Nanoparticles Synthesized by Microorganisms

Tiquia-Arashiro, Sonia; Rodrigues, Debora

Microorganisms capable of synthesizing nanoparticles are prevalent microflora of the terrestrial and marine ecosystems. These microorganisms are involved in biogeochemical cycling of metals in processes such as precipitation (biomineralization), decomposition (bioweathering), and degradation (biocorro-sion). The biosynthesis of metal NPs by microbes is a function of heavy metal toxicity resistance mechanisms. Resistance mechanisms range from redox enzymes that convert toxic metal ions to inert forms, structural proteins that bind protein, or through the use of efflux proteins that transport metal ions by proton motive force, chemiosmotic gradients, or ATP hydrolysis, which work together to coordinate synthesis nanoparticle synthesis. This chapter focuses on the biological systems; bacteria, fungi, actinomycetes, and algae for utilization in nanotechnology, especially in the development of a reliable and eco-friendly processes for the synthesis of metallic nanoparticles. The rich microbial diversity points to their innate potential for acting as potential biofactories for nanoparticles synthesis.

Chapter 1 Nanoparticles Synthesized by Microorganisms Abstract Microorganisms capable of synthesizing nanoparticles are prevalent microflora of the terrestrial and marine ecosystems. These microorganisms are involved in biogeochemical cycling of metals in processes such as precipitation (biomineralization), decomposition (bioweathering), and degradation (biocorrosion). The biosynthesis of metal NPs by microbes is a function of heavy metal toxicity resistance mechanisms. Resistance mechanisms range from redox enzymes that convert toxic metal ions to inert forms, structural proteins that bind protein, or through the use of efflux proteins that transport metal ions by proton motive force, chemiosmotic gradients, or ATP hydrolysis, which work together to coordinate synthesis nanoparticle synthesis. This chapter focuses on the biological systems; bacteria, fungi, actinomycetes, and algae for utilization in nanotechnology, especially in the development of a reliable and eco-friendly processes for the synthesis of metallic nanoparticles. The rich microbial diversity points to their innate potential for acting as potential biofactories for nanoparticles synthesis. 1.1 Introduction Nanoparticles (NPs) fall within the size range of 0.1–100 nm and are capable of exhibiting a range of ideal properties such as near identical strength (e.g. resistance to crushing), active surfaces, which have important catalytic properties; and discrete energy levels that can yield some important tailoring of electronic properties (Daniel and Astruc 2004; Kato 2011). While the chemical compositions of NPs are important, the morphology of the NPs (size and shape) and its surface/colloidal properties are equally essential. For example, smaller size NPs are known to have more antimicrobial activity than larger NPs (Chwalibog et al. 2010). With regard to drug delivery, the smaller the NP, the longer it will remain in the circulatory system and therefore have a greater chance of being distributed among the target sites (Gaumet et al. 2008). NPs in general can be synthetically formed or occur naturally (e.g. by microbial biosynthesis) within the environment. In both instances, variations in the morphology of the resulting NPs are common. NPs can be found as © Springer International Publishing AG 2016 S. Tiquia-Arashiro and D. Rodrigues, Extremophiles: Applications in Nanotechnology, DOI 10.1007/978-3-319-45215-9_1 smtiquia@umich.edu 1 2 1 Nanoparticles Synthesized by Microorganisms nanospheres, nanorods, nanocubes, nanoplates, nanobelts, nanotetrapods, and nanoprisms. These can be loosely grouped into face-centered cubic, cuboctahedron, icosahedrons, regular decahedron, star decahedron, marks decahedron, and round decahedrons (Yacaman et al. 2001). Another extremely important aspect of NPs in addition to their morphology is their composition. For instance NPs containing metal ions have many beneﬁcial properties, which are becoming increasingly more common in new technology and processes. For example, a recent review on silver nanoparticles (AgNP) (Sweet and Singleton 2011) covers the usage of AgNP in a wide range of applications including food storage, photonics, information storage, electronic and optical detection systems, therapeutics, diagnostics, photovoltaics, and catalysts. However, despite the signiﬁcant advantages of NPs being formed with metals such as silver, the challenge of synthetically controlling the shape of metal NPs has met with variable success, making manipulation aimed at a certain size and/or shape of metal NPs difﬁcult. Microorganisms capable of synthesizing NPs are especially prevalent microflora of the terrestrial and marine ecosystems. It is well known that microbes are involved in biogeochemical cycles of metals in processes such as precipitation (biomineralization), decomposition (bioweathering), and degradation (biocorrosion). Central to each ecological process is the mobilization, distribution, and