Accepted Manuscript Review Biogenic synthesis of nanoparticles: A review Deepali Sharma, Suvardhan Kanchi, Krishna Bisetty PII: DOI: Reference:
S1878-5352(15)00314-7 http://dx.doi.org/10.1016/j.arabjc.2015.11.002 ARABJC 1795
To appear in:
Arabian Journal of Chemistry
Received Date: Accepted Date:
25 August 2015 5 November 2015
Please cite this article as: D. Sharma, S. Kanchi, K. Bisetty, Biogenic synthesis of nanoparticles: A review, Arabian Journal of Chemistry (2015), doi: http://dx.doi.org/10.1016/j.arabjc.2015.11.002
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Biogenic synthesis of nanoparticles: A review Deepali Sharma, Suvardhan Kanchi*, Krishna Bisetty† *†Department of Chemistry, Durban University of Technology, P.O. Box 1334, Durban 4000, South Africa
*†Corresponding author(s): *†Department of Chemistry, Durban University of Technology, P.O. Box 1334, Durban 4000, South Africa Tel.: +27-31-3736008/2311; Fax: +27-86-6740243. E-mail address:
[email protected] (S. Kanchi),
[email protected] (K. Bisetty)
Biogenic synthesis of nanoparticles: A review Deepali Sharma, Suvardhan Kanchi*, Krishna Bisetty† *†Department of Chemistry, Durban University of Technology, P.O. Box 1334, Durban 4000, South Africa
Abstract The nature act like a large “bio-laboratory” comprising of plants, algae, fungi, yeast etc. which are composed of biomolecules. These naturally occurring biomolecules have been identified to play an active role in the formation of nanoparticles with distinct shapes and sizes thereby acting as a driving force for the designing of greener, safe and environmentally benign protocols for the synthesis of nanoparticles. The present review targets the comparative biogenic synthesis and mechanisms of nanoparticles using algae and waste materials (agro waste in the presence of biomolecules. The use of waste materials not only reduces the cost of synthesis but also minimizes the need of using hazardous chemicals and stimulates ‘green synthesis’. It also focuses on the computational aspects of binding of biomolecules to nanoparticles and some of the applications of the biosynthesized nanoparticles in biomedical, catalysis and biosensors fields.
KEYWORDS: Nanoparticles, green synthesis, algae, waste materials, computational chemistry ______________________________________________________________________________ *†Corresponding author. Tel.: +27-31-3736008/2311; fax: +27-86-6740243. E-mail address:
[email protected] (S. Kanchi),
[email protected] (K. Bisetty)
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Contents 1. Introduction………………………………………………………………………………..3 2. Capping agents and their types……………………………………………………………6 2.1.
Biomolecules………………………………………………………………………6
2.2.
Polysaccharides……………………………………………………………………8
3. Understanding the role of capping agents/biomolecule binding to NPs via computational techniques………………………………………………………………...9 3.1.
Facet specific binding of biomolecules for programmed biomimetic synthesis of NPs……………………………………………………..11
4. Synthesis of NPs using Algae…………………………………………………………....13 4.1.
Metallic NPs……………………………………………………………………..13
4.2.
Metal oxide NPs…………………………………………………………………21
4.3.
Mechanism of biosynthesis of NPs using Algae…………………………………26
5. Synthesis of NPs using waste material…………………………………………………..26 5.1.
Metallic NPs……………………………………………………………………..27
5.2.
Metal oxide NPs…………………………………………………………………33
5.3.
Mechanism of waste material mediated synthesis of NPs……………………….34
5.4.
Advantage of waste materials in the synthesis of NPs…………………………..35
6. Applications of biogenic synthesis of NPs………………………………………………36 6.1.
Biomedical applications…………………………………………………………36
6.2.
Catalytic applications……………………………………………………………39
6.3.
Biosensing applications………………………………………………………….41
7. Conclusions………………………………………………………………………………42 8. Future prospects………………………………………………………………………….43 Acknowledgements………………………………………………………………………….44 References…………………………………………………………………………………...45
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1. Introduction In the past decade there has been a marked increase in the field of fabrication of nanoparticles with controlled morphologies and remarkable features making it an extensive area of research. The synthesis of nanoparticles (NPs) with control over particle size, shape and crystalline nature has been one of the main objectives in chemistry that could be used for potential applications, such as bio-medical, biosensor, catalyst for bacterial biotoxin elimination and lower cost electrode (Antonyraj et al., 2013; Frances et al., 2009; Staniland, 2007). The nanoparticles (NPs) having at least one dimension less than 100 nm such as nanosheets, nanotubes and nanowires have gained much attention because of their promising applications (Fu et al., 2013; Kwon & Bard, 2012; Roy et al., 2013). Other than their unique physical and chemical properties, NPs act as a bridge between bulk materials and atomic or molecular structures. Therefore, they are good candidate for applications including medical, catalysis, electrochemistry, biotechnology, and trace-substance detection (Luo et al., 2006; Murthy, 2007; Wen et al., 2011; Xia et al., 2013). Different synthetic methods have been employed for the preparation of NPs with diverse morphology and size. Although these methods have resulted in superior NPs but still a key understanding of improved manufacturing process is required which could be exploited at the industrial and commercial level to have better built, long lasting, cleaner, safer and smarter products like home appliances, communication technology, medicines, transportation, agriculture and industries. Therefore, the main focus is to design NPs using environmentally benign approaches. These provide solutions to growing challenges related to environmental issues. Nature has provided ways and insight into the synthesis of advanced nanomaterials. It has now been reported in literature that biological systems can act as the ‘bio-laboratory’ for the 3
production of pure metal and metal oxide particles at the nanometer scale using biomimetic approach. Various microorganisms, such as bacteria (Shivaji et al., 2011; Stephen & Macnaughtont, 1999), fungi (Chan & Mat Don, 2013; Syed et al., 2013), yeast (K et al., 2011), plant extracts (Akhtar et al., 2013) and waste materials (Kanchi et al., 2014), have acted as ecofriendly precursors for the synthesis of NPs with potential applications. The biological approach which includes different types of microorganisms has been used to synthesize different metallic NPs, which has advantages over other chemical methods as this is greener, energy saving and cost effective. The coating of biological molecules on the surface of NPs makes them biocompatible in comparison to the NPs prepared by chemical methods (Hakim et al., 2005; Mukherjee et al., 2001; Tripp et al., 2002). The biocompatibility of bioinspired NPs offers very interesting applications in biomedicine and related fields (Huang et al., 2015). The biogenic methods leads to the designing of NPs with interesting morphologies and varied sizes (Riddin et al., 2010; Schröfel et al., 2014), for example Ag NPs in the size range of 25±12 nm have been prepared by exposing fungal biomass (Verticillium) to the aqueous solution of Ag+ ions, where the NPs were not toxic since the biomass (fungal cells) continues to grow. The NPs were found to grow on the surface of mycelia as a result of electrostatic interaction between the Ag+ ions and negatively charged carboxylate groups of enzymes present in the cell wall of the fungus (Mukherjee et al., 2001). Pt NPs of definite shape and size have been prepared from the cell-soluble protein extract of sulfate reducing bacteria (Riddin et al., 2010). These NPs fabricated via biogenic enzymatic process were superior to those synthesized via chemical methods as the use of expensive chemicals was limited and they possessed higher catalytic activity. An industrially important fungus, Penicillium rugulosum, was used to synthesize
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uniform sized Au NPs, which is easier to handle as compared to other bacteria and yeast (Mishra et al., 2012). Extracts from plant leaf, root, latex, seed and stem have also been used for the synthesis of NPs as they act as stabilizing or reducing agents. The leaf extracts of Jasminum sambac were employed to prepare stable Au, Ag, Au-Ag alloy NPs (Yallappa et al., 2015). Regarding the morphology control, triangular, hexagonal and spherical shaped Au NPs were prepared using hot water, olive leaf extract at a high reaction temperature as compared to NPs synthesized via chemical methods (Khalil et al., 2012). Iron-polyphenol (Fe-P) NPs with a photocatalytic activity against Acid Black-194 dye were synthesized from Australian native leaves of Eucalyptus tereticornis, Melaleuca nesophila and Rosemarinus officinalis (Wang et al., 2014). It was noted that the polyphenols present in the three plants reacted with ferric chloride (FeCl3) solution to form chelated ferric-polyphenols NPs and also led to different shapes of Fe-P NPs. Among different biological systems used for NP synthesis, various forms of algae are now being currently used as model systems as these have tremendous ability of bioremediation of toxic metals thereby converting them into more pliable forms. Also, these are competent in the fabrication of diverse metal and metal oxide NPs (Patel et al., 2015). The biosynthesis of NPs using alage and waste materials is an emerging and upcoming research. The present review discusses the synthesis and the comparison of the better prospects of using algae or waste material for the efficient designing of NPs (Figure 1).
