Facile Access to Wearable Device via Microfluidic Spinning of Robust

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Facile Access to Wearable Device via Microfluidic Spinning of Robust and Aligned Fluorescent Microfibers Tingting Cui, Zhijie Zhu, Rui Cheng, Yu-long Tong, Gang Peng, Cai-feng Wang, and Su Chen* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Jiangsu Key Laboratory of Fine Chemicals and Functional Polymer Materials, Nanjing Tech University, 5 Xin Mofan Road, Nanjing 210009, P. R. China

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ABSTRACT: Microfluidic spinning technology (MST) has drawn much attention owing to its ideal platform for ordered fluorescent fibers, along with their large-scale manipulation, high efficiency, flexibility, and environment friendliness. Here, we employed the MST to fabricate a series of uniform fluorescent microfibers. By adjusting the microfluidic spinning parameters, the as-prepared microfibers of different diameters are successfully obtained. For more practice, these regular arranged fibers could be formed to versatile fluorescent codes by using various microfluidic chips. Also, these versatile fluorescent fibers could be further weaved into a white fluorescent film via continuous and cross-spinning process, which could be applied in a white light emitting diode (WLED) and a wearable device. Besides, we investigated the MST-directed microreactors to carry out green synthesis of CdSe quantum dots (QDs) fibers by the knot of Y-type microfluidic chip. The as-prepared CdSe QDs show nice optical property and are good candidate as phosphors in WLED. This strategy offers a facile and environment-friendly route to fluorescent hybrid microfibers and might open their potential application in optical devices, security, and fluorescent coding. KEYWORDS: aligned fibers, microfluidic spinning, in suit microreactor, fluorescent coding, wearable device

1. INTRODUCTION Superfine micro/nanofibers or fiber microreactors have gained widespread attention in recent years due to their significant potential applications in tissue engineering,1 sensor,2 and wearable device.3 The ability to fabricate fibers in an ordered fashion at nano or micro size is highly desirable. Recently, much effort has been devoted to the preparation of highly compact flexible and lightweight fluorescent fiber film.4 Especially, the most reported methods in this respect focused on electrospinning,5−8 melt spinning,9−11 centrifugal spinning,12 and wet spinning.13−15 Among these techniques, electrospinning is the most versatile method for fabricating fibers with various polymers, such as poly(methyl methacrylate),16 poly(L-lactide),17 poly(vinyl alcohol),18,19 and polystyrene.20 However, it is normally hard to generate ordered arrays by these methods. Although recently some emerging methods (continuous draw spinning,21 spray printing,22 and inkjet printing23) have prepared ordered microfiber arrays, these devices are still costly, time consuming, and complicated. If performed in a controlled and reliable manner, such methods would benefit various research efforts by involving flexible and wearable fabrics, such as flexible displays,24 health monitors,25 wearable detectors,26,27 and smart coding.28 Thus, seeking an available and simple way for ordered microfiber arrays is the top concern. Alternatively, microfluidic spinning as a new generation of fiber fabrication method is widely studied owing to its simplicity, safety, diversity, low cost, and scalable manufacturing process.29−32 In this respect, Lee et al. applied a © XXXX American Chemical Society

microfluidic system consisting of a digital and programmable flow control that mimicked the silk-spinning process of spiders to fabricate alginate fibers with highly ordered structures.33,34 Gu et al. employed microfluidic spinning method to prepare alginate microfibers with tunable, morphological, structural, and chemical features.35 Förster et al. reported a microfluidicproduced collagen fiber with extraordinary mechanical property.36 Wang et al. used a simple gas-in-water microfluidic method to prepare cavity microfibers for large-scale water collection.37 Our previous work has constructed a series of functional microfibers with fluorescent38,39 or conductive performance40 via microfluidic spinning. However, only few examples reported on production of light-emitting diode (LED) fluorescent films for wearable and portable devices via microfluidic spinning. In this work, we demonstrated a method that combined microfluidic spinning with different chips for the preparation of ordered fibers with diameters ranging from 0.8 to 20 μm. First, we successfully constructed a white fluorescent film for preparing wearable device by using fluorescent quantum dots (QDs) hybrid fiber film (Scheme 1a). The fluorescent fiber film prepared by this method has excellent optical property and transparency (∼84%), along with good flexibility and mechanical properties (mechanical stretching of ∼190%). Received: July 16, 2018 Accepted: August 16, 2018 Published: August 16, 2018 A

DOI: 10.1021/acsami.8b11926 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Scheme 1. Preparation of Fluorescent Fiber Films and Its Applications in Coding, WLED, and Wearable Ringa

a (a) Schematic illustration of the fabrication of fluorescent fiber films and its application in wearable ring. (b) Schematic illustration of preparation of CdSe QDs/PVP fibers by in situ reaction under the nodes of Y-type microfluidic chip and then milling into CdSe/PVP fluorescent powders to be used in WLED. (c) Schematic representation of ordered fluorescent coding (red, green, and blue).