chemical modiﬁcation that govern metal speciation and ultimately toxicity (Gadd 2010). As a consequence of these ecological processes, microbes are often subjected to toxic levels of heavy metals, which unless managed, may induce cell death. For example, silver toxicity can occur through interaction with thiol groups of membrane-bound proteins including enzymes involved in respiration, leading to disruption of the cellular membranes, and subsequent disruption of proton motive force through an inability to maintain a proton gradient. This is thought to promote uncoupling of the respiratory chain from oxidative phosphorylation, due to disruption of electron transport (Holt and Bard 2005). Uncontrolled respiration promotes superoxide and hydroxyl radical formation, leading to induction of SOS response and ultimately cell death. Similar metal toxicity responses are observed with other metals, for example, cadmium (Ahmad et al. 2002), and NP biosynthesis would appear to be a common byproduct of metal resistance. The biosynthesis of metal NPs by microbes is a function of heavy metal toxicity resistance mechanisms, whereby toxic heavy metals are converted to nontoxic species and precipitated as metal clusters of nanoscale dimension and deﬁned shape (Narayanan and Sakthivel 2010). Resistance mechanisms range from redox enzymes that convert toxic metal ions to inert forms, structural proteins that bind protein (Gadd 2010), or through the use of efflux proteins that transport metal ions by proton motive force, chemiosmotic gradients, or ATP hydrolysis (Nies 2003). It is proposed that such mechanisms work to coordinate synthesis. This chapter provides an overview of the different types of NPs and the different microorganisms that synthesize them. The general scheme of the formation of metallic nanoparticles through biosynthesis is shown in Fig. 1.1. The nanoparticles are produced either intracellularly or smtiquia@umich.edu 1.1 Introduction 3 Fig. 1.1 The formation of the metal nanoparticles (Me-NPs) during biosynthesis. Source Mittal et al. (2013). Copyright © 2013, Elsevier. Reproduced with permission extracellularly (Rangarajan et al. 2014). In case of the intracellular synthesis the nanoparticles are produced inside the bacterial cells by the reductive pathways of the cell wall and accumulated in the periplasmic space of the cell. The nanoparticles are produced extracellularly when the cell wall reductive enzymes or soluble secreted enzymes are extracted outside the cell and are involved in the reductive process of metal ions. One of the enzymes involved in the biosynthesis of metal nanoparticles is the nitrate reductase which reduces the metal ions (Me+1) to the metallic form (Me0). This enzyme is a NADH- and NADPH-dependent enzyme. He et al. (2007) described the hypothetical mechanism for gold nanoparticles biosynthesis carried out by Rhodopseudomonas capsulate. These bacteria are known to secrete cofactor NADH- and NADH-dependent enzymes that can be responsible for the biological reduction of Au3+ to Au0 and the subsequent formation of gold nanoparticles (Fig. 1.2). This reduction is initiated by electron transfer from the NADH by NADH-dependent reductase as electron carrier during which the gold ions gain electrons and are therefore reduced to Au0. smtiquia@umich.edu 4 1 Nanoparticles Synthesized by Microorganisms Fig. 1.2 Hypothetical mechanism for silver and gold nanoparticles biosynthesis. Source He et al. (2007). Copyright © 2007, Elsevier. Reproduced with permission 1.2 Metallic Nanoparticles Metallic nanoparticles have fascinated scientist for over a century and are now heavily utilized in biomedical sciences and engineering. They are a focus of interest because of their huge potential in nanotechnology. Metallic nanoparticles have possible applications in diverse areas such as electronics, cosmetics, coatings, packaging, and biotechnology. For example, nanoparticles can be induced to merge into a solid at relatively lower temperatures, often without melting, leading to improved and easy-to-create coatings for electronics applications. Typically, NPs possess a wavelength below the critical wavelength of light. This renders them transparent, a property that makes them very useful for applications in cosmetics, coatings, and packaging. Metallic NPs can be attached to single strands of DNA nondestructively. This opens up avenues for medical diagnostic applications. Nanoparticles can traverse through the vasculature and localize any target organ. This potentially can lead to novel therapeutic, imaging, and biomedical applications. Today these materials can be synthesized and modiﬁed with various chemical functional groups which allow them to be conjugated with antibodies, ligands, and drugs of interest and thus opening a wide range of potential applications in biotechnology, magnetic separation, and preconcentration of target analytes, targeted drug delivery, and vehicles for gene and drug delivery and more importantly diagnostic imaging (Mody et al. 2010). Some typical metal nanoparticles produced by microorganisms are summarized in Table 1.1. smtiquia@umich.edu 1.2 