Synthesizing and Playing with Magnetic Nanoparticles: A

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX-XXX

pubs.acs.org/jchemeduc

Synthesizing and Playing with Magnetic Nanoparticles: A Comprehensive Approach to Amazing Magnetic Materials Anne-Laure Dalverny,† Géraldine Leyral,† Florence Rouessac,†,‡ Laurent Bernaud,† and Jean-Sébastien Filhol*,†,‡ †

Chemistry Department, Faculté des Sciences, Université de Montpellier, Place E. Bataillon, 34095 Montpellier Cedex 5, France Institut Charles Gerhardt Montpellier UMR 5253 CNRS-UM-ENSCM, Université de Montpellier, Place E. Bataillon, 34095 Montpellier Cedex 5, France

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S Supporting Information *

ABSTRACT: Magnetic iron oxide nanoparticles were synthesized and stabilized using ammonium cations or poly(vinyl alcohol) to produce amazing materials such as safer aqueous ferrofluids, ferrogels, ferromagnetic inks, plastics, and nanopowders illustrating how versatile materials can be produced just by simple modifications. The synthesis is fast, reliable, and efficient. It demonstrates many material properties such as magnetism, ferrofluidity, gelation, and so forth.

KEYWORDS: First-Year Undergraduate/General, Second-Year Undergraduate, Inorganic Chemistry, Inquiry-Based/Discovery Learning, Magnetic Properties, Physical Properties, Colloids, Undergraduate Research, Polymer Chemistry Matériaux à Propriétés Remarquables” by about 400 first- and second-year undergraduate students since 2008. The first goal of this laboratory project is for the student to learn how to synthesize, extract, and stabilize colloidal MNPs. The second goal is to use an inquiry-based approach to show how an initial object such as MNPs can be modified to produce new materials with different interesting properties. In a typical 3 h lab session, around 25 students are teamed into groups of two or three. Each group performs the MNPs synthesis, then adds different stabilizers and compares the results with the others. During the final “Genius Session”, some of these groups apply the different skills they have learned to the synthesis of a new magnetic material, by either changing the surfactant or the magnetic material and/or applying it to a new system. The last part occurs about one month later and is dedicated to a 10−15 min oral presentation, about their Genius Session, followed by 5−10 min of questioning that focuses on evaluating the knowledge acquired by the students on materials, nanoparticles, magnetism, and surfactants.

or more than 10 years the “Chimie: Science Magique” program has been designing high impact laboratory experiments, involving undergraduate students in the research process.1,2 This approach was applied to a study of superparamagnetic iron oxide nanoparticles (SPIONs) that allow the formation of the well-known aqueous ferrofluids.3−6 The classical ferrofluid synthesis was initially mastered by the undergraduates during the basic training, and then, the participants were challenged in a “Genius Session” to use the acquired basic skills to synthesize new materials, with modified properties from the initial ferrofluid with the following goals: (i) obtaining a less toxic ferrofluid that can be used as an ink, and (ii) changing its texture either by transforming the fluid into a gel or creating a magnetic solid material. These new syntheses are then added in the following years to the basic training part leading to new “Genius” challenges and continuous improvement of the proposed synthesis. This article presents a number of materials that can be produced from iron oxide magnetic nanoparticles (MNPs) that have been developed in the different “Genius” sessions in the last 10 years before being implemented into the main course. Many of these syntheses are simple enough to be performed at the high school level with limited laboratory equipment.

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COURSE ORGANIZATION

Received: April 28, 2017 Revised: September 27, 2017

The synthesis of these magnetic materials has been performed in the biannual course “Chimie Science Magique: Synthèse de © XXXX American Chemical Society and Division of Chemical Education, Inc.

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DOI: 10.1021/acs.jchemed.7b00277 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Figure 1. (A) TBA-stabilized aqueous ferrofluid. (B) Ferromagnetic ink interacting with a magnet after drying. (C) Piece of magnetic PVA plastic in interaction with a magnet. (D) Solid TBA-NNPs in interaction with a magnet after drying and fragmentation.

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formed. This “phagocytor goo” can be tested by approaching a magnet that will be phagocyted (see Figure 2). Magic Ink: Using Ferrofluid as Ferromagnetic Ink/ Paint. PVA-stabilized nanoparticles in water can be used as a magnetic ink. For that purpose, 2−5 mL of the PVA solution (40 g·L−1) was added to the MNPs to form a homogeneous nearly nontoxic ink that can be used to create magnet-sensitive writing or drawing (see Figure 1B). Making Your Own Magnetic Plastic. The MNPs are mixed with 40 mL of the PVA solution (40 g·L−1). The resulting solution is left to evaporate in a mold for a few days resulting in a PVA-magnetite plastic (see Figure 1C). Making Your Own Magnetic Nanopowder. One of the previous TBA-MNP ferrofluids is allowed to dry under a fume hood at room temperature for a few days until a magnetic black solid is formed. The solid is maintained in a closed container (to prevent MNPs dispersion) that is shaken until the magnetic solid becomes a fine powder. The powder shows a behavior similar to that of the ferrofluid, presenting spikes that are a visualization of the magnetic field lines (see Figure 1D).

