Preparation and characterization of silicaâgold coreâshell nanoparticles

RESEARCH PAPER

Preparation and characterization of silica–goldcore–shell nanoparticles

Thi Ha Lien Nghiem • Tuyet Ngan Le •

Thi Hue Do • Thi Thuy Duong Vu •

Quang Hoa Do • Hong Nhung Tran

Received: 3 July 2013 / Accepted: 19 October 2013 / Published online: 30 October 2013

� Springer Science+Business Media Dordrecht 2013

Abstract Silica–gold core–shell nanoparticles

(NPs) were prepared by gold ion plating on hydro-

philic-functionalized silica core NPs using formalde-

hyde as a reducing reagent. The monodisperse silica

particles were first prepared by a sol–gel method,

while the ultrafine gold colloids (diameter 1–2 nm)

were synthesized by the reduction of chloroauric acid

with tetrakis(hydroxymethyl)phosphonium chloride.

The growth and attachment of the gold NPs onto the

functionalized surface of the silica NPs with average

diameter ranging from 40 to 180 nm, using a low-

temperature-mediated route, were systematically

investigated. The coverage of the gold NPs and

clusters on the surface of the silica NPs have been

evaluated by means of UV–Vis/near-infrared spec-

troscopy and transmission electron microscopy. The

surface plasmon resonance absorption spectra from

550 to 1,000 nm of the core–shell NPs can be

effectively controlled by the surface gold coverage

or the silica core NP’s size.

Keywords Gold nanoshells � Silica

nanoparticles (silica NPs) � Surface plasmon

resonance (SPR) � Coating

Introduction

Gold nanoshells typically consist of a low dielectric

spherical core nanoparticle (NP), such as silica or

polystyrene, coated with a thin layer of gold with a

thickness ranging from a few nanometers to a few tens

of nanometers. Gold nanoshells are plasmonic mate-

rials with intense absorbing and scattering properties.

The surface plasmon resonance (SPR) of gold nano-

shells can be finetuned over the visible to near-infrared

(NIR) spectrum region by adjusting the relative core/

shell ratio (Pham et al. 2002; Loo et al. 2005).

Moreover, the SPR effect of gold nanoshells, which

operates within the spectral range of 700–900 nm,

matches with the optical requirements of tissue. Gold

colloids or gold nanoshells can also be conjugated

with biological molecules (Horisberger and Vauthey

1984; Hirsch et al. 2003; Nghiem et al. 2010).

Therefore, the application of gold nanoshells has been

of increasing interest in the biomedical field, incorpo-

rating diagnostic and therapy research studies that

include optical labeling for tumor cell imaging,

controlled drug delivery, and plasmonic photothermal

therapy (Hirsch et al. 2003; Loo et al. 2005).

Halas Group has been the pioneers in developing

metallic nanoshells on dielectric nanospheres (Olden-

burg et al. 1998). Many studies followed, which

investigated various core materials such as silica,

polystyrene, and magnetic NPs (Pham et al. 2002;

Hofmeister et al. 2002; Shi et al. 2005; Yong et al.

2006; Levin et al. 2009; Wang et al. 2010; Bardhan

T. H. L. Nghiem (&) � T. N. Le � T. H. Do �T. T. D. Vu � Q. H. Do � H. N. Tran

Institute of Physics, Vietnam Academy of Science and

Technology, 18 Hoang Quoc Viet Road, Cau Giay

District, Hanoi, Vietnam

e-mail: [email protected]

123

J Nanopart Res (2013) 15:2091

DOI 10.1007/s11051-013-2091-6

et al. 2010; Liang et al. 2011). To date, the seed-

mediated growth method has been widely employed

for the synthesis of metallic nanoshells on core NPs. It

involves four steps: (i) the synthesis and surface

functionalization of silica core NPs, (ii) the synthesis

of ultrafine gold colloids in solution, (iii) the prepa-

ration of the ‘‘seed’’ particles by attaching ultrafine

gold colloids onto the silica core, and finally (iv) the

‘‘growth’’ of a gold layer onto the seed particles, which

takes place in the bulk gold ion-plating solution, until a

complete shell is obtained. However, this process is

highly dependent on experimental conditions such as

the surface properties of the silica core NPs, the size of

the gold NPs, and pH of the gold ion-plating solution

(Kah et al. 2008; Bardhan et al. 2010; Liang et al.

