PAPER www.rsc.org/nanoscale | Nanoscale

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A new CoFe2O4–Cr2O3–SiO2 fluorescent magnetic nanocomposite

Chandan Borgohain,ab Kula Kamal Senapati,ab Debabrata Mishra,b Kanak Ch. Sarmac and Prodeep Phukan*a

Received 27th April 2010, Accepted 2nd July 2010

DOI: 10.1039/c0nr00281j

A combined sonochemical co-precipitaion method has been developed for the synthesis of a CoFe2O4–

Cr2O3–SiO2 magnetic nanocomposite. The synthesis involved the pre-synthesis of CoFe2O4–Cr2O3

nanoparticles, which were subsequently coated with SiO2 by treatment with tetraethyl orthosilicate. It

was observed that the as-prepared CoFe2O4–Cr2O3–SiO2 nanocomposite exhibits photoluminescence

properties without the addition of any external fluorescent marker. The fluorescent magnetic

nanoparticles (FMNPs) had a typical diameter of 30� 5 nm and a saturation magnetization of 5.1 emu

g-1 at room temperature. This as-prepared nanocomposite was used for staining cultured HeLa cells for

fluorescence imaging.

Introduction

Magnetic nanoparticles have drawn significant attention in

recent years from both a fundamental point of view and for

applications in material science.1–4 Such particles are in high

demand as new nanoscale technologies are beginning to change

the scientific landscape in terms of medical diagnosis, treatment

and prevention.5–7 Cobalt ferrite (CoFe2O4) is one of the most

extensively studied ferrites and has been exploited for its

potential utilization as an active component for high-density

magnetic storage, spintronic devices and for the fabrication of

sensors for biomedical applications and hyperthermia.8–10 Soler

et al.11 have shown that CoFe2O4 has high structural stability

and is reliable as a magnetic drug carrier in biological appli-

cations. Chromium oxide (Cr2O3) on the other hand is an

important material since it has a high melting temperature, is

resistance to oxidation and exhibits interesting optical, elec-

trical and magnetic properties.12,13 Magnetically CoFe2O4–

Cr2O3 is a two-phase exchanged coupled system consisting of

a ferromagnet (CoFe2O4) biased by an antiferromagnet (Cr2O3)

and such systems with ferro-antiferromagnetic coupling have

been extensively studied in light of magnetoresistive read-head

applications.14,15

Recently, several types of nanoparticle have been used for

bioanalysis. These include dye-doped nanoparticles,16 quantum

dots (QDs),17 lanthanide (Ln3+) doped nanoparticles,18

magnetic nanoparticles19 and gold nanorods.20 These nano-

particles have their own unique properties and have been

adapted for different applications in the field of bioanalysis.

Compared with conventional organic dyes, luminescent nano-

particles are preferred as probes for bioapplications because of

their photostability and strong luminescence. For instance, QD-

integrated magnetic nanoparticles have been actively studied

due to their excellent optical properties such as having narrow

emission bands, continuous broad-band absorption and a high

aDepartment of Chemistry, Gauhati University, Guwahati, 781014 Assam,India. E-mail: pphukan@yahoo.com; Fax: +91-361-2700311; Tel: +91-361-2570535bIndian Institute of Technology Guwahati, Guwahati, 781039 Assam, IndiacDepartment of Instrumentation and USIC, Gauhati University, Guwahati,781014, Assam, India

2250 | Nanoscale, 2010, 2, 2250–2256

resistance to photobleaching in comparison to organic dyes.

However the separation between the magnetic nanoparticles

and the QDs is controlled using a layer-by-layer (LBL)

approach21 to prevent quenching. Such nanoparticles are

usually large (70–100 nm) affecting their stability. Moreover,

this method uses reagents such as polyelectrolytes which are

expensive.

For many applications such as magnetic recording and tar-

geted drug delivery, particles with a higher magnetic moment and

larger particle size are required. Furthermore it is difficult to

produce non-agglomerated nanocrystals of ferromagnetic

nanoparticles. Often larger particles (�50 nm), just below the

superparamagnetic threshold, are more suitable for applications

such as targeted drug delivery as these parameters may influence

drug loading, drug release, stability, toxicity and biological fate.

Much effort has been made to synthesize cobalt ferrite with well-

defined properties. These include important examples such as

mechanochemical methods,22 sonochemical reactions,23 co-

precipitation,24 micro-emulsion procedures,25 and others.26–32

One of the major disadvantages in most of these techniques is the

lower degree of crystallinity in the resulting material, leading in

turn to significant spin misalignment which reduces the net

magnetic moment of the particle.

