A novel green synthesis and characterization of silver nanoparticles using gum tragacanth and evaluation of their potential catalytic reduction activities with methylene blue and Congo red dyes
© The Author(s). 2016
Received: 14 March 2016
Accepted: 26 July 2016
Published: 13 August 2016
A facile and eco-friendly method for green synthesis of silver nanoparticles (AgNPs) has been developed using gum tragacanth (GT) (Astragalus gummifer), an abundantly available natural phyto-exudate in India, employing a novel method of ultrasonication process.
Silver nanoparticles were prepared by the reduction of silver nitrate solution by the aqueous extract of gum tragacanth by ultrasonication method at 45 °C for about 45 min. The aqueous extract of the gum acts as a reducing as well as stabilizing agent.
The resultant AgNPs were characterized by ultraviolet-visible (UV-Vis) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD) techniques. The influence of gum concentration and silver precursor concentration on the synthesis of AgNPs was studied. The role and effectiveness of nanoparticles in the catalytic reduction of a cationic dye methylene blue (MB), and an anionic azo dye Congo red (CR), were also studied. FTIR analysis revealed that –OH groups present in the gum matrix might be responsible for the reduction of Ag+ into AgNPs. The X-ray diffraction studies indicated that the resulting AgNPs were highly crystalline with face-centered cubic geometry. TEM studies showed that the average particle size of the synthesized AgNPs was 18 ± 2 nm.
The study highlights the green synthesis of GT-capped AgNPs and the rapid reduction of carcinogenic and toxic contaminants such as MB and CR with the help of GT-capped AgNPs in an eco-friendly manner.
KeywordsGum tragacanth Silver nanoparticles Ultra-sonication Catalytic reduction Methylene blue Congo red
Metal nanomaterials, in recent times, play a crucial role in a multitude of applications as they were proved to exhibit remarkable improvements in their electronic, physical, chemical, optical, and biological properties when compared to their pure metal counterparts (Thakkar et al. 2010). These colloidal metal nanoparticles possess unique physical properties owing to their extremely small size and very high surface/volume ratio (Pandey et al. 2016). The extremely small size of these metal nanoparticles offers a very high reactive surface area that contributes to their significant improvement in the properties in comparison to their metal compatriots (Guo and Wang 2007). The extremely small size of nanoparticles, especially in the case of silver nanoparticles (AgNPs), finds their utility in chemical applications such as catalysis, reduction, stabilization, and colorimetric sensors (Murugadoss and Chattopadhyay 2008; Rastogi et al. 2014). The biological applications involving the integration of nanoparticles with biological molecules led to the development of therapeutic medicines, diagnostic tools, antimicrobial agents, drug delivery systems, bioimaging tools, labeling agents, therapies in cancer, etc. (Roy et al. 2015, Song and Kim 2009, Kokila et al. 2015, Daniel and Astruc 2004).
The AgNPs are generally synthesized using various methods such as chemical, photochemical, electrochemical, laser ablation, γ-irradiation, and biological conversion (Solomon et al. 2007; Callegari et al. 2003; Pyatenko et al. 2004; Li et al. 2007; Raju et al. 2014). The most popular method by which AgNPs are synthesized is by reducing agents such as sodium borohydride and sodium citrate. Most of these synthetic methods are hazardous since they employ toxic chemicals leading to environmental degradation (Mehndiratta et al. 2013) and biological hazards. Much effort is being focussed, in recent times, by researchers to mitigate the impact of these harmful chemicals’ utilization through the integration of “green chemistry principles” for the synthesis of metal nanomaterials using plant extracts (Parveen et al. 2016), biosurfactants, etc. There are many natural biopolymers used for metal nanoparticle synthesis such as gum kondagogu (Rastogi et al. 2014), salmalia malabarica gum (Murali krishna et al. 2015) guar gum (Pandey et al. 2012, Pandey and Mishra 2011, Pandey and Mishra 2013, Pandey et al. 2013, Pandey and Mishra 2014, Pandey and Mishra 2016), chitosan (Wang et al. 2015), gum ghatti (Kora et al. 2012) and cyclodextrin (Maciollek and Ritter 2014). These biopolymers act as stabilizing as well as reducing agents. The characteristics of the metal nanoparticles were further improved when these metal nanoparticles were reacted with natural gums such as gum tragacanth (GT) (Raveendran et al. 2003) due to the increase in the number of active sites, i.e., reactive functional groups on nanoparticles through capping process.
