Biogenic Silver Nanoparticles by Cacumen Platycladi Extract

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Biogenic Silver Nanoparticles by Cacumen Platycladi Extract: Synthesis, Formation Mechanism, and Antibacterial Activity Jiale Huang,†,§ Guowu Zhan,†,§ Bingyun Zheng,†,‡ Daohua Sun,*,† Fenfen Lu,† Yuan Lin,† Huimei Chen,† Zhouding Zheng,† Yanmei Zheng,† and Qingbiao Li*,† †

Department of Chemical and Biochemical Engineering, and National Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, and Key Lab for Chemical Biology of Fujian Province, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China ‡ Department of Environment & Life Sciences, Putian University, Putian 351100, P. R. China

bS Supporting Information ABSTRACT: Biosynthesis of Ag nanoparticles (AgNPs) by Cacumen Platycladi extract was investigated. The AgNPs were characterized by ultravioletvisible absorption spectroscopy (UVvis), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and X-ray diffraction (XRD). The results showed that increasing the initial AgNO3 concentration at 30 or 60 °C increased the mean size and widened the size distribution of the AgNPs leading to red shift and broadening of the Surface Plasmon Resonance absorption. The conversion of silver ions was determined by atomic absorption spectroscopy (AAS) and to discuss the bioreductive mechanism, the reducing sugar, flavonoid, saccharide, protein contents in the extract, and the antioxidant activity were measured using 3,5-dinitrosalicylic acid colorimetric; Coomassie brilliant blue; phenol-sulfuric acid; rutin-based spectrophotometry method; and 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical-scavenging assay methods. The results showed that the reducing sugars and flavonoids were mainly responsible for the bioreduction of the silver ions and their reductive capability promoted at 90 °C, leading to the formation of AgNPs (18.4 ( 4.6 nm) with narrow size distribution. Finally, the antibacterial activity of the AgNPs against E. coli and S. aureus was assessed to determine their potential applications in silver-loaded antibacterial materials. This work provides useful technical parameters for industrialization of the biosynthetic technique and further antibacterial application of the AgNPs. Furthermore, the elucidation of bioreductive mechanism of silver ions by measuring the change of the biomolecular concentrations in plant extract exemplifies understanding the formation mechanism of such biogenic AgNPs.

1. INTRODUCTION Proliferation of nanotechnology has opened up novel fundamental and applied frontiers in materials science and engineering in the past decade. As building blocks in nanotechnology, nanostructures of well-defined compositions, shapes, and sizes have been synthesized through various methods. However, conventional physical and chemical methods are not only energy intensive (because of stringent conditions) but equally environmentally unfriendly, due to the use of toxic solvents or additives. Compared with the conventional methods, bioreduction methods (based on microorganisms or plants) have been demonstrated (in recent years) to be cost-effective and environmentally benign and yet produce highly stable nanoparticles (NPs). Driven by this growing impetus (of green chemistry), considerable efforts have been directed toward the biosynthesis of inorganic nanoparticles (NPs). such as Ag,13 Au,4,5 Pd,5,6 CaCO3,7,8 BaCO3,8 SrCO3,9 ZrO2,10 TiO2,11 SiO2,11,12 PbCO3,13 CdCO3,13 BaTiO3,14,15 Fe3O4,16 CuAlO2,17 CdS18,19 and Sb2O320 using microorganisms. The use of bacteria,13,5,6 fungi,4,718 actinomycete,8 and genetically engineered E. coli19 have been exploited in the intracellular and extracellular assembly of stabilized NPs under mild conditions without auxiliary capping agents.21 However, the processing of NPs by intracellular biosynthesis is generally difficult21 despite the use of bacteria-supported Pd catalysts, and the screening of extracellular biosynthesis with microorganisms is extensive. In contrast, living r 2011 American Chemical Society

plants, plant extracts, and plant biomass have recently attracted attention as alternative candidates that are simple but effective for the extracellular synthesis of metal NPs.22,23 For example, alfalfa has been used to prepare Ag nanoparticles (AgNPs),24 and broths of geranium,25 neem,26 and Emblica officinalis fruit27 as well as Aloe vera leaves28 have been employed to synthesize AgNPs. Some studies broadened the scope by using extracts of Chlorella vulgaris,29 Capsicum annuum L.,30 coffee,31 tea,31 Camellia sinensis,32 Eclipta leaf33 and Azadirachta indica leaf,34 and sundried Cinnamomum camphora leaf.35,36 Very recently, in our study, single crystalline Ag nanowires were synthesized using broth of Cassia fistula leaf.37 Mostly, biosyntheses of AgNPs using plant extracts are carried out at room temperature, resulting in low reaction rates and conversions of silver ions. Rapid biosynthesis of metal NPs by plant extract has hitherto received less reportage. Song et al. demonstrated the rapid biosynthesis of AgNPs using several plant extracts.38 However, detailed reports on the effect of reaction temperature and initial AgNO3 concentration on the shape and size of AgNPs have not been demonstrated. Furthermore, the Received: November 22, 2010 Accepted: June 21, 2011 Revised: May 31, 2011 Published: June 21, 2011 9095

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Figure 1. UVvis absorption spectra of the AgNPs over reaction time by reducing (a) 1, (b) 3, and (c) 5 mM aqueous AgNO3 with the C. Platycladi extract at 30 °C, respectively. (d) shows the change of SPR absorption intensity over reaction time. All samples were measured without dilution.

formation mechanism of biologically synthesized AgNPs is currently not fully understood.38 In this work, we synthesized AgNPs by reducing aqueous AgNO3 of various concentrations at different temperatures with Cacumen Platycladi (C. Platycladi) extract as reducing and protecting agents without any additive protecting the AgNPs from aggregating or template shaping the AgNPs. In contrast to microorganisms, the use of the C. Platycladi extract previously unexploited for bioreduction of silver ions and based on rapid pretreatment procedures can circumvent laborious biological screening and cultivation. The C. Platycladi extract was readily available for the bioreduction as it was obtained from arborvitae that is widely naturalized in Asia. The effects of reaction temperature, reaction time, and initial concentration of AgNO3, etc., on the conversion of silver ions, morphology, mean size, and size distribution were investigated. Ultravioletvisible absorption spectroscopy (UVvis), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and X-ray diffraction (XRD) were used to characterize the AgNPs and atomic absorption spectroscopy (AAS) was adopted to determine the concentration of the residual silver ions (after the reaction). To elucidate the formation mechanism the extracts before and after the reaction were analyzed by Fourier transform infrared spectroscopy (FTIR). The contents of the reducing sugars, flavonoids, saccharides, proteins, and antioxidant activity before and after the reaction were also measured by 3,5-dinitrosalicylic acid colorimetric, rutin-based spectrophotometry, phenol-sulfuric,

