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Grafting poly(4-vinylpyridine) onto gold nanorods toward functionalplasmonic core–shell nanostructures†

DongXiang Li,ab Yu Jin Jang,a Jieun Lee,‡a Ji-Eun Lee,a Saji Thomas Kochuveedua and Dong Ha Kim*a

Received 14th July 2011, Accepted 18th August 2011

DOI: 10.1039/c1jm13302k

Ligand exchange of CTAB-protected gold nanorods (AuNRs) with disulfide initiator was found to

usually cause nanorod aggregation, but such aggregation could be prevented under the steric hindrance

of pre-anchored poly(ethylene glycol) thiol. The obtained initiator-modified AuNRs were well-

dispersed and could initiate the in situ atom-transfer radical polymerization (ATRP) of 4-vinylpyridine,

resulting in poly(4-vinylpyridine)-grafted AuNRs as core–shell structures (AuNR@PVP). These

polymer/Au nanocomposites displayed pH-responsive surface plasmon resonance changes because of

the protonation and deprotonation of pyridine groups. The coordinative polymer shells allowed these

structures to be employed as nanosupports for transition metal ions such as platinum ions, which could

be reduced to Pt nanoparticles embedded on the surfaces of the AuNRs. Bimetallic nanostructures of

Pt-decorated AuNR@PVP nanocomposites exhibited typical catalytic activity for methanol oxidation.

1. Introduction

Core–shell polymer-functionalized gold nanoparticles (AuNPs)

have been studied for use in electronics, optics, catalysis and

biology due to their unique surface plasmon resonances

(SPRs).1–3 Polymer shells grafted onto the surfaces of AuNPs can

greatly enhance the nanoparticles’ stability through steric

hindrance and can allow the engineering of various functional

properties in the resulting composite NPs. AuNPs modified by

stimuli-responsive polymers have been shown to demonstrate

significantly enhanced properties.4,5 For example, AuNPs

modified with poly(N-isopropyl acrylamide) showed highly

temperature-dependent optical transmittance, SPR band shift

and steric occupation.6–8 AuNPs have displayed pH-dependent

morphologies after modification with pH-sensitive poly(4-

vinylpyridine) (PVP).9 Solvent- and salt- responsive polymer-

modified AuNPs have also been reported.10,11

The existence of two distinct transverse and longitude SPR

modes in asymmetric gold nanorods (AuNRs), a unique type of

low-dimensional Au nanomaterial, results in the particles having

interesting and useful properties.12 They have been employed in

a range of applications such as optical contrast agents for dark

fields,13–15 drug release controllers for biodelivery,16,17 near-

aDepartment of Chemistry and Nano Science, Ewha Womans University,52, Ewhayeodae-gil, Seodaemun-gu, Seoul, 120-750, Korea. E-mail:dhkim@ewha.ac.krbCollege of Chemistry and Molecular Engineering, Qingdao University ofScience and Technology, Qingdao, China

† Electronic supplementary information (ESI) available. See DOI:10.1039/c1jm13302k

‡ Current address: Battery Research & Development, LG Chem, Ltd.,Daejeon 305-380, Korea.

This journal is ª The Royal Society of Chemistry 2011

infrared fluorescence diagnostic imaging18,19 and photothermal/

photodynamic therapy.20,21 They have also been employed as

modulated photothermal recording media and two-photon-

fluorescence reading materials for data storage.22 Poly(ethylene

glycol) is usually used as a biocompatible protecting agent to

modify AuNRs in biological systems,23–25 and polystyrene has

been used to as a modifier to allow the assembly of well-defined

nanostructures.26,27 Poly(N-isopropyl acrylamide)-grafted

AuNRs have been reported to be sensitive to heat and near-

infrared-light for photothermal-controlled drug release,28,29 and

an amphiphilic block copolymer of PVP-b-polystyrene(PS)-b-

PVP was also employed to cap AuNRs for interfacial entrap-

ment.30 However, a rational approach to the design and fabri-

cation of pH-sensitive and coordinative PVP-grafted AuNRs has

not yet been reported.

This work reports the fabrication of poly(4-vinylpyridine)-

grafted AuNRs (AuNR@PVP) through the immobilization of

disulfide initiator and surface-initiated atom-transfer radical

polymerization (ATRP) on the surfaces of AuNRs (Scheme 1).

