Flexible Janus nanofibers: a feasible route to realize simultaneously tuned magnetism and enhanced color-tunable luminescence bifunctionality

Abstract

Novel magnetic-fluorescent bifunctional Janus nanofibers with high luminescent intensity and tunable luminescence color have been successfully fabricated by electrospinning technology using a specially designed parallel spinneret. The Janus nanofiber is composed of Fe₃O₄/polyvinyl pyrrolidone (PVP) as one strand of the nanofiber and [Dy(BA)₃phen + Eu(BA)₃phen]/PVP as the other strand of the nanofiber. The morphology and properties of the final products have been investigated in detail by X-ray diffractometry (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM) and photoluminescence (PL) spectroscopy. The results reveal that the Janus nanofibers possess superior magnetic and luminescence properties due to their special nanostructure. Tunable colors from greenish blue to white to yellowish pink can be realized in the flexible Janus nanofibers by varying the mass ratio of Dy(BA)₃phen to Eu(BA)₃phen, and furthermore, it is the first time that white-light-emitting flexible Janus nanofibers have been achieved using rare earth complexes as luminescent centers. The impact of different amounts of Fe₃O₄ nanoparticles on the luminescence color and intensity of the Janus nanofibers is in-depth investigated. The new type of magnetic and color-tunable bifunctional Janus nanofibers have potential applications in the fields of bio-medicine, nanotechnology, and color displays, etc. due to their excellent magnetic-fluorescent properties, tunable color and flexibility.

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Introduction

Nowadays, color-tunable luminescent materials have attracted considerable attention owing to their wide range of applications, such as light-emitting diodes (LEDs), transducers, resonators, flat panel displays and full-color displays.^1–5 Generally, color-tunable luminescent materials and white light-emitting materials are prepared by using Dy³⁺ and Eu³⁺ ion (rare earth ion) doped inorganic compounds as the luminescent centers due to the abundant emission colors based on their 4f electrons transitions, such as NaGaF₄:Dy³⁺,Eu³⁺ nanophosphors, SrAl₂O₄:Dy³⁺,Eu³⁺ phosphors, and GdNbO₄:Dy³⁺,Eu³⁺ phosphors, etc.^6–9 As far as we know, there are no reports concerning the preparation of Dy(BA)₃phen and Eu(BA)₃phen complexes doped one-dimensional (1D) composite nanofibers. Therefore, it is a worthwhile subject of study to explore new-typed of 1D color-tunable luminescent and white light-emitting nanomaterials.

Magnetic-luminescent bifunctional composite nanomaterials have been applied in medical diagnostics, optical imaging, nanodevice, etc.^10–14 In recent years, some preparations of Fe₃O₄@rare earth (RE) complex core–shell structure nanoparticles (NPs) have been reported.^15–19 At present, some 1D magnetic-luminescent bifunctional nanomaterials have been prepared, including Fe₂O₃/Eu(DBM)₃(Bath)/PVP composite nanofibers, Fe₃O₄/Eu(BA)₃phen/PMMA composite nanoribbons, etc.^20,21 In their papers, however, magnetic-luminescent bifunctional composite nanomaterials usually have poor fluorescence properties because Fe₃O₄ or Fe₂O₃ NPs directly contacts with the RE luminescent compounds, which restrict the use of these magnetic–luminescent bifunctional nanomaterials to promising extensive photophysical applications and practical uses. Therefore, luminescent and magnetic materials should be effectively isolated to avoid direct contact if the strong luminescence of the magnetic and color-tunable bifunctional composite nanofibers is achieved. We were inspired by the reports on the Janus particles.²² Janus particles have two distinguished surfaces/chemistries on the two sides. Upon the unique feature of the asymmetry dual-sided Janus structure, we have successfully designed and fabricated the Janus nanofibers in our previous work.^23–25

Electrospinning represents an outstanding technique to process viscous solutions or melts into continuous fibers or ribbons with 1D nanostructure.^26–28 The electrospun products have been applied in many areas such as filtration, optical and chemical sensors, biological scaffolds, electrode materials, drug delivery materials, photocatalysts and nanocables.^29–35 Accordingly, here we employed electrospinning method to prepare magnetic and color-tunable bifunctional [Fe₃O₄/PVP]//[(Dy(BA)₃phen + Eu(BA)₃phen)/PVP] Janus nanofibers with new 1D structure in this paper. For the Janus nanofibers, its one strand nanofiber is composed of template PVP containing Fe₃O₄ NPs (namely Fe₃O₄/PVP nanofiber), and the other strand nanofiber consists of PVP containing Dy(BA)₃phen and Eu(BA)₃phen complexes (namely [Dy(BA)₃phen + Eu(BA)₃phen)]/PVP nanofiber). This new-type 1D nanostructure can successfully realize the effective separation of Fe₃O₄ NPs from the RE complexes (include Dy(BA)₃phen and Eu(BA)₃phen complexes), and this great morphology of bifunctional nanofibers will be obtained with excellent magnetism and color-tunable luminescence. To the best of our knowledge, the novel magnetic and color-tunable bifunctional Janus nanofibers have not been found in any literature. Full characterization and detailed studies of the magnetic and color-tunable properties of these Janus nanofibers were discussed.