Metallic Nanoparticles 5 Table 1.1 Metal nanoparticles synthesized by microorganisms Microorganism Type nanoparticle synthesize Size (nm) Shape Reference Bacteria Actinobacter spp. Magnetite 10–40 Bacillus lichineformis Silver 50 Bacillus cereus Silver 4–5 Not available Not available Spherical Brevibacterium casei Gold, silver 10–50 Spherical Clostridium thermoaceticum Corynebacterium glutamicum Desulfovibrio desulfuricans Enterobacter sp. Cadmium sulﬁde Silver Not available 5–50 Not available Irregular Palladium 50 Spherical Mercury 2–5 Spherical Escherichia coli Gold 20–30 Escherichia coli Klebsiella pneumoniae Lactobacillus spp. Cadmium telluride Gold 2-3 5–32 Gold, Silver Triangles, hexagons Spherical Not available Not available Not available Not available Spherical Bharde et al. (2005) Kalimuthu et al. (2008) Babu and Gunasekaran (2009) Kalishwaralal et al. (2010) Sweeney et al. (2004) Gurunathan et al. (2009) Lloyd et al. (1998) Sinha and Khare (2011) Gericke and Pinches (2006) Bao et al. (2010) Malarkodi et al. (2013) Nair and Pradeep (2002) Husseiny et al. (2007) Klaus et al. (1999) Kasheﬁ and Lovley (2000) Pseudomonas aeruginosa Pseudomonas stutzeri Gold Not available 15–30 Silver 200 nm Pyrobaculum islandicum Uranium (VI), Technetium (VII), Chromium (VI), Cobalt (III), Manganese (IV) Gold Not available 5–15 Spherical Rhodopseudomonas capsulate Shewanella algae Gold 10–20 Spherical Gold 10–20 Shewanella oneidensis Shewanella algae Gold 12–17 Not available Spherical Rhodococcus sp. Platinum 5 smtiquia@umich.edu Not available Ahmad et al. (2003b) He et al. (2007) Konishi et al. (2007a) Suresh et al. (2011) Konishi et al. (2007b) (continued) 6 1 Nanoparticles Synthesized by Microorganisms Table 1.1 (continued) Microorganism Type nanoparticle synthesize Size (nm) Shape Reference Shewanella sp. Thermonospora Selenium Silver 181–221 8 Ureibacillus thermosphaericus Fungi Aspergillus flavus Silver 50–70 Spherical Not available Not available Lee et al. (2007) Ahmad et al. (2003a) Juibari et al. (2011) Silver 8–9 Spherical Aspergillus fumigatus Silver 5–25 Spherical Candida utilis Gold Fusarium oxysporum Silver Not available 5–50 Not available Spherical Fusarium oxysporum Silicon 5–10 Spherical Fusarium oxysporum Titanium 6–13 Spherical Neurospora crassa Gold, silver/gold 32, 20–50 Spherical Phaenerochaete chrysosporium Trichoderma viride Silver 50–200 Pyramidal Silver 5–40 Spherical Verticillium luteoalbum Verticillium sp. Gold Silver Not available 25–32 Not available Spherical Yarrowia lipolytica Gold 15 Triangles Vigneshwaran et al. (2007) Bhainsa and D’Souza (2006) Gericke and Pinches (2006) Senapati et al. (2004) Bansal et al. (2005) Bansal et al. (2005) Castro-Longoria et al. (2011) Vigneshwaran et al. (2006) Fayaz et al. (2010) Gericke and Pinches (2006) Senapati et al. (2004) Agnihotri et al. (2009) Algae Chlorella vulgaris Silver 9–20 Oscillatoria willei Silver 100–200 Not available Spherical Plectonemaboryanum Gold <10–25 Cubic Plectonema boryanum UTEX 485 Sargassum wightii Gold 10–6 µm Octahedral Gold 8–12 Planar smtiquia@umich.edu Jianping et al. (2007) Mubarak-Ali et al. ( 2011) Lengke et al. (2006a) Lengke et al. (2006b) Singaravelu et al. (2007) (continued) 1.2 Metallic Nanoparticles 7 Table 1.1 (continued) Microorganism Type nanoparticle synthesize Size (nm) Shape Reference Spirulina platenensis Silver 11.6 Spherical Pterochladia capillacae Jania rubins Silver 7 (average) Spherical Silver 12 (average) Spherical Ulva faciata Silver 7 (average) Spherical Colpmenia sinusa Silver 20 (average) Spherical Mahdieh et al. (2012) El-Raﬁe et al. (2013) El-Raﬁe et al. (2013) El-Raﬁe et al. (2013) El-Raﬁe et al. (2013) 1.2.1 Gold Nanoparticles Gold nanoparticles (AuNOs) have attracted attention in biotechnology due to their unique optical and electrical properties, high chemical and thermal ability, and good biocompatibility and potential applications in various life sciences related applications including biosensing, bioimaging, drug delivery for cancer diagnosis and therapy (Jiang et al. 2012). Covalently modiﬁed gold nanoparticles have attracted a great deal of interest as a drug delivery vehicles. Their predictable and reliable surface modiﬁcation chemistry, usually through gold-thiol binding, makes the desired functionalization of nanoparticles quite possible and accurate. A variety of therapeutic molecules have been attached in this manner, including various oligonucleotides for gene therapy, bacterial compounds, and anti-cancer drugs. (Jiang et al. 2012). Recently, many advancements were made in biomedical applications with better biocompatibility in disease diagnosis and therapeutics. AuNPs could be prepared and conjugated with many functionalizing agents, such as polymers, surfactants, ligands, dendrimers, drugs, DNA, RNA, proteins, peptides and oligonucleotides. Overall, Au-NPs would be a promising vehicle for drug delivery and therapies. AuNPs can be produced by microorganisms such as bacteria, fungi and algae (Table 1.1). Bacteria have been used to synthesize AuNPs. For example, microbial synthesis of gold nanoparticles was achieved by Konishi et al. (2007a) using