Yucca-derived synthesis of gold nanomaterial and their catalytic potential
© Krishnamurthy et al.; licensee Springer. 2014
Received: 15 September 2014
Accepted: 5 November 2014
Published: 23 November 2014
AuNPs ranging in 20 to 300 nm size were synthesized at a room temperature using Yucca filamentosa leaf extract. Diverse nanomaterial morphologies were obtained by varying the extract concentration, reaction pH, and temperature. While low volumes of extract (0.25 and 0.5 mL) induced the formation of microscale Au sheets with edge length greater than 1 μm, high volumes yielded spherical particles ranging from 20 to 200 nm. Varying pH of the solution significantly influenced the particle shape with the production of largely spherical particles at pH 5 to 6 and truncated triangles at pH 2. Separation of multidimensional nanostructures was achieved using a novel method of sucrose density gradient centrifugation. The catalytic function of Yucca-derived AuNPs was demonstrated by degradation of a wastewater dye: methylene blue using spectrophotometric measurements over time. Treatment with Au nanosheets and spheres demonstrated methylene blue degradation approximately 100% greater than the activity in control at 60 min.
KeywordsSpherical AuNPs Anisotropic Au nanosheets Photocatalysis Gradient separation
Nanoparticles have attracted much attention in recent years due to their unique optoelectronic and physicochemical properties. Their potential applications range from the use in medicine, optoelectronic devices, sustainable energy systems, and food to catalytic functions. A number of green chemical methods employing plant, plant extract, microbes, antioxidants, and ionic liquids have been proposed for the synthesis of environmentally benign metal nanoparticles. One of the simplest methods for gold nanoparticle (AuNP) synthesis is using citrate as a reducing agent for gold salts. However, a major limitation using citrate for synthesis is that AuNPs greater than 50 nm are generally polydispersed in nature. Furthermore, the use of organic solvents and capping agents for controlling the morphology may not always render the synthesized AuNPs as biocompatible and non-toxic.
Biological systems, either employing plants or microbes, can be a manufacturing route once pathways are delineated, standardized, and scaled up. The benefit of ‘green chemistry’ can be two-fold. First and foremost, it is an environment-friendly, less intrusive technique with negligible contribution to the generation of hazardous substances. In addition, such biocompatible nanomaterials can be expected to be less reactive to biomolecules, cells, human health, or environment because these products carry surface coatings of biological origin[10, 11]. Elemental gold (Auo) is considered inert and has been used in food and traditional medicine. However, recent in vivo studies highlight potential toxicity of engineered AuNPs[12, 13]. It has been reported that the toxicity of (gold) nanoparticles depends on the size, shape, crystal structure, and surface radicles/conjugates.
Recently, various plant products such as biomass of wheat, oat, alfalfa, and leaf extracts of geranium, lemongrass, neem, tamarind, Aloe vera, or fruit extract of Emblica officinalis were employed for rapid extracellular reduction of ionic Au (III) to Au (0). In planta fabrication of AuNPs has been also reported before[10, 15]. Most studies on biomimetic fabrication of nanoparticles were limited by the uncontrolled synthesis of polydisperse nanoparticles and difficulty in separation of particles of various shapes and sizes. Furthermore, physicochemical and optoelectronic properties of the nanomaterial are a strong function of particle shape and size. Especially, non-spherical AuNPs are known for their optical properties due to their ability to exhibit multiple absorption bands. Au nanoprisms, for example, are very promising since they have found applications in cancer therapeutics and biosensing[17, 18]. Apart from biomedical applications, AuNPs can be effectively used in applications such as catalysis[11, 19] and optical sensing. Therefore, producing biologically derived nanostructures of a desirable size and morphology is critical to their commercial applicability. In light of the above observation, the objectives of this study were to determine the optimal conditions of (i) plant extract concentration, (ii) potassium tetrachloroaurate concentration, (iii) incubation time, (iv) reaction pH, (v) reaction temperature, and (vi) fractionation method for the production of nanomaterial using Yucca filamentosa leaf extract. This investigation also demonstrates the catalytic function of a suitable nanostructure using reduction of a wastewater dye.
Preparation of Y. filamentosa leaf powder and extract
Extracts from Hemerocallis fulva (daylily), Ilex verticillata (Hollyberry), Hedera helix (English ivy), and Y. filamentosa (Yucca) were separately prepared and reacted under conditions of a 24-h incubation, 1 mM KAuCl4 concentration, temperature of 37°C, and with no adjustment in pH. Based on variable spectrophotometric measurements, the plant extract showing the highest absorbance, Y. filamentosa was chosen for studies in this investigation. (Additional file1: Figure S1c) Y. filamentosa leaves was collected form a local plant nursery, washed to remove any impurities, and dried in an oven for 2 days at 60°C. The leaves were cut into small pieces, pulverized, and sieved using a 20-mesh sieve to get a uniform size range. The fine powder obtained was used for the preparation of Y. filamentosa leaf extract (YFLE). Extract stock was prepared by adding 2.5 g of leaf powder to a 500-mL Erlenmeyer flask with 100 mL sterile nanopure water and boiled for 2 min. The boiled mixture was allowed to cool to a room temperature and then filtered using a 0.44-micron millipore sterile filter to remove particulate matter. The same procedure was used for the other plants. Various volumes of the extract (0.25, 0.5, 0.75, 1.0, 1.25, 1.5, and 2.0 mL) were used from the above stock solution. A working volume of 10 mL was maintained for all experiments.
