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Organic-Nanowire-SiO2 Core-Shell Microlasers with Highly Polarized and Narrow Emissions for Biological Imaging Changfu Feng, Zhenzhen Xu, Xu Wang, Hongjuan Yang, Lemin Zheng, and Hongbing Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13387 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 12, 2017

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ACS Applied Materials & Interfaces

Organic-Nanowire−SiO2 Core−Shell Microlasers with Highly Polarized and Narrow Emissions for Biological Imaging Changfu Feng, † Zhenzhen Xu,*,† Xu Wang,‡ Hongjuan Yang, † Lemin Zheng,‡ and Hongbing Fu*, †, § †

Beijing Key Laboratory for Optical Materials and Photonic Devices, Department of Chemistry,

Capital Normal University, Beijing 100048, P. R. China ‡

School of Basic Medical Sciences, and Key Laboratory of Molecular Cardiovascular Sciences

of Ministry of Education, the Institute of Cardiovascular Sciences and Institute of Systems Biomedicine, Peking University Health Science Center, Beijing 100191, P. R. China §

Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry,

Tianjin University, and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China. KEYWORDS: Organic Nanowire, Core-Shell Structure, Intracellular Microlaser, Polarization Probes, Biological Imaging

ACS Paragon Plus Environment

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Abstract: Development of luminescence probes with polarized and narrow emissions simultaneously are helpful to remove multiply scattered light and enable multiplexing detection, but remain challengeable by using conventional organic dyes, fluorescence proteins and quantum dots. Here, we demonstrated a smart one-dimensional microlaser probes (MLPs) by coating a thin layer of silica shell on the surface of organic nanowires (ONWs) of 1,4-dimethoxy-2,5-di[4'(methylthio)styryl]benzene (TDSB), namely, ONW@SiO2 core-shell structures. Different from Fabry-Pérot (FP) cavity formed between two end-faces of semiconductor nanowires, whispering gallery mode (WGM) microresonators are built within the rectangular cross-section of ONW@SiO2 MLPs. This enables a lasing threshold as low as 1.54 µJ/cm2, above which lasing emissions are obtained with a full width at half maximum (FWHM) < 5 nm and a degree of polarization (DOP) > 83%. Meanwhile, small dimensions of ONW@SiO2 MLPs with a sidelength of ca. 500 nm and a length of 3-8 µm help to reduce their perturbations in living cells. With the help of mesoporous silica shells, which provide both high biocompatibility and good photostability, ONW@SiO2 MLPs can be easily introduced into the cell cytoplasm through natural endocytosis. Using their narrow and highly polarized lasing emissions in vitro, we would be possible to tag individual cells using ONW@SiO2 MLPs with high stability.


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1. INTRODUCTION

Recently fluorescence polarization has attracted a great deal of interest in biophotonics, because of its unique ability to remove multiply scattered (depolarized) light and consequently to offer high imaging-contrast and improved tissue-resolution.1-3 In the past decades, various luminescent probes have been developed as indispensable toolkit in cell biology and medical diagnostics, such as organic dyes,4-6 fluorescence proteins,7-9 fluorescent silicon nanoparticles1012

and quantum dots.13-16 On the one hand, these conventional luminescent probes typically emit

unpolarized light, ill-suited to fluorescence polarization applications. On the other hand, their broad emission spectra (typically 30-100 nm) are often indistinguishable from the broad biological background emission and therefore severely limit their applications.17 For instance, discriminable luminescent probes used in multiplexed detection have to exhibit individual emissions narrow enough to nonoverlap with each other. However, rational design and synthesis of luminescent probes with highly polarized and narrow spontaneous emissions simultaneously remain a formidable task so far.17 Microlasers, which require optical feedback structure and stimulated emission of gain material for light amplification, allow spectral narrowing (filtering) via coherent loss and gain.17 Recently, Humar and Yun demonstrated stand-alone cell lasers by embedding gain materials of organic dyes into intracellular whispering gallery mode (WGM) microresonators made by polystyrene beads or natural lipid droplets.17 The characteristics of narrow WGM lasing self-sustained in living cells provide barcode-type cell tagging, tracking, intracellular sensing, and adaptive imaging. Nevertheless, the highly random and disordered molecular packing in WGM microspheres results in unpolarized intracellular microlasers. Furthermore, the large optical microresonator size (> 10 µm) and the relatively high lasing threshold (over thousands of µJ/cm2)