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Fig. 1. Green synthesis of NPs using algae and waste materials 2. Capping agents and their types Capping agents play a very pivotal and versatile role in the NP synthesis. NPs can be functionalized and stabilized using capping agents to impart useful properties by controlling morphology, size and protecting the surface thereby preventing aggregation. Many surfactants have been reported to be used as capping agents for altering the desired shape and size of the NPs but these are difficult to remove and do not easily degrade. Thus, the commercial surfactants are hazardous to the environment (Gittins et al., 2000; Liu et al., 2005). In the view of the limitation possessed by these chemicals, there is an urgent need to use environment friendly capping agents and design green biochemical routes at laboratory and industrial level for the NP synthesis. There are different types of molecules that could act or used as capping agents but some of the broadly classified green capping agents have been discussed below with their potential role. 2.1 Biomolecules The preparation of homogenous NPs using biomolecules has recently gained interest due to their non-toxic nature and not involving harsh synthetic procedures. Amino acids act as an 6
efficient reducing as well as capping agents to synthesize NPs with unique structure. Maruyama and coworkers synthesized Au NPs with the size range of 4-7 nm using amino acids as capping agents. Among 20 different amino acids, they adopted L-histidine which was found to reduce tetraauric acid (AuCl4-) to Au NPs. The concentration of L-histidine was found to effect the size of NPs; higher the concentration smaller the size of NP. Moreover, the amino and carboxy groups present in the amino acids caused the reduction of AuCl4 - and coating of NP surface (Maruyama et al., 2015). In another interesting study, Au nanochains were prepared via facile single step within 15 min in the presence of glutamic acid and histidine amino acids (Polavarapu & Xu, 2008). The oriented attachment mechanism has been proposed where there is spontaneous self organization of the adjacent particles at a planar interface as they share a common crystallographic orientation (Figure 2). The binding affinity of amino acids is found to be different for different facets of Au crystal. The fusion of NPs along (111) facet revealed that binding affinity of amino acids along this facet might be weaker as compared to the other facets. The removal of amino acid molecules from (111) facet allows the linear aggregation of particles due to dipole-dipole interactions which arise as a result of the zwitterionic nature of amino acids (Xu et al., 2003).
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Fig. 2. Illustration of Au nanochain and nanowire formation through dipole-dipole interaction due to the zwitterionic nature of amino acids. Reproduced with permission (Polavarapu & Xu, 2008). 2.2 Polysaccharides Polysaccharides are a class of polymeric carbohydrate molecules with repeating units of mono or disaccharides linked together by glycosidic linkages. They act as capping agents in the NP synthesis as they are low cost, hydrophilic, stable, safe, biodegradable and non-toxic. The synthesis is carried out in the presence of water as a solvent thus, eliminating the use of toxic solvents (Akhlaghi et al., 2013; Duan et al., 2015). One of the distinguishing features of polysaccharides is that they sharply accelerate the kinetics of sol-gel processes due to their catalytic effect (Boury & Plumejeau, 2015). They not only have been found to modify the structure and morphology of TiO2 but have induced a different phase where rutile phase has been obtained in the presence of chitosan whereas anatase in the presence of starch (Bao et al., 2013). Dextran is a complex branched polysaccharide composed of many glucose molecules with chains of varying lengths. It is hydrophilic, biocompatible, non-toxic and used for coating 8
of many metal NPs (Virkutyte & Varma, 2011). Spherical Au NPs of size ~15 nm were synthesized in water using natural honey which acted as reducing as well as protecting agent. Fructose present in the honey was supposed to act as a reducing agent whereas proteins were responsible for the stabilization of the NPs (Philip, 2009). Cheng and coworkers synthesized Ag NPs within the size range of 2-14 nm using aminocellulose as reducing and capping agent. Aminocellulose is generally referred as aminodoxy derivative bearing nitrogen functional group attached directly to the cellulose backbone. It was predicted that at high temperature, reduction of Ag+ ions to Ag (0) was brought by cellulose (Cheng et al., 2013). Thus, polysaccharides have come up as one of the renewable green alternative for the fabrication of NPs replacing toxic chemicals thereby saving the environment from their hazardous effects. 3. Understanding the role of capping agents or biomolecule binding to NPs via computational techniques With the introduction of different force field parameters and supercomputing, computational field has given a different prospective of comprehending the interaction of biomolecules with NPs and minutely study the mechanistic details and supporting experiments. Different approaches like density functional theory (DFT), molecular dynamics (MD) simulations, docking studies are being employed to study interactions which yield accurate results. The interaction of biomolecules especially peptides on the metal surface has been predicted to result in the stabilization of nanostructures and hence, improvising their applicability as sensors, biomedical devices and electronics. Phage-display libraries have been formed to evolve peptides that can bind specifically onto the surface of semiconductor materials depending on the crystallographic orientation and composition. The phage display approach identifies the 9
physical linkage between peptide-substrate interactions. Peptides could lead to the controlled placement and assembly of molecules thereby broadening the scope of 'bottom-up' approach for the synthesis of NPs (Whaley et al., 2000). Amino acid residues present in the biomolecules bounded to (111) facet of gold surface resulting in the formation of gold nanoplates with thickness of 30 nm (Shao et al., 2004). Thus, amino acids help in the biomimetic fabrication of nanostructures. Feng et al. explored the adsorption mechanism of amino acids and surfactants on to the (111) surface of gold using molecular dynamic simulation with the application of intermolecular potential CHARMM-METAL (Feng et al., 2011). The molecules adsorbed onto the surface with energy between -3 and -26 kcal mol-1 and it correlated with the preferential degree of coordination of polarizable atoms (O, N, C) to multiple epitaxial sites. The amino acids containing planar side groups like Arg, Trp, Gln, Met, Tyr, Asn, and PPh3 with sp2 hybridization adsorbed strongly indicating a correlation with molecular size and geometry. The movement of the strongly bonded amino acids has been attributed to hopping mechanism where surface attached guanidinium group of Arginine moved from one favourable coordination site to another on the metal surface in the order of picoseconds (Figure 3).
