Preparation and characterization of bimodal magnetofluorescent nanoprobes for biomedical application

Fe3O4superparamagnetic nanoparticles are firstly prepared through the thermal decomposition of Fe oleate precursors and coated with a mesoporous silica shell using the Stöber method, and

Preparation and Characterization of Bimodal Magnetofluorescent Nanoprobes for

Biomedical Application *

LEI Jie-Mei(雷洁梅)1, XU Xiao-Liang(许小亮)1**, LIU Ling(刘玲)1,

YIN Nai-Qiang(尹乃强)1, ZHU Li-Xin(朱立新)2 **

1Department of Physics, University of Science and Technology of China, Hefei 230026

2Center Laboratory, First Affiliated Hospital, Anhui Medical University, Hefei 230022

(Received 18 June 2012)

Magnetic-fluorescent bifunctional Fe3O4/SiO2-CdTeS nanocomposites are synthesized Fe3O4superparamagnetic nanoparticles are firstly prepared through the thermal decomposition of Fe oleate precursors and coated with a mesoporous silica shell using the Stöber method, and the silica surface is then modified with positively charged amino groups by adding 3-aminopropyltrimethoxysilane Finally, negatively charged CdTeS quantum dots are linked and assembled onto the positively charged surface of Fe3O4/SiO2through electrostatic interactions X-ray diffraction, transmission electron microscopy, photoluminescence spectroscopy, and magnetometry are applied

to characterize the nanocomposites The results show that the bifunctional nanocomposites combine the opti-cal properties of near-infrared CdTeS quantum dots with the superparamagnetic properties of Fe3O4 perfectly, expressing the potential application as a biocompatible magnetofuorescent nanoprobe for in vivo labelling

PACS: 78.30.Fs, 75.70.Cn, 87.64.kv DOI: 10.1088/0256-307X/29/9/097803

In the past decades, magnetic nanoparticles such

as magnetite (Fe3O4) have attracted considerable

in-terest in various fields of biomedicine, ranging from

clinical diagnosis to cancer diagnosis by magnetic

res-onance imaging (MRI), drug targeting, cell separation,

and hyperthermia.[ 1−7 ] One of the most important

characteristics of Fe3O4at nanoscale (∼10 nm) for

ap-plications is its superparamagnetic property

Mean-while, semiconductor quantum dots (QDs) have also

been widely studied due to their unique optical

prop-erties, such as strong size-dependent emission

wave-length, continuous excitation spectrum, excellent

nar-row emission spectrum, and high stability against

pho-tobleaching compared with organic dyes.[ 8−13 ] The

most attractive application for QDs is as fluorescent

probes for bioimaging Along with the research and

development of magnetic nanoparticles and

semicon-ductor QDs, respectively, magnetic-fluorescent

bifunc-tional nanostructures have also drawn considerable

increasing attention recently, because this

nanocom-posite probe has both favorable magnetic and

fluo-rescent properties in the same structure.[14−18] For

example, Xu and co-workers[ 17 ] have successfully

as-sembled semiconductor quantum dots around

silica-coated superparmagnetic Fe3O4 nanoparticles, and

the nanocomposites exhibit magnetic and

photolumi-nescent properties simultaneously However, Fe3O4

nanoparticles prepared by using the co-precipitation

method have bad uniformity and monodispersion In

addition, the semiconductor QDs were firstly

syn-thesized in organic phase and then transferred to

the water by another modification step before

link-ing Fe3O4 nanoparticles The method is relatively complicated and will lead to fluorescence decrease of QDs as well Therefore, how to obtain monodis-perse high-quality paramagnetic nanoparticles and water-dispersed QDs with high fluorescent intensity still makes a lot of sense for us We have obtained highly uniform and monodispersed Fe3O4 nanoparti-cles through thermal decomposition[ 20 ] of iron oleates

in organic phase, and water-dispersed alloyed semi-conductor QDs (CdTeS) with high photoluminescence intensity by using a facile one-pot method Compared with semiconductor QDs, the emission wavelengths

of alloyed semiconductor QDs (CdTeS) can be easily tuned from visible to near-infrared by changing the reaction time.[ 12 ] In this Letter, we just use CdTeS alloyed QDs with an emission peak at 625 nm The typical procedure for the synthesis of the Fe3O4/SiO2 -CdTeS magnetic/fluorescent nanocomposites is shown