the mesophilic bacterium Shewanella algae with H2 as the electron donor. The authors used varying pH conditions in their study. When the solution pH was 7, gold nanoparticles of 10–20 nm were synthesized in the periplasmic space of S. algae cells. Interestingly, when the solution pH was decreased to 1, larger-sized gold nanoparticles (50–500 nm) were precipitated extracellularly (Konishi et al. 2004). In an analogous study, He et al. (2007) showed that the bacteria Rhodopseudomonas capsulata produces gold nanoparticles of different sizes and shapes. He et al. incubated R. capsulata biomass and aqueous chloroauric acid (HAuCl4) solution at pH values ranging from 7 to 4.16 They found that at pH 7, spherical gold nanoparticles in the smtiquia@umich.edu 8 1 Nanoparticles Synthesized by Microorganisms range of 10–20 nm were formed. In contrast, at pH 4, a number of nanoplates were produced (He et al. 2007). In both of these studies, the solution pH was an important factor in controlling the morphology of biogenic gold particles and location of gold deposition. These observations are in line with the ﬁndings of Klaus et al. (1999) who observed that variations in incubation conditions lead to variations in particle size. Of note, gold nanoparticles can be used for a variety of applications (e.g., direct electrochemistry of proteins) (Liangwei et al. 2007). The synthesis of gold nanoparticles by two novel strains of Arthrobacter sp. 61B and Arthrobacter globiformis 151B isolated from basalt rocks in Georgia was studied by Kalabegishvili et al. (2012). Their study has shown that the extracellular formation of nanoparticles took place after 1.5–2 days. They noted that the concentration of gold particles accumulated by increases in bacterial biomass. Gericke and Pinches (2006) reported intracellular gold production by Pseudomonas stutzeri NCIMB 13420, Bacillus subtilis DSM 10 and Pseudomonas putida DSM 291. He et al. (2007) demonstrated that the bacterium Rhodopseudomonas capsulata is capable of producing gold particles extracellularly. The gold nanoparticles produced are stable in solution. In the study conducted by Malarkodi et al. (2013), the extracellular biosynthesis of gold nanoparticles is achieved by an easy biological procedure using Klebsiella pneumoniae as the reducing agent. After exposing the gold ions to K. pneumoniae, rapid reduction of gold ion is observed and leads to the formation of gold nanoparticles in colloidal solution (Figs. 1.3 and 1.4). The method exploits a Fig. 1.3 Transmission electron microscopy and photo of gold nanoparticles prepared by using K. pneumoniae a 100 nm and b 50 nm. TEM images of gold nanoparticles using K. pneumoniae a and b. Source Malarkodi et al. (2013). Copyright © 2013, Springer. Reproduced with permission smtiquia@umich.edu 1.2 Metallic Nanoparticles 9 Fig. 1.4 a Ultraviolet-visible-near infrared spectra of gold nanoparticles synthesized by exposing various amounts of Candida albicans cytosolic extract to a ﬁxed volume (5 mL) of HAuCl4 solution (10–3 M), keeping the ﬁnal volume (10 mL) of reaction mixture for 24 h. b Representative ultraviolet-visible-near infrared spectra depicting kinetics of the reaction of 1 mL of C. albicans cytosolic extract with 10 mL of aqueous HAuCl4 solution for speciﬁed time periods. The incubation mixture was scanned in the ultraviolet range to analyze characteristic peaks. c Color development as a function of surface plasmon resonance in C. albicans cytosolic extract-mediated synthesis of gold nanoparticles. (a) HAuCl4 aqueous solution, (b) Incubation of 5 mL of HAuCl4 aqueous solution with 1 mL of C. albicans cytosolic extract keeping the ﬁnal volume of reaction mixture at 10 mL, (c) Incubation of HAuCl4 aqueous solution (5 mL) with 3 mL of C. albicans cytosolic extract, making the ﬁnal volume of reaction mixture 10 mL by adding 2 mL of deionized water, (d) 5 mL of C. albicans cytosolic extract incubated with 5 mL of aqueous HAuCl4 solution. Source Chauhan et al. (2011). Copyright © 2011, Dovepress. Reproduced with permission cheap and easily available biomaterial for the synthesis of metallic nanoparticles (Malarkodi et al. 2013). Fungi are taking the center stage of studies on biological generation of metallic nanoparticles because of their tolerance and metal bioaccumulation ability (Sastry et al. 2003). A distinct advantage of using fungi in nanoparticle synthesis is the ease in their scale-up (e.g., using a thin solid substrate fermentation method). Given that fungi are extremely efﬁcient secretors of extracellular enzymes, it is thus possible to easily obtain large-scale production of enzymes. Further advantages of using a fungal-mediated green approach for synthesis of metallic nanoparticles include economic viability and ease in handling biomass. However, a signiﬁcant drawback of using these bio-entities in nanoparticles synthesis is that the genetic manipulation of eukaryotic organisms as a means of overexpressing speciﬁc enzymes (e.g. the ones identiﬁed in synthesis of metallic nanoparticles) is relatively much more difﬁcult than that in prokaryotes. Sastry and coworkers have reported