and Coomassie brilliant blue methods as well as 2,2-diphenyl1-picrylhydrazyl (DPPH) free radical-scavenging assay. Besides the synthesis of different metal nanostructures, applications of such nanostructures have received tremendous attention recently. AgNPs have been recognized as new generation antimicrobials due to their excellent antimicrobial property and stability3942 and the increase of bacterial resistance to antibiotics. It was demonstrated that AgNPs exhibit more efficient antimicrobial activity than silver ions and other silver salts,43 and the bactericidal properties of AgNPs are size-dependent.44 As a novel approach to AgNPs, plant-based biosynthesis has been explored to produce AgNPs with antibacterial activity. Several antibacterial evaluations of such biogenic AgNPs have been carried out very recently using standard paper disk or broth medium methods.4548 However, the antibacterial effect of silver ions was not ruled out as incomplete reduction of silver ions toxic to the tested strains was neglected at room temperature in the previous studies.4548 Therefore, to explore the potential application of the biogenic AgNPs in the development of antimicrobial materials, the antibacterial activity of the AgNPs was evaluated by agar plate and broth medium methods with E. coli (gram-negative bacteria) and S. aureus (gram-positive bacteria) as test strains.49 Shape and size of the AgNPs, which may affect their downstreaming applications, by the biosynthesis using the C. Platycladi extract were closely associated with the biochemical components in the extract. To our knowledge, this is the first report on 9096

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Figure 2. UVvis absorption spectra of the AgNPs over reaction time by reducing (a) 1, (b) 3, and (c) 5 mM aqueous AgNO3 with the C. Platycladi extract at 60 °C, respectively. (d) shows change of SPR absorption intensity over reaction time. All samples were diluted with 30 deionized water before taking spectra.

elucidation of bioreductive mechanism of silver ions by measuring the change of the total concentrations of reducing sugars, flavonoids, saccharides, and proteins in plant extract and the antioxidant activity of plant extract, which therefore exemplifies understanding the formation mechanism of such biogenic AgNPs. Importantly, synthesis of the AgNPs by rapid bioreduction of silver ions by the C. Platycladi extract and their antibacterial test in this work may provide useful technical parameters for industrialization of the biosynthetic technique and further antibacterial application of the AgNPs.

2. EXPERIMENTAL SECTIONS 2.1. Preparation of exrtact. Sundried C. Platycladi was purchased from Xiamen Jiuding Drugstore, China. The biomass (for the reduction) was milled, and the powder was screened with a 20 mesh sieve. 6.0 g of the powdered extract was dispersed in 200 mL of deionzied water with a blender (Philip) for 9 min to obtain the extract, which was then centrifuged at 5000 rpm for another 9 min to remove residual insoluble biomass. Then the supernatant solution was reserved in a refrigerator (6 °C) and 0.03 g/mL used for the synthesis. 2.2. Synthesis of Silver Nanoparticles. Silver nitrate (AgNO3) was purchased from Sinopharm Chemical Reagent Co. Ltd., China and was used as received. Ten mL of the

C. Platycladi extract in a double necks round flask (25 mL), with a condenser for refluxing the reaction solution, was first heated (at constant temperature) in an oil bath under vigorous magnetic stirring for 10 min with subsequent intermittent addition of 0.101, 0.309, or 0.525 mL of 0.1 M aqueous AgNO3 until the end of the reaction. 2.3. TEM Analysis. AgNPs samples for TEM were prepared by placing a drop of silver hydrosol on carbon coated copper grids and allowing for the complete evaporation of water. The images were taken, and SAED analysis was performed on a Tecnai F30 microscope. The average size and size distribution of the nanoparticles were estimated with the same method as in our previous work.35 2.4. AAS Analysis. To completely precipitate the AgNPs, silver hydrosol was centrifuged at 14000 rpm for 20 min and the concentration of the residual silver ions in the supernatant measured by atomic absorption spectrophotometer (Pgeneral, China); with the conversion of silver ions calculated by Q ¼

C0 Cf 100% C0

ð1Þ

where Q denotes the conversion of silver ions; C0 is the initial concentration of aqueous AgNO3; and Cf is the final concentration of silver ions. 9097



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Figure 3. UVvis absorption spectra of the AgNPs over reaction time by reducing (a) 1, (b) 3, and (c) 5 mM aqueous AgNO3 with the C. Platycladi extract at 90 °C, respectively. (d) shows change of SPR absorption intensity over reaction time. All samples were diluted with 30 deionized water before taking spectra.

2.5. UVVis Analysis. UVvis analysis of the reaction solutions was carried out at room temperature on UNICAM UV-300 spectrophotometer (Thermo Spectronic, USA) at a resolution of 1 nm. 2.6. FTIR Analysis. The extract before and the resulting solution after the reaction was completely dried at 60 °C, and the dried biomass was analyzed by an FTIR spectrophotometer (Avatar 660, Nicolet, USA). 2.7. Biomolecular Components. The following methods were adopted to measure the contents of biomolecular components in the C. Platycladi extract before and after 1 h reaction.50 The content of the reducing sugars was determined using the 3,5dinitrosalicylic acid colorimetric method, the flavonoids by the spectrophotometry method (with rutin as the standard sample), and the saccharides by the phenol-sulfuric acid method; the proteins were assayed by the Coomassie brilliant blue method; and the antioxidant activity was investigated by the 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical-scavenging assay in a process regulated by its discoloration5052 (each parameter was assayed in duplicates). 2.8. Antibacterial Performance. The antibacterial activity of the AgNPs was evaluated against E. coli (gram-negative) and S. aureus (gram-positive) bacteria by culturing in Muller Hinton (MH) broth medium and used as inoculums for the study. Agar plate and broth medium methods were adopted to test the

antibacterial activity of the AgNPs. For the former method, an appropriate amount of E. coli and S. aureus were inoculated on agar plates and exposed to the AgNPs with different concentrations. The plates were later incubated at 37 °C for 24 h, the number of colonies were counted, and the results of three plates were averaged. In the latter method, the (two test) strains were inoculated with 109 CFU, added to several MH broth media flasks (100 mL), and supplemented with 0; 5; 15; 35; and 50 ppm of AgNPs. The growth rates were determined by measuring the optical density at 600 nm intervals (UNICAM UV-300 spectrophotometers).