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Fig. 3. Representative snapshot of Arg on the Au 111 surface. The guanidinium group is found most of the time in a favorable coordination pattern with numerous epitaxial sites which leads to strong adsorption (pink highlights). Diffusion on the surface occurs by stepwise hopping of the guanidinium group to similar epitaxial environments. In the depicted conformation, Ca and Cg point upwards from the metal surface. Most water molecules are not shown for ease of visualization and the chloride counter ion is located outside the visible range near the metal surface. Metal atoms are shown in decreasing diameter from the top atomic layer toward adjacent atomic layers. Reproduced with permission (Feng et al., 2011). This adsorption mechanism could give the knowledge to control the assembly and growth of NPs. 3.1. Facet-specific binding of biomolecules for programmed biomimetic synthesis of NPs The bio-inspired routes have helped researchers to achieve controllable morphology and size of NPs and come up with promising applications. This depends on the interaction of capping agent (biomolecules) with a particular facet of a metal or metal oxide crystal. It is therefore, important to fully understand and identify the facet specific interaction of a biomolecules with inorganic materials in order to produce NPs with hierarchical structures. The question still remains about the specificity of a biomolecule and its adsorption on the surface of a particle. Ramakrishnan et al. employed DFT method for investigating the interactions between facet specific amino acids (Phe, Asn, Gln, Ser, Pro, Leu, Thr) and Pt (111) and Pt (100) crystallographic facets. Their results indicated that the electrostatic interactions were responsible for the binding of amino acids onto the Pt surface. Phe, Ser and Pro amino acids preferably adsorbed on the Pt (111) facet whereas Asn, Leu and Thr were Pt (100) facet specific. The charge transfer and exchange processes alongwith dispersive effects caused the interaction of amino acids and Pt surface (Ramakrishnan et al., 2015). Peptides have been found to have additional complexation capability in the formation of ZnO crystal. The effect of ZnO binding peptide (G-12, GLHVMHKVAPPR) and its derivative GT-16 (GLHVMHKVAPPRGGGC) on 11
the growth and morphology of ZnO crystal was studied by Liang et al. (Liang et al., 2011; SolaRabada et al., 2015). The morphology of the ZnO crystal was altered by G-12 and GT-16 via adsorption-growth inhibition mechanism (Figure 4). The aspect ratio of ZnO was found to be reduced in the presence of G-12 and GT-16. The inhibition of diameter growth along a-axis in presence of G-12 and reduction of length along c-axis in presence of both GT-16 and G-12 led to the hypothesis: (i) G-12 was able to adsorb preferentially on (0001) plane as well as (1010) plane. (ii) Selective adsorption of GT-16 on (0001) plane. (iii) The selectivity of GT -16 was due to presence of GGGC tag not present in G-12. This gave an insight into the desired tuning of ZnO morphology by the selective adsorption characteristics of the biomolecules onto the crystal faces.
Fig. 4. Adsorption-growth-inhibition mechanism. The length and thickness of the growth arrows reflects the growth rate of the crystals along their respective axis. Reproduced with permission (Sola-Rabada et al., 2015). A recent study revealed the binding behavior of two peptides (EAHVMHKVAPRP, EM-12 and mutant EAHVCHKVAPRP, EC-12) on the formation of ZnO from solution (Liang, 12
2011). It was proposed that binding of ZnO did not depend on hydrophobicity but on the ZnO recognition of specific amino acid alignments in peptides. EM-12 suppressed the crystal growth in the (0001) direction with Met-His or His-Lys sequence of amino acid. It was also observed that with increase in the concentration of EM-12, there was delay in ZnO formation. This was attributed to the speculation that amino acids with side chain functional groups interact with Zn 2+ via electrostatic interactions and strongly influence the morphology. They reduce Zn 2+ concentration in the solution thereby delaying the ZnO formation (Gerstel et al., 2006). EC-12 was found to have higher Zn2+-complexation capability due to presence of cysteine which is a strong binder and thus, retained more Zn2+ in the solution. The morphology of the ZnO crystals was different for EC-12 as compared to EM-12: (i) As concentration of EC-12 was increased, the crystal remained twinned. (ii) Along the c-axis the diameter of the crystal was not uniform. (iii) The mushroom like morphology was obtained at higher concentration of EC-12 as the crystal planes were not well defined. These observations confirm the facet specificity of biomolecules in modulating the morphology of the NPs. 4. Synthesis of NPs using algae 4.1. Metallic NPs The controlled growth of NPs in solution is believed to be kinetically controlled process where low energy faces of any crystal results into a particular shape. The energy and growth rate of a crystal can be controlled by the introduction of a suitable templating agent or a surfactant which lowers the interfacial energy (Chiu et al., 2013; Xia et al., 2003). Till now, different commercial surfactants have been used as templates and capping agents for the synthesis of NPs 13
with varied morphologies. But the problem is the removal and complete biodegradation of these chemicals. Nowadays, more and more research is converged on to the green synthesis employing environment friendly and bioinspired approaches. By knowing the capability of naturally occurring biomolecules to modify the shape or size of a crystal, NPs of superior quality can be manufactured. The use of different species of algae in the synthesis of metallic NPs has stimulated the researchers to come up with 'nature-friendly' methodologies. Ag NPs have gained widespread attention due to their efficient antibacterial activities. Therefore, synthesis is carried out in the presence of microalgae where the metabolites excreted by the algal culture cause the reduction of silver ions. The NPs have tunable optical properties directed by the particle size (Merin et al., 2010). The reduction of silver nitrate was observed in the presence of seaweed Chaetomorpha linum. The metabolites (flavonoids and terpenoids) present in the extract were found to be effective capping, stabilizing agents and resulted in the formation of NPs with an average size of 30 nm having potential applications in the field of medicine (Kannan et al., 2013). Biomolecules like polysaccharides present in algal species also play an important role in controlling the size and desired shape of Ag NPs. Pterocladia, capillacae, Jania rubins, Ulva faciata, and Colpmenia sinusa are the species of the marine algae which are efficient in assisting the synthesis of polydispersed and spherical Ag NPs capable of immobilizing on cotton fabrics. Thus, they act as antimicrobial agents (El-Rafie et al., 2013). Composite of nano Ag-CaCO3 were manufactured via microalgae as a bio-template using efficient and ecofriendly approach. The microalga (Chlorella sp. KR-1) was used for the mineralization of carbon dioxide (CO2) to form calcium carbonate (CaCO3) microspheres (Sahoo et al., 2014). The use of alga was advantageous in two ways: (i) Nucleation and crystal growth was accelerated due to the presence of negative charge on the 14
surface of the cell (ii) large scale synthesis at a very low cost. The surface charge of the microalgae cells was found to be responsible for the formation of CaCO3 particles. Due to electrostatic interactions, the positively charged Ca 2+ ions agglomerated on the surface of negatively charged algal cells and thus, were driving force for the initiation of nucleation process. Also, the concentration of Ca2+ ions had a profound effect on the size of microspheres. The higher the concentration, larger was the size of the CaCO 3 microspheres keeping the constant concentration of algal cells and favouring heterogenous nucleation. The synthesized microspheres were used as economical matrix for the synthesis of Ag NPs. The composite exhibited efficient antimicrobial activity against model bacteria such as Escherichia coli, Psychrobacter alimenterius and Staphylococcus euroum thereby finding its way to be commercialized as paint additives. The Figure 5 illustrates the formation of CaCO3 microspheres and nano Ag- CaCO3 composite.