in Fig.1 Hydrophobic Fe3O4 nanoparticles capped with oleic acid and oleyamine were transferred to the aqueous solution using cetyltrimethylammonium bro-mide (CTAB) and then coated with silica CTAB served not only as the secondary stabilizing surfactant for the transfer of nanocrystals to aqueous phase but also as the organic template for the formation of the mesoporous silica shell.[6,16] These mesoporous silica-coated Fe3O4 nanoparticles were then modified by 3-aminopropyltrimethoxysilane (APS) to terminate the silica surface with amino groups The negatively charged MPA-capped CdTeS QDs were finally electro-statically assembled onto the surface of APS modified silica-coated Fe3O4nanoparticles This biocompatible

* Supported by the National Natural Science Foundation of China under Grant Nos 50872129 and 81172082.

** Corresponding author Email: xlxu@ustc.edu.cn; lx-zhu@163.com

© 2012 Chinese Physical Society and IOP Publishing Ltd

magnetofuorescent nanoprobe retained favorable

mag-netic and fluorescent properties simultaneously, which

can be applied to targeted drug delivery,

hyperther-mia, bioimaging, as well as bioseparation by a single

material

Oleic acid and

oleymine coated

CTAB coated

Mesoporous silica

Magnetic/Fluorescent Nanocomposites

CTAB

CTAB

Silica coating

Electrostatic self-assembly

Fig 1 Synthetic procedure of Fe 3 O 4 /SiO 2 -CdTeS

mag-netic/fluorescent nanocomposites.

Iron (III) chloride hexadydrate (FeCl3·6H2O, 99%),

cetyltrimethylammonium bromide (CTAB), sodium

oleate (99%), oleic acid, ethanol (99.7%), ethyl

ac-etate (99.5%), trichloromethane (99%), ammonia

so-lution (25%–28%), tetraethyl orthosilicate (TEOS),

n-hexane (97%), Tellurium (Te,99%), cadmium

chlo-ride (CdCl2), sodium borohydride (NaBH4,96%) and

sodium hydroxide (NaOH,96%) were obtained from

the Sinopharm Chemical Reagent Co., Ltd, China

Oleylamine was purchased from Aldrich The

3-mercaptopropionic acid (MPA, 99%) was received

from Acros Organics All chemicals were used with

no further purification

Iron oxide nanoparticles (Fe3O4) stabilized by oleic

acid and oleyamine were prepared through the

ther-mal decomposition method.[ 20 ]Firstly, iron-oleate was

synthesized by reacting metal chlorides and sodium

oleate according to the reported method.[ 1 ]Then, 3.6 g

of iron-oleate, 1.91 mL of oleylamine and 0.64 mL of

oleic acid were mixed together and were heated at

120∘C for one hour After that, the dark solution was

quickly heated up to 200∘C and kept at this

tempera-ture for two hours under a nitrogen atmosphere

Fi-nally, the solution was further heated up to 300∘C for

another two hours Nitrogen gas was gently blown

through the reaction system in the whole process to

remove the trace hydrate vapor during the heating

The resulting solution containing Fe3O4nanoparticles

was then cooled to room temperature and abundant

ethanol was added to the solution to precipitate Fe3O4

nanoparticles The Fe3O4 nanoparticles were

sepa-rated by centrifugation and washed with hexane and

ethanol several times

CdTeS alloyed QDs with MPA as the

stabiliz-ing agent were synthesized through the

hydrother-mal route[12] according to previously published

litera-ture Briefly, 127 mg of tellurium powder was reacted

with 80 mg of sodium borohydride (NaBH4) in 2 mL

of deionized water for preparation of sodium

hydro-gen telluride (NaHTe) solution Then, the fresh

pre-pared solutions of NaHTe were swiftly injected into

100 mL N2-saturated CdCl2solution with the presence

of MPA at PH 9.0 under vigorous stirring The molar ratio of [CdCl2]:[MPA]:[NaHTe] was fixed at 1:1.8:0.5 Finally, a 35 mL of the mixture precursor solution was sealed in a Teflon-lined stainless steel autoclave and maintained at 180∘C for 90 min After a hydrocooling process, the particles was precipitated by centrifuga-tion with ethanol and redispersed in 35 mL deionized water

Fe 3 O 4

2

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

(d)

(c)

(b)

(a)

*

Fig 2 XRD patterns of Fe 3 O 4 nanoparticles (a),

Fe 3 O 4 /SiO 2 nanoparticles (b), CdTeS QDs (c), and

Fe 3 O 4 /SiO 2 -CdTeS nanocomposites (d).