3. RESULTS AND DISCUSSION 3.1. Biosyntheses of Silver Nanoparticles. 3.1.1. UVVis Absorption. Comparative study was carried out to investigate the

effect of initial AgNO3 concentration and the reaction temperature on the shape and size disperity of the AgNPs, with the same extract. It is generally recognized that UVvis absorption spectroscopy could be used to examine the shape and size of metal NPs.53 Figure 1(ac) shows the UVvis absorption spectra of the AgNPs as a function of reaction time produced by the reduction of 1, 3, and 5 mM aqueous AgNO3 with the extract at 30 °C. The frequency and width of the Surface Plasmon Resonance (SPR) absorption depend on the shape and size of the metal 9098



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Figure 4. TEM images of the AgNPs by reducing (ab) 1, (cd) 3, and (ef) 5 mM aqueous AgNO3 with the C. Platycladi extract for 24 h at 30 °C, respectively.

NPs as well as the dielectric constant and the surrounding medium of the metal NPs.54 The figure also indicates that the SPR peaks of the AgNPs are centered respectively on 433, 445, and 455 nm, implying that the AgNPs were mainly spheroidal and that their SPR spectra broadened with the increase of the initial AgNO3 concentration (C0). The evolution of the SPR absorption intensity as a function of reaction time is presented in Figure 1d, revealing that the absorption intensity increased over time until stability. In addition, given the same reaction time, the intensity increased with the increase of the initial AgNO3 concentration as higher AgNO3 concentration resulted in higher concentration of the AgNPs. Figure 2(ac) shows the evolution of the UVvis absorption spectra at 60 °C (with the same reaction time and reducing agents as before), indicating SPR peaks at 418, 425, and 429 nm, respectively; with Figure 2d shows the changes of the SPR absorption intensity over time at different initial AgNO3 concentration (C0). It took only 30 min to reach a stable SPR absorption intensity for C0 = 1 mM but 240 min for both C0 = 3 and 5 mM (Figure 2d). Comparing the SPR absorption spectra for the same initial AgNO3 concentration at 30 and 60 °C, the peaks at 60 °C are sharper and significantly underwent blue shift from 30 to 60 °C. Obviously, the concentrations of the AgNPs synthesized at 60 °C were much higher than 30 °C as the bioreduction of silver ions was greatly accelerated by switching reaction temperature from 30 to 60 °C. Moreover, the SPR peaks of the AgNPs significantly underwent blue shift, suggesting that the same particle shape, medium dielectric constant, and mean diameter of metal NPs strongly influence the SPR band in the

aqueous solution.55 The size reduction of the metal NPs may have also induced the blue shift leading to the narrow size distribution at 60 °C. The UVvis absorption spectra of the AgNPs synthesized at 90 °C are shown in the Figure 3(a—c) with the SPR peaks at 411, 425, and 431 nm for C0 = 1, 3, and 5 mM, respectively. Given the same reaction time, the concentrations of the AgNPs synthesized at 90 °C were also much higher than 30 °C and slightly higher than 60 °C. In addition, the absorption intensity tended to increase as the reaction time prolonged, and the intensity increased with the increase of the initial AgNO3 concentration. The wavelengths of the maximum absorption peak indicate AgNPs with spheroidal shape, and Figure 3d shows the correlation curves of the SPR absorption intensity over reaction time. For C0 = 1 mM, it took only 30 min for the SPR absorption intensity to stabilize, but the SPR absorption intensity for C0 = 3 and 5 mM did not increase after 150 min. Comparing the SPR absorption peaks (30 and 90 °C), those at 90 °C exhibited notable blue shift at about 20 nm; and there is a slight blue shift between the peaks at 60 and 90 °C, elucidated with detailed characterization of the AgNPs by TEM and the statistics of size distribution. 3.1.2. Size Distribution. Figure 4 is the TEM images of the AgNPs at 30 °C, with a—b being the images of the AgNPs synthesized with C0 = 1 mM. The AgNPs are almost spheroidal in shape with a wide size distribution (between 50 and 100 nm, although many are less than 10 nm), and c—d are the images for C0 = 3 mM which also indicate almost spheroidal shapes with broad size distribution (but with some observed silver plates); nevertheless, the proportion of AgNPs between 50 and 100 nm 9099



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in diameter increased, compared with those in a—b. Parts e—f of Figure 4 are the images for C0 = 5 mM that are predominantly spheroidal and plate-like shapes with wide size distribution still (some larger than 100 nm and others smaller than 10 nm). These results are consistent with that of the UVvis spectra in Figure 1 (a—c). Increasing the initial AgNO3 concentration increased the mean size and widened the size distribution of the AgNPs leading to red shift and broadening of the SPR absorption. Figure 5 shows the images at 60 °C with a—b being the images of the AgNPs synthesized at C0 = 1 mM, manifesting that most of them was spheroidal. The insert SAED pattern of the AgNPs in Figure 5b indicates that they are polycrystalline. By comparison (reactions at 30 and 60 °C), the proportion of AgNPs between 50 and 100 nm largely decreased for the higher temperature (60 °C) with Figure 5c—d also showing a low proportion of AgNPs (in the same range) at 60 °C for C0 = 3 mM; while parts e—f of Figure 5 are the images for C0 = 5 mM, indicating that the formation of Ag nanoplates and relatively larger AgNPs are suppressed at 60 °C. The results of Figures 4 and Figures 5 were consistent with the UVvis spectra (Figures 1 and Figures 2), revealing that increasing the reaction temperature leads to AgNPs with narrow size distribution. Figure 6 represents the TEM images of the AgNPs from aqueous AgNO3 with different initial concentrations (C0 = 1, 3, and 5 mM) at 90 °C. The insert SAED patterns (in b, d, and f) indicate that they were polycrystalline; XRD analysis was thus carried out to determine the crystallinity of the AgNPs. The typical XRD pattern shows distinct Bragg reflections (Figure S1 in the Supporting Information) which may be indexed on the