EXPERIMENTAL DETAILS

Magnetite Nanoparticle Synthesis

Classical Funnel Synthesis. The first step of all the following syntheses is the synthesis of magnetite nanoparticles. This is done using a mix of 1/3 FeII and 2/3 FeIII with a dropwise addition of an NH3 solution following the classical approach described in refs 3 and 7. See the Supporting Information (SI) for details. One-Pot, 1 min Synthesis. A simplification of the experimental protocol is possible with the quick addition of the NH3 solution in three equal portions. Surprisingly, such a simple synthesis remains quite efficient in producing a colloidal suspension of MNPs with both efficiency and reproducibility: The success ratio is at least equal to that of the traditional method, leading to a strongly magnetic product. We use this procedure as a quick alternative method in the case of failure of the standard one. The MNPs are then washed, followed by a sequence of magnetic decantation and elimination of the nonmagnetic phase. Magnetic Materials Synthesis

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Less Toxic Ferrofluid. To obtain a ferrofluid with the characteristic spiking effect under an applied magnetic field, a high density of nanoparticles needs to be dispersed into the water solvent. This is done by magnetic decantation and drying followed by addition of a few drops of a solution of tetrabutylammonium (TBA) hydroxide as seen in Figure 1A. The use of TBA rather than tetramethylammonium hydroxide results in a rather less toxic reactant and aqueous ferrofluid. Phagocytor Goo: Example of Ferrogel. A soluble polymer can be used to stabilize nanoparticles.8 A good example is poly(vinyl alcohol) (PVA) that is commonly used for many applications ranging from hospital bags to soluble plastic used for dishwasher tablets.9 The MNPs formed in the previous step are mixed with PVA and stirred until a homogeneous black solution is obtained. To achieve a ferrogel, a cross-linker such as borax is added and stirred until the gel is

HAZARDS

The toxicity of Fe(II) and Fe(III) chloride is moderate, but FeCl2 is corrosive. TBA hydroxide is a strong base and should be handled with the necessary personal protections. PVA is considered to be nontoxic in moderate quantities as it is used as a coating agent in pharmaceutical and dietary supplements. Borax is not acutely toxic, and its global toxicity is considered mild except in the cases of long chronic exposure or ingestion of large quantities. In all cases, the use of standard personal protection is strongly advised. There is no known concern about MNPs’ specific acute toxicity, but we would strongly advise prevention of their dissemination. See the Supporting Information for details. B




20% (for an average of 16 groups/year for 5 years (2008− 2013)) showing that some improvement was possible. Therefore, for the “Genius Sessions”, a subject entitled “Improving and Hacking Nanomagnetic Materials” was given. The purpose of these sessions was to give the students an insight into the scientific and engineering method with different goals to achieve: (1) improving the MNPs synthesis (a) to get a faster and more reliable process (b) by using safer reactants and thus obtaining a less toxic ferrofluid; (2) producing different magnetic products with new properties leading to different applications. Fast Magnetic Nanoparticle Formation

The initial work was about the formation of MNPs by investigating the duration of NH3 solution addition. It was found by the student that when addition is too long (t > 10 min), a reddish black precipitate tends to appear, leading to a solid with weak magnetic properties: a too slow addition probably leads to a separate precipitation of Fe(OH)2 and Fe(OH)3 rather than Fe3O4. Faster addition (t < 5 min) usually leads to magnetite with good magnetic properties. Surprisingly, a very fast addition (in a few seconds) is quite efficient in forming MNPs even if there is some inhomogeneity in the results. Safer Ferrofluid

The second challenge is about changing the toxic TMA cation for a safer molecule and trying to reduce the alkalinity of the obtained ferrofluid. The students have checked the parent systems and found the following: (i) Tetraethylammonium ion is nearly as toxic as TMA. (ii) The tetrapropylammonium ion seems to be safer, but not as well-studied. (iii) The TBA cation has a lower toxicity14 and is quite commonly used as a phase transfer agent. The corrosive effect was expected to be reduced by introducing a minimal quantity of TBA(OH), far lower than the classical synthesis leading to a theoretical hydroxide concentration at around 0.1−0.2 mol L−1. This is still dangerous for the eyes but reduces possible skin burns in case of an accident. The experimentally measured pH after a minimum amount of TBA is added is in fact around 8, far less alkaline than expected as most of the hydroxide OH− reacts with the ammonium ion NH4+ still present or is adsorbed on the large surface area of the MNPs. New Material Development: Ferrogel, Ferromagnetic Ink Plastic, and Nanopowder

