Controlled Synthesis and Fluorescence Tracking of Highly Uniform Poly(N-isopropylacrylamide) Microgels

Summary

Non-stirred precipitation polymerization provides a rapid, reproducible prototyping approach to the synthesis of stimuli-sensitive poly(N-isopropylacrylamide) microgels of narrow size distribution. In this protocol synthesis, light scattering characterization and single particle fluorescence tracking of these microgels in a wide-field microscopy setup are demonstrated.

Abstract

Stimuli-sensitive poly(N-isopropylacrylamide) (PNIPAM) microgels have various prospective practical applications and uses in fundamental research. In this work, we use single particle tracking of fluorescently labeled PNIPAM microgels as a showcase for tuning microgel size by a rapid non-stirred precipitation polymerization procedure. This approach is well suited for prototyping new reaction compositions and conditions or for applications that do not require large amounts of product. Microgel synthesis, particle size and structure determination by dynamic and static light scattering are detailed in the protocol. It is shown that the addition of functional comonomers can have a large influence on the particle nucleation and structure. Single particle tracking by wide-field fluorescence microscopy allows for an investigation of the diffusion of labeled tracer microgels in a concentrated matrix of non-labeled microgels, a system not easily investigated by other methods such as dynamic light scattering.

Introduction

Stimuli-sensitive poly(N-isopropylacrylamide) (PNIPAM) microgels ^1,2 have attracted continuous interest over the past two decades due to their potential in various smart applications. Demonstrated use cases include switchable emulsion stabilizers ^3-8, microlenses ⁹, cell culture substrates for easy cell harvesting ^10,11, and smart carriers for low molecular weight compounds and other biomedical uses ¹². From a fundamental research point of view these particles have been proven to be useful for investigating subjects such as colloidal interactions ^13-15 and polymer-solvent interactions ^16-1....

Protocol

1. Microgel Synthesis

NOTE: N-isopropylacrylamide (NIPAM) was recrystallized from n-hexane. Other reagents were used as received.

1. Conventional Batch Synthesis of Poly(NIPAM) Matrix Microgels
   1. Dissolve 1.8 g NIPAM and 24 mg N,N'-bisacrylamide (BIS) in 245 ml filtered (0.2 µm regenerated cellulose (RC) membrane filter) double distilled water in a 500 ml three-neck round bottom flask equipped with a reflux condenser, a stirrer and a rubber septum.
   2. Insert a thermometer and a 120 mm needle for the nitrogen input through the septum.
   3. Heat the solution to 60 °C, while stir....

Results

The number of PNIPAM microgel particles in the batch, and thus the final particle volume, is determined early in the reaction during the nucleation phase ²⁰ Hydrophobic co-monomer dye methacryloxyethyl thiocarbamoyl rhodamine B influences the nucleation by reducing the particle number density in the batch. The decrease in particle concentration for two different initial NIPAM concentrations can be seen as increase in.......

Discussion

Addition of small amounts of functional comonomer can have a significant effect on the particle size and structure of the PNIPAM derived microgels. Simultaneous small-scale test tube polymerization is a good method to account for such changes, and helps to rapidly find the right reactant compositions for target particle size for upscaling the reaction as needed. The mass of the particles is approximately exponentially dependent on the .......

Materials

Name	Company	Catalog Number	Comments
Acetone	VWR Chemicals	KRAF13455
Bisacrylamid	AppliChem	A3636
n-Hexane	Merck	104374
N-Isopropylacrylamide	Fisher Scientific	AC412785000	recrystallized from n-hexane
Methacryloxyethyl thiocarbamoyl rhodamine B	Polysciences	23591
Potassium peroxodisulfate	Merck	105091
Silicone oil 47 V 350	VWR Chemicals	83851
Toluene	Sigma Aldrich	244511
F12 Refrigerated/heating circulator	Julabo	9116612
Microscope	Olympus	IX83
XY(Z) Piezo System	Physik Instrumente	P-545.3R7
100x Oil immersion objective	Olympus	UPLSAPO
QuadLine Beamsplitter	AHF Analysentechnik	F68-556T
Cobolt Jive 150 laser	Cobolt	0561-04-01-0150-300
Multimode Fiber	Thorlabs	UM22-600
iXON Ultra 897 EMCCD camera	Andor	DU-897U-CS0-BV
Laser goniometer	SLS Systemtechnik	Mark III
CF40 Cryo-compact circulator	Julabo	9400340
Laser goniometer system	ALV GmbH	ALV / CGS-8F
Multi-tau corretator	ALV GmbH	ALV-7004
Light scattering electronics	ALV GmbH	ALV / LSE 5004
Photon counting module	PerkinElmer	SPCM-CD2969	2 units in pseudo cross-correlation mode
633 nm HeNe Laser	JDS Uniphase	1145P
F32 Refrigerated/heating circulator	Julabo	9312632