basis of the face-centered cubic (fcc) structure of silver, implying essentially, crystalline AgNPs. Figure 6 shows that AgNPs prepared at 90 °C are evidently smaller with narrower size distribution and uniform spheroidal shape, which is validated by Figure 7 (representing the size distribution of AgNPs at 60 and 90 °C). 3.1.3. Conversion of Silver Ions. Figure 8 (a-c) represents the variation in the conversion of silver ions with reaction time at 30, 60, and 90 °C, respectively, with Figure 8a representing 80.8%, 27.8%, and 19.1% conversions of silver ions within 1 h at 30 °C for C0 = 1, 3, and 5 mM, respectively. The conversions increased to 94.1%, 89.1%, and 83.6%, respectively, with prolonged reaction time of 24 h, showing that the incomplete reduction of silver ions within 24 h at 30 °C, even for C0 = 1 mM. From an engineering perspective, this is one disadvantage of the biosynthetic protocol of the AgNPs (at room temperature) that needs to be addressed. Furthermore (as mentioned above), the AgNPs synthesized at 30 °C possessed wide size distribution and polydispersed shapes, leading to heterogeneous nucleation (a characteristic of a secondary nucleation); allowing for the formation of spheriodal nanoparticles with wide size distribution for C0 = 1 mM, but for C0 = 3 and 5 mM, the Ostwald ripening effect was promoted, resulting in anisotropic silver nanostructures. When the reaction temperature was raised from 30 to 60 °C (as shown in Figure 8b), the conversions were 83.6% for C0 = 1 mM, 52.6% for C0 = 3 mM, and 32.6% for C0 = 5 mM after 10 min, increasing to 98.2%, 91.3%, and 82.0%, respectively, in 6 h. The conversions after 10 min at 90 °C were 91.5% for C0 = 1 mM, 84.4% for C0 = 3 mM, and 46.4% for C0 = 5 mM (Figure 8c), and the conversion at all the initial concentrations would reach 100% 9100



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Figure 7. Statistic size of the AgNPs by reducing aqueous AgNO3 with the C. Platycladi extract at 60 and 90 °C, respectively.

after 3 h. Therefore, the reaction temperature was a very critical factor for the bioreduction. The TEM results and size distribution statistics show that AgNPs with narrow size distribution were synthesized at 90 °C for the same initial AgNO3 concentrations, suggesting that higher temperature not only leads to AgNPs with uniform sizes but also ensures higher conversion in a short time. Therefore, for rapid synthesis of the AgNPs, a reaction temperature of 90 °C was used. 3.2. Formation Mechanism of the Silver Nanoparticles. Since the bioreduction of silver ions and the formation of AgNPs

are closely related with the biomolecular components of the extract, four main categories of the biomolecules (reducing sugars, flavonoids, saccharides and proteins) before and after the reaction were quantitatively examined to explore the formation mechanism of the AgNPs. The contents of the reducing sugars and saccharides before the reaction were much higher than those of the flavonoids and proteins (Figure 9), but the contents of both the reducing sugars and flavonoids decreased significantly after the reaction, while those of the saccharides and proteins changed only slightly (Figure 9) in most cases. Therefore, the reducing sugars and flavonoids were the two main components responsible for the bioreduction of the silver ions, compared with the saccharides and proteins. It is worth noting that the difference in the contents of the reducing sugars and flavonoids before and after the reaction were affected by initial AgNO3 concentration (C0) and reaction temperature (Figure 9). In the case of C0 = 1 mM, the two components (flavonoids and reducing sugars) were again responsible for the bioreduction except that the content of the reducing sugars changed a little after 1 h reaction at 30 °C; with similar behavior observed for C0 = 3 and 5 mM at 30 and 60 °C. The bioreduction of the silver ions by flavonoids and reducing sugars was promoted at higher temperature (90 °C), and for C0 = 5 mM, the difference in the content of the flavonoids before and after the reaction at 30 and 60 °C was much less significant than that at 90 °C. Some flavonoids such as amentoflavone, cupressuflavone, 7,700 -di-Omethylcupressuflavone, hinokiflavone, etc. in C. Platycladi have been identified (Figure S2 in the Supporting Information).56 Moreover, it is well-known that reducing sugars widely exist in 9101



Figure 8. Change of Ag+ conversion over reaction time by reducing aqueous AgNO3 with the C. Platycladi extract at (a) 30, (b) 60, and (c) 90 °C, respectively.

plant materials.52 Therefore, the flavonoids and some reducing sugars might play an important role in reducing silver ions to silver atoms in the bioreduction by the C. Platycladi extract. Natural antioxidants with strong reductive ability for the removal of free radicals (such as DPPH radicals) have already been extracted from a number of plants.57 In a previous study, we showed that the capability of plant extracts to reduce chloroaurate ions increased with the enhancement of their ability for free radical removal,52 which we regarded as antioxidant activity.51 Similarly therefore, antioxidant activity can be used to evaluate the reductive ability of the extract for the bioreduction of the silver ions. The extract exhibited very high initial antioxidant activity before the reaction (Figure 10). For C0 = 1 and 3 mM, the bioreduction of the silver ions at 60 and 90 °C were curbed by insufficient AgNO3 precursor; consequently, the antioxidant activity was not changed (notably) after 1 h reaction time (Figure 10). In contrast, the difference in the antioxidant activity of the extract before and