Fig. 5. (a) Schematic illustration of the synthetic steps for porous CaCO 3 microsphere by CO2 mineralization pathway in the presence of microalgae. Ag NPs were formed on the porous CaCO3 using AgNO3 as precursor followed by reduction. (b) Speculated mechanism for surfacedirected mineralization of CaCO3 . The negatively charged microalgae surface aggregates Ca2+ ion via electrostatic interaction prior to nucleation. Nucleation occurs on supplying CO 32- from 15
CO2 hydration resulting in the formation nano CaCO3. Reproduced with permission (Sahoo et al., 2014). With the advent of nanotechnology, toxicity issues related to the NPs are also of great concern. The in vitro and in vivo studies have been carried out to predict the toxicity levels of NPs to different biological membranes and human cells that could be administered as drugs. NP interactions are reported to be size dependent (Kim et al., 2006). NPs with size of ca. 10 nm have been observed to induce greater cell death than large sized NPs (50 to 100 nm) (Carlson et al., 2008; Gorth et al., 2011). The biosynthesis of metallic NPs via algae provides a greener route with no toxicity levels. The proteins present in the algae membranes act as better templating agents and also stabilize the NPs thereby enhancing their possibility to be used as nanomedicine. Marine microalgae are constantly exposed to metal salts present in the water. Therefore, these are widely effective in reducing metal salts to NPs. The biggest challenge in nanochemistry is the fabrication of monodispersed NPs which is difficult to achieve even in the presence of long chain surfactant molecules. Algae provide a green, fast, cheap route and act as a potential ‘nanoreserves’ for varied sustainable NPs (Baker et al., 2013). Moreover, these can be easily cultured in laboratories. Degradation of hazardous organic chemicals and dyes is a crucial problem. Many techniques like activated carbon sorption, electrocoagulation, UV degradation and redox treatments are used but these are not found to be very efficient and cost effective. Therefore, there is an urgent need of improved ways to degrade chemicals as well as treatment of wastewater.
Ag nanocatalyts degrading methyl orange dye were synthesized from Hypnea
musciformis at room temperature (Ganapathy Selvam & Sivakumar, 2015; Kumar et al., 2011). Similarly, Ag NPs with face centered cubic crystalline structure have been synthesized from 16
green alga Enteromorpha flexuosa and these demonstrated antibacterial activity against gram negative and positive bacteria (Yousefzadi et al., 2014). The extracellular synthesis of monodispersed Au NPs have been achieved in a short duration from marine alga Sargassum wightii Greville. The alga caused the reduction of auric chloride solution and the AuNPs were stable in solution which is important from biological prospective (Singaravelu et al., 2007). Stoechospermum marginatum is brown algae with a high metal binding capacity and constitutes proteins, vitamins, amino acids, fatty acids, minerals and trace elements. Au NPs were prepared from this algal biomass with a reaction time of 10 min (Arockiya Aarthi Rajathi et al., 2012). Au NPs have been reported to be intracellularly synthesized in the suspensions encapsulated within silica gels in the presence of Klebsormidium flaccidium algal cells giving rise to “living” bio-hybrid material. TEM images show algal cells before and after the addition of gold (Figure 6). The cells are surrounded by the colloidal silica which is not in direct contact with the cells thereby maintaining the ability to synthesize Au NPs intracellularly. The proximity of silica cells with cell membrane was found to be dependent on the physiological state of algae. This pathway proves to be one of the promising 'green' route for the synthesis of NPs (Sicard et al., 2010).
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Fig. 6. TEM images of Kf cells within silica before (a) and after (b) gold addition. Gold colloids could be found within the cells in the thylakoids (c), in the cell EPS (d) and in the surrounding silica gel (e) ED pattern of dark dots with examples of attribution to the cfc Au structure (f). Reproduced with permission (Sicard et al., 2010). The diversity in the algae species has led to their exploitation and even the edible forms of algae are used in the synthesis of metallic NPs. Au NPs have paved a way in the field of science and technology due to their enormous vital applications as optical, electronic, catalytic, biosensors and drug carriers. They exhibit excellent antimicrobial and antioxidant properties as their surface is functionalized (Ghosh et al., 2008; Hu et al., 2006; Mikami et al., 2013; Nair & Pradeep, 2002; Pingarrón et al., 2008; Zhao et al., 2010). The chemical methods employ solvents which are toxic in nature and pose a limitation for the use of NPs in the medical field. The biomatrix of fresh water, edible red alga Lemanea fluviatilis has been used to accomplish the synthesis of Au NPs. The proteins present in the alga acted as templating as well as stabilizing agent, thereby avoiding the use of surfactants which are difficult to remove (Sharma et al., 2014b). Prasiola crispa is another fresh water green algae which is used for the one step biosynthesis of Au NPs in the size range of 5 to 25 nm via reduction of chloroauric acid (Sharma et al., 2014a). It has been reported that algal morphology along with high pH, metal ion concentration and biomass concentration are important factors for the monodispersity of the NPs. Though with time, on exposure of NP the toxicity levels against alga increase, but the synthesized NPs show no toxicity when tested with cell lines of normal human cell (Parial & Pal, 2015). Thus, algal-NPs interactions are very important considering the effective use of NPs in drug delivery (Figure 7).
