Stable Aqueous Suspensions of Manganese Ferrite Clusters with Tunable Nanoscale Dimension and Composition

Summary

We report a one-pot hydrothermal synthesis of manganese ferrite clusters (MFCs) that offers independent control over material dimension and composition. Magnetic separation allows rapid purification while surface functionalization using sulfonated polymers ensures the materials are non-aggregating in biologically relevant medium. The resulting products are well positioned for biomedical applications.

Abstract

Manganese ferrite clusters (MFCs) are spherical assemblies of tens to hundreds of primary nanocrystals whose magnetic properties are valuable in diverse applications. Here we describe how to form these materials in a hydrothermal process that permits the independent control of product cluster size (from 30 to 120 nm) and manganese content of the resulting material. Parameters such as the total amount of water added to the alcoholic reaction media and the ratio of manganese to iron precursor are important factors in achieving multiple types of MFC nanoscale products. A fast purification method uses magnetic separation to recover the materials making production of grams of magnetic nanomaterials quite efficient. We overcome the challenge of magnetic nanomaterial aggregation by applying highly charged sulfonate polymers to the surface of these nanomaterials yielding colloidally stable MFCs that remain non-aggregating even in highly saline environments. These non-aggregating, uniform, and tunable materials are excellent prospective materials for biomedical and environmental applications.

Introduction

The inclusion of manganese as a dopant in an iron oxide lattice can, under the appropriate conditions, increase the material's magnetization at high applied fields as compared to pure iron oxides. As a result, manganese ferrite (Mn_xFe_3-xO₄) nanoparticles are highly desirable magnetic nanomaterials due to their high saturation magnetization, strong response to external fields, and low cytotoxicity^1,2,3,4,5. Both single domain nanocrystals as well as clusters of....

Protocol

1. Synthesis of MFCs with control over MFCs' overall diameter and ferrite composition

1. Wash and thoroughly dry all glassware to be used in the synthesis. The amount of water in the synthesis impacts the dimensions of the MFCs, so it is crucial to ensure the glassware has no residual water in it^16,26.
   1. To wash the glassware, rinse with water and detergent and scrub with a flask brush to remove debris. Thoroughly rinse to remove all detergent and finish with a rinse of deionized water.
   2. To dry the glassware, shake water droplets off the surface of the glasswar....

Results

After hydrothermal treatment, the reaction mixture turns into a viscous black dispersion as can be seen in Figure 1. What results after purification is a highly concentrated MFC solution that behaves like a ferrofluid. The fluid in the vial responds within seconds when placed near a handheld magnet (<0.5 T), forming a macroscopic black mass that can be moved around as the magnet is placed at different locations.

This synthesis yields products whose dimension an.......