In this report we describe a method for the synthesis of

a Cr2O3-doped CoFe2O4 nanoparticles coated with a monolayer

of SiO2. A combined co-precipitation sonochemical technique

has been developed in order to obtain highly crystalline magnetic

nanoparticles of 30� 5 nm particle size. It was observed that this

nanocomposite shows photoluminescence properties without the

extra addition of any fluorescent marker. The use of magnetic

fluid nanoparticles (MFNPs) was tested for bio-imaging human

cervical cancer cells (HeLa). While no studies of such CoFe2O4–

Cr2O3–SiO2 nanoparticles for in vitro and in vivo applications

have been reported so far, such conjugates may been used as

bioprobes in which the fluorescent part may be used as an

effective tool for imaging biological cells while the magnetic part

may be used as a therapeutic agent for performing hyperthermia

treatment. The development of such a MFNP with properties for

bio-imaging and for the transport of pharmaceuticals to specific

sites in the body constitutes a powerful tool for gene/drug

therapy.33

This journal is ª The Royal Society of Chemistry 2010

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Experimental

The synthesis of the CoFe2O4–Cr2O3–SiO2 magnetic nano-

composite was achieved in three successive steps. Initially

uncapped CoFe2O4 was synthesized which was then coated with

Cr2O3. Subsequent treatment of the CoFe2O4–Cr2O3 nano-

particles with trietylorthosilicate produced the CoFe2O4–Cr2O3–

SiO2 magnetic nanocomposite. The procedure is described

below.

Synthesis of CoFe2O4 nanoparticles

Two aqueous solutions of FeCl3 (1.5 g, 9.3 mmol, 50 mL) and

CoCl2$6H2O (1 g, 4.2 mmol, 50 mL) were mixed in a 200 mL flat

bottom flask and placed in an ultrasonic bath. An aqueous KOH

solution (3 M, 25 mL) was added dropwise under an argon

atmosphere with continuous ultrasonic irradiation (frequency 40

kHz at 40 kW). Prior to mixing, all these three solutions were

sonicated for 30 min to remove dissolved oxygen. The tempera-

ture of the sonicator bath was raised up to 60 �C and the mixture

was sonicated for a further 30 min in air. The reaction mixture

was centrifuged (14 000 rpm) at ambient temperature for 15 min.

The mixture was further subjected to successive sonication (30

min) and centrifugation (15 min) five times. The black precipitate

was then separated, washed with copious amounts of distilled

water followed by ethanol and kept overnight in an incubator at

60 �C for ageing. The precipitate was further dried in an oven at

100 �C for one hour and subsequently kept under high vacuum

(10�2 bar) for one hour. Finally, the black particles were placed in

50 mL of dry ethanol and subjected to successive sonication (30

min) and centrifugation (15 min) repeatedly until a brown

solution appeared. The precipitate was separated, dried and used

for further modification.

Synthesis of the CoFe2O4–Cr2O3 nanocomposite

The coating of a layer of Cr2O3 on the surface of CoFe2O4

nanoparticles was achieved by premixing a dispersion of

CoFe2O4 nanoparticles in deionised water with appropriate

molar ratios of Cr(OAc)3$H2O in a round-bottom flask in an

ultrasonic bath. An aqueous solution of NaOH was added to the

mixture in the presence of ultrasonic irradiation (frequency 40

kHz at 40 kW). Prior to mixing, all these four solutions were

degassed by sonication for 30 min. The temperature of the son-

icator bath was raised to 60 �C and the mixture was sonicated for

a further 30 min in air. Brown precipitate formation was

observed during this time. The reaction mixture was centrifuged

(10 000 rpm) at ambient temperature for 15 min. The brown

precipitate was then separated, washed with copious amounts of

distilled water followed by ethanol and kept overnight in an

incubator at 60 �C for ageing. The precipitate was further dried

in an oven at 100 �C for one hour and subsequently kept under

high vacuum (10�2 bar) for one hour. Finally, the particles were

placed in 50 mL of dry ethanol and subjected to successive

sonication (30 min) and centrifugation (15 min) repeatedly until

a brown solution appeared. The precipitate was separated, dried

and held at 1000 �C in a muffle furnace for 10 hours to obtain

a fine black powder. Energy-dispersive X-ray spectroscopy

(EDX) analysis at this point showed excellent agreement between

This journal is ª The Royal Society of Chemistry 2010

expected and observed values of the constituent elements which

therefore confirmed the formation of CoFe2O4–Cr2O3.

