Facile Access to Wearable Device via Microfluidic Spinning Robust

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Facile Access to Wearable Device via Microfluidic Spinning Robust and Aligned Fluorescent Microfibers Tingting Cui, Zhijie Zhu, Rui Cheng, Yu-Long Tong, Gang Peng, Cai-Feng Wang, and Su Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11926 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 19, 2018

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ACS Applied Materials & Interfaces

Facile Access to Wearable Device via Microfluidic Spinning Robust and Aligned Fluorescent Microfibers Tingting Cui, Zhijie Zhu, Rui Cheng, Yu-long Tong, Gang Peng, Cai-feng Wang, 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

E-mail: [email protected]

KEYWORDS

Aligned fibers, microfluidic spinning, in-suit micro-reactor, fluorescent coding, wearable device.

ABSTRACT

Microfluidic spinning technology (MST) has drawn much attention owing to its ideal platform towards ordered fluorescent fibers, along with their largely-scale 1

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manipulation, high efficiency, flexibility, and environmental-friendly. Here, we employed the MST to fabricate a series of uniform fluorescent microfibers. By adjusting the microfluidic spinning parameters, the as-prepared microfibers with various diameters are successfully obtained. For more practice, these regular arranged fibers could be formed to versatile fluorescent codes by use of various microfluidic chips. Also, these versatile fluorescent fibers could further weave to a white fluorescent film via a continuous and crossing spinning process, which could be applied in white light emitting diode (WLED) and the wearable device. Besides, we investigated 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 a nice optical property, and are a good candidate as phosphors in WLED. This strategy offers a facile and environmental-friendly route to fluorescent hybrid microfibers, and might open its potential application in optical devices, security and fluorescent coding.

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 ordered fashion at nano or micro-size is highly desirable. Recently, much effort has been devoted to the 2


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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, polymethylmethacrylate (PMMA),16 poly(L-lactide) PLLA,17 polyvinyl alcohol (PVA),18-19 and polystyrene (PS) fibers.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) can prepare 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 high 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 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 3



Stephan et al. reported a microfluidic-produced 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 towards wearable and portable devices via microfluidic spinning.

In this work, we demonstrated a method that combined microfluidic spinning with different chips for preparation of ordered fibers with diameters ranging from 0.8 µm to 20 µm. Firstly, 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%). 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 PVP hybrid fluorescent microfibers would generate immediately. Principally, the proposed approach not only break through the limitation of in-situ reaction, but also offered a green pathway to facilely (within 30 min) and large-scale (6 cm×6 cm) access CdSe QDs fiber films under mild reaction temperature (110 oC), which is quite 4


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lower than that of traditional method for preparing CdSe QDs (about 230 oC).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 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 comparing to the latest methods (digital regulation method,34 cut-off 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.

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Scheme 1. Preparation of fluorescent fiber films and its applications in coding, WLED and the wearable ring. (a) Schematic illustration of 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 milled into form CdSe/PVP fluorescent powders for using in the WLED. (c) Schematic represention of ordered fluorescent coding (red, green and blue).

2. RESULES 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). 6


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Meanwhile, the diameters of obtained microfibers could be easily adjusted by rotation speed of substrate and flow rate of solution. Compared with conventional electronic spinning technology, this method does not need to use the 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 be reached about 1413 m (L=πdvt) according to the parameters (v: rotational speed 250 r/min, t: time is 30 min, d: frame length is 6 cm). Furthermore, we also designed a ten-needle microfluidic chip which 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 are easy to occur, which is caused by a stream of liquid flow arisen 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 reducing the surface tension. Accordingly, we carried out various experiments on the relationship between the fiber diameter and spin speed. When the microfluidic spinning forward speed is lower than 250 µm/s, adjacent fibers tend to adhered together. On contrast, with the increase of spin speed, fibers are arranged in an orderly network. But the space of fibers are different according to different forward speeds, as shown in Figure 1b (600 µm/s, 450 µm/s, 350 µm/s) and microscope images of Figure S2. From the Figure 1c, it have been found that the fiber diameter is determined by rotational 7



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 r/min and the forward speed at 700 µm/s, the fiber diameters increase from 0.8 µm to 20 µm with the PVP concentrations, increasing from 16 wt% to 26 wt%, respectively. By adjusting parameters during MST, the diameters of fibers and fiber spaces could be controlled and the extra-long PVP/QDs fibers with various diameters have been done.

