Phase Diagram Characterization Using Magnetic Beads as Liquid Carriers

Summary

Here, we present a protocol to investigate multi-component phase diagrams using externally controlled magnetic beads as liquid carriers in a lab-in-tube approach. This approach can aid in applications that seek to gather further information on phase change in complex liquid systems.

Abstract

Magnetic beads with ~1.9 µm average diameter were used to transport microliter volumes of liquids between contiguous liquid segments with a tube for the purpose of investigating phase change of those liquid segments. The magnetic beads were externally controlled using a magnet, allowing for the beads to bridge the air valve between the adjacent liquid segments. A hydrophobic coating was applied to the inner surface of the tube to enhance the separation between two liquid segments. The applied magnetic field formed an aggregate cluster of magnetic beads, capturing a certain liquid amount within the cluster that is referred to as carry-over volume. A fluorescent dye was added to one liquid segment, followed by a series of liquid transfers, which then changed the fluorescence intensity in the neighboring liquid segment. Based on the numerical analysis of the measured fluorescence intensity change, the carry-over volume per mass of magnetic beads has been found to be ~2 to 3 µl/mg. This small amount of liquid allowed for the use of comparatively small liquid segments of a couple hundred microliters, enhancing the feasibility of the device for a lab-in-tube approach. This technique of applying small compositional variation in a liquid volume was applied to analyzing the binary phase diagram between water and the surfactant C12E5 (pentaethylene glycol monododecyl ether), leading to quicker analysis with smaller sample volumes than conventional methods.

Introduction

Magnetic beads (MBs) on the order of 1 micrometer in diameter have been used^1,2 quite often in microfluidic-based applications, particularly for biomedical devices. In these devices, MBs have offered capabilities such as cell and nucleic acid separation, contrast agents, and drug delivery, to name a few. The combination of external (magnetic field) control and droplet-based microfluidics has enabled³ control of immunoassays using small volumes (<100 nl). MBs have also shown promise when used for liquid handling⁴. This approach uses the MBs to transport biomolecules between liquid segments within a tube separated by an air valve. Th....

Protocol

1. Preparation of One-Time Use Materials in Device

1. Preparation of tube
   1. Cut tubing into 15 cm segments. Tubing has 1.6 mm inner diameter and 3.2 mm outer diameter.
   2. Hang tube segments vertically using tape. Place paper towel underneath tubes to collect the excess fluoropolymer solution.
   3. Inject 100 µl of fluoropolymer solution into top opening of each tube segment using syringe, such that it will come into contact with entire circumference on the inner-wall.
   4. Allow tube segments to hang in place for 1 hr to remove excess amount of fluoropolymer solution.
   5. Clean out any fluoropolymer solution....

Results

Using the Lab-in-Tube approach for transporting µl-volume amounts of liquid with magnetic beads along with MATLAB for numerical analysis, average liquid carry-over volumes, as a function of magnetic bead mass, were found (Figure 2). Higher mass of magnetic beads provides higher carry-over volume in the rate of 2-3 µl/mg. The experimental setup (Figure 1) was used to observe phase change within the H₂O/C12E5 binary system. Since the H₂O/C12E5 system is well-known and.......

Discussion

In most common techniques for phase diagram investigation, multiple samples with different compositions and ratios need to be prepared and have to reach thermodynamic equilibrium which causes a lengthy process and a significant amount of material. Some challenges can be resolved by DIT (diffusive interfacial transport) method using flat capillary and the infrared analysis method, but none of them can resolve all challenges with low cost investment.

The feasibility of using magnetic beads as li.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
AccuBead	Bioneer Inc.	TS-1010-1	Magnetic beads
C12E5 Surfactant	Sigma-Aldrich	76437
Thermo Scientific Nalgene 890	Fisher Scientific	14176178
Cube Magnet	Apex Magnets	M1CU
Polarizer Film	Edmund Optics	38-493
Teflon AF	Dupont	400s1-100-1	Fluoropolymer solution
Keyacid Red Dye	Keystone	601-001-49	Fluorescent dye
Luer-Lock	Cole-Parmer	T-45502-12	Female
Luer-Lock	Cole-Parmer	T-45502-56	Male
Syringe	Fisher Scientific	14-823-435	3 ml
Syringe Pump	Stoelting	53130
Stereo Microscope	Nikon	SMZ-2T
Inverted Microscope	Nikon	Eclipse Ti-U	The filter cube used had an excitation wavelength range from 540-580 nm and a dichroic mirror at 585 nm, allowing for photoemission ranging from 593-668 nm.
Balance	Denver Instruments	PI-225D
Microscope-Mounted Camera	Motic	5000