Fabrication of Polymer Microspheres for Optical Resonator and Laser Applications

Summary

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

Abstract

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

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 forth^{1,2,3,4,5,6,7,8,9}. In particular, π-conjugated polymers have inherent luminescent and charge conducting properties that are beneficial t....

Protocol

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....

Results

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.......

Discussion

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.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
polystyrene	Aldrich	132427-25G
sodium dodecylsulfate	Kanto Kagaku	372035-31
tetrahydrofuran	Wako	206-08744
chloroform	Wako	038-18495
methanol	Wako	139-13995
Poly(9,9-di-n-octylfluorenyl-2,7-diyl)	Aldrich	571652-500MG
Poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMOPPV)	Aldrich	546461-1G
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 Microscope	Nicon	Eclipse LV-N
laser_405 nm	Hutech	DH405-10-5
Spectrometer	Lambda Vision	LV-MC3/T
Homogenizer	Microtech Nichion	Physcotron NS-360D
micromanipulation	Microsupport	Quick Pro QP-3RH