A facile method for the preparation of bifunctional Mn:ZnS/ZnS/Fe3O4 magnetic and fluorescent nanocrystals

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Bifunctional magnetic and fluorescent core/shell/shell Mn:ZnS/ZnS/Fe 3 O 4 nanocrystals were synthesized in a basic aqueous solution using 3-mercaptopropionic acid (MPA) as a capping ligand. The structural and optical properties of the heterostructures were characterized by X-ray diffraction (XRD), dynamic light scattering (DLS), transmission electron microscopy (TEM), UV–vis spectroscopy and photoluminescence (PL) spectroscopy. The PL spectra of Mn:ZnS/ZnS/Fe 3 O 4 quantum dots (QDs) showed marked visible emission around 584 nm related to the 4 T 1 → 6 A 1 Mn 2+ transition. The PL quantum yield (QY) and the remnant magnetization can be regulated by varying the thickness of the magnetic shell. The results showed that an increase in the thickness of the Fe 3 O 4 magnetite layer around the Mn:ZnS/ZnS core reduced the PL QY but improved the magnetic properties of the composites. Nevertheless, a good compromise was achieved in order to maintain the dual modality of the nanocrystals, which may be promising candidates for various biological applications.

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# A facile method for the preparation of bifunctional Mn:ZnS/ZnS/Fe3O4 magnetic and fluorescent nanocrystals

Houcine Labiadh 1, Tahar Ben Chaabane 1, Romain Sibille 2, Lavinia Balan 3

and Raphaël Schneider *4

Full Research Paper [Open Access](http://www.beilstein-journals.org/bjnano/about/openAccess.htm)

Address:
1Unité de Recherche Synthèse et Structure de Nanomatériaux UR 11

ES 30, Université de Carthage, Faculté des Sciences de Bizerte,
7021 Jarzouna, Bizerte, Tunisia, 2 Institut Jean Lamour (IJL),

Université de Lorraine, CNRS, UMR 7198, CNRS, BP 70239, 54506
Vandoeuvre-lès-Nancy Cedex, France, 3 Institut de Science des

Matériaux de Mulhouse (IS2M), CNRS, UMR 7361, 15 rue Jean
Starcky, 68093 Mulhouse, France, and 4 Université de Lorraine,

Laboratoire Réactions et Génie des Procédés (LRGP), UMR 7274,

CNRS, 1 rue Grandville, BP 20451, 54001 Nancy Cedex, France

Email:
Raphaël Schneider * [- raphael.schneider@univ-lorraine.fr](mailto:raphael.schneider@univ-lorraine.fr)

- Corresponding author

Keywords:

aqueous synthesis; bifunctional nanocrystals; magnetite; Mn-doped

ZnS; quantum dots

_Beilstein J. Nanotechnol._ 2015, _6,_ 1743–1751.

[doi:10.3762/bjnano.6.178](http://dx.doi.org/10.3762%2Fbjnano.6.178)

Received: 05 May 2015

Accepted: 25 July 2015

Published: 17 August 2015

Associate Editor: J. J. Schneider

License and terms: see end of document.

## Abstract

characterized by X-ray diffraction (XRD), dynamic light scattering (DLS), transmission electron microscopy (TEM), UV–vis spec
troscopy and photoluminescence (PL) spectroscopy. The PL spectra of Mn:ZnS/ZnS/Fe3O4 quantum dots (QDs) showed marked
visible emission around 584 nm related to the 4 T1 → 6 A1 Mn 2+ transition. The PL quantum yield (QY) and the remnant magnetization can be regulated by varying the thickness of the magnetic shell. The results showed that an increase in the thickness of the

Fe3O4 magnetite layer around the Mn:ZnS/ZnS core reduced the PL QY but improved the magnetic properties of the composites.
Nevertheless, a good compromise was achieved in order to maintain the dual modality of the nanocrystals, which may be promising

candidates for various biological applications.