Experimental sections

Materials

Polyvinyl pyrrolidone (PVP, M_w ≈ 90 000), benzoic acid (BA), 1,10-phenanthroline (phen), FeCl₃·6H₂O, FeSO₄·7H₂O, NH₄NO₃, polyethyleneglycol (PEG, M_w ≈ 20 000), ammonia, anhydrous ethanol, oleic acid (OA) and N,N-dimethylformamide (DMF) were of analytical grade. The purity of Eu₂O₃ and Dy₂O₃ was 99.99%. All chemicals were directly used as received without further purification.

Preparation of Fe₃O₄ NPs by coprecipitation method

Fe₃O₄ NPs were obtained via a facile coprecipitation synthetic method,³⁶ and PEG was used as the protective agent to prevent the particles from aggregation. One typical synthetic procedure was as follows: 8.0800 g of Fe(NO)₃·9H₂O, 2.7800 g of FeSO₄·7H₂O, 4.0400 g of NH₄NO₃, and 1.9000 g of PEG were added into 100 mL of deionized water to form uniform solution under vigorous stirring at 50 °C. To prevent the oxidation of Fe²⁺ ions, the reactive mixture was kept under argon atmosphere. After the mixture had been bubbled with argon for 30 min, 0.1 mol L⁻¹ of NH₃·H₂O was added dropwise into the mixture to adjust the pH value above 11. Then the system was continuously bubbled with argon for 20 min at 50 °C, and black precipitates were formed. The precipitates were collected from the solution by magnetic separation, washed with the mixed solution of deionized water and anhydrous ethanol for three times, and then dried in an electric vacuum oven at 60 °C for 12 h.

To improve the monodispersity, stability and solubility of Fe₃O₄ NPs in the spinning solution, the as-prepared Fe₃O₄ NPs were coated with OA as below: 2.0000 g of the as-prepared Fe₃O₄ NPs were ultrasonically dispersed in 100 mL of deionized water for 20 min. The suspension was heated to 80 °C under argon atmosphere with vigorous mechanical stirring for 30 min, and then 1.5 mL of OA was slowly added into the above suspension. Reaction was stopped after heating and stirring the mixture for 40 min. The precipitates were removed from the solution by magnetic separation, and then dried in an electric vacuum oven at 60 °C for 6 h.

Synthesis of Dy(BA)₃phen and Eu(BA)₃phen complexes

Dy(BA)₃phen powders were synthesized according to the traditional method as described in the ref. 37 1.8650 g of Dy₂O₃ was dissolved in 10 mL of concentrated nitric acid and then crystallized via evaporation of excess nitric acid and water by heating, and Dy(NO₃)₃·6H₂O powders were acquired. Dy(NO₃)₃ ethanol solution was prepared by adding 20 mL of anhydrous ethanol into the above Dy(NO₃)₃·6H₂O. 3.6600 g of BA and 1.8000 g of phen were dissolved in 200 mL of ethanol. The Dy(NO₃)₃ ethanol solution was then added into the mixed solution of BA and phen with magnetic agitation for 3 h at 60 °C. The precipitates were collected by filtration and dried at 60 °C for 12 h. The synthetic method of Eu(BA)₃phen complex was similar to the above method, except that the used dosages of Eu₂O₃, BA and phen were 1.7600 g, 3.6640 g and 1.8020 g, respectively.

Preparations of spinning solutions for fabricating [Fe₃O₄/PVP]//[(Dy(BA)₃phen + Eu(BA)₃phen)/PVP] Janus nanofibers and Fe₃O₄/[Dy(BA)₃phen + Eu(BA)₃phen]/PVP composite nanofibers

Two different kinds of spinning solutions were prepared to fabricate Janus nanofibers. The spinning solution for one strand nanofiber of Janus nanofibers consisted of Dy(BA)₃phen, Eu(BA)₃phen, PVP, and DMF (denoted as spinning solutions A). A series of [Dy(BA)₃phen + Eu(BA)₃phen]/PVP spinning solutions with different mass percentages of Eu(BA)₃phen were prepared. Herein, the mass percentage of Dy(BA)₃phen to PVP was settled as 10% in all the spinning solutions A, which was because the luminescent intensity of Dy(BA)₃phen in PVP matrix was the strongest, as indicated in Fig. S1, ESI.† In order to fabricate color-tunable Janus nanofibers, the mass percentages of Eu(BA)₃phen to PVP were settled as 0%, 0.5%, 1%, 2%, and 3%, respectively. Dy(BA)₃phen and Eu(BA)₃phen complexes were added into 4.0000 g of DMF, and then 1.0000 g of PVP powder was dissolved into the above solutions under magnetic stirring for 12 h.

The other spinning solution for one strand nanofiber of Janus nanofibers was composed of oleic acid modified Fe₃O₄ NPs, PVP and DMF (denoted as spinning solutions B). In order to investigate the impact of Fe₃O₄ NPs on the magnetic and fluorescent properties of the Janus nanofibers, various amounts of Fe₃O₄ NPs were introduced into the spinning solutions B, as summarized in Table 1.