Surface modification of CoFe2O4–Cr2O3 nanoparticles

The third synthesis step involves silica coating of the as-prepared

CoFe2O4–Cr2O3 nanoparticle surfaces by the hydrolysis of tet-

raethyl orthosilicate (TEOS).34 The CoFe2O4–Cr2O3 nano-

particles (0.2 g) were dispersed in a mixture of ethanol (20 mL),

water (9 mL) and ammonia (25%, 0.5 mL) under ultrasonication

and then TEOS (0.5 mL) was added to the mixture. After three

hours, the precipitate was isolated by centrifugation, washed

with ethanol and water several times and dried at 80 �C under

vacuum for two hours.

Cell labeling

In the cell-labeling process, the cells were cultured in a 25 cm2

glass culture vial and the culture medium was a mixture of

DMEM (Dulbecco’s modified Eagle’s medium), 10% inactivated

fetal bovine serum, 50 units mL�1 of penicillin, 40 mg mL�1 of

streptomycin, and 0.3 mg mL�1 of L-glutamine. Cells were

cultured at 37 �C in a humidified atmosphere that was supplied

with 5% CO2. When the number of cells reached �105 cells per

vial, the culture medium was removed, and a mixture of 0.2 mL

of nanoparticles in PBS buffer (2 mg mL�1) was added to the

culture vial. After incubation for 20 h at 35 �C in 5% CO2 ,�95%

air incubator, the culture vial was washed 16 times using PBS

buffer and was subjected to transmission electron microscopy

(TEM) and fluorescence imaging.

Cell viability experiments

The antiproliferative assay was carried out using a standard

methylthiazoltetrazolium bromide (MTT) assay. The cells (5 �104 cells in each 100 mL medium well) were plated in 0.07%

DMSO as a control in 96-well plates and were incubated for 24 h.

At the end of the treatments, each well was treated with MTT (10

mL), and after incubation for two hours, the absorption at 570

nm was read with a microplate reader.

Characterization

To investigate the formation of CoFe2O4–Cr2O3 nano-

composites, IR studies and X-ray diffraction patterns were

recorded on a Perkin–Elmer RXI FT-IR spectrometer using KBr

pellets and on a Bruker AXD D8 using Cu Ka radiation (l ¼1.54178 �A). The samples for TEM were prepared by drying an

ethanol dispersion of the particles on a carbon-coated copper

grid. The particles were imaged on a 200 kV, JEOL JEM2100

transmission electron microscope. Quantitative elemental anal-

ysis was carried out with an Oxford energy-dispersive X-ray

spectrometer mounted on the transmission electron microscope.

The magnetic properties of the as-prepared CoFe2O4, CoFe2O4–

Cr2O3 and CoFe2O4–Cr2O3–SiO2 nanoparticles were investi-

gated using a vibrating sample magnetometer (Lakeshore 7410).

UV–visible spectra and fluorescent spectra of the nanoparticles

were recorded using a Varian Cary50 biospectrophotometer and

Edinburgh FSP920 Instruments respectively. Quantum yield

evaluations were made by comparing the integrated intensity of

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the luminescence of the sample with a standard solution of

quinine sulphate. The UV–visible absorbance spectrum of the

reference solvent and the sample solution were noted at the

excitation wavelength of 260 nm and 360 nm. Cell labeling

experiments were conducted under fluorescence microscopy

using a Carl Zeiss, LSM 510 meta confocal laser scanning

microscope.

Results and discussion

Structural and morphological analysis

To understand the mechanisms behind the magnetic properties,

we carried out detailed magnetic and microstrucural studies. The

fluorescent properties of the nanoparticles were studied using

a time-resolved steady-state photoluminescence spectrometer.

Pure magnetic nanoparticles however may not be very useful in

practical applications as they are likely to form large aggregates

and their magnetic properties would change or they may undergo

rapid biodegradation when they are exposed to biological

systems. To remove these drawbacks we coated the nanoparticles

with a layer of SiO2.

Fig. 1(a) shows the XRD pattern of the CoFe2O4–Cr2O3

nanocomposite. The diffraction peaks and relative intensities of

the pattern match well with the cubic spinel structure of CoFe2O4

(JCPDS–International center diffraction data, PDF cards 3-864

and 22-1086) and of Cr2O3 (JCPDS–International center

diffraction data, PDF cards 06-0504).