Dense hexadecyltrimethylammonium bromide (CTAB) bilayers

on the nanorod surfaces would hinder the ligand exchange and

lead to the nanorod aggregation in the direct ligand-exchange

with disulfide initiator. To solve this problem, poly(ethylene

glycol) (PEG) thiol was first introduced to aqueous AuNRs to

produce PEG-modified AuNRs (AuNR@PEG). This allowed

the NRs to be well-dispersed in organic solvents and provided

steric hindrance during subsequent initiator modification. The

disulfide initiator could penetrate into the PEG chains in organic

solvent, first replacing the CTAB layer under polymeric

hindrance before replacing the PEG chains, resulting in initiator-

modified AuNRs (AuNR@initiator). Subsequent ATRP of 4-

vinylpyridine (4VP) was performed on the NRs’ surfaces under

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Scheme 1 The stepwise surface modification and polymer grafting strategy from CTAB-protected AuNRs to PEG-modified AuNRs (AuNR@PEG),

initiator-modified AuNRs (AuNR@initiator), PVP-grafted AuNRs (AuNR@PVP) and nanoparticle-loaded AuNRs (AuNR@PVP-NP) core@shell

nanostructures.

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a catalytic system of CuBr/CuBr2/PMDETA. The final

AuNR@PVP nanocomposites had soft polymer shells and

showed pH-responsive SPR changes in aqueous solutions. When

they were used as coordinative supports for transition metals, the

resulting bimetallic Pt-loaded AuNR@PVP nanohybrids showed

typical electrocatalytic activity.

2. Experimental

2.1 Materials

Hexadecyltrimethylammonium bromide (CTAB, H9151),

HAuCl4$3H2O, NaBH4, AgNO3, ascorbic acid (AA), and poly

(ethylene glycol) thiol (Mw ¼ 5000 g mol�1) (for nanorod

synthesis and modification), and N,N,N0,N0,N0 0-pentam-

ethyldiethylenetriamine (PMDETA), CuBr, CuBr2 and 4-vinyl-

pyridine (4VP) (for ATRP), were purchased from Sigma-

Aldrich. The ATRP initiator bis[2-(2-bromoisobutyryloxy)

undecyl] disulfide was prepared from 11-mercapto-1-undecanol

and 2-bromo-2-methylpropionyl bromide through a modified

procedure reported elsewhere.31 4-Vinylpyridine (4VP) was

treated by filtrating through an alumina column to remove the

polymerization resistor. Other commercially available chemicals

and solvents were purchased and used without purification.

Purified water (resistance > 18.2 MU) was prepared through an

ultrapure water purification system.

2.2 Synthesis of gold nanorods

AuNRs were synthesized by a two-step, seed-mediated, surfac-

tant-assisted approach in aqueous solution.32–34 First, the Au

seed solution was prepared by adding sodium borohydride (5

mM, 0.48 mL) to a mixed solution of CTAB (0.20 M, 2.0 mL)

and hydrogen tetrachloroaurate (0.5 mM, 2.0 mL) with stirring.

Then, the Au seed solution (0.12 mL) was slowly added to

a growth solution (100 mL) of CTAB (0.10 M), hydrogen tet-

rachloroaurate (0.5 mM), ascorbic acid (0.6 mM) and silver

nitrate (0.04–0.12 mM). The NRs were allowed to grow for ca. 12

hours at 27 �C before being collected by centrifugation, decan-

tation and redispersion.

16454 | J. Mater. Chem., 2011, 21, 16453–16460

2.3 Initiator immobilization on the nanorods’ surfaces

PEG thiol was used to modify the nanorods in an aqueous

suspension before the immobilization of disulfide initiator on the

nanorods’ surfaces. In detail, 2.0 mL PEG thiol solution (0.25

mM) was introduced into 2.0 mL concentrated aqueous AuNR

suspension (�5 nM), followed by overnight stirring at 25–30 �C.The PEG-modified AuNRs were separated by centrifugation and

redispersed in 4 mL initiator solution (2 mM) in DMF. After 3–5

h of ligand exchange reaction, AuNR@initiator samples were

obtained by centrifugation.