Table 1 Compositions of spinning solutions for preparing [Fe₃O₄/PVP]//[(10%Dy(BA)₃phen + n%Eu(BA)₃phen)/PVP] Janus nanofibers (S_a1–a5, S_b1–b5 and S_c1–c5) and Fe₃O₄/[Dy(BA)₃phen + Eu(BA)₃phen]/PVP composite nanofibers (S_d), in which n% represents the mass percentage of Eu(BA)₃phen to PVP

Samples	Spinning solutions	Compositions
		Dy(BA)3phen/g	Eu(BA)3phen/g	Fe3O4/g	PVP/g	DMF/g	n%
Sa1	A1	0.1000	0		1.0000	4.0000	0
	B1			0.2000	1.0000	4.0000
Sa2	A2	0.1000	0.0050		1.0000	4.0000	0.5
	B1			0.2000	1.0000	4.0000
Sa3	A3	0.1000	0.0100		1.0000	4.0000	1.0
	B1			0.2000	1.0000	4.0000
Sa4	A4	0.1000	0.0200		1.0000	4.0000	2.0
	B1			0.2000	1.0000	4.0000
Sa5	A5	0.1000	0.0300		1.0000	4.0000	3.0
	B1			0.2000	1.0000	4.0000
Sb1	A1	0.1000	0		1.0000	4.0000	0
	B2			0.5000	1.0000	4.0000
Sb2	A2	0.1000	0.0050		1.0000	4.0000	0.5
	B2			0.5000	1.0000	4.0000
Sb3	A3	0.1000	0.0100		1.0000	4.0000	1.0
	B2			0.5000	1.0000	4.0000
Sb4	A4	0.1000	0.0200		1.0000	4.0000	2.0
	B2			0.5000	1.0000	4.0000
Sb5	A5	0.1000	0.0300		1.0000	4.0000	3.0
	B2			0.5000	1.0000	4.0000
Sc1	A1	0.1000	0		1.0000	4.0000	0
	B3			1.0000	1.0000	4.0000
Sc2	A2	0.1000	0.0050		1.0000	4.0000	0.5
	B3			1.0000	1.0000	4.0000
Sc3	A3	0.1000	0.0100		1.0000	4.0000	1.0
	B3			1.0000	1.0000	4.0000
Sc4	A4	0.1000	0.0200		1.0000	4.0000	2.0
	B3			1.0000	1.0000	4.0000
Sc5	A5	0.1000	0.0300		1.0000	4.0000	3.0
	B3			1.0000	1.0000	4.0000
Sd		0.1000	0.0050	0.2000	2.0000	8.0000

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For comparison, Fe₃O₄/[Dy(BA)₃phen + Eu(BA)₃phen]/PVP composite nanofibers (S_d) were also fabricated by mixing spinning solution A₂ and spinning solution B₁ together at the volume ratio of 1 : 1 and electrospun via the traditional single-nozzle electrospinning method. This fabrication process of the Fe₃O₄/[Dy(BA)₃phen + Eu(BA)₃phen]/PVP composite nanofibers is an easy way to realize the preparation of the magnetic–fluorescent bifunctional nanofibers.

The compositions and contents of all these spinning solutions, and the products produced by corresponding spinning solutions were listed in Table 1.

Electrospinning equipments for fabricating Janus nanofibers

[Fe₃O₄/PVP]//[(10%Dy(BA)₃phen + n%Eu(BA)₃phen)/PVP] Janus nanofibers were prepared using an electrospinning setup with a homemade parallel spinneret, as indicated in Fig. 1. Two same sized stainless steel needles were used, with the outer diameters of 1.260 mm and inner diameters of 0.900 mm. The two kinds of spinning solutions were respectively loaded into each syringe, and the spinneret was settled vertically. A flat iron net was put about 14–16 cm away from the tip of the plastic nozzle to collect the Janus nanofibers. A positive direct current (DC) voltage of 13–14 kV was applied between the spinneret and the collector. The electrospinning process was carried out at ambient temperature of 22–24 °C and relative air humidity of 44–48%. Both the flow rates of the two spinning solutions were measured to be 0.133 mL min⁻¹. Meanwhile, Fe₃O₄/[Dy(BA)₃phen + Eu(BA)₃phen]/PVP composite nanofibers (S_d), as a contrast sample, were also prepared to study the superiority of the structure of Janus nanofibers by using the traditional single-spinneret electrospinning setup, and the other spinning parameters were the same as those for the fabrication of the Janus nanofibers.

Fig. 1 Schematic diagrams of the equipments for electrospinning Janus nanofibers.

Characterization

The as-prepared Fe₃O₄ NPs, [Fe₃O₄/PVP]//[(Dy(BA)₃phen + Eu(BA)₃phen)/PVP] Janus nanofibers and Fe₃O₄/[Dy(BA)₃phen + Eu(BA)₃phen]/PVP composite nanofibers were identified by an X-ray powder diffractometer (XRD, Bruker, D8 FOCUS) with Cu Kα radiation. The operation voltage and current were kept at 40 kV and 20 mA, respectively. The morphology and internal structure of samples were observed by a field emission scanning electron microscope (FESEM, XL-30) and a transmission electron microscope (TEM, JEM-2010), respectively. The elements analysis for the Janus nanofibers was performed by an energy dispersive spectrometer (EDS, Oxford ISIS 300) attached to the FESEM. The fluorescent properties of the samples were investigated by a Hitachi photoluminescence (PL) spectrophotometer F-7000. The ultraviolet-visible spectrum was determined by suing a UV-1240 ultraviolet-visible spectrophotometer. Then, the magnetic measurements were performed by using a vibrating sample magnetometer (VSM, MPMS SQUID XL). All the measures were performed at room temperature.

