Electrospinning fabrication and characterization of magnetic-upconversion fluorescent bifunctional core–shell nanofibers

Abstract

Novel magnetic-upconversion fluorescent bifunctional core–shell nanofibers have been successfully fabricated by coaxial electrospinning technology. NaYF₄:Yb³⁺,Er³⁺ and Fe₃O₄ nanoparticles (Nps) were incorporated into polyvinylpyrrolidone (PVP) and electrospun into core–shell nanofibers with Fe₃O₄/PVP as core and NaYF₄:Yb³⁺,Er³⁺/PVP as the shell. The morphology and properties of the final products were investigated in detail by X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, vibrating sample magnetometer, and fluorescence spectroscopy. The core contained magnetic Nps was ca. 100 nm in diameter, and the shell scattered with NaYF₄:Yb³⁺, Er³⁺ Nps was ca. 80 nm in thickness. Fluorescence emission peaks of Er³⁺ in the [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers were observed. Compared with Fe₃O₄/NaYF₄:Yb³⁺,Er³⁺/PVP composite nanofibers, the luminescent intensity of the [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers was much higher, because the Fe₃O₄ Nps were only distributed in the core of the core–shell nanofibers, thus the manufactured core–shell nanofibers possessed excellent magnetic properties. The new type magnetic-upconversion fluorescent bifunctional [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers have many potential applications in display device, nanorobots, protein determination, and target delivery of drug owing to their excellent magnetism and fluorescence.

Graphical Abstract

A novel magnetic-upconversion fluorescent bifunctional core-shell nanofibers are fabricated by coaxial electrospinning technique. Because the Fe₃O₄ nanoparticles are only distributed in the core of the core-shell nanofibers, they barely decrease the fluorescent emission intensity of the core-shell nanofibers, and strong upconversion fluorescent of the magnetic-upconversion coaxial nanofibers can be achieved.

Introduction

Electrospinning (Gai et al. 2013a, b; Li et al. 2013a, b, c; Ma et al. 2013c; Wang et al. 2011) is an outstanding technique to process viscous solutions or melts into continuous fibers with diameters ranging from micrometer to submicron or nanometer. This method has not only attracted extensive academic investigations but has also been applied in many areas such as filtration (Sambaer et al. 2011), optical and chemical sensors (Corres et al. 2011), biological scaffolds (Sell et al. 2011), electrode materials (Chen et al. 2011), and nanocables materials (Song et al. 2011).

Magnetic-fluorescent nanomaterials have been applied in medical diagnostics and optical imaging, etc. At present, some preparations of Fe₃O₄-RE complex core–shell structure Nps have been reported (Lu et al. 2010; Peng et al. 2011; Wang et al. 2010c; Feng et al. 2010; Gai et al. 2010). In order to obtain new morphologies of magnetic-fluorescent nanomaterials, the fabrication of one-dimensional magnetic-fluorescent nanomaterials is an urgent subject of study. In recent years, some different types of electrospun upconversion fluorescent nanofibers (Hou et al. 2011, 2012a, b, 2013) are reported, which can be used for drug delivery, cell imaging, fluorescent lamps, color displays, etc. Some magnetic nanofibers prepared by electrospinning are also emerged in the references (Wang et al. 2010a; Meng et al. 2012). However, there are few reports in the literature about the study of the magnetic-upconversion fluorescent bifunctional nanofibers. By now, the studies of Fe₂O₃/Eu(DBM)₃(Bath)/polyvinyl pyrrolidone (PVP) composite nanofibers (Wang et al. 2010b) and Fe₃O₄/Eu(BA)₃phen/PVP composite nanofibers (Ma et al. 2012b) obtained via electrospinning are emerged in the literatures. It has been proven that the existence of Fe₃O₄ or Fe₂O₃ will greatly decrease the luminescence of rare earth compounds if Fe₃O₄ or Fe₂O₃ directly contacts with the rare earth luminescent compounds. Therefore, if a strong luminescence of magnetic-luminescent bifunctional nanofibers is to be achieved, rare earth complex must be effectively isolated from Fe₃O₄ Nps to avoid direct contact. Nanostructure of core–shell nanofibers may help to realize this academic idea. Of the core–shell nanofibers, the core is composed of template PVP containing Fe₃O₄ Nps, and the shell consists of PVP containing rare earth compounds, thus it is expected that bifunctional core–shell nanofibers with excellent magnetism and luminescence will be obtained. Moreover, the obtained magnetic-upconversion fluorescent bifunctional core–shell nanofibers have not been reported in the literature.