Figure 1. Continuous spinning process of fiber film. (a) The photograph of MST process and the prepared fiber film, which is wrapped around a hollow three-dimensional (3D)-printed poly(lactic acid) (PLA) frame. (b) The optical images of the fiber arrays with different spacing and diameters according to different forward speeds. (c) The relationship between the diameters of fiber and rotational speeds, forward speeds of microfluid machine, and PVP concentrations of spinning solutions.

And then, we developed a microfiber reactor platform for green synthesis of QDs via microfluidic spinning technology (MST). When two kinds of ions (Cd2+ and Se2−) met at the knot of a Y-type microchip, CdSe QDs poly(vinylpyrrolidone) (PVP) hybrid fluorescent microfibers would be generated

immediately. Principally, the proposed approach not only breaks through the limitation of in situ reaction but also offers a green pathway for facile (within 30 min) and large-scale (6 cm × 6 cm) access CdSe QDs fiber films under mild reaction temperature (110 °C), which is quite lower than that of B


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Figure 2. Schematic illustration of the fluorescent coding by microfluidic chip and spinning process. (a) A microfluidic chip with three individual channels. (b) The scheme of fluorescent coding. (c) The schematic illustration of a 123123-type fluorescent coding array (1, red; 2, blue; 3, green). (d) The optical images of a fluorescent coding array. Schematic illustration of (e) 112233-type fluorescent coding. (f) 121312323-type fluorescent coding.

traditional method for preparing CdSe QDs (about 230 °C).41 Then, the as-produced CdSe QDs fibers were milled into phosphor powders for preparing the white light emitting diode (WLED). The WLED shows color rendering index (CRI) of 72 and wide color gamut coordinates (0.3251, 0.2667) (Scheme 1b). Finally, we developed a controllable microfluidic-directed method for facilely constructing a series of fluorescent codings. The method is a simple, low cost, and efficient, which contains various coding information compared to the latest methods (digital regulation method,34 cutoff lithography42) (Scheme 1c). It is worth noting that these fluorescent fiber films might find their potential applications in optical devices, especially for security and fluorescent codings. These findings might allow microfluidic spinning to provide easy-to-perform platform for microreactors, ordered array codings, and wearable devices.

reducing the surface tension. Accordingly, we carried out various experiments on the relationship between fiber diameter and spin speed. When the microfluidic spinning forward speed is lower than 250 μm/s, adjacent fibers tend to adhere together. In contrast, with increase in the spin speed, the fibers are arranged in an orderly network. But, the space between the fibers is different for different forward speeds, as shown in Figure 1b (600, 450, and 350 μm/s) and the microscopic images of Figure S2. Figure 1c shows that the fiber diameter is determined by the rotational speeds, forward speeds, and spinning solution concentrations. The fiber diameters also decrease with increasing rotational speeds and forward speeds while maintaining the PVP concentration at 24 wt %. Furthermore, when we fixed the rotation speed at 450 rpm and the forward speed at 700 μm/s, the fiber diameters increased from 0.8 to 20 μm, with the PVP concentrations increasing from 16 to 26 wt %, respectively. By adjusting the parameters during MST, the diameters of the fibers and the fiber spaces could be controlled and the extra-long PVP/QDs fibers with various diameters could be generated. 2.2. MST-Directed Fluorescent Codings. In practice, it is desirable to obtain ordered fiber arrays by MST. So, we make use of this method to fabricate fluorescent codings (Figure 2). Recently, fluorescent codings as an information carrier have attracted more and more attention. Other approaches, including digital regulation method34 and cutoff lithography,42 have been reported for preparing various codings. However, these reported methods are complicated, costly, and not easily portable, limiting their further applications. To this end, we propose a controllable fluorescent coding method using a microfluidic chip, which applied soft polymer PVP and QDs to produce various fluorescent coding arrays (123123, 112233, 121312321 types) based on different microfluidic chips. In a typical run, three kinds of spinning solutions (QDs/PVP with red and green fluorescence and carbon dots (CDs)/PVP with blue fluorescence) were injected into three individual syringes, which were connected to a three-channel microfluidic chip (Figure 2a). Subsequently, the red, green, and blue fluorescent spinning solutions were simultaneously extruded from the syringes and the rotating device drove the rotation of the receiver so that the droplets