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Fig. 7. Microphotographs of Au3+ exposed algal filaments showing changes in morphological and reproductive behavior at different time points. (a) Akinete formation within 12 h; (b) series of akinetes at 24 h; (c) Au NP deposition started in vegetative cells and akinetes after 24 h; (d) cell wall thickening after 48 h; (e) giant cell with complete Au NP deposition after 48 h; (f) pyknotic cell after 48 h; (g) Au NP deposition at the periphery of cells; (h) Degradation of chlorophyll and Au NP deposition within cell after 48 h; (i) purple colored filament after 72 h. Scale bars, 20 μm (inset showing control filaments; scale bar, 40 μm). Reproduced with permission (Parial & Pal, 2015). In summary, Table 1 depicts the compositions, shape and size of the NPs and the corresponding bio-materials adopted as the reduction agents reported during the last decade. Table 1. Algae mediated synthesis of metallic NPs. Composition of NPs Au
Species of Algae
Brown, Sargassum muticum
Size (nm)
5.42 ± 1.18
19
Morphology
Spherical
Citation
(Namvar et al., 2015)
Au
Tetraselmis kochinensis
5-35
Spherical & triangular
Au
Brown, cava
30 ± 0.25
Spherical & triangular
Ag
Ag
Ag
Au
Ecklonia
Caulerpa racemosa
Brown, Cystophora moniliformis
5-25
50-100
Spherical & Triangular
(Senapati et al., 2012)
(Venkatesan et al., 2014)
(Kathiraven et al., 2015)
Spherical (Prasad et al., 2013)
Chlamydomonas reinhardtii
5-35
Chlorella vulgaris
2-10
20
Round /rectangular
(Barwal et al., 2011)
Spatial array of self assembled Structures (Annamalai & Nallamuthu, 2015)
CdS
Phaeodactylum tricornutum
-
NA (Scarano & Morelli, 2003)
Au
Brown, Padina gymnospora
53-67
Spherical
(Singh et al., 2013)
Au
Brown, Fucus vesiculosus
Varied
Spherical
(Mata et al., 2009)
2-lines ferrihydrite nanoparticles
Euglena gracilis
0.6-1.0
Spherical
(Brayner et al., 2012)
NA=not available 4.2. Metal oxide NPs Metal oxides are widely explored and studied class of inorganic solids due to a wide variety of structures, properties and exceptional phenomenon exhibited by the NPs. Transition metal oxides have been used in numerous industrial applications. NPs, nano-powders and 21
nanotubes play a significant role in industry, environmental remediation, medicine and even in household applications. One dimensional (1D) metal oxide nanostructures are ideal systems for exploring size and morphology dependent applications and have become the focus of current research efforts in the field of nanoscience and nanotechnology. Metal oxides are commonly available and present in different forms possessing special shapes, composition, structures, chemical and physical properties (Zhai et al., 2009). Different synthetic methods like hydrothermal, solvothermal, microwave, vapour deposition, seed mediated, spray pyrolysis, wet-chemical have been employed for the preparation of metal oxide NPs with diverse morphology and size (Gotić & Musić, 2008; Hayashi & Hakuta, 2010; Kim et al., 2003; Leonelli & Lojkowski, 2007). The reports on the biosynthesis of metal oxide NPs are very few. The research is now being shifted from conventional synthetic methods to biosynthesis process where microorganism could be employed for synthesizing NPs. Very few articles have reported the biosynthesis approaches, especially using algae. Copper oxide NPs have been prepared from brown alga, Bifurcaria bifurcata extract with a dimension between 5 to 45 nm (Figure 8). The NPs were stable in solution which is promising for their use in biomedical applications. These NPs exhibited antibacterial activity against bacterial strains of Enterobacter aerogenes and Staphylococcus aureus (Abboud et al., 2014).
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Fig. 8. TEM image of CuO NPs synthesized from the alga extract. Reproduced with permission (Abboud et al., 2014). The extracts of green seaweed Caulerpa peltata, red Hypnea Valencia and brown Sargassum myriocystum were also used for the biosynthesis of ZnO NPs. The NPs with different morphologies like spherical, triangular, radial, hexagonal, rod and rectangular were formed with size range of 76 to 186 nm. The water soluble pigments present in the extract were found to be responsible for the reduction and stabilization of the NPs (Nagarajan & Arumugam Kuppusamy, 2013). AFM study of ZnO NPs revealed that change in temperature parameter remarkably affected the size and morphology of the particles (Figure 9).
Fig. 9. AFM results of ZnO NPs 2D and 3D images. (a) Unfiltered AFM image showing topographical 2D image of ZnO NPs (b) 3D image of synthesized (c) Filtered 2D AFM image of ZnO nanoparticles (d) particle size distribution of ZnO NPs. Reproduced with permission (Nagarajan & Arumugam Kuppusamy, 2013). The polysaccharides present in the aqueous extract of Sargassum muticum caused the reduction of ferric chloride solution, thereby leading to the formation of ferric oxide (Fe 3O4) NPs with cubic morphology and average diameter of 18±4 nm (Mahdavi et al., 2013). Sargassum muticum is 23
algal seaweed which is also used as functional food consisting of large quantities of lipids, minerals, vitamins and many other bioactive substances like polysaccharides, proteins and polyphenols. These biomolecules are effective against cancer, diabetes, thrombosis, obesity, and other degenerative diseases (Miyashita, 2009; Namvar et al., 2012; Nishino et al., 1999; Perez et al., 1998) and also act as reducing as well as capping agents. ZnO nanomaterials are considered to be biocompatible for the medical applications. Therefore, research is carried out to develop algae mediated one step, green and ecofriendly approach. Recent report on the synthesis via aqueous extract of brown alga Sargassum muticum have indicated the formation of pure ZnO NPs in the size range of 30 to 57 nm with hexagonal crystal structure (Azizi et al., 2014). Francavilla et al., designed a feasible protocol for the synthesis of ZnO NPs using Gracilaria gracilis, an edible form of alage (Francavilla et al., 2014). This protocol involved the solid state grinding of zinc precursors including a biomass extracted from microalga followed by thermal decomposition in the absence of any solvent. The synthesized NPs were found to degrade phenol efficiently (Figure 10A). ZnOAG were agar synthesized and ZnO AA alginic acid as compared to commercial P25 Evonik. The photocatalytic activity was enhanced in the case of agar synthesized (~52% phenol degradation, Figure 10B) whereas the degradation of phenol was less than 35% in the case of ZnOAA. The contribution to enhanced photocatalytic activity in this case is still under study but the methodology could be applied widely for the synthesis of other metal oxide NPs.
24
A
B
Fig. 10. (A) Phenol degradation efficiency (measured as the relative concentration of phenol (C/Co ) over time) of ZnO AA 1 : 16 alginic acid, ZnO AG 1 : 2 ex. agar and P25 Evonik, (B) Photocatalytic degradation curves of phenol for ZnO AA 1 : 16 alginic acid, ZnO AG 1 : 2 ex. agar and P25 Evonik. Reproduced with permission (Francavilla et al., 2014). The mechanism for the formation of NPs in the presence of algae is yet to be fully understood. The algal membranes consist of biomolecules like polysaccharides, proteins and enzymes which catalyze the reduction of metal salt precursors into metal or metal oxide NPs. Since, these are large molecules and amphiphatic in nature, they act as surfactant molecules which causes not only concentration buildup of the surfactant at the surface and reduction of the surface tension, but also the orientation of the molecule at the surface (Bianchi et al., 2006; Liu et al., 2014). They act as capping agents and thus, reduce the interfacial energy. The interaction of these biomolecules with the NPs is also an important aspect under consideration.
25
4.3. Mechanism of biosynthesis of NPs using Algae With the availability of sophisticated biochemical techniques and instruments, it is easy to identify the role and interaction of a particular biomolecule like polysaccharides, proteins, enzymes present in the organism with NPs thereby comprehending the mechanism. The simple mechanism in Figure 11 explains that enzymes and functional groups present in the cell walls of algae form complexing agents with the precursors thereby, causing reduction and deposition of metal/metal oxide NPs at ambient conditions (Crookes-Goodson et al., 2008; Gade et al., 2008).