Fig. 1 (a) XRD pattern and (b) FT-IR spectra of the as-prepared

CoFe2O4–Cr2O3 particles.

2252 | Nanoscale, 2010, 2, 2250–2256

The crystallite size (D) of the nanoparticles were determined

using the Scherrer formulae35 on the (311) peak of CoFe2O4 and

was found to be 30 nm. The FT-IR spectra of the material

(Fig. 1b) showed peaks at 611, 938, 1037, 1180, 1338 and 1586

cm�1 corresponding to CoFe2O4 and peaks at 575, 624, 923,

1155, 1528 and 1638 cm�1 corresponding to CoFe2O4–Cr2O3.

The peak obtained at 611 cm�1 in the CoFe2O4 samples is

attributed to the Fe–O or the Co–O bond.36 In the CoFe2O4–

Cr2O3 samples the peak at 575 cm�1 corresponds to the Cr–O

bond.37

The structural composition and crystallinity of the cobalt

ferrite nanoparticles was further examined using TEM. Fig. 2

shows the TEM image of the cobalt–ferrite nanocrystals depos-

ited on a carbon-coated copper grid. A TEM image of the

CoFe2O4–Cr2O3 nanoparticle is shown in Fig. 2a and Fig. 2b

shows the TEM image of the CoFe2O4–Cr2O3–SiO2 nano-

composite. The SAED pattern (Fig. 2d) obtained from TEM

showed CoFe2O4–Cr2O3–SiO2, with the rings corresponding to

reflections from the planes of CoFe2O4, Cr2O3 and SiO2. The

average size of the nanoparticles from the TEM analysis

(Fig. 2a–b) was found to be 30 � 5 nm.

Magnetic properties

It is well known that the magnetic properties of CoFe2O4 depend

on the chemical nature of Co, since Fe3+ species are evenly

distributed in the structure at tetrahedral and octahedral inter-

stices and are antiferromagnetically coupled. Such coupling

cancels the moment contribution from Fe3+ and hence the

moment contribution is solely dependent on Co2+.38 Incorpora-

tion of antiferromagnetic Cr2O3 and SiO2 in the Co–Fe–O matrix

may cause changes in the magnetic properties of the material.

The Magnetisation–Hysteresis (M–H) loop was taken at room

temperature with a maximum applied field of � 2 T. From the

hysteresis loop, both saturation magnetization, coercivity and

retentivity values were extracted.

Fig. 3 shows that the prepared CoFe2O4–Cr2O3–SiO2 particles

are ferromagnetic at room temperature with a saturation

magnetization of 5.1 emu gm�1 and coercivity of 482 Oe. It has

recently been demonstrated by Jordan et al.39 that large coer-

civity magnetic nanoparticles have the ability to self-heat when

irradiated by electromagnetic irradiation. This is known as

magnetic fluid hyperthermia (MFH)40 and can act as a thera-

peutic agent by itself. It can be seen from Table 1 that incorpo-

ration of Cr2O3 and SiO2 into the Co–Fe–O matrix has very little

effect on the coercivity, however the magnetization and the

retentivity decrease rapidly. Therefore by controlling the quan-

tity of Cr2O3 and SiO2 in the Co–Fe–O matrix, magnetic prop-

erties of the nanocomposite can be tailored to suite different

biomedical applications.

Spectral analysis

We have performed room-temperature optical absorption and

photoluminescence to show the optical properties of the nano-

composite compared to its individual component (Cr2O3). Fig. 4

(a–d) shows the UV–visible absorption and fluorescence emis-

sion spectra of the CoFe2O4–Cr2O3–SiO2 particles. The sharp

This journal is ª The Royal Society of Chemistry 2010

Fig. 2 TEM images. (a) TEM image of CoFe2O4–Cr2O3,(b) TEM image

of CoFe2O4–Cr2O3–SiO2 nanocomposite, (c) HRTEM images of

CoFe2O4–Cr2O3–SiO2 nanocomposite and (d) SAED pattern of

CoFe2O4–Cr2O3–SiO2 nanocomposite.

Fig. 3 Hysteresis curves of CoFe2O4, CoFe2O4–Cr2O3 and CoFe2O4–

Cr2O3–SiO2 nanocomposites.