2. RESULTS AND DISCUSSION 2.1. MST-Directed Fibers with Controllable Microfiber Diameters. The present approach was focused on fabrication of the extra-long PVP/QDs fibers via the MST, which could produce continuous and uniform microfibers. The extra-long microfiber was received by a frame and microfluidic device (Figure 1a). Meanwhile, the diameters of obtained microfibers could be easily adjusted by the rotation speed of the substrate and the flow rate of the solution. Compared with conventional electronic spinning technology, this method does not need to use high voltage or other auxiliary measures. Although the size of the frame is only 6 cm × 6 cm, we calculated the theoretical length of the as-prepared fiber to reach about 1413 m (L = πdvt) according to the parameters (v: rotational speed 250 rpm, t: time is 30 min, d: frame length is 6 cm). Furthermore, we also designed a 10-needle microfluidic chip that could produce uniform microfibers continuously and simultaneously, along with high spinning productivity and efficiency. Its preparation process is shown in Figure S1 and Video S1. In the process of building up one-dimensional microfibers, ubiquitous cracks occur easily due to a stream of liquid flow originating from Rayleigh instability and high fiber surface tensions. To solve this, we added a special surfactant (sodium dodecyl sulfate, SDS) into the spinning precursor for C


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Figure 3. Preparation and properties of white fluorescent fiber film. (a) The diagram of red, green, and blue three primary colors (three-channel microfluidic chip) composite into white fluorescent fiber film. (b) Emission spectra of fluorescent fiber film. Inset: the sample image. (c) The UV− vis transmission spectra of the fiber films. Inset: (d) mechanical properties of the fiber films. (e) Fourier transform infrared (FT-IR) spectra of the pure PVP (curve 1), pure poly(amidoamine) (PAMAM) (curve 2), and PVP/PAMAM (curve 3). (f) The scheme of WLED by attaching composite fluorescent film on a UV chip.

pulled into fibers by traction. And, the three-pump propulsion speeds were set at the same flow rate (0.1 mL/h). Figure 2b and Video S2 show the preparation process of fluorescent codings. And, the well-aligned 123123-type fluorescent codings are indicated in Figure 2c,d. As can be seen, the as-prepared fibers are continuously and regularly spun onto substrates, as well as the three kinds of fluorescent fibers (blue, green, and red) are separated by equal distances. Moreover, the fiber spacing can be adjusted by adjusting the advance speeds of the spinning device. Figure S3 presents the ordered arrangement of fibers in detail. Similarly, by using the six-channel and ninechannel microfluidic chip, the 112233-type and 121312323type fluorescent codings are achieved (Figure 2e,f). More importantly, the codings are flexible, programmable, and woven, allowing them to be applied in flexible wearable devices. Also, the coding information is rich and safer owing to the dual responses (fluorescent response and digital controllability). Therefore, a series of digital codes, along with various fluorescent patterns, are created that have various potential applications, such as biosensing,43 anti-counterfeiting,44 forensic labeling,45 and flexible wearable devices. 2.3. Construction of White Fluorescent Fiber Film via MST. In another attempt, we applied a three-channel microfluidic chip to prepare white fluorescent fiber film based on aforementioned MST. The process is achieved by

combining the two CdSe QDs/PVP precursor solutions (λem1 = 610 nm, red; λem2 = 545 nm, green) and the CDs/PVP precursor solution (λem3 = 450 nm, blue) into a microfluidic channel and spinning them simultaneously onto a receiver. Figure 3 shows the fabrication procedure of the fluorescent films and their physiochemical properties. As indicated in Figure 3a, the three precursor solutions converge at the node of the three-microfluidic chip and form a white fluorescent stream under ultraviolet light, based on the principle of three primary colors. With the aid of the traction force of the rotating machine, the droplets are pulled into fibers and then assembled into a white fluorescent fiber film. Finally, we peeled off the white fluorescent film from the fiber receiver for preparing a wearable ring. The actual preparation process is shown in Figure S4. The scanning electron microscopy (SEM) images of the white fluorescent fiber mesh at different scale bar are demonstrated in Figure S5. It is observed that the fluorescent fibers are orderly arrayed, which is further confirmed by the corresponding microscopic images (Figure S6). Then, we investigated the fluorescent properties of the asprepared fiber film by photoluminescence (PL) spectrum. As indicated in Figure 3b, three emission peaks are noticed at 445 nm (blue), 522 nm (green), and 595 nm (red). The corresponding image of the film is shown in the inset of Figure 3b, which shows white fluorescent under ultraviolet D


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Figure 4. In situ preparation of CdSe QDs fiber films by MST. (a) Schematic illustration of in situ synthesis of CdSe QDs by a Y-type microfluidic chip. Fluorescence microscopy images of (b) green, (c) yellow, and (d) red monolayer CdSe/PVP fluorescent fibers. (e) PL emission spectra of green, yellow, and red CdSe/PVP fluorescent fibers (λ = 542, 571, and 638 nm, respectively). (f) HRTEM image of CdSe/PVP QDs fiber. (g) The EDS spectrum of CdSe/PVP fiber powder. (h) Fluorescence lifetimes of green, yellow, and red CdSe/PVP fluorescent fibers. (i) Sample image of fluorescent fiber array and fluorescent microscopy images of (j) green, (k) yellow, and (l) red CdSe/PVP fluorescent fibers array.

