Electrospinning synthesis of Fe3O4/Eu(DBM)3phen/PVP multifunctional microfibers and their structure, luminescent and magnetic properties

Multifunctional Fe3O4/Eu(DBM)3phen/PVP ((DBM: dibenzoylmethane, phen: 1,10-phenanthroline, PVP: polyvinyl pyrrolidone) microfibers were constructed by simple electrospinning process. The structure and morphology of the microfibers were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images. The diameters of pure PVP microfibers and the microfibers doped only with Fe3O4 nanoparticles (NPs) were uniformly distributed, with an average diameter of about 360 nm. When 3% Eu(DBM)3phen complex and Fe3O4 NPs were both added to the precursor for electrospinning, the microfibers became very inhomogeneous in diameter. The photoluminescent properties of pure Eu(DBM)3phen complex and composite microfibers were also studied. The characteristic emission peaks of Eu3+ appeared in the composite microfibers. The intensities of emission and excitation spectra gradually decrease with adding more Fe3O4 NPs. The unit mass of the pure europium complex in some composite microfibers gave stronger luminescence than the pure europium complex. The fluorescence lifetime of 5D0 state in the composite microfibers is longer than that of pure europium complex. Additionally, the magnetic properties of Fe3O4 NPs and the composite microfibers were investigated. The saturation magnetization of the composite microfibers was smaller than that of pure Fe3O4 NPs.


Introduction
Recently, with the increasing demand for integrated devices, single functional materials have been difficult to meet the needs of modern science and technology. Therefore, multifunctional composites including biomedicine, environmental protection, material science, and many other fields have gradually aroused more and more interests among scientists [1][2][3]. Among such composites, multifunctional luminescent-magnetic composites simultaneously possess excellent magnetism, luminescence, and have been widely used in biomedical applications such as magnetic resonance imaging (MRI), targeted drug delivery, cell labeling and separation [4][5][6]. Zhuang et al. synthesized multifunctional NPs encoded with quantum dots and magnetic NPs for cell tagging and MRI [7]. Li et al. synthesized biomimetic immunofluorescent magnetic multifunctional nanoprobes for isolation and analysis of tumor cell subpopulations [8]. Fu et al. synthesized multifunctional NaYF 4 :-Yb,Er@PE 3 @Fe 3 O 4 nanocomposites for magneticfield-assisted upconversion imaging guided photothermal therapy of cancer cells [9].
In recent years, organic dyes, quantum dots and rare-earth complexes have been employed as luminescent materials in the luminescent-magnetic composites. However, organic dyes and quantum dots have limited use in large-scale applications due to their short lifetime photobleaching and potential toxicity to cells [10,11]. Rare-earth complexes have some excellent luminescent properties, such as a long fluorescence lifetime, sharply spiked emission spectra, large stokes shift, and high quantum yield [12]. However, pure rare-earth complexes usually do not have good thermal stability, mechanical stabilities, and machinability, which limits the prospects of these complexes in a wide range of photophysical and other practical applications. To overcome these shortcomings, rare-earth complexes were usually incorporated into organic, inorganic, or organicinorganic hybrid matrixes, such as mesoporous materials, sol-gel silica, and polymers [13][14][15].
Magnetic nanomaterials have attracted increasing interest of scientists owing to their potential applications, such as biomacromolecule separation, drug delivery and release and MRI [16,17]. Among the magnetic materials, Fe 3 O 4 has the advantages of high saturation magnetization, non-toxicity, and good biocompatibility, and is widely used in the field of biomedicine [18]. Therefore, Fe 3 O 4 NPs have been employed as magnetic materials in the luminescentmagnetic composites.
Based on the above analysis, in this work, Fe 3 O 4 / Eu(DBM) 3 phen/PVP multifunctional microfibers were synthesized via electrospinning process. The synthesis procedure of the multifunctional microfibers is shown in Fig. 1 microfibers were prepared via electrospinning process. 0.5 g PVP was dissolved in 4.5 g ethanol at a mass concentration of 10%. The solution was stirred for 24 h and then 0.015 g Eu(DBM) 3 phen complex was dissolved in that solution under magnetic stirring for 10 min. The mass percentage of Eu(DBM) 3phen complex to PVP is 3%. A certain amount of Fe 3 O 4 NPs were dispersed in the solution under ultrasonic. The mass ratios of Fe 3 O 4 NPs to PVP for various samples were 0%, 10%, 20%, 30%, respectively. The electrospinning process was carried out in air at room temperature, similar to that depicted before [29]. The composite microfibers with Fe 3 O 4 NPs accounting for 0%, 10%, 20% and 30% of PVP mass were labeled as samples 0F3E, 10F3E, 20F3E and 30F3E, respectively. For comparison, pure PVP microfibers and the composite microfibers containing 20% Fe 3 O 4 NPs of PVP were prepared by the same method, and the composite microfibers were labeled as sample 20F0E.