In this paper, we employ coaxial electrospinning technique to prepare magnetic-upconversion fluorescent bifunctional [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers. Both the magnetic and fluorescent Nps are nontoxic (Sun et al. 2012). PVP is used as the fiber template and is a stable and biocompatible material (Siepmann et al. 2010). The upconversion fluorescence properties of [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers are studied through comparison with NaYF₄:Yb³⁺,Er³⁺/PVP nanofibers and Fe₃O₄/NaYF₄:Yb³⁺,Er³⁺/PVP composite nanofibers. The magnetism properties of the core–shell nanofibers are also studied. Some new meaningful results are obtained.

Experimental sections

Chemicals

PVP K90 (Mw ≈ 90,000), Y₂O₃, Yb₂O₃, Er₂O₃, NaF, ethyleneglycol (EG), PVP K30 (Mw ≈ 30,000), FeCl₃·6H₂O, FeSO₄·7H₂O, NH₄NO₃, polyethyleneglycol (PEG, Mw ≈ 20,000), ammonia, anhydrous ethanol, CHCl₃, DMF, and deionized water were used. All the reagents are of analytical grade. The purity of Y₂O₃, Yb₂O₃, and Er₂O₃ are 99.99 %. Deionized water is homemade.

Preparation of Fe₃O₄ Nps

Fe₃O₄ Nps were obtained via a facile coprecipitation synthetic method, and PEG was used as the protective agents to prevent the particles from aggregation. One typical synthetic procedure was as follows: 5.4060 g of FeCl₃·6H₂O, 2.7800 g of FeSO₄·7H₂O, 4.0400 g of NH₄NO₃, and 1.9000 g of PEG 20000 were added into 100 ml of deionized water to form a uniform solution under vigorous stirring at 50 °C. To prevent the oxidation of Fe²⁺, the reactive mixture was kept under argon atmosphere. After the mixture had been bubbled with argon for 30 min, 0.1 mol/L of NH₃·H₂O was added dropwise into the mixture, until the pH value was above 11. Then the system was continuously bubbled with argon for 20 min at 50 °C, and a black precipitate formed. The precipitates were collected from the solution by magnetic separation, washed with deionized water three times, and then dried in an electric vacuum oven for 12 h at 60 °C.

Synthesis of NaYF₄:Yb³⁺,Er³⁺ Nps

NaYF₄:Yb³⁺,Er³⁺ Nps were synthesized via a modified ethyleneglycol refluxing method, and PVP K30 was used as a surfactant. 0.8807 g of Y₂O₃, 0.3940 g of Yb₂O₃, and 0.0382 g of Er₂O₃ (Y:Yb:Er = 78:20:2, molar ratio) were dissolved in 50 ml of concentrated nitric acid and then crystallized by evaporation of excess nitric acid through heating process, and Ln(NO₃)₃·6H₂O (Ln = Y, Yb and Er) was acquired. Ln(NO₃)₃ ethylene glycol solution (solution I) was prepared by adding 50 ml of ethylene glycol into the above Ln(NO₃)₃·6H₂O. 5.0388 g of NaF and 5.0000 g of PVP K30 were dissolved in a mixed solution of 30 mL of deionized water and 120 mL of ethylene glycol (solution II) in a 250-mL three-necked flask with a backflow device. After having continuously bubbled solution II with argon for 30 min, solution I was added into solution II, and then the mixture was raised to 180 °C for 3 h. The as-prepared white precipitates were separated by a centrifuge, washed with deionized water for three times, and then dried in an electric vacuum oven for 12 h at 60 °C.

Fabrication of magnetic-upconversion fluorescent bifunctional core–shell nanofibers via electrospinning

In the preparation of inner electrospinning precursor solution, 3.0000 g of the as-prepared Fe₃O₄ Nps were dispersed ultrasonically 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 2 mL of oleic acid was slowly added. Reaction was stopped after heating and stirring the mixture for 40 min. The precipitates were collected from the solution by magnetic separation, washed with methyl alcohol for three times, and then dried in an electric vacuum oven for 6 h at 60 °C. All of the products were dispersed in 20.0000 g of CHCl₃ to form a stable emulsion, and then 0.5000 g of PVP K90 was slowly added into the emulsion with stirring to increase its viscosity for electrospinning process.