To attach the fiber film to the wearable device, it needs to be peeled off from the fiber receiver after the spinning process. Therefore, to improve the mechanical properties of the fiber film, G3 poly(amidoamine) (PAMAM) dendrimer was added. Compared with pure PVP fiber film, the tensile strength of CDs/PVP/PAMAM fiber film increased from 0.941 to 2.089 MPa, whereas the elongation at break remains essentially constant (Figure 3d). This could be attributed to the intermolecular hydrogen bonds. As indicated by FT-IR spectra in Figure 3e, the absorption peak of 3432 cm−1 shifts to 3375 cm−1, the deformation vibrational peak of N−H shifts from 1496 to 1476 cm−1 and the stretching vibration peak of CO shifts from 1664 to 1644 cm−1 after PAMAM doping. This change may be attributed to the hydrogen-bond interaction between PVP and G3 PAMAM dendrimers. This result also reveals that a novel hydrogen bond has been formed in the composite film, leading to an obvious enhancement of mechanical stiffness and tensile property of this film. After attaching the fluorescent film (been attached on a flexible poly(ethylene terephthalate) (PET) substrate) on a UV chip, the white light is observed and the wearable ring can be realized (Figure 3f). The detailed assembly process of the wearable ring involves the following three steps. In the first step, four UV chips are inserted on a 16 cm × 2 cm thin and flexible sheet and then connected to four UV chips with a copper wire to form a series circuit. In the second step, the prepared white fluorescent fiber film based on the PET film was attached to the area of the four ultraviolet chips and both ends of the fiber film were fixed on the sheet. Finally, the positive and negative ends of the copper wire were soldered to the positive and negative ends of the supercapacitor, respectively, and the ultraviolet light emitted by the ultraviolet chip first penetrated the PET film and then the white fluorescent film to emit white light. Therefore, from the

radiation. It is worth noting that the mass ratio of three different fluorescent phosphors should be precisely controlled (red/green/blue = 5:7:32). Figure S7 is the electroluminescence (EL) spectrum of the WLED fabricated by the fiber film. Three peaks are obviously perceived, which correspond to blue CDs/PVP (445 nm), green CdSe QDs/PVP (522 nm), and red CdSe QDs/PVP (595 nm). Its color rendering index (CRI) was determined to be 70.9 at 350 mA, and the color coordinate was realized at (0.3281, 0.2711) (Figure S8), belonging to the white light region. These results suggest that the white fluorescent films could be successfuly done and act as wearable device. However, it is well established that fluorescent fibers with unique optics properties have been developed into fluorescent labeling,46 fluorescent detection,47 and fluorescent encoding.48 However, it is still rare that fluorescent fibers are directly used as fluorescent materials for preparing WLED. The transmittance and mechanical properties of white fluorescent films are key parameters for their practical applications in LED. We studied the effect of fluorescent substances on the transmittance of fluorescent films. As indicated in Figure 3c, the pure PVP film exhibits high transparency of up to 84% in the visible spectrum. On the other hand, the CdSe QDs/PVP and CDs/PVP composite fluorescent films still have transparency of 75%, accounted for by the incorporation fluorescent phosphors. The inset in Figure 3c shows the sample images of the pure PVP fiber film and the QD/PVP fiber film. Moreover, Figure S9 shows the flexibility and lightweight properties of the fiber film, which could effortlessly stand on a leaf. Figure S10 shows the flexibility of the as-prepared films in the wearable device with different bend angles; it is noticed that the bending angle can reach 180°, indicating good flexibility of the white fluorescent fiber film. E


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Figure 5. Fluorescent properties of fluorescent fiber membranes and their optical applications. (a) The sample image of CdSe QDs/PVP fluorescent fiber film. (b) The digital photos of CdSe/PVP powders under UV light. (c) EL spectrum of WLED based on CdSe/PVP powders operated at 350 mA. Inset: photographs of WLED under daylight. (d) Placement of the WLED emission spectra on the CIE 1931 chromaticity chart. (e) The photograph of as-prepared WLED device in the dark. (f) The sample image of wearable WLED ring.

