars indicate 50 nm.

Nanoscale, 2010, 2, 1489–1493 | 1491

solutions showed dramatic changes in color from orange to green

(Fig. 4(b) and 5(b)). The TEM images clearly indicated the

formation of silver shells on the gold nanorods. It was shown

that the both procedures examined in this work were useful

methods to realize controllable and reproducible silver-shell

formation on gold nanorods.

Even on long and short gold nanorods (aspect ratio ¼ 7 and 4)

and spherical gold nanoparticles, uniform core–shell nanorods

could be prepared using the described methods (Fig. S8 in the

ESI†). Consequently, the slow reactions of the AgCl particles or

the AgNO3 solutions at low pH are useful to control the growth

of anisotropic silver shells on various gold nanoparticles. Using

these methods, we can control the anisotropic synthesis of silver

shells on gold nanorods with uniform shapes.

Fig. 6 indicates the characteristics of our methods. The vertical

and horizontal axes of Fig. 6 are the transverse diameters and

longitudinal lengths of the nanoparticles. Aspect ratios (trans-

verse width/longitudinal length) are shown as five straight lines.

If there were spherical (isotropic) nanoparticles of different

particle sizes, they would be plotted on the line of the aspect ratio

¼ 1, that is, the most-inclined line. In the case of the synthesis of

the silver nanorods that were reported in previous papers,33,34 the

shapes of these nanorods are distributed in the green area. The

longitudinal lengths of the silver nanorods are shown to be

controlled over a wide range, but the transverse diameters are

restricted from 10 to 35 nm. Xia and co-workers reported

preparations of silver nanowires35–38 and silver nanobars.50 The

blue and yellow areas in Fig. 6 indicate the distribution of the

silver nanowires and nanobars produced by these groups. In the

previous cases, it was shown that the control of transverse

diameters was not as good as that of the longitudinal length.

Consequently, the green, blue, and yellow areas are long rect-

angles in the horizontal direction.

In Fig. 6, the sizes of our core–shell nanorods by the two

methods are also plotted. The methods used herein control the

thickness of the silver shells in the transverse direction of the

core–shell nanorods. In the longitudinal direction, growth of the

silver shells is suppressed. This anisotropic shell formation can be

seen in the TEM images in Fig. 1(b), 4(c), 5(c), and S6–8.† The

plots of our core–shell nanorods (within the red quadrangle)

Fig. 6 Schematic illustration of controllability of anisotropic metal

nanoparticles. The size of the gold–silver core–shell particles is control-

lable in the transverse direction (red quadrangle). It is very different from

the size distributions of the previous methods, shown as green, blue, and

yellow rectangles.33–38,47 Red stars are plotted from the TEM images in

Fig. S8 in the ESI.†

1492 | Nanoscale, 2010, 2, 1489–1493

indicate very different controllability from the previous results

(green, blue, and yellow rectangles). Thus, our methods propose

a novel strategy to manipulate the shapes of anisotropic silver

nanoparticles. In our methods, various gold nanoparticles can be

used as seed particles (Fig. S8 in the ESI†). In particular, as seed

nanoparticles, gold nanorods can be prepared over wide ranges

without degrading their uniformities (gray rectangle).51 As such,

various gold nanorods can be used as core particles for the

formation of silver shells, and this will facilitate the design of

desirable shapes for core–shell particles.

Experimental

Gold nanorods in a CTAB solution, which were prepared by

a photochemical method,52 were obtained from a joint research

project of Dai-Nihon-Toryo Co. Ltd. and Mitsubishi Materials

Corp. The aspect ratio of the gold nanorods was about 5 (51 � 7

nm and 9.7 � 1.1 nm in longitudinal and transverse directions,

respectively). Longer (73 � 11 nm � 10 � 1.7 nm, aspect ratio ¼7) and shorter (42 � 5 nm � 9.2 � 1.2 nm, aspect ratio ¼ 4) rods

were also used. CTAB and CTAC were obtained from Tokyo

Kasei and used without further purification. The gold nanorod

solution was centrifuged at 8000 � g for 60 min, and the

precipitates were redispersed in a CTAC solution (80 mM). This

procedure was repeated twice, and then the concentration of the

gold nanorods was set to be 33 mM (0.22 mM as Au atoms). The

absorbance of the nanorod solution was 0.5 at 900 nm and

corresponds to the top peak of the longitudinal SP band. An

appropriate amount of the nanorod solution (0.1–2.4 mL) and an

ascorbic acid solution (100 mL, 0.5 mL) was added in a CTAC

solution (10 mL, 80 mM). This is the reaction solution used for

the formation of the silver shells. The AgCl particles or a silver

nitrate solution was added to the reaction solution. AgCl

nanoparticles were prepared by the addition of silver nitrate (17

mg) in a CTAC solution (10 mL, 80 mM). Dynamic light scat-

tering measurements indicated that the sizes of the AgCl particles

were distributed between 500–950 nm. A sodium hydroxide

solution (0.5 M, 0.06–4.0 mL) and silver ions (10 mM, 0.25 mL)

were added to the reaction solution with vigorous stirring. The

sodium hydroxide solution was added to control the pH of the

reaction solutions.