Characterizations
The data of XRD, the energy-dispersive analysis of x-ray (EDS), The infrared (IR) absorption spectra, FE-SEM and TEM images were measured according to our previous work [30]. The fluorescence dynamics, excitation and emission spectra were recorded using an Edinburgh FLS980 spectrophotometer. The hysteresis loops of the samples were measured on a physical property measurement system (PPMS-9 T) manufactured by Quantum Design, Inc.

Structure and morphology of Fe 3 O 4 / Eu(DBM) 3 phen/PVP multifunctional microfibers
The XRD patterns of different samples are shown in Fig. 2 NPs was estimated to be 83 nm, and the value was larger than that calculated using the Debye-Scherrer's equation. The Debye-Scherrer's equation calculated the crystallite size, while SEM image gave the particle size. A particle may consist of more than one crystal. In addition, lattice distortion can also cause the width of the diffraction peak. Therefore, the crystallite size estimated using the Debye-Scherrer's equation was smaller. Figure 4 shows the SEM images of pure PVP fibers, samples 20F0E, 0F3E and 20F3E. The diameters of pure PVP microfibers were uniformly distributed, with an average diameter of about 360 nm. The diameters of the microfibers doped only with Fe 3 O 4 nanoparticles were still uniformly distributed (Fig. 4b), and the average diameter was consistent with that of the pure PVP microfibers. However, the surface of the microfibers became less smooth, and some humps appeared (inset of Fig. 4b), which were caused by Fe 3 O 4 NPs in the microfibers. When 3% Eu(DBM) 3 phen complex and Fe 3 O 4 NPs were both added to the precursor for electrospinning, the microfibers became very inhomogeneous in diameter (Fig. 4c, d). The diameters of the microfibers were between 100 and 700 nm. The addition of europium complex may change the conductivity and viscosity of precursor solution and make the size of microfibers ununiform.
In order to further prove that Fe 3 O 4 NPs have been successfully doped into the composite microfibers, TEM images of sample 20F3E are given in Fig. 5. As shown in Fig. 5, Fe 3 O 4 NPs in the composite microfibers can be clearly seen, indicating that Fe 3 O 4 NPs were successfully doped into PVP microfibers. Table 1 shows the energy spectrum test results of sample 20F3E. There are iron and europium elements in the composite microfibers, which also indicates that Fe 3 O 4 NPs were successfully doped into the microfibers.
FTIR spectra of the different samples are shown in Fig. 6. For the PVP fibers, a broad band at about 3400 cm -1 corresponding to the vibrations of hydroxyl groups appeared. The PVP fibers contain trace hydroxyl group due to the electrospinning solution.
The broad band at about 2900 cm -1 , and the peaks at 1649 cm -1 , 1421 cm -1 , 1279 cm -1 were attributed to the C-H, and C=O, C-N, C-C vibrations, respectively [32]. For pure Eu(DBM) 3 phen complex, except for the vibrations of C-H (3000 cm -1 ), C=O (1601 cm -1 ), C-N (1400 cm -1 ), some absorption bands (607 cm -1 , 725 cm -1 , 853 cm -1 , 937 cm -1 , 1059 cm -1 ) due to the vibrations of the benzenoid ring appeared. However, the vibrations of the benzenoid ring disappeared in the IR spectra of samples 20F3E and 0F3E. The FTIR spectra of the composite microfibers were similar to that of pure PVP fibers, indicating that Eu(DBM) 3phen complex in the composite microfibers was encapsulated in the PVP matrix.