Biosynthesis of AuNPs
Potassium chloroaurate (KAuCl4) was purchased from Sigma-Aldrich, St. Louis, MO, USA. For kinetic studies, 1 mM KAuCl4 solution was reacted to 1 mL of YFLE at a room temperature (22°C to 24°C) and pH 4.2 for 6 to 48 h. The following conditions were varied for the optimization of the process of AuNP synthesis: (a) YFLE volumes: 0.25, 0.5, 0.75, 1.0 , 1.25, and 1.5 mL; (b) KAuCl4 concentrations: 0.5, 1, 1.5, and 2 mM; (c) pH: pH 1 to pH 6; (d) temperature regime (10°C to 100°C): 10°C, 20°C, 30°C, 40°C, and 100°C.
Characterization of AuNPs
The presence of synthesized AuNPs was monitored periodically using Perkin Elmer’s LAMBDA 35 UV/vis spectrophotometer (LAMBDA 35 UV–vis spectrophotometer, Perkin Elmer, Waltham, MA, USA). For analysis, 0.1 mL of samples was diluted to 1 mL using nanopure water. The UV–vis spectra were monitored as a function of reaction time, biomaterial dosage, pH, and KAuCl4 concentrations. Transmission electron microscope (TEM) images were obtained in order to characterize the size and morphology of the synthesized AuNPs. One part of AuNP stock was diluted with five parts of nanopure water. Five microliters of the diluted AuNP solution was placed on formvar-coated copper grids and allowed to dry over a hot plate. Samples were randomly scanned through using a 120-CX TEM (JEOL JEM, Peabody, MA, USA) at 100 kV. X-ray diffraction was carried out on a Rigaku Miniflex-II (Rigaku, Shibuya-ku, Japan), with a step size of 0.02 using Cu K α radiation.
Purification and sucrose gradient separation of synthesized biogenic AuNPs
The AuNPs synthesized using plant extract were initially passed though gel filtration column and eluted using distilled water to remove impurities from AuNP colloid. The purified AuNPs were then separated using sucrose gradient centrifugation. A discontinuous sucrose gradient was prepared by making a five-layer step gradient of different concentration of sucrose (10%, 30%, 40%, 50%, 60%, and 70%) in a 2.4-mL polycarbonate ultracentrifuge tube. The tube was added first with 0.5 mL of 70% sucrose, followed by subsequent layering with 0.3 mL of 60%, 50%, 40%, 30%, and 10% sucrose solutions. Typically, 0.2 mL of Au colloid was sonicated in a bucket type sonicator at 80% amplitude with 10 s on-cycle and 10 s off-cycle, for 10 min, and layered on top of the gradient and centrifuged at 500 rpm for 25 min in an ultracentrifuge (SORVALL RC M120EX, Thermo Fisher Scientific, Waltham, MA, USA ). Fractions of the gradient (0.2 mL each) were collected and characterized using TEM following gradient separation.
Catalytic activity of AuNPs
A heteropolyaromatic dye, methylene blue (MB), was used to evaluate the photocatalytic activity of the various gold nanoparticles. Gold nanoparticles synthesized were diluted to the absorbance of 0.2 at 520 nm using nanopure water. A mixture of stannous chloride and methylene blue was used as a control. Experimental samples contained 3 mL of 1 mM sodium dodecyl sulfate, 10 μL methylene blue, 50 μL stannous chloride, and 100 μL of diluted AuNP solution synthesized under different conditions. Before exposure to illumination, the suspension was stirred in the dark for 10 min to ensure the establishment of adsorption/desorption equilibrium of MB on the sample surfaces. For visible irradiation, direct sunlight was used and all experiments were conducted under similar conditions on the sunny days. Between 12 noon and 1.00 pm (25°C to 27°C), the UV–vis absorption was measured using Perkin Elmer’s LAMBDA 35 UV/vis spectrophotometer (LAMBDA 35 UV/vis spectrophotometer, Perkin Elmer, Waltham, MA, USA). Statistical analysis of photocatalytic activity was analyzed by a one-way analysis of variance (ANOVA) using Microsoft Office Excel 2013. In order to determine the significance, all experimental values were compared to control. Values obtained were the mean of three experiments and were considered significant when p <0.05.