Spectral changes of each solution were monitored by sampling

a small portion of the reaction solutions in a thin optical cell

(optical path length: 1 mm). TEM observations were performed

using a JEM 2010 (JEOL, operated at 120 kV). A high-resolution

image was obtained by JEM-2010FEF (JEOL, operated at

200 kV).

Conclusion

The uniform Au–Ag core–shell nanorods were obtained by

retarding the silver shell growth on gold nanorods. The colloidal

solutions of the Au–Ag core–shell nanorods showed the four

extinction bands that originated from the anisotropic silver shell

formation. The spectroscopic characters and the TEM images of

the core–shell nanorods indicated that our methods gave very

reproducible and controllable core–shell nanorods. This work

provides a novel strategy to facilitate the precise control of the

anisotropic silver-shell formation. The monodisperse anisotropic


silver nanoparticles, which have never previously been produced,

will contribute to novel optical phenomena that can be induced

by anisotropic silver nanostructures.

Acknowledgements

This work was supported in part by a KAKENHI (Grant-in-Aid

for Scientific Research) on the Priority Area ‘‘Strong Photon-

Molecule Coupling Fields (No. 470)’’ and a Grant-in-Aid for the

Global COE Program, ‘‘Science for Future Molecular Systems’’

from the Ministry of Education, Culture, Sports, Science and

Technology, Japan.

Notes and references

1 S. Kalele, S. W. Gosavi, J. Urban and S. K. Kulkarini, Curr. Sci.,2006, 91, 1038–1052.

2 C. Wang, S. Peng, R. Chan and S. Sun, Small, 2009, 5, 567–570.3 Y. Yang, J. Shi, G. Kawamura and M. Nogami, Scr. Mater., 2008, 58,

862–865.4 Y. Huang, Y. Yang, Z. Chen, X. Li and M. Nogami, J. Mater. Chem.,

2008, 43, 5390–5393.5 A. M. Gobin, M. H. Lee, N. J. Halas, W. D. James, R. A. Drezek and

J. L. West, Nano Lett., 2007, 7, 1929–1934.6 M. Tsuji, N. Miyamae, S. Lim, K. Kimura, X. Zhang, S. Hikino and

M. Nishio, Cryst. Growth Des., 2006, 6, 1801–1807.7 S. S. Shankar, A. Rai, A. Ahmad and M. Sastry, J. Colloid Interface

Sci., 2004, 275, 496–502.8 C.-C. Huang, Z. Yang and H.-T. Chang, Langmuir, 2004, 20, 6089–

6092.9 L. R. Hirsch, R. J. Stafford, J. A. Bankson, S. R. Sershen, B. Rivera,

R. e. Price, J. D. Hazle, N. J. Halas and J. L. West, Proc. Natl. Acad.Sci. U. S. A., 2003, 100, 13549–13554.

10 L. R. Hirsch, J. B. Jackson, A. Lee, N. J. Halas and J. L. West, Anal.Chem., 2003, 75, 2377–2381.

11 L. Lu, H. Wang, Y. Zhou, S. Xi, H. Zhang, J. Hu and B. Zhao, Chem.Commun., 2002, 144–145.

12 K. L. Kelly, E. Coronado, L. L. Zhao and G. C. Schatz, J. Phys.Chem. B, 2003, 107, 668–677.

13 U. Kreibig and M. Vollmer, Optical Properties of Metal Clusters,Springer, Berlin, 1994.

14 J. Turkevitch, P. C. Stevenson and J. Hillier, Discuss. Faraday Soc.,1951, 11, 55.

15 B. L. Cusing, V. L. Kolesnichenko and C. J. O’Connor, Chem. Rev.,2004, 104, 3893–3946.

16 R. Shukla, V. Bansal, M. Chaudhary, A. Basu, R. R. Bhonde andM. Sastry, Langmuir, 2005, 21, 10644–10654.

17 P. K. Jain, I. H. El-Sayed and M. A. El-Sayed, Nano Today, 2007, 2,18–29.

18 M. Faraday, Philos. Trans. R. Soc. London, 1857, 147, 145.19 J. E. Millstone, G. S. M�etraux and C. A. Mirkin, Adv. Funct. Mater.,

2006, 16, 1209–1214.20 C.-J. Huang, Y.-H. Wang, P.-H. Chiu, M.-C. Shih and T.-H. Meen,

Mater. Lett., 2006, 60, 1896–1900.


