Biosynthesis of Iron and Silver Nanoparticles at Room Temperature

Dec 6, 2010 - E-mail: [email protected]. Cite this:Langmuir 27 ... Citation data is made available by participants in Crossref's Cited-by Linking ...
0 downloads 12 Views 2MB Size
Download PDF
pubs.acs.org/Langmuir © 2010 American Chemical Society

Biosynthesis of Iron and Silver Nanoparticles at Room Temperature Using Aqueous Sorghum Bran Extracts Eric C. Njagi,† Hui Huang,† Lisa Stafford,† Homer Genuino,† Hugo M. Galindo,† John B. Collins,‡ George E. Hoag,‡ and Steven L. Suib*,†,§ †

Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269-3060, United States, ‡ VeruTEK Technologies, Inc., 65 West Dudley Town Rd, Suite 100, Bloomfield, Connecticut 06002, United States, and §Department of Chemical Engineering, and Institute of Materials Science, University of Connecticut, U-3060, 55 North Eagleville Rd., Storrs, Connecticut 06269-3060, United States Received August 11, 2010. Revised Manuscript Received November 22, 2010 Iron and silver nanoparticles were synthesized using a rapid, single step, and completely green biosynthetic method employing aqueous sorghum extracts as both the reducing and capping agent. Silver ions were rapidly reduced by the aqueous sorghum bran extracts, leading to the formation of highly crystalline silver nanoparticles with an average diameter of 10 nm. The diffraction peaks were indexed to the face-centered cubic (fcc) phase of silver. The absorption spectra of colloidal silver nanoparticles showed a surface plasmon resonance (SPR) peak centered at a wavelength of 390 nm. Amorphous iron nanoparticles with an average diameter of 50 nm were formed instantaneously under ambient conditions. The reactivity of iron nanoparticles was tested by the H2O2-catalyzed degradation of bromothymol blue as a model organic contaminant.

1. Introduction Metallic nanoparticles have attracted tremendous interest due to their unique optoelectronic and physicochemical properties. Their applications include use in biosensing,1 media recording,2 optics,3 catalysis,4 and environmental remediation.5-10 Metallic nanoparticles of specific sizes and morphologies can be readily synthesized using chemical and physical methods.11-15 However, these methods employ toxic chemicals as reducing agents, organic solvents, or nonbiodegradable stabilizing agents and are therefore potentially dangerous to the environment and biological systems.16 Moreover, most of these methods entail intricate controls or nonstandard conditions making them quite expensive. The biosynthesis of nanoparticles has been proposed as a costeffective environmental friendly alternative to chemical and physical *Corresponding author. E-mail: [email protected]. (1) Ren, X.; Meng, X.; Chen, D.; Tang, F.; Jiao, J. Biosens. Bioelectron. 2005, 21, 433–437. (2) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989–1991. (3) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729–7744. (4) Moreno-Ma~nas, M.; Pleixats, R. Acc. Chem. Res. 2003, 36, 638–643. (5) Kanel, S.; Manning, B.; Charlet, L.; Choi, H. Environ. Sci. Technol. 2005, 39, 1291–1298. (6) Cao, J.; Elliott, D.; Zhang, W. J. Nanopart. Res. 2005, 7, 499–506. (7) Chen, S.; Hsu, H.; Li, C. J. Nanopart. Res. 2004, 6, 639–647. (8) Gillham, R. W.; O’Hannesin, S. F. Ground Water 1994, 36, 958–967. (9) Lin, Y.; Wen, C.; Chen, F. Sep. Purif. Technol. 2008, 64, 26–30. (10) Wang, C. B.; Zhang, W. X. Environ. Sci. Technol. 1997, 31, 2154–2156. (11) Sun, Y.; Mayers, B.; Herricks, T.; Xia, Y. Nano Lett. 2003, 3, 955–960. (12) Srikanth, H.; Hajndl, R.; Chirinos, C.; Sanders, J. Appl. Phys. Lett. 2001, 79, 3503–3505. (13) Valle-Orta, M.; Diaz, D.; Santiago-Jacinto, P.; Vazquez-Olmos, A.; Reguera, E. J. Phys. Chem. B 2008, 112, 14427–14434. (14) Guo, L.; Huang, Q.; Li, X.; Yang, S. Phys. Chem. Chem. Phys. 2001, 3, 1661–1665. (15) Alqudami, A.; Annapoorni, S. Plasmonics 2007, 2, 5–13. (16) Nadagouda, M. N.; Hoag, G.; Collins, J.; Varma, R. S. Cryst. Growth Des. 2009, 9, 4979–4983. (17) Ahmad, A.; Mukherjee, P.; Mandal, D.; Senapati, S.; Khan, M. I.; Kumar, R.; Sastry, M. J. Am. Chem. Soc. 2002, 124, 12108–12109. (18) Ahmad, A.; Senapati, S.; Khan, M. I.; Kumar, R.; Sastry, M. Langmuir 2003, 19, 3550–3553.