Results and discussion

XRD analysis

The phase compositions of the Fe₃O₄ NPs, [Fe₃O₄/PVP]//[(Dy(BA)₃phen + Eu(BA)₃phen)/PVP] Janus nanofibers (S_b2) and Fe₃O₄/[Dy(BA)₃phen + Eu(BA)₃phen]/PVP composite nanofibers (S_d) were characterized by means of XRD analysis, as shown in Fig. 2. The XRD patterns of the as-prepared Fe₃O₄ NPs are conformed to the cubic structure of Fe₃O₄ (PDF 74-0748), and no characteristic peaks are observed for other impurities such as Fe₂O₃ and FeO(OH). The diffraction peaks of Fe₃O₄ in the Janus nanofibers and composite nanofibers are weaker than those of Fe₃O₄ NPs due to the existence of amorphous PVP and RE complexes in the Janus nanofibers and composite nanofibers. These results demonstrate that the Janus nanofibers and composite nanofibers contain Fe₃O₄ NPs.

Fig. 2 XRD patterns of Fe₃O₄ NPs, [Fe₃O₄/PVP]//[(Dy(BA)₃phen + Eu(BA)₃phen)/PVP] Janus nanofibers and Fe₃O₄/[Dy(BA)₃phen + Eu(BA)₃phen]/PVP composite nanofibers.

Morphology and structure

The morphology of the as-prepared Fe₃O₄ NPs was observed by means of SEM, as presented in Fig. 3A. The Fe₃O₄ NPs are spherical in shape, and the mean diameter of them is 22.50 ± 0.25 nm (Fig. 3B). As shown in Fig. 3C, each [Fe₃O₄/PVP]//[(Dy(BA)₃phen + Eu(BA)₃phen)/PVP] Janus nanofiber consists of two nanofibers assembled side-by-side, and the mean diameter of individual nanofiber in the Janus nanofibers is ca. 500 nm. Moreover, some Fe₃O₄ NPs aggregates are faintly visible in one individual nanofiber, whereas the other individual nanofiber is extremely smooth. EDS spectrum shown in Fig. 3D reveals that the Janus nanofibers are composed of elements C, N, O, Dy, Eu, Fe and Au, in which Dy, Eu and Fe elements respectively come from Dy(BA)₃phen, Eu(BA)₃phen and Fe₃O₄ NPs, and the Au peak comes from gold conductive film plated on the surface of the sample for SEM observation. Fig. 3E shows the TEM image of a Janus nanofiber. One can see that Fe₃O₄ NPs are only dispersed in one strand nanofiber. The SEM image of the Fe₃O₄/[Dy(BA)₃phen + Eu(BA)₃phen]/PVP composite nanofibers was shown in Fig. 3F. It can be seen that each single composite nanofiber is independent, and the diameter of the composite nanofibers is ca. 500 nm. From the above analyses, we can confirm that the [Fe₃O₄/PVP]//[(Dy(BA)₃phen + Eu(BA)₃phen)/PVP] Janus nanofibers have been successfully fabricated.

Fig. 3 SEM image (A) and histogram of diameter (B) of the Fe₃O₄ NPs; SEM image (C), EDS analysis (D) and TEM image (E) of [Fe₃O₄/PVP]//[(Dy(BA)₃phen + Eu(BA)₃phen)/PVP] Janus nanofibers; and SEM image (F) of Fe₃O₄/[Dy(BA)₃phen + Eu(BA)₃phen]/PVP composite nanofibers.

Photoluminescence properties

In order to illustrate the advantage of the nanostructure of the magnetic–fluorescent bifunctional Janus nanofibers, the PL spectra of [Fe₃O₄/PVP]//[(10%Dy(BA)₃phen + 0.5%Eu(BA)₃phen)/PVP] Janus nanofibers (S_a2) and Fe₃O₄/[10%Dy(BA)₃phen + 0.5%Eu(BA)₃phen]/PVP composite nanofibers (S_d) were contrasted, as shown in Fig. 4. One can see that excitation and emission intensity of the Janus nanofibers are much stronger than those of Fe₃O₄/[Dy(BA)₃phen + Eu(BA)₃phen]/PVP composite nanofibers. This result from can be attributed to the isolation of RE complexes from Fe₃O₄ NPs. From the UV-vis absorbance spectrum of Fe₃O₄ NPs illustrated in Fig. 12B, it is observed that Fe₃O₄ NPs absorb light at ultraviolet wavelengths (<400 nm) much more strongly than visible range (400–700 nm). As illustrated in Fig. 5, Dy(BA)₃phen, Eu(BA)₃phen and Fe₃O₄ NPs are promiscuously dispersed in the Fe₃O₄/[Dy(BA)₃phen + Eu(BA)₃phen]/PVP composite nanofibers. The exciting light in the composite nanofibers has to pass through Fe₃O₄ NPs to reach and excite Dy(BA)₃phen and Eu(BA)₃phen complexes. In this process, a large part of the exciting light has been absorbed by Fe₃O₄ NPs, and thus the exciting light is much weakened when it reaches the Dy(BA)₃phen and Eu(BA)₃phen complexes. Similarly, the emitting light emitted by Dy(BA)₃phen and Eu(BA)₃phen complexes also has to pass through Fe₃O₄ NPs and is absorbed by them. Consequently, both the exciting and emitting light are severely weakened. For the [Fe₃O₄/PVP]//[(Dy(BA)₃phen + Eu(BA)₃phen)/PVP] Janus nanofibers, Dy(BA)₃phen, Eu(BA)₃phen and Fe₃O₄ NPs are separated in their own strand, so that the exciting light and emitting light in the [Dy(BA)₃phen + Eu(BA)₃phen]/PVP strand will be little affected by Fe₃O₄ NPs. The overall effect is that the [Fe₃O₄/PVP]//[(Dy(BA)₃phen + Eu(BA)₃phen)/PVP] Janus nanofibers possess much higher fluorescent performance than the Fe₃O₄/[Dy(BA)₃phen + Eu(BA)₃phen]/PVP composite nanofibers.