## Introduction

Semiconductor nanocrystals with a diameter of approximately

1–10 nm, also referred to as quantum dots (QDs), have attracted

great attention due to their unique optical and electronic prop
erties, which are not observed in bulk semiconductor materials

[1-3]. Compared to conventional organic dyes, QDs possess

many advantages, including a broad absorption with a narrow

photoluminescence (PL) spectra, low photobleaching, high PL

quantum yield (QY), tunable emission from the visible to

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infrared wavelengths, and high resistance to chemical degrad
ation [3,4]. Such characteristics originate from their large

surface-to-volume ratio and from confinement phenomena

resulting in an atomic-like electronic structure with discrete

energy levels. Thus, QDs have been widely studied for their

fundamental properties and applications, mostly employed as

emitters for biolabelling [3,5], light emitting diodes [6,7] or

solar cells [8]. Conventional QDs systems have a core/shell

architecture. The shell, generally constituted of a wide band gap

material such as ZnS, prevents degradation and preserves the

optical properties [3,4].

Magnetic nanoparticles have many advantages, especially for

biological applications. They can, for example, be used as a

contrast agent in magnetic resonance imaging or as therapeutic

agents in magnetic fluid hyperthermia [9,10]. Several successful

applications such as targeted drug delivery, bioseparation,

biodetection, and labelling and sorting of cells [11-14] are based

on the unique feature of magnetic nanoparticles to respond well

to magnetic control.

The combination of magnetic and optical properties in a single,

nanostructured material has attracted increased attention

because of the advantageous properties of such nanoparticles.

Such fluorescent and magnetic properties allow nanoparticles to

be guided with a magnetic force both in vivo and in vitro and

provide detailed information on their biodistribution using a

fluorescence microscope [15-17]. Such bifunctional nanoparti
cles would enable simultaneous biolabelling/imaging and cell

sorting/separation. Over the last decades, different strategies

have been developed toward this goal such as epitaxial

heterocrystalline growth, co-encapsulation of preformed

QDs and magnetic particles in silica beads, doping of QDs with

transition metal ions, conjugation between magnetic chelates

or magnetic nanocrystals with QDs (e.g., using avidin and

biotin) [18,19]. However, there are still some challenges to

overcome such as the complexity in the preparation, which

involves multistep synthesis and purification stages or the

instability and aggregation of the nanocomposites in aqueous

solution. Moreover, some of the nanocomposite particles are

rather large (70–200 nm), limiting their use in biological appli
cations.

Epitaxial heterocrystalline growth is generally conducted by

coating a ferro- or ferri-magnet that has an ordering tempera
ture well above 300 K (e.g., FePt, Fe2O3, or Fe3O4) with a
semiconductor shell (CdSe or CdS) of thickness between

2–7 nm resulting in either spherical core/shell nanoparticles or

heterodimers [20-22]. The PL QY of the resulting bifunctional

nanoparticles is generally low (typically <5%) due to the

quenching effect of the magnetic domain [20,21].

_Beilstein J. Nanotechnol._ 2015, _6,_ 1743–1751.

Herein, we describe a facile synthesis of epitaxial heterocrys
talline nanoparticles, consisting of an Fe3O4 shell surrounding a
core/shell of Mn-doped ZnS/ZnS QDs. The choice of the

Mn:ZnS/ZnS system was motivated by its very low toxicity and

thus potential use in various biological applications such as

fluorescent labelling [5,23,24]. Mn:ZnS/ZnS QDs were first

prepared in aqueous solution in the presence of the MPA ligand.

The Fe3O4 layer was next grown on the preformed Mn:ZnS/
ZnS QDs in aqueous solution. The resulting bifunctional, core/

shell/shell Mn:ZnS/ZnS/Fe3O4 QDs exhibited superparamagnetism and fluorescence properties, which are discussed herein.

The approach described in this study is anticipated to be useful

and cost-effective for biological and biomedical applications

requiring both fluorescence and magnetic characteristics.