264 DOI: 10.1021/la103190n

methods. Consequently, nanomaterials have been synthesized using microorganisms17-21 and plant extracts.22-28 The use of plant extracts for synthesis of nanoparticles is potentially advantageous over microorganisms due to the ease of scale up, the biohazards, and elaborate process of maintaining cell cultures.24,29,30 Gold and silver nanoparticles have been synthesized using various plant extracts including hibiscus (Hibiscus rosa sinensis) leaf extract,31 neem (Azadirachta indica) leaf broth,23 black tea leaf extracts,32 Indian gooseberry (Emblica officinalis) fruit extract,24 sundried camphor (Cinnamomum camphora) leaves,33 and Aloe vera plant extract.25 The biosynthesis of iron oxides and antimony trioxide (Sb2O3) using plant extracts has also been reported.20,34 Recently, silver and iron nanoparticles of various sizes and morphologies have been synthesized using coffee and green tea (19) Shahverdi, A. R.; Minaeian, S.; Shahverdi, H. R.; Jamalifar, H.; Nohi, A. Process Biochem. (Amsterdam, Neth.) 2007, 42, 919–923. (20) Jha, A. K.; Prasad, K.; Prasad, K. Biochem. Eng. J. 2009, 43, 303–306. (21) Bharde, A. A.; Parikh, R. Y.; Baidakova, M.; Jouen, S.; Hannoyer, B.; Enoki, T.; Prasad, B. L. V.; Shouche, Y. S.; Ogale, S.; Sastry, M. Langmuir 2008, 24, 5787–5794. (22) Shankar, S.; Ahmad, A.; Sastry, M. Biotechnol. Prog. 2003, 19, 1627–1631. (23) Shankar, S. S.; Rai, A.; Ahmad, A.; Sastry, M. J. Colloid Interface Sci. 2004, 275, 496–502. (24) Ankamwar, B.; Damle, C.; Ahmad, A.; Sastry, M. J. Nanosci. Nanotechnol. 2005, 5, 1665–1671. (25) Chandran, S. P.; Chaudhary, M.; Pasricha, R.; Ahmad, A.; Sastry, M. Biotechnol. Prog. 2006, 22, 577–583. (26) Nadagouda, M. N.; Varma, R. S. Green Chem. 2008, 10, 859–862. (27) Nadagouda, M. N.; Castle, A. B.; Murdock, R. C.; Hussain, S. M.; Varma, R. S. Green Chem. 2010, 12, 114–122. (28) Hoag, G. E.; Collins, J. B.; Holcomb, J. L.; Hoag, J. R.; Nadagouda, M. N.; Varma, R. S. J. Mater. Chem. 2009, 19, 8671–8677. (29) Bar, H.; Bhui, D. K.; Sahoo, G. P.; Sarkar, P.; De, S. P.; Misra, A. Colloids Surf., A 2009, 339, 134–139. (30) Sathishkumar, M.; Sneha, K.; Kwak, I. S.; Mao, J.; Tripathy, S. J.; Yun, Y.-S. J. Hazard. Mater. 2009, 171, 400–404. (31) Philip, D. Physica E: Low Dimens. Syst. Nanostruct. 2010, 42, 1417–1424. (32) Begum, N. A.; Mondal, S.; Basu, S.; Laskar, R. A.; Mandal, D. Colloids Surf., B 2009, 71, 113–118. (33) Huang, J.; Li, Q.; Sun, D.; Lu, Y.; Su, Y.; Yang, X.; Wang, H.; Wang, Y.; Shao, W.; He, N.; Hong, J.; Chen, C. Nanotechnology 2007, 18, 105104–105115. (34) Herrera-Becerra, R.; Zorrilla, C.; Ascencio, J. A. J. Phys. Chem. C 2007, 111, 16147–16153.