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limit their further application in biophotonics.17-18 Semiconductor nanowires (SNWs), whose faceted structure naturally forms an axial Fabry-Pérot (FP) cavity between two wire end-faces, are excellent candidates for realization of miniaturized and low threshold lasers.19-22 Yang and co-workers developed nanowire-based single-cell electrophysiology and endoscopy.23 However, synthesis of SNWs generally requires expensive high-temperature procedures and complicated post-synthesis surface treatments.19, 24 We recently reported organic nanowire (ONWs) lasers of 1,4-dimethoxy-2,5-di[4'-(methylthio)styryl]benzene (TDSB) self-assembled from solution. Herein, we developed smart one-dimensional microlaser probes (MLPs) by coating ONWs of TDSB with a silica shell, giving rise to ONW@SiO2 core-shell structures. Experimental and theoretical results reveal that ONW@SiO2 core-shell MLPs form WGM microresonator within its rectangular cross-section with a side-length (W) at the wavelength scale of ca. 500 nm and irrelevant to its length (L) of 3-8 µm. The small size combining with water-soluble and biocompatible mesoporous silica shell eliminate perturbations of ONW@SiO2 MLPs in cells. The high-quality built-in WGM cavity lead to a low lasing threshold 1.54 µJ/cm2, above which lasing action was demonstrated by superlinear output intensity and spectral narrowing25 with FWHM decreasing from 22 nm for spontaneous emission to 5 nm for laser emission. Owning to helical propagation of WGM lasing emission along the length direction, ONW@SiO2 MLPs show a perfect emission polarization with a DOP > 83%. Using their narrow and highly polarized lasing emission in vitro, we would be possible to tag individual cells using ONW@SiO2 MLPs with high stability. 2. EXPERIMENTAL SECTION

Scheme 1 presents our synthesis strategy for the ONW@SiO2 MLPs. This strategy starts with the formation of different sizes of ONWs of TDSB in cetyltrimethyl ammonium bromide


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(CTAB) solution through a simple reprecipitation method. Then, through a surfactant-template approach, a mesostructured CTAB/silica composite was deposited on the ONWs, resulting in well-dispersed ONW@SiO2 microstructures with good biocompatibility. Although the ONWs can be readily prepared based on simple solution self-assembly of the TDSB molecules, the exactly size control of the ONWs remains a challenge, especially for the preparation of the ONWs with high monodisperse and small size (L ˂ 10 µm) in high throughput.22, 26 Here, we prepared monodisperse and size-tunable ONWs induced by CTAB micelles. In a typical preparation, 50 µL of stock solution (2 mM) in tetrahydrofuran (THF) was injected into the mixed poor solvent of ethanol and water in the presence of CTAB (2 mL, CCTAB = 0.9 mM) at room temperature under shaking.27 By controlling the volume ratio of ethanol and deionized water from 1 : 1 to 2 : 0, the sizes of ONWs can be adjusted from 480 nm to 55 µm (Figure S1). During the subsequent encapsulation procedure, the ONWs with positively charged CTAB in surface acting as “growth seeds” promoted the formation of mesoporous silica shells via electrostatic interactions.28 Generally, ammonia could be added as the catalyst to speed up the hydrolysis of tetraethyl orthosilicate (TEOS) and deposition process. During a typical procedure, 40 µL of TEOS/ethanol (0.3 g/L) was added into 2 mL ONWs suspension under stirring quickly. Then 100 µL of ammonia aqueous solution (2.8%) was slowly added with continuously stirring for 50 minutes. The green solid obtained by microcrystals containing ONWs core and the silica shell (ONW@SiO2 core-shell structures) was collected by centrifugation and washed by water for several times. After the washing process, the CTAB templates can be removed successfully. The supernatant was transparent and colorless, indicating that ONWs keep stable the whole time in the coating produce.