## Results and Discussion

Synthesis and structural/microstructural characterization
The design of the fluorescent and magnetic Mn:ZnS/ZnS/Fe3O4
QDs is shown schematically in Figure 1. The Fe3O4 shell is
grown in aqueous solution on the surface of MPA-capped core/

shell Mn:ZnS/ZnS nanocrystals prepared via the nucleation
doping strategy [5,23-25]. Various amounts of a 0.1 M FeCl3
aqueous solution and of the MPA ligand (1, 1.5, 2 or 3 mL)

were injected dropwise at 100 °C to the preformed Mn:ZnS/ZnS

QDs dispersion solution to form magnetic Fe3O4 layers of
different thicknesses. These Mn:ZnS/ZnS/Fe3O4 samples are
respectively labelled (1), (1.5), (2) and (3) hereafter.

X-ray diffraction (XRD) patterns of Mn:ZnS/ZnS, Mn:ZnS/

ZnS/Fe3O4 and Fe3O4 nanocrystals are shown in Figure 2. All
the diffraction peak positions of the Fe3O4 black powder
prepared in the absence of Mn:ZnS/ZnS QDs are in accordance

with the data reported in the literature (see JCPDS record

number 99-100-7775) and correspond to the magnetite Fe3O4
spinel-like structure (Figure 2f) [26,27]. Note that this sample

was prepared using FeCl3 as a precursor in the presence of the
MPA ligand, which probably also acts as a reducing agent to
provide the necessary amount of Fe 2+ to form magnetite. For

the Mn:ZnS/ZnS and Mn:ZnS/ZnS/Fe3O4 (1), (1.5), (2) and (3)
nanocrystals, the diffraction peaks match perfectly with the

(111), (220), and (311) crystalline planes of the cubic ZnS

phase (JCPDS record number 99-100-0108) [26]. The XRD

patterns of Mn:ZnS/ZnS/Fe3O4 QDs (2) and (3) exhibit additional, low intensity peaks that were not observed in the case of

Mn:ZnS/ZnS/Fe3O4 (1) and (1.5) nanocrystals. These peaks
correspond to magnetite and were detectable only when the

thickness of the magnetic shell increased. For the Mn:ZnS/ZnS/

Fe3O4 (3) sample, the appearance of additional peaks of very
low intensity originating from hematite α-Fe2O3 (JCPDS record
number 99-100-0140) can also be observed [26]. Since the

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_Beilstein J. Nanotechnol._ 2015, _6,_ 1743–1751.

Figure 1: Schematic of the core/shell/shell Mn:ZnS/ZnS/Fe3O4 QD synthesis.

surface of finely divided materials is highly reactive, partial oxi
dation of Fe3O4 into Fe2O3 may have taken place during the
handling of the nanocrystals [28,29]. The crystallites sizes of

the Mn:ZnS/ZnS/Fe3O4 nanoparticles were calculated using the
Scherrer formula based on the width of the most intense (111)

diffraction peak (Table 1).

Figure 2: XRD patterns of (a) Mn:ZnS/ZnS, (b–e) Mn:ZnS/ZnS/Fe3O4
(1), (1.5), (2) and (3) samples, respectively and (f) Fe3O4 nanocrystals.
The reference line spectrum (bottom) shows the relative intensities of
the reflections for bulk ZnS. The diffraction peaks of hematite
(α-Fe2O3) in the XRD pattern f are labelled with an asterisk.

Figure 3 shows transmission electron microscopy (TEM) and

high-resolution TEM (HR-TEM) images of the Mn:ZnS/ZnS

and Mn:ZnS/ZnS/Fe3O4 (1.5) materials. All Mn:ZnS/ZnS core/
shell nanocrystals were nearly monodisperse and of spherical

shape. The average diameter of these QDs was determined to be

3.8 ± 0.7 nm (Figure 3a,b,e). The TEM and HR-TEM micro
graph of Mn:ZnS/ZnS/Fe3O4 (1.5) crystals shows almost
spherical particles with an average size of 4.4 ± 0.9 nm

(Figure 3c,d,f). The measured diameters fit well with the calcu
lated crystallite sizes deduced from the application of the

Scherrer formula (Table 1). The increase in size observed from

3.8 ± 0.7 nm to 4.4 ± 0.9 nm confirms the growth of the iron

oxide shell around the Mn:ZnS/ZnS core and indicates that the

thickness of the Fe3O4 layer is approximately 0.6 nm for
Mn:ZnS/ZnS/Fe3O4 (1.5) QDs.