Published on Web 12/06/2010

Langmuir 2011, 27(1), 264–271

Njagi et al.

extracts.26-28 Stable polydispersed spherical silver nanoparticles with sizes ranging from 5 to 100 nm were obtained using aqueous coffee and tea extracts.26 Iron nanoparticles of various sizes and morphologies (spherical, platelets, and nanorods) were also formed instantaneously using aqueous tea extracts.27,28 The size and crystallinity (hexagonal metallic iron, amorphous iron, and R-Fe2O3) of the synthesized iron nanoparticles were found to depend on the concentration of the tea extract in the reaction mixture.27 The synthesized iron nanoparticles were found to be nontoxic when compared with iron nanoparticles prepared using conventional NaBH4 reduction protocols.27 The synthesized iron nanoparticles also effectively catalyzed the H2O2 degradation of bromothymol blue, a model compound for environmental remediation.28 The iron nanoparticles were effectively capped by the tea polyphenols, extending their lifetime. Although the synthesis of metallic nanoparticles using plant materials has been demonstrated, a rapid, cost-effective biosynthetic protocol for bulk synthesis of stable metallic nanoparticles has not been developed. Such a method is important for the full potential of these nanomaterials in environmental remediation and other technological applications to be realized. Most biological methods reported in the literature use dilute (e10-3 M) metal ion solutions forming low concentrations of colloids.23-25,31,34 In this study, we report the biosynthesis of iron and silver nanoparticles via a single-step, room-temperature reduction of iron and silver ions using aqueous hybrid sorghum (Sorghum spp) bran extracts. The reduction of the metal ions using aqueous sorghum bran extracts is rapid and results in moderately stable colloids. Specialty sorghums contain high levels of diverse phenolic compounds that can function as both reducing and capping agents in the synthesis of metallic and metal oxide nanoparticles. These phenolic compounds are water-soluble, nontoxic, and biodegradable, affording a green synthesis process. Specialty hybrid sorghums with high levels of freely extractable phenolic compounds are potentially more cost-effective and advantageous than the use of coffee and tea for bulk synthesis of metallic nanoparticles. Most sorghum varieties are drought and heat tolerant, making sorghum economical to produce.35 On the contrary, the production of coffee and tea requires very specific climatic conditions, longer maturity times, and expensive processing.36,37 Further, sorghum bran is primarily used as a feedstock and for production of alcohol and other industrial products while coffee and tea are commonly used as behavioral stimulants and for other therapeutic uses.28,35 The performance of the synthesized iron nanoparticles in the degradation of organic contaminants was investigated using bromothymol blue (BTB) as a model organic contaminant. BTB is a useful probe molecule for free radical oxidation that is easily monitored using UV-vis spectroscopy due to its concentration-dependent absorption in the visible region. BTB has been used as a model compound to study TiO2-mediated photocatalytic degradation treatment of wastewater contaminated with dye effluents.38 Bromothymol blue has also been used as a probe compound to study the effects of microwave irradiation and auxiliary oxidants (ozone and UV) on TiO2-assisted photodegradation of organic contaminants in water.39 BTB is a useful probe molecule that can only be chemically degraded via free-radical pathways unlike methylene blue, which undergoes direct oxidation by persulfate.28 (35) (36) (37) (38) (39) 1632.

Awika, J. M.; Rooney, L. W. Phytochemistry 2004, 65, 1199–1221. DaMatta, F. M.; Ramalho, J. D. C. Braz. J. Plant Physiol. 2006, 18, 55–81. Carr, M. K. V. Exp. Agric. 1972, 8, 1–14. Haque, M. M.; Muneer, M. Dyes Pigm. 2007, 75, 443–448. Park, S. H.; Kim, S.; Seo, S.; Jung, S. Nanoscale Res. Lett. 2010, 5, 1627–

