Janus nanofiber array pellicle: facile conjugate electrospinning construction, structure and bifunctionality of enhanced green fluorescence and adjustable magnetism

A [Fe3O4/polyvinyl pyrrolidone (PVP)]//[Tb(BA)3phen/PVP] Janus nanofiber array pellicle (denoted JNAP) was successfully constructed by facile conjugate electrospinning without twisting for the first time. The JNAP offers the dual-functionality of fluorescence and magnetism. This technology entirely solves the dilemma of the magnetic spinning dope and fluorescent spinning dope being easily mixed together during the parallel electrospinning process, as it achieves complete segregation of magnetic nanoparticles and fluorescent molecules. Moreover, conjugate electrospinning without twisting has fewer requirements on the viscosity of the spinning dope compared with parallel electrospinning, in which the two spinning dopes should have the same viscosity. It was satisfactorily found that the JNAP has higher fluorescence intensity than the corresponding non-aligned Janus nanofiber pellicle. The magnetism of the JNAP could be tailored by changing the doping amount of the Fe3O4 NPs. The JNAP has potential applications in nanotechnology and biomedicine, etc., due to its enhanced green fluorescence and adjustable magnetism. In addition, this design concept and manufacturing process provide a facile way for preparing other one-dimensional Janus nanomaterials with multifunctionality.


Introduction
It is becoming harder to meet the needs of many emerging practical technologies using single-functional materials alone. Therefore, research into the preparation and performance of multifunctional materials has become increasingly important in the realm of materials science and technology. [1][2][3] Fluorescent-magnetic bifunctional nanomaterials have attracted much attention of researchers due to their potential uses in cell imaging, cancer studies, sensing and biomolecular detection, etc. [4][5][6][7][8][9] For instance, uorescent-magnetic bifunctional nanomaterials are ideal candidates for drug delivery because the loaded drug can be transported to a specic location using magnetic navigation, and meanwhile, the real-time position of the drug can be monitored by measuring uorescence signals emitted from the uorescent-magnetic bifunctional nanomaterial. 10,11 As another example, uorescent-magnetic bifunctional nanomaterials possess the property of dual-mode imaging, including uorescent imaging and magnetic resonance imaging, which is benecial for biological detection. 12,13 However, researchers in this eld must face the plight that if the magnetic nanoparticles and uorescent molecules are directly mixed, the uorescence intensity of the sample will be greatly reduced. 14,15 Hence, exceptional structures need to be designed and constructed to separate magnetic nanoparticles from uorescent molecules in an integrated system. Inspired by Janus, a god in Roman mythology, an exceptive "Janus structure" has been proposed by several research groups. 16,17 Researchers have proposed that magnetic nanoparticles and uorescent molecules should be respectively added to both sides of a Janus material in order to isolate two different substances from each other and reduce the detrimental inuence of magnetic nanoparticles on the uorescence intensity. 18,19 Magnetic-uorescent bifunctional Janus nanobers are typical Janus materials that realize separation of magnetic nanoparticles from uorescent molecules. According to the literature, Janus nanobers can be gained by parallel electrospinning, where two kinds of spinning dopes are respectively loaded into two syringes to generate magnetic-uorescent bifunctional Janus nanobers using a specially-made parallel spinneret under electrospinning. [20][21][22][23][24][25][26][27][28][29] Xi, et al. 30,31 fabricated exible magnetic-photoluminescent bifunctional Janus nanobers by parallel electrospinning. It has been proven that such Janus nanobers have stronger uorescence intensity than the counterpart composite nano-bers. However, parallel electrospinning still has a drawback in that it is difficult to fulll complete segregation of the magnetic nanoparticles and the uorescent molecules, since the two spinning dopes are easily mixed at the outlet of spinneret. 32 Consequently, it is a pressing subject of study to develop a new technique to overcome this drawback.
Conjugate electrospinning is an excellent technique for constructing one-dimensional nanomaterials. This technique can be divided into two kinds: conjugate electrospinning with twisting [33][34][35] and that without twisting. 36,37 Conjugate electrospinning with twisting is used to prepare nanober yarns which can be applied to tissue repair, nerve regeneration and electrically conductive material, etc. [38][39][40] Recently, this method has been utilized to construct magnetic-uorescent bifunctional nanober yarns. Fan, et al. used conjugate electrospinning with twisting to fabricate heterogeneous nanober yarns to effectively separate magnetic nanoparticles from uorescent molecules. 41,42 To date, there are only a few studies on conjugate electrospinning without twisting, which mainly focus on constructing materials for photocatalysis, and those with mechanical and waterproof properties. [43][44][45] A magnetic-uorescent bifunctional Janus nanober array pellicle built by conjugate electrospinning without twisting has not been reported.
In this work, polyvinyl pyrrolidone (PVP), Fe 3 O 4 nanoparticles (NPs) and Tb(BA) 3 phen were respectively used as a template, magnetic material and uorescent compound. Tb(BA) 3 phen possesses excellent uorescence properties due to the unique f-f transition of Tb 3+ . It has become one of the most important uorescent materials at present. Fe 3 O 4 NPs can be widely used in many elds, such as targeted therapy, magnetically-controlled switches, electronics and biological processes due to their unique superparamagnetism, good biocompatibility and high permeability. A [Fe 3 O 4 /PVP]//[Tb(BA) 3 phen/PVP] Janus nanober array pellicle (abbreviated as JNAP) with magnetic-uorescent bifunctionality was constructed by conjugate electrospinning without twisting (called conjugate electrospinning hereinaer). To highlight the excellent performance of the JNAP, a series of comparative samples were also constructed by conjugate electrospinning and parallel electrospinning. Finally, the as-prepared samples were systematically characterized using the relevant test instruments, and several new ndings were obtained.

