Manganese Oxide Nanoparticle Synthesis by Thermal Decomposition of Manganese(II) Acetylacetonate

Summary

This protocol details a facile, one-pot synthesis of manganese oxide (MnO) nanoparticles by thermal decomposition of manganese(II) acetylacetonate in the presence of oleylamine and dibenzyl ether. MnO nanoparticles have been utilized in diverse applications including magnetic resonance imaging, biosensing, catalysis, batteries, and waste water treatment.

Abstract

For biomedical applications, metal oxide nanoparticles such as iron oxide and manganese oxide (MnO), have been used as biosensors and contrast agents in magnetic resonance imaging (MRI). While iron oxide nanoparticles provide constant negative contrast on MRI over typical experimental timeframes, MnO generates switchable positive contrast on MRI through dissolution of MnO to Mn²⁺ at low pH within cell endosomes to ‘turn ON’ MRI contrast. This protocol describes a one-pot synthesis of MnO nanoparticles formed by thermal decomposition of manganese(II) acetylacetonate in oleylamine and dibenzyl ether. Although running the synthesis of MnO nanoparticles is simple, the initial experimental setup can be difficult to reproduce if detailed instructions are not provided. Thus, the glassware and tubing assembly is first thoroughly described to allow other investigators to easily reproduce the setup. The synthesis method incorporates a temperature controller to achieve automated and precise manipulation of the desired temperature profile, which will impact resulting nanoparticle size and chemistry. The thermal decomposition protocol can be readily adapted to generate other metal oxide nanoparticles (e.g., iron oxide) and to include alternative organic solvents and stabilizers (e.g., oleic acid). In addition, the ratio of organic solvent to stabilizer can be changed to further impact nanoparticle properties, which is shown herein. Synthesized MnO nanoparticles are characterized for morphology, size, bulk composition, and surface composition through transmission electron microscopy, X-ray diffraction, and Fourier-transform infrared spectroscopy, respectively. The MnO nanoparticles synthesized by this method will be hydrophobic and must be further manipulated through ligand exchange, polymeric encapsulation, or lipid capping to incorporate hydrophilic groups for interaction with biological fluids and tissues.

Introduction

Metal oxide nanoparticles possess magnetic, electric, and catalytic properties, which have been applied in bioimaging^1,2,3, sensor technologies^4,5, catalysis^6,7,8, energy storage⁹, and water purification¹⁰. Within the biomedical field, iron oxide nanoparticles and manganese oxide (MnO) nanoparticles have proven utility as contrast agents in magnetic resonance imaging (MRI)

Protocol

1. Glassware and tubing assembly – to be performed only the first time

NOTE: Figure 1 shows the experimental setup for MnO nanoparticle synthesis with numbered tubing connections. Figure S1 shows the same setup with the main glassware components labeled. If there is a mismatch between the chemical resistant tubing and the glass connection size, cover the glass connection first with a short piece of smaller tubing before adding the chemical resistant tubing to make the connections snug.

1. Secure the air-free nitrogen tank to the wall close to a chemical fume hood using app....

Results

To confirm successful synthesis, MnO nanoparticles should be assayed for size and morphology (TEM), bulk composition (XRD), and surface composition (FTIR). Figure 2 shows representative TEM images of MnO nanoparticles synthesized using decreasing ratios of oleylamine (OA, the stabilizer) to dibenzyl ether (DE, the organic solvent): 60:0, 50:10, 40:20, 30:30, 20:40, 10:50. Ideal TEM images consist of individual nanoparticles (shown as dark rounded octagons in Figure 2

Discussion

The protocol herein describes a facile, one-pot synthesis of MnO nanoparticles using Mn(II) ACAC, DE, and OA. Mn(II) ACAC is utilized as the starting material to provide a source of Mn²⁺ for MnO nanoparticle formation. The starting material can be easily substituted to enable production of other metal oxide nanoparticles. For example, when iron(III) ACAC is applied, Fe₃O₄ nanoparticles can be generated using the same nanoparticle synthesis equipment and protocol described