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3. RESULTS AND DISCUSSION

Figure 1a and b show SEM images of ONW@SiO2 MLPs synthesized with the use of prefabricated ONWs as the cores prepared in the mixed poor solvent of ethanol/deionized water (v : v = 3 : 2). SEM results also show the smooth outer surfaces and uniform rod-like shapes with a rectangular cross-section (Figure S2) and their width (W) of 0.3 – 0.6 µm uniformly distributed along the entire length (L) of about 5 µm, similar to the pristine ONWs.22 Figure 1c shows a typical TEM image of the prepared ONW@SiO2 MLPs, which reveals clearly a core/shell structure with a uniform thickness of 110 ± 20 nm over the entire surface of each ONW. Almost no uncoated ONWs were observed, demonstrating the efficiency of the coating method. Moreover, the etching of ONWs was not significant enough to be observed during the coating process. The core-shell structure is especially distinct in the high-magnification TEM image (Figure 1d). The corresponding energy–dispersive X-ray spectroscopy line analysis of crosssection (Figure 1f) demonstrates that Si and O elements distribute in the whole nanowire with relatively higher content in the shell area. Two peaks of the elements Si/O reveal that the thickness of the SiO2 layer extends around 0.1µm, equal to the results of the TEM images. Moreover, the two peaks are separated by a distance of 0.45 µm, equal to the width of the ONW observed in the Figure 1d. Note that S is the special element of ONW@SiO2 MLP which only exist in the ONW core. All these results demonstrate that ONW@SiO2 MLPs with a core-shell structure have been successfully fabricated by our proposed self-template method. The XRD (Figure 1e) measurement reveals that there is no significant difference with regard to the crystal structure and crystallinity between the ONW and ONW@SiO2. The diffraction peaks are consistent with the simulated XRD pattern of ONW with a monoclinic space group (P21/c)


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according to the published crystal structure data.26 This is another piece of clear evidence for that ONWs keep stable the whole time in the coating produce. Figure S3 shows the normalized diffused reflection absorption and photoluminescence (PL) spectra of ensemble ONW@SiO2 MLPs placed on a glass substrate (red line), in comparison with those of pristine ONWs (black line). It should be note that there is no significant difference between the spectral features of ONWs and ONW@SiO2 MLPs, which reveals that the coating procedure has no obvious effect on the optical properties of the ONWs. The ONW@SiO2 MLPs exhibit strong green PL with bright PL spots at the tips and weaker PL from the bodies under excitation of unfocused UV light (330-380 nm), which is the typical features of an active optical waveguide (Figure S4). We next test the ability of the ONW@SiO2 MLPs to be as optical microcavities using a home-made µ-PL system described detailedly in our earlier work. 25 Figure 2a shows that the twelve ONW@SiO2 MLPs in the glass substrate can operate simultaneously at a pump density of 20 µJ/cm2, which is above the lasing thresholds of each one. Representative µ-PL spectra of selected ONW@SiO2 MLP ① with W = 0.79 µm (L = 6.9 µm) and ② with W = 0.50 µm (L = 4 µm) under different pump density (P) are shown in the Figure 2d and 2e. The zoom-out SEM images of the ONW@SiO2 MLP ① and ② were showed in Figure S5. Figure 3d shows the emission spectra at different pump densities obtained from ONW@SiO2 MLP ①. When the pump density exceeds a threshold Pth of 1.59 µJ/cm2, strong laser emission emerges as a set of sharp peaks around 500 nm. The FWHM of the emission peak dramatically decreases from 22 nm below the threshold to 5 nm above the threshold. The narrow emission bandwidth is important for distinguishing its spectrum from the broad backgroud emission and preventing emission cross-talk among probes, particularly for in multiplexed


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biological detection. The lasing emission spectrum presents periodic interference peaks with a dominant lasing wavelength λmax = 501.00 nm and a spacing of ∆λ = 1.35 nm, which shows an evidence of the optical microcavity. Moreover, these interference peaks exhibit a FWHM of ca. 0.56 nm at 501.00 nm, giving rise to Q as high as 895. The inset of Figure 2d shows the integrated intensities as a function of pump density, indicating distinctly a threshold Pth of 1.59 µJ/cm2. The relatively low threshold energy of the small ONW@SiO2 MLP means that these optical microcavities can be integrated into the single cells and generate laser light without compromising their biological function.18,

