Electrospun Flexible Coaxial Nanoribbons Endowed With Tuned and Simultaneous Fluorescent Color-Electricity-Magnetism Trifunctionality

In order to develop new-typed multifunctional nanocomposites, fluorescent-electrical-magnetic trifunctional coaxial nanoribbons with tunable fluorescent color, including white-light emission, have been successfully fabricated via coaxial electrospinning technology. Each stripe of coaxial nanoribbon is composed of a Fe3O4/PMMA core and a [Eu(BA)3phen+Dy(BA)3phen]/PANI/PMMA (PMMA = polymethyl methacrylate, BA = benzoic acid, phen = phenanthroline, polyaniline = PANI) shell. X-ray diffractometry (XRD), field emission scanning electron microscopy (FE-SEM), biological microscopy (BM), vibrating sample magnetometry (VSM), energy dispersive spectrometry (EDS), Hall effect measurement system and photoluminescence (PL) spectroscopy were employed to characterize the coaxial nanoribbons. Emitting color of the coaxial nanoribbons can be tuned by adjusting the contents of Dy(BA)3phen, Eu(BA)3phen, PANI and Fe3O4 in a wide color range of blue-white-orange under the excitation of 273-nm single-wavelength ultraviolet light. The coaxial nanoribbons simultaneously possess excellent luminescent performance, electrical conduction and magnetism compared with the counterpart composite nanoribbons. Furthermore, the electrical and magnetic performances of the coaxial nanoribbons also can be tunable by adding different quantities of PANI and Fe3O4 nanoparticles, respectively. The obtained coaxial nanoribbons have promising applications in many areas, such as electromagnetic interference shielding, microwave absorption, molecular electronics, biomedicine, future nanomechanics and display fields.

Scientific RepoRts | 5:14052 | DOi: 10.1038/srep14052 Synthesis of Eu(BA) 3 phen and Dy(BA) 3 phen complexes. Eu(BA) 3 phen powders were synthesized according to the traditional method as described in the reference 27 . 1.7600 g of Eu 2 O 3 was dissolved in an amount of concentrated nitric acid and then crystallized via evaporation of excess nitric acid and water by heating, and Eu(NO 3 ) 3 ·6H 2 O was acquired. Eu(NO 3 ) 3 ethanol solution was prepared by adding amount of anhydrous ethanol into the above Eu(NO 3 ) 3 ·6H 2 O. 3.6640 g of BA and 1.8020 g of phen were dissolved in ethanol. The Eu(NO 3 ) 3 ethanol solution was then added dropwise into the mixture solution of BA and phen with magnetic agitation at 60 °C for 3 h. The precipitates were collected by filtration and washed for three times using ethanol, and then dried in an electric oven at 60 °C for 12 h. The synthetic method of Dy(BA) 3 phen complex was similar to the above method, except that the using dosages of Dy 2 O 3 , BA and phen were 1.8650 g, 3.6600 g and 1.8000 g, respectively.
Preparation of PMMA. PMMA used in this study was prepared by oxidative polymerization of MMA 28 . MMA (100 mL) and BPO (0.1000 g) were mixed in a 250 mL three-necked flask with a backflow device and stirred vigorously at 90-95 °C. When the viscosity of the solution reached a certain value just like that of glycerol, the heating was stopped and it was left to naturally cool down to room temperature. The obtained gelatinous solution was then loaded into test tubes, and the influx height was 5-7 cm. After that, the tubes were put in an electric vacuum oven for 48 h at 50 °C, and the gelatinous solution was then solidified. Finally, the temperature in the oven was raised to 110 °C for 2 h to terminate the reaction. The weight-average molecular weight and the degree of polymerization (DP) value of as-prepared PMMA are 9.7 × 10 4 and 9.7 × 10 2 , respectively. Fe 3 O 4 NPs were obtained via a facile coprecipitation synthetic method 29 , and PEG was used as the protective agent to prevent the particles from aggregating. One typical synthetic procedure was as follows: 5.4060 g of FeCl 3 ·6H 2 O, 2.7800 g of FeSO 4 ·7H 2 O, 4.0400 g of NH 4 NO 3 and 1.9000 g of PEG were added into 100 mL of deionized water to form a uniform solution under vigorous stirring at 50 °C. To prevent the oxidation of Fe 2+ , 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 NH 3 ·H 2 O was dropwise added 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 black precipitates were formed. The precipitates were collected from the solution by magnetic separation, washed for three times with deionized water, and then dried in an electric vacuum oven at 60 °C for 12 h. The as-prepared Fe 3 O 4 NPs were then coated with oleic acid (OA) as below: 2.0000 g of the as-prepared Fe 3 O 4 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 mL of OA was dropwise 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 ethyl alcohol for three times, and then dried in an electric vacuum oven for 6 h at 60 °C.