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after the reaction for C0 = 5 mM in the same reaction time was significant with the increase in temperature (60 to 90 °C), but the antioxidant activity of the extract was not changed significantly in 1 h reaction at 30 °C for C0 = 1, 3, and 5 mM due to the very slow reaction at room temperature. In this work, the soluble biomolecules were extracted by the intensified mixing of the C. Platycladi biomass and deionized water. Measurement of the antioxidant activity indicated that the extract exhibited excellent reductive ability for bioreduction of silver ions with facile biosynthesis of well dispersed AgNPs with narrow size distribution. Therefore, the pretreatment procedures for the biomass (C. Platycladi) and the biosynthesis of the AgNPs could be extended to other biomasses for mass-production of metal NPs. Figure 11, the FTIR spectrum of the extract, gives information regarding the chemical transformation of the functional groups involved in the reduction of the silver ions. Some pronounced absorbance bands centered at 640, 1059, 1420, 1610, 2925, and 3402 cm1 were observed in the region 5004000 cm1. Among them, the absorbance bands at 640, 1059, 1420, 1610, 2925, and 3402 cm1 were associated with the stretch vibration of CO, CH, C=C, CH2, and OH, respectively.58 These absorbance bands could be attributed to the reducing sugars, flavonoids (amentoflavone, cupressuflavone, 7, 700 -di-O-methylcupressuflavone, hinokiflavone, etc.), saccharides, and proteins in the extract, while the absorbance band at 640 cm1 could be regarded as a ‘fingerprint’ of the biomolecules.58 In our previous study,35 the disappearance of the band at 1109 cm1 after the bioreduction showed that polyols (from the sundried Cinnamomum camphora) were mainly responsible for the reduction of the silver and chloroaurate ions. In this present study, the band at 1059 cm1 may be the result of the CO groups of the reducing sugars, flavonoids, saccharides, and proteins in the extract. The reducing sugars among the saccharides acted as reductants while the other saccharides as protecting agents. Undoubtedly, the flavonoids, i.e. amentoflavone, cupressuflavone,56 7,700 -di-Omethylcupressuflavone,56 hinokiflavone,56 were also excellent reductants for reduction of silver ions. The proteins in the C. Platycladi extract did not work in the reduction of silver ions as they were much weaker than the reducing sugars and flavonoids. Our previous research excluded the possibility for proteins to be reductants in the biosynthesis of Au NPs by foliar broths.52 Therefore, the band at 1059 cm1 weakened after the bioreduction as the reducing sugars and flavonoids were involved in the reduction of silver ions. Based on the FTIR spectra and the biomolecular components of the extract, our interest was in understanding the effect of the extract on the formation mechanism of the AgNPs especially, due to the pronounced differences in shapes at different initial AgNO3 concentrations (C0) and reaction temperature. In the polyol synthesis (extensive study of silver nanostructures), both oxygen and nitrogen atoms (of the pyrrolidone unit) facilitated the protection of the silver nanostructures by the adsorption of PVP onto their surfaces.59 Similarly, the oxygen atoms of the biomolecular components in our extract facilitated the stabilization of the AgNPs by the adsorption of the components onto the particle surface; in which the reducing sugars, flavonoids, saccharides, and proteins played a dual role as reducing and protecting agents (biomolecules). At 30 °C and for C0 = 1 mM, the flavonoids were mainly responsible for the reduction of the silver ions implying that the other components might largely have acted as protecting agents of the AgNPs, producing well protected (during the growth process) and spheroidal AgNPs as Ag atoms were slowly but 9102



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Figure 9. (a) Reducing sugar, (b) flavonoid, (c) saccharide, and (d) protein contents in the extract before reaction and after 1 h reaction.

Figure 10. Antioxidant activity of the extract before reaction and after 1 h reaction.

limitedly produced. However, for C0 = 3 and 5 mM, the accelerated reduction of the silver ions was by the reducing sugars; the flavonoids showed strong reductive ability, while the other components exhibited slightly changed protecting ability resulting in the excessive production of Ag atoms and the anisotropic growth of the AgNPs, leading to the formation of plate-like AgNPs due to, perhaps, the capping action of the proteins on the silver nuclei.29 Silver nuclei may form through homogeneous nucleation at 90 °C. Remarkable activation of the reducing

Figure 11. Typical FTIR spectra of the biomass (a) before and (b) after bioreduction of silver ions.

sugars and flavonoids at 90 °C led to the rapid reduction and the subsequent homogeneous nucleation of the silver nuclei which was formed simultaneously at the commencement of the bioreduction(with little subsequent nucleation) leading to the formation of AgNPs with narrow size distribution. The silver nuclei produced at 60 °C were more homogeneous than those at 9103


Industrial & Engineering Chemistry Research 30 °C but less homogeneous than those at 90 °C. Therefore, with the same initial AgNO3 concentrations at 30, 60, and 90 °C, the AgNPs produced at 90 °C possessed the narrower size distribution. 3.3. Antibacterial Activity of the Silver Nanoparticles. 3.3.1. Solid Agar Plate Test. The AgNPs were prepared by reducing aqueous AgNO3 (5 mM) with the extract at 90 °C. The mean diameter of the AgNPs, which were well dispersed in silver hydrosol, was 18.4 ( 4.6 nm. AAS analysis indicates that the conversion of silver ions was 100% ruling out the possible effect of silver ions on the antibacterial properties. The inhibitory percentage of the AgNPs against E. coli and S. aureus at different Ag concentrations (in the agar plates) shows that the higher the Ag concentration, the higher the inhibitory (Figure 12). Against E. coli, the minimal inhibitory concentration (MIC) was 1.4 ppm and minimal bactericidal concentration (MBC) of 27 ppm, while the MIC against S. aureus was 5.4 ppm. Figure S3 (in the Supporting Information) shows the growth of fewer colonies grew on the agar plate with the addition of more AgNPs. Obviously, the growth of E. coli was completely inhibited in the presence of the AgNPs with Ag concentration (CAg) of 27 ppm. Sondi et al. prepared approximately 12 nm AgNPs by reducing AgNO3

Figure 12. Correlative curves between Ag concentration and inhibitory percentage of the AgNPs against E. coli (2) and S. aureus (4), respectively.

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solutions with ascorbic acid with Daxad 19 as a stabilizing agent and obtained an inhibitory of about 95% (comparable with our results of 100%) against E. coli with the same Ag concentration (CAg = 27 ppm) in the agar plates.60 In contrast, the AgNPs had an insignificant effect on the growth of S. aureus (gram-positive bacteria) with a MIC of 5.4 ppm, almost 5 times less than that against E. coli. The antibacterial activity was more pronounced against E. coli than S. aureus; at CAg = 27 ppm, the inhibitory to E. coli was 100% and 16.1% to S. aureus. Even for CAg = 54 ppm, the inhibitory against S. aureus was only 46.3% (Figure S4 in the Supporting Information indicates this phenomenon). The color of the AgNPs hydrosol was dark brown which made it extremely difficult to count the S. aureus colonies (the color covered the colonies) with the increased Ag concentration. 3.3.2. Broth Medium Test. Figure 13 shows the growth curves of E. coli and S. aureus in MH broth medium in the presence of the AgNPs at different Ag concentrations. It indicates that the bacterial growth strongly depended on the Ag concentration in the broth medium; for CAg = 5 ppm, the value of OD600 was close to zero implying ineffective bacterial growth inhibition. However, strong inhibitory action against E. coli was observed at high Ag concentration; for CAg = 15 ppm, the inhibitory time was 10 h and 11 h for CAg = 35 ppm (Figure 13a). For CAg = 50 ppm, the OD600 remained close to zero suggesting that all the bacteria were killed (similar results were obtained against S. aureus) (Figure 13b). Sathishkumar et al. also demonstrated that the E. coli strain could be completely killed by the AgNPs (CAg = 50 ppm) synthesized by Cinnamon zeylanicum bark extract and powder.45 The AgNPs might adsorb onto the E. coli strain through interaction between the nanoparticles and negatively charged functional groups.49 Shrivastava et al. reported that the interaction of AgNPs was much less with the S. aureus strain.49 However, in this work, the strong antibacterial effect of the biogenic AgNPs showed that they also dramatically interacted with the S. aureus strain in broth medium. In recent years, antibacterial materials impregnated with AgNPs such as coatings,61,62 textiles,63 and wood flooring64 have attracted much interest. The antibacterial activity of the biogenic AgNPs showed the AgNPs might have potential application in such antibacterial materials. Immobilization of the AgNPs onto the materials might be achieved by sonochemical, solgel methods, etc.6164 The biosynthesis by C. Platycladi extract endows the AgNPs with low-cost and green route for production of antibacterial materials.