2011). For in vivo medical applications, large core

NPs with a diameter in the range of 100–550 nm are

typically used for synthesizing gold nanoshells as the

large particle size affords quick clearance from the

blood, by the liver and spleen, and limited diffusion

into the tissue (von Maltzahn et al. 2009; Zhang et al.

2009; Cho et al. 2009, 2010). Only a few reported

medical applications benefit from the use of nano-

shells with diameters of less than 60 nm.

Several simple strategies have been reported for the

synthesis of 3-aminopropyltriethoxylsilane (APTES)-

functionalized silica core NPs that can be employed

further to form core–shell NPs. In general, the silica

NPs are prepared in a highly alkaline medium using a

sol–gel method or the Stober et al. route (1968). This

generates silica NPs with sizes greater than 100 nm.

The functionalization of the silica NPs, with amine

moieties, is then performed, typically at elevated

processing temperatures.

In this study, we report a simple approach to form

gold shells, with variable thicknesses, on silica core

NPs with variable diameters ranging from 40 to

180 nm at room temperature (25 �C), sequentially

functionalized with APTES in one pot. Even, the silica

core NPs were synthesized by the Stober route. The

experimental results show that the sizes of tetra-

kis(hydroxymethyl)phosphonium chloride (THPC)–

gold NPs which have been prepared in the presence of

citrate buffer are more stable than those of the THPC–

gold NPs prepared using conventional method (with-

out citrate). Our approach expresses a useful method to

obtain gold nanoshell NPs for biochemical and

biological applications. The results are very

interesting and would provide new insights into the

development of new synthetic strategies on gold-

capped NPs.

Materials and methods

Reagents

Tetraethylorthosilicate (TEOS, 99 %), ammonium

hydroxide (NH4OH, 29 % in water), and glutathione

(GSH) were purchased from Sigma–Aldrich. APTES

(99 %); ethanol, tetrachloroauric acid trihydrate

(HAuCl4�3H2O, 99.9 %), THPC (80 % solution in

water), sodium hydroxide (NaOH, 99 %), trisodium

citrate dihydrate (Na3C6H5O7�2H2O, 99 %), potas-

sium carbonate (99 %), and formaldehyde (37 %

solution in water) were purchased from Merck.

Deionized water was used in all experiments.

Materials preparation

Synthesis of amino-functionalized silica NPs

Silica NPs with variable sizes were first prepared,

using sol–gel, by varying the amount of NH4OH

solution. To 30 mL ethanol, a known volume of

NH4OH solution (1.8–2.8 mL at increment of 0.2 mL)

was added under vigorous stirring for 30 min. 300 lL

TEOS was then added to the six respective solutions,

containing varying amounts of NH4OH solution,

under continuous stirring. Aliquots of the resulting

silica NPs’ suspension solutions were either repeat-

edly washed with water to produce silica NPs with

surface hydroxyl groups (OH) or continuously reacted

by adding 10 lL APTES to the respective NPs’

suspension solutions, for 2 h, to generate amino-

functionalized silica NPs. The latter NPs were then

washed thrice with water, via centrifugation, and then

redispersed in water before use.

Preparation of THPC–gold NPs

Gold NPs were prepared according to the procedure as

reported (Duff et al. 1993; Pham et al. 2002) and are

referred to as THPC–gold NPs. 0.4 mL NaOH (1 M),

3 mL trisodium citrate (68 mM), and 1 mL THPC

of 9 J Nanopart Res (2013) 15:2091

123

(85 mM) were added to 42 mL water. After stirring

for 5–10 min, 2 mL HAuCl4 (25 mM) was added to

the solution. A color change from yellow to dark

brown indicated the formation of the THPC–gold NPs.