2.4 Surface-initiated ATRP on the AuNRs

The ATRP of 4VP monomers on the surfaces of the AuNRs was

performed in co-solvents of DMF and water/2-propanol by an

elsewhere published method.35–38 In detail, CuBr (14.3–57.2 mg)

and CuBr2 (1–4 mg) were added to a round-bottom flask and

degassed by three freeze–pump–thaw cycles under an N2 atmo-

sphere. Then, a nitrogen-purged mixture of AuNR@initiator

(1–2 mg) dissolved in DMF (3.5 mL), PMDETA (0.208–0.416

mL), water (0.5 mL), and a certain amount of sacrificial initiator

(ethyl a-bromoisobutyrate, 1–10 mL) was injected into the flask

through a syringe, followed by the addition of nitrogen-purged

4VP (2.10 g) in DMF (1.0 mL) with stirring. The reaction was

allowed to progress for ca. 24 h at 40 �C and terminated by

opening the system to air. The AuNR@PVP nanocomposites

were purified by more than three cycles of centrifugation and

were redispersed in DMF.

2.5 Platinum nanoparticle formation on the AuNR surface

A certain amount of the AuNR@PVP nanocomposites was

dispersed in 2 mL of dilute HCl solution (pH 3.0) and 0.01–

0.05 mL of 1 mM H2PtCl6 solution was added dropwise with

drastic shaking. Then, the nanorods with adsorbed Pt

precursor were separated by centrifugation, followed by the

addition of 10 mM NaBH4 under shaking. The final Pt

nanoparticle-loaded AuNR@PVP was separated and cleaned

by centrifugation.

This journal is ª The Royal Society of Chemistry 2011

Fig. 1 TEM images of CTAB-protected AuNRs (A and B), PEG-

modified AuNRs (C and D) and initiator-modified AuNRs (E and F) at

different magnifications. Insets show photographs of the respective

AuNR dispersions in DMF, respectively.

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2.6 Characterization and instrumentation

UV-vis absorption and FT-IR spectra were recorded on a Varian

Cary 5000UV-Vis-NIR spectrophotometer and aVarianFTS800

spectrometer, respectively. TEM imageswere obtained by a JEOL

2011 transmission electron microscopy electron microscope.

X-Ray photoelectron spectroscopy (XPS) measurements were

performed on a PHI 5800 ESCA system using a monochromatic

aluminium X-ray source (Al Ka 1486.6 eV). The particle size was

measured at 25 �C on a dynamic light scattering (DLS) system of

the Malvern Zetasizer Nano-ZS apparatus. The electrochemical

study was performed on an Autolab PGSTAT302N potentiostat

at room temperature. Electrode potentials were measured and

reported against a saturated calomel electrode (SCE), while the

counter electrode was Pt plate. The working electrode was

prepared by dropping the nanocomposite samples on glassy

carbon electrodes (GCEs) of 3 mm diameter, and after drying at

room temperature, 0.05% nafion solution in 2-propanol was used

to fix the catalyst samples.

3. Results and discussion

3.1 Immobilizing disulfide initiator on the nanorods’ surfaces

under PEG hindrance

Uniform AuNRs with different aspect ratios were synthesized

by a conventional seed-mediated, surfactant-assisted growth

method.32–34 Micelles of the surfactant, CTAB, were employed as

soft templates for the anisotropic growth of the Au nanocrystals,

and the use of silver nitrate (AgNO3) enhanced their uniformity

because reduced silver atoms could selectively cover side facets of

the NRs, enhancing anisotropic growth.39 Varying the concen-

tration of AgNO3 allowed a series of AuNRs with variable aspect

ratios to be produced (see Fig. S1 in the ESI†). These AuNRs

were stabilized effectively by CTAB bilayers on their surfaces.

Their stability was such that centrifuged AuNRs could be

redispersed in water even at low CTAB contents.25,40

AuNRs with average dimensions of 14 nm � 56 nm (i.e. an

aspect ratio of 4.0) were selected for surface modification by

ligand exchange to immobilize the initiator for subsequent

ATRP. Although AuNPs are known to be readily modified by

sulfur-containing compounds viaAu–S interactions, there is little

work on the surface modification of AuNRs. The direct addition

of a small volume of disulfide initiator in DMF to aqueous

AuNRs has been reported to produce initiator-modified

AuNRs.28 However, this approach produced mainly aggregated

NRs after centrifugation. The ‘‘round-trip’’ method was then

tested: AuNRs were transferred into organic solvent by an

alkanethiol assisted phase-transfer,41,42 which was expected to

facilitate their modification by the hydrophobic disulfide initi-

ator. However, AuNR aggregates were observed after the phase-

transfer because maintaining sufficient separation of the NRs

was difficult as they passed through the inorganic/organic

interface. Altering the NRs’ surface functionality using the

biocompatible polymer PEG thiol has been reported for the

modification of AuNRs for biological applications, with PEG-

modified AuNRs (AuNR@PEG) showing good stability and

dispersity.23–25 The amphiphilic PEG could allow AuNR@PEG

to be well-dispersed in organic solvents such as DMF, THF, and

alcohols. The organic dispersity and steric hindrance afforded by

This journal is ª The Royal Society of Chemistry 2011

the polymer chains can allow alkanethiol surface modification by

ligand exchange without aggregation.43 Therefore, PEG thiol

was employed for the first modification of the AuNRs, then

disulfide initiator surface modification was performed in DMF

under polymer hindrance (Scheme 1).