Preparation of oleic acid modified Fe 3 O 4 NPs.
Preparations of spinning solutions for fabricating coaxial nanoribbons. Two different kinds of spinning solutions were prepared to fabricate coaxial nanoribbons. The spinning solution for the shell (denoted as spinning solution I) of coaxial nanoribbons was composed of Dy(BA) 3 phen, Eu(BA) 3 phen, PANI, PMMA, DMF and CHCl 3 , and detailed preparation process for spinning solution I was as following: certain amount of ANI and CSA, and 0.5 g of PMMA were dissolved in the mixed solution of 0.3 g of DMF and 6 g of CHCl 3 with magnetic stirring for 48 h at room temperature (defined as solution A). Meanwhile, APS was used as an oxidant and dispersed into a mixed solution of 0.6 g of DMF and 3 g of CHCl 3 with magnetic stirring for at least 2 h at room temperature (defined as solution B). Then solution A and B were both cooled down to 0 °C in an ice-bath. Subsequently, solution B was added dropwise into solution A under magnetic stirring. The final mixture was allowed to react at 0 °C for 24 h to produce PANI by the polymerization of aniline 30,31 . Then certain amounts of Eu(BA) 3 phen and Dy(BA) 3 phen complexes were added into the mixture under magnetic stirring for another 12 h at room temperature, thus spinning solution I for the shell was prepared.
Compared to Eu(BA) 3 phen, Dy(BA) 3 phen has weaker luminescence intensity. Therefore, in this study, we need firstly find the optimum concentration of Dy(BA) 3 phen in the shell to guarantee the luminescence intensity of Dy(BA) 3 phen to reach maximum, and then different amounts of Eu(BA) 3 phen are introduced into the shell to realize tunable color. In order to find the optimum concentration of Dy(BA) 3 phen, a series of Dy(BA) 3 phen/PANI/PMMA composite nanoribbons were fabricated. For performing this study, the mass percentage of ANI to PMMA was settled as 30%, the mass percentages of Dy(BA) 3 phen to PMMA were varied from 120% to 240%. The corresponding samples were marked as a, b, c, d and e. The compositions and contents of these composite nanoribbons were listed in Table 1. Based on photoluminescence analysis discussed in section "Photoluminescence property", the mass percentage of Dy(BA) 3 phen to PMMA settled as 180% was adopted to prepare [Fe 3 O 4 /PMMA]@ {[Dy(BA) 3 phen+ Eu(BA) 3 phen]/PANI/PMMA} coaxial nanoribbons. The dosages of materials used for preparing spinning solution I were shown in Table 2.
The other spinning solution for the core of coaxial nanoribbons consisted of certain amounts of OA modified Fe 3 O 4 NPs, 0.5 g of PMMA, 9 g of CHCl 3 and 0.9 g of DMF (denoted as spinning solution II). Fe 3 O 4 NPs were dispersed in DMF and CHCl 3 with the assist of ultrasonics for 15 min, and then PMMA was added into the above solution under mechanical stirring. In order to investigate the impact of Fe 3 O 4 NPs on the properties of coaxial nanoribbons, various contents of Fe 3 O 4 NPs were introduced into spinning solution II. The compositions and contents of the spinning solutions were summarized in Table 3. The obtained coaxial nanoribbons were denoted as S ax @S by (x = 1-9; y = 1-3) according to the corresponding spinning solution I and II.

Preparations of tunable multicolor and white-light emissions luminescent-electrical-magnetic trifunctional coaxial nanoribbons.
A homemade coaxial electrospinneret was used in this study.