Fig. 4 Excitation spectra monitored at 574 nm (A), 616 nm (B) and emission spectra (C) of [Fe₃O₄/PVP]//[(10%Dy(BA)₃phen + 0.5%Eu(BA)₃phen)/PVP] Janus nanofibers and Fe₃O₄/[10%Dy(BA)₃phen + 0.5%Eu(BA)₃phen]/PVP composite nanofibers when the mass ratios of Fe₃O₄ NPs to PVP were respectively settled at 0.2 : 1.

Fig. 5 Schematic diagrams of the exciting light and emitting light in Fe₃O₄/[Dy(BA)₃phen + Eu(BA)₃phen]/PVP composite nanofiber and [Fe₃O₄/PVP]//[(Dy(BA)₃phen + Eu(BA)₃phen)/PVP] Janus nanofiber.

To get the color-tunable Janus nanofibers, the mass percentage of Dy(BA)₃phen to PVP were settled as 10%, and the mass percentage of Eu(BA)₃phen to PVP were respectively varied from 0%, 0.5%, 1.0%, 2.0% to 3.0% in [(Dy(BA)₃phen + Eu(BA)₃phen)/PVP] strand nanofiber. Fig. 6A demonstrates the excitation spectra of the samples monitored at 574 nm (the characteristic emission peak of Dy³⁺), and the mass ratio of Fe₃O₄ NPs to PVP was fixed as 0.2 : 1 (S_a1–a5). Broad excitation bands extending from 200 to 350 nm are observed in various samples, and the strongest peaks at 273 nm are assigned to the π → π* electron transition of the ligands.¹² The excitation intensity is decreased with adding more Eu(BA)₃phen complexes. Fig. 6B shows the excitation spectra of various samples monitored at 616 nm (the characteristic emission peak of Eu³⁺). Similarly, the strongest excitation peaks are also located at 273 nm. The excitation intensity is increased along with adding more Eu(BA)₃phen complexes. Thus, one can see that both Dy(BA)₃phen and Eu(BA)₃phen complexes can be simultaneously and most effectively excited using 273 nm single-wavelength ultraviolet light.

Fig. 6 Excitation spectra of [Fe₃O₄/PVP]//[(10%Dy(BA)₃phen + n%Eu(BA)₃phen)/PVP] Janus nanofibers monitored at 574 nm (A) and 616 nm (B) when the mass ratio of Fe₃O₄ NPs to PVP was fixed at 0.2 : 1 (S_a1: n = 0, S_a2: n = 0.5, S_a3: n = 1.0, S_a4: n = 2.0, S_a5: n = 3.0).

Fig. 7 and 8 indicate the excitation spectra of the [Fe₃O₄/PVP]//[(Dy(BA)₃phen + Eu(BA)₃phen)/PVP] Janus nanofibers containing different amounts of Fe₃O₄ NPs (S_b1–b5: Fe₃O₄ : PVP = 0.5 : 1; S_c1–c5: Fe₃O₄ : PVP = 1 : 1). One can see that both the Dy(BA)₃phen and Eu(BA)₃phen complexes can be also simultaneously and most effectively excited using 273 nm single-wavelength ultraviolet light. The excitation intensity of Dy³⁺ (574 nm) is decreased with adding more Eu(BA)₃phen complexes. On the contrary, the excitation intensity of Eu³⁺ (616 nm) is increased along with introducing more Eu(BA)₃phen complexes. In addition, by comparing the intensities of the excitation spectra among Fig. 6A, 7A and 8A, as well as Fig. 6B, 7B and 8B, one can see that the intensity is decreased with introducing more Fe₃O₄ NPs into the Fe₃O₄/PVP strand nanofiber.

Fig. 7 Excitation spectra of [Fe₃O₄/PVP]//[(10%Dy(BA)₃phen + n%Eu(BA)₃phen)/PVP] Janus nanofibers monitored at 574 nm (A) and 616 nm (B) when the mass ratio of Fe₃O₄ NPs to PVP was fixed at 0.5 : 1 (S_b1: n = 0, S_b2: n = 0.5, S_b3: n = 1.0, S_b4: n = 2.0, S_b5: n = 3.0).

Fig. 8 Excitation spectra of [Fe₃O₄/PVP]//[(10%Dy(BA)₃phen + n%Eu(BA)₃phen)/PVP] Janus nanofibers monitored at 574 nm (A) and 616 nm (B) when the mass ratio of Fe₃O₄ NPs to PVP was fixed at 1 : 1 (S_c1: n = 0, S_c2: n = 0.5, S_c3: n = 1.0, S_c4: n = 2.0, S_c5: n = 3.0).