21 B. D. Busbee, S. O. Obare and C. J. Murphy, Adv. Mater., 2003, 15,414–416.

22 Y.-Y. Yu, S.-S. Chang, C.-L. Lee and C. R. C. Wang, J. Phys. Chem.B, 1997, 101, 6661–6664.

23 Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kimand H. Yan, Adv. Mater., 2003, 15, 353–389.

24 C. Burda, X. Chen, R. Narayanan and M. A. El-Sayed, Chem. Rev.,2005, 105, 1025–1102.

25 M. E. Stewart, C. R. Anderton, L. B. Thompson, J. Maria,S. K. Gray, J. A. Rogers and R. G. Nuzzo, Chem. Rev., 2008, 108,494–521.

26 P. C. Lee and D. Meisel, J. Phys. Chem., 1982, 86, 3391–3396.27 A. Hatta, T. Ohshima and W. Su€etaka, Appl. Phys. A: Mater. Sci.

Process., 1982, 29, 71–75.28 K. Aslan, J. R. Lakowicz and C. D. Geddes, Anal. Bioanal. Chem.,

2005, 382, 926–933.29 A. Taleb, C. Petit and M. P. Pileni, Chem. Mater., 1997, 9, 950–959.30 Y. Sun and Y. Xia, Science, 2002, 298, 2176–2179.31 A. Tao, P. Sinsermsuksakul and P. Yang, Angew. Chem., Int. Ed.,

2006, 45, 4597–4601.32 R. Jin, Y. C. Cao, E. Hao, G. S. M�etraux, G. C. Schatz and

C. A. Mirkin, Nature, 2003, 425, 487–490.33 N. R. Jana, L. Gearheart and C. J. Murphy, Chem. Commun., 2001,

617–618.34 K. K. Caswell, J. N. Wilson, U. H. F. Burnz and C. J. Murphy, J. Am.

Chem. Soc., 2003, 125, 13914–13915.35 Y. Sun, B. Gates, B. Mayers and Y. Xia, Nano Lett., 2002, 2, 165–168.36 Y. Sun, B. Mayers, T. Herricks and Y. Xia, Nano Lett., 2003, 3, 955–

960.37 Y. Sun, Y. Yin, B. T. Mayers, T. Herricks and Y. Xia, Chem. Mater.,

2002, 14, 4736–4745.38 Y. Sun and Y. Xia, Adv. Mater., 2002, 14, 833–837.39 B. Pietrobon, M. McEachran and V. Kitaev, ACS Nano, 2009, 3, 21–

26.40 B. Wiley, Y. Sun and Y. Xia, Acc. Chem. Res., 2007, 40, 1067–1076.41 J. P�erez-Juste, I. Pastoriza-Santos, L. M. Liz-Marz�an and

P. Mulvaney, Coord. Chem. Rev., 2005, 249, 1870–1901.42 Y. Xiang, X. Wu, D. Liu, Z. Li, W. Chu, L. Feng, K. Zhang, W. Zhou

and S. Xie, Langmuir, 2008, 24, 3465–3470.43 C. S. Ah, S. D. Hong and D.-J. Jang, J. Phys. Chem. B, 2001, 105,

7871–7873.44 Z. Yang and H.-T. Chang, Nanotechnology, 2006, 17, 2304–2310.45 M. Liu and P. Guyot-Sionnest, J. Phys. Chem. B, 2004, 108, 5882–

5888.46 E. C. Cho, R. H. C. Camargo and Y. Xia, Adv. Mater., 2010, 22, 744–

748.47 Y. Okuno, K. Nishioka, N. Nakashima and Y. Niidome, Chem. Lett.,

2009, 38, 60–61.48 Y. Xiang, X. Wu, D. Liu, X. Jiang, W. Chu, Z. Li, Y. Ma, W. Zhou

and S. Xie, Nano Lett., 2006, 6, 2290–2294.49 B. Nikoobakht and M. A. El-Sayed, Langmuir, 2001, 17, 6368–6374.50 B. J. Wiley, Y. Chen, J. M. McLellan, Y. Xiong, Z.-Y. Li, D. Ginger

and Y. Xia, Nano Lett., 2007, 7, 1032–1036.51 C. J. Murphy, T. K. Sau, A. M. Gole, C. J. Orendorff and J. Gao, J.

Phys. Chem. B, 2005, 109, 13857–13870.52 Y. Niidome, K. Nishioka, H. Kawasaki and S. Yamada, Chem.

Commun., 2003, 2376–2377.

Nanoscale, 2010, 2, 1489–1493 | 1493

Documents

Au Ag Core Shell Nanorods