Method Article

Fabrication of Polymer Microspheres for Optical Resonator and Laser Applications

DOI:

10.3791/55934

⸱

June 2nd, 2017

 ,  ,  ,  , 
1Faculty of Pure and Applied Sciences, University of Tsukuba, 2Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), University of Tsukuba, 3Center for Integrated Research in Fundamental Science and Technology (CiRfSE), University of Tsukuba

In This Article

Summary

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Protocols for the synthesis of microspheres from polymers, the manipulation of microspheres, and micro-photoluminescence measurements are presented.

Abstract

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This paper describes three methods of preparing fluorescent microspheres comprising π-conjugated or non-conjugated polymers: vapor diffusion, interface precipitation, and mini-emulsion. In all methods, well-defined, micrometer-sized spheres are obtained from a self-assembling process in solution. The vapor diffusion method can result in spheres with the highest sphericity and surface smoothness, yet the types of the polymers able to form these spheres are limited. On the other hand, in the mini-emulsion method, microspheres can be made from various types of polymers, even from highly crystalline polymers with coplanar, π-conjugated backbones. The photoluminescent (PL) properties from single isolated microspheres are unusual: the PL is confined inside the spheres, propagates at the circumference of the spheres via the total internal reflection at the polymer/air interface, and self-interferes to show sharp and periodic resonant PL lines. These resonating modes are so-called "whispering gallery modes" (WGMs). This work demonstrates how to measure WGM PL from single isolated spheres using the micro-photoluminescence (µ-PL) technique. In this technique, a focused laser beam irradiates a single microsphere, and the luminescence is detected by a spectrometer. A micromanipulation technique is then used to connect the microspheres one by one and to demonstrate the intersphere PL propagation and color conversion from coupled microspheres upon excitation at the perimeter of one sphere and detection of PL from the other microsphere. These techniques, µ-PL and micromanipulation, are useful for experiments on micro-optic application using polymer materials.

Introduction

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Polymer nano/micro-sized particles are widely used for a variety of applications, including as catalyst support, column chromatography fillers, drug delivery agents, fluorescent probes for cell tracking, optical media, and so forth1,2,3,4,5,6,7,8,9. In particular, π-conjugated polymers have inherent luminescent and charge conducting properties that are beneficial to optical, electronic, and optoelectronic applications using polymer spheres10,11,12,13,14, especially laser applications using soft organic materials15,16,17. For example, the three-dimensional integration of spheres with several hundred nanometer diameters forms colloidal crystals, which show photonic band gaps at a certain wavelength18,19. When light is confined in the intersphere periodic structure, lasing action appears at the middle of the stop band. On the other hand, when the size of the spheres increases to the several-micrometer scale, light is confined inside a single microsphere via total internal reflection at the polymer/air interface20. Propagation of the light wave at the maximum circumference results in interference, leading to the appearance of a resonant mode with sharp and periodic emission lines. These optical modes are so-called "whispering gallery modes" (WGMs). The term "whispering gallery" originated from St. Paul's Cathedral in London, where sound waves propagate along the circumference of the wall, allowing whispers to be heard by a person on the other side of the gallery. Because the wavelength of light is on the sub-micrometer scale, which is far smaller than sound waves, such a large dome is not necessary for the WGM of light: tiny, micrometer-scale, well-defined vessels, such as microspheres, microdiscs, and microcrystals, fulfill the WGM conditions.

Equation 1 is a simple form of the WGM resonating condition21:

nπd = lλ      (1)

where n is the refractive index of the resonator, d is the diameter, l is the integer number, and λ is the wavelength of the light. The left part of (1) is the optical path length through one circle propagation. When the optical path coincides with the integer multiple of the wavelength, resonance occurs, while at the other wavelength, the light wave is diminished upon rounding.