The THPC–gold solutions were stored in the dark at

4 �C before use.

Preparation of gold ion-plating solution

The gold-plating solution was prepared a few days

before shell synthesis, as detailed in Liang et al. (2011).

The pH of the gold-plating solution was adjusted to an

optimum value between 8.0 and 9.0 as reported in the

literature (Kah et al. 2008; Liang et al. 2011). 28 mg of

potassium carbonate was first dissolved in 100 mL

water under stirring for 10 min, to which 1.5 mL

HAuCl4 (25 mM) was added. The initial light yellow

solution gradually became colorless after 30 min,

indicating the formation of gold hydroxide solution.

The resulting gold-plating solution was stored for at

least 3 days in a dark and cool area before use.

Synthesis of seed particles: attachment of THPC–gold

colloids to APTES-functionalized silica NPs

To ensure uniform particle dispersion, the respective

suspension solutions (2 mL) containing the APTES

grafted on different-sized silica NPs were first soni-

cated for 10 min. An excess volume of THPC–gold

solution was added under vigorous stirring. The

solutions were then allowed to sit for 2–3 h. The

APTES-functionalized silica NPs with surface-

attached THPC–gold colloids were retrieved via

centrifugation and then redispersed in 2 mL water.

The resulting particles are referred to as the seed

particles.

Preparation of silica–gold core–shell NPs

Gold nanoshells were prepared as reported previously

(Pham et al. 2002). To grow the gold shell, a

suspension solution containing the prepared THPC–

gold NP-decorated APTES-functionalized silica NPs

was added to 10 mL of the prepared gold-plating

solution, and the resulting mixture was stirred (vortex

stirrer). 15 lL formaldehyde (37 %) was added to the

mixture to induce the reduction of gold hydroxide, to

gold, onto the gold-decorated APTES-functionalized

silica core NPs. The thickness of the gold shell was

regulated by varying the volume of gold-decorated

silica NP’s suspension solution relative to the volume

of the gold-plating solution (gold hydroxide) that was

kept constant. A color change from colorless to red,

purple, blue, or green, depending on the amount of

gold-decorated APTES-functionalized-silica NP’s

suspension solution, which occurred within 5 min,

indicated the formation and complete growth of the

shell. To prevent aggregation of the gold nanoshells,

the suspension solutions were mixed with 15 lL GSH

(1 mM) to create a passive coating layer on the gold

nanoshells.

Characterization methods

The absorption spectra (200–1,100 nm) were recorded

on a Jasco V-570 UV–Vis/NIR spectrophotometer.

Transmission electron microscopy (TEM) images of

the silica NPs and gold nanoshells were obtained on a

JEOL JEM 1011 microscope at an acceleration

voltage of 80 kV. The samples were prepared by drop

coating the sample dispersion onto an amorphous

carbon film supported on a 200-mesh copper grid. The

polydispersity index (PDI) values and average size of

the bare and APTES-functionalized silica NPs were

measured by dynamic light scattering (DLS) tech-

nique with an angle scattering of 173� (Nano ZS,

Malvern).

Results and discussion

Formation of gold nanoshells

Silica NPs

The morphologic features of the silica NPs, obtained

from a sol–gel method, are viewed from the TEM

images in Fig. 1. The sizes of the spherical particles

were regulated by the amount of ammonia used during

the synthesis: a corresponding increase in the average

diameter from 40 to 180 nm was obtained with the

increasing amounts of ammonia from 1.8 to 2.8 mL.

The average diameters of the silica NPs, as measured

by DLS, before functionalization, were in accordance

with the estimated sizes from TEM analysis. The high

monodispersity of the NPs was confirmed from the

low measured PDI values (\0.100; Malvern Instru-

ments 2007).

