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

Bifunctional magnetic and fluorescent core/shell/shell Mn:ZnS/ZnS/Fe3O4 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/Fe3O4 quantum dots (QDs) showed marked visible emission around 584 nm related to the 4T1 → 6A1 Mn2+ 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 properties, which are not observed in bulk semiconductor materials [1][2][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 infrared wavelengths, and high resistance to chemical degradation [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][12][13][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][16][17]. Such bifunctional nanoparticles 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 applications.
Epitaxial heterocrystalline growth is generally conducted by coating a ferro-or ferri-magnet that has an ordering temperature well above 300 K (e.g., FePt, Fe 2 O 3 , or Fe 3 O 4 ) with a semiconductor shell (CdSe or CdS) of thickness between 2-7 nm resulting in either spherical core/shell nanoparticles or heterodimers [20][21][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].
Herein, we describe a facile synthesis of epitaxial heterocrystalline nanoparticles, consisting of an Fe 3 O 4 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 Fe 3 O 4 layer was next grown on the preformed Mn:ZnS/ ZnS QDs in aqueous solution. The resulting bifunctional, core/ shell/shell Mn:ZnS/ZnS/Fe 3 O 4 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  [26,27]. Note that this sample was prepared using FeCl 3 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/Fe 3 O 4 (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/Fe 3 O 4 QDs (2) and (3) exhibit additional, low intensity peaks that were not observed in the case of Mn:ZnS/ZnS/Fe 3 O 4 (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/ Fe 3 O 4 (3) sample, the appearance of additional peaks of very low intensity originating from hematite α-Fe 2 O 3 (JCPDS record number 99-100-0140) can also be observed [26]. Since the   surface of finely divided materials is highly reactive, partial oxidation of Fe 3 O 4 into Fe 2 O 3 may have taken place during the handling of the nanocrystals [28,29]. The crystallites sizes of the Mn:ZnS/ZnS/Fe 3 O 4 nanoparticles were calculated using the Scherrer formula based on the width of the most intense (111) diffraction peak (Table 1).     [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 T 1 → 6 A 1 ligand field transition [23,24,31].  Table 2.
The highest value of 2.67 emu/g was obtained for the sample Mn:ZnS/ZnS/Fe 3 O 4 (2). This value was remarkably lower than the saturation magnetization of bulk phase Fe 3 O 4 (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, H C , 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] [34,35].
The zero-field-cooled (ZFC) and field-cooled (FC) measurements were performed under an applied field of 500 Oe   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 behavior 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/Fe 3 O 4 (1) at 20 K for Mn:ZnS/ZnS/Fe 3 O 4 (2). These results are in agreement with the size increase of the nanocrystals [36].

Conclusion
In summary, we have developed a low cost and efficient aqueous-based route to prepare core/shell/shell Mn:ZnS/ZnS/  work may pave a reliable way for constructing imaging probes with good performance and low toxicity for biological applications.

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 = Kλ/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 Zetasizer 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 photoluminescence 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 determined relative to rhodamine 6G in ethanol (QY = 94%).
The magnetic measurements were carried out using the physical 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 magnetization 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 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.