Fig. 1 shows TEM images of CTAB-protected AuNRs (top),

PEG-modified AuNRs (AuNR@PEG) (middle) and initiator-

modified AuNRs (AuNR@initiator) (bottom) together with

photographs of the respective solutions (insets). The CTAB-

protected AuNRs in Fig. 1A and B are well-dispersed and

assembled on carbon film on TEM copper grids. The average

separation between neighboring NRs is ca. 5 nm, while the

thickness of dense CTAB bilayer is estimated to be around 2 nm

according to the molecule size. This should be ascribed to the

electrostatic repulsion between the two NRs due to the positive

cetrimonium ions on the NR surfaces. The AuNR@PEG

(Fig. 1C and D) showed increased average separation between

neighboring NRs of ca. 20 nm. Such a large distance should be

ascribed to the steric hindrance of the long PEG chains (Mw

5000, structural unit number: 113) although only a small visible

polymer layer could be found on the AuNR surface by TEM.

Fig. 1E and F show reduced average separation between the

AuNR@initiator of only 2 nm, in accordance with the doubled

length of the initiator molecules. Although all three samples were

stained with phosphotungstic acid, a soft corona is only

observable around the AuNR@PEG particles, reflecting the

existence of the polymer layer.

FT-IR spectra were obtained to analyze the compositions of

the AuNR@PEG, AuNR@initiator and AuNR@PVP nano-

composites (see Fig. S2 in the ESI†). The broad peak around

3500 cm�1 originated from moisture in the samples. The peaks

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between 2960 and 2820 cm�1 represent the asymmetric stretching

bands of CH3 and CH2 groups, respectively. For the AuNR@

PEG sample, the large peak at 1086 cm�1 corresponds to char-

acteristic ether (C–O–C) bond stretching of PEG,31 while the one

centered at 1466 cm�1 is related to the dasym(C–H) and dsym(C–H)

of CH3–N+ moiety in CTAB.40 The whole profile of the

AuNR@PEG sample seems to be a combination of PEG and

CTAB, possibly reflecting a coexistence of these two compo-

nents. Obviously, the FT-IR profile of the AuNR@initiator

spectrum is very similar to that of the pure initiator, especially

focused on the characteristic absorbance of the ester carbonyl

stretching at 1736 cm�1, indicating the presence of the initiator on

the nanorods’ surfaces.

The chemical identities of AuNR@PEG and AuNR@initiator

were further analyzed by X-ray photoelectron spectroscopy

(XPS). The survey scan profiles in Fig. 2A confirm that both

samples contained Au, Ag, C, and Br. The presence of elemental

Ag verifies that the nanorods’ surfaces were covered by Ag

during their synthesis.39 Deconvolution of the peaks in the C1s

spectrum of AuNR@PEG (Fig. 2B) shows three discernible

peaks with binding energies of 284.8, 286.1 and 286.5 eV. The

former two peaks are attributable to carbon atoms in C–H/C–C

and C–N bonds in CTAB, and their ratio of 15 : 4.5 is in

consistent with the theoretical value (15 : 4). The latter peak has

the highest binding energy and can be ascribed to carbon atoms

in the glycol moieties of PEG. This verifies the coexistence of

CTAB and PEG on the NR surface in good agreement with the

FT-IR results. Quantitative evaluation of the peaks’ relative

areas shows that the molar ratio of adsorbed PEG to CTAB is ca.

1 : 23. Although the PEG grafting density is much smaller than

CTAB molecules, the factual mass ratio of PEG to CTAB on the

NR surface can be deduced to be 0.6 : 1 considering the large

molecular weight of PEG polymer. Possibly, PEG–thiol

exchanges a small part of the CTAB molecules and brings more

mass onto the NR surface, thus a visible layer on the NR surface

is discerned under TEM observation. In comparison, the

deconvoluted C1s spectrum of AuNR@initiator (Fig. 2B) shows

different carbon components. The intensity of the original peak

Fig. 2 XPS survey scan (A) and C 1s deconvolution (B), Au 4f (C) and

Ag 3d (D) analyses of AuNR@PEG and AuNR@initiator.

