Biogenic synthesis of nanoparticles: A review

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 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 affect 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 and 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 (Fig. 2). The binding affinity of amino acids is found to be different for different facets of Au crystal. The fusion of NPs along (1 1 1) 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 (1 1 1) 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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Figure 2

Illustration of Au nanochain and nanowire formation through dipole–dipole interaction due to the zwitterionic nature of amino acids. Reproduced with permission (Polavarapu and Xu, 2008).

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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 and 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 of many metal NPs (Virkutyte and 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 alternatives for the fabrication of NPs replacing toxic chemicals thereby saving the environment from their hazardous effects.

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 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 (1 1 1) and Pt (1 0 0) 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 (1 1 1) facet whereas Asn, Leu and Thr were Pt (1 0 0) facet specific. The charge transfer and exchange processes along with 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 the growth and morphology of ZnO crystal was studied by Liang et al. (2011), Sola-Rabada et al. (2015). The morphology of the ZnO crystal was altered by G-12 and GT-16 via adsorption-growth inhibition mechanism (Fig. 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 the presence of G-12 and reduction of length along c-axis in the presence of both GT-16 and G-12 led to the following hypothesis:

1. G-12 was able to adsorb preferentially on (0 0 0 1) plane as well as on (1 0 1 0) plane.

2. Selective adsorption of GT-16 on (0 0 0 1) plane.

3. The selectivity of GT-16 was due to presence of GGGC tag not present in G-12.

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Figure 4

Adsorption–growth–inhibition mechanism. The length and thickness of the growth arrows reflect the growth rate of the crystals along their respective axis. Reproduced with permission (Sola-Rabada et al., 2015).

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This gave an insight into the desired tuning of ZnO morphology by the selective adsorption characteristics of the biomolecules onto the crystal faces.

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 et al., 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 (0 0 01) 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 Zn²⁺ via electrostatic interactions and strongly influence the morphology. They reduce Zn²⁺ 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:

1. As concentration of EC-12 was increased, the crystal remained twinned.

2. Along the c-axis the diameter of the crystal was not uniform.

3. 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.

4.1 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 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 such as 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 was manufactured via microalgae as a bio-template using efficient and eco-friendly 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 the following two ways:

1. Nucleation and crystal growth were accelerated due to the presence of negative charge on the surface of the cell.

2. 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 Ca²⁺ 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 CaCO₃ microspheres keeping the constant concentration of algal cells and favoring 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. Fig. 5 illustrates the formation of CaCO₃ microspheres and nano Ag–CaCO₃ composite.

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Figure 5

(a) Schematic illustration of the synthetic steps for porous CaCO₃ 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 surface-directed mineralization of CaCO₃. The negatively charged microalgae surface aggregates Ca²⁺ ion via electrostatic interaction prior to nucleation. Nucleation occurs on supplying CO₃²⁻ from CO₂ hydration resulting in the formation of nanoCaCO₃. Reproduced with permission (Sahoo et al., 2014).

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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 (50–100 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 potential ‘nano-reserves’ 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 such as 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 nanocatalysts degrading methyl orange dye were synthesized from Hypnea musciformis at room temperature (Ganapathy Selvam and Sivakumar, 2015; Kumar et al., 2011). Similarly, Ag NPs with face centered cubic crystalline structure have been synthesized from 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 has 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 (Fig. 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’ routes for the synthesis of NPs (Sicard et al., 2010).

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Figure 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).

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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 and 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 freshwater, 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 freshwater green algae which is used for the one step biosynthesis of Au NPs in the size range of 5–25 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 and Pal, 2015). Thus, algal-NPs interactions are very important considering the effective use of NPs in drug delivery (Fig. 7).

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Figure 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 and Pal, 2015).