The equipment for the electrospinning process is presented in Fig. 1. Spinning solution I was loaded into the outer plastic syringe while spinning solution II was loaded into the inner plastic syringe. A piece of flat iron net was used as a collector and put about 10 cm away from the nozzle tip. The positive terminal of a direct current (DC) high voltage power supply was connected to the carbon electrode which was immersed into the spinning solution II, and the negative terminal was connected to the iron net. Positive DC voltage of 6 kV was applied between the nozzle and the collector to generate coaxial nanoribbons under the ambient temperature of 20-25 °C, and the relative humidity of 45-50%.    The morphology and size of the coaxial nanoribbons were observed by a field emission scanning electron microscope (FESEM, XL-30) equipped with an energy-dispersive X-ray spectrometer (EDS). The internal structure of the coaxial nanoribbons was observed by a biological microscope (CVM500E). The measurements of photoluminescence (PL) spectra and the luminescence decay curves were performed by a HITACHI F-7000 fluorescence spectrophotometer using a 150 W Xe lamp as the excitation source, and scanning speed was fixed at 1200 nm·min −1 . The excitation and emission slits were both set to 5.0 nm. Then, the magnetic performance of Fe 3 O 4 NPs, coaxial nanoribbons and composite nanoribbons was measured by a vibrating sample magnetometer (VSM, MPMS SQUID XL). The conductive property was detected by Hall effect measurement system (ECOPIA HMS-3000). The ultraviolet-visible spectra of samples were determined by a UV-1240 ultraviolet-visible spectrophotometer. All the measures were performed at room temperature.

Results and Discussion
Crystallization behavior. XRD   Photoluminescence property. In order to find appropriate content of Dy(BA) 3 phen, a series of Dy(BA) 3 phen/PANI/PMMA composite nanoribbons were fabricated by electrospinning using different spinning solutions indicated in Table 1. The excitation and emission spectra of Dy(BA) 3 phen/PANI/ PMMA composite nanoribbons are provided in Fig. 4. From the excitation spectra (Fig. 4A, left), a broad excitation band extending from 200 to 350 nm is observed from each sample when monitoring wavelength is 574 nm. The strongest peak at 273 nm assigned to the π → π * electron transition of the ligands could also be identified. Characteristic emission peaks of the Dy(BA) 3 phen are observed under the excitation of 273-nm ultraviolet light, which are ascribed to the energy levels transitions of 4 F 9/2 → 6 H 15/2 (481 nm) and 4 F 9/2 → 6 H 13/2 (574 nm). One can see that the photoluminescence intensity of Dy(BA) 3 phen/ PANI/PMMA composite nanoribbons is increased with adding more Dy(BA) 3 phen. In order to further discuss the variation trend, the intensities of predominant emission peaks at 481 nm and 574 nm versus different mass percentages of Dy(BA) 3 phen to PMMA are plotted in Fig. 4B. Obviously, the fluorescence intensity only slightly increases with introducing more Dy(BA) 3 phen than the mass percentage of 180%. Therefore, the mass percentage of Dy(BA) 3 phen to PMMA settled as 180% was adopted to prepare To study the color-tunable property of coaxial nanoribbons, the mass percentages of Eu(BA) 3 phen to PMMA were varied from 0 to 5% (samples S ax @S b1 , x = 1-7), while the mass percentage of PANI to PMMA was settled as 30% and the mass ratio of Fe 3 O 4 to PMMA was fixed as 1:1. Figure 5A shows excitation spectra of coaxial nanoribbons monitored at 574 nm, where 574 nm is the characteristic emission wavelength of Dy 3+ . Figure 5B demonstrates the excitation spectra of the samples monitored at 616 nm, where 616 nm is the characteristic emission wavelength of Eu 3+ . The strongest peak at 273 nm assigned to the π → π * electron transition of the ligands could be also identified, and the excitation intensity is increased with introducing more Eu(BA) 3 phen. The blue shift of excitation peak of Dy 3+ is probably due to the strong absorption of Eu(BA) 3 phen around 273 nm, resulting in the decrease of light absorption of Eu(BA) 3 phen around 