From Rust To High Tech: Semi-Synthesis Of A Ferrofluid Using FeO Nanoparticles

Lisa M. Stuber^†, Elizabeth M. Rachford^†, Christopher S. Jordan^†, Scott J. Mitchell^†, Crystal Tabron^‡, Thomas J. Manning^‡,*

Georgia Governor’s Honors Program, Chemistry section, Valdosta State University, Valdosta, Georgia, 31698 and^. Department of Chemistry, Valdosta State University, Valdosta, Georgia, 31698, tmanning@valdosta.edu

Received July 20, 2004. Accepted May 5, 2005.

Abstract:Metal oxide nanoparticles are an important material in nanotechnology. Ferrofluids, which are typically made from iron oxide (Fe₃O₄) nanoparticles, have found commercial applications and are widely used in academic research and educational activities. This study describes the novel semisynthesis of a ferrofluid using commercially available iron oxide nanoparticles (FeO) as the starting material. It forms the spiked solution indicative of a ferrofluid. Three educational goals were the driving force behind the development of this experiment: (a) students work with a cutting edge nanotechnology material that is safe, economical, and can be completed in a three-hour laboratory session; (b) the results of the experiments can be clearly observed without the need for expensive or research-grade equipment (i.e, high-resolution transmission electron microscopy, scanning tunneling microscopy, etc.); and (c) it emphasizes some of the basic concepts taught in a general chemistry class, such as oxidation and reduction and stoichiometry.

[*][†][‡]Historical

Ferrofluids are a highly viscous fluid made up of aqueous magnetite. This magnetite is very responsive to magnets assuming a spiked structure when near a magnetic field. Magnetite is made up of tiny, nanosized bits of Fe₃O₄, and the size of magnetite’s constituents contributes largely to its ability to conform to magnetic fields (Figure 1).

Ferrofluids have very unique physical properties, thus, they are useful for a plethora of different applications. One such application is in the classroom; ferrofluids have often been used in an educational setting. Many of these projects pertain to their synthesis or application, for example, one project highlights the reactions between Fe²⁺ and Fe³⁺ ions that form magnetite. The experiment presented in this manuscript is used mainly in a teaching setting to demonstrate the concepts of stochiometry, oxidation states, crystal structure, colloidal properties, and magnetism [2]. Instructors are able to lead their classes in such experiments because information is available about the preparation and illustration of hydromagnetics [3]; however, experimenting with nanosized materials, such as ferrofluids, is not limited to the classroom. A program at the University of Wisconsin, Madison allows interns to work with schoolchildren in a museum setting and to educate them about science and, specifically, nanotechnology [4].

Research and experimentation with ferrofluids is not limited to iron-based fluids; different methods of synthesizing bimetallic ferrofluids have been researched. The magnetic and catalytic properties of these bimetallic ferrofluids, the determination of their structure, and the morphology of the materials have also been studied [5]. Discussions have arisen concerning the synthesis of magnetic colloids and magnetic latexes, including such properties as their ferrite content [6]. Magnetic fluids and magnetorheol fluids are being reviewed [7] as has the synthesis of iron oxide-based magnetic nanomaterials and composites, including different methods of synthesizing nanoparticles and nanocomposites [8].

Magnetic fluids are useful in medicine and biotechnology [9] and may soon be of some use in the diagnosis of Alzheimer’s disease [10]. Magnetic microcarriers also have uses in medical applications, such as targeted drug delivery [11]. These magnetic fluids, which can be either electrically conductive or electrically nonconductive, have many applications outside of the medical field. Ferrofluids have been used as carrier fluids [12] and have been used to synthesize copolymers to be used as magnetic dispersion stabilizers [13]. Ferrofluids have been used as imaging agents [14] and as magnetically powered pumps [15] as well. Ferrofluids have also been useful for maintaining vacuum seals [16].

As evidenced by the above discussion, ferrofluids are useful in many areas, for instance, medicine, industry, and education. Students attending Valdosta State University requested a research project in nanotechnology. It was necessary to find a project that was economical and did not require expensive equipment, demonstrated visible results, and was safe and relevant. Thus, we spawned the question: Can a ferrofluid be made from commercially available iron oxide nanoparticles in a three-hour laboratory?

Experimental

Many different approaches were attempted in an effort to answer the question asked above, and one succeeded. The optimum procedure involved in the semisynthesis is as follows. First, 200 mg of iron oxide nanoparticles were weighed and placed in an empty 150-mL beaker. Then, 10 mL of 3% hydrogen peroxide were added dropwise to the nanoparticles at room temperature. The beaker was swirled by hand during the addition. Once the additions were complete, the resulting mixture was allowed to sit for 30 min to ensure that the

Figure 1. Three pictures of ferrofluids. Ferrofluids show their characteristic spiked structure when in the presence of a magnetic field, and with the appropriate placement of magnets, they can also be manipulated to make columns or spikes [1].