Table 1: Calculated crystallite diameters and measured hydrodynamic diameters.

Sample Diameter

(nm) a

Hydrodynamic diameter

(nm) b

Mn:ZnS/ZnS 3.5 ± 1.0 11 ± 1.0
Mn:ZnS/ZnS/Fe3O4 (1) 4.2 ± 1.0 22 ± 4.7
Mn:ZnS/ZnS/Fe3O4 (1.5) 3.9 ± 1.0 23 ± 2.9
Mn:ZnS/ZnS/Fe3O4 (2) 4.0 ± 1.0 24 ± 7.5
Mn:ZnS/ZnS/Fe3O4 (3) 4.5 ± 1.0 39 ± 8.0

aDetermined using the Scherrer equation; bDetermined by DLS measurements.

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_Beilstein J. Nanotechnol._ 2015, _6,_ 1743–1751.

Figure 3: TEM and HR-TEM micrographs of (a) Mn:ZnS/ZnS and (b) Mn:ZnS/ZnS/Fe3O4 (1.5) nanocrystals, and (c) and (d) are the corresponding
size distributions.

The nanoparticles were characterized by their hydrodynamic

diameter before and after the Fe3O4 coating using dynamic light
scattering (DLS) (Table 1). The average hydrodynamic diame
ters, _d_ H, are larger than the particles sizes determined using
TEM because of the solvation layer around the QDs in aqueous

solution. The lowest _d_ H value of 11 nm was obtained for the
Mn:ZnS/ZnS QDs. The _d_ H increased with the thickness of the
magnetic shell (Table 1). The highest _d_ H (39 nm) was observed
for Mn:ZnS/ZnS/Fe3O4 (3) nanocrystals. The DLS measurements clearly confirmed the growth of the Fe3O4 shell around
the Mn:ZnS/ZnS core. Note also that these functionalized

nanoparticles could be easily redispersed in aqueous medium

after purification and drying (Figure 4a). The room temperature,

magnetic harvesting of the heterostructured nanocrystals could

be conducted for all Mn:ZnS/ZnS/Fe3O4 materials prepared.
Figure 4b illustrates the sensitive magnetic response of the

Mn:ZnS/ZnS/Fe3O4 (2) sample.

PL properties of Mn:ZnS/ZnS/Fe3O4 QDs
The optical properties of Mn:ZnS/ZnS/Fe3O4 samples were
studied by UV–vis absorption spectroscopy and PL spec
troscopy (Figure 5a,b). As can be seen, the absorbance of

Mn:ZnS/ZnS/Fe3O4 crystals (Figure 5a) shifted to longer wavelengths with increasing thickness of the magnetic layer. The PL

Figure 4: Colloidal dispersion of Mn:ZnS/ZnS/Fe3O4 (2) nanoparticles
dispersed in water (a) in the absence of and (b) in the presence of a
permanent magnet.

spectrum of Mn:ZnS/ZnS/Fe3O4 nanocrystals exhibits two
bands: a broad and very weak one centered at approximately

450 nm and another dominant one at approximately 584 nm

(visible, orange wavelength region). The first emission is asso
ciated with transitions involving vacancy states of the ZnS host
material [24,30], while the second one originates from the Mn 2+

dopant, which is excited via energy transfer of the ZnS host followed by the dipole forbidden 4 T1 → 6 A1 ligand field transition [23,24,31].

1746

Figure 5: (a) Absorption and (b) photoluminescence spectra of
Mn:ZnS/ZnS and Mn:ZnS/ZnS/Fe3O4 (1), (1.5), (2) and (3) nanocrystals. (c) Digital photograph of colloidal dispersions of Mn:ZnS/ZnS/
Fe3O4 nanocrystals with increasing Fe3O4 shell thickness.