Firstly, 7.5 mg of oil-soluble (hydrophobic) Fe3O4

nanoparticles were transferred to aqueous solu-tion using CTAB;[16] and 0.5 mL of the resulting

Fe3O4/CTAB aqueous solution were diluted with

10 mL of water Then 0.3 mL of NH4OH aqueous solution, 0.03 mL of tetraethylorthosilicate (TEOS), and 0.5 mL of ethyl acetate (EtOAc) were continu-ously added to the diluted magnetite nanoparticles aqueous solution under stirring The reaction was continued under stirring for 30 s, and then aged for three hours The Fe3O4/SiO2nanoparticles were col-lected by centrifugation and washed with water and ethanol several times, and then redispersed in 10 mL

of ethanol After that, 0.05 mL of APS was added

to 10 mL of Fe3O4/SiO2 ethanol solution for amino-functionalization The solution was gently stirred at

60∘C for 12 h Finally, centrifugation was used to collect the amino-functionalized particles and remove the unbounded The amino-functionalized Fe3O4/SiO2 nanoparticles were conjugated with the as-prepared MPA-functionalized CdTeS QDs through bonding be-tween carboxyl groups on QDs and amino groups on silica shells In brief, 2 mL of as-synthesized MPA-stabilized CdTeS QDs were injected to 10 mL of func-tionalized Fe3O4/SiO2 aqueous solution under me-chanical stirring After 6-h reaction at room temper-ature, the magnetic/fluorescent nanocomposites were collected by magnetic decantation

X-ray diffraction patterns were determined using

a rotating anode x-ray diffractometer (XRD,

MX-PAHF, Cu K𝛼 radiation) The morphology of the

samples was investigated by transmission electronic

microscopy (TEM, JEOL JEM-2010 (HT)) and

high-resolving transmission electron microscopy (HRTEM,

JEM-2010 FET(UHR)) Photoluminescence spectra

were obtained using a Sahimadzu RF-5301PC

spec-trofluorophotometer with an excitation wavelength of

470 nm The magnetic properties of samples were

measured by using a superconducting quantum

inter-ference device magnetometer (SQUID, MPMS XL-7)

80 nm

(a)

(c)

10 nm

3.50 A

(b)

80 nm

(d)

20 nm

30 nm

5 7 9 10 11 12 0

10 20 30

Particle size (nm)

Fig 3 TEM images of Fe 3 O 4 nanoparticles Inset:

par-ticle size distribution of Fe 3 O 4 nanoparticles (a), HRTEM

image of CdTeS QDs (b), Fe 3 O 4 /SiO 2 nanoparticles(c),

Fe 3 O 4 /SiO 2 -CdTeS nanocomposites (d).

XRD patterns as shown in Fig.2 were used to

confirm the crystal structure of the as-synthesized

nanoparticles All of the diffraction peaks of Fe3O4

in Fig.2(a) can be perfectly indexed to the

character-istic (220), (311), (400), (422), (511), and (440) peaks

(marked with asterisks) of the cubic spinel magnetite

structure of Fe3O4 The size of Fe3O4 was calculated

to be about 8 nm in diameter by using the

Scher-rer equation 𝐷 = 𝑘𝜆/𝛽 cos(2𝜃), where 𝐷 (in nm) is

the size of the nanocrystal, 𝜆 is the wavelength (in

nm) of the x-ray (𝜆 = 0.154178 nm in our

experi-ment), 2𝜃 is the angle at which the peak is observed,

and 𝛽 (in radian) is the full width at the half

max-imum of the peak given by the XRD pattern In

Fig.2(b), besides the characteristic peaks of the Fe3O4

as shown Fig.2(a), a strong and broad peak around

2𝜃=23∘derived from the amorphous mesoporous silica

can be observed, suggesting the successfully coating of

amorphous mesoporous silica on the surface of Fe3O4

nanoparticles All of the diffraction peaks of CdTeS

QDs in Fig.2(c) are between the diffraction peaks

of the cubic CdTe and CdS structures, indicating

that the CdTeS alloy QDs were synthesized.[12] The

XRD pattern of the final Fe3O4/SiO2-CdTeS

mag-netic/fluorescent nanocomposites shown in Fig.2(d)

proves that the material is a mixture of Fe3O4, SiO2,

and CdTeS Furthermore, the much stronger peaks of

the CdTeS phase in the pattern than those of Fe3O4

phase can be attributed to the larger proportion of CdTeS in the final nanocomposites

0.0 0.2 0.4 0.6 0.8 1.0

Wavelength (nm)

Fig 4 PL spectra of CdTeS QDs (solid line) and

Fe 3 O 4 /SiO 2 -CdTeS nanocomposites (dashed line) The insets are photographs of Fe 3 O 4 /SiO 2 -CdTeS nanocom-posites under normal (left) and ultraviolet (right) irradia-tion.

