Production of Iron Oxide Nanoparticles by a Biosynthesis Method: An

Production of iron oxide nanoparticles is made by a biosynthesis mechanism. Particularly ..... balls) with spaces well filled by O atoms (darker balls...
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J. Phys. Chem. C 2007, 111, 16147-16153

16147

Production of Iron Oxide Nanoparticles by a Biosynthesis Method: An Environmentally Friendly Route Rau´ l Herrera-Becerra,† Cristina Zorrilla,† and Jorge A. Ascencio*,‡ Dpto. Materia Condensada, Instituto de Fı´sica, UniVersidad Nacional Auto´ noma de Me´ xico, A. P. 20-364, Distrito Federal, C.P. 01000, Me´ xico, and Instituto de Ciencias Fı´sicas, UniVersidad Nacional Auto´ noma de Me´ xico, A.P. 48-3, CuernaVaca, Morelos. C. P. 62210, Mexico ReceiVed: March 21, 2007; In Final Form: July 19, 2007

Production of iron oxide nanoparticles is made by a biosynthesis mechanism. Particularly, the generation of cubic-based clusters of wuestite (Fe0.902O) and magnetite (Fe3O4) is demonstrated with the aid of a microscopy analysis. Transmission electron microscopy characterization of the nanoparticles at controlled pH conditions during the synthesis allows determination of an optimal size distribution around 3.1 nm with a standard deviation of 2.1 nm. High-resolution transmission electron microscopy images and electron energy loss spectroscopy were used in order to elucidate the type of structure and associated oxidation state for the Fe atoms.

1. Introduction Among the primary goals of nanotechnology is the improvement of production methods, especially those that are simpler and cleaner. The objectives are the production of size-controlled clusters, increasing the yield of desirable nanoparticles, and the reduction of the subsequent pollutant contribution, particularly, biosynthetic methods have been used to produce different types of particles1-10 and even nanorods;11-13 however, they have been focused mainly on metallic1-6,11-13 and bimetallic aggregates.7-10 Just a few reports have evaluated the use of biosynthesis14,15 to make metal oxides, but none in scale ranges of less than 10 nm, where nanoparticles have more than 50% of their atoms on the surface. Consequently, they show new properties, even though this type of particle is one of the most important ones for future applications in catalysis, sensors, photon emission, and others applications.16-18 Additionally, iron oxides have attracted a great deal of attention among specialists because of their multivalent oxidation states,19 a large set of possible polymorphisms,20 and especially at the nanometric scale, their characteristic structural changes.21 These materials have a considerable potential to be applied as sensors, in catalysis,22 as high-density magnetic recording media,23 for targeted drug delivery in clinical trials,24 and as substrates in cancer treatment methods.25 The application of these nanostructures in medicine involves major requirements because oxide nanoparticles must be superparamagnetic, with a discrete distribution and narrow size distribution in a range smaller than 20 nm, which is a condition for maintaining these particular physical and chemical properties.26-28 In fact, overcoming the boundary to produce nanoparticles smaller than 10 nm based on ferrites implies the possibility to use a novel set of properties that are not achievable through clusters of sizes grater than 10 nm.29-31 A recent work by Redl * To whom correspondence should be addressed. E-mail: ascencio@ fis.unam.mx. Phone: +52 777 3291785. Fax: +52 777 3175388. Website: http://www.paginasprodigy.com/jascenciogtz/. † Instituto de Fı´sica, Universidad Nacional Auto ´ noma de Me´xico. ‡ Instituto de Ciencias Fı´sicas, Universidad Nacional Auto ´ noma de Me´xico.

