Biosynthesis of Stable Iron Oxide Nanoparticles in ... - ACS Publications

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Biosynthesis of Stable Iron Oxide Nanoparticles in Aqueous Extracts of Hordeum vulgare and Rumex acetosa Plants Valentin V. Makarov,*,† Svetlana S. Makarova,‡ Andrew J. Love,§ Olga V. Sinitsyna,∥ Anna O. Dudnik,⊥ Igor V. Yaminsky,⊥ Michael E. Taliansky,§ and Natalia O. Kalinina*,† †

Belozersky Institute of Physico-Chemical Biology and ‡Biological Faculty, ∥Chemical Faculty, and ⊥Physical Faculty, Lomonosov Moscow State University, 119992 Moscow, Russia § The James Hutton Institute, Invergowrie, Dundee, DD2 5DA, Scotland, United Kingdom ABSTRACT: We report the synthesis and characterization of amorphous iron oxide nanoparticles from iron salts in aqueous extracts of monocotyledonous (Hordeum vulgare) and dicotyledonous (Rumex acetosa) plants. The nanoparticles were characterized by TEM, absorbance spectroscopy, SAED, EELS, XPS, and DLS methods and were shown to contain mainly iron oxide and iron oxohydroxide. H. vulgare extracts produced amorphous iron oxide nanoparticles with diameters of up to 30 nm. These iron nanoparticles are intrinsically unstable and prone to aggregation; however, we rendered them stable in the long term by addition of 40 mM citrate buffer pH 3.0. In contrast, amorphous iron oxide nanoparticles (diameters of 10−40 nm) produced using R. acetosa extracts are highly stable. The total protein content and antioxidant capacity are similar for both extracts, but pH values differ (H. vulgare pH 5.8 vs R. acetosa pH 3.7). We suggest that the presence of organic acids (such oxalic or citric acids) plays an important role in the stabilization of iron nanoparticles, and that plants containing such constituents may be more efficacious for the green synthesis of iron nanoparticles.

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INTRODUCTION In recent years, the biosynthesis of nanoparticles has been considered a more environmentally sound and safer and more cost-effective alternative to chemical and physical methods of production.1−3 Plant extracts are particularly promising for “green” production since they are freely available, cheap, and offer simplicity of use and scalability.4,5 Plant extracts contain a potent array of antioxidants such as polyphenols,6−8 reducing sugars,9 nitrogenous bases, and amino acids,10 which can reduce metal ions in a metal salt solution.11 Initial reduction of the metal ions leads to the formation of nucleation centers which sequester additional metal ions and also incorporate neighboring nucleation sites, eventually leading to the formation of nanoparticles. Such particles are associated with organic substances from plant extracts, which serve to improve particle stability. Moreover, these nanoparticles have also been reported to demonstrate no toxicity in comparison to those metal nanoparticles produced using traditional chemical methods.5,11,12 While there are many published studies on the production of silver and gold particles using extracts of various plants such as sorghum (Sorghum spp),5 hibiscus (Hibiscus rosa sinensis),13,14 Aloe vera,15 black tea (Camellia sinensis),16 coffee (Coffea arabica),17 and fruit extracts,18 effective synthesis of iron nanoparticles using such techniques is much more difficult and is consequently infrequently reported. This is due to the fact that reduced iron is more readily oxidized in solution than its gold or silver counterparts, leading to greater particle instability. © XXXX American Chemical Society

Furthermore, iron nanoparticles (when they do form) have a high propensity to aggregate into agglomerates in order to lower the energy associated with the large surface area to volume ratio, a phenomenon which is likely exacerbated by the low surface charge. In spite of these obstacles, it has previously been reported that amorphous nanoparticles of zerovalent iron can be synthesized19 using extracts of some kinds of tea,20−22 coffee,19,20 and sorghum.5 Iron nanoparticles of various sizes and morphology, such as spheres, platelets, and nanorods, formed instantaneously in aqueous tea extracts.16,20−22 The concentration of the tea extract determined these structures, which were found to contain hexagonal metallic iron, amorphous iron, Fe3O4 and Fe2O3.16,19,21 In addition, 40 to 50 nm spherical amorphous iron (iron oxide) nanoparticles were synthesized using aqueous sorghum (Sorghum spp) bran extracts, which are known to contain high levels of freely extractable phenolic compounds.5 The biosynthesized iron nanoparticles were found to be nontoxic when compared with iron nanoparticles prepared using conventional NaBH 4 reduction protocols,16 and were functional as demonstrated through their use in the degradation of organic contaminants in a model system.5 Although production of iron nanoparticles using plant extracts has been reported, this technology needs further improvement in order to obtain stable nanoparticles of controlled size and morphology, which would be conducive to Received: March 28, 2014

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were obtained using the He−Ne laser (633 nm). Curves were fitted using Dispersion Technology Software (DTS) version 5.10.

large-scale synthesis of iron nanoparticles for environmental remediation and hazardous waste treatment applications. In this work, we characterized the formation of iron nanoparticles using extracts of Hordeum vulgare and Rumex acetosa, and also describe approaches and considerations on how to enhance the stability of plant produced iron nanoparticles.