Figure 1. Continuous spinning process of fiber film. (a) The photograph of MST 8


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

2.2 MST directed fluorescent codings.

In practice, it is desirable to obtain the ordered fiber arrays by MST. Then, 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. Much approaches, including digital regulation method,34 cut-off lithography,42 have been reported for preparing various codings. However, these reported methods are complicated, costly and not easy to be portable, limiting their further applications. To this end, we propose a controllable fluorescent coding method by 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 typical run, three kinds of spinning solutions (QDs/PVP with red, 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 are simultaneously extruded from the syringes, the rotating device drives the rotation of

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the receiver so that the droplets pull into fibers by the action of 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, 2d. 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 arranged in an equal distance. Moreover, the fiber spacing can be adjusted by adjusting the advance speeds of the spinning device. The Figure S3 presents the ordered arrangement of fibers in detail. Similarly, by using the six-channel and nine-channel microfluidic chip, the 112233-type and 121312323-type fluorescent codings are achieved (Figure 2e, 2f). More importantly, the codings are flexible, programmable and woven, allowing them to apply in flexible wearable devices. Also, the coding information is rich and safer owing to the dual responses (fluorescent response, digital controllability). Therefore, a series of digital codes, along with various fluorescent patterns are done, which have various potential applications, such as biosensing,43 anti-counterfeiting,44 forensic labelling,45 and flexible wearable devices.

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Figure 2. The 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.

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 11



node of 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 to the white fluorescent fiber film. Finally, we peeled off the white fluorescent film from the fiber receiver for preparing wearable ring. The actual preparation process was shown in Figure S4. The 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 microscope images (Figure S6). Then, we investigated the fluorescent properties of the as-prepared 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), respectively. The corresponding image of the film is shown in the inset of Figure 3b, which shows white fluorescent under ultraviolet radiation. It is worth noting that the mass ratio of three different fluorescent phosphors should be precisely controlled (red: green: blue=5:7:32). The Figure S7 is the 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 (522nm) and red CdSe QDs/PVP (595nm) respectively. 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 as-prepared white fluorescent films could be 12


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successfully done and act as wearable device. Although 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 a 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 among visible spectrum and its transparency is up to 84 %. On the other side, the CdSe QDs/PVP and CDs/PVP composite fluorescent films still have transparency with 75 %, which is accounted for the incorporation fluorescent phosphors. And the insert in Figure 3c shows the sample images of the pure PVP fiber film and the QD/PVP fiber film. Moreover, the Figure S9 shows the flexibility and lightweight properties of the fiber film, which could effortlessly stand on a leaf. And the Figure S10 shows the flexibility of the as-prepared films in the wearable device with the different bend angles, from this, it is noticed that the bending angle can reach to 180o, indicating good flexibility of the white fluorescent fiber film.

Since the fiber film needs to be peeled off from the fiber receiver after the spinning 13



process, it is inevitable that the fiber film will be undertaken too much force. Therefore, in order 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 has increased from 0.941 MPa to 2.089 MPa, while the elongation at break keeps 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 cm-1 to 1476 cm-1, and the stretching vibration peak of C=O shifts from 1664 cm-1 to 1644 cm-1after PAMAM doping. This change may be contributed to the hydrogen bond interaction between PVP and G3 PAMAM dendrimers. This result also reveals that a novel hydrogen bond have 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 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 completed in the following three steps. First step, insert four UV chips on a 16 cm×2 cm thin and flexible sheet, then connect the four UV chips with a copper wire to form a series circuit. 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 14


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positive and negative ends of the copper wire were soldered to the positive and negative ends of the supercapacitor, the ultraviolet light emitted by the ultraviolet chip penetrated the PET film and then penetrated to the white fluorescent film to emit white light. Therefore, from the assemble process, it is found that this structure can effectively alleviate the light dispersion brought by the direct contact of the phosphor in the 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 be installed as shown in images of wearable device of Figure S11.

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Figure 3. The 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. Insert:(d) Mechanical properties of the fiber films. (e) FT-IR spectra of the pure PVP (curve 1), pure PAMAM (curve 2), PVP/ PAMAM (curve 3). (f) The scheme of WLED by attaching composite fluorescent film on a UV-chip.