Table 1 Effect of the incorporation of antiferromagnetic Cr2O3 andSiO2 into the Co–Fe–O matrix on its magnetic properties

NanocompositeCoercivity/Oe

Magnetization/mass emu gm�1

Retentivity/emu gm�1

CoFe2O4 539 64.97 17.49CoFe2O4–Cr2O3 529 24.74 6.62CoFe2O4–Cr2O3–SiO2 482 5.10 1.79

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peak at 200 nm in the UV–visible spectra indicates that the

colloids are well dispersed.

The UV–visible spectra of CoFe2O4–Cr2O3and Cr2O3 showed

absorption bands at around 260 and 360 nm. The bandgap

corresponding to the absorption bands was calculated by plot-

ting (ahn)2 vs. hn (Fig. 4b) using the relationship:41

ahn ¼ const (hn � Eg)n (1)

The value of a is obtained from the equation:42

a ¼ 2:3026

�A

t

�(2)

Where A is the absorption and t is the thickness of the sample.

Fig. 4 (a–b) Absorption spectra of CoFe2O4–Cr2O3–SiO2 at different

concentrations, (c–d) absorbance of nanocomposites in DMEM over

a period of 24 hrs.

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The extrapolation (Fig. 4b) of the straight line to a2 ¼ 0 gives

the value of the band-gap energy.

To further investigate absorption bands, the fluorescent

properties of the nanoparticles were measured using a steady-

state time-resolved spectrofluorometer at the wavelengths 260

and 360 nm for excitation scans and 350–600 nm for emission

scans (Fig. 5). We did not observe any prominent fluorescence

peaks for the as-prepared CoFe2O4–Cr2O3–SiO2 nanoparticles

Fig. 5 (a–b) Fluorescence emission spectra of CoFe2O4–Cr2O3–SiO2

particles excited at 260 and 360 nm; (c) emission spectra of quinine

sulphate in 0.1 mol L�1 H2SO4 solution and CoFe2O4–Cr2O3–SiO2

dispersion in water at 0.1 ABS excited at 370 nm; (d–e) Absorbance of the

nanocomposites in DMEM over a period of 24 h.

2254 | Nanoscale, 2010, 2, 2250–2256

(Fig. 5a). However, when the sample was annealed at 1000 �C,

a broad band at 460 nm appeared in the fluorescence spectrum,

(Fig. 5b). This was attributed to lattice defects, such as interval

atoms, displacement atoms and line defects resulting from grain

boundary diffusion between CoFe2O4 and Cr2O3. The presence

of defects in the nanocomposite is also supported by the

HRTEM images of the sample (Fig. 2c). Such lattice defects are

responsible for the luminescent properties of the nano-

composite.43

The fluorescent efficiency of the nanocomposite was measured

using quinine sulphate (FR ¼ 0.55)44 in 0.1 mol L�1 H2SO4

solution as a standard using the relationship:45

F ¼ FR

�IntARn2

IntRAn2R

�(3)

Where F is the quantum yield, Int is the area under the

emission peak (on a wavelength scale), A is absorbance at the

excitation wavelength of 370 nm, and n is the refractive index of

the sample (we have taken the value of nR ¼ 1.338,45 n ¼1.34639.46 FR ¼ 0.5547). The subscript R denotes the respective

values of the reference substance.

The fluorescence spectrum of the solutions were recorded in

a 20 mm fluorescence cuvette of constant slit width. In order to

minimize reabsorption effects, absorbances at 0.1 above the

excitation wavelength (at 370 nm) were used.48 The quantum-

yield of the FMNP sample was found to be 0.0354 (Fig. 5c),

which is appreciably good for bio-imaging.

The stability of the CoFe2O4–Cr2O3–SiO2 suspension was

inferred from optical absorbance measurements taken at

different intervals of time as studied by Gonz�alez-Caballero

et al.49 The optical absorbance was studied at 360 nm as a func-

tion of time. We scanned the entire range of wavelengths from

310 nm to 450 nm for different intervals of time. All suspensions

contained 0.1 g L�1 of CoFe2O4–Cr2O3–SiO2 particles in

a mixture of DMEM, Sodium bicarbonate and water. Fig. 5(d–e)

displayed the scanning spectra taken at 3 hour intervals up to 24

hours. From the spectra it was evident that there was no signif-

icant change in absorbance with time which revealed the stability

of the suspension of the ferrite particles. However, the overall

trend of A to decrease with time (DA ¼ 0.04), due to magnetic

interactions between the as-synthesized particles, demonstrates

their stability.