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RESULTS Preparation and Characterization of Iron Nanoparticles in Extracts of Hordeum vulgare. Addition of Hordeum vulgare (member of the Gramineae family of monocotyledonous plants) extracts to 0.1 M ferric (III) chloride produced an instantaneous color change in the solution from yellow to intense brown, indicating the formation of iron-containing nanoparticles.19 This phenotypic change correlated well with the absorption spectra data, such that the absorption peak at 360 nm of the ferric (III) chloride was shifted to 405 nm after the addition of the plant extracts, with the 405 nm peak being indicative of iron nanoparticle formation (Figure 1A).20 Although these spectra are broadly

EXPERIMENTAL SECTION

Plant Extract Preparation and Characterization. Hordeum vulgare (member of the Gramineae family of monocotyledonous plants) and Rumex acetosa (member of the Polygonaceae family of dicotyledonous plants) plants were grown in controlled conditions with 17 °C night and 25 °C day cycles, with the day length lasting 16 h. Leaves of four-week-old plants were used. Plant sap extracts were prepared by grinding fresh leaf material in liquid nitrogen into a fine powder. Distilled deionized water was added to the grindate at a ratio of 1:2.5 (w/v); this was mixed and then boiled for 30 min. The extracts were passed through two layers of muslin, and the liquid was centrifuged for 10 min at 16 000 g. The supernatant was then passed through 0.22 μm filters (Millipore) and was used in later nanoparticle synthesis reactions. The Folin−Ciocalteu method (phosphorus−molybdenum−tungsten reagent)23 was used with slight modifications to determine the antioxidant capacity of the plant extracts. One gram of leaf material was ground in liquid nitrogen and then thawed in 2.5 mL of 80% ethanol, prior to boiling for 30 min. The extract was filtered through two layers of miracloth, and centrifuged at 13 000 rpm for 10 min. The final volume of the extract was adjusted to 2.5 mL. The reaction mixture consisted of 40 μL of the extract, 40 μL of Folin−Ciocalteu reagent, and 320 μL of H2O. The control mixture consisted of 40 μL of the 80% ethanol instead of the plant extract. Subsequently the absorbance at 720 nm of the reactions was measured using a Shimadzu UV-1601 spectrophotometer (Shimadzu scientific Instruments, Inc., Japan). Using a calibration curve prepared from a solution of hydroquinone (y = 0.084x + 0.0046, R2 = 0.9966), the total antioxidant capacity of the extracts was expressed as a milligram hydroquinone equivalent per gram fresh weight. pHs of the extracts were measured using a portable pH-meter (Hanna Instruments; Germany) and the protein concentrations were determined using absorption spectroscopy methods. Synthesis and Characterization of Iron Nanoparticles. For nanoparticle synthesis, 0.1 M iron salt (FeCl3·6H2O, Panreac) was continually mixed with plant extracts in a 1:1 ratio at room temperature for 30 min. Subsequently, the synthesized Fe NPs were characterized using TEM, absorbance spectroscopy, SAED, EELS, XPS, and DLS methods. A LEO912 AB OMEGA transmission electron microscope (TEM) operating at 100 kV was used in tandem for image capture and selected area electron diffraction analysis (SAED) and electron energyloss spectra (EELS) data acquisition. The nanoparticle samples were diluted 1:10 or 1:100 and dropped onto Formvar coated copper grids, which were then air-dried. Electron diffraction patterns and electron energy-loss spectra were measured simultaneously in the TEM in order to determine the nanoparticle crystal structure and chemical composition. The absorption spectra in the 390−600 nm range were measured in cells with an optical path of 1 cm, using a Hitachi UV-2600 spectrophotometer. The spectra of the intact plant extract were used as a baseline and subtracted from the spectra of a mixture of extract and synthesized nanoparticles. Synthesized nanoparticles were lyophilized prior to X-ray photoelectron spectrum (XPS) analysis, which was carried out using a Kratos Axis Ultra DLD system fitted with an Al Kα X-ray source. The pass energy was 160 and 40 eV for the survey and the narrow regions, respectively. Spectral calibration was performed by setting the C 1s component at 284.8 eV. For dynamic light scattering (DLS) analysis, samples of iron nanoparticles from plant extracts were loaded into 1 cm cells of the Zetasizer Nano ZS (Malvern Instruments, UK), and measurements