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With the width decreasing to 0.50 µm, the

ONW@SiO2 MLP ② shows a similar laser action with a dominant lasing wavelength λmax = 500.90 nm, a spacing of ∆λ = 1.97 nm and a threshold at Pth = 1.74 µJ/cm2 (Figure 2e). The dominant lasing wavelength and the spacing of the periodic interference peaks differ between different ONW@SiO2 MLPs (Figure 2d, e and Figure S7), which indicates that ONW@SiO2 MLPs have significant potential in barcode-type cell tagging and tracking. A plot of 1/W of ONW@SiO2 MLPs versus the mode spacing ∆λ at 500 nm is shown in Figure 2f and Figure S6. The best-fit line (red line) is distinctly linear. This confirms that the mode spacing is related to the width of ONW@SiO2 MLPs rather than its length. This serves as an evidence for the microlaser based on radial cavity rather than axial cavities of the ONW@SiO2 MLPs. Furthermore, the lasing threshold for different W of ONW@SiO2 MLPs is shown in Figure 2f and Figure S7. The best-fit line (black line) of width versus the lasing threshold is approximately 1/W2, which shows that the ONW@SiO2 MLP operates as WGM lasing. This demonstrates that the WGM resonator actually exists along the four lateral-faces of ONW@SiO2 MLPs. It can be seen from the Figure 2a that bright lasing spots were observed at the two ends of the ONW@SiO2 MLPs. All results indicate that the laser light generated in the


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local radial cavity can propagate along the axial nanowires and leak out of tips, which is typically helically propagating mode similar to InP nanowires and InGaAs/GaAs nanopillars. 3032

To identify the cavity mode within the ONW@SiO2 MLPs, the simulation of local electric field |E|2 distribution in the microcavity was performed by using the finite-difference timedomain (FDTD) method. The width/height and length of the cavity are set as 1.2/0.9 µm and 8 µm in the model in FDTD simulations in Figure 3a. The lasing wavelength is located around 500 nm. The refractive indexes of ONWs and SiO2 are 1.95 and 1.4, respectively. The electric field distribution of the rectangular cross section (Figure 3b) and longitudinal section (Figure 3d) were simulated. The calculated result in Figure 3b shows an efficient 4-WGM mode pattern in the rectangular cross section with nearly all the electric field |E|2 intensities are limited inside in the inner surface of four edges of ONW. Simultaneously, a standing wave pattern of the optical mode along the nanowire axis in the longitudinal section is clearly shown in Figure 3d. Different from Fabry-Pérot (FP) cavity formed between two end-faces of SNWs, a helically propagating 4WGM mode pattern microresonator built within the rectangular cross-section of ONW@SiO2 MLPs is identified. 4-WGM mode microresonators with strong self-cavity optical confinement afford high-Q value and efficiently diminish the size of gain materials required for lasing, are promising for intracellular lasing when internalized by cells. With the experiment instrument set-up shown in supporting information, we measured the emission spectra of a single ONW@SiO2 MLP (W= 0.72 µm, L= 5.5 µm) with the polarization directions of the pump light and the polarizer perpendicular/parallel to the MLP-axial direction, while keeping the pump intensity at 2 Pth. Figure 4a shows that the lasing with highly structured spectra occur when the pump light is perpendicular and the polarizer is parallel to the MLP-axial