2.4 Green synthesis of CdSe QDs by fiber microreactors

This MST was also utilized as a microreactor platform for 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 Se2at the node of a Y-type microfluidic chip (Figure S12), and the scheme illustration was shown in Figure 4a. Then, as-prepared fluorescent fibers were annealed at 110 oC to accelerate solvent evaporation and promote the CdSe QDs generation. It is well known that traditional methods for preparing CdSe QDs not only require high reaction temperatures (230-280 oC) and long reaction times (2-4 h), but also likely to cause environmental pollution own to the remaining 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 oC). Even more noteworthy was that these QDs fluorescent fiber film are produced in-situ via MST and so far the method for achieving in situ reaction reported 16


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relatively few. Interestingly, when we adjust the concentrations of Se2- and Cd2+ in spinning precursor, various fluorescent CdSe QDs could be synthesized in-situ. The morphology of CdSe QDs fibers are characterized by optical microscope and SEM, as shown in Figure 4b-4d. As can be seen, the overall diameter along the fiber axis is uniform, and the corresponding SEM images are shown in the insert of Figure 4b, 4c, 4d. From SEM images, it is noticed that the fiber surface is smooth owe to addition of the surfactant during spinning process. In the short period of time, it has experienced rapid solvent evaporation and fiber curing process. Moreover, the humidity and temperature of the current system is optimized at 70 % and 60 oC, respectively. Under this condition, solvent (ethanol) may evaporate instantly in air, along with polymer precursor easily spun into uniform fibers. The fluorescence spectra of CdSe QDs with green, yellow and red fluorescence are shown in Figure 4e, respectively. The emission peaks of these CdSe QDs fibers are at 542 nm, 571 nm and 638 nm with narrow half peaks. We also employed high resolution transmission electron microscopy (HRTEM) analysis. From Figure 4f, it can be seen that CdSe QDs have a good monodispersity and lattice fringe, the average particle size is about 4 nm. The energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) are all validates that the composition of QDs is cadmium and selenium (Figure 4g and Figure S13). Table S1 shows the operating parameters for inductively coupled plasma mass spectrometry (ICP-MS), and according to the formula of C×V×F/m (C: 17



Instrument, V: Volume, F: Dilution factor, m: Mass), the result shows that Cd and Se contents are 19615 mg/kg and 15452 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 that the obtained product has obvious diffraction peaks at a value of 2θ, 25.5o, 42.4o, 49.8o, corresponding to the (111), (220), (311) crystal planes of the cubic CdSe, respectively. And their fluorescence lifetime values were 4.0 ns, 4.4 ns 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 microscope image shows that the obtained fluorescent fiber grid has a uniform spacing distribution at a scale bar of 50 µm (Figure 4j-4l). This method enables QDs synthesizing to be carried out at ultrafine scales and mild reaction conditions in a controllable fashion, which is potentially important for the development of an environmentally friendly and ligand-free route to fluorescent QDs, may offer fundamental insight into chemistry in confined microreactors.

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Figure 4. In-situ preparation of CdSe QDs fiber films by MST. (a) Schematic illustration of in situ synthesising of CdSe QDs by a Y-type microfluidic chip. Fluorescence microscopy images of (b) green, (c) yellow, (d) red monolayer CdSe/PVP fluorescent fibers. (e) PL emittion spectra of green, yellow and red CdSe/PVP fluorescent fibers. (λ=542 nm, 571 nm, and 638 nm). (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, fluorescent microscopy images of (j) green, (k) yellow, (l) red CdSe/PVP fluorescent fibers array.

2.5 The application of QDs derived from fiber-microreactors

We further investigated the as-prepared CdSe QDs phosphors to apply in LED and wearable devices. These phosphors derived from fiber microreactors, and should have good compatability with spinning fibers. As illustrated in Figure 5, bright yellow 19



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

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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) Palacement 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 wearble WLED ring.