et al. allows identification of multiple possibilities of generating and manipulating nanoparticles and even nanostructured arrays, where magnetic and electronic properties are dramatically determined by the composition and morphology of the clusters and ordered systems.31 Besides, the size control for small nanoparticles would represent the use of quantum size effects, and it could greatly enhance the magnetic and electronic properties impacting deeply on the new technologies and its possible applications, as it has been shown for one-dimensional nanostructures.32-34 Hence, even when several methods have been used to generate iron oxide nanoparticles,26-39 the production of wellcontrolled-size iron oxide nanoparticles has been quite limited. Researchers have reported the use of passivation agents, such as surfactants, ligands, polymers, or dendrimers, to delimitate the nanoparticle surface.35-38 Additionally, few works have reported the possibility to generate high-quality clusters by thermal decomposition of Fe(CO)5 as an iron precursor in hot surfactant solutions, which derive into iron particles followed by an oxidation process that produces maghemite nanoparticles using a chemical reagent.39 Similar works have demonstrated that a high-temperature (250 °C) is necessary to cause oxidation in order to produce maghemite and magnetite nanometric particles from iron acetylacetonate in phenyl ether in the presence of alcohol and surfactant.40 Also, the synthesis of iron oxide nanoparticles at room temperature has been reported; however, the size range of production establishes a distribution of 5-11 and 19 nm for maghemite and magnetite structures, respectively.41 In this work, we successfully implemented a green chemistry method to obtain biosynthesized iron oxide nanoparticles with sizes of less than 5 nm, reporting the optimum parameters for the synthesis and a full characterization of the generated clusters. In order to determine the structure of the nanoparticles, transmission electron microscopy (TEM)-related techniques were used. Electron energy loss spectroscopy (EELS) and highresolution TEM (HRTEM) gave the elemental composition and internal structure of the nanoparticles. All of this allowed confirmation of the production of nanoparticles smaller than 5

10.1021/jp072259a CCC: $37.00 © 2007 American Chemical Society Published on Web 10/06/2007

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Figure 1. TEM analysis of samples produced with different pH conditions. Low-magnification images of (a) pH ) 3, (b) pH ) 5, (c) pH ) 7, and (d) pH ) 10 samples, with their corresponding (e) size distribution plot and (f) a common EELS spectrum with the evidence of Fe and O signals at ∼92 and ∼524 eV, respectively.

nm with preferential cubic arrays, which derive into magnetiteand wuestite-like clusters. 2. Experimental Methods Nanostructures of FexOy were prepared using the biosynthesis method.1,6-13,42 In order to reduce Fe ions to produce clusters, a homogeneous suspension of powdered milled alfalfa (which had been previously washed, dried, milled, washed twice in HCl 0.01 M, washed again up to neutral pH, and re-dried) was prepared with a concentration of 5 mg/mL in water using an ultrasonic treatment for 15 min. The pH of the solution was controlled by means of a buffer solution, fixing the values at 3, 5, 7, and 10. After placing the mixture in an ultrasonic bath for 15 min, it was left for another 10 min without agitation and then centrifuged to 5000 rpm for 15 min. A solution of 3 × 10-4 M of FeNH4(SO4)2‚12H2O, was prepared in deionized water. Both solutions were mixed homogeneously using an ultrasonic bath for 20 min. Later, the mixed solution was kept at 25 °C for 0.5 h and centrifuged at 5000 rpm for 20 min. Finally, the reaction mixture was decanted and kept at rest for 48 h. Electron microscopy characterization included transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and EELS (electron energy loss spectroscopy). The samples were prepared by adding a drop of the solution on carbon-coated copper grids, and then, they were left for drying. Electron microscopy was performed in a JEOL JEM-2010F FasTem microscope at IFUNAM, equipped with analytical devices. High-resolution images were obtained under several different conditions, and the images were analyzed by obtaining digital spectra by FFT (fast Fourier transforms) to achieve more-precise measurement.