The emission spectra of [Fe₃O₄/PVP]//[(10%Dy(BA)₃phen + n%Eu(BA)₃phen)/PVP] Janus nanofibers (from samples S_a1 to S_a5) were plotted in Fig. 9. Characteristic emission peaks of the Dy(BA)₃phen and Eu(BA)₃phen are observed under the most effective excitation of 273 nm ultraviolet light. The blue emission and yellow-green one centering at 481 nm and 574 nm originate respecting from the energy level transition ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 of Dy³⁺ ions.³⁸ Meanwhile, two red emitting peaks locating at 592 nm and 616 nm can also be observed, which are respectively ascribed to the energy levels transitions of the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ of Eu³⁺ ions, and it is observed that the emission peak at 592 nm is much lower than that at 616 nm.¹²

Fig. 9 Comparison among the emission spectra of [Fe₃O₄/PVP]//[(10%Dy(BA)₃phen + n%Eu(BA)₃phen)/PVP] Janus nanofibers containing different mass percentage of Eu(BA)₃phen complexes when the mass ratio of Fe₃O₄ NPs to PVP was fixed at 0.2 : 1 (S_a1: n = 0, S_a2: n = 0.5, S_a3: n = 1.0, S_a4: n = 2.0, S_a5: n = 3.0).

It is interesting and reasonable to suggest that the PL intensity of the Eu³⁺ ions is observed to increase, whereas that of the Dy³⁺ ions is simultaneously found to decrease monotonically with the increase of Eu(BA)₃phen concentration. In order to clearly depict the variation trend, the intensities of the characteristic emission peaks of each sample versus different samples were plotted in the inset of Fig. 9. The variation of the PL intensity of the Eu³⁺ and Dy³⁺ can be attributed to the energy distribution. Since the energy that the matrix absorbs and the content of Dy(BA)₃phen are constant, more energy is assigned to Eu³⁺ with the increase of Eu(BA)₃phen content, thus leading to stronger fluorescence peaks at 592 and 616 nm. Meanwhile, on the contrary, the energy which is assigned to Dy³⁺ is reduced and the fluorescence peaks at 481 and 574 nm are relevantly weakened. Fig. 10 and 11 respectively show the fluorescent emission spectra of the Janus nanofibers from the samples S_b1 to S_b5 and S_c1 to S_c5. Similar variable regularity can be observed among Fig. 9–11, but the overall intensity is decreased with adding more Fe₃O₄ NPs.

Fig. 10 Comparison among the emission spectra of [Fe₃O₄/PVP]//[(10%Dy(BA)₃phen + n%Eu(BA)₃phen)/PVP] Janus nanofibers containing different mass percentage of Eu(BA)₃phen complexes when the mass ratio of Fe₃O₄ NPs to PVP was respectively fixed at 0.5 : 1 (A) and 1 : 1 (B) (S_b1: n = 0, S_b2: n = 0.5, S_b3: n = 1.0, S_b4: n = 2.0, S_b5: n = 3.0, S_c1: n = 0, S_c2: n = 0.5, S_c3: n = 1.0, S_c4: n = 2.0, S_c5: n = 3.0).

Fig. 11 CIE chromaticity diagram for [Fe₃O₄/PVP]//[(10%Dy(BA)₃phen + n%Eu(BA)₃phen)/PVP] Janus nanofibers when the mass ratio of Fe₃O₄ NPs to PVP was respectively fixed at 0.2 : 1 (A), 0.5 : 1 (B), and 1 : 1 (C), together with their corresponding photographs upon excitation by 273 nm ultraviolet light (n = 0, 0.5, 1.0, 2.0, 3.0).

Generally, color can be represented by the Commission Internationale de L'Eclairage (CIE) 1931 chromaticity coordinates. The CIE chromaticity coordinates for the samples, together with their corresponding photographs upon excitation at 273 nm ultraviolet light, were provided in Table 2 and Fig. 11. As the content of Eu(BA)₃phen complexes increases from 0 to 3%, one can see that the fluorescent color of the obtained Janus nanofibers can be easily tuned from greenish blue (S_a1, S_b1, S_c1), white (S_a2, S_b2, S_c2), pale pink (S_a3, S_b3, S_c3), pink (S_a4, S_b4, S_c4) and eventually to yellowish pink (S_a5, S_b5, S_c5). In particular, it is gratify to see that the desirable white emission can be realized by the co-doping of Dy(BA)₃phen and Eu(BA)₃phen complexes into PVP nanofiber. The results show that the emission color of Janus nanofibers can be turned by adjusting the amount of Eu(BA)₃phen complexes.

