Microwave Synthesis of Fe2O3(oleic acid)n Nanoparticles

References:

H. M. Jensen et al, PNAS 2010.

Materials needed:

  • Iron(III) chloride hexahydrate, Sigma-Aldrich, 44944-50g, stored at room temp.
  • Sodium oleate, Sigma-Aldrich, 03880-10g, stored at -20C
  • Oleic acid, Sigma-Aldrich, 01008-25g, stored at -20C
  • Millipore water
  • Absolute ethanol
  • Hexanes

Containers needed:

  • 20 mL reaction vial with micro stir bar (Biotage)
  • 50 mL centrifuge tube (Falcon) 

Equipment needed:

  • Biotage microwave synthesizer
  • Centrifuge
  • Sonicator

Special Safety Concerns:

  • During the microwave synthesis, significant pressure (~20 atm) builds up within the vial. Thus it is essential to use a reaction vial designed for high pressure and to tightly crimp the lid on it.
  • Be aware that nanoparticles may pose additional health concerns beyond their bulk counterparts, so limit your exposure by doing all steps involving aspiration (pipetting, decanting, etc) in an externally-vented hood.

Waste Disposal:

Critical Steps:

The wash steps are essential for 

Procedure:

  1. Thaw oleic acid in warm water bath.

  2. Weigh iron (III) chloride hexahydrate (MW = 270.3g/mol, 543.72 mg, 2.01 mmol).

  3. Weigh sodium oleate (1826.51 mg, 6.00 mmol). Exercise care in doing so, it is a light-weight clumpy powder which is easy to accidentally disperse.

  4. Add 5 mL absolute ethanol to scintillation vial. Add 1 mL oleic acid. Vortex to mix.

  5. Add iron(III) chloride hexahydrate to a 20 mL microwave reaction vial (Biotage) containing an egg-shaped ~1cm long micro stir bar.

  6. Add 8.0 mL H2O (Millipore) to microwave reaction vial.

  7. Stir the reaction at ~700 rpm for 5 min at 25 °C. A clear, yellow solution should appear as the iron(III) chloride dissolves in water.

  8. Using a glass powder filter, transfer the sodium oleate into the reaction. Even with stirring, it will sit as a white powder atop the yellow solution.

Figure 1. Reaction after addition of oleic acid (step 9; left), after stirring for 1 h at 25 °C (step 10; center), and after being heated to 180 °C for 15 m (step 11; right).

  1. Add oleic acid to the stirring solution. The sodium oleate will dissolve upon contact with the oleic acid to produce a deep red-orange color (see Fig. 1 left).

  2. Cover the solution and stir for 1 h at 25 °C. The microwave vial should contain an opaque, dark reddish brown organic top layer and a clear, pale yellow aqueous bottom layer (see Fig. 1 center).

  3. The vial was crimped shut, transferred to a microwave synthesizer (Biotage Initiator 8), and heated at 80 °C for 4 h, giving an opaque, dark reddish brown organic top layer and clear, colorless aqueous bottom layer in the microwave vial.

  4. The reaction was heated at 180 °C for 15 min, yielding an opaque, dark reddish brown organic top layer containing a reddish brown precipitate, and a clear, colorless aqueous bottom layer in the reaction vial (see Fig 1, right).

  5. The contents of the vessel were transferred to a 50 mL centrifuge tube (Falcon). The tube was centrifuged at 3000 rcf for 30 min at 25 °C (Eppendorf Centrifuge, Model 5702), affording a dark, reddish brown pellet at the bottom of the tube.

  6. The liquid portion was decanted off, then 5.0 mL H2O (Millipore) was added to the pellet.

  7. The tube was sonicated for 5 min at 25 °C, giving a cloudy, reddish brown solution.

  8. The tube was centrifuged at 3000 rcf for 15 min at 25 °C, giving a dark, reddish brown pellet in the bottom of the tube and a milky orange supernatant(see Fig 2, left).

  9. The water was decanted off, and 5.0 mL absolute ethanol was added to the pellet.

  10. The tube was sonicated for 5 min at 25 °C, giving a cloudy, reddish brown solution.

  11. The vial was centrifuged at 3000 rcf rpm for 15 min at 25 °C, giving a dark, reddish brown pellet in the bottom of the tube and a transparent yellow supernatant (see Fig 2 center).

  12. The ethanol was decanted off, and 50 mL hexanes was added to the remaining pellet, the tube was sealed with parafilm, and the tube vortexed for 1 min at 25 °C, giving a transparent, dark reddish brown solution (see Fig 2 right).

  13. The tube was centrifuged at 3000 rcf for 5 min at 25 °C, giving a clear, dark reddish brown hexanes solution containing smaller, soluble Fe2O3 nanoparticles, and a reddish brown pellet containing larger, insoluble Fe2O3 nanoparticles and Fe2O3 nanoparticle aggregates.

  14. The hexanes solution was decanted off into a new 50 mL centrifuge tube and filtered through an Acrodisc Syringe Filter (PTFE membrane, 0.22 um, Pall Corporation), giving a clear, dark reddish brown solution containing Fe2O3(oleic acid)n nanoparticles.

  15. Dynamic Light Scattering (DLS) measurements (Malvern Instruments) performed in hexanes at 20 °C showed that the particles were narrowly distributed, with a mean diameter of 13 nm.

 

Figure 2. Nanoparticle containing solution after water wash (step 16; left), after ethanol wash (step 19; center), and after vortexing in hexanes (step; right).

Notes:

Since this reaction is neither air nor moisture sensitive, it can sit at any point between steps.

Reaction Scheme:

 

 

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