Langmuir 2011, 27(1), 264–271

Article

2. Experimental Methods 2.1. Materials and Chemicals. Iron(III) chloride hexahydrate (FeCl3 3 6H2O, Fluka; >98%), bromothymol blue (Sigma-Aldrich; ACS reagent), gallic acid (Sigma-Aldrich, 97.5-102.5%), sodium carbonate (Sigma-Aldrich, >99.0%), Folin-Ciocalteu’s phenol reagent (Sigma-Aldrich, 2 N), hydrogen peroxide (H2O2, Fisher Scientific; 30% solution), and silver nitrate (AgNO3, Alfa Aesar; 99.9%) were used in this study without further purification. All aqueous solutions were made using distilled deionized water (DDW). Specialty sorghum bran (Sorghum spp) powder was obtained from VeruTEK Technologies Inc. (Bloomfield, CT). Sorghum bran extracts are rich in polyphenols, policosanols, and other reductive biomolecules containing multiple hydroxyl functional groups such as flavonoids and phenolic acids.35,40 These phenolic compounds have reduction potential values ranging from 0.3 to 0.8 V and are mainly responsible for bioreduction of metal ions and stabilization of the resultant nanoparticles.27,28 2.2. Preparation of Iron and Silver Nanostructures. Sorghum bran powder (8.3 g) was extracted with 125 mL of DDW at 25, 50, and 80 °C while being continuously stirred for 30 min. After extraction, samples were centrifuged at 8000 rpm for 30 min. The resultant supernatant was collected, filtered, and stored at -20 °C before use. The total phenolic content of the aqueous sorghum bran extracts was determined by the Folin-Ciocalteu method using gallic acid as a standard phenolic compound as described by Slinkard and Singleton.41 Absorbance was measured at 765 nm using a Jasco V530 UV-vis spectrophotometer, and results were expressed as mg/L gallic acid equivalent (GAE). The total phenolic content of the extracts was 2010, 2375, and 2520 mg/L GAE for sorghum bran extracted at 25, 50, and 80 °C, respectively. This indicates that the extraction efficiency increased with increase of extraction temperature. The pH values of the aqueous sorghum extracts were 6.30, 6.12, and 5.86 for sorghum extracted at 25, 50, and 80 °C, respectively. Iron nanoparticles were prepared by adding 0.1 M FeCl3 solution to the sorghum bran extract (supernatant at ambient temperature) in a 2:1 volume ratio. The mixture was hand shaken for 1 min and allowed to stand at room temperature for 1 h. The same procedure was used to synthesize silver nanoparticles using 0.1 M AgNO3 solution. 2.3. Characterization of Nanoparticles. Morphological studies were performed using field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM). The FE-SEM micrographs were obtained using a JEOL 6335F FE-SEM microscope equipped with a Thermo Noran energy dispersive spectroscopy (EDS) detector. The HR-TEM images were obtained using a JEOL 2010 FasTEM microscope equipped with an energy dispersive spectroscopy (EDS) detector. The powder X-ray diffraction (XRD) patterns were recorded using a Scintag 2000 PDS diffractometer with Cu KR radiation, a beam voltage of 45 kV, and current of 40 mA. The UV-vis absorption spectra were obtained using a Jasco V530 UV-vis spectrophotometer. The elemental analysis of the samples was performed using energy dispersive spectroscopy (EDS). The zeta (ζ) potential measurements were carried out using a Zetasizer Nano ZS 90 (Malvern Instruments). 2.4. Degradation of Bromothymol Blue. The bromothymol blue solution (500 mg/L) was prepared by dissolving 50 mg of bromothymol blue in 100 mL of DDW. Blanks were prepared by adding 7.3, 15, and 30 μL of the colloidal iron nanoparticles to 3 mL of DDW to make solutions with iron concentrations of 0.16, 0.33, and 0.66 mM, respectively. Samples were prepared by mixing 3 mL of the 500 mg/L (0.8 mM) bromothymol blue and 200 μL of 30% H2O2 (0.68 M) in a cuvette. The cuvette was then (40) Hahn, D. H.; Faubion, J. M.; Rooney, L. W. Cereal Chem. 1983, 60, 255–259. (41) Slinkard, K.; Singleton, V. L. Am. J. Enol. Vitic. 1977, 28, 49–55.

DOI: 10.1021/la103190n

265

Article

Njagi et al.

Figure 1. XRD patterns of silver nanoparticles synthesized using sorghum bran extracted at 25, 50, and 80 °C.

Figure 2. UV-vis spectra of colloidal silver nanoparticles synthesized using sorghum bran extracted at (a) 25, (b) 50, and (c) 80 °C. inserted in the spectrophotometer, and the iron nanoparticles were added to the solution and quickly mixed with a pipet. Scans were started immediately after the addition of iron, and the solution was left untouched until completion. Absorbance was monitored at the wavelength of maximum absorption (λmax = 431 nm).

3. Results 3.1. Preparation of Silver Nanoparticles. Metallic silver nanoparticles formed within a few minutes at room temperature when silver nitrate solution was added to the aqueous sorghum 266 DOI: 10.1021/la103190n

extracts. The solutions turned yellow-brown, indicating the formation of silver nanoparticles, which absorb radiation in the visible region (ca. 380-450 nm).42 The pH values of the colloidal silver nanoparticles synthesized using sorghum extracted at 25, 50, and 80 °C were 3.46, 3.45, and 3.44, respectively. 3.1.1. Crystallographic Structures. The XRD patterns of synthesized silver nanoparticles are shown in Figure 1. The nanoparticles are highly crystalline with diffraction peaks corresponding (42) Huang, H.; Yang, X. Carbohydr. Res. 2004, 339, 2627–2631.