Figure 2. An electron microscope image of FeO nanoparticles that are used as the starting material for ferrofluids (image courtesy of Nanophase, Inc).

redox reaction went to completion. The beaker containing the sample was then swirled for 5 min by hand. A magnetic stirrer was avoided because FeO nanoparticles adhere to the surface of magnetic stir bars. After swirling, the mixture was allowed to sit for several minutes, and, then, 1 mL of tetramethylammonium hydroxide (TMAH) was added dropwise and the mixture was swirled by hand. The TMAH drops were added approximately every 20 s. The beaker was subsequently covered with Parafilm and allowed to sit for approximately 45 min. A Nd magnet was placed on the exterior of the beaker while the liquid was decanted in order to hold the magnetic solid. The remaining gelatinous material is the ferrofluid.

Various optimizations were tested, including the addition of different molar ratios of the oxidizing agent H₂O₂ to the nanoparticles (H₂O₂/FeO; 0:1, 2:1, 3:1, 4:1, 8:1) with and without tetramethyl-ammonium hydroxide. Hydrogen peroxide (eq 1) was used as an oxidizing agent in an attempt to oxidize Fe²⁺ (bound as an oxide in the FeO nanoparticle) into Fe³⁺ (eq 2). It was determined that a 3 to 1 ratio of hydrogen peroxide to iron oxide nanoparticles gave the best ferrofluid based on visual observations. Ferrofluids made with published protocols [2] or purchased from commercial vendors (i.e., Ferrotec Educational Material, Nashua, NH) are black in appearance. The ferrofluid produced with this procedure is dark-brown in appearance.

Iron oxide nanoparticle powder was obtained from Nanophase Technologies Corporation (NTC Part#: FE-0800-007-025, batch #: BEBF2201, Illinois, USA). Tetramethylammonium hydroxide (25% in water) was obtained from Acros Organics, and 3% hydrogen peroxide was used as the oxidizing agent. The magnets used were purchased from Edmund Scientific.

The students were given safety instructions on working with nanoparticles, acids, and oxidizing agents. Nanoparticles are solids but can become airborne very easily. The nanoparticles were weighted out and handled until they were in solution in a hood as a precaution.

Results and Discussion

Ferrofluids can be relatively expensive and must be used frugally. This study proposes a simple method of synthesizing a ferrofluid from iron oxide nanoparticles that can be purchased in bulk and used economically in a laboratory setting. In addition to low cost, students are able to directly interact with ferrofluids and learn about their chemistry. Other synthetic approaches require a slightly more complicated procedure [2]. Depending on the level of theory and type of instrumentation available, this synthesis can be adapted to a number of classes, from a simple demonstration in a high school or first-year undergraduate general chemistry class to a laboratory exercise in general chemistry or physical chemistry classes.

This exercise was conducted as an exploratory laboratory in a summer honors program for high school students (Georgia Governor’s Honors Program, summer, 2004) and was assessed as a successful approach when the students were able to make a ferrofluid. A general chemistry class (CHEM1212) at Valdosta State University used this laboratory experience to emphasize oxidation numbers, redox chemistry, different forms of iron oxide, and the specifics of nanoparticles including surface-area calculations and sedimentation rates in solution. Students were given a content-based quiz and were also asked to rate their increase in knowledge and their degree of interest in the subject matter. Simply discussing iron oxide in terms of rusting was less appealing to students when compared to working with FeO nanoparticles in a nanotechnology hands-on exercise (see Figure 2).

The ferrofluid produced with this process (see Figure 3) is dark brown in appearance and results in smaller spikes than observed with a commercial ferrofluid that is prepared with the traditional method [2]. Four possible reasons were offered to the students. First, the FeO nanoparticles are bigger (30 to 35 nm) than the commercial nanoparticles (5 to 10 nm). Second, we used H₂O₂ to shift some Fe(II) to Fe(III), but we probably did not have the same ratio of Fe(III)/Fe(II) that the commercial brand does. Third, with a different Fe(III)/Fe(II) ratio, the attachment of the surfactant (tetramethylammonium hydroxide) might have been less efficient. Fourth, with larger particles, a different Fe(III)/Fe(II) ratio, and the different efficiency of attaching the surfactant, there was a greater chance of our particles aggregating.

Figure 3. Ferrofluids produced by FeO nanoparticles have the spikes that indicate a ferrofluid is present. The Nd magnet (red-green) sits under the glass dish and holds the ferrofluid in place.

The visualization of a properly produced ferrofluid and its subsequent interaction with a magnetic field captures a student’s attention. They will experiment with different shapes and magnetic configurations for an extended period of time. The students had access to approximately 50 magnets to visualize the physical properties of the ferrofluid. These included neodymium magnets, which are both powerful and economical, and bar and horseshoe magnets, which can be used to produce different magnetic-field configurations. The students were able to see first-hand the effects of a magnetic field on matter. Typically, after the solution is decanted from the ferrofluid material (gelatinous), the remaining material is spread thinly over a glass slide or a watch glass. Thin layers of the material produce a better spike than a single large clump of the material.