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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	Species of Algae	Size (nm)	Morphology	Citation
Au	Brown, Sargassum muticum	5.42 ± 1.18	Spherical	Namvar et al. (2015)
Au	Tetraselmis kochinensis	5–35	Spherical and triangular	Senapati et al. (2012)
Au	Brown, Ecklonia cava	30 ± 0.25	Spherical and triangular	Venkatesan et al. (2014)
Ag	Caulerpa racemosa	5–25	Spherical and Triangular	Kathiraven et al. (2015)
Ag	Brown, Cystophora moniliformis	50–100	Spherical	Prasad et al. (2013)
Ag	Chlamydomonas reinhardtii	5–35	Round/rectangular	Barwal et al. (2011)
Au	Chlorella vulgaris	2–10	Spatial array of self assembled Structures	Annamalai and Nallamuthu, (2015)
CdS	Phaeodactylum tricornutum	-	NA	Scarano and 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 their NPs. Transition metal oxides have been used in numerous industrial applications. NPs, nano-powders and 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 such as hydrothermal, solvothermal, microwave, vapor deposition, seed mediated, spray pyrolysis, wet-chemical have been employed for the preparation of metal oxide NPs with diverse morphology and size (Gotić and Musić, 2008; Hayashi and Hakuta, 2010; Kim et al., 2003; Leonelli and 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 and 45 nm (Fig. 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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Figure 8

TEM image of CuO NPs synthesized from the alga extract. Reproduced with permission (Abboud et al., 2014).

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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 such as spherical, triangular, radial, hexagonal, rod and rectangular were formed with size range of 76–186 nm. The water soluble pigments present in the extract were found to be responsible for the reduction and stabilization of the NPs (Nagarajan and Arumugam Kuppusamy, 2013). AFM study of ZnO NPs revealed that change in temperature parameter remarkably affected the size and morphology of the particles (Fig. 9).

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Figure 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 and Arumugam Kuppusamy, 2013).

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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 (Fe₃O₄) NPs with cubic morphology and average diameter of 18 ± 4 nm (Mahdavi et al., 2013). S. muticum is algal seaweed which is also used as functional food consisting of large quantities of lipids, minerals, vitamins and many other bioactive substances such as 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 eco-friendly approach. Recent report on the synthesis via aqueous extract of brown alga S. muticum has indicated the formation of pure ZnO NPs in the size range of 30–57 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 algae (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 (Fig. 10A). ZnO_AG 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, Fig. 10B) whereas the degradation of phenol was less than 35% in the case of ZnO_AA. 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.

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Figure 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).

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The mechanism for the formation of NPs in the presence of algae is yet to be fully understood. The algal membranes consist of biomolecules such as polysaccharides, proteins and enzymes which catalyze the reduction of metal salt precursors into metal or metal oxide NPs. Since, these are large molecules and amphiphilic 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.

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 such as polysaccharides, proteins, enzymes present in the organism with NPs thereby comprehending the mechanism. The simple mechanism in Fig. 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). The different sources of waste materials for the green synthesis of NPs was schematically represented in Fig. 12.

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Figure 11

Mechanism of biosynthesis of NPs using algae.

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Figure 12

Type of waste materials employed/could be employed in the synthesis of NPs.

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5.1 Metallic NPs

The environment also acts as a ‘treasure’ of waste materials especially food waste which can be successfully utilized for the biosynthesis of NPs. Recent literature reports the use of biodegradable food waste for manufacturing different NPs. The food waste material contains different organic compounds such as polyphenols, flavonoids, carotenoids 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 acts 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 (Fig. 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.

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Figure 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 are represented in the form of a bar graph and plotted using means ± S.E. of triplicate determinations. Reproduced with permission (Yang et al., 2014).

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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 25–35 nm were formed by reduction of silver ions into NPs in the presence of extract from grape seeds (Fig. 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 3429.14, 2954.88, 2928.66, 2855.29, 1613.67, 1524.71, 1445.81, 1380.06, 1281.05, 1103.14, 1053.63, 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, 2928.66 cm⁻¹, 2,855.29 cm⁻¹ referred to C–H stretching vibration mode 1613.67 cm⁻¹, 1,524.71 cm⁻¹ referred to vibration of cyclobenzene 1103.14 cm⁻¹, and 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 3429.14 to 3436.10 cm⁻¹ indicating the formation of Ag NPs (Fig. 14B).

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Figure 14

(A) TEM image of Ag NPs, (B) FTIR spectra of grape seed extract and Ag NPs. Reproduced with permission (Xu et al., 2015).

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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 (Fig. 15) (Kahrilas et al., 2014).

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Figure 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).

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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 such as glycine alanine and uronic acid (Arias et al., 1991; Wong et al., 1984). The application of EMS as reported in the 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 were monitored by absorption and fluorescence spectral changes in addition to change in the color. 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 Au³⁺ solution indicating the removal of Au³⁺ from the solution. Also a color change was visible from white to yellow showing the adsorption of Au³⁺ on the membrane surface (Fig. 16).

