Nanotechnology is the science and technology controlling the matter at the nanoscale. The applications encompassing the nanobiotechnology fields are drug delivery (Venkatpurwar et al., 2011), bioremediation (Nair and Pradeep, 2007), biosensors (Bindhu and Umadevi, 2014), food packaging (Kanmani and Rhim, 2014), cosmetics (Saraceno et al., 2013) and water treatment (Pradeep and Anshup, 2009).
Nanoparticles (NPs) are particles ? 100 nm in each spatial dimension (Narayanan and Sakthivel, 2011a). The physicochemical properties of nanoparticles vary with the size and shape. Some of the important size dependent properties are surface plasmon resonance, quantum confinement and paramagnetism. The NPs are important to scientific community as they link the gap between the bulk materials and atomic or molecular structures (Thakkar et al., 2010).
Nanoparticles are commonly produced using “top-down” and “bottom-up” approaches (Mohanpuria et al., 2008). In “top-down” approach, the larger materials are steadily reduced to nano size using physical means. In “bottom-up” approach, atom-by-atom, molecule-by-molecule or cluster-by-cluster atoms are amassed into structures of nanometer range using biological or chemical means. The imperfection on the surface structure of nanoparticles is the major disadvantage of the top-down method. The defects in the surface structure can have a substantial impact on physical properties and surface chemistry of the nanoparticles (Thakkar et al., 2010). Hence the bottom up approach is advantageous as it results in obtaining homogenous composition of nanoparticles with fewer surface defects.
The unique properties of noble metal nanoparticles are optical, electrical, catalytic and antimicrobial activity. The nanoparticles are remarkable because of the ease in its synthesis, chemical stability and chemical modifications (Jain et al., 2007). Silver (Ag) and gold (Au) nanoparticles are widely used in the emerging interdisciplinary field of nanobiotechnology.
Nobel metal nanoparticles are synthesized using various physicochemical methods. The physical methods like UV irradiation, aerosol, and lithography are commonly used for bulk production of nanoparticles. However, the physical methods are expensive and utilize enormous energy. The wet chemical methods employ the use of hazardous chemicals such as potassium bitartrate, sodium borohydride (NaBH4), poly-N-vinyl pyrrolidone (PVP) and hydroxylamine for the synthesis of nanoparticles. The use of harmful chemicals and use of high energy for synthesis of nanoparticles limits its applications in nanobiotechnology field (García-Barrasa et al., 2011; Narayanan and Sakthivel, 2011a). Therefore, the soft synthesis of clean, safe, biocompatible and environmentally benign nanoparticles is pivotal for commercial and biological applications.
The use of biomolecules, microorganisms, plant extracts and biodegradable polymers are attractive bio-based systems for the synthesis of nanoparticles. They not only act as reducing agent but also play a dual role of passivation and functionalization (Dahl et al., 2007). The implementation of principles of green chemistry by the usage of nature’s synthetic machinery is an imperative approach to the synthetic process.
The organisms ranging from unicellular to complex multicellular organisms are capable of synthesizing metal nanoparticles either intracellularly or extracellularly (Narayanan and Sakthivel, 2010a; 2011a). The unicellular organism like diatoms synthesizes silica (SiO2) nanoparticles intracellularly (Nassif and Livage, 2011). The magnetotactic bacteria like Magneto spirillum, Magneto tacticum produces magnetite (Fe3O4) nanoparticles of ~ 50 nm dimension (Philipse and Maas, 2002). The intracellular synthesis of silver and gold nanoparticles by diverse bacteria like Acinetobacter sp. SW 30 (Wadhwani et al., 2014), Geobacillus sp. (Correa-Llanten et al., 2013) and Pseudomonas stutzeri (Klaus et al., 1999) exhibited polyhedral, hexagonal, spherical and hexagonal shaped morphology. Fungi like Aspergillus flavis, Verticillium sp., Trichothecium sp. and Volvariella luteoalbum are able to produce silver and gold nanoparticles intracellularly (Narayanan and Sakthivel, 2010). The intracellular formation of Ag and Au nanoparticles by the live alfalfa plant was first reported by Gardea-Torresdey et al (2002); Gardea-Torresdey et al (2003).