Figure 13. Growth curves of (a) E. coli and (b) S. aureus in the presence of the AgNPs at different Ag concentrations, respectively. 9104



4. CONCLUSIONS The synthesis of Ag nanoparticles (AgNPs) by reducing AgNO3 with the C. Platycladi extract was investigated; the formation mechanism was discussed, and the antibacterial activities were evaluated. Increasing the initial AgNO3 concentration at 30 or 60 °C increased the mean size and widened the size distribution of the AgNPs leading to red shift and broadening of the SPR absorption. The biomolecules (reducing sugars, flavonoids, saccharides, and proteins in the extract) played a dual role as reducing and protecting agents; the reducing sugars and flavonoids were mainly responsible for the bioreduction of silver ions, compared with saccharides and proteins. Moreover, bioreduction of silver ions by flavonoids and reducing sugars could be promoted at higher temperature, especially at 90 °C and resulting in the formation of AgNPs (18.4 ( 4.6 nm) with narrow size distribution through homogeneous nucleation. The AgNPs exhibited excellent antibacterial activity against E. coli and S. aureus at the Ag concentration of 50 ppm; we therefore propose their potential application in fabricating silver-loaded antibacterial materials. This work provides useful technical parameters for industrialization of the biosynthetic technique and further antibacterial application of the AgNPs. Furthermore, the elucidation of bioreductive mechanism of silver ions by measuring the change of the biomolecular concentrations in plant extract exemplifies understanding the formation mechanism of such biogenic AgNPs. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figures S1S4. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Authors

*Phone: (+86) 592-2189595. Fax: (+86)592- 2184822. E-mail: [email protected] (Q.L.) and [email protected] (D.S.). Author Contributions §

The first two authors contributed equally to this work.

’ ACKNOWLEDGMENT This work was supported by the national Natural Science Foundation of China (Nos. 21036004, 20776120, and 20976146), the national 863 Program of China (No. 2007AA03Z347), the Natural Science Foundation of Fujian Province of China (Grant Nos. 2010J05032 and 2010J01052), and the Fundamental Research Funds for Central Universities (No.2010121051). We thank Dr. Abdul-Rauf Ibrahim for editing the English. ’ REFERENCES (1) Klaus, T.; Joerger, R.; Olsson, E.; Granqvist, C. G. Silver-based crystalline nanoparticles, microbially fabricated. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 13611. (2) Fu, J. K.; Liu, Y. Y.; Gu, P. Y.; Tang, D. L.; Lin, Z. Y.; Yao, B. X.; Weng, S. Z. Spectroscopic characterization on the biosorption and bioreduction of Ag(I) by Lactobacillus sp. A09. Acta Phys.-Chim. Sin. 2000, 16, 779. (3) Mukherjee, P.; Ahmad, A.; Mandal, D.; Senapati, S.; Sainkar, S. R.; Khan, M. I.; Parishcha, R.; Ajaykumar, P. V.; Alam, M.; Kumar, R.; Sastry, M. Fungus-mediated synthesis of silver nanoparticles and their immobilization in the mycelial matrix: A novel biological approach to nanoparticle synthesis. Nano Lett. 2001, 1, 515.

ARTICLE

(4) Mukherjee, P.; Ahmad, A.; Mandal, D.; Senapati, S.; Sainkar, S. R.; Khan, M. I.; Ramani, R.; Parischa, R.; Ajayakumar, P. V.; Alam, M.; Sastry, M.; Kumar, R. Bioreduction of AuCl4 ions by the fungus, Verticillium sp. and surface trapping of the gold nanoparticles formed. Angew. Chem., Int. Ed. 2001, 40, 3585. (5) Yong, P.; Rowson, N. A.; Farr, J. P. G.; Harris, I. R.; Macaskie, L. E. Bioreduction and biocrystallization of palladium by Desulfovibrio desulfuricans NCIMB 8307. Biotechnol. Bioeng. 2002, 80, 369. (6) De Windt, W.; Aelterman, P.; Verstraete, W. Bioreductive deposition of palladium (0) nanoparticles on Shewanella oneidensis with catalytic activity towards reductive dechlorination of polychlorinated biphenyls. Environ. Microbiol. 2005, 7, 314. (7) Rautaray, D.; Ahmad, A.; Sastry, M. Biosynthesis of CaCO3 crystals of complex morphology using a fungus and an actinomycete. J. Am. Chem. Soc. 2003, 125, 14656. (8) Rautaray, D.; Ahmad, A.; Sastry, M. Biological synthesis of metal carbonate minerals using fungi and actinomycetes. J. Mater. Chem. 2004, 14, 2333. (9) Rautaray, D.; Sanyal, A.; Adyanthaya, S. D.; Ahmad, A.; Sastry, M. Biological synthesis of strontium carbonate crystals using the fungus Fusarium oxysporum. Langmuir 2004, 20, 6827. (10) Bansal, V.; Rautaray, D.; Ahmad, A.; Sastry, M. Biosynthesis of zirconia nanoparticles using the fungus Fusarium oxysporum. J. Mater. Chem. 2004, 14, 3303. (11) Bansal, V.; Rautaray, D.; Bharde, A.; Ahire, K.; Sanyal, A.; Ahmad, A.; Sastry, M. Fungus-mediated biosynthesis of silica and titania particles. J. Mater. Chem. 2005, 15, 2583. (12) Bansal, V.; Ahmad, A.; Sastry, M. Fungus-mediated biotransformation of amorphous silica in rice husk to nanocrystalline silica. J. Am. Chem. Soc. 2006, 128, 14059. (13) Sanyal, A.; Rautaray, D.; Bansal, V.; Ahmad, A.; Sastry, M. Heavy-metal remediation by a fungus as a means of production of lead and cadmium carbonate crystals. Langmuir 2005, 21, 7220. (14) Bansal, V.; Poddar, P.; Ahmad, A.; Sastry, M. Room-temperature biosynthesis of ferroelectric barium titanate nanoparticles. J. Am. Chem. Soc. 2006, 128, 11958. (15) Jha, A. K.; Prasad, K. Ferroelectric BaTiO3 nanoparticles: Biosynthesis and characterization. Colloids Surf., B 2010, 75, 330. (16) Bharde, A.; Rautaray, D.; Bansal, V.; Ahmad, A.; Sarkar, I.; Yusuf, S. M.; Sanyal, M.; Sastry, M. Extracellular biosynthesis of magnetite using fungi. Small 2006, 2, 135. (17) Ahmad, A.; Jagadale, T.; Dhas, V.; Khan, S.; Patil, S.; Pasricha, R.; Ravi, V.; Ogale, S. Fungus-based synthesis of chemically difficult-tosynthesize multifunctional nanoparticles of CuAlO2. Adv. Mater. 2007, 19, 3295. (18) Ahmad, A.; Mukherjee, P.; Mandal, D.; Senapati, S.; Khan, M. I.; Kumar, R.; Sastry, M. Enzyme mediated extracellular synthesis of CdS nanoparticles by the fungus, Fusarium oxysporum. J. Am. Chem. Soc. 2002, 124, 12108. (19) Kang, S. H.; Bozhilov, K. N.; Myung, N. V.; Mulchandani, A.; Chen, W. Microbial synthesis of CdS nanocrystals in genetically engineered E-coli. Angew. Chem., Int. Ed. 2008, 47, 5186. (20) Jha, A. K.; Prasad, K.; Prasad, K. A green low-cost biosynthesis of Sb2O3 nanoparticles. Biochem. Eng. J. 2009, 43, 303. (21) Mandal, D.; Bolander, M. E.; Mukhopadhyay, D.; Sarkar, G.; Mukherjee, P. The use of microorganisms for the formation of metal nanoparticles and their application. Appl. Microbiol. Biotechnol. 2006, 69, 485. (22) Mohanpuria, P.; Rana, N. K.; Yadav, S. K. Biosynthesis of nanoparticles: technological concepts and future applications. J. Nanopart. Res. 2008, 10, 507. (23) Kumar, V.; Yadav, S. K. Plant-mediated synthesis of silver and gold nanoparticles and their applications. J. Chem. Technol. Biotechnol. 2009, 84, 151. (24) Gardea-Torresdey, J. L.; Gomez, E.; Peralta-Videa, J. R.; Parsons, J. G.; Troiani, H.; Jose-Yacaman, M. Alfalfa sprouts: A natural source for the synthesis of silver nanoparticles. Langmuir 2003, 19, 1357. (25) Shankar, S. S.; Ahmad, A.; Sastry, M. Geranium leaf assisted biosynthesis of silver nanoparticles. Biotechnol. Prog. 2003, 19, 1627. 9105