Adding the magnetic Fe3O4 shell to the surface of Mn:ZnS/ZnS
QDs progressively reduced the fluorescence intensity, which

became very low especially for the Mn:ZnS/ZnS/Fe3O4 (3) material (Figure 5b,c). Note that the PL QY decreased from 10.8%

for the Mn:ZnS/ZnS QDs, to 3.24% for the Mn:ZnS/ZnS/Fe3O4
(1) material. A continuous decrease of the PL QY was further

observed when increasing the thickness of the Fe3O4 shell and
reached 0.65% for Mn:ZnS/ZnS/Fe3O4 (3). These results probably originate (1) from structural defects related to the notable

_Beilstein J. Nanotechnol._ 2015, _6,_ 1743–1751.

discrepancy between the lattice parameters of the cubic zinc

blende phase of the ZnS core material ( _a_ = 5.4093 Å) and the

cubic spinel phase of the magnetite shell ( _a_ = 8.3970 Å) and (2)

from the partial quenching of the Fe3O4 paramagnetic shell
originating from the changes in the electronic density on the

surface of Mn:ZnS/ZnS QDs.

Magnetic properties of Mn:ZnS/ZnS/Fe3O4
nanocrystals
The magnetization curves, _M_ versus μ0 _H_, of Mn:ZnS/ZnS/
Fe3O4 nanocrystals at 300 K and 2 K are shown in Figure 6. At
300 K, a strong paramagnetic signal was observed for the

Mn:ZnS/ZnS/Fe3O4 samples (1.5), (2) and (3). The magnetization values recorded at the maximum applied magnetic field of

9 T, _M_ 9T, are given in Table 2.

The highest value of 2.67 emu/g was obtained for the sample

Mn:ZnS/ZnS/Fe3O4 (2). This value was remarkably lower than
the saturation magnetization of bulk phase Fe3O4 (90 emu/g),
which could be related to the quantum confinement effects of

nanocrystals and to the diamagnetic contribution of the ZnS

core.

At 2 K, all bifunctional nanoparticles exhibited hysteresis with

remnance magnetization, _M_ R, at 9 T and coercivity, HC, indicating a dominant ferromagnetic nature of the iron oxide layer.

The magnetic characteristics ( _M_ R, _M_ 9T, and _H_ C) of the samples
are given in Table 2. One can observe that the hysteresis loops

are not saturated even for fields up to ±9 T; this could be due to

frozen spins at the surface of the nanocrystals as reported in

previous works [32,33]. The coercive fields increased with the

iron oxide layer thickness, where the highest _H_ C value of 0.18 T
was measured for Mn:ZnS/ZnS/Fe3O4 QDs (3). Thus, the
nanocrystals become magnetically harder with an increasingly

open hysteresis loop with increasing thickness of the magnetite

shell.

The values of _M_ R and _M_ 9T change as well in the same manner
for Mn:ZnS/ZnS/Fe3O4 QDs (1, 1.5 and 2). Surprisingly, the
Mn:ZnS/ZnS/Fe3O4 (3) crystals exhibited reduced _M_ R and _M_ 9T
values, which were lower than those measured for (1.5) and (2)

samples. However, its coercive field was the highest (Table 2).

This likely originates from the contribution of the hematite

(α-Fe2O3) impurity in the Mn:ZnS/ZnS/Fe3O4 (3) sample, as
observed in the XRD pattern. Among bulk iron oxides (Fe3O4,
α-Fe2O3 and γ-Fe2O3), hematite exhibits the lowest saturation
magnetization of 0.3 emu/g with a relatively large coercivity of

0.17 T at room temperature [34,35].

The zero-field-cooled (ZFC) and field-cooled (FC) measure
ments were performed under an applied field of 500 Oe

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_Beilstein J. Nanotechnol._ 2015, _6,_ 1743–1751.

Figure 6: (a–d) Magnetization loops of fluorescent, magnetic nanocrystals Mn:ZnS/ZnS/Fe3O4 (1, 1.5, 2 and 3). The units of magnetization are given
in emu/g of the total weight, and in emu/mole of Fe3O4.

Table 2: Magnetic characteristics of Mn:ZnS/ZnS/Fe3O4 nanocrystals.