This paper introduces several experimental methods to prepare microspheres for WGM resonators from conjugated polymers in solution: vapor diffusion22,23,24,25,26,27,28,29,30, mini-emulsion31, and interface precipitation32. Each method has unique characteristics; for example, the vapor diffusion method affords well-defined microspheres with very high sphericity and smooth surfaces, but only low-crystallinity polymers can form these microspheres. On the other hand, for the mini-emulsion method, various kinds of conjugated polymers, including high-crystalline polymers, can form spheres, but the surface morphology is inferior to that obtained from the vapor diffusion method. The interface precipitation method is preferable for creating microspheres from dye-doped, non-conjugated polymers. In all cases, the selection of the solvent and the non-solvent plays an important role in the formation of spherical morphology.

In the second half of this paper, µ-PL and micro-manipulation techniques are presented. For the µ-PL technique, microspheres are dispersed on a substrate, and a focused laser beam, through a microscope lens, is used to irradiate a single isolated microsphere24. The generated PL from a sphere is detected by a spectrometer through the microscope lens. Moving the sample stage can vary the position of the excitation spot. The detection point is also variable by tilting the collimator optics of the excitation laser beam with respect to the optical axis of the detection path28,32. To investigate intersphere light propagation and wavelength conversion, the micro-manipulation technique can be used32. To connect several microspheres with different optical properties, it is possible to pick up one sphere using a micro-needle and put it on another sphere. In conjunction with the micromanipulation techniques and the µ-PL method, various optical measurements can be carried out using conjugated polymer spheres, which are prepared by a simple self-assembly method. This video paper will be useful to readers who wish to use soft polymer materials for optical applications.

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Protocol

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1. Fabrication Protocols of Polymer Microspheres

  1. Vapor Diffusion Method
    1. Dissolve 2 mg of conjugated polymers, such as P1 (poly[(9,9-dioctylfluorene-2,7-diyl)-alt-(5-octylthieno[3,4-c]pyrrole-4,6-dione-1,3-diyl)])28 and P2 (poly[(N-(2-heptylundecyl)carbazole-2,7-diyl)-alt-(4,8-bis[(dodecyl)carbonyl]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)])28, in 2 mL of chloroform (a good solvent) in a 5-mL vial.
    2. Put 5 mL of methanol (a poor solvent) in a 50 mL vial.
    3. Put the 5 mL vial containing the chloroform solution of the polymer into the 50 mL vial containing methanol.
    4. Cap the 50 mL vial and keep it for 3 days at 25 °C to allow for the precipitation of the polymer microspheres.
  2. Mini-Emulsion Method
    1. Dissolve 5 mg of conjugated polymers, such as poly[9,9-di-n-octylfluorenyl-2,7-diyl] (PFO) and poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMOPPV), in 1 mL of chloroform.
    2. Dissolve 30 mg (~50 mM) of sodium dodecyl sulfate (SDS) in 2 mL of deionized water.
    3. Add 100 µL of the chloroform solution of the polymer to 2 mL of water containing SDS.
    4. Stir the chloroform/water mixture vigorously using an ultra-high-speed homogenizer at 30,000 rpm for 2 min to emulsify the solution.
    5. Keep it for 1 day without capping the vial to evaporate the chloroform.
    6. Centrifuge the dispersion in a 1.5 mL microcentrifuge tube for 5 min at 2,200 x g. Remove the supernatant aqueous solution containing SDS.
    7. Add 2 mL of deionized water and shake vigorously.
    8. Repeat step 1.2.6 and 1.2.7 thrice to wash out the residual SDS.
  3. Interface Precipitation Method
  4. Dissolve 200 µg of polystyrene (PS) and 10 µg of fluorescent dye (boron dipyrrin, BODIPY) to 0.2 mL of tetrahydrofuran (THF).
  5. Gently pour the THF solution onto 1 mL of the water layer.
  6. Keep the two-layer separated THF/water for 6 h without capping the vial to allow for the precipitation of the polymer microspheres.