3. Results and Discussion Biosynthesis is well established for the production of small metallic particles, with dimensions controlled by the pH of the solution used during the bioreduction process.1,6-13 The characteristic sizes obtained by our group using the alfalfa (Medicago satiVa) biomass have been around 4 nm for other metals. In the case of Fe ions, the reduction did not reach into metal clusters, as we will show in this work. In this way, the first parameters to evaluate are the size and the elements present in the nanoparticles. In Figure 1, a general analysis of the studied samples can be observed, considering a concentration of 3 × 10-4 mol/gr and varying the pH values from 3 to 10. In Figure 1a-d, low-magnification transmission electron microscopy images are shown for the different pH conditions, with similar contrasts and a distribution of sizes lower than 10 nm but with apparently different densities of small clusters. In fact, a size distribution plot (Figure 1e) denotes the frequency of each particle size for different pH samples. It clearly distinguishes that for pH ) 3, there is a multimodal distribution with a mean of 6.2 nm and a standard deviation of 3.4 nm, in contrast to the pH ) 5 samples, where the standard deviation is reduced to 2.5 nm but the mean size is increased to 7.2. The smallest particles were found for solutions at pH ) 7 and 10, where particles around 4.1 and 3.6 nm were respectively recognized as average, with standard deviation values of 1.9 and 1.6 nm too. Hence, the reduction of metallic ions to generate particles is verified, but the composition of these clusters has to be studied, and in order to do it, we used EELS and lattice spacing measurements to identify the elements and the structures, respectively. In Figure 1f, a common EELS spectrum is shown, with a magnification of the region with energies higher than

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Figure 2. HRTEM analysis of iron oxide nanoparticles produced at pH ) 3, showing wuestite-like structures at (a) [11-3] and (b) [1-1-1] zone axes. In both cases, a magnified image of the selected area with the interplanar distances and the corresponding FFT spectrum with the angles and associated families of planes are shown.

400 eV; in the first region, the signal of Fe can be distinguished, and in the second region, the intensity of the lost energy associated with O has lower intensity and consequently must be evaluated for just a few dozens counts. As a consequence of the signals observed in the EELS spectrum, we can consider that both Fe and O are present in the particles, even when the oxidation state cannot be directly concluded.43 It should be noticed that the tendency to produce smaller and more controlled particles is improved by increasing the value of the pH (7 and 10 are better than 3 and 5), as opposed to the results obtained for metallic clusters previously reported. The effect of the nature of Fe and also the chemical potential required to reduce an oxide system must be important, where the proton density in the solution appears to be more favorable in contrast to metal nanoparticles. In order to obtain a better understanding of the produced structures, higher-resolution images were obtained and analyzed by a crystallographic methodology. In the next figures, a sequence of examples for the structure determination of the clusters is shown, selecting examples with clear evidence for each pH condition. Figure 2 shows high-resolution TEM micrographs of different regions of samples produced at pH ) 3. In Figure 2a, a particle with a diameter around 10 nm is analyzed, observing that it tends to show a rounded profile; a detailed scheme is shown from the “i” selected area, where the measured interplanar distances are shown with values of 0.259, 0.261, and 0.221 nm, while the angles of 73 and 33° can be clearly observed in the FFT spectrum (in order to reduce the measuring error, the values are obtained from the FFT where the information limit is ∼0.17 nm for the used microscope). With these values, we can identify that the planes correspond to the (422), (242), and (-440) for a particle based on a wuestite-like iron oxide (Fe0.902O cubic phase with a ) 1.29 nm44) at the [11-3] zone axis. Moreover, in Figure 2b, several particles can be distinguished, even when the profile is not fully established because they form irregular shapes; an area is selected, where point resolution is found. In a major magnification analysis of the “ii” area, a hexagonal section with distances between planes of 0.229, 0.230, and 0.229 nm and angles observed in the FFT of 59 and 61°