J Nanopart Res (2013) 15:2091 of 9

123

As observed in the TEM images, the morphologic

features of the silica NPs were retained following

functionalization with APTES. Although a slight

increase in the PDI values was observed for the

APTES-functionalized samples, PDI values for all

samples remained below 0.100 except for the APTES-

grafted on the smallest-sized silica NPs sample

(PDI = 0.192). The onset of aggregation was noted

in the smallest-sized silica NPs sample. As a result, a

slightly larger particle size value (92 nm), as measured

by DLS, was obtained. As the diameter of the silica

NPs decreased to 40 nm, the resulting APTES–silica

NPs were more prone to aggregation. The onset of

aggregation can be explained by the increasing

amounts of amino-functional groups, relative to the

amount of OH groups, present on the surface of the

silica NPs, possibly resulting in excessive amounts as

the size of the silica NPs is reduced. To overcome

aggregation, determination of optimally grafted AP-

TES layers onto the surface of the silica NPs,

according to their sizes, is necessary.

It can be explained by the potential f of silica NPs.

Indeed, the values of the potential f of all silica NPs

samples increase after functionalized process by

APTES (from -43 mV to more than -25 mV). For

example, the values of the potential f of the function-

alized silica NPs with sizes of 120, 90, 60, 40 nm are

recorded to be -25, -24, -20, -10 mV, respectively.

The potential f is more positively related to the

presence of larger number of amine groups and lesser

number of hydroxyl groups over the surfaces of the

APTES-functionalized silica NPs (Vaidya et al. 2011).

THPC–gold NPs

Figure 2 shows the morphologic properties of ultrafine

gold NPs synthesized according to the procedure in the

presence of citrate ions. The prepared THPC–gold NPs

were slightly smaller in size (1–2 nm; Duff et al. 1993)

than the reported gold NPs (1–3 nm; Pham et al. 2002).

The prepared THPC–gold NPs remained stable over a

period of 30 days. The presence of citrate ions in the

THPC–gold solution regulated the pH of the solution

(*7.5) that in turn conferred the high stability of the

THPC–gold NPs. Plasmon absorption spectroscopy

also confirmed the ultrafine size and the high stability

Fig. 1 TEM images of silica NPs prepared by sol–gel with the

increasing amounts of ammonia and corresponding sizes of

a 40 ± 3 nm, b 65 ± 4 nm, c 80 ± 5 nm, d 110 ± 5 nm, and

e 140 ± 5 nm, before functionalization with APTES, and f–j the respective silica NPs obtained after functionalization with

APTES. The same scale bar (100 nm) applies to all images

Fig. 2 HTEM images of THPC–gold NPs prepared in a citrate

buffer solution at 30 days after synthesis


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of the THPC–gold NPs. Only slight changes in the

plasmon absorption spectra, in terms of the shape and

intensity, were observed between the THPC–gold NPs

samples after 1 and 30 days, respectively, of prepara-

tion (Fig. 3). As well known, the peaks at ca. 350 nm

may be attributed of molecular transmission for the

size between 10 and 50 atoms (especially for the cluster

of 13 atoms: Au13). The peak of ca. 510 nm on the

plasmon absorption spectra appears for the bigger-

sized THPC–gold NPs (about 300–500 atoms; Kreibig

and Vollmer 1995). Therefore, THPC–gold NPs

prepared in a citrate buffer solution are highly stable.