the smtiquia@umich.edu 10 1 Nanoparticles Synthesized by Microorganisms extracellular synthesis of gold nanoparticles by fungus Fusarium oxysporum (Mukherjee et al. 2002). They reported the intracellular synthesis of gold nanoparticles by fungus Verticillium sp. as well (Mukherjee et al. 2001a). The extracellular and intracellular biosynthesis of gold nanoparticles by fungus Trichothecium sp. was reported by Absar et al. (2005). The gold ions react with the Trichothecium sp. fungal biomass under stationary conditions, resulting in the rapid extracellular production of nanoparticles; reaction of the biomass under shaking conditions resulted in intracellular production of the gold nanoparticles. Chauhan et al. (2011) reported biogenic synthesis of gold nanoparticles using cytosolic extract of Candida albicans. The study revealed that the shape and the size of nanoparticles formed govern the characteristic features of their spectra (Figs. 1.5 and 1.6). This technique can be extended for rapid, speciﬁc, and cost-effective detection of various cancers, hormones, pathogenic microbes, and their toxins if a speciﬁc antibody is available. There are few reports of algae being used as a “biofactory” for synthesis of metallic nanoparticles. Singaravelu et al. (2007) adopted a systematic approach to study the synthesis of metallic nanoparticles by Sargassum wightii. This is the ﬁrst Fig. 1.5 Extracellular synthesis of K. pneumoniae biomass and gold chloride mixed with biomass. K. pneumoniae biomass (a), 1 mM gold chloride mixed with biomass at the beginning of reaction showing a greenish-brown color change (b), and after 1 day of reaction showing a dark-purple in color change (c). Source Malarkodi et al. (2013). Copyright © 2013, Springer. Reproduced with permission smtiquia@umich.edu 1.2 Metallic Nanoparticles 11 Fig. 1.6 Representative transmission electron micrographs of gold nanoparticles synthesized using various amounts of Candida albicans cytosolic extract. Transmission electron micrographs of gold nanoparticles generated upon incubation of 5 mL of HAuCl4 (10−3 M) with 1 mL of C. albicans cytosolic extract and making up ﬁnal volume of reaction mixture (10 mL) using deionized water for a 12 h and b 24 h. c and d represent transmission electron microscopic images of nanoparticles obtained as a result of reduction of 5 mL of HAuCl4 solution (10−3 M) by 5 mL of cytosolic extract after 12 h and 24 h, respectively. Source Chauhan et al. (2011). Copyright © 2011, Dovepress. Reproduced with permission report in which a marine alga has been used to synthesize highly stable extracellular gold nanoparticles in a relatively short time period compared with that of other biological procedures. Indeed, 95 % of the bioreduction of AuCl−4 ions occurred within 12 h at stirring condition (Singaravelu et al. 2007). The authors extended their report to the formation of palladium and platinum nanoparticles starting with their corresponding metallic chloride—containing salts (Singaravelu et al. 2007). Rajeshkumar et al. (2013, 2014) reported synthesis of gold particles by marine brown algae Tubinaria conoides. The nanoparticles showed antibacterial activity against Streptococcus sp., Bacillus subtilis, Klebsiella pneumoniae. smtiquia@umich.edu 12 1 Nanoparticles Synthesized by Microorganisms Southam and Beveridge (1996) have demonstrated that gold particles of nanoscale dimensions may readily be precipitated within bacterial cells by incubation of the cells with Au3+ ions. Monodisperse gold nanoparticles have been synthesized by using alkalotolerant Rhodococcus sp. under extreme biological conditions like alkaline and slightly elevated temperature conditions (Ahmad et al. 2003b). Lengke et al. (2006a) claimed the synthesis of gold nanostructures in different shapes (spherical, cubic, and octahedral) by ﬁlamentous cyanobacteria from Au(I)-thiosulfate and Au(III)-chloride complexes and analyzed their formation mechanisms (Lengke et al. 2006a, b). Nair and Pradeep (2002) reported the growth of nanocrystals and nanoalloys using Lactobacillus. Some other typical gold nanoparticles produced by microorganisms are summarized in Table 1.1. 1.2.2 Silver Nanoparticles Silver nanoparticles (AgNps), like their bulk counterpart, show effective antimicrobial activity against Gram-positive and Gram-negative bacteria, including highly multiresistant strains such as methicillin-resistant Staphylococcus aureus (Panacek et al. 2006). Silver nanoparticles synthesis by microorganisms is a result of their defense mechanism. The resistance caused by the microorganism for silver ions in the environment is responsible for its nanoparticle synthesis. The silver ions in nature are highly toxic to microorganisms. As a defense mechanism, the microorganism utilizes its cellular machinery to transform the reactive silver ions to stable silver atoms. Parameters such as temperature and pH play an important role in their production. It is now known that more nanoparticles are synthesized under alkaline conditions than under acidic conditions (Saklani and Jain 2012). The reduction of Ag+ to colloidal silver by microorganisms in aqueous