Fig. 11. Mechanism of biosynthesis of NPs using algae. 5. Synthesis of NPs using waste materials
Fig. 12. Type of waste materials employed/could be employed in the synthesis of NPs. 26
5.1. Metallic NPs The environment also acts as a ‘treasure’ of waste materials specially food waste which can be successfully utilized for the biosynthesis of NPs. Recent literature reports the use of biodegradable food waste for the manufacturing different NPs. The food waste material contains different organic compounds like polyphenols, flavonoids, caretanoids and vitamins (Kim et al., 2012) which act as templating agents. The various functional groups present in these compounds cause the reduction of metal precursors. Thus, the waste act as a ‘biofactory’. Au NPs have been successfully synthesized using mango peel extract. The reaction rate was found to be faster as compared to other plant extracts. The particles were monodispersed in the size range of 6.03±2.77 to 18.01±3.67 nm and had no biological toxicity on the normal kidney cells (CV-1) of African green monkeys as well as normal human fetal lung fibroblast cells (WI-38) (Yang et al., 2014). The cytotoxicity of the NPs was assessed by treating them with cells at different concentrations (0, 20, 40, 80 and 160 μg mL -1) for 24 h (Figure 13). It was observed that Au NPs had no biological toxicity event at higher concentration of 160 μg ml -1. The plausible reason was the functionalization of NPs with the organic moieties present the extract of mango peel.
27
Fig. 13. Cell viability assay of the synthesized Au NPs with different sizes (6.037±2.77 nm and 18.01±73.67 nm) against CV-1 and WI-38 cells. The data is represented in the form of a bar graph and plotted using means ± S.E of triplicate determinations. Reproduced with permission (Yang et al., 2014). Wine industry produces a lot of grape waste which is a source of ample of organic compounds that lead to the reduction of metals to NPs. The spherical and polygonal shaped Ag NPs with an average diameter of 25 to 35 nm were formed by reduction of silver ions into NPs in the presence of extract from grape seeds (Figure 14A). These NPs showed effective antibacterial activity against gram negative and gram positive bacteria (Xu et al., 2015). FTIR spectrum of grape seed extract depicted peaks at 3,429.14, 2,954.88, 2,928.66, 2,855.29, 1,613.67, 1,524.71, 1,445.81, 1,380.06, 1,281.05, 1,103.14, 1,053.63, 820.03, 773.61 cm−1 where the peak at: 3429.14 cm−1 corresponded to stretching vibration of O-H functional group 954.88 cm−1, 2,928.66 cm−1, 2,855.29 cm−1 referred to C-H stretching vibration mode 1,613.67 cm−1, 1,524.71 cm−1 referred to vibration of cyclobenzene 1,103.14 cm−1, 1,053.63 cm−1 due to C-O vibration of phenols. The changes in the position of absorption bands were noticed in the FTIR spectrum of Ag NPs. There were O-H stretching vibration shifts from 3,429.14 to 3,436.10 cm-1 indicating the formation of Ag NPs (Figure 14B).
B
A
28
Fig. 14. (A) TEM image of Ag NPs, (B) FTIR spectra of grape seed extract and Ag NPs. Reproduced
with permission (Xu et al., 2015). Microwave assisted methods have been used to synthesize Ag NPs with an average diameter of 7.36 ± 8.06 nm from orange peel extract. The compounds present in the extract acted as capping molecules. The less agglomeration of spherical shaped NPs was observed (Figure 15) (Kahrilas et al., 2014).
Fig. 15. TEM images of Ag NPs by orange peel extract at two magnification levels. White arrows indicate the presence of a thin organic layer surrounding the Ag NPs. Reproduced with permission (Kahrilas et al., 2014). Chicken eggshell membrane (ESM) is one of the nature's gift which goes waste. This has been utilized for the synthesis of fluorescent Au NPs via single step under ambient conditions (Devi et al., 2012). ESM is double layered membrane composed of water-insoluble glycoproteins such as collagen (types I, V and X), and amino acids like glycine alanine and uronic acid (Arias et al., 1991; Wong et al., 1984). The application of EMS as reported in literature includes the recovery of heavy metals, template for macroporous materials and biosensing of enzyme immobilized Au-ESM (Lee et al., 2009; Zheng et al., 2010). The reduction process and particle formation was monitored by absorption and fluorescence spectral changes in addition to change 29
in the colour. The changes were observed in absorption spectrum after impregnating EMS with 10-4 M solution of HAuCl4. A strong maximum was observed at 295 nm due to the initial Au 3+ solution indicating the removal of Au3+ from the solution. Also a colour change was visible from white to yellow showing the adsorption of Au3+ on the membrane surface (Figure 16).
A
B
Fig. 16. (A) UV-Vis spectra of the reaction of ESM with chloroauric acid solution (10 -4 M), recorded as a function of time. Curves (1) blank HAuCl4 (2) 1 h (3) 6 h (4) 1 day (5) 3 days (6) 6 days and (7) 7 days (B) Photographs of (a) bare eggshell membrane and pink colored impregnated eggshell membrane with 10 -4 M solution, (b) blue colored impregnated eggshell membrane with 10 -2 M metal ion solution and (c) membranes impregnated with 10 -6 (1), 10-4 (2) and 10-2 M HAuCl4 (3) solutions for 7 days. Reproduced with permission (Devi et al., 2012). Annona squamosa (Annonaceae) or custard apple is an edible fruit of tropical origin widely found in America and Asia and is reported to have medicinal, antimicrobial, insecticidal and anti-cancerous properties (Dwivedi & Gopal, 2010; Madhumitha et al., 2012; Mukhlesur Rahman et al., 2005; Wagner et al., 1980). The fruit pulp is consumed but peels are discarded. Water soluble ketone and hydroxyl groups are present in the peel which are found to be responsible for the reduction of silver ions and formation of Ag NPs. These groups provide stability to NPs by forming a thin layer on the surface (Kumar et al., 2012). 30
Table 2 summarizes the compositions, size, and morphology of the NPs synthesized using waste materials. Table 2. Waste material mediated synthesis of metallic NPs. Composition of NPs
Waste Material
Cellulose
Cotton fibers
Silicon carbide
Electronic (compact char
Ag
Ag
Size (nm)
Morphology
40-90
Spherical
40-90
Spherical
discs
Citation
(Fattahi Meyabadi et al., 2014)
(Rajarao et al., 2014)
Satsuma 5-20 mandarin(Citrus unshiu) peel extract
Spherical
Industrial waste
Nanorods
(Basavegowda & Rok Lee, 2013)
milk -
31
(Sivakumar et al., 2013)
Fe
Citrine juices
Au
Spherical, cylindrical, irregular
(Machado et al., 2014)
Grape skin, stalk 20-25 and seeds
Quasi-spherical
(Krishnaswamy et al., 2014)
Au
Rice bran
50-100
Spherical
(Malhotra et al., 2014)
Pd
Watermelon rind
96
Spherical
(Lakshmipathy et al., 2015)
N-CNTs Poultry (Nitrogen doped Feather carbon nanotubes
3-300
chicken -
(Gao et al., 2014)
32