To obtain an even less toxic compound, the stabilization of MNPs was tried with the use of a soluble PVA polymer.15 The problem with a PVA solution is that the required volume to obtain a large stable colloidal solution is important as high MNPs concentration seems to induce partial PVA reticulation, as found for other compounds.16 A ferrofluid is obtained but with less impressive properties as the spiking effect3,7 is not observed due to a lower MNPs concentration. Therefore, the students have achieved a partial success by forming a nontoxic ferrofluid, even if the properties were not as impressive as the TBA-stabilized ones. This new ferrofluid was used as a starting point for the following Genius sessions (of the following semesters) to create new materials. The first material was obtained via the direct use of the PVA-ferrofluid as an ink/paint to create magnetic writing in a way to mimic the commercial use of ferrofluid as an antiforgery ink. This allows for the production of magnetic paint as shown in Figure 1B. The other possibility is to let the PVA-MNPs solution evaporate until a

Figure 2. Synthesized ferrogel interacting with a magnet over a time scale of approximately 3 min.

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TEACHING AND LEARNING OUTCOMES

Background and Goals

The classical synthesis of an aqueous ferrofluid using the approach of ref 3. began in 2008 in one of the basic lab training sessions of “Chimie Magique”. The safety issues10,11 linked with the tetramethylammonium (TMA) hydroxide were underlined because of the toxicity of both the reactant and the resulting ferrofluid. Indeed this material should be considered extremely toxic with a TMA LD50 as low as 2−3 mg·kg−1 for humans.10,12,13 It is also corrosive and should therefore be handled with great care. Furthermore, the failure rate for the magnetite/ferrofluid synthesis was initially found to be around C




black magnetic plastic remains. This is done in a silicon mold in order to produce the magnetic plastic forms. Finally, between the liquid ink and the solid plastic, the students have produced a ferrogel17 with fascinating behavior just by adding a borax solution to reticulate the MNPs stabilized by PVA.18 This produces the so-called “magnetic goo phagocytor”. Its behavior makes it look alive: When it is put a centimeter away from a strong magnet, it will fully flow around the magnet and cover it totally. The last material leads to a powder of magnetic nanoparticles with properties that are even better than those of iron filings for the visualization of magnetic fields.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: jean-sebastien.fi[email protected]. ORCID

Jean-Sébastien Filhol: 0000-0002-3681-9267 Notes

The authors declare no competing financial interest.

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Achievement of the Goals

This teaching approach with first- and second-year undergraduate students proved to be very positive for both the laboratory work, and for the final oral presentations. As the synthesis is very fast and reliable, many MNPs batches can be produced in a typical 3 h laboratory session allowing each group to synthesize all of the ferrofluids, ferrogel, magnetic ink, powder, and plastic compounds. This enables careful monitoring of the students’ laboratory skills, such as the preparing of solutions, synthesizing, washing and transferring nanoparticles, and dispersing and stabilizing a colloidal suspension, but also of the theoretical skills and knowledge such as the use of potential−pH diagrams, colloids, chemical safety, etc. Furthermore, the Genius session has allowed the evaluation the students’ ability to transpose an experimental setup to new problems such as changing the dispersant for safety and creating materials with new properties among others. The Genius sessions were evaluated by an external board of high school/university teachers and research personnel that were always pleased to observe a high involvement of the students in both the accomplished work and presentation. The developed syntheses such as the less toxic TBA-stabilized MNPs, the “phagocytor goo”, or the ferroplastic were so successful with the students that they are now fully integrated in our basic training.

ACKNOWLEDGMENTS This synthesis was optimized for the “Chimie Science Magique” program (www.chimiemagique.fr) and sponsored by Faculté des Sciences/Université Montpellier, CNRS, and Institut Charles Gerhardt Montpellier. Finance for the laboratory experiments was in part provided by the Labex Chemisyst. The authors want to thank Dr. Naseem Ramsahye for his help in improving the quality of this manuscript.