16456 | J. Mater. Chem., 2011, 21, 16453–16460

between 286 and 287 eV is significantly decreased, indicating that

the PEG content is either decreased or lacking. The three

component C1s peaks of AuNR@initiator around 284.8, 286.3

and 289 eV are ascribed to carbon atoms in C–H/C–C, C–O/C–S/

C–Br and C]O bonds, respectively, at a ratio of 11 : 3.56 : 0.84.

This corresponds to the theoretical value of 11 : 3 : 1 in the ini-

tiator’s chemical structure,8 implying an absence of PEG moie-

ties. Fig. 2C and D show Au 4f and Ag 3d spectra of

AuNR@PEG and AuNR@initiator, respectively. The binding

energy of Au atoms after initiator modification increased by 0.3

eV and that of silver atoms increased by 0.2 eV. These changes

are considered to result from the loss of the dense CTAB layers

from the metal surfaces. The binding energy of Br3d in

AuNR@PEG was observed as 69.2 eV (see Fig. S3 in the ESI†),

different from that in AuNR@initiator (68.2 eV), reflecting

differences in valence state. A weak signal from sulfur atoms is

observable at around 162 eV in the spectrum of the AuNR@

initiator sample, verifying the presence of initiator moieties.

These XPS results combined with the TEM images are indicative

of the successful surface modification of the AuNRs by disulfide

initiator under the steric hindrance of the amphiphilic polymer.

The XPS results show that CTAB moieties remained on the

nanorods after PEG grafting and that the grafting density of

PEG was relatively low, in accordance with reported results.43

Despite their low grafting density, the long amphiphilic PEG

chains on the NRs’ surfaces allowed subsequent surface modi-

fication by disulfide initiator in organic solvent. This implies that

the PEG chains acted as mediate, temporary steric ligands that

could prevent the irreversible aggregation of the NRs that would

occur because of the disassembly of the CTAB bilayers during

alkanethiol ligand exchange.43 Considering the Au–S bond

stability, the small initiator molecules can be assumed first to

penetrate the loose PEG chains and replace the CTABmolecules,

forming an assembled monolayer on the nanorods’ surfaces.

During this process, steric hindrance by the PEG chains will

prevent the aggregation of the NRs. Alkanethiol molecules then

replace the anchored PEG chains due to the large amount of

initiator in the system and the low PEG grafting density.

Therefore, the immobilization of the initiator on the AuNRs can

be performed without their aggregation.

Fig. 3A shows UV-vis absorbance spectra obtained during the

sequential ligand exchange process. The dashed line in Fig. 3A

represents the spectrum of a dilute aqueous solution of freshly

prepared AuNRs, with characteristic transverse and longitude

SPR bands observed around 509 and 781 nm, respectively.

Aqueous AuNR@PEG (dot-dashed line) showed the two SPR

peaks slightly blue-shifted by 2 nm and 9 nm, respectively,

possibly ascribable to the partial disassembly of the CTAB

bilayer on the NRs caused by PEG grafting. Subsequent redis-

persal of AuNR@PEG in DMF red-shifted the SPR peaks to 511

and 795 nm due to the change of the surrounding medium’s

refractive index (black solid line). Initiator modification of the

nanorods resulted in no discernible change in the UV-vis spec-

trum (red solid line).

3.2 Surface-initiated ATRP

The ATRP of 4VP for free polymers in 2-propanol or aqueous

media was optimized by K. Matyjaszewski and his

This journal is ª The Royal Society of Chemistry 2011

Fig. 3 (A and B): UV-vis absorption spectra of AuNRs after ligand

exchange (A) and of AuNR@PVP after ATRP (B); (C and D): TEM

images of AuNR@PVP prepared at 40 �C in DMF with a co-solvent of

(C) H2O and (D) 2-propanol. Scale bar in (C and D): 200 nm. The inset in

(B) shows the AuNR@PVP suspension in DMF.

Fig. 4 UV-vis spectra (A and C) and hydrodynamic particle size (B and

D) of the AuNR@PVP nanocomposites as redispersed from water-

washed neutral samples (A and B) and as redispersed from pH 3.0 HCl-

washed samples (C and D).