As is known, the easy aggregation of nanoparti-cles to form large clusters can be due to their high surface energy However, Fe3O4 nanoparticles more easily aggregate to bigger particles owing to the mag-netic attraction between particles In our experiment, oleic acid and oleyamine, as the capping agents, tend

to adsorb on the particular high-energy facets of the nanoparticles Their overall specific surface energy was more or less reduced, thereby, reducing their ten-dency to aggregate The topography of the oleic acid and oleyamine-stabilized Fe3O4 MNPs is shown in Fig.3(a) The TEM image clearly demonstrates that the Fe3O4nanoparticles are highly uniform in particle-size distribution and no aggregation can be observed The average diameter of the observed Fe3O4 nanopar-ticles was measured to be about 8 nm (as shown in the inset of Fig.3(a)), which is nearly the same as their crystallite size estimated by XRD using Scherrer’s for-mula Figure 3(b) provides HRTEM overviews of the as-prepared CdTeS alloyed QDs emitting at 625 nm, indicating that the QDs were spherical particles with good monodispersity The as-prepared CdTeS alloyed quantum dots show a lattice plane distance of 3.50 Å, which is intermediate between cubic CdTe (3.74 Å) and cubic CdS (3.36 Å) The existence of lattice planes

on the HRTEM image confirmed the crystallinity of the as-prepared CdTeS alloyed QDs Fe3O4 nanopar-ticles were coated with a silica shell by taking a sol–gel approach through the hydrolysis and condensation of TEOS, relying on the well-known Stöber method The TEM image of Fe3O4/SiO2, as shown in Fig.3(c), re-veals that the rod-like mesoporous silica outside Fe3O4

nanoparticles is synthesized Each rod-like silica shell contains one or more monodisperse Fe3O4 nanopar-ticles and the major axis rod-like mesoporous silica

is around 100 nm, which is within the applicable size

range for drug and gene delivery.[ 16 , 21 ] CTAB on the

magnetic nanoparticles plays an important role in the

formation of rod-like silica shell CTAB alone in

wa-ter makes rod-like micelles, and therefore it is thought

that rod-like silica shell will grow due to CTAB being

a soft template.[22] The mesoporous silica structures

are considered as the result of removal of CTAB

or-ganic templates by a small amount of ethyl acetate

during sol-gel reaction.[ 16 ] These rod-like mesoporous

silica shells were functionalized with APS, and the

end of amino group in APS makes the silica positively

charged While QDs were functionalized by MPA, the

end of carbocyl group in MPA makes the QDs

nega-tively charged Driven by the electrostatic interaction

between amino-terminated Fe3O4/SiO2 nanoparticles

and carboxyl-terminated CdTeS QDs, the CdTeS QDs

can be firmly linked onto the rod-like mesoporous

sil-ica shells The TEM image of Fe3O4/SiO2-CdTeS

magnetic/fluorescent is shown in Fig.3(d), which

confirms the structure of Fe3O4/SiO2-CdTeS

mag-netic/fluorescent nanocomposite nanoparticles That

is to say, a mass of CdTeS QDs was anchored onto the

surface of rod-like mesoporous silica shell with several

Fe3O4 nanoparticles as cores

-40

-30

-20

-10

0

10

20

30

40

Fe 3 O 4

Fe 3 O 4 /SiO 2 -CdTeS

Fe 3 O 4 /SiO 2

Magnetic field (Oe)

Fig 5 Hysteresis loop of Fe 3 O 4 nanoparticles,

Fe 3 O 4 /SiO 2 nanoparticles and Fe 3 O 4 /SiO 2 -CdTeS

nanocomposites Inset: the picture of Fe 3 O 4 /SiO 2

-CdTeS nanocomposites separated by a magnet in the

solution under ultraviolet light.