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, flexible wear. More importantly, MST also provide a microreaction platform for facilely (less than 30 min) synthesizing of CdSe QDs under mild reaction 21



conditions (110 oC). Accordingly, highly-ordered fluorescent coding fibers were achieved by a three individual channel microfluidic chip. 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, versatile, high efficiency and environmental-friendly chemical process in 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 SECTIONS

Materials: Polyvinyl pyrrolidone (PVP Mn=13000000, K88-96), G3 PAMAM dendrimer were synthesized as previously reported.50 Ethanol (99.7 %), sodium dodecyl sulfate (SDS Mw=288.38, Alfa Aesar). Cadmium acetate dehydrate (Cd (Ac) 2.2 H2O, 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

Fabrication of microfluidic spinning device: The device was fabricated by Nanjing 22


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Janus New-Materials Co. Ltd, and it has two main parts. One part is a syringe pumps 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 sample moving in a constant speed, which leads to equidistant distance between each microarray.

Preparation of spinning solutions: PVP of 4 g was dissolved in 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), respectively. After well mechanically stirring, three kinds of fluorescent spinning solutions were obtained, then loaded it into three 15 mL syringes, respectively.

The preparation of the well-aligned fluorescent coding: The three kinds of syringes with different spinning solution were connected with the three-channel microfluidic chip, respectively. The three channels were independent and have the same channel diameter. The inner and outer diameter of the channels were 0.5 mm and 0.6 mm at the present experiments. Then the three syringes were simultaneously 23



extruded, and peristaltic pump propulsion speed was set at 0.1 mL/h. Finally, the spacing of fibers arrays was controlled by adjusting the forward speed of substrate, and then the red, green, blue fluorescent PVP/QDs hybrid well-aligned coding fibers were obtained.

The preparation of white fluorescent film and wearable ring: Connect three syringes to the three-channel microfluidic chip. (The channel inner and outer diameters of chip is 0.5 mm and 0.6 mm, respectively). Then, the three syringes with the precursor was placed in the syringe pump, and the flow speed of the syringe pumps all set 0.05 mL/h. When the three spinning solutions were converged at the node of the three-channel microfluidic chip to form white fluorescent stream. A collector (PET film was pasted on the 3D-printed PLA frame) was mounted on a step motor and was 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 white fluorescent film on the collector. And peeled off the white fluorescent fiber film with PET film 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 WLED to construct a wearable ring which prepared by copper wire connected them in series. And the prepared wearable ring was connected to a wearable supercapacitor made by our previous work.

24


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In situ preparation of CdSe/PVP fluorescent fiber: 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, mixed well to prepare Cd2+/PVP spinning solution. Similarly, 0.00132 g of Se powder was added to 1 mL of TBP to form Se-TBP solution. 1 g Se-TBP was then mixed with the 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, and the chip was then mounted on a microfluidic spinning machine. Two kinds of precursors spinning solution in the Y-shaped channel at the node in-situ reaction of CdSe/PVP droplets, 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 oC), after 30 min, the fluorescent CdSe QDs/PVP fibers were obtained. Finally, crushed the fluorescent fiber film into CdSe/ PVP powder for preparation WLED.

Preparation of a 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. Afterwards, the prepared yellow fluorescent CdSe/PVP powder was applied to the blue chip, and then use 0.5 g silicone (OE-6550A:OE-6550B=1:1) to 25



encapsulate the CdSe/PVP powder, finally, put it in a vacuum chamber to remove bubble. The CdSe/PVP powder were attached on the LED chip and thermally cured for 1 h in a LED vacuum drying oven at 120 oC. A ZWL-600 INSTRUMENT (Zvision Co. LTD) with an integral sphere was used to measure the relevant optical performances.

Measurements: The microstructure and morphology of the microfibers were observed by scanning electron microscopy (SEM) with a QUANTA 200 (Philips-FEI,Holland) instrument at 20.0 kv, which prove that the size of fiber and array structure. The microfibers were also observed by an inverted fluorescence microscope (SFM-30I, Shanghai), which prove that the fluorescent coding properties and well-aligned structure of fibers. HRTEM observation 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 MCT detector (Thermo Electron Corporation,USA). IR microscopy data were collected using reflection mode. Transparence of the fiber films were tested by PerkinELmer Lambda 900 UV-vis spectrometer with the wavelength 350 nm to 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 26


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(XRD) patterns were obtained from a D5005 X-ray diffractometer (Siemens AG,Munich, Germany) from 5o to 60o at a scanning speed of 0.05o/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).

Supporting Information

Materials and method. This material is available free of charge via the Internet at the http://pubs.acs.org.

Acknowledgements

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

Conflict of interest

The authors declare no conflict of interest

27



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Facile Access to Wearable Device via Microfluidic Spinning Robust

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