Excitation and emission spectra
The excitation and emission spectra of various samples are shown in Fig. 7. The intensity of the emission spectra was weaker than that of the excitation spectra. The emission spectra of all the samples were expanded by a factor of 10. Compared with the composite microfibers, the excitation and emission spectra of pure europium complex were the strongest. It can be obviously seen that the intensities of emission and excitation spectra gradually decrease with adding more Fe 3   which resulted in a decrease in the excitation and emission spectra. Nevertheless, the composite microfibers can also emit bright red light (Fig. 8a), although the luminescence is weaker than that of the pure Eu(DBM) 3 phen complex (Fig. 8b).
As shown in Fig. 7, in the pure complex, a wide excitation band ranging from 230 to 510 nm appears. In the composite microfibers, it is interesting to observe that the excitation band is divided into two parts, with peaks at about 274 and 357 nm,  respectively. This suggests that the site symmetry of the composite microfibers is reduced [33]. The excitation peaks of 7 F 0 -5 D 2 and 7 F 1 -5 D 1 appear in the excitation spectra of pure Eu(DBM) 3 phen complex, but disappear in the excitation spectra of composite fibers. This indicates that in the composite microfibers, the f-f inner shell transitions were quenched by the transfer of nonradiative energy from the highly excited state to some uncertain defect levels, replacing the nonradiative relaxation to 5 D 0 process. The emission spectra of different samples were tested under the excitation of 365 nm light. The emission peak positions of the composite microfibers were the same as that of the europium complex. The intensities of emission spectra gradually decrease with adding more Fe 3 O 4 NPs. The characteristic emission peaks of Eu(DBM) 3 phen complex were observed, which located at 579, 590 and 612 nm. These peaks were attributed to the 5 D 0 -7 F 0 (579 nm), 5 D 0 -7 F 1 (590 nm), and 5 D 0 -7 F 2 (612 nm) energy level transitions of Eu 3? , and the red-light emission at 612 nm is dominant.
The relative fluorescence intensity ( 5 D 0 -7 F 2 ) in different samples are listed in Table 2. By comparing the intensity and the concentration of different samples, we can conclude that the unit mass of europium complex in samples 0F3E, 10F3E and 20F3E gave stronger luminescence than the pure europium complex, implying the improvement of outer luminescence efficiency in samples 0F3E, 10F3E and 20F3E [34]. Owing to the strong light absorption ability of Fe 3 O 4 NPs, with adding more Fe 3 O 4 NPs, the unit mass of the pure europium complex gave weaker and weaker luminescence. The unit mass of europium complex in sample 30F3E gave weaker luminescence than that of pure Eu(DBM) 3 phen complex due to the influence of Fe 3 O 4 NPs on luminescence. When europium complexes are added to PVP matrix, PVP inhibits the vibrational transition. Consequently, more energy was transferred to Eu 3? , leading to the improvement of photoluminescence.  Table 2, the exponential lifetimes are 396,

Possible energy transfer process
The energy level diagram of phen, DBM, Eu 3? and the possible energy transfer mechanism is shown in Fig. 10. As shown in Fig. 10, both DBM and phen absorbed the 365 nm excited light and populated their excited singlet states. The excited phen and DBM molecules undergo a nonradiative transition to their longer lived triplet state. The energy transfer also occurred between the two ligands due to the short distance. Furthermore, the intramolecular energy transfer efficiency from organic ligands to Eu 3? is the most important factor influencing the luminescence properties of Eu(DBM) 3 phen complex. The larger the energy difference between the ligands and Eu(DBM) 3 phen complex, the lower the luminescence efficiency of Eu(DBM) 3 phen complex [36]. The energy transfer from phen to Eu 3? was not very efficient. The energy was transferred from phen to DBM. Then, the partial energy from the absorption of DBM and the transfer of phen was transferred to 5 D 1 and 5 D 0 excited state of Eu 3? . The luminescence lifetime of 5 D 0 level is much longer than that of 5 D 1 level. Therefore, it is easy for the electrons of 5 D 1 level to relax to 5 D 0 level through energy relaxation. Finally, the radiative transition ( 5 D 0 -7 F J ) was achieved. Some energy absorbed by the ligands was lost by non-radiative transition.

D 1
Eu 3+  3 phen complex and composite microfibers were also studied. The characteristic emission peaks of Eu(DBM) 3 phen complex were observed, which located at 579, 590 and 612 nm, and the red-light emission at 612 nm is dominant. The luminescence of the composite microfibers is weaker than that of the pure europium complex.
With adding more Fe 3 O 4 NPs, the unit mass of the pure Eu(DBM) 3 phen complex gave weaker luminescence. The fluorescence lifetime of the composite fibers is longer than that of pure Eu(DBM) 3 phen complex. This new kind of microfibers possess both magnetism and luminescence, which makes it a material with great potential for application in targeted drug delivery, biomedicine and biochemistry and so on. 212102210312, 20B140009, 19A430020 and 21A140016).

Funding
The financial support from the National Natural Science Foundations of China, Grant No (51802139, 11905096 and 51801092). Natural Science Foundation of Henan Province, Grant No (18B140007, 20B140009, 19A430020 and 21A140016).

Declarations
Conflict of interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.