Seed particles: THPC–gold-decorated APTES-

functionalized silica NPs

Figure 4 shows the morphology of the resulting

particles following THPC–gold NPs surface

attachment on bare silica NPs and APTES-function-

alized silica NPs (diameter 140 ± 5 nm). As sug-

gested from Fig. 4a, no favorable adsorption between

the bare silica NPs and THPC–gold NPs takes place as

both surfaces are negatively charged under the

synthesis conditions employed. In contrast, surface

attachment of the gold NPs onto the amino-function-

alized silica NPs was observed (Fig. 4b, c). A uniform

deposition of discrete THPC–gold NPs (diameter

1–2 nm) on APTES-functionalized silica NPs was

observed when deposition was conducted in a citrate

buffer solution (Fig. 4b). However, when deposition

was carried out conventionally (in the absence of

citrate ions), gold clusters (size 4–6 nm) were addi-

tionally observed (Fig. 4c). These findings demon-

strate that the presence of citrate ions suppresses the

aggregation of the THPC–gold NPs by maintaining the

pH of the gold solution, thereby facilitating a regular

deposition of discrete gold NPs onto the silica core

NPs and unchanged the thickness of the gold layer.

Moreover, the successful attachment of the THPC–

gold NPs onto the amino-functionalized silica NPs,

through strong covalent interactions (Grabar et al.

1995; Sato et al. 1997), supports the successful

grafting of APTES onto the silica NPs.

Shi et al. reported that the use of large-sized gold

NPs attached onto core NPs generates gold nano-

shells with a thick and rough shell surface. In

contrast, the use of smaller-sized gold NPs produce

nanoshells with a smoother texture (Shi et al. 2005).

With the expectation to achieve a smooth texture,

the gold nanoshells, in this study, were grown using

the seed particles prepared in a citrate buffer

solution (THPC–gold-decorated APTES-functional-

ized silica NPs that feature gold NPs with a size of

1–2 nm, Fig. 4b).

300 400 500 600 7000

1

2

3

4

5

6

Wavelength (nm)

Abs

orba

nce

1day 30 days

THPC-gold in citrate buffer

Fig. 3 Plasmon absorption spectra of THPC–gold solution

measured after (broken line) 1 day and (solid line) 30 days of

preparation

Fig. 4 TEM images of THPC–gold-decorated on a bare silica NPs, in citrate buffer solution, b APTES-functionalized silica NPs, in

citrate buffer solution, and c APTES-functionalized silica NPs in water. Scale bars are 100 nm for all images


123

Fig. 5 TEM images of silica NPs with varying sizes of

a 40 ± 3 nm, b 65 ± 4 nm, c 80 ± 5 nm, d 110 ± 5 nm,

e 140 ± 5 nm, and f 180 ± 5 nm coated with gold shells. Each

frame shows the development of the nanoshells at different

stages of the deposition process: (i) bare APTES-functionalized

silica, (ii) gold-decorated APTES-functionalized silica, (iii)

partial gold shell growth, and (iv) complete gold shell formation


123

Gold–silica core–shell formation

The reaction rate for reducing gold ion to gold atom is

expected to be the same for all the prepared samples as

the pH, and amounts of gold-plating solution and

formaldehyde solution used were kept constant. The

reduction of gold (from the gold-plating solution) onto

the THPC–gold-decorated APTES-functionalized sil-

ica NPs seed particles occurred upon the addition of

formaldehyde and was completed within 3–5 min,

generating a complete gold shell onto the silica NPs.

The time of reduction is defined at the moment when

there is any variation in their plasmon absorption

spectra by using GSH to stamp out the reduction of

gold in different time (from 0.25 to 30 min). The

extent of gold shell coverage and the thickness of the

shell were dependent on the size of silica NPs cores

and the concentration of the THPC–gold-decorated

APTES-functionalized silica NPs seed particles. Fig-

ure 5 shows the development of the gold nanoshells

using silica core NPs of variable sizes of 40 ± 3,

65 ± 5, 80 ± 4, 110 ± 5, 140 ± 5, and 180 ± 5 nm.

Each frame shows the development of the nanoshells

at different stages of the coating process: (i) bare

APTES-functionalized silica, (ii) gold-decorated AT-

PES-functionalized silica, (iii) partial gold shell

growth, and (iv) complete gold shell formation. The

gold coverage, at different stages of the shell growth,

was uniform regardless of the size of the silica NPs.