60 °C, respectively. Under these conditions, solvent (ethanol) may evaporate instantly, with the polymer precursor easily spun into uniform fibers as well. The fluorescence spectra of CdSe QDs with green, yellow, and red fluorescence are shown in Figure 4e. The emission peaks of these CdSe QDs fibers are at 542, 571, and 638 nm with narrow half peaks. We also employed a high-resolution transmission electron microscopy (HRTEM) analysis. From Figure 4f, it can be seen that CdSe QDs have a good monodispersity and lattice fringe, with the average particle size of about 4 nm. The energy-dispersive Xray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) all validate that the composition of QDs is cadmium and selenium (Figures 4g and S13). Table S1 shows the operating parameters for inductively coupled plasma mass spectrometry (ICP-MS); according to the formula C × V × F/ m (C: instrument, V: volume, F: dilution factor, m: mass), the result shows that Cd and Se contents are 19 615 and 15 452 mg/kg, respectively. Moreover, we further use X-ray power diffraction (XRD) to verify the results of the existence of CdSe QDs. As Figure S14 shows, the obtained product has obvious diffraction peaks at a 2θ value of 25.5, 42.4, and 49.8°, corresponding to the (111), (220), and (311) crystal planes of the cubic CdSe, respectively. And, their fluorescence lifetime values were 4.0, 4.4, and 5.3 ns (green, yellow, and red), respectively (Figure 4h). We further make use of MST to facilely weave the CdSe QDs-loaded grid fiber film (Figure 4i). The corresponding microscopic image shows that the obtained fluorescent fiber grid has a uniform spacing distribution at a scale bar of 50 μm (Figure 4j−l). This method of in situ synthesising of QDs by MST at ultrafine scales and mild reaction conditions can provide fundamental insight into chemistry in confined microreactors. 2.5. Application of QDs Derived from Fiber Microreactors. We further investigated the as-prepared CdSe QDs phosphors to be applied in LED and wearable devices. These phosphors are derived from fiber microreactors and should have good compatibility with the spinning fibers. As illustrated in Figure 5, bright yellow fluorescence can be clearly observed

assembling process, it is found that this structure can effectively alleviate light dispersion caused by direct contact of phosphor in conventional LED package and has poor absorption and heat dissipation and low reliability series problems. Besides, such a device has a simple structure and is easy to install, as shown in the images of wearable devices in Figure S11. 2.4. Green Synthesis of CdSe QDs by Fiber Microreactors. This MST was also utilized as a microreactor platform for the preparation of CdSe QDs. In this case, the uniform CdSe QDs fluorescent fibers were continuously obtained through in situ reaction of two precursor spinning solutions containing Cd2+ and Se2− at the node of a Y-type microfluidic chip (Figure S12), and the scheme illustration is shown in Figure 4a. Then, the as-prepared fluorescent fibers were annealed at 110 °C to accelerate solvent evaporation and promote CdSe QDs generation. It is well known that traditional methods for preparing CdSe QDs not only require high reaction temperatures (230−280 °C) and long reaction times (2−4 h) but are also likely to cause environmental pollution owing to a number of Cd2+ residual liquids.49 Therefore, the MST offered a green pathway to facile (within 30 min) and large-scale (6 cm × 6 cm) access to CdSe QDs fiber film under mild reaction temperature (below 110 °C). Even more noteworthy was the fact that these QDs fluorescent fiber films are produced in situ via MST and the reported methods for achieving in situ reaction are relatively few. Interestingly, when we adjust the concentrations of Se2− and Cd2+ in the spinning precursor, various fluorescent CdSe QDs could be synthesized in situ. The morphology of the CdSe QDs fibers are characterized by optical microscopy and SEM, as shown in Figure 4b−d. As can be seen, the overall diameter along the fiber axis is uniform, and the corresponding SEM images are shown in the inset of Figure 4b−d. From the SEM images, it can be noticed that the fiber surface is smooth owing to the addition of surfactant during the spinning process. In the short period of time, it experienced rapid solvent evaporation and fiber curing process. Moreover, the humidity and temperature of the current system are optimized at 70% and F