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coworkers,36,44 and the similar ATRP from macroinitiator was

performed in organic media.38 Moreover, the ATRP of 4VP

initiated from macro/nano surfaces was also reported in DMF or

DMF/2-propanol mixture.35,45 Based on these previous works,

herein the ATRP on the surface of AuNR@initiator was carried

out at 40 �C in the presence of a CuBr/CuBr2/PMDETA cata-

lytic system.35–37 The relative amount of DMF, the major

solvent, was maintained at 90% (v/v) to allow good dispersity of

the AuNR@initiator particles, and a small amount of mixed

2-propanol/water solvent was added to provide protic solvent

conditions for the ATRP reaction.36,38 Because the nucleophilic

monomer, 4VP, has a preferred coordination to copper ions

which results in catalyst deactivation, a relatively large amount

of PMDETA was added to coordinate the catalyst first to

decrease complexing between copper and pyridine moieties. The

UV-vis spectrum of thusly obtained AuNR@PVP in DMF is

shown in Fig. 3B. The SPR bands do not show noticeable

changes in either the transverse and longitude modes when

compared with those of AuNR@initiator. The photograph (the

inset in Fig. 3B) shows a uniform suspension of nanocomposites

of a color similar to that in the previous steps. Fig. 3C and D

show TEM images of the AuNR@PVP nanocomposites when

10% water or 2-pronaol, respectively, were used as the co-

solvent. The nanocomposites were well-dispersed after ATRP

and the average thickness of the polymer shells was estimated to

be ca. 3 nm in both cases, with no obvious difference when water

was used instead of 2-propanol.

The FT-IR spectrum of AuNR@PVP shows a similar profile

to the obtained free PVP polymers (Fig. S2†), in which the

characteristic peaks at 1596, 1555, and 1414 cm�1 represent the

pyridyl ring-stretching vibrations, indicating a successful poly-

merization of 4VP monomer. The molecular weight (Mw) and its

distribution of surface-grafted polymers were demonstrated to be

essentially identical to those of free polymers generated from the

added free initiators in the surface-initiated ATRP.46 Herein, the

Mw of grafted PVP on the NR surface in the typical reaction was

determined to be 2541 g mol�1 with a polydispersity of 1.87

measured by gel permeation chromatography using DMF as

This journal is ª The Royal Society of Chemistry 2011

a solvent and eluent. This Mw value is in accordance with the

estimated thickness of polymer layer on the NR surface.

3.3 pH response of AuNR@PVP nanocomposites

The AuNR@PVP nanocomposites showed good dispersity in

DMF, THF and alcohols. They were easily dispersible in dilute

HCl solution, but could not be dispersed in pure water, indi-

cating that the NRs show a pH-dependent dispersity. Therefore,

the AuNR@PVP nanocomposites were washed with water to

discard residual DMF by centrifugation, and the obtained

precipitates were redispersed with dilute HCl solution from pH

2.5 to pH 6.5. Fig. 4A shows the UV-vis spectra of the

AuNR@PVP nanocomposites at different pH, which displays

a pH-dependent SPR absorbance of the obtained nano-

composites. With increasing the pH, both the transverse and

longitude SPR bands show a small change as pH < 3.0 but

a dramatic change as pH > 3.5, which should be ascribed to the

protonation/deprotonation of pyridinyl groups. The particle size

is also determined by dynamic light scattering, the result of which

is shown in Fig. 4B. The particle size at pH 2.5 and 3.0 was about

50 nm in accordance with the TEM results, and a drastic increase

was found as pH changed from 3.0 to 3.5, showing an obvious

response to pH change. These phenomena are similar to those

observed in the PVP grafted AuNPs.35

On the other hand, to study the responsive behavior of the

surface-grafted polymers from an extended state, the

AuNR@PVP nanocomposites were first treated with dilute HCl

solution and redispersed in pH 3.0 HCl solution to get

a concentrated well-dispersed suspension. Then, a small volume

of such suspensions was diluted with pH 2.0 HCl solution and the

calculated volume of water or NaOH solution to obtain samples

with pH 2.0, 3.4, 3.7, 4.0 and 5.6. Fig. 4C shows the UV-vis

spectra of these AuNR@PVP suspensions which were equili-

brated for 2 h. The transverse and longitudinal SPRmodes of the

AuNR@PVP in the aqueous suspension were observed at

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around 510 nm and 750 nm, respectively. However, only