Figure 4 shows the PL spectra of CdTeS before

(solid line) and after (dashed line) deposition on the

surfaces of Fe3O4/SiO2nanoparticles The free CdTeS

QDs in aqueous solution have a fluorescent

emis-sion peak at 625 nm However, the emisemis-sion peak of

Fe3O4/SiO2-CdTeS nanocomposites is slightly

blue-shifted (around 7 nm) as compared to the free CdTeS

QDs in aqueous solution, which have also been

re-ported in other papers.[ 15 , 23 , 24 ] Based on the

previ-ous work,[25] there are strong interactions between

Fe3O4MNPs and QDs This kind of interaction would

lead to energy transfer between the Fe3O4 MNPs and

QDs, and hence influences the PL properties of the nanocomposites The interaction is extremely sen-sitive to the separation distance between these two kinds of materials and the influence will be drastically diminished with the increase of distance In our exper-iment, shown in the inset of Fig.4, it can be concluded that the Fe3O4/SiO2-CdTeS nanocomposites are well dispersed in the aqueous solution and show strong flu-orescent emission under UV irradiation

Figure 5 shows the hysteresis loop of the oleic acid and oleyamine stabilized Fe3O4 nanoparticles, the Fe3O4/SiO2 nanoparticles, and the Fe3O4/SiO2 -CdTeS magnetic/fluorescent nanocomposites, where the magnetic field was cycled between −8 and 8 kOe

at 300 K It is illustrated that the saturation magne-tization (𝑀𝑠) of oleic acid and oleyamine stabilized

Fe3O4 nanoparticles with the diameter of about 8 nm was 38.26 emu/g, and zero coercivity was found How-ever, the value of 𝑀𝑠 of Fe3O4/SiO2 decreases to 13.57 emu/g, which results from the formation of a silica shell on the surface of Fe3O4 This reduction of the 𝑀𝑠 value could be attributed to the lower mass ratio of the magnetic component in the Fe3O4/SiO2 sample Hence, the thicker the silica shell, the lower the 𝑀𝑠 value.[ 26 ] As shown in Fig.5, the 𝑀𝑠 value

of Fe3O4/SiO2-CdTeS magnetic/fluorescent nanocom-posites further reduces to 9.09 emu/g because of the coating of CdTeS QDs on the surface of Fe3O4/SiO2 These phenomena demonstrate that both the thick sil-ica shell and CdTeS QDs surrounding can affect the magnetic properties of Fe3O4/SiO2-CdTeS nanocom-posites and lead to a lower 𝑀𝑠 value The thicker silica shell can screen the effect of Fe3O4 MNP to the

PL intensity of QDs on the outside shell but reduced the 𝑀𝑠 values of magnetic/fluorescent nanocompos-ites simultaneously Therefore, the SiO2 should be neither too thick nor too thin to retain favorable mag-netic and fluorescent properties of Fe3O4/SiO2-CdTeS magnetic/fluorescent nanocomposites As shown the inset of Fig.5, when a magnetic bar is placed near the solution, the nanocomposites are attracted and ac-cumulate toward the magnet while the bulk solution becomes a clear phase, indicating that magnetic sepa-ration occurs The aggregated magnetic/fluorescent nanocomposites under ultraviolet light show strong fluorescence intensity, illustrating the Fe3O4/SiO2 -CdTeS magnetic/fluorescent nanocomposites retain favorable magnetic and fluorescent properties at the same time in our experiment These results suggest that our nanocomposites can find potential applica-tions as magnetofluorescent nanoprobes in magnetic guiding and separation, targeted drug delivery, hyper-thermia, bioimaging, etc

In summary, we have synthesized superparam-agnetic Fe3O4 nanoparticles with good monodisper-sity and water-soluble CdTeS quantum dots (QDs)

with high fluorescent intensity separately Then

water-soluble magnetic fluorescent multifunctional

nanocomposite particles of about 100 nm in size are

prepared through electrostatic assembly between

pos-itively charged amino-modified Fe3O4/SiO2

nanopar-ticles and negatively charged CdTeS quantum dots

(QDs) Although the magnetization intensity

de-creases at a certain extent because of the appearance

of SiO2and QD layers, the multifunctional

nanocom-posites still retain favorable magnetic and fluorescent

properties simultaneously, which can be further

ex-plored as a magneto-fluorescent nanoprobe for

appli-cations in biomedicine areas

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