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sample to move at a constant speed, in turn, leading to equidistance between the microarrays. 4.3. Preparation of Spinning Solutions. 4 g PVP was dissolved in the ethanol of 15 g, and then 0.5 g SDS and 1 g G3 PAMAM were added to the mixture to prepare 26.67 wt % PVP solution at ambient temperature. And, the 26.67 wt % mixed solution (20 g) was added to fluorescent CdSe QDs solution (0.5 g), green fluorescent CdSe QDs solution (0.7 g), and blue CDs (3.2 g). After well mechanical stirring, three kinds of fluorescent spinning solutions were obtained, which were then loaded into three 15 mL syringes. 4.4. Preparation of the Well-Aligned Fluorescent Coding. The three kinds of syringes with different spinning solutions were connected to the three-channel microfluidic chip. The three channels were independent and have the same channel diameter. The inner and outer diameters of the channels were 0.5 and 0.6 mm in the present experiments. Then, the three syringes were simultaneously extruded and the peristaltic pump propulsion speed was set at 0.1 mL/h. Finally, the spacing of the fibers arrays was controlled by adjusting the forward speed of the substrate; then, the two CdSe QDs/PVP fluorescent fibers (λem1 = 610 nm, red; λem2 = 545 nm, green) and the CDs/PVP fluorescent (λem3 = 450 nm, blue) coding fibers were obtained. 4.5. Preparation of White Fluorescent Film and Wearable Ring. Connect three syringes to the three-channel microfluidic chip. (The inner and outer channel diameters of microfluidics chips are 0.5 mm and 0.6 mm, respectively.) Then, the three syringes with the precursor were placed in the syringe pump and the flow speed of the syringe pumps was set at 0.05 mL/h. Then, the three spinning solutions were converged at the node of the three-channel microfluidic chip to form a white fluorescent stream. A collector (PET film pasted on the 3D-printed PLA frame) was mounted on a step motor and rotated onto the substrate. The rotating substrate provides a force to draw fibers directly from the reservoir. Then, the white fluorescent fiber was assembled to form a white fluorescent film on the collector. And, the white fluorescent fiber film with the PET film was peeled from the collector. Finally, a flexible PET-based white fluorescent fiber film was placed on the UV chip for preparing WLED. In addition, using the as-prepared four WLEDs to construct a wearable ring, which is prepared by copper wire connected in series. And, the prepared wearable ring was connected to a wearable supercapacitor made according to our previous work. 4.6. In Situ Preparation of CdSe/PVP Fluorescent Fiber. A 2.0 g 26.67 wt % PVP solution and 0.2 g cadmium acetate dihydrate ethanol solution (0.2 moL/L) were transferred into a 5 mL beaker and mixed well to prepare Cd2+/PVP spinning solution. Similarly, 0.00132 g Se powder was added to 1 mL TBP to form Se−TBP solution. One gram Se−TBP was then mixed with 26.67 wt % PVP solution to form a Se2−/PVP precursor spinning solution. The two spinning solutions were then separately injected into a 15 mL syringe with 20 G stainless steel needle. The two syringes were connected to a Y-type microfluidic chip, which was then mounted on a microfluidic spinning machine. The in-situ reaction of Cd2+/PVP precursor spinning solution and Se2−/PVP precursor spinning solution generated CdSe/PVP droplets at the junction of Y-type microfluidic chip, and then by adjusting the parameters of the spinning machine, and through the driven force by the spinning machine to prepared CdSe/PVP fiber. Then, the fiber was placed on a heating stage (110 °C) and fluorescent CdSe QDs/PVP fibers were obtained after 30 min. Finally, the fluorescent fiber film was crushed into CdSe/PVP powder for the preparation WLED. 4.7. Preparation of WLED. Blue GaN-based LED chips (Shenzhen xinda optoelectronic Co. Ltd) with the peak wavelength centered at 460 nm were attached on the bottom of the LED bases. Two threads on the LED were prepared to connect to the power supply. Afterward, the prepared yellow fluorescent CdSe/PVP powder was applied to the blue chip, then 0.5 g silicone (OE-6550A/OE6550B = 1:1) used to encapsulate the CdSe/PVP powder, and, finally, put in a vacuum chamber to remove bubble. The CdSe/PVP powder was attached to the LED chip and thermally cured for 1 h in a LED vacuum drying oven at 120 °C. A ZWL-600 INSTRUMENT (Zvision

when exposed under UV light (Figure 5a). Then, these yellow fluorescent fibers are crushed into fluorescent powders as phosphors to fabricate WLEDs (Figure 5b). Then, the yellow phosphors are doped into a 460 nm blue LED chip for the preparation of WLED. The blue light emitted by the CaN chip can emit white light through the yellow fluorescent powders, as indicated in the inset of Figure 5c. Figure 5c is the EL spectrum of WLED and shows two peaks corresponding to a blue CaN chip (460 nm) and a yellow fluorescent fiber film (572 nm). And, its color rendering index (CRI) was determined to be 72 at 350 mA and the color coordinate was realized at (0.3251, 0.2667) (Figure 5d), belonging to the white light range. Figure 5e shows that the bright white light generated by a LED lamp illuminated an image in the dark. These results demonstrate the successful construction WLED based on high PL performance of CdSe/PVP nanocomposite. Therefore, yellow QDs CdSe/PVP fiber film can not only serve as a candidate material for preparing WLED, but it also has great potential application in other optical devices. For example, we developed this WLED to facilely prepare a robust wearable ring (Figure 5f and Video S4), which employed four blue chips connected by a copper wire. Figure S15 shows the images of the wearable ring before and after lighting. This finding might offer an available route path for the construction of flexible wears.