Industrial & Engineering Chemistry Research (26) Shankar, S. S.; Rai, A.; Ahmad, A.; Sastry, M. Rapid synthesis of Au, Ag, and bimetallic Au core-Ag shell nanoparticles using Neem (Azadirachta indica) leaf broth. J. Colloid Interface Sci. 2004, 275, 496. (27) Ankamwar, B.; Damle, C.; Ahmad, A.; Sastry, M. Biosynthesis of gold and silver nanoparticles using Emblica officinalis fruit extract, their phase transfer and transmetallation in an organic solution. J. Nanosci. Nanotechnol. 2005, 5, 1665. (28) Chandran, S. P.; Chaudhary, M.; Pasricha, R.; Ahmad, A.; Sastry, M. Synthesis of gold nanotriangles and silver nanoparticles using Aloe vera plant extract. Biotechnol. Prog. 2006, 22, 577. (29) Xie, J. P.; Lee, J. Y.; Wang, D. I. C.; Ting, Y. P. Silver nanoplates: From biological to biomimetic synthesis. ACS Nano 2007, 1, 429. (30) Li, S. K.; Shen, Y. H.; Xie, A. J.; Yu, X. R.; Qiu, L. G.; Zhang, L.; Zhang, Q. F. Green synthesis of silver nanoparticles using Capsicum annuum L. extract. Green Chem. 2007, 9, 852. (31) Nadagouda, M. N.; Varma, R. S. Green synthesis of silver and palladium nanoparticles at room temperature using coffee and tea extract. Green Chem. 2008, 10, 859. (32) Vilchis-Nestor, A. R.; Sanchez-Mendieta, V.; Carnacho-Lopez, M. A.; Gomez-Espinosa, R. M.; Camacho-Lopez, M. A.; ArenasAlatorre, J. A. Solventless synthesis and optical properties of Au and Ag nanoparticles using Camellia sinensis extract. Mater. Lett. 2008, 62, 3103. (33) Jha, A. K.; Prasad, K.; Kumar, V.; Prasad, K. Biosynthesis of silver nanoparticles using Eclipta leaf. Biotechnol. Prog. 2009, 25, 1476. (34) Prathna, T. C.; Chandrasekaran, N.; Raichur, A. M.; Mukherjee, A. Kinetic evolution studies of silver nanoparticles in a bio-based green synthesis process. Colloids Surf., A 2011, 377, 212. (35) Huang, J. L.; Li, Q. B.; Sun, D. H.; Lu, Y. H.; Su, Y. B.; Yang, X.; Wang, H. X.; Wang, Y. P.; Shao, W. Y.; He, N.; Hong, J. Q.; Chen, C. X. Biosynthesis of silver and gold nanoparticles by novel sundried Cinnamomum camphora leaf. Nanotechnology 2007, 18, 105104. (36) Huang, J. L.; Lin, L. Q.; Li, Q. B.; Sun, D. H.; Wang, Y. P.; Lu, Y. H.; He, N.; Yang, K.; Yang, X.; Wang, H. X.; Wang, W. T.; Lin, W. S. Continuous-flow biosynthesis of silver nanoparticles by lixivium of sundried Cinnamomum camphora leaf in tubular microreactors. Ind. Eng. Chem. Res. 2008, 47, 6081. (37) Lin, L. Q.; Wang, W. T.; Huang, J. L.; Li, Q. B.; Sun, D. H.; Yang, X.; Wang, H. X.; He, N.; Wang, Y. P. Nature factory of silver nanowires: Plant-mediated synthesis using broth of Cassia fistula leaf. Chem. Eng. J. 2010, 162, 852. (38) Song, J. Y.; Kim, B. S. Rapid biological synthesis of silver nanoparticles using plant leaf extracts. Bioproc. Biosyst. Eng. 2009, 32, 79. (39) Rai, M.; Yadav, A.; Gade, A. Silver nanoparticles as a new generation of antimicrobials. Biotechnol. Adv. 2009, 27, 76. (40) Sharma, V. K.; Yngard, R. A.; Lin, Y. Silver nanoparticles: Green synthesis and their antimicrobial activities. Adv. Colloid. Interface 2009, 145, 83. (41) Martinez-Castanon, G. A.; Nino-Martinez, N.; MartinezGutierrez, F.; Martinez-Mendoza, J. R.; Ruiz, F. Synthesis and antibacterial activity of silver nanoparticles with different sizes. J. Nanopart. Res. 2008, 10, 1343. (42) Panacek, A.; Kvitek, L.; Prucek, R.; Kolar, M.; Vecerova, R.; Pizurova, N.; Sharma, V. K.; Nevecna, T.; Zboril, R. Silver colloid nanoparticles: Synthesis, characterization, and their antibacterial activity. J. Phys. Chem. B 2006, 110, 16248. (43) Lok, C. N.; Ho, C. M.; Chen, R.; He, Q. Y.; Yu, W. Y.; Sun, H. Z.; Tam, P. K. H.; Chiu, J. F.; Che, C. M. Proteomic analysis of the mode of antibacterial action of silver nanoparticles. J. Proteome Res. 2006, 5, 916. (44) Morones, J. R.; Elechiguerra, J. L.; Camacho, A.; Holt, K.; Kouri, J. B.; Ramirez, J. T.; Yacaman, M. J. The bactericidal effect of silver nanoparticles. Nanotechnology 2005, 16, 2346. (45) Sathishkumar, M.; Sneha, K.; Won, S. W.; Cho, C. W.; Kim, S.; Yun, Y. S. Cinnamon zeylanicum bark extract and powder mediated green synthesis of nano-crystalline silver particles and its bactericidal activity. Colloids Surf. B-Biointerf. 2009, 73, 332. (46) Krishnaraj, C.; Jagan, E. G.; Rajasekar, S.; Selvakumar, P.; Kalaichelvan, P. T.; Mohan, N. Synthesis of silver nanoparticles using