1.0000 g of the as-prepared NaYF₄:Yb³⁺,Er³⁺ Nps were dispersed into the mixture solution of 1.0000 g of PVP K90 and 6.6000 g of DMF as the shell electrospinning precursor solution. In this precursor solution, the mass ratio of NaYF₄:Yb³⁺,Er³⁺ Nps and PVP K90 was 1:1. Another two shell electrospinning precursor solutions with different NaYF₄:Yb³⁺,Er³⁺ Nps contents were also prepared to perform comparison experiments, in which 0.5000 and 0.3333 g of NaYF₄:Yb³⁺,Er³⁺ Nps were respectively added into the mixture solution of 1.0000 g of PVP K90 and 6.6000 g of DMF, and the mass ratios of NaYF₄:Yb³⁺,Er³⁺ Nps and PVP K90 were 1:2 and 1:3, respectively. A homemade coaxial dual-spinneret was used in this study. As shown in Fig. 1, the inner fluid was injected into the inner plastic syringe, while the shell fluid was loaded into the outer one. A flat iron net was used as a fiber collector and put about 14 cm away from the spinneret. A positive direct current (DC) voltage of 13 kV was applied between the spinneret and the collector to generate stable, continuous PVP-based core–shell nanofiber at ambient temperature of 22–24 °C and the relative humidity of 44–48 %.

Fig. 1

Schematic diagram of the homemade coaxial electrospinning spinneret and the electrospinning setup, in which the inset shows the detail of core–shell Taylor cone

Fabrication of Fe₃O₄/NaYF₄:Yb³⁺,Er³⁺/PVP composite nanofibers and NaYF₄:Yb³⁺,Er³⁺/PVP nanofibers

In order to demonstrate the structure advantages of the magnetic-upconversion fluorescent bifunctional [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers, Fe₃O₄/NaYF₄:Yb³⁺,Er³⁺/PVP composite nanofibers were also fabricated via traditional single-spinneret electrospinning setup as a comparison sample. The fabrication of the Fe₃O₄/NaYF₄:Yb³⁺,Er³⁺/PVP composite nanofibers is a simple way of realizing the preparation of the magnetic-upconversion fluorescent bifunctional nanomaterial which mixes the Fe₃O₄ Nps, NaYF₄:Yb³⁺,Er³⁺ Nps, and PVP K90 together in a certain ratio as the precursor solution and electrospun into composite nanofibers. The precursor solution was prepared by mixing the inner and shell precursor solution for the fabrication of [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers at the volume ratio of 1:1, and the other spinning parameters were the same as they were in the fabrication of the core–shell nanofibers.

Meanwhile, NaYF₄:Yb³⁺,Er³⁺/PVP nanofibers, which have the same component as the shell of the [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers, were also prepared to study the impact of magnetic core of the [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers on the fluorescent property of the core–shell nanofibers. The NaYF₄:Yb³⁺,Er³⁺/PVP nanofibers were electrospun using the shell precursor solution for the fabrication of [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers by traditional single-spinneret electrospinning setup, and the other spinning parameters were also the same as they were in the fabrication of the core–shell nanofibers.

Characterization

The as-prepared samples 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 were observed by a field emission scanning electron microscope (FESEM, XL-30) and a transmission electron microscope (TEM, JEM-2010), respectively. The fluorescent properties were investigated by Hitachi fluorescence spectrophotometer F-7000 with a 980-nm diode laser. Then, the magnetic performances were measured by a vibrating sample magnetometer (VSM, MPMS SQUID XL). All the measurements were performed at room temperature.