Sample _M_ 9T (2 K, 300 K)

(emu/g)

_M_ R (2K) (emu/g) Coercive field

(T)

_T_ B
(K)

Mn:ZnS/ZnS/Fe3O4 (1) 4.82, — 0.21 0.08 6
Mn:ZnS/ZnS/Fe3O4 (1.5) 9.30, 2.33 1.45 0.13 12
Mn:ZnS/ZnS/Fe3O4 (2) 7.05, 2.67 1.61 0.15 20
Mn:ZnS/ZnS/Fe3O4 (3) 4.35, 1.21 0.77 0.18 21

between 2–300 K. The ZFC and FC curves of Mn:ZnS/ZnS/

Fe3O4 (1, 1.5 and 2) crystals are given in Figure 7. The ZFC
curve reached a temperature maximum corresponding to the

blocking temperature ( _T_ B) of the sample. The result of the ZFC
and FC measurements confirmed the superparamagnetic behav
ior of the nanocrystals [36]. Below the blocking temperature,

the material is ferromagnetic, and above _T_ B, it is superparamagnetic [33].

The values for the _T_ B are given in Table 2. _T_ B increased with
increased iron oxide shell thickness starting at 6 K for Mn:ZnS/

ZnS/Fe3O4 (1) at 20 K for Mn:ZnS/ZnS/Fe3O4 (2). These
results are in agreement with the size increase of the nanocrys
tals [36].

## Conclusion

In summary, we have developed a low cost and efficient

aqueous-based route to prepare core/shell/shell Mn:ZnS/ZnS/

Fe3O4 nanocrystals possessing both fluorescent and magnetic
properties. Our results showed that increasing the thickness of

the iron oxide shell around the Mn:ZnS/ZnS QDs improved the

magnetic properties of the heterostructured nanomaterials but

negatively affected their luminescence efficiency, and vice

versa. The Mn:ZnS/ZnS/Fe3O4 (1) sample, which exhibited the
best fluorescence quantum yield, also exhibited limited

magnetic characteristics. Nevertheless, samples Mn:ZnS/ZnS/

Fe3O4 (1.5) and Mn:ZnS/ZnS/Fe3O4 (2) are a good compromise to maintain both fluorescence and magnetic properties. We

therefore believe that the synthetic protocol developed in this

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_Beilstein J. Nanotechnol._ 2015, _6,_ 1743–1751.

Figure 7: FC and ZFC thermal variation of the susceptibility of the samples measured in a magnetic field of 500 Oe: (a) Mn:ZnS/ZnS/Fe3O4 (1),
(b) Mn:ZnS/ZnS/Fe3O4 (1.5), Mn:ZnS/ZnS/Fe3O4 (c) (2) and (d) Mn:ZnS/ZnS/Fe3O4 (3). The blocking temperature, _T_ B, is indicated in each graph.

work may pave a reliable way for constructing imaging probes

with good performance and low toxicity for biological applica
tions.

## Experimental

Chemicals
Zinc sulfate heptahydrate (ZnSO4·7H2O, 99.99%), manganese
acetate tetrahydrate (Mn(OAc)2·4H2O, 99%), iron(III) chloride
hexahydrate (FeCl3·6H2O, 99%), sodium sulfide nonahydrate
(Na2S·9H2O, 98+%), 3-mercaptopropionic acid (MPA, 99%)
and isopropanol (HPLC grade) were used as received without

additional purification. All solutions were prepared using MilliQ ® water (18.2 MΩ·cm, Millipore Corporation) as the solvent.

Synthesis of Mn:ZnS/ZnS QDs

The Mn:ZnS/ZnS nanoparticles were prepared in basic aqueous

solution at 100 °C using MPA as a ligand. The synthesis was

carried out according to the procedure detailed in our previous

work [37]. The obtained colloidal solution of Mn:ZnS/ZnS QDs

was labelled as solution A. The Mn:ZnS/ZnS QDs were precipi
tated by adding isopropanol, collected by centrifugation, and

washed three times with isopropanol, then dried in vacuum. The

obtained nanocrystals could be dispersed in water to form a

stable and clear colloidal solution.