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Chemicals and Gases
Benzyl ether (DE)	Acros Organics	AC14840-0010	Concentration: 99%, 1 L
Drierite	W. A. Hammond Drierite Co. LTD	23001	Drierite 8 mesh, 1 lb
Ethanol	Decon Laboratories	2701	200 proof, 4 x 3.7 L
Hexane	Macron Fine Chemicals	5189-08	Concentration:  ≥98.5%, 4 L
Hydrochloric acid	VWR	BDH3030-2.5LPC	Concentration: 36.5 - 38.0 % ACS, 2.5 L
Manganese (II) acetyl acetonate (Mn(II)ACAC)	Sigma Aldrich	245763-100G	100 g
Nitrogen gas tank	Airgas	NI R300	Research 5.7 grade nitrogen, size 300 cylinder
Nitrogen regulator	Airgas	Y11244D580-AG	Single stage brass 0-100 psi analytical cylinder regulator CGA-580 with needle outlet
Oleylamine (OA)	Sigma Aldrich	O7805-500G	Concentration: 70%, technical grade, 500 g
Silicone oil	Beantown Chemical	221590-100G	100 g
Equipment
Centrifuge	Beckman-Coulter	Avanti J-E	JA-20 fixed-angle aluminum rotor, 8 x 50 mL, 48,400 x g
Hemisphere mantle	Ace Glass Inc.	12035-17	115 V, 270 W, 500 mL, temperature up to 450 °C
Hot plate stirrer	VWR	97042-642	120 V, 1000 W, 8.3 A, ceramic top
Temperature controller	Yokogawa Electric Corporation	UP351
Temperature probe	Omega	KMQXL-040G-12	Immersion probe, temperature up to 1335 °C
Vacuum oven	Fisher Scientific	282A	120 V, 1800 W, temperature up to 280 °C
Vortex mixer	Fisher Scientific	02-215-365	120 V, 50/60 Hz, 150 W
Water bath sonicator	Fisher Scientific	FS30H	Ultrasonic power 130 W, 3.7 L tank
Tools and Materials
Dumont tweezer	Electron Microscopy Sciences	72703D	Style 5/45, Dumoxel, 109 mm, for picking up TEM grids
Dumont reverse tweezer	Ted Pella	5748	Style N2a, 118 mm, NM-SS, self-closing, holding TEM grids in place for sample preparation
Mortar and pestle	Amazon	BS0007	BIPEE agate mortar and pestle, 70 X 60 X 15 mm labware
Nalgene™ Oak Ridge tubes	ThermoFisher Scientific	3139-0050	Polypropylene copolymer, 50,000 x g, 50 mL, pack of 10
Scintillation vials	Fisher Scientific	03-337-4	20 mL vials with white caps, case of 500
TEM grids	Ted Pella	01813-F	Carbon Type-B, 300 mesh, copper, pack of 50
Glassware Setup
4-neck round bottom flask	Chemglass Life Sciences	CG-1534-01	24/40 joint, 500 mL, #7 chem thread for thermometers
6-port vacuum manifold	Chemglass Life Sciences	CG-4430-02	480 nm, 6 ports, 4 mm PTFE stopcocks
Adapter	Chemglass Life Sciences	CG-1014-01	24/40 inner joint, 90°
Condenser	Chemglass Life Sciences	CG-1216-03	24/40 joint, 365 mm, 250 mm jacket length
Drierite 26800 drying column	Cole-Parmer	EW-07193-00	200 L/hr, 90 psi
Funnel	Chemglass Life Sciences	CG-1720-L-02	24/40 joint, 100 powder funnel, 195 mm OAL
Interlocked worm gear hose clamp	Grainger	16P292	1/2" wide stainless steel clamp, 3/8" to 7/8" diameter, to secure condenser tubing, 10 pack
Keck clips	Kemtech America Inc	CS002440	24/40 joint
Metal claw clamp	Fisher Scientific	05-769-7Q	22cm, three-prong extension clamps
Metal claw clamp holder	Fisher Scientific	05-754Q	Clamp regular holder
Mineral oil bubbler	Kemtech America Inc	B257040	185 mm
Rotovap trap	Chemglass Life Sciences	CG-1319-02	24/40 joints, 100 mL, self washing rotary evaporator
Rubber stopper	Chemglass Life Sciences	CG-3022-98	24/40 joints, red rubber
Tubing for air/water	McMaster-Carr	6516T21	Clear Tygon PVC for air/water, B-44-3, 1/4" ID, 1/16" wall, 25 ft
Tubing for air/water	McMaster-Carr	6516T26	Clear Tygon PVC for air/water, B-44-3, 3/8" ID, 1/16" wall, 25 ft
Tubing for chemicals	McMaster-Carr	5155T34	Clear Tygon PVC for chemicals, E-3603, 3/8" ID, 1/16" wall, 50 ft
Analysis Programs
XRD analysis program	Malvern Panalytical	N/A	X'Pert HighScore Plus
FTIR analysis program	Varian, Inc.	N/A	Varian Resolutions Pro