Figure 1. Characterization of iron-containing nanoparticles synthesized in Hordeum vulgare extract. (A) Absorption spectra of ironcontaining nanoparticles (red line) and 0.1 M aqueous solution of FeCl3 (blue line). The spectra of the intact plant extract were used as a baseline. (B) TEM micrographs of nanoparticles (Scale bar 200 nm).

similar to those observed in other studies (which used tea and sorghum extracts to produce iron nanoparticles5,19,20) there were slight differences in the maximal absorbance peaks and gradients of the traces in these reactions, which is likely due to variations in the aggregation state and valency of the iron nanoparticles produced in the different studies. B

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Studies on the Stability of Iron Nanoparticles in H. vulgare Extracts. Previous studies have found that the ζpotential of the iron particles produced using extracts of sorghum are positive and range from 0.11 to 0.15 mV.24 However, this ζ-potential is around 2-fold less than that required for a colloidal solution to be regarded as stable, and as such suggests that the individual particles of the colloid can interact, leading to aggregation of the nanoparticles into large complexes.24,25 With regard H. vulgare synthesis reactions, aggregation of iron nanoparticles occurred rapidly, such that 100−200 nm aggregates were detected as early as 1 h after the initiation of synthesis. Around 5 days after initiation, the size of the aggregates increased to 500−600 nm (Figure 3A). From the literature it is known that citrate ions bind to the surface of iron nanoparticles and significantly increase their surface charge, and thus enhance their stability by inhibiting aggregation.26 We have found that addition of 40 mM citrate buffer pH 3.0 to the synthesized nanoparticles prevented aggregation (Figure 3B) and rendered them stable for at least a week. Formation of Stable Iron Nanoparticles in the Extracts of Rumex acetosa. Given the stabilizing effect of citrate on the reactions, it is very likely that extracts from plants that have copious amounts of low molecular weight organic acids may be very effective in inhibiting aggregation of the iron nanoparticles. To test this assumption we synthesized iron nanoparticles using the oxalate-rich extract of Rumex acetosa leaves. The reaction solution changed to an intense brown color, indicating successful formation of the iron nanoparticles, which was confirmed by the characteristic 420 nm peak in the absorption spectrum (Figure 4A). TEM and SAED analysis demonstrated that the preparation contained electron dense amorphous particles up to 40 nm in diameter (Figure 4B). Dynamic light scattering analysis demonstrated that nanoparticles (average diameter 50 nm) from Rumex acetosa were stable for at least a week (Figure 5). It can be assumed that the organic components in the R. acetosa extract can act as capping agents since, like the effect of citrate, the nanoparticles were found to have no change in their adsorptive characteristics or turbidity with time. The XPS spectrum of iron-containing nanoparticles from Rumex acetosa extract was similar to that of nanoparticles produced using H. vulgare extract, such that it also displayed lines of carbon, oxygen, iron, nitrogen, and chlorine, and moreover, it verified the presence of tri- and divalent iron atoms in the nanoparticle (presumable mixed iron oxidation state). In particular, the XPS spectrum of 2p-electrons of iron (711.4 eV) and the intensive shakeup satellite, which was shifted 6 eV relative to the main peak, were typical for trivalent iron (Figure 6A and B). At the same time (additionally) the shoulder at 709.6 eV points to the existence of divalent iron. These results are in good agreement with previous reports.27,28 Thus, nanoparticles biosynthesized in extracts of Rumex acetosa contain the iron oxide Fe3O4 with iron atoms in combined (II and III) oxidation states. Properties of H. vulgare and R. acetosa Extracts That Influence Iron Nanoparticle Formation. As indicated above there are differences in iron nanoparticle stability between R. acetosa and H. vulgare extracts. These differences may be due to variation in the pH, total protein content, and reducing power between these species. We measured these parameters and found that some properties were very similar in both plant extracts; they each have the same content of reducing chemical compounds and total protein (Table 1). This suggested that stability of iron nanoparticles may be influenced by pH. The