Discussion

This work demonstrates a modified polyol synthesis of manganese ferrite nanocrystals clustered together into uniform nanoscale aggregates²⁹. In this synthesis, iron(III) chloride and manganese(II) chloride undergo a forced hydrolysis reaction and reduction, forming molecular Mn_xFe_3-xO₄. These ferrite molecules form primary nanocrystals under the high temperature and high pressure in the reactors, ultimately assembling into spherical aggregates termed her.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
0.1 Micron Vaccum Filtration Filter	Thermo Fisher Scientific	NC9902431	for filtration of aggregated clusters after synthesis and surface coating to achieve a uniform solution
2-Acrylamido-2-methylpropane sulfonic acid (AMPS, 99%)	Sigma-Aldrich	282731-250G	reagent used in copolymer to surface coat nanoclusters and functionalize them for biological media
2,2′-Azobis(2-methylpropionitrile) (AIBN)	Sigma-Aldrich	441090-100G	reagent used in copolymer making as the free ridical generator
4-Morpholineethanesulfonic acid, 2-(N-Morpholino)ethanesulfonic acid (MES)	Sigma-Aldrich	M3671-250G	acidic buffer used to stabilize nanocluster surface coating process
Acrylic acid	Sigma-Aldrich	147230-100G	reagent used in copolymer to surface coat nanoclusters and functionalize them for biological media; anhydrous, contains 200 ppm MEHQ as inhibitor, 99%
Analytical Balance	Avantor	VWR-205AC	used to weigh out solid chemical reagents for use in synthesis and dilution
Digital Sonifier and Probe	Branson	B450	used to sonicate nanocluster solution during surface coating to break up aggregates
Dopamine hydrochloride	Sigma-Aldrich	H8502-25G	used in surface coating for ligand exchange reaction
Ethylene glycol (anhydrous, 99.8%)	Sigma-Aldrich	324558-2L	reagent used as solvent in hydrothermal synthesis of nanoclusters
Glass Vials (20mL)	Premium Vials	B1015	container for nanocluster solution during washing and surface coating as well as polymer solutions
Graduated Beaker (100mL)	Corning	1000-100	container for mixing of solid and liquid reagents during hydrothermal synthesis (to be transferred into autoclave reactor before oven)
Handheld Magnet	MSC Industrial Supply, Inc.	92673904	1/2" Long x 1/2" Wide x 1/8" High, 5 Poles, Rectangular Neodymium Magnet low strength magnet used to precipitate nanoclusters from solution (field strength is increased with steel wool when needed)
Hydrochloric acid (ACS grade, 37%)	Fisher Scientific	7647-01-0	for removing leftover nanocluster debris and cleaning autoclave reactors for next use
Hydrothermal Autoclave Reactor	Toption	TOPT-HP500	container for finished reagent mixture to withstand high temperature and pressure created by the oven in hydrothermal synthesis
Iron(III) Chloride Hexahydrate (FeCl3·6H2O, ACS reagent, 97%)	ACS	236489-500G	reagent used in synthesis of nanoclusters as source of iron (III) that becomes iron (II) in finished nanocluster product (keep dry and weigh out quickly to avoid water contamination)
Labware Washer Brushes	Fisher Scientific	13-641-708	used to wash and clean glassware before synthesis
Magnetic Stir Plate	Thermo Fisher Scientific	50093538	for mixing of solid and liquid reagents during hydrothermal synthesis
Manganese chloride tetrahydrate (MnCl2·4H2O, 99.0%, crystals, ACS)	Sigma-Aldrich	1375127-2G	reagent used in synthesis of nanoclusters as source of manganese
Micropipette (100-1000μL)	Thermo Fisher Scientific	FF-1000	for transferring liquid reagents such as water and manganese chloride
N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC)	Sigma-Aldrich	25952-53-8	used in surface coating to assist in ligand exchange of copolymer (keep bulk chemical in freezer and diluted solution in refrigerator)
N,N-Dimethylformamide (DMF)	Sigma-Aldrich	227056-2L	reagent used in copolymer making as the solvent
Polyacrylic acid sodium salt (PAA, Mw~6,000)	PolyScience Inc.	06567-250	reagent used in hydrothermal synthesis to initially coat the nanoclusters (eventually replaced in surface coating step)
Poly(ethylene glycol) methyl ether acrylate	Sigma-Aldrich	454990-250ML	reagent used in copolymer to surface coat nanoclusters and functionalize them for biological media; average Mn 480, contains 100 ppm BHT as inhibitor, 100 ppm MEHQ as inhibitor
Reagents Acetone, 4L, ACS Reagent	Cole-Parmer	UX-78920-66	used as solvent to precipitate nanoclusters during washing
Single Channel Pipette, Adjustable 1-10 mL	Eppendorf	3123000080	for transferring ethylene glycol and other liquids
Steel Wool	Lowe's	788470	used to increase the magnetic field strength in the vial to aid in precipitation of nanoclusters for washing and surface coating
Stirring Bar	Thomas Scientific	8608S92	for mixing of solid and liquid reagents during hydrothermal synthesis
Table Clamp	Grainger	29YW53	for tight sealing of autoclave reactor to withstand high pressure of oven during hyrothermal synthesis
Urea (ACS reagent, 99.0%)	Sigma-Aldrich	U5128-500G	reagent used in hydrothermal synthesis to create a basic solution
Vaccum Filtration Bottle Tops	Thermo Fisher Scientific	596-3320	for filtration of aggregated clusters after synthesis and surface coating to achieve a uniform solution
Vacuum Controller V-850	Buchi	BU-V850	for filtration of aggregated clusters after synthesis and surface coating to achieve a uniform solution
Vacuum Oven	Fisher Scientific	13-262-51	used to create high temperature and pressure needed for nanocluster formation in hydrothermal synthesis