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direction. Meanwhile, the relatively weak emission intensity can be detected in the other configurations in Figure 4a, b. Further anisotropic study of the lasing process in ONW@SiO2 MLPs shows that the lasing output is strongly polarization-dependent with high polarization purity. In the absence of the polarizer, we systematically adjust the angle β between the polarization of the excitation laser and the axial-direction of the ONW@SiO2 MLP from 0 to 360° using a half-wave plate (Scheme S2). It can be seen from Figure S11 that the integrated PL intensities oscillate with a period of 180°, between the minimum at β = 0 or 180° and the maximum at 90 or 270°. The result is consistent with that the transition dipole moment of the TDSB molecules is along the molecular long axes,22 which forms an angle of 90° to the length direction of ONW. Then we keep the β = 90° and measure the emission spectra as a function of the detection polarization angle θ, which is defined as the angle between the polarization of the emission and the MLP-axial direction (Scheme S2). As shown in Figure 4c, the sharp microcavity lasing peak (500 nm) is highly polarized with the intensity maximizing in the length direction (θ = 0°) and minimizing in the width direction (θ = 90°). We define the degree of the polarization as DOP = (Imax - Imin)/(Imax + Imin), obtain DOP = 0.827 ± 0.05 from the fit. And the results of polarized emission of ONW@SiO2 MLPs with different sizes show almost same DOP ≈ 0.83 , indicating that there is no dependence on the ratio of length/width (Figure S10). The narrow and highly polarized lasing emissions from the ONW@SiO2 MLPs may find great potentials in biological applications. It is worth noting that the polarization dependence of the input excitation laser (400 nm) and the output emission microlaser (500 nm) shows an exact anti-correlated relationship, when we keep the β = 90°. That’s mean the polarization of the emission output is parallel to the length direction, which is perpendicular to the transition dipole moment of the TDSB molecules in


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ONW@SiO2 MLPs. It is reported that the polarization of the cavity modes in SNWs is determined by the waveguide modes they couple to.23, 33 It can be seen from the Figure 3b and the Figure S8 that the fundamental mode is mostly a standing wave pattern in the longitudinal section with dominant electric filed polarized in the length direction, in excellent agreement with the experiment results. For the application of these ONW@SiO2 structures as bio-microlaser probes, they should remain their photophysical properties while they are water-soluble. After centrifugating and vacuum-drying, the ONW@SiO2 MLPs are still re-dispersible in water for a stable suspension. The size and optical properties of ONW@SiO2 MLPs remained the same after being stored at room temperature for several months. The photostability of florescence probes is critically important in live cell imaging. Under 400 nm fs laser irradiation with at very high excitation density of P = 3 Pth, the PL intensity of ONW dropped by 20% from its initial value after 2000 s (red line in Figure 5a); in sharp contrast, the PL intensity of the ONW@SiO2 MLP with the same size shows no apparent fatigue (Figure 5a). This indicates the coating of the SiO2 shell provides an effective strategy to improve the photostability of ONWs. To evaluate the biocompatibility of ONW@SiO2 MLPs with different sizes around 5 µm and 10µm, RAW 264.7 cells were employed to test the cytotoxicities of these ONW@SiO2 MLPs by using a Cell counting Kit-8 colorimetric assay. As shown in Figure 5b and Figure S14, when the ONW@SiO2 MLPs have been added to a culture medium with immune cells for 24 h, the viability of RAW 264.7 remained over 90% incubation with ONW@SiO2 MLPs, even at a concentration as high as 28 µg/mL. This result shows that the as-prepared ONW@SiO2 MLPs are almost not toxic for cells and are suitable for biological applications.


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To examine the luminescence imaging capability of ONW@SiO2 MLPs in live cells, Raw 264.7 macrophages were chosen for cells to internalize ONW@SiO2 MLPs with the size about 5 µm. The small size combining with water-soluble and biocompatible mesoporous silica shell eliminate perturbations of ONW@SiO2 MLPs in cells. Macrophages cells were incubated with 1 mL of fresh medium and 8 µL PBS containing ONW@SiO2 MLPs (20µg/mL) for 24 h and investigated them by confocal laser scanning microscopy. Intense intracellular luminescence was observed under excitation of 400 nm (Figure 6). Overlays of bright field and confocal PL images show strong green PL in the intracellular region, suggesting that ONW@SiO2 MLP was internalized into the cells rather than merely staining the membrane surface (Figure S13). Figure S13 also shows that ONW@SiO2 MLPs with the size about 10 µm can be internalized by macrophages as bioimaging probes. We then study the ability of the ONW@SiO2 MLP as intracellular microlasers by our home-made µ-PL system. When we keep the polarization of the excitation laser perpendicular to axial-direction of the ONW@SiO2 MLP (β = 90°), strong laser emission emerges as a set of multiple sharp peaks with the FWHM of 7 nm above thresholds with a low pump intensity (Figure 6a-2). When we adjust the angle β to 0°, only a very weak spontaneous emission is detected at the same pump intensity. Excitingly, intriguing phenomenon was found in our subsequent work. Due to the small size, each cell can engulf multiple ONW@SiO2 MLPs. Figure 6b show the bright-field image of cells containing two mutually perpendicular ONW@SiO2 MLPs named ONW@SiO2 MLP ③ and ONW@SiO2 MLP ④. When the excitation polarization was fixed perpendicular (90°) to the ONW@SiO2 MLP ③, the ONW@SiO2 MLP ③ in Figure 6b-2 produced a bright green laser emission and the ONW@SiO2 MLP ④ in Figure 6b-2 appeared fainter with a very weak spontaneous emission, and vice versa ( Figure 6b-3). It is reasonable to predict that the additional