5.2. Metal oxide NPs As compared to metallic NPs, synthesis of metal oxide NPs using agro and food waste is yet to be explored widely. Thus, this opens up an upcoming area of research where eco-friendly and fast methods can be developed thereby achieving the potential applications of the metal oxide NPs. The tea waste template has been used for the fabrication of magnetic iron oxide (Fe3O4) NPs in the size range of 5 to 25 nm with cuboid/pyramid morphology. These NPs proved to be very efficient in removing arsenic metal from water and could be used up to five adsorption cycles (Lunge et al., 2014). Another material which is considered as garbage is egg-shells from the baking, food processing and baking industry. This is considered to have no food value, but it has been used for the synthesis of hydroxyapatite NPs. Eggshells contain calcium carbonate (94%), magnesium carbonate (1%), calcium phosphate (1%) and organic matter (4%). The high calcium content present assists in the formation of hydroxyapatite NPs (Rivera et al., 1999; Wu et al., 2013). Banana is a favourite food worlwide and according to statistics which are not complete, more than 100 million tons of banana is consumed world-wide every year (Venkateswarlu et al., 2013) and the peel is dumped as garbage. This is composed of biopolymers like cellulose, hemicelluloses, pectin, lignin and proteins and could be efficiently used for the synthesis of NPs. Hydroxyapatite NPs have been synthesized using banana peel. Pectin present in the peel play a major role in the surface modification of the NPs (Chanakya et al., 2009; Happi Emaga et al., 2008; Morra et al., 2004; Rodríguez-Ambriz et al., 2008). The assynthesized NPs exhibited antibacterial activity against gram positive and negative bacteria (Gopi et al., 2014). Banana peel extract has also been used successfully to synthesize Mn3O4 NPs having super-capacitive properties (Yan et al., 2014). Mn3O4 is one of most stable oxide of manganese which possesses broad range of interesting properties ranging from catalysis to high 33
density magnetic storage medium (Wang et al., 2009). SEM image depict the agglomerated particles with an average diameter of 20-50 nm (Figure 17A). Figure 17B represent cyclic voltammetric (CV) profiles which indicate high electrochemical reversibility of the as synthesized NPs. 93% of specific capacitance (SC) was maintained after 2000 cycle numbers of charge discharge process thereby confirming the superior stability of Mn3O4 electrode (Figure 17C).
Fig. 17. (A) SEM image of Mn3O4 NPs (B) Cyclic voltammograms of Mn3O4 (C) Cycle performance
during 2000 cycles at a current density of 0.3 A g-1 (the inset shows charge–discharge curves for the first (left) and last (right) five cycles. Reproduced with permission (Yan et al., 2014).
5.3. Mechanism of waste material mediated synthesis of NPs 34
The cell walls of food waste materials contain cellulose, hemicelluloses, pectins, lignins, proteins and biodegradable polysaccharides. The waste materials also contain phytochemicals like polyphenols, carotenoids, flavonoids, dietary fibres and essential oils. These act as templating agents in the synthesis of NPs thereby determining their morphology and size. The biomolecules cause the reduction of metal salts into metal or metal oxide NPs (Heim et al., 2002). The plausible mechanism for the synthesis of NPs using waste materials has been diagrammatically shown in Figure 18.
Fig. 18. Mechanism of formation of NPs in the presence of waste materials. 5.4 Advantage of waste material in the synthesis of NPs Green synthesis has led to the fabrication of many inorganic NPs especially, metal NPs. Although different species of algae have been used for NP synthesis but waste materials (food and agro waste) have been found to be best suited for preparation of NPs of diverse 35
morphologies and sizes. The different biomolecules present in the waste act as templating agents and thus, lead to the fast, completely green and cost effective approach. This waste is easily available and does not require rigorous processing methods. These can be directly used in the synthesis of NPs and also pave the way for the management of the waste. Whereas, the preparation of algae culture requires time and cost. There is a limitation of maintaining the cultures over a period of time. The toxicity factors of some of the species of the algae is also to be considered before using them for the synthesis of NPs as certain compounds present could induce toxicity to the NPs and thus limit their use for biomedical applications. Moreover, limited morphologies could be obtained via algae mediated synthesis of NPs as can be seen in Tables 1 and 2. Thus, the main challenges related to algae mediated synthesis of NPs are (Seabra & Durán, 2015): (i) reproducibility of the methods needs to be improved. (ii) limitation of scaling up of the synthesis methods. (iii) complete elucidation of mechanism of formation of NPs. (iv) size control and monodispersity of NPs. 6. Applications of biogenic synthesis of NPs The applications of biosynthesized NPs range from biomedical to photocatalytic and sensors. The properties of these NPs are different from the NPs synthesized via other conventional and chemical methods since, no capping agents or surfactants are involved. Thus, NPs synthesized using algae and waste materials exhibit a broad new spectrum of potential applications. 6.1. Biomedical applications
36
Extensive research is going on and vast literature is available on the antimicrobial activity of NPs. Ag NPs have attracted much attention as an efficient antimicrobial and biocompatible agent. These can effectively bind with the cell wall which is required for better antimicrobial activity (Eckhardt et al., 2013). Ag NPs possess higher antibacterial activity against E.coli than S. aureus due the structural difference of the cell wall (Feng et al., 2000; Jung et al., 2008). Being super paramagnetic in nature, iron and iron oxide NPs find extensive usage in biomedical applications like cell labeling, tissue repair, magnetic resonance imaging (MRI), and drug delivery (Catherine & Adam, 2003; Pankhurst et al., 2003). Au NPs have proved to be important tool in many potential biomedical applications including an emerging alternative for life threatening diseases and also used in DNA modeling (Gupta & Gupta, 2005; Khan et al., 2013). Au NPs with different sizes display optical properties necessary for biosensor applications, especially in cancer nanotechnology. PEG coated Au NPs maximize the tumour damage as compared to Tumor necrosis factor-alpha (TNF-α), a cytokine which has anticancer efficacy, but limited therapeutic applications (Cai et al., 2008; van Horssen et al., 2006; Visaria et al., 2006). Au NPs exhibit high antibacterial activity due to their small size and high surface area. These NPs suppress the respiratory chain enzymes which are vital for the cell wall synthesis of bacteria thereby leading to death or static growth. Spherical Au NPs synthesized from protein extract of blue green alga, S. platensis have been reported to show inhibitory action against B. subtilis and S. aureu. Since, gram positive bacteria have thick peptidoglycan layer, NPs adhere to the membrane and break the bonds thereby entering inside the microorganism (Uma Suganya et al., 2015). Figure 19 depicts the damage caused to bacterial membrane on treatment with Au NPs. 37
Fig. 19. TEM images of (A) B. subtilis (B) S. aureus cells treated with S. platensis protein protected Au NPs in agar medium for 1 h. Reproduced with permission (Uma Suganya et al., 2015). Thus, functionalized NPs could be modified for use in advanced medical applications with greener methods. Banana peels rich in lignin, cellulose, pectins and hemicullose act an excellent template for the synthesis of NPs. The peel extract has been used to synthesize Ag NPs which showed efficacious antimicrobial activity against pathogenic bacteria (Bacillus subtilis, Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli) and yeast (Candida albicans) (Ibrahim, 2015). The mechanism for the interaction of NPs with the specific membrane has been illustrated in Figure 20. As soon as NP comes in contact with the membrane of pathogenic 38
microorganisms, there is dissolution and release of metal cations which inhibit respiratory enzymes and ATP production. There is ROS (reactive oxygen species) production which disrupts membrane integrity and other transport processes (Nel et al., 2009).