Langmuir 2011, 27(1), 264–271

Njagi et al.

Article

Figure 3. FE-SEM images of silver nanoparticles prepared using sorghum bran extracted at (a) 25, (b) 50, and (c) 80 °C.

to the face-centered cubic (fcc) phase of metallic silver. The calculated lattice constant (a = 4.0602 A˚) is in good agreement with the reported value (JCPDS 4-783; a = 4.0862 A˚). The patterns lack diffraction peaks corresponding to impurities or oxides except for a broad bump about 2θ=25° most likely from organic moieties present in the extracts.16,33 3.1.2. UV-vis Absorption. The UV-vis spectra of diluted (10) colloidal silver nanoparticles are shown in Figure 2. An intense surface plasmon resonance (SPR) peak is observed at 390 nm. All spectra presented a minimum at ∼320 nm. Colloidal silver nanoparticles synthesized using the sorghum bran extracted at 80 °C had the most intense SPR peak, suggesting a higher number of absorbing particles, attributable to increased extraction efficiency with temperature. 3.1.3. Morphology of Silver Nanoparticles. Figure 3 shows the FE-SEM images of silver nanostructures synthesized using the sorghum extracts. The nanoparticles appear to aggregate and form nanoclusters inside the extract matrix. However, these nanoclusters are well separated from each other. The nanoparticles are largely uniform with a narrow size distribution. The highresolution transmission electron microscope (HR-TEM) micrographs (Figure 4) show that the silver nanoparticles are spherical with an average diameter of about 10 nm. 3.1.4. Composition of Silver Nanoparticles. The elemental composition of powdered samples was determined using FESEM equipped with an EDS detector. Figure 5 shows a representative EDS spectrum of the synthesized silver nanoparticles. The spectrum shows the nanoparticles are primarily composed of silver with the only noticeable contaminant being phosphorus presumably from the sorghum extract. 3.2. Preparation of Iron Nanoparticles. On adding ferric chloride solution to the aqueous sorghum extracts, the solutions instantaneously turned from pale yellow to dark brownish, indicating the formation of iron nanoparticles.27 The pH values of iron nanoparticles prepared using sorghum extracted at 25, 50, and 80 °C were 2.24, 2.20, and 2.10, respectively. The as-synthesized colloidal iron nanoparticles had ζ potentials of 14.6, 13.3, and 11.4 for samples prepared using sorghum extracted at 25, 50, and 80 °C, respectively. Sedimentation was not observed after a week of storage. This suggests that the synthesized iron nanoparticles were stabilized by the polyphenols in the sorghum extracts. 3.2.1. Crystallographic Structures. The XRD patterns of iron nanoparticles are shown in Figure 6. The patterns lack distinct diffraction peaks, suggesting that the iron nanoparticles Langmuir 2011, 27(1), 264–271

Figure 4. TEM images of silver nanoparticles synthesized using sorghum bran extracted at (a) 25, (b) 50, and (c) 80 °C.

Figure 5. EDS spectrum of silver nanoparticles prepared using sorghum bran extracted at 80 °C.

are amorphous. In the XRD patterns of silver, broad humps are present at about 2θ = 25°, which can be attributed to organic materials in the matrix. 3.2.2. UV-vis Absorption. The UV-vis spectra of the iron nanoparticles are shown in Figure 7. There is continuous absorption in the visible range. Nadagouda et al.27 obtained similar UV-vis spectra for amorphous iron nanoparticles synthesized using tea polyphenols. Colloidal samples prepared with sorghum extracted at 25 and 80 °C had the least and most absorbance in the whole visible range, respectively. 3.2.3. Morphology of Iron Nanoparticles. The FE-SEM images of iron nanoparticles are shown in Figure 8. These images suggest that the particles agglomerate to form irregular clusters. The average diameter of the iron nanoparticles calculated from HR-TEM micrographs (Figure 9) is about 50 nm. The selected area electron diffraction (SAED) micrographs (insets of Figure 9) have no diffraction rings or spot patterns but show a diffuse ring, confirming the amorphous nature of the iron nanoparticles. DOI: 10.1021/la103190n

267

Article

Njagi et al.

Figure 6. XRD patterns of iron nanoparticles synthesized using sorghum bran extracted at 25, 50, and 80 °C.