Electrospinning process
Two different types of spinning dopes (named spinning dope A and spinning dope B) were used to prepare the Janus nano-bers. Spinning dope A, with uorescence properties, was prepared as follows: PVP (1 g) was fully dissolved in ethanol (7 g) under magnetic stirring, and then a certain amount of Tb(BA) 3 phen complex was uniformly dispersed in the solution to form spinning dope A. Spinning dope B, with magnetic properties, was fabricated as follows: Fe 3 O 4 NPs were dispersed in ethanol (7 g) under ultrasonication for 20 min, and then PVP (1 g) was added into the above suspension under mechanical stirring. The actual ingredients of spinning dope A and B are respectively shown in Tables 1 and 2. A device diagram for constructing the [Fe 3 O 4 /PVP]// [Tb(BA) 3 phen/PVP] JNAP by conjugate electrospinning is presented in Table 3. The spinning dopes A and B were separately loaded into two syringes with plastic spinnerets. The angle between the syringe and the horizontal line was ca. 45 in the conjugate electrospinning device. To obtain the array pellicle, a rotating drum was used as a collector. The corresponding spinning dopes and detailed spinning conditions are summarized in Table 3.
For comparison, a [Fe 3 O 4 /PVP]//[Tb(BA) 3 phen/PVP] Janus nanober non-array pellicle (referred to as JNNP) was prepared by conjugate electrospinning with a wire mesh as the collector, and a parallel electrospinning-made [Fe 3 O 4 /PVP]//[Tb(BA) 3phen/PVP] Janus nanober array pellicle (named P-JNAP) was constructed by using spinning dopes S A3 and S B1 . Another spinning dope was prepared by evenly mixing equal volumes of  Table 3.

Characterization methods
The phase compositions of Fe 3 O 4 NPs, JNAP, JNNP, HNAP and P-JNAP were analyzed using X-ray power diffraction (XRD) with Cu Ka radiation. The operation voltage and current were respectively kept at 40 kV and 20 mA. The morphology of the JNAP was observed using a eld-emission scanning electron microscope (FESEM), equipped with energy-dispersive X-ray spectroscopy (EDS). A Hitachi uorescence spectrophotometer F-7000 was used to investigate the uorescence of different samples when the excitation and emission slits were 2.5 nm and 2.5 nm. The magnetic properties of the samples were measured using a vibrating sample magnetometer (VSM).

Results and discussion
Phase compositions

Morphological and structural analyses
SEM images, EDS spectra and diameter distribution histograms of the Janus nanobers in JNAP are given in Fig. 2. As seen in Fig. 2a, the nanobers are in good directional alignment and form an array pellicle. It can be seen in Fig. 2b that exceptivestructured Janus nanobers are obtained, with every single Janus nanober being composed of two nanobers bound together side-by-side. As shown in Fig. 2b and c, the diameter of each single nanober in the Janus nanobers is almost equal (the average diameter of a single nanober is ca. 720 nm). The EDS spectrum of the JNAP (Fig. 2d) indicates that the Janus nanobers are composed of the elements C, N, O, Fe, Tb and Pt. The Pt peak is attributed to the conductive lm sprayed on the surface of the sample for SEM observation. Fig. 2e presents the EDS line-scan analysis results for the Janus nanobers, where Fe and Tb elements indicate Fe 3 O 4 and Tb(BA) 3 phen, respectively. The Fe and Tb elements are respectively found in two single nanobers, further demonstrating that the Janus nano-bers were successfully constructed by conjugate electrospinning and the goal of segregating magnetic nanoparticles and uorescent molecules has been realized.