3. CONCLUSIONS In summary, a rapid and facile MST strategy has been developed to construct a variety of fluorescent fiber films with versatile applications, including fluorescent coding, WLED, and flexible wear. More importantly, MST also provides a microreaction platform for facilely (less than 30 min) synthesizing CdSe QDs under mild reaction conditions (110 °C). Accordingly, highly ordered fluorescent coding fibers were achieved by three individual channel microfluidic chips. Moreover, single continuous fibers with limitless length and tunable diameter can be constructed by MST, and the fibers can be arranged into arrays and grids. Therefore, the advantage of this MST-directed microfiber lies in its flexible controllability, simplicity, versatility, high efficiency, and environmentfriendly chemical process of the preparation of fluorescent fibers and white films. Our results show that this facile approach based on different microfluidic chip may be applied to generate new hybrid materials with prospective applications in optical microsensor arrays, anisotropic hybrid microfibers, security devices, and biological materials. 4. EXPERIMENTAL SECTION 4.1. Materials. Poly(vinylpyrrolidone) (PVP Mn = 13 000 000, K88-96), G3 PAMAM dendrimer were synthesized as previously reported.50 Ethanol (99.7%) and sodium dodecyl sulfate (SDS Mw = 288.38, Alfa Aesar) were used. Cadmium acetate dehydrate (Cd(Ac) 2.2H2O, 98.5%), selenium powder (Se, 99.9%), tri-butyl phosphine (TBP, 90%), and deionized water were purchased from standard commercial suppliers; all chemicals were used without any further purification. CDs were prepared according to our previous work.51 4.2. Fabrication of Microfluidic Spinning Device. The device was fabricated by Nanjing Janus New-Materials Co. Ltd and has two main parts. One part is a syringe pump for stable velocity generation and the other is a hollow PLA frame made by a 3D printer fixed on a roller for fiber receiver (Figure S16). The fiber receiver employs a mechanical motor generally used for directly drawing fibers in combination with motor’s mechanical stretch and pump’s impetus. Rotary motion was fixed on a single-axis stepping stage, allowing the G


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Polyhydroxyalkanoate (PHA) Scaffolds for Tissue Engineering. ACS Biomater. Sci. Eng. 2017, 3, 3064−3075. (2) Guo, J.; Liu, X.; Jiang, N.; Yetisen, A. K.; Yuk, H.; Yang, C.; Khademhosseini, A.; Zhao, X.; Yun, S.-H. Highly Stretchable, Strain Sensing Hydrogel Optical Fibers. Adv. Mater. 2016, 28, 10244− 10249. (3) Rui, K.; Wang, X. S.; Du, M.; Zhang, Y.; Wang, Q. Q.; Ma, Z. Y.; Zhang, Q.; Li, D. S.; Huang, X.; Sun, G. Z.; Zhu, J. X.; Huang, W. Dual-Function Metal-Organic Framework-Based Wearable Fibers for Gas Probing and Energy Storage. ACS Appl. Mater. Interfaces 2018, 10, 2837−2842. (4) Xu, L.-L.; Wang, C.-F.; Chen, S. Microarrays Formed by Microfluidic Spinning as Multidimensional Microreactors. Angew. Chem., Int. Ed. 2014, 53, 3988−3992. (5) Lin, S.; Bai, X.; Wang, H.; Wang, H.; Song, J.; Huang, K.; Wang, C.; Wang, N.; Li, B.; Lei, M.; Wu, H. Roll-to-Roll Production of Transparent Silver-Nanofiber-Network Electrodes for Flexible Electrochromic Smart Windows. Adv. Mater. 2017, 29, No. 1703238. (6) An, S.; Jo, H. S.; Kim, D.-Y.; Lee, H. J.; Ju, B.-K.; Al-Deyab, S. S.; Ahn, J.-H.; Qin, Y.; Swihart, M. T.; Yarin, A. L.; Yoon, S. S. SelfJunctioned Copper Nanofiber Transparent Flexible Conducting Film via Electrospinning and Electroplating. Adv. Mater. 2016, 28, 7149− 7154. (7) Yan, X.; Yu, M.; Zhang, L. H.; Jia, X. S.; Li, J. T.; Duan, X. P.; Qin, C. C.; Dong, R. H.; Long, Y. Z. A Portable Electrospinning Apparatus Based on a Small Solar Cell and a Hand Generator: Design, Performance and Application. Nanoscale 2016, 8, 209−213. (8) Luo, C. J.; Stoyanov, S. D.; Stride, E.; Pelan, E.; Edirisinghe, M. Electrospinning Versus Fibre Production Methods: From Specifics to Technological Convergence. Chem. Soc. Rev. 2012, 41, 4708−4735. (9) Li, J.; Li, Y.; Zhang, J.; Li, G.; Liu, X.; Li, Z.; Liu, X. Q.; Han, Y. X.; Zhao, Z. Nano Polypeptide Particles Reinforced Polymer Composite Fibers. ACS Appl. Mater. Interfaces 2015, 7, 3871−3876. (10) Persson, M.; Lorite, G. S.; Cho, S. W.; Tuukkanen, J.; Skrifvars, M. Melt Spinning of Poly(Lactic Acid) and Hydroxyapatite Composite Fibers: Influence of the Filler Content on the Fiber Properties. ACS Appl. Mater. Interfaces 2013, 5, 6864−6872. (11) Qin, C. C.; Duan, X. P.; Wang, L.; Zhang, L. H.; Yu, M.; Dong, R. H.; Yan, X.; He, H. W.; Long, Y. Z. Melt Electrospinning of poly(Lactic Acid) and Polycaprolactone Microfibers by Using a Hand-Operated Wimshurst Generator. Nanoscale 2015, 7, 16611− 16615. (12) Ren, L.; Pandit, V.; Elkin, J.; Denman, T.; Cooper, J. A.; Kotha, S. P. Large-Scale and Highly Efficient Synthesis of Micro- and NanoFibers with Controlled Fiber Morphology by Centrifugal Jet Spinning for Tissue Regeneration. Nanoscale 2013, 5, 2337−2345. (13) Bai, H.; Ju, J.; Zheng, Y.; Jiang, L. Functional Fibers with Unique Wettability Inspired by Spider Silks. Adv. Mater. 2012, 24, 2786−2791. (14) Yu, Y.; Wen, H.; Ma, J.; Lykkemark, S.; Xu, H.; Qin, J. Flexible Fabrication of Biomimetic Bamboo- Like Hybrid Microfibers. Adv. Mater. 2014, 26, 2494−2499. (15) Reiser, B.; Gerstner, D.; Gonzalez-Garcia, L.; Maurer, J. H. M.; Kanelidis, I.; Kraus, T. Spinning Hierarchical Gold Nanowire Microfibers by Shear Alignment and Intermolecular Self-Assembly. ACS Nano 2017, 11, 4934−4942. (16) Chen, H. T.; Lin, H. L.; Chen, I. G.; Kuo, C. Conducting Silver Networks Based on Electrospun Poly(Methyl Methacrylate) and Silver Trifluoroacetate. ACS Appl. Mater. Interfaces 2015, 7, 9479− 9485. (17) Mujica-Garcia, A.; Hooshmand, S.; Skrifvars, M.; Kenny, J. M.; Oksman, K.; Peponi, L. Poly(Lactic Acid) Melt-Spun Fibers Reinforced with Functionalized Cellulose Nanocrystals. RSC Adv. 2016, 6, 9221−9231. (18) Liu, K.; Sun, Y. H.; Lin, X. Y.; Zhou, R. F.; Wang, J. P.; Fan, S. S.; Jiang, K. L. Scratch-Resistant, Highly Conductive and HighStrength Carbon Nanotube-Based Composite Yarns. ACS Nano 2010, 4, 5827−5834.