Results and discussion
Synthesis of AuNPs over reaction time
It was observed that the surface plasmon resonance (SPR) band of AuNPS was initially centered at 511 nm and steadily increased as a function of reaction time. A remarkable shift in the peak was noticed as the reaction time advanced from 6 to 24 h. A shift of peak to 545 nm was observed by the end of 24 h, which may be due to an increase in AuNPs size with respect to time. After an incubation period of 24 h, there was no significant increase or shift in SPR peak observed, indicating the complete reduction of Au3+ to Au0 form. UV-visible spectrophotometry is commonly used to monitor the nanomaterial concentration present in a reaction mixture.
Effects of extract volume and KAuCl4 concentration on AuNPs synthesis
Effects of pH and temperature on AuNP biosynthesis using YFLE
AuNP synthesis by Yucca extract was carried out under a range of pH conditions (pH 1 to 6) (Additional file1: Figure S4). TEM analysis demonstrated the efficient formation of AuNPs at acidic pH (pH 3 to 5) (Additional file1: Figure S4A (c-e)). It is evident from Additional file1: Figure S4A (a-f) that decreasing pH of the solution to pH 2 induced an aggregation of AuNPs with as much as 40% of truncated triangles (Additional file1: Figure S5). No nanoparticle formation was observed at pH 1 (Additional file1: Figure S4A). This is probably due to the leaching of gold and denaturation of the biomolecules in a highly acidic condition. The nanoparticles formed at acidic pH (pH 2 and 3) were polydisperse (Additional file1: Figure S4A (b-c)). It is interesting to note that the nanoparticle size decreased with an increase in pH of the solution. Starnes et al. (10) and Sneha et al. (22) observed a similar pattern. Reaction mixtures at a pH 5 to 6 led to the formation of nanoparticles in the range of 10 to 100 nm (Additional file1: Figure S4A e-f). Also increase in pH directly correlated with the formation of spherical particles, almost reaching monodispersity at pH 6. Truncated triangles UV–vis spectral peaks also confirmed the pattern observed above (Additional file1: Figure S4B). The SPR peak significantly dropped at pH 6, indicating a poor yield of AuNPs. Findings in the present study are in agreement with another report wherein pH 3 to 5 was found to be an optimal range. Low pH leads to protonation and neutralization of carboxyl groups in the extract promoting the interaction between the AuCl4- ion and the biomolecule. However, the nanoparticles continue to grow until reductant and biomolecule are available. Thus, the enhanced interaction of AuCl4- ions with biomolecules in acidic pH also contributes to the growth of crystals[22, 32]. This explains the polydispersity in the size of nanoparticles at this range of pH (3 to 5). No studies were possible beyond pH 6 due to the precipitation of gold salt in the solution.
In order to control size of nanoparticle, the effect of different temperatures was scrutinized on AuNP synthesis. TEM analysis revealed the change in the size and shape of the synthesized AuNPs at different temperatures (Additional file1: Figure S6 (a-d)). It is evident from TEM images that the nanoparticles synthesized at 10°C were flocculated and aggregated, the size exceeding a micrometer (Additional file1: Figure S7). As the reaction temperature reached 20°C, the shape of nanoparticles became mainly anisotropic in nature (Additional file1: Figure S6b). AuNPs synthesized at higher temperatures were polydisperse (Additional file1: Figure S6 (c-d). The nanoparticle reduction was rapid at 100°C and the process was complete with few minutes of reaction. The majority of AuNPs synthesized at this temperature were in the size range of 20 to 40 nm, and the particles synthesized were mainly spherical or elliptical in shape along with some tiny triangular nanoparticles (Additional file1: Figure S6d). This pattern of nanoparticle growth at low temperature, obtained in this study, is reasonable, since nucleation and growth is a kinetically driven process. Hence, the rate of reaction slows down at a low temperature which allows nucleation and crystal growth at a slow rate. Whereas, at high temperature, due to a rapid rate of reaction, the primary nucleation is immediately followed by a secondary reduction on the primary nuclei; hence, particle growth is stalled. Furthermore, the high rate of reaction also leads to the fast consumption of available ions, reducing them to zero valent form[22, 28].
Separation of biogenic AuNPs by sucrose density gradient centrifugation
Catalytic activity of AuNPs
We have, for the first time, demonstrated the difference in photocatalytic activity of size-dependent biogenic nanoparticles synthesized under different physicochemical conditions. Microscale anisotropic Au sheets synthesized using 0.5 mL plant extract exhibited better photocatalytic activity than their spherical counter parts. Although a number of reports show the synthesis of biogenic metal nanoparticles, few reports present the practical application of nanoparticles synthesized through biological means. Furthermore, the ease of separation of AuNPs using a sucrose gradient centrifugation is an improvement to research using biological methods for nanoparticle fabrication.
We thank Dr. John Andersland for the TEM analysis at Western Kentucky University (WKU). This research was supported in part by the grant from the National Science Foundation (Award No. MCB-1158507) to S.V.S., and partial financial support from WKU-RCAP Category I grant is also acknowledged.
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