solutions is a stepwise process. First various complexes of Ag+ are reduced to metallic silver atoms (Ag0). This is followed by the agglomeration of Ag0 into oligomeric clusters (Sharma et al. 2008). It is these clusters that eventually lead to the formation of colloidal AgNPs (Kapoor et al. 1994; Sharma et al. 2008). The low molecular weight peptide, glutathione (GSH) and proteins like metallothioneins and phytochelatins, enzymes such as oxidoreductases, NADH-dependent reductases, nitroreductases, NADH-dependent nitrate reductases (NRs) and cysteine desulfhydrases have been shown to be responsible for nanocrystal formation in yeast, bacteria, and fungi. These microbes are known to reduce the Ag+ ions to form silver nanoparticles, most of which are found to be spherical particles (Mukherjee et al. 2001b; Ahmad et al. 2003a; Fayaz et al. 2010). In one of the earliest studies in this technology, Slawson et al. (1992) found that a silver-resistant bacterial strain isolated from silver mines, Pseudomonas stutzeri AG259, accumulated AgNPs within the periplasmic space. Of note, the particle size ranged from 35 to 46 nm (Slawson et al. 1992). Interestingly, Klaus et al. (1999) observed that when this bacterium was placed in concentrated aqueous solution (50 mM), particles of larger size (200 nm) were formed. Klaus et al. (1999) group attributed the difference in smtiquia@umich.edu 1.2 Metallic Nanoparticles 13 particle size (in comparison with the report of Slawson et al. 1992) to the differences in cell growth and metal incubation conditions. Klaus et al. (1999) have shown that the bacterium Pseudomonas stutzeri AG259, isolated from a silver mine, when placed in a concentrated aqueous solution of silver nitrate, played a major role in the reduction of the Ag+ ions and the formation of silver nanoparticles (AgNPs) of well-deﬁned size and distinct topography within the periplasmic space of the bacteria (Klaus et al. 1999). An important application of such a bacterium would be in industrial Ag recovery. Intriguingly, the exact mechanism(s) of AgNPs synthesis by this bacterium is still unclear. However, the molecular basis of biochemical synthesis of AgNPs from Morganella sp. RP-42, an insect midgut isolate (Parikh et al. 2008). Parikh et al. (2008) observed that Morganella sp. RP-42 when exposed to silver nitrate (AgNO3) produced extracellular crystalline AgNPs of size 20 ± 5 nm. Three gene homologues (silE, silP, and silS) were identiﬁed in silver-resistant Morganella sp. The homologue of silE from Morganella sp. showed 99 % nucleotide sequence similarity with the previously reported gene, silE, which encodes a periplasmic silver-binding protein (Parikh et al. 2008). This is the only report that elucidates the molecular evidence of silver resistance in bacteria, which could be linked to synthesis mechanism. In another study, Nair and Pradeep (2002), exposed common Lactobacillus strains present in buttermilk to large concentrations of metal ions to produce microscopic gold, silver, and gold-silver alloy crystals of well-deﬁned morphology. The bacteria produced these intracellularly and, remarkably, the cells preserved their viability even after crystal growth (Nair and Pradeep 2002) Notably, even cyanobacteria have been observed to produce AgNPs. For example, the biosynthesis of AgNPs has been successfully conducted using Plectonema boryanum UTEX 485, a ﬁlamentous cyanobacterium (Lengke et al. 2007). The authors posit that the mechanisms of AgNPs production via cyanobacteria could involve metabolic processes from the use of nitrate at 25 °C and/or organics released from the dead cyanobacteria at 25° (Fig. 1.7) to 100 °C (Fig. 1.8). AgNPs were synthesized in the form of a ﬁlm or produced in solution or accumulated on the cell surface of fungi, Verticillium, Fusarium oxysporum, or Aspergillus flavus, were employed (Mukherjee et al. 2001b; Senapati et al. 2004; Bhainsa and D’Souza 2006; Vigneshwaran et al. 2007; Jain et al. 2011). Mukherjee et al. (2001b) studied the synthesis of intracellular AgNPs using the fungus Verticillium. The authors observed that exposure of the fungal biomass to aqueous Ag+ ions results in the intracellular reduction of the metal ions and formation of 25 ± 12 nm AgNPs. A shortcoming of the study by Mukherjee et al. (2001b) was that the metallic nanoparticles were synthesized intracellularly. Indeed, when the site of nanoparticle synthesis is intracellular, downstream processing becomes difﬁcult and often defeats the purpose of developing a simple and cheap process. In this regard, Bhainsa and D’Souza (2006) have reported extracellular biosynthesis of AgNPs using the ﬁlamentous fungus Aspergillus fumigatus and the synthesis process was rapid. AgNPs were formed within minutes of silver ion coming in contact with the cell ﬁltrate. The ultraviolet-visible (UV-Vis) spectrum of the aqueous medium containing Ag+ ion showed a peak at 420 nm corresponding with smtiquia@umich.edu 14 1 Nanoparticles Synthesized by Microorganisms Fig. 1.7 TEM micrographs of whole mounts of cyanobacterial cells cultured in the presence of AgNO3 at 25 °C and 28 days. a Precipitation of