Table 2 Comparison among the CIE chromaticity coordinates (x, y) for the Janus nanofibers excited by 273 nm ultraviolet light

Sample no.	Sample composition		CIE coordinates (x, y)	Em (color)
	(n%)	Fe3O4 (g) : PVP (g)
Sa1	0	0.2 : 1	(0.259, 0.305)	Greenish blue
Sb1		0.5 : 1	(0.261, 0.311)
Sc1		1 : 1	(0.268, 0.318)
Sa2	0.5	0.2 : 1	(0.312, 0.303)	White
Sb2		0.5 : 1	(0.324, 0.319)
Sc2		1 : 1	(0.345, 0.325)
Sa3	1.0	0.2 : 1	(0.362, 0.314)	Pale pink
Sb3		0.5 : 1	(0.371, 0.308)
Sc3		1 : 1	(0.383, 0.318)
Sa4	2.0	0.2 : 1	(0.430, 0.319)	Pink
Sb4		0.5 : 1	(0.441, 0.328)
Sc4		1 : 1	(0.455, 0.333)
Sa5	3.0	0.2 : 1	(0.456, 0.319)	Yellowish pink
Sb5		0.5 : 1	(0.464, 0.320)
Sc5		1 : 1	(0.479, 0.327)

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Fig. 12A gives the fluorescent emission spectra (excited by 273 nm) of [Fe₃O₄/PVP]//[10%Dy(BA)₃phen/PVP] Janus nanofibers containing different amounts of Fe₃O₄ NPs. The [10%Dy(BA)₃phen]/PVP strand nanofibers in the Janus nanofibers were fabricated using spinning solution A₁, and the Fe₃O₄/PVP strand nanofiber were fabricated using spinning solution B₁ (Fe₃O₄ : PVP = 0.2 : 1), B₂ (Fe₃O₄ : PVP = 0.5 : 1) and B₃ (Fe₃O₄ : PVP = 1 : 1), respectively. One can see that the emission intensity of the Janus nanofibers is decreased with adding more Fe₃O₄ NPs into the Fe₃O₄/PVP strand nanofiber due to the light absorption of Fe₃O₄. Moreover, as indicated in Fig. 12C, one can see that the CIE coordinates have slightly variations towards the direction of red color with introducing more Fe₃O₄ NPs. This phenomenon results from that the low-wavelength light is more absorbed by Fe₃O₄ NPs than long-wavelength light, as depicted in Fig. 12B. In other word, red light (616 nm) is less absorbed by Fe₃O₄ NPs compared with cyan light (481 nm). In the inset of Fig. 12A, a standard used for comparison when the mass percentage of Fe₃O₄ to PVP was fixed at 0.2 : 1 (S_a1), compared with the other emission intensity of Janus nanofibers when the mass percentage of Fe₃O₄ to PVP was fixed at 0.5 : 1 and 1 : 1, respectively. Reduce the degree of the fluorescent intensity at 481 nm is much stronger than the fluorescent intensity at 574 nm with the increase of the amount of Fe₃O₄ NPs introduced into Fe₃O₄/PVP nanofiber. In this case, adding more Fe₃O₄ NPs leads to more intense absorption of cyan light, whereas the red light is not absorbed so much. Consequently, the fluorescent color of Janus nanofibers becomes more red with more Fe₃O₄ NPs. Similar phenomena can be observed in the Janus nanofibers containing Eu(BA)₃phen complexes, as seen in Fig. 13 and 14. The above results indicate that the as-obtained Janus nanofibers can exhibit tunable color and white luminescence in the visible region by changing the content of Eu(BA)₃phen under the excitation of single-wavelength ultraviolet light. In addition, fluorescent color of Janus nanofibers is also influenced by Fe₃O₄ NPs.

Fig. 12 Comparison of emission spectra (A); UV-vis absorbance spectrum of Fe₃O₄ NPs (B); and CIE chromaticity diagram (C) for [Fe₃O₄/PVP]//[10%Dy(BA)₃phen/PVP] Janus nanofibers containing different mass ratios of Fe₃O₄ NPs.

Fig. 13 Comparison of emission spectra (left) and CIE chromaticity diagram (right) for [Fe₃O₄/PVP]//[(10%Dy(BA)₃phen + 0.5%Eu(BA)₃phen)/PVP] Janus nanofibers containing different mass ratios of Fe₃O₄ NPs.

Fig. 14 Comparison of emission spectra (left) and CIE chromaticity diagram (right) for [Fe₃O₄/PVP]//[(10%Dy(BA)₃phen + 3%Eu(BA)₃phen)/PVP] Janus nanofibers containing different mass ratios of Fe₃O₄ NPs.

The fluorescence decay curves (Fig. 15) of [Fe₃O₄/PVP]//[(10%Dy(BA)₃phen + n%Eu(BA)₃phen)/PVP] Janus nanofibers containing different mass percentage of Eu(BA)₃phen complexes when the mass ratios of Fe₃O₄ to PVP were respectively settled at 0.2 : 1 (S_a1–a5), 0.5 : 1 (S_b1–b5) and 1 : 1 (S_c1–c5) are used to calculate the lifetime and to investigate the fluorescence dynamics of these samples. The samples are excited by 273 nm ultraviolet light and monitored at 574 nm. It is found that the curves follow the single-exponential decay:

I_t = I₀ exp(−t/ι)

where I_t is the intensity at time t, I₀ is the intensity at t = 0 and ι is the decay lifetime. The obtained average lifetime values (τ/ms) of the samples are shown in Fig. 15(A–C). It is obvious that the fluorescence lifetime of the ⁴F_9/2 → ⁶H_13/2 transition of Dy³⁺ ions (λ_em = 574 nm) in the Janus nanofibers is extended with increase in the content of Eu(BA)₃phen complex. The possible reasons for this result are as follows. The relative content of Dy(BA)₃phen complex in the fibers is reduced with introducing more Eu(BA)₃phen. Thus the distance among Dy³⁺ in Dy(BA)₃phen molecular clusters and/or nanoparticles in the Janus nanofibers is increased, resulting in reduction of energy transfer among Dy³⁺ to Dy³⁺ and elongated fluorescence lifetime of Dy³⁺.³⁹

Fig. 15 Fluorescence decay dynamics of the ⁴F_9/2 → ⁶H_13/2 transitions (λ_em = 574 nm) in Janus nanofibers doped with different mass percentage of Eu(BA)₃phen complexes when the mass ratio of Fe₃O₄ NPs to PVP was respectively fixed at 0.2 : 1 (A), 0.5 : 1 (B) and 1 : 1 (C).