By using transmission electron microscopy (TEM) it was found that electron-dense spherical particles with a diameter of up to 30 nm were formed in the H. vulgare extracts (Figure 1B). The content of the iron atoms in the nanoparticles from H. vulgare is evidenced by their high electron density, as illustrated by the characteristic iron peak in the electron energy loss spectroscopy (EELS) traces (data not shown). The iron nanoparticles, according to selected area electron diffraction analysis (SAED), had a diffraction pattern characteristic of amorphous particles (Figure 1B), which is consistent with the previously published data that used sorghum and tea extracts.5,19,20 In the overview XPS spectrum of iron-containing nanoparticles produced from H. vulgare extracts (Figure 2A),

Figure 2. XPS spectra of iron-containing nanoparticles produced in H. vulgare extract measured using a Kratos Axis Ultra DLD system. (A) Overview XPS spectrum of iron-containing nanoparticles. (B) XPS spectrum of iron 2p-electrons of iron-containing nanoparticles.

there are lines of carbon, oxygen, iron, nitrogen, and chlorine. In the XPS spectrum of iron 2p-electrons (Figure 2B), there are two distinct states of the iron atoms at 711.3 and 709.5 eV, which correspond to the binding energy electron components of Fe 2p3/2. Such binding energies are typical for tri- and divalent iron atoms, respectively. The mixed iron oxidation state was also illustrated by mild shakeup satellites for tri- and divalent iron atoms. A similar pattern, for example, is observed in the Fe3O4 iron oxide, where iron atoms are contained in combined (II and III) oxidation state. Thus, apparently particles forming in the barley extracts represent iron oxide (II, III) nanoparticles. C

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Figure 3. Size distribution of iron oxide nanoparticles produced in H. vulgare extract measured by dynamic light scattering (DLS) methods. (A) Aggregation of iron nanoparticles over time. (B) Stabilization of nanoparticles in the presence of 40 mM citrate buffer (pH 3.0).

are a mixture of iron oxides doped with hydroxides,19,21,31 but a few of the studies succeeded in obtaining nanoparticles with zerovalent iron.20,32 In our report the nanoparticles synthesized using extracts of Hordeum vulgare and Rumex acetosa plants are a mixture of iron oxides, without the presence of zerovalent iron. Iron nanoparticles have great capacity to aggregate and become unstable, phenomena which are typically a result of their low surface charge. This problem can be rectified by increasing their surface charge via the addition of citrate ions which form a “citrate coat” around the particles and prevent them from interacting. While this method has been used previously for stabilizing chemically synthesized iron nanoparticles,26 we have demonstrated that it also has utility in greatly prolonging the stability of iron nanoparticles produced using H. vulgare. However, we also considered that iron nanoparticles may be stabilized more naturally by synthesizing them in plant extracts which are known to contain high concentrations of low molecular weight organic acids, such as citrate, malate, and oxalate; in this case, the organic acids perform an analogous function to the citrate by coating the particles and preventing them from aggregating. We demonstrated that the iron

pH value of extracts of R. acetosa and H. vulgare were 3.7 and 5.8, respectively. This difference in pH is likely a consequence of increased amounts of low molecular weight organic acids in R. acetosa, constituents which also play a probable role in enhancing the stability of iron nanoparticles.

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DISCUSSION The main difference between iron nanoparticles and those of noble metals such as silver or gold is the high reactivity of iron. Silver or gold particles are relatively inert and stable and thus can be easily produced using plant extracts.7,29,30 In contrast, iron ions are readily oxidized and reduced by interacting with a wide range of different chemical compounds; a factor which complicates the synthesis of stable iron nanoparticles using plant extracts.29,30 However, to date, there are several publications that demonstrate the possibility of synthesizing colloidal iron using extracts of black tea and sorghum.5,19,20 These iron nanoparticles are of an amorphous nature, which is quite distinct from the crystalline structure observed with the noble metal particles produced using the same plant extracts.5,16 Our findings agree with these data. In most studies where the nature of biosynthesized iron particles were investigated in detail, it was shown that these particles typically D

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Figure 4. Characterization of iron-containing nanoparticles synthesized in Rumex acetosa extract. (A) Absorption spectra of ironcontaining nanoparticles (red line) and 0.1 M aqueous solution of FeCl3 (blue line). The spectra of the intact plant extract were used as a baseline. (B) TEM micrographs of nanoparticles (Scale bar 200 nm).