3.2.4. Elemental Composition. Figure 10 shows a representative EDS spectrum of the iron nanoparticles. The iron nanoparticles are primarily composed of iron. The spectrum also reveals the presence of residual chloride ions and phosphorus impurities. Oxygen is also present suggesting surface oxidation of the iron nanoparticles. 3.2.5. Catalytic Studies. Figure 11 shows the degradation of bromothymol blue with time using 2% hydrogen peroxide and various concentrations of the synthesized iron nanoparticles. The initial concentration of bromothymol blue was 500 mg/L. Degradation of bromothymol blue did not occur in the presence of only 2% H2O2, indicating there was no direct oxidation pathway by peroxide. However, degradation of BTB occurred in the presence of 2% H2O2 and iron nanoparticles, suggesting degradation via free radical pathways. The degradation of BTB was fastest in the presence of 2% H2O2 and 0.66 mM iron nanoparticles leading to a 90% decrease in the concentration of BTB within 30 min. In the presence of 2% H2O2 and 0.33 mM iron nanoparticles about 60% of BTB was degraded in 30 min. The degradation was significantly slower in the presence of 2% H2O2 and 0.16 mM iron nanoparticles with only about 20% BTB being degraded after 30 min. Thus, these results indicate that higher iron concentrations accelerated the degradation of bromothymol blue.

4. Discussion When AgNO3 solution was added to the sorghum extract, the solution immediately turned from pale yellow to yellow-brownish, indicating the formation of silver nanoparticles.42 The yellowishbrownish color results from absorption by colloidal silver nanoparticles in the visible (380-450 nm) region of the electromagnetic spectrum due to the excitation of surface plasmon vibrations.42,43 The ultraviolet-visible absorption spectra of colloidal silver nanoparticles exhibited characteristic surface plasmon resonance (SPR) bands centered at 390 nm. The location of the SPR peak on (43) Ahmad, A.; Mukherjee, P.; Senapati, S.; Mandal, D.; Khan, M. I.; Kumar, R.; Sastry, M. Colloids Surf., B 2003, 28, 313–318.

268 DOI: 10.1021/la103190n

the lower end of the absorption range (380-450 nm) indicates that the colloidal dispersion was primarily composed of small spherical silver nanoparticles.44,45 The small sizes (∼10 nm) and the spherical nature of the silver nanoparticles were confirmed by our TEM results. All spectra presented a minimum at ∼320 nm that corresponds to the wavelength at which the real and imaginary parts of the dielectric function of silver almost vanish.46 The plasmon bands are broad with an absorption tail in the longer wavelengths due to the size distribution of the particles.47 Similar UV-vis spectra were obtained for silver nanoparticles prepared using hibiscus (Hibiscus rosa sinensis) leaf extract and neem (Azadirachta indica) leaf broth.23,31 The intensity of the absorption bands increased with the temperature of extraction of the sorghum bran, suggesting an increase in the concentration of silver nanoparticles. This is in agreement with our phenolic content values that increased with extraction temperature, which suggests that phenolic compounds are the principal reducing agents. The synthesized silver nanoparticles were highly crystalline with diffraction peaks corresponding to the face-centered cubic (fcc) phase of metallic silver. The five diffraction peaks present in the spectra at 2θ values of 38.35°, 44.49°, 64.68°, 77.58°, and 81.66° were indexed to the (111), (200), (220), (311), and (222) reflections of the fcc structure of metallic silver. No extra diffraction peaks were present, suggesting that the synthesized silver was essentially pure. The high purity and the low toxicity of aqueous sorghum extracts render the synthesized silver nanoparticles potentially attractive for biological applications. Morphological studies using FE-SEM showed that silver nanoparticles formed nanoclusters inside the extracts. HR-TEM showed these spherical nanoparticles (44) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881–3891. (45) Petit, C.; Lixon, P.; Pileni, M. J. Phys. Chem. 1993, 97, 12974–12983. (46) Elechiguerra, J. L.; Burt, J. L.; Morones, J. R.; Camacho-Bragado, A.; Gao, X.; Lara, H. H.; Yacaman, M. J. J. Nanobiotechnol. 2005, 3, 6. (47) Vigneshwaran, N.; Ashtaputre, N. M.; Varadarajan, P. V.; Nachane, R. P.; Paralikar, K. M.; Balasubramanya, R. H. Mater. Lett. 2007, 61, 1413–1418.

Langmuir 2011, 27(1), 264–271

Njagi et al.