silver nanoparticles on the cyanobacterial surface. b TEM micrograph of a thin section of cyanobacteria cells with nanoparticles of silver deposited inside the cell. c, d Spherical nanoparticles of silver precipitated in solution. e TEM-SAED diffraction powder-ring pattern consistent with crystalline nanoparticles of Ag with a possible trace of silver sulﬁde (*). d spacings of 0.235, 0.204, 0.144, and 0.123 nm corresponding to reflections 111, 200, 220, and 311, respectively. f TEM-EDS for area D. Scale bars in A, B, C, and D are 0.5 and 0.2 nm and 25 and 50 nm, respectively. Source Lengke et al. (2007). Copyright © 2007, American Chemical Society. Reproduced with permission smtiquia@umich.edu 1.2 Metallic Nanoparticles 15 Fig. 1.8 TEM micrographs of whole mounts of cyanobacterial cells cultured in the presence of AgNO3 at 100 °C and 28 days. a Silver nanoparticles encrusted on cyanobacterial cells. b TEM micrograph of a thin section of cyanobacteria cells with nanoparticles of silver deposited inside the cell. c Octahedral, spherical, and anhedral nanoparticles of silver precipitated in solution. d Octahedral silver platelets. e TEM-SAED diffraction powder-ring pattern consistent with crystalline nanoparticles of Ag; d spacings of 0.235, 0.204, 0.144, and 0.123 nm correspond to reflections 111, 200, 220, and 311, respectively. f TEM-EDS for area B. Scale bars in A, B, C, and D are 1, 0.05, and 0.1 and 50 nm, respectively. Source Lengke et al. (2007). Copyright © 2007, American Chemical Society. Reproduced with permission smtiquia@umich.edu 16 1 Nanoparticles Synthesized by Microorganisms the plasmon absorbance of AgNPs. The crystalline AgNPs were well dispersed in the range of 5–25 nm. Remarkably, the nanoparticles present in the aqueous medium were quite stable, even up to 4 months of incubation at 25 °C. Mukherjee et al. (2008) demonstrated green synthesis of highly stabilized nanocrystalline silver particles by a nonpathogenic and agriculturally important fungus, Trichoderma asperellum. An interesting aspect of this study is the mechanism of formation of AgNPs. The process of growing nanoparticles comprises two key steps: bioreduction of AgNO3 to produce AgNPs followed by stabilization and/or encapsulation of the same by a suitable capping agent. The size of the silver particles using TEM and XRD studies is found to be in the range 13–18 nm. These nanoparticles are found to be highly stable and even after prolonged storage for over 6 months they do not show signiﬁcant aggregation (Fig. 1.9). A mechanism behind the formation of silver nanoparticles and their stabilization via capping has been investigated using FTIR (Fig. 1.8) and surface-enhanced resonance Raman spectroscopy (Fig. 1.9). Both the spectra exhibit a broad intense band at *3400 cm−1 with overlapping shoulders on either side assigned to the N–H stretching frequency arising from the peptide linkages present in the proteins Fig. 1.9 High-resolution transmission electron micrograph of drop-cast silver nanoparticles preserved for over 6 months. Inset: a low-resolution micrographs showing size of the particulates, b histogram of particle size distribution as obtained from TEM and c SAED pattern recorded on the same sample. Source Mukherjee et al. (2008). Copyright © 2008, IOP Science. Reproduced with permission smtiquia@umich.edu 1.2 Metallic Nanoparticles 17 Fig. 1.10 FTIR spectra of the cell-free extract a before addition of AgNO3 and b after removal of silver nanoparticles by centrifugation. Source Mukherjee et al. (2008). Copyright © 2008, IOP Science. Reproduced with permission of the extract (Fig. 1.10). Upon deconvolution, the side bands were respectively identiﬁed to be the overtone of the amide-II band (*3270 cm−1) and the stretching frequency of the O–H band (*3600 cm−1) possibly arising from the carbohydrates and/or proteins present in the sample. However, it can arise from the aqueous solvent as well. It may be observed that the intensity of the ﬁrst two bands diminishes signiﬁcantly in (b), indicating a decrease in the concentration of the peptide linkages in the solution. The spectra also exhibit an intense band at *1640 cm−1 and a broad asymmetric band at *2100 cm−1, the latter assigned to the N–H stretching band in the free amino groups of biomacromolecules and a low intensity peak at *2600 cm−1 due to S–H stretching vibrations. It may be noted that the intensity of the band at *2100 cm−1 remains almost unchanged in the two spectra while that due to S–H stretching shifts towards lower wavenumbers in (b). Upon deconvolution, the band at *1640 cm−1 is found to be a superposition of three different bands centred at *1550, 1640 and 1670 cm−1, respectively assigned to the amide-II band, carbonyl and carboxylic C=O stretching bands of the peptide linkages (Mukherjee et al. 2008). Figure 1.11 shows the aforesaid Raman spectrum which clearly exhibits an intense band at *240 cm−1 identiﬁed to be due to stretching vibrations of Ag–N bonds and two broad bands at *1350 and 1565 cm−1 attributed, respectively, to symmetric and asymmetric C=O stretching vibrations of the CO2 ions apart from a few weak features