Co. Ltd) with an integral sphere was used to measure the relevant optical performances. 4.8. Measurements. The morphology of the microfibers were observed by scanning electron microscopy (SEM) with a QUANTA 200 (Philips-FEI, Holland) instrument at 20.0 kv. The microfibers were also observed by an inverted fluorescence microscope (SFM-30I, Shanghai), which prove the fluorescent coding properties and wellaligned structure of the fibers. HRTEM was performed with a JEOL JEM-2010 TEM. Time-resolved fluorescence decay curves were achieved on an Edinburgh FL 900 photocounting system. IR images were performed on a Thermo Scientific Nicolet In10 infrared microscope equipped with a liquid nitrogen cooled mercury cadmium telluride detector (Thermo Electron Corporation). IR microscopy data were collected using reflection mode. Transparency of the fiber films was tested by PerkinElmer Lambda 900 UV−vis spectrometer at the wavelength 350−650 nm. Photoluminescence (PL) spectra were measured on a Varian Cary Eclipse spectrophotometer at room temperature. A 350 nm laser beam was chosen as the excited light source from a Xe lamp with the voltage of 650 V. The X-ray diffraction (XRD) patterns were obtained from a D5005 X-ray diffractometer (Siemens AG, Munich, Germany) from 5 to 60° at a scanning speed of 0.05°/s. The atomic ratios were obtained using the X-ray photoelectron spectroscopy (XPS) (PHI 5000 VersaProbe). The inductively coupled plasma mass spectrometry (ICP-MS) were obtained from a Optima 7000DV ICP-MS (PerkinElmer company).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b11926. Ten-channel microfluidic chip (Figure S1); microscope images of the fiber arrays (Figure S2); metallographic microscope images of fluorescent encoding (Figure S3); operating parameters for inductively coupled plasma mass spectrometry (ICP-MS) (PDF) Spinning process of 10-channel microfluidic chip (AVI) Spinning process of three-channel microfluidic chip (AVI) Sample image of white fluorescent fiber film (AVI) Image of before and after lighting the wearable ring (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Cai-feng Wang: 0000-0003-4667-2120 Su Chen: 0000-0002-3799-469X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (21474052 and 21736006), the National Key Research and Development Program of China (project No. 2016YFB0401700), Fund of State Key Laboratory of Materials-Oriented Chemical Engineering (ZK201704 and ZK201716), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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REFERENCES

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