Fluorescence performance
A series of JNAP samples (the mass ratio of Fe 3 O 4 NPs to PVP was 0.5 : 1, and the mass percentages of Tb(BA) 3 phen to PVP were 5%, 10%, 15%, 20% and 25%, respectively) were prepared to nd the optimal mass percentage of Tb(BA) 3 phen to PVP.  transition at 545 nm (green light) is the predominant emission peak. The uorescence intensity of the JNAP rst enhances and then weakens with the increase of Tb(BA) 3 phen content, as revealed in Fig. 3. When the mass percentage of Tb(BA) 3 phen to PVP is 15%, the JNAP achieves the highest uorescence intensity. When the mass percentage of Tb(BA) 3 phen to PVP exceeds 15%, the distribution of the Tb(BA) 3 phen becomes denser in the polymer matrix, resulting in stronger non-radiative transitions amongst the Tb 3+ ions, which causes obvious reductions in the uorescence intensity. Therefore, the optimum mass percentage of Tb(BA) 3 phen to PVP is 15%. Physical photographs of the JNAP are displayed in Fig. 4. From Fig. 4a-c, it can be seen that the JNAP can be easily bent by hand and also possesses the ability of self-recovery, which proves that the JNAP is exible. Fig. 4d shows a camera photograph of the JNAP under 275 nm UV illumination in darkness, indicating that the JNAP can emit green uorescence.
The uorescence decay curves of Tb 3+ ions in the JNAP samples doped with different amounts of Tb(BA) 3 phen are shown in Fig. 5. The excitation wavelength is set to be 275 nm and the monitoring wavelength is 545 nm. It is generally known that the uorescence decay curves follow the single-exponential decay: 48 where I t signies the intensity at time t, I 0 represents the intensity at t ¼ 0 and i symbolizes the decay lifetime. The average lifetime values (i/ms) of the JNAP are revealed in Fig. 5. As the content of Tb(BA) 3 phen increases, the uorescence lifetime of the 5 D 4 / 7 F 5 (545 nm) transition is gradually decreased. The introduction of more Tb(BA) 3 phen leads to a reduced distance between Tb 3+ ions in Tb(BA) 3 phen molecular clusters in the JNAP. Thus, the energy transfer among Tb 3+ ions is increased, and the uorescence lifetime of Tb 3+ is shortened. 49 A series of JNAP samples (the mass percentage of Tb(BA) 3phen to PVP was 15% and the mass ratios of Fe 3 O 4 NPs to PVP were 0.5 : 1, 1 : 1, 1.5 : 1, and 2 : 1, respectively) were prepared to explore the effect of different amounts of Fe 3 O 4 NPs on the uorescence intensity. The intensities of the excitation and emission peaks gradually decrease along with increasing content of Fe 3 O 4 NPs added into the JNAP, as illustrated in Fig. 6. As previously reported, dark-colored Fe 3 O 4 NPs can absorb visible light (400 nm < l < 700 nm) and ultraviolet light (l < 400 nm). 41 Therefore, Fe 3 O 4 NPs can absorb the excitation   light and emission light of the JNAP, and the degree of absorption becomes stronger as the content of Fe 3 O 4 NPs is increased. Moreover, the more Fe 3 O 4 NPs added into the JNAP, the darker the color of the products and the more intense the absorption of excitation emission light by the Fe 3 O 4 NPs, as revealed in Fig. 7, giving rise to lower uorescence intensity.
Furthermore, the uorescence intensities of samples with the same ingredients and content prepared by different methods were compared to highlight the superiority of the JNAP. The JNAP has the highest uorescence intensity compared with three comparative samples. It has stronger uorescence intensity than the P-JNAP, and HNAP possesses the lowest uorescence intensity, as shown in Fig. 8. This outcome can be explained by three aspects: the special Janus structure, the arrangement of Janus nanobers, and the electrospinning method. Fig. 9 presents schematic graphs of the excitation and emission light for the JNAP, JNNP, P-JNAP and HNAP. As seen from Fig. 9a, the JNAP is composed of aligned Janus nanobers, and the Janus nanobers are closely arranged. The Janus nanobers in the upper layer can directly absorb excitation light and the produced emission light can be directly emitted without refraction or reection, which is the most effective mode for uorescence. Furthermore, the Fe 3 O 4 NPs and Tb(BA) 3 phen molecules are absolutely segregated, leading to a reduced effect of the Fe 3 O 4 NPs on the uorescence performance. As depicted in Fig. 9b, the Fe 3 O 4 NPs and Tb(BA) 3 phen molecules are also completely segregated in the JNNP. The surface of the JNNP is loose due to the unordered arrangement of Janus nanobers. In this case, some of the excitation light can pass through the gaps   Paper in the upper layer and thereby excite the Janus nanobers in lower layer. In this process, the Janus nanobers in the upper layer can absorb some of the excitation light, and thus, the excitation light arriving to the Janus nanobers in the lower layer is weakened. For the same reason, the emission light emitted from the lower layer is also absorbed by the upper layer in the JNNP. Hence, the uorescence intensity of the JNNP is lower than that of the JNAP. P-JNAP was prepared by parallel electrospinning. Fig. 10 describes the actual situation of two spinning dopes in parallel spinneret during the parallel electrospinning process. Inside the parallel spinneret, two spinning dopes mutually diffuse at the contact interfaces, so that diffusion regions exist at the contact interfaces between nanobers in the Janus nanobers. This results in an incomplete separation of Fe 3 O 4 NPs from Tb(BA) 3 phen molecules in the Janus nanobers. Thus, the uorescence intensity of P-JNAP is lower than that of JNAP. As seen in Fig. 9d, the HNAP is totally made up of homogeneous nanobers where Fe 3 O 4 NPs and Tb(BA) 3 phen molecules are directly mixed, so that the uorescence intensity of the HNAP is the lowest. The above new ndings thoroughly prove that conjugate electrospinning has more advantages than parallel electrospinning in the fabrication of Janus nanobers.