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Figure 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).

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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 and 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).

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	Size (nm)	Morphology	Citation
Cellulose	Cotton fibers	40–90	Spherical	Fattahi Meyabadi et al. (2014)
Silicon carbide	Electronic compact disks char	40–90	Spherical	Rajarao et al. (2014)
Ag	Satsuma mandarin(Citrus unshiu) peel extract	5–20	Spherical	Basavegowda and Rok Lee (2013)
Ag	Industrial milk waste	–	Nanorods	Sivakumar et al. (2013)
Fe	Citrine juices	3–300	Spherical, cylindrical, irregular	Machado et al. (2014)
Au	Grape skin, stalk and seeds	20–25	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 (Nitrogen doped carbon nanotubes)	Poultry chicken Feather	–		Gao et al. (2014)

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 5–25 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 favorite food worldwide and according to statistics which are not complete, more than 100 million tons of bananas is consumed worldwide every year (Venkateswarlu et al., 2013) and the peel is dumped as garbage. This is composed of biopolymers such as 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 plays 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 as-synthesized 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 oxides of manganese which possesses broad range of interesting properties ranging from catalysis to high density magnetic storage medium (Wang et al., 2009). SEM image depicts the agglomerated particles with an average diameter of 20–50 nm (Fig. 17A). Fig. 17B represents 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 (Fig. 17C).

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Figure 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).

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5.3 Mechanism of waste material mediated synthesis of NPs

The cell walls of food waste materials contain cellulose, hemicelluloses, pectins, lignins, proteins and biodegradable polysaccharides. The waste materials also contain phytochemicals such as polyphenols, carotenoids, flavonoids, dietary fibers 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 Fig. 18.

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Figure 18

Mechanism of formation of NPs in the presence of waste materials.

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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 waste materials (food and agro waste) have been found to be best suited for preparation of NPs of diverse 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 are 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 as follows (Seabra and Durán, 2015):

1. Reproducibility of the methods needs to be improved.

2. Limitation of scaling up of the synthesis methods.

3. Complete elucidation of mechanism of formation of NPs.

4. Size control and monodispersity of NPs.

6.1 Biomedical applications

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 to 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 such as cell labeling, tissue repair, magnetic resonance imaging (MRI), and drug delivery (Catherine and 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 have been used in DNA modeling (Gupta and 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 tumor 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). Fig. 19 depicts the damage caused to bacterial membrane on treatment with Au NPs.

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Figure 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).

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Thus, functionalized NPs could be modified for use in advanced medical applications with greener methods.

Banana peels rich in lignin, cellulose, pectins and hemicellulose 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 (B. subtilis, S. aureus, Pseudomonas aeruginosa, and E. coli) and yeast (Candida albicans) (Ibrahim, 2015).

The mechanism for the interaction of NPs with the specific membrane has been illustrated in Fig. 20. As soon as NP comes in contact with the membrane of pathogenic microorganisms, there is dissolution and release of metal cations which inhibit respiratory enzymes and ATP production. There is reactive oxygen species (ROS) production which disrupts membrane integrity and other transport processes (Nel et al., 2009).

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Figure 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).

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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 (Fig. 21A). Thus, these NPs could be used for the removal of pollutants in the water (Magnacca et al., 2014). The effect 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 (Fig. 21B).

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Figure 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).

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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 up to eight cycles because of high stability (Liang et al., 2014). ESM fibers 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 wastes 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 caused the reduction of toxic pollutant 4-nitrophenol to 4-aminophenol (Dauthal and Mukhopadhyay, 2012).

Spherical Pd NPs synthesized within green microalgae (Chlorella vulgaris) acted as a catalyst for Mizoroki–Heck cross-coupling reaction (Fig. 22). They were synthesized via photosynthetic reaction and thus provide a green route of synthesis for other NPs (Eroglu et al., 2013).

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Figure 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 oxidized as a result of aerobic culture conditions. Reproduced with permission (Eroglu et al., 2013).

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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 min in the presence of Pd NPs with a reusability of 2–3 times (Parker et al., 2015).

6.3 Biosensing applications

The biosensing applications of algae and waste mediated synthesized NPs are 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 have 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).