that imply reflections of three similar (440) family types of planes for a wuestite-like structure at the [1-1-1] zone axis can be distinguished. In this micrograph, the presence of an extra point in the FFT spectrum (shown with the arrow) in the same orientation as the one associated with the analyzed particle but with a smaller distance (bigger in the reciprocal space) can also be observed. The origin of this is identified in the micrograph by the arrow, where planes of compression are observed while the shape tends to follow an axial growth direction opposite to the place where the hexagonal particle is located. When the samples were obtained with pH ) 5 conditions, the nanoparticles showed a slightly different behavior, as can be seen in Figure 3. An interesting particle, with a diameter of 8.4 nm, is observed in Figure 3a, which shows two domains generated by an internal twin. In the analysis of the “i” selected area, the distances of the bigger domain are evaluated, observing that interplanar distances are 0.243, 0.244, and 0.211 nm, with characteristic angles of 70 and 55°. These data match a magnetite-like structure (Fe3O4 cubic system with a ) 0.849 nm40) at the [0-11] zone axis, where the reflections correspond to the (222) and (400) families of planes. From the FFT analysis, the extra reflections, marked with the arrow, are associated with the second domain (marked with an arrow in the micrograph too), which has no extra distinguishable planes. Furthermore, a second particle with diameter of 12 nm is studied and shown in Figure 3b. It is clear that the contrast of the particle denotes a rounded profile and homogeneous structure, while the corresponding interplanar distances found for this particle are 0.226, 0.225, and 0.228 nm, with an angle of 60° between them. These parameters match with the planes (440), (044), and (4-40) of a wuestite-like particle at the [-110] axis zone. A comparable analysis was made for the sample obtained using a solution of pH ) 7, which is denoted in Figure 4, where a couple of examples are shown with characteristic clusters. From the micrograph of Figure 4a, where multiple particles can be found, an area with a particle of square profile is selected, with diameter of 3.6 nm and internal dotting contrast that

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Figure 3. HRTEM analysis of iron oxide nanoparticles produced at pH ) 5, showing (a) a single twin magnetite-like structure at the [0-11] zone axis and (b) a wuestite-like cluster with a rounded profile and with the orientation [1-1-1] parallel to the electron beam. In both cases, a magnified image of the selected area with the interplanar distances and the corresponding FFT spectrum with the angles and associated families of planes are shown.

Figure 4. HRTEM analysis of iron oxide nanoparticles produced at pH ) 7, showing a couple of cubic nanoparticles at the [001] orientation, with structure of (a) wuestite and (b) magnetite. In both cases, a magnified image of the selected area with the interplanar distances and the corresponding FFT spectrum with the angles and associated families of planes are shown.

involves interplanar distances of 0.255 and 0.258 nm, while from the FFT, the angle between dots is 90°. These parameters are characteristic of the (500) families of planes and imply a [001] zone axis of a wuestite-like cluster. In a different region, we can find a particle of 7 nm with similar square dotting contrast, which also matches the [001] orientation but with smaller distances (2.15 and 2.13 nm), which represents a magnetite-like nanostructure. At this pH condition, the amount of particles obtained is increased, denoting a better efficiency. The excellent opportunity of using a similar orientation of nanoparticles with a different crystalline structure and distinct interplanar distances induces important consequences. However, it must also be noticed that the generation of differences in the internal contrast of the particles (as it can be observed in the micrographs) is another parameter to be consider for a full

characterization of small clusters. Particularly, the new field emission gun generation microscopes have the sensibility to reproduce a significant contrast to identify the scattering variations for heavier elements. The direct comparison for similar orientations in the case of samples produced at pH ) 10 allows us to recognize the type of generated clusters, as shown in Figure 5, where a couple of nanoparticles near 2.6 nm are shown; both of them can be associated to a cubic-like array, and they are observed in an orientation near [10-1], which produces a hexagonal profile and hexagonal internal contrast. However, even when the measured angles for both are quite similar, the obtained interplanar distances are different, as is the dotting contrast. For the particle of Figure 5a, the measured distances are 0.239, 0.243, and 0.201 nm, while for Figure 5b,