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advantages of ONW@SiO2 MLP for biological applications also include removing multiply scattered (depolarized) light, as well as high imaging-contrast and improved tissue-resolution. What's more, we can use ONW@SiO2 MLPs with different sizes to tag individual cells. The dominant lasing wavelength λmax and the spacing between the two peaks in the lasing spectrum ∆λ differ between different ONW@SiO2 MLPs. For the ONW@SiO2 MLPs with length of 3-5 µm in Figure S15, the λmax was within a range of 3 nm and the width of interval accommodated the typical wavelength variation ∆λ of 0.5 nm in the Figure S15. Considering that the resolution of our spectrometer is 0.01 nm and the temporal stability of λmax and ∆λ (0.22 nm for λmax and 0.02 nm for ∆λ, Figure S12), we can individually distinguish (3 nm/0.22 nm) × (0.5 nm/0.02 nm) ≈ 300 cells. If the range of sizes was enlarge to 3-15 µm in the Figure S6, more than 3000 individual cells can be distinguished. Considering that each cell can engulf multiple ONW@SiO2 MLPs, the highly polarized and narrow emissions of the ONW@SiO2 MLPs microlasers are promising as efficient fluorescent probes for tracking of large numbers of cells in vivo cell tracking. Macrophages are of much consequence to the immune system and can internalize foreign objects that measure up to nearly twice their own diameter.18 By contrast, two different types of cancer cells, HCT116 and LOVO, were chosen for cells to internalize ONW@SiO2 MLPs with the size about 5 µm. HCT116 and LOVO readily internalized ONW@SiO2 MLPs (Figure S16), even though they are cell lines without unusually obvious capacity for endocytosis. The method of using microlaser probes is adequate for a large variety of different cell types. 4. CONCLUSIONS

In conclusion, we demonstrate a silica-coated organic-nanowire microlaser probe, i.e., ONW@SiO2 core-shell MLP, for biological imaging. The high-quality WGM microresonators


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built within the rectangular cross-section of ONW@SiO2 MLPs leads to a low lasing threshold of 1.54 µJ/cm2, above which lasing emissions are developed with a FWHM < 5 nm and a DOP > 83%. Meanwhile, small dimensions of ONW@SiO2 MLPs with a side-length of ca. 500-nm and a length of 3-8 µm help to reduce their perturbations in living cells. With the help of mesoporous silica shells, which provide both high biocompatibility and good photostability, ONW@SiO2 MLPs can easily be introduced into the cell cytoplasm through natural endocytosis. Using their narrow and highly polarized lasing emissions in vitro, we would be possible to tag individual cells using ONW@SiO2 MLPs with high stability.

ASSOCIATED CONTENT

Supporting Information. Experimental details, PL and SEM images, diffused reflection absorption and PL spectra, µ-PL spectra for the cavity mode, polarized PL spectra, simulated polarized electric field intensity profiles, dependence of polarization on the ratio of length/width, stability of ONW@SiO2 MLPs in cells, spectra with a barcode-type label, cancer cell labeling experiments and the efficiency of ONW@SiO2 MLPs internalization rate. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author *Email: [email protected], [email protected], hbfu@ cnu.edu.cn Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This work was supported by the National Natural Science Foundation of China (Grant Nos. 21190034, 91222203, 21273251, 21221002, 91333111, 21503139, and 21673144), the National Natural Science Foundation of China (Grant Nos. 2162011), project of State Key Laboratory on Integrated Optoelectronics of Jilin University (IOSKL2014KF16), project of Beijing Municipal Education Commission (KM20161002800 8), project of Construction of Innovative Teams and Teacher Career Development for Universities and Colleges Under Beijing Municipality (IDHT20140512), the National Basic Research Program of China (973) and 2013CB933500, and the Chinese Academy of Sciences. REFERENCES