Synthetic dyes are extensively used in a variety of industries such as textiles, paper, polymers, adhesives, ceramics, construction, cosmetics, food, glass, paints, ink, soap, and pharmaceuticals (Zalikha et al. 2012). Dye residues form a group of glaring and grave contaminants since they impart color to the wastewater even at a very low concentration level. The catalytic reduction studies of these dyes assume greater significance in the present context as most of these dyes are known to be toxic when inhaled or ingested orally and pose health hazards such as skin and eye irritation in humans. The adverse effects of these synthetic dyes to the environment include the ability to deplete oxygen in the surface waters and streams thereby affecting the very sustenance of aquatic flora and fauna and causing an inhibitory effect on the photosynthetic activity of plants. The anaerobic degradation products of azo dyes are amines which are very toxic, carcinogenic, and mutagenic in nature. The effluents from industries containing these hazardous dyes need to be removed before they are let out for on-land disposal purposes. The treatment technologies such as adsorption, ion exchange (Karcher et al. 2002), filtration, electrode deposition, and chemical precipitation are being widely used to remove the dye contaminants from water. An attempt has been made to alleviate the detrimental effects of these chemicals on the surrounding environment by trying to reduce the dye concentration in the treated effluents such as methylene blue (MB) and Congo red (CR) by employing GT-capped AgNPs as a nanocatalyst. The previous studies on the reduction of MB and CR by using gold and silver nanoparticles (Wanyonyi et al. 2014; Ganapuram et al. 2015) as nanocatalysts were also reported.
GT is a naturally occurring complex, an acidic polysaccharide obtained from the sap of the genus Astragalus, including A. adscendens, A. gummifer, A. brachycalyx, and A. tragacanthus, a native genus of Middle Eastern countries and West and South Asia. GT is a highly viscous, odorless, tasteless, water-soluble mixture of highly branched and heterogeneous hydrophilic polysaccharides obtained from the exudate of roots and bark of the plant which can be dried and powdered (Kora and Arunachalam 2012). The molecular weight of a typical gum is reported to be about 8.4 × 105 g/mol. The biopolymer is a mixture of two polysaccharide fractions viz. water-soluble tragacanthin and water-swellable tragacanthic acid or bassorin. The tragacanthin comprises about 30–40 % in the gum and is a neutral, highly branched, type II arabinogalactan comprising (1→6)- and (1→3)-linked core chains containing galactose and arabinose units and side groups of (1→2)-, (1→3)-, and (1→5)-linked arabinose units occurring as a monosaccharide or oligosaccharides. Bassorin, a pectin component, has a chain of (1→4)-linked α-D-galacturonic acid units, some of which are substituted at O-3 with β-D-xylopyranosyl units and some of these being terminated with D-galactose or L-fucose (Tischer et al. 2002; Hassan et al. 2013; Chenlo et al. 2010). GT is also recognized as a food-grade additive by the Indian Bureau of Standards under Indian Standard IS 7238: 1997.
The present study has been taken up to explore the innumerable potential possibilities that have been foreseen with the green synthesis of AgNPs with GT as a reducing, capping, and stabilizing agent. Also, the attributes of GT which prompted us to get into the study are (i) widespread natural availability, (ii) nontoxic and noncarcinogenic nature, (iii) excellent reducing and stabilizing capability, (iv) compliance with eco-friendly green synthesis principles, and (v) efficient catalytic reduction of MB and CR. The synthesized AgNPs were characterized for ultraviolet-visible (UV-Vis), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM) parameters.
Silver nitrate (AgNO3), NaBH4, CR, and MB, all of analytical grade, were purchased from E. Merck Limited, Mumbai, India, and GT (99 % purity) was purchased from Loba Chemie, Mumbai, India.
Synthesis of AgNPs using GT
A 0.5 % (weight/volume (w/v)) homogeneous gum stock solution was prepared by dissolving 0.5 g of GT in 100 mL of deionized water and stirring overnight at room temperature. The solution was then centrifuged at 6000 rpm for 10 min to separate out any undissolved matter, and the supernatant is used for the experiments to be carried out in the study. A 0.5 % (w/v) solution of AgNO3 is prepared by dissolving 0.5 g of analytical grade AgNO3 in 100 mL of deionized water. Then, 5 mL of gum extract was mixed with 5 mL of AgNO3 solution in a boiling tube, and this mixture was subjected to ultra sonication at 45 °C temperature for a 45-min reaction time (Firdhouse et al. 2012). The conversion of the colorless reaction mixture to the characteristic clear yellow color indicates the formation of AgNPs. A series of samples were prepared by varying the concentration of AgNO3 (0.1 to 0.5 %) while keeping the concentration of GT (0.5 %) constant. Likewise, the second set of samples were prepared by varying the concentration of GT (0.1 to 0.5 %), keeping the concentration of AgNO3 constant (0.5 %) at the same sonication conditions as mentioned earlier (45 °C for 45 min).
Characterization of silver nanoparticles
The UV-Vis spectral analysis of GT-AgNP solution was carried out on a dual-beam UV-Vis spectrophotometer (Shimadzu-3600, Kyoto, Japan). The FTIR spectra of GT alone and GTcapped AgNPs were recorded using FTIR spectrophotometer (model: IRAffinity-1, Shimadzu Corporation, Japan) equipped with attenuated total reflectance (ATR) accessory in the scanning range of 650–4000 cm−1. XRD studies of GT-AgNPs were carried out on X’pert Pro MRD X-ray diffractometer (PANalytical BV, The Netherlands) operating at 40 kV and a current of 30 mA at a scan rate of 0.388 min−1. TEM analysis was conducted for the morphology and size distribution measurements of the GT-AgNPs using a transmission electron microscope (model: 1200 EX, JEOL Ltd., Japan) operated at an accelerating voltage of 200 kV, casting nanoparticle dispersion on carbon-coated copper grids and allowing for drying at room temperature. The particle size distribution was measured by using ImageJ software analysis of TEM micrographs. Zeta potential was determined using dynamic light scattering (Malvern instrument Ltd., Malvern, UK).