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Figure 5. HRTEM analysis of iron oxide nanoparticles produced at pH ) 10, showing a couple of cubic nanoparticles at the [10-1] orientation, with the structure of (a) magnetite and (b) wuestite. In both cases, a magnified image of the selected area with the interplanar distances and the corresponding FFT spectrum with the angles and associated families of planes are shown.

these are 0.248, 0.25, and 0.216 nm, matching magnetite- and wuestite-like structures, respectively. Even the zigzag contrast of Figure 5a is characteristic of the magnetite, where the difference in the scattering proportions induces a smaller resolved position for the points, while in Figure 5b, the points are well defined, with homogeneous bright points. The understanding of the differences of the internal contrast is associated with the atomic distribution, including the crystalline array and the element present, because of the kind of involved electron scattering in the microscope chamber. In Figure 6a, the different iron oxide cluster models for wuestite-, magnetite-, and hematite-based crystals can be observed. The observation of these models in a couple of orientations allows one to establish the regular distribution of Fe atoms (light gray balls) with spaces well filled by O atoms (darker balls) for wuestite. In the case of the magnetite system, the cations produce compact patterns, with a square array of atomic groups in case of the [001] orientation, while for the [110] zone axis, the produced scattering can be associated more to a zigzag-like pattern. This behavior of magnetite is particularly interesting because this crystal, also a fcc close-packed lattice of oxygen anions, is an inverse spinel. The lattice array is based on tetragonal and octahedral interstitial cation sites, with just a quarter of filled sites; it means that for each unit cell, 8 Fe2+ cations sit in the octahedral sites, while 16 Fe3+ cations are distributed between the tetragonal and octahedral sites homogeneously, which can be associated with the observed zigzag contrast for this structure. Finally, in the case of the hematite model, the characteristic hexagonal atomic distribution at the [001] orientation is observed, while for [110], the rectangular array is clearly identified to be mainly induced by the distribution of the Fe3+ cations, which produce most of the electron scattering that can be observed as HRTEM contrast. On the basis of the general study over the different samples and a minimum of 30 micrographs per sample, the analysis of multiple nanoparticles allows determination of the tendency to produce a different kind of cluster. In Figure 6b, a plot of the frequency for each pH condition is shown. From the plot, and the exposed micrographs, it is clear that the main two crystalline arrays associated to the nanoparticles are wuestite and magnetite

phases. Wuestite used to be the most frequently found structure for pH ) 3, followed by the magnetite, and the order changes for pH ) 5, 7, and 10, where the magnetite amount is higher than that of wuestite. However, in all of the cases, the presence of both phases simultaneously is characteristic of more than 94% of the evaluated clusters, but the amount of magnetite increases from 36 to 51 to 58 to 65 (for pH ) 3, 5, 7 and 10, respectively). Besides, hematite has a production of a maximum of 6% for the pH ) 3 samples but minimal presence in most of the cases. This analysis allows one to determine that the main phases that coexist in the samples are wuestite and magnetite, which are cubic-based systems. These systems are common for small nanoparticles due to the tendency to produce spherical morphologies, which used to be the lowest-energy surface configurations. Moreover, in both cases, they show an oxidation state for the iron of Fe2+; because wuestite is based on Fe2+O2- and magnetite on Fe2+Fe3+O2-, this allows identification of a preferential reduction of iron into Fe2+ during the biosynthesis process. From the obtained results and the previous works, where alfalfa has been used by our group1,6,8-13 and some other colleagues,41 we can identify that the biosynthesis process is induced by the reduction of metal ions in the following way:

FeCl3 + H2O f [Fe(H2O)n]3+ + H2O and by considering that the tannins associated to the alfalfa (garlic acid) derive into radical tannins “R” (see Supporting Information for more details), which act as reduction agents as