Results and discussion

XRD analyses

The phase compositions of the Fe₃O₄ Nps, NaYF₄:Yb³⁺,Er³⁺ Nps, and [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers were characterized by means of XRD analysis, as shown in Fig. 2. The XRD patterns of the as-prepared Fe₃O₄ Nps conform 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 XRD patterns of the NaYF₄:Yb³⁺,Er³⁺ Nps could be indexed to the cubic NaYF₄ (PDF 06-0342). The XRD analysis result of [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers demonstrates that the obtained core–shell nanofibers contain both Fe₃O₄ Nps and NaYF₄:Yb³⁺,Er³⁺ Nps. Because the relative intensities of the XRD peaks of Fe₃O₄ Nps are much weaker than those of the NaYF₄:Yb³⁺,Er³⁺ Nps and Fe₃O₄ Nps are in the core of the core–shell nanofibers, it can be seen that the diffraction peaks of Fe₃O₄ are not obvious in the [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers, although the content of Fe₃O₄ Nps is richer than that of NaYF₄:Yb³⁺,Er³⁺ Nps in core–shell nanofibers.

Fig. 2

XRD patterns of the Fe₃O₄ Nps, NaYF₄:Yb³⁺,Er³⁺ Nps, and [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers

Morphology and structure

The morphology of the as-prepared Fe₃O₄ Nps was observed by means of SEM, as presented in Fig. 3a. The size distribution of the spherical Fe₃O₄ Nps is almost uniform, and the particle size of the Nps is 8–10 nm. The NaYF₄:Yb³⁺,Er³⁺ Nps were characterized by means of TEM observation, as shown in Fig. 3b. The NaYF₄:Yb³⁺,Er³⁺ Nps are spherical in shape and are 20–25 nm in particles size. Each NaYF₄:Yb³⁺,Er³⁺ nanoparticle is coated by a low-contrast shell due to the coating of the amorphous surfactant PVP K30.

Fig. 3

SEM image of Fe₃O₄ Nps (a) and TEM image of NaYF₄:Yb³⁺,Er³⁺ Nps (b)

The morphology and inner structure of [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers were characterized by SEM and TEM observation, respectively. As seen from Fig. 4a, the surface of the [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers is not smooth, and many Nps with the diameter of ca. 25 nm which are consistent with the size of NaYF₄:Yb³⁺,Er³⁺ Nps can be observed on the surface of the core–shell nanofibers. The diameter of the core–shell nanofibers is ca. 260 nm. Figure 4b reveals the inner structure of the [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers, one can see that a large number of Nps with the diameter of ca. 10 nm is scattered throughout the core of the core–shell nanofibers, and the core diameter of the core–shell nanofibers is ca. 80 nm. The shell of the core–shell nanofibers is ca. 100 nm in thickness and is composed of Nps with the diameter of ca. 25 nm, and some of the nanoparticle aggregates protrude out of the surface of the core–shell nanofibers. These Nps can be identified to be NaYF₄:Yb³⁺,Er³⁺ Nps due to the same particle size. Through the SEM and TEM analyses in Fig. 4a,b, we can confirm that the [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers were successfully prepared.

Fig. 4

SEM image (a) and TEM image (b) of [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers; and SEM image (c) and TEM image (d) of Fe₃O₄/NaYF₄:Yb³⁺,Er³⁺/PVP composite nanofibers

The morphology and inner structure of the Fe₃O₄/NaYF₄:Yb³⁺,Er³⁺/PVP composite nanofibers were shown in Fig. 4c,d. It can be seen that the diameter of the composite nanofibers is ca. 260 nm, and Fe₃O₄ and NaYF₄:Yb³⁺,Er³⁺ Nps are dispersed randomly in the composite nanofibers. The surface of the composite nanofibers is more smooth than that of the [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers, which may be because the NaYF₄:Yb³⁺,Er³⁺ Nps have larger dispersion region in the Fe₃O₄/NaYF₄:Yb³⁺,Er³⁺/PVP composite nanofibers (260 nm in diameter) than in the [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers (80 nm in thickness). That is, the Fe₃O₄/NaYF₄:Yb³⁺,Er³⁺/PVP composite nanofibers can provide adequate space to store the NaYF₄:Yb³⁺,Er³⁺ nanoparticle aggregates.