273 nm. Figure 6A displays the emission spectra of sample S ax @S b1 (x = 1-7). Upon excitation with 273-nm ultraviolet light, coaxial nanoribbons exhibit several main emission bands, whose positions locate at     481 nm, 574 nm, 592 nm and 616 nm. It is found that the emissions at 481 and 574 nm are due to the energy levels transitions of 4 F 9/2 → 6 H J/2(J = 15,13) of Dy 3+ and the peaks at 592 and 616 nm are corresponding to the energy levels transitions of 5 D 0 → 7 F 1 and 5 D 0 → 7 F 2 of Eu 3+ , respectively. It is interesting and reasonable to suggest that the emission intensity of Eu 3+ is increased, whereas that of the Dy 3+ is simultaneously found to decrease monotonically with the increase of Eu 3+ ions 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 Fig. 6B. The variation of the PL intensity of the Eu 3+ and Dy 3+ can be attributed to the energy distribution. Since the energy that the matrix absorbs and the content of Dy(BA) 3 phen are constant, more energy is assigned to Eu 3+ with the increase of Eu(BA) 3 Table 4 and Fig. 7. It is found that the emitting color of coaxial nanoribbons (sample S ax @S b1 x = 1-7) could be tuned by adjusting the mass ratio of Eu(BA) 3 phen complexes in a wide color range of blue-white-orange. Among all the coaxial nanoribbons, the coaxial nanoribbons with 0.7% Eu 3+ (sample S a4 @S b1 ) have coordinates of x = 0.344, y = 0.337 which are close to those of standard white light (x = 0.333, y = 0.333), indicating that the as-obtained coaxial nanoribbons can emit warm white-light color. It is gratify to see that the warm white emission can be selectively realized by the co-doping of Dy(BA) 3 phen and Eu(BA) 3 phen complexes into coaxial nanoribbons.

phen content,
The fluorescence lifetime curves of Dy 3+ emission at 574 nm and Eu 3+ emission at 616 nm in samples S ax @S b1 (x = 1-7) under the excitation of 273-nm ultraviolet light are shown in Fig. 8 (A,B). From Fig. 8(A,B), one can see that all the luminescent decay lifetimes fit the single exponential rule by the following equation as the Fig. 8 (A,B) depicts.
Where I t and I 0 are the luminescence intensities at times t and 0, respectively, t is the decay time and τ is the lifetime. Figure 8A shows that fluorescence decay lifetime of the 4 F 9/2 → 6 H 13/2 transitions (λ em = 574 nm) in coaxial nanoribbons is extended with increasing in Eu 3+ concentration, while fluorescence decay lifetime of the 5 D 0 → 7 F 2 transitions (λ em = 616 nm) decreases, as shown in Fig. 8B. On one hand, the relative content of Dy(BA) 3 phen complex in the coaxial nanoribbons is reduced with introducing more Eu(BA) 3 phen. Thus the distance among Dy 3+ in Dy(BA) 3 phen molecular clusters and/ or nanoparticles in the coaxial nanoribbons is increased, resulting in that the energy transfer among Dy 3+ to Dy 3+ is reduced and the fluorescence lifetime of Dy 3+ is prolonged. On the other hand, more Meanwhile, the coaxial nanoribbons containing different amounts of PANI and Fe 3 O 4 NPs were fabricated to research the effect of adding different contents of Fe 3 O 4 NPs (samples S a3 @S b1 , S a3 @S b2 , S a3 @ S b3 , as illustrated in Fig. 9) and PANI (samples S a3 @S b1 , S a8 @S b1 , S a9 @S b1 , as shown in Fig. 10) on the fluorescent properties of the coaxial nanoribbons. As shown in Fig. 9 (A-C), the excitation and emission intensity of coaxial nanoribbons are decreased with the increase of Fe 3 O 4 NPs content. Figure 9D is the CIE chromaticity coordinate diagram of coaxial nanoribbons with different Fe 3 O 4 NPs contents under the excitation of 273-nm ultraviolet light. It demonstrates that the emitting color of the coaxial nanoribbons shifts with introducing more Fe 3 O 4 NPs. Similarly, when the amount of PANI is increased, the excitation and emission intensity of coaxial nanoribbons are decreased as illustrated in Fig. 10 (A-C).  The CIE chromaticity coordinates for the samples and their corresponding photographs upon excitation at 273-nm ultraviolet light are provided in the Fig. 10D. It is found that the emitting color of coaxial nanoribbons could be shifted by adjusting the mass ratio of PANI.