R + [Fe(H2O)n]3+ + H2O f [Fe(H2O)n]2+ + H+ + R-OH pH

[Fe(H2O)n]2+ + 2R- + H+ + OH- 98 Fe2+O2- + Fe2+Fe3+O2- + R-H + R-OHConsidering this last step as dependent on the pH conditions, both kinds of crystals are generated according to the major proportion of Fe2+. It must be noticed that even when the

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Figure 6. (a) Structural models for the observed crystalline arrays of the produced clusters and (b) structural distribution plot for the different pH samples.

bioreduction process is sustained in the presence of radical tannins, the internal competition for charge during the solution synthesis must be the main parameter to control the size, and consequently, the possibility of a better separation of the alfalfa components would allow for an improvement in the synthesis control conditions. Conclusions From the low-magnification analysis, the successful production of small iron oxide nanoparticles by means of the biosynthesis method can be established, where the variation of pH conditions allows control of the size distribution of clusters smaller than 10 nm. The optimum conditions for the smallest particles and with the minimum dispersion were fixed for pH ) 10, where nanoparticles of around 3.6 nm were produced, while a standard deviation of 1.6 nm was calculated. The analysis of elemental composition by using EELS allowed determination of the presence of Fe and O elements; however, the structure and the corresponding iron oxide species were established using a deep analysis of atomistic distribution by HRTEM images and FFT spectra. The main produced clusters are of the magnetite and wuestite types, which are fcc crystalline systems, considered the most stable for multiple materials at the scale of the quantum dots. However, these are also based on the presence of Fe2+ cations. The optimum conditions to produce small nanoparticles also lead to the generation of magnetite clusters coexisting with wuestite-like clusters but with a significant preference for magnetite when the pH ) 7 and 10.

This method is simple and requires no thermal treatment, and the induced pollution is minimal; therefore, we successfully demonstrate that green chemistry mechanisms can be used for the production of these types of nanostructures, which will be fundamental in multiple applications of nanotechnology. Acknowledgment. The authors acknowledge the financial support from DGAPA with grant IN120006-3. We also thank Mr. Luis Rendon for the technical operation of the transmission electron microscopy operation. References and Notes (1) Ascencio, J. A.; Rincon, A. C.; Canizal. G. J. Phys. Chem. B 2005, 109, 8806. (2) Mukherjee, P.; Ahmad, A.; Mandal, D.; Senapati, S.; Sainkar, S. R.; Khan, M. I.; Arishcha, R.; Ajaykumar, P. V.; Alam, M.; Kumar, R.; Sastry, M. Nano Lett. 2001, 1, 10. (3) Rautaray, D.; Ahmad, A.; Sastry, M. J. Am. Chem. Soc. 2003, 125, 48. (4) Sastry, M.; Ahmad, A.; Khan, M. I.; Kumar, R. Curr. Sci. 2003, 85, 2. (5) Shankar, S. S.; Rai, A.; Ankamwar, B.; Singh, A.; Ahmad, A.; Sastry, M. Nat. Mater. 2004, 3, 482. (6) Ascencio, J. A.; Canizal, G.; Medina-Flores, A.; Bejar, L.; Tavera, L.; Matamoros, H.; Liu, H. B. J. Nanosci. Nanotechol. 2006, 6, 1044. (7) Liu, H. B.; Canizal, G.; Schabes-Retchkiman, P. S.; Ascencio, J. A. J. Phys. Chem. B 2006, 110, 12333. (8) Ascencio, J. A.; Mejia, Y.; Liu, H. B.; Angeles, C.; Canizal, G. Langmuir 2003, 19, 5882. (9) Schabes-Retchkiman, P. S.; Canizal, G.; Herrera-Becerra, R.; Zorrilla, C.; Liu, H. B.; Ascencio, J. A. Opt. Mater. 2006, 29, 88. (10) Juarez-Ruiz, E.; Pal, U.; Lombardero-Chartuni, J. A.; Medina, A.; Ascencio, J. A. Appl. Phys. A 2007, 86, 441.

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