Magnetic properties
Typical hysteresis loops of the Fe 3 O 4 NPs, the JNAP with different mass ratios of the Fe 3 O 4 NPs and comparative samples are exhibited in Fig. 11, and corresponding saturation magnetization results are summarized in Table 4. It is known that the saturation magnetization of a magnetic compound material depends on the doping mass ratio of the magnetic material. As can be seen in Table 4, the saturation magnetization of the Fe 3 O 4 NPs reaches 37.99 emu g À1 . As the content of the Fe 3 O 4 NPs increases in the JNAP, the saturation magnetization of the JNAP also increases from 8.93 to 18.99 emu g À1 . Thus, the JNAP possesses adjustable magnetism via changing the doping amount of the Fe 3 O 4 NPs. The magnetism of the comparative samples is similar to that of the JNAP (prepared by S A3 /S B1 ) owing to the same content of Fe 3 O 4 NPs in the samples.    Formation mechanism for the JNAP Schematic graphs of the formation mechanism of Janus nano-bers and the JNAP are presented in Fig. 12. Under the effect of an electrostatic eld, the spinning dopes in the two spinnerets form two bundles of continuous nanobers with positive and negative charges aer positive and negative power are applied to the two spinnerets, respectively, as seen in Fig. 12a. Here, an electric eld is formed between the two spinnerets when positive and negative charges accumulate at the tip of the two spinnerets, and the directions of the electric eld lines are indicated by the dotted line (from the positive pole to the negative pole), as shown in Fig. 12b. Magnetic nanobers and uorescent nanobers attract each other along the path of the electric eld. Aer the two kinds of nanober bundles meet in the middle of the two spinnerets, the electrical charges in the two kinds of nanober bundles are neutralized, and the formed Janus nanobers move downward by gravity. Due to electrostatic repulsion, the same nanobers (e.g. magnetic nanober and uorescent nanober) cannot get together to form a Janus nanober in this process. Therefore, [Fe 3 O 4 /PVP]//[Tb(BA) 3phen/PVP] Janus nanobers are obtained. Fig. 12c describes the process of collecting an electrically neutral Janus nanober bundle with the rotating drum. At this point, the Janus nano-bers are affected by the vortex generated by the rotating drum in the experimental environment, so that the Janus nanobers shake slightly and are drawn to and wrap around the rotating drum to form the JNAP.

Conclusions
In

Conflicts of interest
There are no conicts of interest to declare.