a shoulder peak around 900 nm emerged with increasing pH as

a result of the AuNRs’ aggregation, which might represent a slow

aggregation of a small part of the nanocomposites. Such a pH

response of the well dispersed nanocomposites at low pH where

the grafted PVP chains are on the extended state is much

different to the pH-dependent dispersity of the aggregated

nanocomposites at a neutral condition where the grafted PVP

chains are collapsed on the NR surface. Moreover, such a pH-

response behavior in a well-dispersed state was much smaller

than that of the PVP-grafted AuNPs.9 This reduction in the

degree of the pH response of the well-dispersed AuNR@PVP

nanocomposites should be mainly attributed to the low molec-

ular weight of the grafted PVP chains, which will make it difficult

for the NRs to connect together and form aggregates. In addi-

tion, the more complex nature of the rod assembly than the

spherical particle assembly may also be partially responsible for

such observations. Correspondingly, the particle size change

detected by DLS in Fig. 4D is in accordance with the UV-vis

results.

TEM images of the AuNR@PVP nanocomposites redispersed

from a neutral condition were shown in Fig. 5. At pH 2.5 and 3.0,

similar well-dispersed morphologies of the assembled NRs were

observed (Fig. 5A and B). At pH 3.5, the majority of the NRs

were aggregated (Fig. 5C). At a higher pH from 4.1 to 6.5, many

similar larger aggregates were observed as shown in Fig. 5D.

These aggregates are much larger than the particle size indicated

by DLS measurements, so they should be formed during the

sample drying due to the collapsed hydrophobic polymer shells.

Such pH-responses explored by several means including SPR

change, particle size variation, and TEM morphology should be

attributed to the protonation and deprotonation of the pyridinyl

groups in the PVP shells. The protonation of pyridine groups at

low pH forms pyridinium ions, rendering positive charge to the

polymer chains,47,48 thus the nanocomposites are well-dispersed

through the electrostatic repulsion. As pH increases, the depro-

tonation of the pyridinium ions leaves uncharged pyridine

groups and allows the nanocomposites to aggregate due to the

absence of electrostatic repulsion. Such pH-responsive

morphologies are in agreement with the SPR response as shown

in Fig. 4A and B. However, TEM morphologies of the

Fig. 5 TEM images of AuNR@PVP nanocomposites redispersed in

aqueous HCl solutions at pH 2.5 (A), 3.0 (B), 3.5 (C), and 4.1 (D).

16458 | J. Mater. Chem., 2011, 21, 16453–16460

AuNR@PVP samples redispersed from pH 3.0 suspensions did

not show much difference from pH 2.0 to 4.0, and at a higher pH

of 5.6 many large aggregates of NRs were found according to the

UV-vis and DLS results.

3.4 Potential applications as nanosupports

AuNRs were considered as an especially unique material for

multi-functional nanostructure assembly. Similar to the near-

infrared-light heating and controlling of AuNRs in cancer

therapy,28,29 the adjustable-light-wave controlled nanodevices

could be expected based on the modulatable longitudinal SPR

absorbance of the AuNRs with different aspect ratios. Particu-

larly, the AuNR@PVP nanostructures can likely be used as

efficient nanosupports for many transition metal catalysts due to

the coordinative pyridyl shells, and thus should have significant

impact on potential future catalytic applications. Herein, the

pyridyl-containing shells of the AuNR@PVP nanocomposites

were employed as coordinative supports for transition metal ions

and nanoparticles. Chloroplatinic acid solution was incubated

with the AuNR@PVP nanocomposites, allowing Pt ions to

penetrate the PVP shells. After subsequent reduction with

sodium borohybride, bimetallic Pt NP-loaded AuNR@PVP

nanocomposites (AuNR@PVP-Pt) resulted. TEM images

(Fig. 6A and B) of these bimetallic nanocomposites show that

2–3 nm Pt NPs were densely decorated on the AuNRs’ surfaces.

Such bimetallic Au–Pt nanostructures have been reported to be

significant electrocatalytic materials in direct methanol fuel

Fig. 6 (A and B): TEM images of nano-Pt-loaded AuNR@PVP nano-

composites at different magnifications; (C–F): cyclic voltammograms of

glass-carbon electrodes modified with AuNR@PVP (C and E) and Pt

NPs-decorated AuNR@PVP nanocomposites (D and F) in 0.5 M H2SO4

solution in the absence (C and D) and presence (E and F) of 1 M

methanol. Scale bars in (A) and (B) are 100 nm and 10 nm, respectively.