Article

Figure 7. UV-vis spectra of colloidal iron nanoparticles prepared using sorghum bran extracted at (a) 25, (b) 50, and (c) 80 °C.

Figure 8. FE-SEM images of iron nanoparticles synthesized using sorghum bran extracted at (a) 25, (b) 50, and (c) 80 °C.

are largely uniform with an average diameter of about 10 nm, which is within the size range critical for their biological applications.44 HR-TEM results reveal the size of silver nanoparticles prepared using sorghum extracted at 50 °C is smaller compared to the size of silver nanoparticles prepared using sorghum extracted at 25 and 80 °C. Nadagouda et al.27 observed that the concentration of phenolic compounds is critical to the size and morphology of metallic nanoparticles. Thus, the concentration of phenolic compounds in the sorghum extracts might be the key determinant of the size of the resultant nanoparticles. The UV-vis spectra of iron nanoparticles synthesized using aqueous sorghum extracts showed continuous absorption in the visible range. Nadagouda et al.27 obtained similar UV-vis spectra for amorphous iron nanoparticles synthesized using aqueous tea extracts. The powder XRD patterns of iron nanoparticles did not have distinct diffraction peaks, suggesting that the synthesized iron nanoparticles were amorphous. The amorphous nature of Langmuir 2011, 27(1), 264–271

Figure 9. TEM images of iron nanoparticles synthesized using sorghum bran extracted at (a) 25, (b) 50, and (c) 80 °C.

the iron nanoparticles was confirmed by the SAED analysis. The HR-TEM micrographs show that the iron nanoparticles are well separated from each other, suggesting effective capping by watersoluble heterocyclics present in the sorghum extracts. The HR-TEM results also revealed that the iron nanoparticles synthesized using sorghum extracts were spherical with sizes ranging from 40 to 50 nm. Nadagouda and co-workers27 have synthesized iron nanoparticles of different sizes and morphologies using aqueous tea extract. The sizes and morphologies of the synthesized iron nanoparticles were found to be dependent on the DOI: 10.1021/la103190n

269

Article

Njagi et al.

Figure 10. EDS spectrum of iron nanoparticles prepared using sorghum bran extracted at 80 °C.

Figure 11. Degradation of BTB over time with iron nanoparticles catalyzed H2O2: (a) control (BTB with 2% hydrogen peroxide (HP) solution); (b) BTB treated with 0.16 mM (as Fe) iron nanoparticles and 2% HP; (c) BTB treated with 0.33 mM (as Fe) iron nanoparticles and 2% HP; (d) BTB treated with 0.66 mM (as Fe) iron nanoparticles and 2% HP.

concentration of the tea extract. Elemental analysis using EDS revealed the presence of oxygen in the synthesized iron nanoparticles. Metallic iron nanoparticles often react spontaneously upon exposure to air or water to form iron-iron oxide core-shell nanoparticles.48,49 The pH of the synthesized iron colloids was about 2, suggesting that amorphous iron oxyhydroxide, which is known to form at low pH, might have precipitated.50 Magnetic and spectroscopic experiments are ongoing to determine the composition of the synthesis iron nanoparticles. The EDS results also revealed the presence of phosphorus in the synthesized silver and iron nanoparticles. Phosphorus is vital to plant growth and is found in every living plant cell. Phosphorus ions are known to adsorb on nanoparticles (especially on iron oxyhydroxides) at low (48) Li, X.; Elliott, D. W.; Zhang, W. Crit. Rev. Solid State Mater. Sci. 2006, 31, 111–122. (49) Sohn, K.; Kang, S. W.; Ahn, S.; Woo, M.; Yang, S. Environ. Sci. Technol. 2006, 40, 5514–5519. (50) Furniss, G.; Hinman, N. W.; Doyle, G. A.; Runnells, D. D. Environ. Geol. 1999, 37, 102–106.

270 DOI: 10.1021/la103190n

pH, which would explain the presence of phosphorus in the synthesized nanoparticles.51 Iron nanoparticles are increasingly being utilized in environmental remediation and hazardous waste treatment. However, these nanoparticles exhibits a strong tendency to aggregate leading to rapid sedimentation and limited mobility especially in aquatic environments. Nonstabilized iron nanoparticles are known to settle out of solution in less than 10 min.52 This has limited the use of iron nanoparticles in environmental remediation. The zeta (ζ) potential is commonly used to assess the stability of colloidal systems.52,53 Colloidal systems with ζ potentials greater than (30 mV are normally considered stable.52 The as-synthesized colloidal iron nanoparticles had ζ potentials of 14.6, 13.3, and 11.4 for samples prepared using sorghum bran extracted at 25, 50, (51) Antelo, J.; Avena, M.; Fiol, S.; Lopez, R.; Arce, F. J. Colloid Interface Sci. 2005, 285, 476–486. (52) Sun, Y.; Li, X.; Zhang, W.; Wang, H. P. Colloids Surf., A 2007, 308, 60–66. (53) Sun, Y.; Li, X.; Cao, J.; Zhang, W.; Wang, H. P. Adv. Colloid Interface Sci. 2006, 120, 47–56.