The extracellular syntheses of nanoparticles have wider applications because of its ease in purification. The microbial systems are capable of reduction or precipitation of metal ions to metal nanoclusters. Bacteria like Rhodopseudomonas capsulata, Pseudomonas aeruginosa and Bacillus megatherium DO1 produce gold nanoparticles in the size ranging between 1.9 – 400 nm. Spherical silver nanoparticles were also produced by Aeromonas sp. SH10, Enterobacteriaceae cloacae, Klebsiella pneumonia, Bacillus licheniformis and Escherichia coli. Fungi, Actinomycetes and yeast synthesize nanoparticles both intracellularly and extracellularly (Narayanan and Sakthivel, 2010a).
The plant extracts like Azadirachta indica (Shankar et al., 2004), Aloe vera (Chandran et al., 2006), Capsicum annum L (Li et al., 2007) Citrus limon (Prathna et al., 2011a), Perilla frutescens (Basavegowda and Lee, 2014) and Tridax procumbens (Bhati-Kushwaha, 2014) have been utilized to synthesize extracellular metal nanoparticles. The use of plants for synthesis of metal nanoparticles is the best source as compared to other biological resources. The down streaming process is easy and moreover it contains various metabolites which would help in the reduction and stabilization of nanoparticles (Prathna et al., 2011a). Plants being a renewable resource are appropriate for large scale synthesis of nanoparticles. Various parts of the plant like seed, roots, stem, bark, leaf, fruit and latex have been used for the synthesis of metallic nanoparticles (Hebbalalu et al., 2013). The aqueous plant extracts has been widely employed for the biosynthesis of Ag and Au nanoparticles (Mittal et al., 2013).
The biomolecules in the plant extracts are capable of reduction of metal ions to nanoparticles in “one pot” synthesis process. This bio-mediated reduction process is rapid, readily carried out at room temperature and easily scalable. The various biomolecules involved in the reduction and stabilization of nanoparticles are alkaloids, polyphenols (Moulton et al., 2010), proteins, terpenoids, flavonoids (Shankar et al., 2004), tartaric acid (Ankamwar et al., 2005), gallic acid (Dubey et al., 2010), citric acid (Prathna et al., 2011a), polysaccharides, ginsenosides (Kumar et al., 2011), glutathiones, chelatins (Jha and Prasad, 2010) and quinones (Vivekanandhan et al., 2009). Plants are renewable biofactories with numerous metabolites to produce stable and biocompatible metallic nanoparticles. The use of plant extract for greener synthesis of nanoparticles is a suitable replacement to physical and chemical methods.
The mangrove biome is an important intertidal wetland ecosystem and is highly productive with several important environmental and economic values (Patra and Mohanta, 2014). They exist in highly marshy area and possess alterations in their physiological process to establish water and salt economy. Hence, they possess chemical constituents which help them to thrive in the extreme environmental conditions.
The metabolites present in mangrove plants possess novel chemical structures and belongs to a wide array of ‘chemical classes.’ The biomolecules like terpenoids, steroids, alkaloids, phenolics, carbohydrates, carotenoids, hydrocarbons, free fatty acids including polyunsaturated fatty acids (PUFAs), lipids, phorbol esters, saponins and pheromones are important phytochemicals present in the plants (Patra and Mohanta, 2014; Sieg and Kubanek, 2013).
The mangrove plants have been used as folklore medicines as antiseptic and insecticide. They have also been used in the treatment of typhoid and hepatitis. They have various medicinal properties like antimicrobial, antiviral larvicidal antifungal, anti-tumor and also used as anti-corrosive agents (Bandaranayake, 1998). The presence of biologically active components owes to the above properties. Hence, the exploration of the chemically rich mangrove plants for the synthesis of metal nanoparticles is a novel approach.
Silver and gold nanoparticles have diverse applications in the biomedical and environmental field. These applications vary from antimicrobial (e.g. antibacterial, antifungal, antiviral and anti-parasite applications) to drug delivery, cancer treatment, medical diagnostics and sensors. The environmental applications include metal bio-sorption and its recovery, catalytic degradation of organic pollutants, biosensors and water treatment (Schrofel et al., 2014; Tran et al., 2013).
The use of noble metal NPs in the treatment of water is of growing interest in nanobiotechnology. They have been used to remove heavy metals, organic pollutants and microbes. Silver based water purifiers are available in market to ensure safe drinking by removal of bacteria and virus. The noble metal nanoparticle based chemistry leads to better water quality.