ARTICLE

Acalypha indica leaf extracts and its antibacterial activity against water borne pathogens. Colloids Surf., B 2010, 76, 50. (47) Nabikhan, A.; Kandasamy, K.; Raj, A.; Alikunhi, N. M. Synthesis of antimicrobial silver nanoparticles by callus and leaf extracts from saltmarsh plant, Sesuvium portulacastrum L. Colloids Surf., B 2010, 79, 488. (48) Veerasamy, R.; Xin, T. Z.; Gunasagaran, S.; Xiang, T. F. W.; Yang, E. F. C.; Jeyakumar, N.; Dhanaraj, S. A. Biosynthesis of silver nanoparticles using mangosteen leaf extract and evaluation of their antimicrobial activities. J. Saudi Chem. Soc. 2011, 15, 113. (49) Shrivastava, S.; Bera, T.; Roy, A.; Singh, G.; Ramachandrarao, P.; Dash, D. Characterization of enhanced antibacterial effects of novel silver nanoparticles. Nanotechnology 2007, 18, 225103. (50) Xu, J. Experiment and guide for biochemistry; China Medical Science and Technology Press: Beijing, 2009. (51) Chen, F. A.; Wu, A. B.; Shieh, P.; Kuo, D. H.; Hsieh, C. Y. Evaluation of the antioxidant activity of Ruellia tuberosa. Food Chem. 2006, 94, 14. (52) Zhou, Y.; Lin, W. S.; Huang, J. L.; Wang, W. T.; Gao, Y. X.; Lin, L. Q.; Li, Q. B.; Lin, L.; Du, M. M. Biosynthesis of gold nanoparticles by foliar broths: roles of biocompounds and other attributes of the extracts. Nanoscale Res. Lett. 2010, 5, 1351. (53) Wiley, B. J.; Im, S. H.; Li, Z. Y.; McLellan, J.; Siekkinen, A.; Xia, Y. A. Maneuvering the surface plasmon resonance of silver nanostructures through shape-controlled synthesis. J. Phys. Chem. B 2006, 110, 15666. (54) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chemistry and properties of nanocrystals of different shapes. Chem. Rev. 2005, 105, 1025. (55) Daniel, M. C.; Astruc, D. Gold nanoparticles: Assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology. Chem. Rev. 2004, 104, 293. (56) Zhou, J. J.; Xie, G. R.; Yan, X. J. Traditional Chinese medicines: Molecular structures, natural sources and applications; Chemical and Industrial Press: Beijing, P. R. China, 2004. (57) Mc Carthy, T. L.; Kerry, J. P.; Kerry, J. F.; Lynch, P. B.; Buckley, D. J. Assessment of the antioxidant potential of natural food and plant extracts in fresh and previously frozen pork patties. Meat Sci. 2001, 57, 177. (58) Stuart, B. Infrared spectroscopy: fundamentals and applications; Wiley-VCH Verlag Gmbh & Co. KGaA: Weinheim, 2004. (59) Xiong, Y. J.; Washio, I.; Chen, J. Y.; Cai, H. G.; Li, Z. Y.; Xia, Y. N. Poly(vinyl pyrrolidone): A dual functional reductant and stabilizer for the facile synthesis of noble metal nanoplates in aqueous solutions. Langmuir 2006, 22, 8563. (60) Sondi, I.; Salopek-Sondi, B. Silver nanoparticles as antimicrobial agent: a case study on E-coli as a model for Gram-negative bacteria. J. Colloid Interface Sci. 2004, 275, 177. (61) Gottesman, R.; Shukla, S.; Perkas, N.; Solovyov, L. A.; Nitzan, Y.; Gedanken, A. Sonochemical coating of paper by microbiocidal silver nanoparticles. Langmuir 2011, 27, 720. (62) Mahltig, B.; Textor, T. Silver containing sol-gel coatings on polyamide fabrics as antimicrobial finish-description of a technical application process for wash permanent antimicrobial effect. Fiber Polym. 2010, 11, 1152. (63) Zhang, F.; Wu, X. L.; Chen, Y. Y.; Lin, H. Application of silver nanoparticles to cotton fabric as an antibacterial textile finish. Fiber Polym. 2009, 10, 496. (64) Kim, S.; Kim, H. J. Anti-bacterial performance of colloidal silvertreated laminate wood flooring. Int. Biodeter. Biodegr. 2006, 57, 155.

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Biogenic Silver Nanoparticles by Cacumen Platycladi Extract

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