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REFERENCES

(1) Leyral, G.; Bernaud, L.; Manteghetti, A.; Filhol, J.-S. Microwave Synthesis of a Fluorescent Ruby Powder. J. Chem. Educ. 2013, 90 (10), 1380−1383. (2) Zitoun, D.; Bernaud, L.; Manteghetti, A.; Filhol, J.-S. Microwave Synthesis of a Long-Lasting Phosphor. J. Chem. Educ. 2009, 86 (1), 72. (3) Berger, P.; Adelman, N. B.; Beckman, K. J.; Campbell, D. J.; Ellis, A. B.; Lisensky, G. C. Preparation and Properties of an Aqueous Ferrofluid. J. Chem. Educ. 1999, 76 (7), 943. (4) Ellis, A. B.; Widstrand, C. G.; Nordell, K. J. Designing and Reporting Experiments in Chemistry Classes Using Examples from Materials Science: Illustrations of the Process and Communication of Scientific Research. J. Chem. Educ. 2001, 78 (8), 1044. (5) Qu, S.; Yang, H.; Ren, D.; Kan, S.; Zou, G.; Li, D.; Li, M. Magnetite Nanoparticles Prepared by Precipitation from Partially Reduced Ferric Chloride Aqueous Solutions. J. Colloid Interface Sci. 1999, 215 (1), 190−192. (6) Torres-Diaz, I.; Rinaldi, C. Recent progress in ferrofluids research: novel applications of magnetically controllable and tunable fluids. Soft Matter 2014, 10 (43), 8584−8602. (7) UW MRSEC Education Group. Synthesis of Aqueous Ferrofluid. http://education.mrsec.wisc.edu/285.htm (accessed Sep 2017). (8) Laurent, S.; Forge, D.; Port, M.; Roch, A.; Robic, C.; Vander Elst, L.; Muller, R. N. Magnetic iron oxide nanoparticles: Synthesis, stabilization, vectorization, physicochemical characterizations, and biological applications. Chem. Rev. 2008, 108 (6), 2064−2110. (9) Ben Halima, N. Poly(vinyl alcohol): review of its promising applications and insights into biodegradation. RSC Adv. 2016, 6 (46), 39823−39832. (10) Lee, C. H.; Wang, C. L.; Lin, H. F.; Chai, C. Y.; Hong, M. Y.; Ho, C. K. Toxicity of tetramethylammonium hydroxide: Review of two fatal cases of dermal exposure and development of an animal model. Toxicol. Ind. Health 2011, 27 (6), 497−503. (11) Lin, C. C.; Yang, C. C.; Ger, J.; Deng, J. F.; Hung, D. Z. Tetramethylammonium hydroxide poisoning. Clin. Toxicol. 2010, 48 (3), 213−217. (12) Anthoni, U.; Bohlin, L.; Larsen, C.; Nielsen, P.; Nielsen, N. H.; Christophersen, C. Tretramine-Occurence in marine organisms and pharmacology. Toxicon 1989, 27 (7), 707−716. (13) Henry, A. J. The toxic principle of courbonia-virgata-Its isolation and identification as a tetramethylammonium salt. Br. J. Pharmacol. Chemother. 1948, 3 (3), 187−188. (14) Dumitrescu, G.; Ciochina, L. P.; Stana, L.; Cretescu, I.; Popescu, R.; Filimon, N. M.; Voia, O. S. Acute effects of tetrabutylammonium

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CONCLUSION The synthesis of new magnetic materials is the direct consequence of the “Chimie Magique” approach. Using only basic skills to produce MNPs, the students were able to make new, less toxic advanced magnetic materials with completely new properties ranging from a nanopowder, a ferrofluid, a ferrogel, a ferromagnetic ink, or plastic, just by changing the surfactant and/ or the amount of solvent, and so forth. The properties of the materials are impressive and easy to obtain and therefore expose the students to the excitement of material sciences. These experiments could be easily done at different levels ranging from high school to graduate students by adapting the theoretical background and goals. It could be an original way to capture an interest for chemistry, and specifically materials chemistry, among high school students and to investigate magnetic colloidal properties with graduate students. In all cases, these experiments are helping students at all levels to gain interest in colloidal and material sciences and on the scientific method to achieve new compounds.

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Instructor notes (PDF, DOCX) Student handout (PDF, DOCX) Video showing experimental techniques (AVI)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00277. D




chloride ionic liquid on the histological structure of liver and kidney in the mouse. Romanian Biotechnological Letters 2014, 19 (1), 8925− 8934. (15) Lin, H.; Watanabe, Y.; Kimura, M.; Hanabusa, K.; Shirai, H. Preparation of magnetic poly(vinyl alcohol) (PVA) materials by in situ synthesis of magnetite in a PVA matrix. J. Appl. Polym. Sci. 2003, 87 (8), 1239−1247. (16) Isokawa, N.; Fueda, K.; Miyagawa, K.; Kanno, K. Demonstration of the Coagulation and Diffusion of Homemade Slime Prepared Under Acidic Conditions without Borate. J. Chem. Educ. 2015, 92 (11), 1886−1888. (17) Zrinyi, M.; Barsi, L.; Buki, A. Deformation of ferrogels induced by nonuniform magnetic fields. J. Chem. Phys. 1996, 104 (21), 8750− 8756. (18) Casassa, E. Z.; Sarquis, A. M.; Van Dyke, C. H. The gelation of polyvinyl alcohol with borax: A novel class participation experiment involving the preparation and properties of a ″slime″. J. Chem. Educ. 1986, 63 (1), 57.

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Synthesizing and Playing with Magnetic Nanoparticles: A

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