The antimicrobial activity of silver and its compound are known from ancient times. The silver nanoparticles (AgNPs) due to its high aspect ratio exhibits effective antimicrobial activity against bacteria, fungi and viruses (Guzman et al., 2012; Kim et al., 2009; Rai et al., 2009; Sun et al., 2005). Ag nanoparticles are active biocides against Gram-positive and Gram-negative bacteria like Escherichia coli, Staphylococcus aureus, Salmonella typhi and Pseudomonas aeruginosa (Birla et al., 2009). Hence, it can be efficiently used in aerosol, drinking water and surface disinfection. Currently, significant interest in the application of AgNPs for water disinfection has risen (Tran et al., 2013). The application of silver based nanomaterials is important to prevent the waterborne diseases caused due to poor quality of drinking water. The incorporation of silver nanoparticles in the polymeric membrane and materials like activated charcoal are ideal for water disinfection (De Gusseme et al., 2011; Kim and Van der Bruggen, 2010; Manjumeena et al., 2014).
Activated charcoal (AC) is a form of carbon obtained from a variety of carbonaceous rich materials such as wood, coal, lignite and coconut shell (Hassler, 1980). They contain well developed internal pores to increase the surface area for adsorption of chemicals. It is mostly used to remove taste and odour forming compounds in drinking water purification. However, the water should be free from faecal matter and total coliforms, which AC cannot ensure (Heidarpour et al., 2011). On the contrary, bacteria may actually prefer to adhere on the surface of AC, using the latter as the carbon source (Kim and Park, 2008). So, it is necessary to prevent bacterial growth on the AC surface. Hence, AC used for water purification should not only have good adsorption capacity but also good antibacterial activity. The impregnation of AgNPs on activated charcoal would impart bactericidal properties to AC and will ensure better water quality.
The use of synthetic pharmaceuticals, dyes, pesticides and explosives has resulted in the production of toxic nitro aromatic contamination in water and soil (Srivastava et al., 2013). Among the nitro aromatic compounds, nitrophenols are the common industrial pollutants. The degradation of nitrophenol using biosynthesized metallic nanoparticle is a common test system to evaluate the catalytic efficiency of nanoparticles (Ai and Jiang, 2013; Narayanan and Sakthivel, 2011b; Sharma et al., 2007). The nanosized Au particles are efficient catalyst because of high activity, selectivity and stability (Baruah et al., 2013; Jain et al., 2007). The AuNPs have better catalytic efficacy as compared to AgNPs owing to the poisoning of the AgNPs surface with the oxide layer (Gangula et al., 2011). Au nanoparticles can catalyze numerous reactions at or even below room temperature. This could aid in the less consumption of energy for many catalytic processes.
The biosynthetic approach for the synthesis of nanoparticles is rapidly growing, but the biosafety issues need to be addressed. In vivo and in vitro test is essential to understand the effect of nanoparticles in humans, animals and environment. The in vitro study involves the use of cell lines and human blood to assess the compatibility of nanoparticles (Tran et al., 2013). However, the complexity increases from in vitro to in vivo models (Li et al., 2012).The hazard identification of nanomaterials in the in vivo models is still in the infant stage. Due to limited studies on the biosafety studies of biosynthesized nanoparticles there is a need to explore its effect on animal models.
The present study demonstrates the synthesis of nanosized silver particles using aqueous plant extract of Rhizophora stylosa. These mangrove plants are rich in wide array of biochemical constituents which makes them a suitable bio-resource for synthesis of metallic nanoparticles as reducing and capping agent. Further, attempts have been made to identify the biomolecules involved in NPs synthesis. The scope of the present work is
ü Synthesis of Silver Nanoparticles from Mangrove plant Rhizophora stylosa.
ü To examine the conversion of Silver Nanoparticles by using UV-Vis spectroscopy in aqueous suspension.
ü To ascertain the role of the bioactive component in the reduction of Silver Nanoparticles by using Fourier Transform Infra-red Spectroscopy (FTIR).
ü To determine the crystalline nature of Silver Nanoparticles by using X-Ray Diffraction (XRD) and Selected-Area Electron Diffraction (SAED) pattern analysis.