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Scheme1. Schematic illustration of fabrication of the core-shell ONW@SiO2 MLPs with the help of CTAB via the self-template strategy


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Figure 1. (a) The SEM image of as-prepared core-shell ONW@SiO2 MLPs on a silicon substrate under low-magnification. (b) The high-magnification SEM image of single ONW@SiO2 MLP. (c) Low-magnification TEM image of ONW@SiO2 MLPs. (d) High-magnification TEM image of an individual ONW@SiO2 MLP. (e) XRD patterns of ONWs and ONW@SiO2 MLPs. (f) Cross sectional compositional line profiles of ONW@SiO2 MLP recorded along the red line marked in panel (d).


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Figure 2. (a) Photoluminescence (PL) microscopy image of ensemble ONW@SiO2 MLPs on a glass substrate excited with 400nm, 150 fs light above the threshold. (b-c) SEM images of the corresponding ONW@SiO2 MLPs with different size labeled as ② and ① in (a), Scale bar, 1 µm. (d-e) The µ-PL spectra of the ONW@SiO2 MLPs (① and ②) in the panel (a) under different excitation density. The insets both show the corresponding dependence of integrated PL intensity on the excitation density. (f) The mode spacing (red) and lasing threshold (black) of the room temperature WGM lasing change with the width (W) of the ONW@SiO2 MLPs. The black curve is the best fit to a 1/W2 function.


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Figure 3. Optical mode simulations of the calculated |E|2 field of ONW@SiO2 MLP. (a) The scheme of the model in FDTD simulations. (b) Cross-section views of the mode profile that are taken at the middle slice of the ONW@SiO2 MLP cavity. (c) Optical ray analysis of WGM cavity in ONW@SiO2 MLP, illustrating the light travelling around due to TIR at the resonator boundary. (d) Standing wave pattern of the optical mode along the ONW@SiO2 MLP axis. Red corresponds to the highest field density and blue is the lowest field density.


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Figure 4. (a-b) The µ-PL spectra from one isolated ONW@SiO2 MLP with pump/emissiondetected polarization combinations at two angles β = 90° (perpendicular to the ONW@SiO2 MLP) and β = 0° (parallel to the ONW@SiO2 MLP), excited at 400 nm. The insets show schematic diagrams of the measurement geometry for an individual ONW@SiO2 MLP and micrographs of lasing spot patterns, Scale bar, 5 µm. (c) The variation of input excitation laser (400 nm) and the output emission microlaser from ONW@SiO2 MLPs (500 nm) with emissiondetected polarization at the angle θ (θ = 0−360°).


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Figure 5. (a)The lasing emission intensities of ONW@SiO2 MLPs and ONWs as a function of 400 nm laser irradiation time under the high excitation density (P = 3 Pth). (b) Viability of RAW 264.7 cells at various concentrations of ONW@SiO2 MLPs with size around 5 µm.


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Figure 6. (a-1) Bright-field images of RAW 264.7 cells containing a single ONW@SiO2 MLP. (a-2, 3) fluorescence images and spectra of the RAW 264.7 cell with pump-detected polarization at two angles β = 90°(perpendicular to the ONW@SiO2 MLP) and β = 0°(parallel to the ONW@SiO2 MLP) , excited at 400 nm above the threshold. (b-1) Bright-field images of RAW 264.7 cells containing two mutually perpendicular ONW@SiO2 MLPs named ONW@SiO2 MLP ③ and ONW@SiO2 MLP ④. (b-2, 3) fluorescence images and spectra of the RAW 264.7 cell with pump-detected polarization perpendicular to the ONW@SiO2 MLP ③ and ONW@SiO2 MLP ④, respectively, excited at 400 nm above the threshold.


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Organic-NanowireâSiO2 CoreâShell Microlasers ... - ACS Publications

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