Magnetic property

The typical hysteresis loops for [Fe₃O₄/PVP]//[(10%Dy(BA)₃phen + 0.5%Eu(BA)₃phen)/PVP] Janus nanofibers containing different mass ratios of Fe₃O₄ NPs and Fe₃O₄/[10%Dy(BA)₃phen + 0.5%Eu(BA)₃phen]/PVP composite nanofibers are shown in Fig. 16, and their saturation magnetizations are listed in Table 3. The saturation magnetization of the Fe₃O₄ NPs is 48.523 emu g⁻¹, as indicated in Fig. S2, ESI.† It is well known that the saturation magnetization of a magnetic composite material depends on the mass percentage of the magnetic substance in the magnetic composite material.¹² It is found that the saturation magnetization of the magnetic–fluorescent Janus nanofibers is increased with the increase of the amount of Fe₃O₄ NPs introduced into the Fe₃O₄/PVP strand. From the Fig. 16, one can see that hysteresis loops for the Janus nanofibers (S_a2) and composite nanofibers (S_d) were nearly overlapped, the saturation magnetization of the Fe₃O₄/[10%Dy(BA)₃phen + 0.5%Eu(BA)₃phen]/PVP composite nanofibers (S_d) is 2.629 emu g⁻¹, which is close to that of the Janus nanofibers sample S_a2 (2.660 emu g⁻¹) because they were both prepared by spinning solution A₂ and spinning solution B₁. Combined the analyses of magnetism and fluorescence, it is found that the Janus nanofibers have the close magnetic property to the Fe₃O₄/[Dy(BA)₃phen + Eu(BA)₃phen]/PVP composite nanofibers, but the fluorescent intensity of the Janus nanofibers is much higher than that of the composite nanofibers, demonstrating that the novel Janus nanofibers are superior than the composite nanofibers.

Fig. 16 Hysteresis loops of [Fe₃O₄/PVP]//[(10%Dy(BA)₃phen + 0.5%Eu(BA)₃phen)/PVP] Janus nanofibers containing different mass ratios of Fe₃O₄ NPs and Fe₃O₄/[10%Dy(BA)₃phen + 0.5%Eu(BA)₃phen]/PVP composite nanofibers.

Table 3 Saturation magnetization (M_s) of [Fe₃O₄/PVP]//[(10%Dy(BA)₃phen + 0.5%Eu(BA)₃phen)/PVP] Janus nanofibers, and Fe₃O₄/[10%Dy(BA)₃phen + 0.5%Eu(BA)₃phen]/PVP composite nanofibers

Samples	Ms/emu g^−1
Janus nanofibers (Fe3O4 : PVP = 0.2 : 1, Sa2)	2.660
Janus nanofibers (Fe3O4 : PVP = 0.5 : 1, Sb2)	5.189
Janus nanofibers (Fe3O4 : PVP = 1 : 1, Sc2)	7.164
Composite nanofibers (Sd)	2.629

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Conclusions

In summary, novel magnetic and color-tunable bifunctional [Fe₃O₄/PVP]//[(Dy(BA)₃phen + Eu(BA)₃phen)/PVP] Janus nanofibers with asymmetry dual-sided structure were successfully synthesized via electrospinning technique using specifically designed spinneret. It is very gratifying to see that the new-typed magnetic–fluorescent bifunctional Janus nanofibers simultaneously possess both high fluorescent intensity and saturation magnetization compared with the simply-mixed Fe₃O₄/[Dy(BA)₃phen + Eu(BA)₃phen]/PVP composite nanofibers. For the Janus nanofibers, tunable colors from greenish blue to yellowish pink can be realized by changing the mass ratio of different RE complexes upon excitation of 273 nm ultraviolet light, and furthermore, it is the first time to obtain white-light-emitting magnetic–luminescent bifunctional 1D nanomaterials. In addition, the color coordinates of the Janus nanofibers have an obvious variation trend of moving to the direction of red color with introducing more Fe₃O₄ nanoparticles. Our work has demonstrated a successful approach to prepare innovation 1D magnetic and color-tunable nanocomposites with controlled luminescent and magnetic properties for potential applications in the realm of future, such as magnetic–luminescent devices, full-color displays, magneto-optic imaging and anti-counterfeit materials, etc.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (NSFC 50972020, 51072026), Specialized Research Fund for the Doctoral Program of Higher Education (20102216110002, 20112216120003), the Science and Technology Development Planning Project of Jilin Province (Grant nos 20130101001JC, 20070402), the Research Project of Science and Technology of Department of Education of Jilin Province “11th 5 year plan” (Grant nos 2010JYT01).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra01644d

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