2. Micro-photoluminescence (µ-PL) Measurement

  1. Sample Preparation
    1. Dilute a suspension of the microspheres prepared in section 1 in a non-solvent (i.e., methanol or deionized water).
    2. Spin-cast one drop (20-30 µL) of the diluted suspension of the microspheres onto a quartz substrate using a spin-coater (typically, 2,000 rpm for 50 s).
    3. Air-dry the resultant casted film until the solvents have evaporated completely (~5 min).
  2. Experimental Setup
    1. Put the quartz substrate (15 x 15 x 0.5 mm3) on the sample stage of an optical microscope.
    2. Find well-defined microspheres that are isolated from other spheres and appropriate for the µ-PL measurement.
    3. Select a laser (i.e., wavelength, continuous wave or pulse, irradiation time, integration, etc.).
    4. Select the magnification of the lens.
  3. Measurements
    1. Use a focused laser beam to irradiate the microsphere. Use the following laser condition: cw or pulsed laser with excitation wavelengths (λex) of 405 nm (cw), 450 nm (cw), 355 nm (pulse laser; frequency, 1 kHz; pulse duration, 7 ns), and 470 nm (pulse laser; frequency, 2.5 MHz; pulse duration, 70 ps).
    2. Record the PL spectrum at the excited spot using a spectrometer with a grating of 300 or 1,200 grooves mm-1.
    3. Take a fluorescent image.
    4. Change the excitation spot by moving the sample stage.
    5. Change the detection spot by tilting the collimator (if necessary).

3. Micromanipulation Technique

  1. Manipulation of Microspheres
    1. Set a quartz substrate upon which the microspheres are immobilized on the sample stage of an optical microscope.
    2. Find a well-defined microsphere appropriate for the µ-PL measurement.
    3. Set a plastic micro-needle on a micro-manipulation apparatus.
    4. Move the micro-needle using a computer-controlled joystick to pick up a microsphere.
    5. Move the microsphere and connect it to another microsphere.
    6. Measure the µ-PL from the connected microsphere.

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Results

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Figure 1 shows schematic representations of the vapor diffusion method (a), mini-emulsion method (b), and interface precipitation method (c). For the vapor diffusion method (Figure 1a), a 5 mL vial containing a CHCl3 solution of polymers (0.5 mg mL-1, 2 mL) was placed in a 50 mL vial containing 5 mL of a non-solvent, such as MeOH. The outside vial was capped and then allowed to stand for 3 days at 25 °C. The...

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Discussion

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The selection of a good solvent and non-solvent is very important for the self-assembly of well-defined microspheres. If the solubility of a polymer is too high, precipitation will not occur. Also, in general, π-conjugated polymers are hydrophobic, so polar non-solvents, such as MeOH, acetonitrile, and acetone, are often used in the vapor diffusion method to minimize the surface energy required to form a spherical shape. The interface precipitation method is often adopted for the preparation of dye-doped polymer mic...

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Disclosures

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The authors declare no competing financial interests.

Acknowledgements

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This work was partly supported by KAKENHI (25708020, 15K13812, 15H00860, 15H00986, 16H02081) from JSPS/MEXT Japan, the Asahi Glass Foundation, and the University of Tsukuba Pre-strategic initiative, "Ensemble of light with matters and life."

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
polystyreneAldrich132427-25G
sodium dodecylsulfateKanto Kagaku372035-31
tetrahydrofuranWako206-08744
chloroformWako038-18495
methanolWako139-13995
Poly(9,9-di-n-octylfluorenyl-2,7-diyl)Aldrich571652-500MG
Poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMOPPV)Aldrich546461-1G
poly[(9,9-dioctylfluorene-2,7-diyl)-alt-(5-octylthieno[3,4-c]pyrrole-4,6-dione-1,3-diyl)] (P1)synthesized-reference 28
poly[(N-(2-heptylundecyl)carbazole-2,7-diyl)-alt-(4,8-bis[(dodecyl)carbonyl]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)] (P2)synthesized-reference 28
fluorescent dye (boron dipyrrin; BODIPY)synthesized-reference 32
Optical MicroscopeNiconEclipse LV-N
laser_405 nmHutechDH405-10-5
SpectrometerLambda VisionLV-MC3/T
HomogenizerMicrotech NichionPhyscotron NS-360D
micromanipulationMicrosupportQuick Pro QP-3RH

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Tags

Whispering Gallery ModesVapor Diffusion MethodMini emulsion MethodInterface PrecipitationMicro photoluminescence TechniqueMicromanipulation TechniqueScanning Electron MicroscopyPhotoluminescent Properties

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