The above results can be explained as the light absorption of Fe 3 O 4 NPs and PANI. From the absorption spectra of Fe 3 O 4 NPs and PANI illustrated in Fig. 11, it is seen that PANI doped PMMA strongly absorb the light in the regions of ultraviolet light (< 400 nm) and 400-800 nm, and Fe 3 O 4 NPs can absorb visible light (400-760 nm) and much more easily absorb the ultraviolet light (< 400 nm). Thus, the exciting light and emitting light are absorbed by the PANI and Fe 3 O 4 NPs, resulting in the decrease in the intensity of excitation and emission peaks. Moreover, the light absorbance becomes stronger with more PANI and Fe 3 O 4 NPs introduced into coaxial nanoribbons. On the other hand, because PANI has different absorbance to different wavelengths of light, as well as Fe 3 O 4 NPs, as seen in Fig. 11, different wavelengths of light emitted from coaxial nanoribbons are unequally absorbed by PANI and     Electrical conductivity analysis. The conductivity of coaxial nanoribbons can be tuned by adjusting the mass percentage of PANI to PMMA. It is found from Table 5 that the electrical conductivities of products are 9.42 × 10 −4 S·cm −1 , 4.26 × 10 −3 S·cm −1 and 1.47 × 10 −2 S·cm −1 when percentages of PANI is 30 wt%, 50 wt % and 70 wt %, respectively. Obviously, the more PANI introduced into the coaxial nanoribbons, the higher electrical conductivity of the electrospun coaxial nanoribbons. As PANI is consecutive in the shell of coaxial nanoribbons and probably forms the conducting network more easily, which renders more efficient charge transport. The conductivity of coaxial nanoribbon (sample S a3 @S b1 ) is nearly slightly bigger than that of Fe 3 O 4 / [Dy(BA) 3  which is close to that of the coaxial nanoribbons (7.23 emu·g −1 , S a3 @S b1 ). By combining the analyses of magnetism, electrical conductivity and fluorescence, it is found that the coaxial nanoribbons have close magnetic property compared with composite nanoribbons, while the fluorescent intensity and electrical conductivity of the coaxial nanoribbons are much higher than those of the composite nanoribbons, and those further demonstrate that the coaxial nanoribbons possess better luminescent-electrical-magnetic performance than the counterpart composite nanoribbons. Based on the above experimental results, we can safely conclude that the shortcomings of the existing luminescence-electricity-magnetism 1D nanomaterials described in the introduction have been greatly overcome.

Conclusions
[Fe 3 O 4 /PMMA]@{[Dy(BA) 3 phen+ Eu(BA) 3 phen]/PANI/PMMA} coaxial nanoribbons with tunable fluorescent color, magnetism and electricity trifunctionality were successfully synthesized by electrospinning technology using a specially designed coaxial spinneret. The core of the coaxial nanoribbons is composed of Fe 3 O 4 NPs and PMMA, and the shell consists of Dy(BA) 3 phen, Eu(BA) 3 phen, PANI and PMMA. Under the excitation of 273-nm single-wavelength ultraviolet light, the emitting color of the coaxial nanoribbons can be tuned in a wide color range of blue-white-orange by adjusting the mass ratio of Dy(BA) 3 phen and Eu(BA) 3 phen complexes. More significantly, warm white luminescent color can be achieved. It is also found that PANI and Fe 3 O 4 NPs affect luminescent color as well. The luminescent intensity, electrical conductivity and magnetic property of the coaxial nanoribbons can be tunable by adjusting the contents of RE complexes, PANI and Fe 3 O 4 NPs, respectively. It is very exciting to see that the coaxial nanoribbons simultaneously possess excellent luminescent performance, electrical conduction  and magnetic properties. The new high-performance luminescent-electrical-magnetic trifunctional coaxial nanoribbons have potential applications in molecular electronics, microwave absorption, color display device and future nanodevice.