Langmuir 2011, 27(1), 264–271

Njagi et al.

and 80 °C, respectively. The magnitude of ζ potentials of the as-synthesized colloidal systems implies an incipient instability with iron nanoparticles in the dispersion bound to aggregates in the long run. However, no noticeable flocculation or sedimentation was observed after storage for a week, suggesting the sorghum extract was able to stabilize the nanoparticles to some extent. This can be attributed to steric stabilization by polyphenols and other water-soluble heterocyclics present in the aqueous sorghum extracts.35,54 This level of stabilization is potentially useful for environmental and electrocatalytic applications where preventing aggregation while providing accessibility to the nanoparticles is vital.14 Iron nanomaterials are very efficient at removing organic contaminants, organic matter (e.g., humic and fulvic acids), and arsenic from ground waters due to their large surface areas, high surface reactivity, and high sorption capacity.55-61 The reactivity of the synthesized iron nanoparticles was tested using bromothymol blue, a useful probe molecule that does not undergo direct oxidation by H2O2 but degrades via free-radical pathways.28 Bromothymol (54) Awika, J. M.; Rooney, L. W.; Wu, X.; Prior, L. R.; Cisneros-Zevallos, L. J. Agric. Food Chem. 2003, 51, 6657–6662. (55) Zhang, Q.; Pan, B.; Zhang, W.; Pan, B.; Zhang, Q.; Ren, H. Ind. Eng. Chem. Res. 2008, 47, 3957–3962. (56) Zouboulis, A. I.; Katsoyiannis, I. A. Ind. Eng. Chem. Res. 2002, 41, 6149– 6155. (57) Thirunavukkarasu, O. S.; Viraraghavan, T.; Subramanian, K. S. Water Air Soil Pollut. 2003, 142, 95–111. (58) Giasuddin, A. B. M.; Kanel, S. R.; Choi, H. Environ. Sci. Technol. 2007, 41, 2022–2027. (59) Ding, C.; Yang, X.; Liu, W.; Chang, Y.; Shang, C. J. Hazard. Mater. 2010, 174, 567–572. (60) Rongcheng, W.; Jiuhui, Q. Water Environ. Res. 2004, 76, 2637–2642(6). (61) Baldrian, P.; Merhautova, V.; Gabriel, J.; Nerud, F.; Stopka, P.; Hruby, M.; Benes, M. J. Appl. Catal., B 2006, 66, 258–264.

Langmuir 2011, 27(1), 264–271

Article

blue degrades rapidly in the presence of iron nanoparticles and H2O2, indicating that the iron nanoparticles act as catalysts for free radical production from H2O2. The catalysis of H2O2 increases with increasing concentrations of iron nanoparticles, leading to increase in the rate of degradation of bromothymol blue. These results suggest that iron nanoparticles synthesized using aqueous sorghum extracts are potentially useful for degradation of organic pollutants.

5. Conclusions The synthesis of iron and silver nanoparticles using aqueous sorghum bran extracts has been demonstrated. These nanoparticles were prepared at ambient conditions with sorghum polyphenols acting as both the reducing and stabilizing agents. Highly crystalline spherical metallic silver nanoparticles of about 10 nm were prepared. These silver nanoparticles were of high purity, making them potentially useful for biological applications. The synthesized iron nanoparticles effectively catalyzed H2O2 degradation of a model organic compound, making them potentially useful for environmental remediation and treatment of hazardous waste. This biological method is potentially attractive for largescale synthesis of metallic and metal oxide nanomaterials. Acknowledgment. We acknowledge financial support from VeruTEK Technologies Inc. and the Department of Energy, Office of Basic Energy Sciences, Division of Geochemical, Biological and Chemical Sciences. The authors also thank Dr. Lichun Zhang for assisting with microscopy experiments and Dr. Raymond Joesten for helpful suggestions. We also thank Aimee Morey-Oppenheim for performing preliminary magnetic measurements.

DOI: 10.1021/la103190n

271