The students were asked to respond to the following prelaboratory questions:

(a) Identify the oxidation number of iron in various complexes (Fe, FeO, Fe₂O₃, Fe₃O₄, etc.) and the redox properties (half reactions, potentials) of H₂O₂, Fe(s), Fe⁺² and Fe⁺³in the aqueous phase. Can H₂O₂ be used to convert Fe⁺² to Fe⁺³?

H₂O₂(aq) + 2H⁺(aq) + 2e^– ↔ 2H₂O(l) Eº = +1.776 V (1)

Fe²⁺(aq) ↔ Fe³⁺(aq) + e^–Eº = –0.771 V(2)

Note that the reduction potentials given are for solutions, and may be different in the solid state.

(b) Describe an existing method of ferrofluid synthesis (students were provided with reference 2) and list three interesting properties of the ferrofluid material.

(c) Describe, both in writing and with a labeled diagram, how a ferrofluid pump works using the World Wide Web as your primary source of information.

Because there is down time between experiments, students were allowed to work on these questions during the laboratory using online computers. Once in the laboratory, the students were given a prelaboratory lecture that discussed additional aspects of FeO nanoparticles and ferrofluids, including a hands-on demonstration of a commercially available ferrofluid being manipulated with different magnets.

The three prelaboratory questions provide students with a background of the basics and applications of ferrofluids from a chemical and engineering perspective. After the students complete the laboratory and the instructor evaluates each ferrofluid sample synthesized, they were asked to describe a current application for ferrofluids and project a future application associated with some aspect of science and engineering. The supporting material contains an example of one description given by a student that hopes to obtain a degree in aerospace engineering.

Conclusion

The uses and applications of ferrofluids in industry, medicine, and education are increasing. These magnetic fluids are unique in their ability to demonstrate the presence and effects of magnetic fields and represent an interdisciplinary topic for general chemistry. This experiment, which can be completed in a three-hour laboratory, allows students to work with and visualize a nanomaterial synthesized by the application of a general chemistry concept (redox). Much of the nanotechnology research results published in the open scientific literature utilize techniques that are very expensive, produce data that can be difficult to interpret by first- and second-year undergraduate students, and can only be performed by a small number of highly trained individuals, which makes their duplication impractical in a educational setting. This synthesis provides a hands-on experience that is economical and safe, builds on oxidation and reduction concepts as outlined in general chemistry, and produces a nanoparticle system that can be clearly visualized with the naked eye.

Acknowledgment. We would like to thank Minako Takeno, a professional artist, for the use of the excellent photographs of the ferrofluids used in this paper and Nanophase, Inc. for the use of the image of the FeO nanoparticles. The students participating in this laboratory exercise were part of the Georgia Governor’s Honors Program. A grant to Valdosta State University from the National Science Foundation’s nanotechnology in Undergraduate Education (NSF-NUE) program supported this work. Students in Dr. Manning’s CHEM1212 (fall, 2004, 3 sections) class are also recognized for working with the ferrofluids for the first time in a general chemistry laboratory at VSU. The authors would also like to thank the reviewers and editor for helpful and constructive comments that strengthened this paper.

Supporting Materials. A current application for ferrofluids and a projected future application associated with some aspect of science and engineering given by a student hoping to obtain a degree in aerospace engineering is included in a Zip File as supporting material (http://dx.doi.org/10.1333s00897050908a).

References and Notes

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4. Payne, A. C.; Widstrand, C. G.; Lisensky, G. C.; Zenner, G. M.; Derenne, T., Krajniak, P.; Crone, W. C. Presented at the 225th National Meeting of the American Chemical Society, New Orleans, LA, March 23–27, 2003.

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10. Halbreich, A.; Roger, J.; Pons, J. N.; De Fatima D. M.; Bacri, J. C. Microspheres, Microcapsules & Liposomes, vol. 3 (Radiolabeled and Magnetic Particulates in Medicine & Biology); Citus Books: London, 2001; pp 459–493.

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12. John, R.; Pillai, N. Janardhan, D. U. S. Patent 6,743,371, June 1, 2004.

13. Riffle, J. S.; Phillips; J. P.; Dailey; J. P. U. S. Patent6,749,844,June 15, 2004.

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15. Weigl, B. H.; Williams, C. L.; Hayenga, J. W.; Bardell, R. L.; Schulte, T. E. U. S. Patent 6,743,399, June 1, 2004.

16. Mitchell, R. J. U. S. Patent 6,740,894,May 25, 2004.

[†] Georgia Governor’s Honors Program, Chemistry section

[‡] Department of Chemistry, Valdosta State University