Upconversion fluorescent performance

The [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers emit bright red luminescence under exciting with a 980 nm diode laser when observed by naked eye. The upconversion luminescence spectra (Fig. 5a) obtained from the core–shell nanofibers show the characteristic red and green upconverted light with red being far more intense than the green one, which means the core–shell nanofibers have good monochromaticity. The peak at 542 nm is ascribed to the transitions from the ⁴S_3/2 levels to the ground state (⁴I_15/2), and the peak at 654 nm is corresponded to the transition from the ⁴F_9/2 to ⁴I_15/2 energy levels of the Er³⁺ doped in the NaYF₄:Yb³⁺,Er³⁺ nanoparticles which were introduced into the core–shell nanofibers. In the upconversion processes, the intensity of each emitting light is in proportion to the infrared excitation power: I_upαPⁿ, where I_up and Pⁿ represent the upconversion luminescence intensity and the infrared excitation intensity, respectively, and n (the slope) is the number of photons required to produce an upconversion photon (Yi et al. 2002). The plots of natural logarithm intensity of the upconversion emission versus natural logarithm pumped power of the core–shell nanofibers (Fig. 5b) were drawn according to the power dependence of upconversion luminescence intensity shown in Fig. 5a. The slopes are 2.220 and 2.318 for the green and red emissions, respectively. These results indicate that two photon is required to produce a visible photon in the core–shell nanofibers.

Fig. 5

Upconversion emission spectra of [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers (a) excited by a 980-nm diode laser with different pumped powers and plots of natural logarithm intensity (b) of the upconversion emission versus natural logarithm pumped power of [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers

The upconversion emission intensities of the [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers containing different amounts of NaYF₄:Yb³⁺,Er³⁺ Nps are investigated by changing the mass ratio of NaYF₄:Yb³⁺,Er³⁺ Nps to PVP in the preparation of the shell electrospinning precursor solution, and the results are shown in Fig. 6. It is obvious that the upconversion emission intensity is increased with the increase of the amount of NaYF₄:Yb³⁺,Er³⁺ Nps introduced into the core–shell nanofibers, and it is possible to tune the upconversion emission intensity of the core–shell nanofibers by introducing various amounts of NaYF₄:Yb³⁺,Er³⁺ Nps.

Fig. 6

Upconversion emission spectra of [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers containing different mass ratios of NaYF₄:Yb³⁺,Er³⁺ Nps

To illustrate the advantages of the core–shell nanostructure of the magnetic-upconversion fluorescent bifunctional nanofibers, the Fe₃O₄/NaYF₄:Yb³⁺,Er³⁺/PVP composite nanofibers and NaYF₄:Yb³⁺,Er³⁺/PVP nanofibers, as the comparative samples, were fabricated in this paper. From the comparison of upconversion emission intensities of the three samples, as indicated in Fig. 7b, it is found that the NaYF₄:Yb³⁺,Er³⁺/PVP nanofibers have the highest upconversion emission intensity. The emission intensity of [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers is only a little bit lower than that of the NaYF₄:Yb³⁺,Er³⁺/PVP nanofibers. The Fe₃O₄/NaYF₄:Yb³⁺,Er³⁺/PVP composite nanofibers obviously have the lowest emission intensity. This phenomenon may result from the strong light absorption of Fe₃O₄ Nps which are mixed into the nanofibers (Ma et al. 2012a, 2013a; Gao et al. 2009). From the absorption spectrum of Fe₃O₄ NPs illustrated in Fig. 7a, it is seen that the Fe₃O₄ NPs can absorb light (400–1,000 nm). Thus, the exciting light and emitting light are absorbed by the Fe₃O₄ Nps and the intensities are decreased. From the contrast of the core–shell nanofibers and NaYF₄:Yb³⁺,Er³⁺/PVP nanofibers, we can safely come to the conclusion that the Fe₃O₄ Nps existed in the core–shell nanofibers barely decrease the upconversion emission intensity of the core–shell nanofibers, indicating that Fe₃O₄ Nps are only in the core of the core–shell nanofibers and not in the shell of the core–shell nanofibers. As a result, the [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers possess superior upconversion fluorescent properties.