The thickness of the gold shells, following complete

coverage, was determined at *15 nm for the differ-

ent-sized silica core NPs, by means of professional

software of Image Process and Analysis Java (Ima-

geJ). As discussed earlier, the smaller-sized APTES-

functionalized silica NPs are prone to aggregation

even before conducting surface attachment with the

THPC–gold NPs. As a result, no discrete gold

nanoshells could be grown on the gold-decorated

APTES-functionalized silica NPs, upon addition of

the reducing agent, formaldehyde. In contrast, a

common gold layer between the fused silica NPs

was observed (Fig. 5a, b). Future study is required to

optimize the growth of the gold shells on the smaller-

sized silica core NPs either by increasing the core/shell

ratio or using smaller gold NPs attached to the silica

core NPs.

Optical characterization of gold nanoshells

Figure 6 shows the plasmon absorption spectra of the

silica–gold core–shell grown from the prepared

THPC–gold-decorated APTES-functionalized silica

NPs’ seed particles. With an increase in surface gold

coverage on the silica NPs, with a specific core size, a

red shift from 580 to 870 nm, generating a broad band

with the appearance of a secondary peak in the shorter

wavelength region, was observed (Fig. 6a). The red

shifts were no longer apparent when surface coverage

was complete. Further gold deposition, which results

in increased shell thickness, conversely causes a blue

shift of the SPR absorption peak with a more intense

secondary peak at the shorter wavelengths (Fig. 6a).

The use of silica NPs with variable core sizes—

whereby surface gold coverage was complete,

400 500 600 700 800 900 1000 1100

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Inte

nsity

Wavelength (nm)

aSilica NPs core: 140 nm

Inscreasing the gold surface coverage on silica NPs

400 500 600 700 800 900 1000 11000.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Inte

nsity

Wavelength (nm)

110 nm 140 nm 180 nm

Silica NPs Core sizesb

Fig. 6 Plasmon absorption spectra of silica–gold core–shell NPs with a varying surfaces of gold coverage (diameter of silica core

140 nm) and b varying sizes of silica NP’s core (complete gold coverage; thickness of gold nanoshell 15 nm)


123

thickness of gold nanoshell: 15 nm—also induced a

shift in the SPR absorption peaks (Fig. 6b). These

results show that the SPRs of the prepared silica–gold

core–shell NPs are strongly dependent on both the

extent of surface gold coverage on the silica core NPs

and the size of the core silica NPs (at a given surface

gold coverage). Hence, by appropriately adjusting

these two properties, it is possible to tailor the SPR

absorption into the 550–1,000 nm spectral region,

which is ideal for optical bio-imaging and plasmonic

photothermal therapy applications, thereby making

these nanoshells suitable for biomedical applications.

Conclusion

Gold shells with tunable thicknesses were grown on

silica NPs, with varying average diameters ranging

from 40 to 180 nm, using a low-temperature-mediated

route. The prepared silica NPs were first grafted with

amine functionalities, which was essential for the

subsequent surface attachment of gold NPs, in a citrate

buffer medium. The presence of citrate ions in solution

was essential to achieve uniform gold coverage. The

surface-attached gold NPs on the amino-functional-

ized silica core NPs served as seeds for the growth of

continuous gold shells. By varying the amount of gold-

decorated APTES-functionalized silica NPs, to tune

the final surface of the gold coverage on the silica NPs,

the SPR of the nanoshells could be adjusted to any

wavelength from the visible to the NIR region of the

electromagnetic spectrum, making them ideal for

biomedical applications.

Acknowledgments Dr. T. Nagao is thanked for helpful

discussions. This study was supported by the Nafosted Grant

No. 103.06-2010.10.

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Preparation and characterization of silicaâgold coreâshell nanoparticles

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Transcript of Preparation and characterization of silicaâgold coreâshell nanoparticles