Preparation of Fe3O4 colloidal solution
A mixture of 20 mL of FeCl3·6H2O (0.1 M) and 20 mL of the
MPA ligand (0.1 M) was placed in a 100 mL flask and the pH

was adjusted to 11 with 2 M NaOH. The obtained solution was

labelled B. The reaction mixture was then heated to reflux for

2 h and then cooled down to room temperature. A black powder

was recovered by centrifugation after adding isopropanol.

Synthesis of bifunctional core/shell/shell
Mn:ZnS/ZnS/Fe3O4 nanocrystals
The bifunctional Mn:ZnS/ZnS/Fe3O4 QDs were prepared from
the Mn:ZnS/ZnS colloidal solution A. 5.5 mL of solution A was

added to 94.5 mL of ultrapure water and the pH was adjusted to

11 using 2 M NaOH. The mixture was heated to 100 °C and a

specific amount of solution B (1, 1.5, 2 or 3 mL) was injected

dropwise, resulting in the formation of Fe3O4 magnetic layers
of different thicknesses covering the Mn:ZnS/ZnS cores. The

samples obtained were labelled Mn:ZnS/ZnS/Fe3O4 (1), (1.5),
(2) and (3), respectively. After cooling to room temperature and

1749

addition of isopropanol, the Mn:ZnS/ZnS/Fe3O4 particles were
collected by centrifugation. The obtained nanocrystals could be

dispersed in water to form stable magnetic and fluorescent

colloidal dispersions.

Experimental techniques

The X-ray powder diffraction data were collected from an

X'Pert MPD diffractometer (Panalytical AXS) with a

goniometer radius of 240 mm and using Cu Kα radiation

(λ = 0.15418 nm). The average particle size was calculated from

the line broadening using the Debye–Scherrer formula,

_D_ = _K_ λ/β·cos(θ), where _D_ is the average crystallite size, _K_ is the

shape factor taken as 0.9, λ is the X-ray wavelength, β is the full

width at half maximum of the Bragg reflection in radians, and θ

is the diffraction angle.

The transmission electron microscopy images were taken by

placing a drop of the particles dispersed in water onto a carbon

film-supported copper grid, and the size and the shape of the

particles were determined using a Philips CM20 instrument

operating at 200 kV.

The DLS measurements were performed on a ZEN 3600 Zeta
sizer Nano ZS. The nanocrystals were dispersed in water by

sonication and transferred to quartz cuvettes using a syringe.

The absorption spectra were recorded on a Thermo Scientific,

Evolution 220 UV–vis spectrophotometer. The photolumines
cence spectra were recorded on a Horiba, Fluoromax-4, Jobin

Yvon spectrofluorimeter. The QY values were determined from

the following equation:

(1)

where _F_, _A_ and _n_ are the measured fluorescence (area under the

emission peak), absorbance at the excitation wavelength and the

refractive index of the solvent, respectively. The PL spectra

were spectrally corrected and the quantum yields were deter
mined relative to rhodamine 6G in ethanol (QY = 94%).

The magnetic measurements were carried out using the phys
ical properties measurement system (PPMS) from Quantum

Design. The hysteresis loops were recorded at 2 K and 300 K in

the range −9 T to +9 T. The thermal variation of the magnetiza
tion was studied using zero-field-cooled (ZFC) and field-cooled

(FC) procedures under an applied magnetic field of 500 Oe. For

the ZFC/FC measurements, the sample was first cooled without

a field from room temperature to 2 K. Thereafter, a 500 Oe

_Beilstein J. Nanotechnol._ 2015, _6,_ 1743–1751.

magnetic field was applied and the magnetic moment was

recorded upon increasing temperature to obtain the ZFC curve.

For the FC curve, the sample was cooled from 300 K under a

field of 500 Oe and the magnetic moment was recorded upon

decreasing temperature.

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A facile method for the preparation of bifunctional Mn:ZnS/ZnS/Fe<sub>3</sub>O<sub>4</sub> magnetic and fluorescent nanocrystals

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