MB and CR were subjected to reduction using sodium borohydride in the presence of AgNPs in order to assess the efficacy of the catalytic activity of the synthesized AgNPs (Suvith and Philip 2014; Junejo et al. 2014). Freshly prepared 1.0 mL of 10 mM sodium borohydride solution was mixed with 1.5 mL of 1 mM MB, and the mixture was made up to 10 mL using double-distilled water and then stirred for 5 min. Similarly, freshly prepared 1.0 mL of 10 mM NaBH4 solution was mixed with 1.5 mL of 1 mM CR, and the solution mixture was made up to 10 mL using double-distilled water and then stirred for 5 min as well. Sufficient quantities of synthesized AgNPs were added to both these solutions and mixed for 30 min with good agitation, and the UV-Vis spectrum of the reaction mixture of MB was recorded at 1-min intervals of time for a period of 12 min and a wavelength range of 400–750 nm at 25 °C whereas, for CR, the absorption spectrum was recorded at a wavelength range between 300 and 650 nm with 1-min intervals of time for a period of 15 min at 25 °C (Ganapuram et al. 2015). The test was conducted in a standard quartz cuvette of about 3.0-mL volume. The rate constant of the redox reaction was dependent on the variation in absorption band at 664 nm as a function of time.
Results and discussion
UV-Vis spectral analysis
Catalytic activity of AgNPs
In the reaction mixture, a higher concentration of NaBH4 was employed as compared to that of both MB and CR, which results in the rise of the pH of the whole system. This, in turn, results in retarding the degradation of BH4 − ions. The liberated H+ ions create a purging atmosphere over the reaction mixture thereby effectively inhibiting the aerial oxidation of reduced MB and CR products. The catalytic reduction aided by metallic nanoparticles is being effectively carried out by transferring electrons from electron donor species BH4 − to electron acceptor species MB and CR mediated by GT-capped AgNPs and thus reduces the activation energy and contributes to the stabilization of the system. As the NaBH4 concentration in the system is much higher as compared to that of MB and CR, the reaction rate can be assumed to be independent of the NaBH4 concentration, and hence, a pseudo-first-order kinetics could be used to evaluate the kinetic reaction rates of the present catalytic reduction systems. A linear correlation between ln(A 0/A t ) (where A 0 is the absorbance at the initial time, A t is the absorbance at the specific reaction time) at the room temperature establishes that the reaction is of pseudo-first order. The pseudo-first-order rate constants (k) for the reduction reactions of MB (Fig. 9c) and CR (Fig. 9f) calculated from the slope are 0.182 and 0.148 min−1, respectively. The catalyst turnover number (TON) and the turnover frequency (TOF) are two important parameters used to determine the catalyst efficiency. In heterogeneous catalysis, the TON is the number of reactant molecules that 1 g of catalyst can convert into products (Lin et al. 2013). The TOF is simply TON/time. For MB and CR dyes using AgNPs as catalysts at a rate of 1.1 g/L dose, the TOF was found to be 2.6 × 1016 and 2.1 × 1016 molecules/g/s, respectively.
In this paper, we report a simple, eco-friendly green synthesis of AgNPs employing naturally occurring and renewable GT as a reducing and stabilizing agent as against using harsher and toxic chemical entities. The nanoparticles so produced here are highly stable as well as reactive due to reactive functional groups present on the attached natural gum entity. The utilization of these green synthesized AgNPs in a plethora of crucial applications such as, among one of many, the catalytic reduction of carcinogenic dyes like MB and CR was reported. As we can see, the greener synthesis of AgNPs, by using exudates from the renewable plant resources, follows the very tenets of green chemistry, and hence, the process can be dubbed as eco-friendly in the true sense. It was very much evident from the experimental results that the catalytic reduction of MB and CR in the presence of NaBH4 with AgNPs as a catalyst was near complete indicating the effectiveness of the nanocatalyst used. The high degree of catalytic ability of GT-capped AgNPs in accelerating the catalytic reduction certainly draws greater importance due to the cleaner and greener approach.
AgNPs, silver nanoparticles; cm, centimeter; CR, Congo red; FTIR, Fourier transform infrared; g, gram; GT, gum tragacanth; MB, methylene blue; min, minute; mL, milliliter; nm, nanometer; rpm, revolutions per minute; TEM, transmission electron microscopy; w/v, weight/volume; XRD, X-ray diffraction
IMK, GBR, and DRK synthesized the AgNPs, BRK collected gum tragacanth and prepared the aqueous gum extract, and DRK characterized the AgNPs using UV-Vis spectroscopy. GBR characterized the AgNPs using X-ray diffraction and TEM. IMK conducted catalytic degradation studies on AgNPs. GVB contributed in analysing the data. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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