Fig. 20. Representation of receptor-mediated uptake. This is the specific biological mechanism for particles interacting with the surface membrane and undergoing cellular uptake. The intrinsic NP characteristics that promote surface binding (roughness, hydrophobicity, cation charge) generally lead to nonspecific binding forces (marked by asterisks) that promote cellular uptake. In contrast, specific receptor-ligand interactions generally lead to endocytic uptake. A combination of nonspecific binding forces on the surface of spiked particles can lead to direct penetration of the surface membrane without the need to involve endocytic compartments. Reproduced with permission (Nel et al., 2009). 6.2. Catalytic applications Biosynthesized NPs exhibit interesting size dependent catalytic properties due to high surface-to-area volume ratio. Pd NPs synthesized using soya leaf extract caused the degradation of azo dyes (Petla et al., 2012). Fe3O4 NPs coated with soluble bio-based products (SBO) efficiently adsorbed crystal violet (CV) dye used as a model pollutant (Figure 21A). Thus, these NPs could be used for the removal of pollutants in the water (Magnacca et al., 2014). The effect
39
of pH on the removal of CV dye with NPs was studied by carrying out sorption experiments. It was observed that as pH was increased the % removal of dye also increased (Figure 21B).
B
A
Fig. 21. (A) Fe3O4 NPs coated with SBO (B) CV removal obtained with NP/0.5 at different pHs. [CV]0 =10 mg L−1 ; [NP/0.5]0=150 mg L−1. Reproduced with permission (Magnacca et al., 2014). Ag NPs with an average diameter of 2.46 nm were prepared from eggshell membrane (ESM) which is considered as food waste. Ag NPs possessed good catalytic activity for the reduction of 4-nitrophenol and could be used upto eight cycles because of high stability (Liang et al., 2014). ESM fibres were treated with Procyanidin (Pro) extracted from grape seeds and skin so that they could act as reductant and stabilizer. Plant extracts are one of the agricultural waste that has become a new face in the green synthesis of NPs. Prunus domestica (plum) fruit extract has been used for the synthesis of spherical Au NPs with an average diameter of 4-38 nm which 40
caused the reduction of toxic pollutant 4-nitrophenol to 4-aminophenol (Dauthal & Mukhopadhyay, 2012). Spherical Pd NPs synthesized within green microalgae (Chlorella vulgaris) acted as a catalyst for Mizoroki–Heck cross-coupling reaction (Figure 22). They were synthesized via photosynthetic reaction and thus provide a green route of synthesis for other NPs (Eroglu et al., 2013).
Fig. 22. Palladium NP synthesis by photosynthetic green microalgae, and their uptake on an electrospun chitosan mat for use as a catalyst in Mizoroki–Heck reactions. The left stage shows a combination of mechanisms taking place within the photosynthetic organisms, resulting in the production of reducing agents. (ADP: adenosine diphosphate, ATP: adenosine triphosphate, Fd: ferredoxin, NADP+: oxidized form of nicotinamide adenine dinucleotide phosphate, NADPH: reduced form of nicotinamide adenine dinucleotide phosphate, PGA: phosphoglycolic acid, RuBisCO: rubilose biphosphate carboxylase). NADPH is likely one of the main reducing agents for the reduction of Na2[PdCl4], which is partially oxidised as a result of aerobic culture conditions. Reproduced with permission (Eroglu et al., 2013). Pd NPs have also been reported to synthesize directly on alginic acid (AA) and seaweed (Laminaria digitata, a brown alga with the common name Oarweed). The reaction of iodobenzene and methyl acrylate was used to determine the catalytic activity. It was noted that 75% yield was obtained after 20 minutes in the presence of Pd NPs with a reusability of 2-3 times (Parker et al., 2015). 41
6.3. Biosensing applications The biosensing applications of algae and waste mediated synthesized NPs is under study and would be preferred over commercially synthesized NPs. Here, in brief, biosensing ability of NPs synthesized from other sources has been discussed which would make NPs derived from algae and waste materials a better choice. Biosynthesized Au NPs have proved to be very important tool for hormone (HCG) detection in pregnant women urine sample (Kuppusamy et al., 2014). Adrenaline acts as a drug which is widely used in the treatment of allergies, heart attack, asthma and cardiac surgery. Therefore, detection of adrenalin is becoming an active area of research from medical point of view. Pt NPs has been acted as a novel biosensor with high sensitivity for the determination of adrenaline (Brondani et al., 2009). Nanoscale Au-Ag alloy prepared via chloroplasts exhibited high electrocatalytic activity for 2-butanone at room temperature thereby providing a platform for the development of biosensor capable of detecting cancer at early stages (Zhang et al., 2012).
7. Conclusions The use of different types of algae in the synthesis of NPs have encouraged the designing of simple, green, cost and time effective approaches thereby, minimizing the use of chemicals and solvents. The polysaccharides, proteins and lipids present in the algal membranes act as capping agents and thus limit the use of non-biodegradable commercial surfactants which are difficult to remove after the synthesis of NPs. But the limitation with the use of algae is that, not all the species can be exploited for the synthesis as some of them contain toxic compounds and moreover, the mechanism for synthesis have not been fully explored yet. This limitation has led to the way for the use of waste materials in the synthesis process of NPs. The waste material, especially fruit waste is easily available and does not require pre-conditioning of the materials. 42
The method of synthesis is very simple, requiring less time and energy and predictable mechanisms. This opens up an opportunity for the use of biodegradable materials especially in the synthesis of metal oxide NPs.
8. Future prospects The biosynthesis of NPs using waste materials will help researchers not only to design safer nanomaterials but also promote the understanding of health and safety considerations of NPs. Useful materials can be produced easily even at reasonable scale because the biomaterial based routes eliminate the need to use toxic chemicals. Considerable efforts are devoted on different capping agents such as biomolecules and polysaccaharides which can act as both chelating/reducing and capping agents for the synthesis of NPs. Therefore, the resulting particles are protected from further reactions and aggregation, which increases their stability and longevity. Greener methods that have been used in NP synthesis are generally single-pot reactions, without the use of additional surfactants, capping agents and templates. Survey of literature reveals that these investigations have been carried out at laboratory scale, whereas there are no reports on ‘pilot plant’ synthesis of NPs using waste materials and algae. We believe that there are good opportunities for developing industrial scale production, where NPs have important applications. Overall, the use of algae and waste materials for green synthesis of NPs/ nanomaterials is an emerging and exciting area of nanotechnology and may have significant impact on further advances in nanoscience. Moreover, with the availability of computational techniques, area of green synthesis and development of nanomaterials will be broadened that could be used in the field of medicine as ‘nanodrugs’ in the near future. Also, these NPs would
43
provide a potential solution for the present energy crisis by finding their use as energy driven devices. Acknowledgements Our grateful acknowledgement goes to the Durban University of Technology and National Research Foundation of South Africa for the financial support.
44
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