Fig. 7

UV–Vis absorbance spectrum of Fe₃O₄ NPs (a) and comparison among upconversion emission spectra (b) of [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers, NaYF₄:Yb³⁺,Er³⁺/PVP nanofibers and Fe₃O₄/NaYF₄:Yb³⁺,Er³⁺/PVP composite nanofibers

A schematic diagram is proposed to explain why the existence of Fe₃O₄/PVP core barely decreases the upconversion fluorescence intensity of the [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers. As seen in Fig. 8, a NaYF₄:Yb³⁺,Er³⁺/PVP nanofiber is divided into three imaginary domains by dash lines. Large quantities of NaYF₄:Yb³⁺,Er³⁺ Nps are dispersed in the PVP K90 based nanofibers. The exciting light will be weakened due to the light absorption of the components of the fiber when it reaches the core domain, and weak emitting light is emitted. Meanwhile, the weak emitting light could barely pass through the components of the fiber to reach the external of the fiber. That is, only the NaYF₄:Yb³⁺,Er³⁺ Nps dispersed in the middle, and surface domains are contributed to the upconversion fluorescence intensity of the fiber. So the core domain of the fiber can be displaced by magnetic core (Fe₃O₄) and almost does not affect the fluorescence performance of the fiber.

Fig. 8

Schematic diagram of the light absorption and emission of a fluorescent nanofiber

Magnetic property

The typical hysteresis loops for Fe₃O₄ Nps, Fe₃O₄/NaYF₄:Yb³⁺,Er³⁺/PVP composite nanofibers and [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers containing different mass ratios of NaYF₄:Yb³⁺,Er³⁺ Nps are shown in Fig. 9, and the saturation magnetizations of them are listed in Table 1. It has been known that the saturation magnetization of a magnetic composite material depends on the mass percentage of the magnetic substance in the magnetic composite material (Ma et al. 2013b). From the results, the saturation magnetization of the [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers is decreased with introducing more NaYF₄:Yb³⁺,Er³⁺ Nps into the core–shell nanofibers because the NaYF₄:Yb³⁺,Er³⁺ Nps are non-magnetic substance. The Fe₃O₄/NaYF₄:Yb³⁺,Er³⁺/PVP composite nanofibers and [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers (NaYF₄:PVP = 1:1) have the similar saturation magnetization which reveals that the two samples have the similar composition proportion.

Fig. 9

Hysteresis loops of Fe₃O₄ Nps, Fe₃O₄/NaYF₄:Yb³⁺,Er³⁺/PVP composite nanofibers and [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers containing different mass ratios of NaYF₄:Yb³⁺,Er³⁺ Nps

Table 1 Saturation magnetization of Fe₃O₄ Nps, Fe₃O₄/NaYF₄:Yb³⁺,Er³⁺/PVP composite nanofibers, and [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers containing different mass ratios of NaYF₄:Yb³⁺,Er³⁺ Nps

By combining the analyses of magnetism and upconversion fluorescence, it is found that the [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers have the similar magnetic property in comparison with the Fe₃O₄/NaYF₄:Yb³⁺,Er³⁺/PVP composite nanofibers, whereas the upconversion emission intensity of the core–shell nanofibers is much higher than that of the composite nanofibers. These conclusions indicate that the core–shell nanofibers have better magnetic-upconversion fluorescent performance than the composite nanofibers.

Conclusions

In summary, uniform magnetic-upconversion fluorescent bifunctional [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers were successfully synthesized by coaxial electrospinning. The diameter of the core–shell nanofibers is ca. 260 nm. The Fe₃O₄/PVP core is successfully coated with NaYF₄:Yb³⁺,Er³⁺/PVP shell as a core–shell nanofiber. It’s very gratifying to see that the [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers possess high saturation magnetization, while the fluorescent emission intensity is barely weakened, because the Fe₃O₄ Nps are only distributed in the core of the [Fe₃O₄/PVP]@[NaYF₄:Yb³⁺,Er³⁺/PVP] core–shell nanofibers. Besides, the design conception and construction technique of the core–shell nanofibers are of universal significance for the fabrication of other magnetic-luminescent core–shell nanofibers. For instance, the fluorescent color of the core–shell nanofibers can be changed by introducing other fluorescent materials into them. The fluorescent and saturation magnetic intensities of the core–shell nanofibers can also be tuned by adding different concentrations of fluorescent material and Fe₃O₄ Nps into them, respectively. The new high-performance magnetic-upconversion fluorescent core–shell nanofibers have potential applications in display device, nanorobots, protein determination, target delivery of drug, etc.