Figure 5. Formation of stable iron-containing nanoparticles in R. acetosa extract. Size distribution over time of iron oxide nanoparticles produced in R. acetosa extract, measured by dynamic light scattering (DLS) methods.

nanoparticles produced using Rumex acetosa, a plant with very high concentrations of oxalate, had very stable particles which did not aggregate. Measurement of the reductive capacity of R. acetosa extract showed that the content of reducing compounds is 1.3 times less than that of the H. vulgare extract. Although R. acetosa extract has lower reductive capacity than H. vulgare, it is possible that it may be more efficient at synthesizing iron nanoparticles since its constituents act as stabilizing agents that can prevent iron nanoparticle aggregation and oxidation, to a greater extent than H. vulgare. It can be assumed that plant extracts with high reductant capacity and a low pH will be best suited for the synthesis of iron nanoparticles. Thus, it is likely that other plant species containing copious amounts of small organic acids such as Rheum spp, Citrus spp, Cornus spp and Berberis spp could be exploited for this purpose. The similar effect of the stabilization of the synthesized iron nanoparticles in sorghum extracts can be explained by steric stabilization by polyphenols and other water-soluble heterocycles present in the aqueous extracts.5 We also could not exclude the possibility that metabolites such as anthraquinones,33 which can act as “capping” agents, may participate in the stabilization of iron nanoparticles synthesized in R. acetosa extracts. In this work we did not study the potential organic component of iron nanoparticles. However, it should be noted that the observed difference between the particle diameters, measured by TEM

and DLS methods, may be explained by the presence of a “coat” of low molecular weight organic compounds around the particles. Iron nanoparticles produced using plant extracts have been shown to have excellent potential in a variety of areas. One of the most important applications for iron nanoparticles is their use as Fenton catalysts. For example it has previously been demonstrated that such nanoparticles, by virtue of their large surface to volume ratio, have superior capacity to catalyze the degradation of bromothymol blue, a model organic contaminant.5 Also, the ability of such nanoparticles to oxidize methylene blue, methylene orange, and monochlorbenzen was demonstrated.21,22 It is important that in acidic solution, the surface of the iron oxide/oxohydroxide corrodes producing ferrous ions, which in turn generate the OH· radicals. These radicals attack bonds in the dye molecules which might be in solution or adsorbed on the catalyst surface.21,22 Thus, the use of plant extracts with acidic pH for the synthesis of nanoparticles might enhance the efficiency of nanoparticles as Fenton catalysts. Iron oxide nanoparticles often possess superparamagnetic properties,19 which in turn makes them E

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30 and 40 nm, respectively. However, iron oxide nanoparticles produced by H. vulgare extract were unstable, and only the addition of citrate buffer (pH 3.0) to the reaction mixture stabilizes these nanoparticles, preventing their aggregation for a week or even longer; whereas iron oxide nanoparticles produced by R. acetosa extract with its intrinsically acidic pH (3.7) can produce highly stable nanoparticles, suggesting that plants rich in low molecular weight organic acids may be more suitable for synthesis of iron nanoparticles. This rapid, simple single-step “green” biosynthesis of iron nanoparticles is attractive as it is environmentally sound and safe, and can produce iron nanoparticles of utility for nanotechnology applications.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]. *E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Education and Science of the Russian Federation, the Federal Target-Oriented Program “Scientific and Scientific-pedagogical Personnel of Innovative Russia”, the event 1.5, Agreement No 14.U02.21.1235. A.J.L. and M.T. were supported by funding from Scottish Government Rural and Environmental Science and Analytical Services Division and the work of V.V.M was funded by the RFBR grant 14-04-01448-a and the Grant of the President of the Russian Federation for supporting of young scientists MK-2072.2014.4.

Figure 6. XPS spectra of iron-containing nanoparticles produced in R. acetosa extract measured using a Kratos Axis Ultra DLD system. (A) Overview XPS spectrum of iron-containing nanoparticles. (B) XPS spectrum of iron 2p-electrons of iron-containing nanoparticles.

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Table 1. Some Biochemical Characteristics of Hordeum vulgare and Rumex acetosa Extracts plant species Hordeum vulgare Rumex acetosa

pH value

concentration of total protein (mg per mL of leaf extract)

concentration of reducing metabolites (mg per g of fresh weight of leaf)

5.8

0.27

0.23 ± 0.04

3.7

0.21

0.17 ± 0.004

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CONCLUSIONS We compared the ability of aqueous extracts of two widespread plants (monocotyledonous Hordeum vulgare and dicotyledonous Rumex acetosa) to synthesize iron nanoparticles from iron salts. We demonstrated that extracts of both plants contain organic components which can reduce iron ions into amorphous iron oxide nanoparticles with diameters of up to F

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