Fluorescence imaging experiments

The combination of nanoscale dimensions, ferromagnetic prop-

erties and fluorescence for these bifunctional nanoparticles has

prompted their use in medical imaging. Optical fluorescent

techniques have high spatial resolution in cellular imaging and

molecular event quantification. To demonstrate the utility of the

fluorescent nanocomposite, we sought to apply it to cellular

imaging. We have carried out our preliminary investigation using

human cervical cancer cells (HeLa) with a mean cell diameter of

14.6 � 0.8 mm as the model cell system and incorporated the

MFNP in the cell.

The as-synthesized SiO2-conjugated CoFe2O4–Cr2O3 nano-

particles were incubated with the HeLa cells, allowing an interac-

tion on the cell surface resulting in the nanoparticles attached to the

outer cell-membrane. The nanoparticles exhibited a surprisingly

This journal is ª The Royal Society of Chemistry 2010

Fig. 6 (a–c) TEM images of cultured HeLa cells; (d) SAED pattern

taken of the internal compartment of the cells; (e–f) pattern and fluo-

rescence image of cultured HeLa cells.

Table 2 Cell viability test for FMNPs

Entry Acontrol Atreated Cell viability Average

1 0.868 0.745 85.83 —2 0.855 0.755 88.30 —3 0.862 0.750 87.01 87.044 0.861 0.756 87.80 —5 0.865 0.746 86.24 —

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high level of cell internalization. While viewing the nanoparticle-

labeled cells using TEM, we were able to follow the nanoparticles

as they transported through the cell membrane (Fig. 6c) and into

internalized compartments (Fig. 6a–b) confirmed by the SAED

pattern (Fig 6d) taken of the cell compartments. The cellular

uptake of the nanoparticles was further confirmed by the inter-

cellular green fluorescence observed by morphology studies of the

HeLa cells under fluorescence microscopy using a confocal laser

scanning microscope (Fig 6e). We observed the morphological

changes in the HeLa cells after treatment with the nanoparticles by

fluorescence microscopic observation. The cells that were treated

with the FMNPs did not show any cell shrinkage or rounded

morphology. We observed a flattened morphology (Fig. 6e) of the

cells, which shows the non-toxicity of the nanoparticles.50

Cell viability experiments

To evaluate the biocompatibility of FMNPs as bio-imaging

probes, we investigated the cytotoxicity of FMNPs using a stan-

dard MTT assay.51 The cells (5 � 104 cells in 100 mL medium

well�1) were plated in 0.07% DMSO media as a control in 96-well

plates and were incubated for 24 h. After 24 h of growth, the

medium was exchanged for the medium that contained the

This journal is ª The Royal Society of Chemistry 2010

nanoparticles. The nanoparticle stock solution (150 mg mL�1)

was prepared in water. From the stock solution, aliquots of

nanoparticles were rapidly added to the culture medium. At the

end of the treatments, each well was treated with MTT (10 mL),

and after incubation for two hours, the absorption at 570 nm was

read with a microplate reader. The cell viability was calculated

using the following equation.

Cell viability ð%Þ ¼�

Atreated

Acontrol

�� 100 (1)

Where Atreated and Acontrol are the absorbance of the treated and

untreated cells, respectively. The cell viability date for concen-

trations of 150 mg mL�1 of the nanoparticles for different sets of

experiments are described in Table 2.

The FMNPs exhibited low toxicity (cell viability ¼ 87%)

towards HeLa cells even at a high concentration of 150 mg mL�1.

Conclusions

In conclusion, a combined sonochemical and co-precipitaion

method has been developed for the synthesis of core–shell

CoFe2O4–Cr2O3–SiO2 magnetic nanocomposites. The as-

prepared CoFe2O4–Cr2O3–SiO2 nanocomposites exhibit photo-

luminescence properties without the addition of any external

fluorescent marker. The fluorescent magnetic nanoparticles had

a typical diameter of 30 � 5 nm and a saturation magnetization

of 5.1 emu g�1 at room temperature. The fluorescent nano-

composites were further utilized for staining the cultured HeLa

cells for fluorescence imaging detection.

Acknowledgements

Financial support from DST (India) for the TEM facility at CIF,

IIT Guwahati (Grant No. SR/S5/NM-01/2005) and a Ramanna

Fellowship to P. Phukan (Grant No. SR/S1/RFPC-07/2006) is

gratefully acknowledged. The authors would also like to

acknowledge the support from IIT Guwahati for the vibrating

sample magnetometer, SEM and XRD facility. We thank

reviewers for their valuable suggestions.

References

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