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cells.49–51 Therefore, the electrocatalytic activity of the

AuNR@PVP nanocomposites before and after being decorated

with Pt NPs was compared. The cyclic voltammetry curves of the

glassy carbon electrodes (GCEs) modified with the AuNR@PVP

nanocomposites and Pt NPs-decorated AuNR@PVP nano-

composites were recorded in 0.5 M H2SO4 at a scan rate range of

25–200 mV s�1 as shown in Fig. 6C and D, respectively. The

typical increase in peak currents was observed with respect to the

scan rate. A small reduction peak around 0.8 V (vs. SCE) was

observed for the AuNR@PVP-modified GCE, indicative of the

reduction of gold oxide on the nanorods’ surfaces; while this

peak was not observed in the case of Pt NPs-decorated

AuNR@PVP nanocomposite and a higher peak emerged around

0.5 V, corresponding to Pt oxide reduction.51,52 The disappear-

ance of the Au oxide reduction peak may suggest that the Au

nanorods’ surfaces were almost completely covered by the small

Pt NPs. Furthermore, the cyclic voltammograms in Fig. 6E and

F showed markedly different results between the AuNR@PVP

and the Pt NPs-decorated AuNR@PVP nanocomposites. The

AuNR@PVP nanocomposites did not exhibit noticeable elec-

trocatalytic activity for the methanol oxidation reaction, while

the Pt NPs-decorated AuNR@PVP nanocomposites showed

typical catalytic behavior for the electro-oxidation of methanol

as indicated by the appearance of an oxidation current in the

positive potential region. The onset potential for the electro-

catalytic oxidation is around 0.34 V (vs. SCE). The current peak

at ca. 0.65 V in the forward scan is attributable to electrocatalytic

methanol oxidation on the surfaces of the bimetallic nano-

composites. In the reverse scan, an oxidation peak is also

observed at ca. 0.44 V, probably associated with the removal of

residual carbon species formed during the forward scan. The

ratio of the forward oxidation current peak (If) to the reverse

current peak (Ib), If/Ib, indicates the catalyst’s tolerance to the

poisoning species, Pt]C]O.53 Higher ratios suggest the effec-

tive removal of the poisoning species from the catalyst’s surface.

The average If/Ib ratio calculated for the AuNR@PVP-Pt

nanocomposites at different scan rates was 2.25, markedly higher

than that of the E-TEK catalyst (0.74).53 The presence of nano-

size Au clusters on Pt catalysts has been reported to be able to

stabilize Pt electrocatalytic activity in fuel cell reactions,52 sug-

gesting an extra advantage of supported Pt NP catalysts on

AuNRs. As an expectation for the catalytic Pt-loaded

AuNR@PVP nanostructure, the AuNRs could be utilized as

irradiation sensors to perform locally photo-thermal heating via

remote irradiation and as such the local heating-controlling of

the catalyst center might be meaningful for some special reaction

system.

4. Conclusion

Surface modification of AuNRs with disulfide initiator was

successfully performed under the steric hindrance of amphiphilic

PEG polymer chains. The PEG moieties acted as mediating

ligands and facilitated the dispersion of the modified NRs in

organic solvents and provided steric hindrance to prevent the

nanorods’ aggregation during subsequent ligand exchange

with alkanethiol. The initiator immobilized on the nanorods’

surfaces could subsequently initiate the ATRP of 4VP to

form poly(4-vinylpyridine)-grafted AuNRs. The AuNR@PVP

This journal is ª The Royal Society of Chemistry 2011

nanocomposites displayed systematic pH-sensitive surface plas-

mon resonance changes and assembly behavior. AuNR@PVP-Pt

bimetallic nanocomposites displayed viable electrocatalytic

activity for methanol oxidation, a typical reaction in direct

methanol fuel cells. Thus, such core–shell nanostructures with

coordinative pyridine shells can be employed as versatile nano-

supports for transition metal catalysts or plasmonic nanodevices.

Acknowledgements

This work was supported by the Mid-career Researcher Program

(20110029409), International Research & Development Program

(2010-00134), SRC Program (20110001334) and the Converging

Research Center Program (2011K000630) through National

Research Foundation of Korea Grant funded by the Korean

Government. D. Li thanks for the financial support by the

National Natural Science Foundation of China (no. 21073102).

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