at *692, 940, 970 and 1050 cm−1 assigned, respectively, to the stretching vibrations of C–S, C–CO2, C–C and Cα–N bonds. Selective enhancement of these Raman bands clearly indicates that C=O bonds of the carboxylate smtiquia@umich.edu 18 1 Nanoparticles Synthesized by Microorganisms Fig. 1.11 Macro-Raman spectrum of silver nanoparticles drop-cast on Si (100) single crystals. Source Mukherjee et al. (2008). Copyright © 2008, IOP Science. Reproduced with permission ions and Ag–N bonds from the free amine groups are lying perpendicular to the nano silver surface and are directly associated with the capping of the same. This is further supported from the fact that both the symmetric and asymmetric stretching bands of CO2 are signiﬁcantly broadened due to distortion in the respective bond angles and bond lengths that have resulted from the strain induced following encapsulation of silver nanoparticles (Mukherjee et al. 2008). The band at 240 cm−1 is direct evidence of the formation of a chemical bond between silver nanoparticles and the nitrogen of the amine group present in the amino acids. The probing technique itself manifests the feasibility of using these nanoparticles as potential templates for surface-enhanced resonance Raman spectroscopy (SERS) (Mukherjee et al. 2008). Some other silver nanoparticles produced by microorganisms are listed in Table 1.1. 1.2.3 Cadmium Nanoparticles The health risks posed by cadmium toxicity have been investigated for over 50 years. Yet knowledge in this area is still expanding, as evidenced by the excellent reviews appearing in this volume. At the level of the organism, cadmium toxicity is associated with liver and kidney injury, osteomalacia, osteoporosis, skeletal deformations, neurological, and other deﬁcits. Cadmium is classiﬁed as a category 1 carcinogen, but is not directly genotoxic or mutagenic in bacteria. It is known to affect genome stability via inhibition of DNA repair and generation of free radical-induced DNA damage. At the cellular level, cadmium induces oxidative stress by depletion of endogenous antioxidants such as glutathione and is associated with mitochondrial damage, induction of apoptosis, and disruption of intracellular calcium signaling. Despite the extensive studies on cadmium toxicity, there smtiquia@umich.edu 1.2 Metallic Nanoparticles 19 continues to be much territory left to cover regarding its mechanism of action, intracellular damage, and environmental exposure. At present, the primary cadmium nanoparticles are those of CdSe or CdTe, encapsulated in various coatings in the form of semiconductor quantum dots (Bao et al. 2010). Semiconductor nano-crystals, which have unique optical, electronic, and optoelectronic properties have potential application in the ﬁeld of nano-electronics. Kumar et al. (2007) synthesized CdSe nanoparticles (9–15 nm) using a fungus, Fusarium oxysporum, in a mixture of CdCl2 and SeCl4. Cui et al. (2009) synthesized CdSe using a yeast strain: Saccharomyces cerevisiae. Pearce et al. (2008) also synthesized CdSe nanoparticles by adding CdCl2O8 to selenide (Se[II]) produced from selenite (Se[IV]) by an anaerobic bacterium: Veillonella atypica. In the latter two studies (Pearce et al. 2008; Cui et al. 2009), cadmium was added after microbial formation of selenide for CdSe synthesis, probably because of its toxicity to the microbes. Consequently, they synthesized CdSe in two-vessel processes consisting of reduction of selenite to selenide and subsequent synthesis of CdSe from selenide and cadmium ion. In contrast, only Kumar et al. (2007) reported a one-vessel process in which the fungus generates CdSe in the co-presence of selenite and cadmium ion, which might improve economic efﬁciency through its simple operation of fewer reaction vessels. Synthesis of CdSe was observed in Pseudomonas sp. RB. by Ayano et al. (2014). Transmission electron microscopy and EDS revealed that this strain accumulated nanoparticles (10–20 nm) consisting of selenium and cadmium inside and on the cells when cultivated in the same medium for the enrichment culture. This report is the ﬁrst report describing isolation of a selenite-reducing and cadmium-resistant bacterium (Ayano et al. 2014). Cadmium telluride (CdTe), an important group II–VI semiconductor material with large exciton Bohr radius (7.3 nm) and narrow bulk band gap of 1.5 eV has shown signiﬁcant potential for LED (energy), FRET (electronics), and biomedical applications (Yang et al. 2009) due to their size dependent properties. These nanoparticles provide excellent photostability, narrow emission and high quantum yield in comparison with organic dyes and therefore explored in live cell bio-imaging (Pan and Feng 2009). Syed and Ahmad (2013) uses the fungus Fusarium oxysporum to synthesize highly fluorescent extracellular CdTe (quantum dot) nanoparticles. The process utilizes Cd and Te precursors in a very dilute form and allows bottom-up, one-step preparation of nanoparticles. Different techniques were employed for their characterization such as SAED and XRD which conﬁrmed the crystalline nature of biosynthesized nanoparticles. These biosynthesized nanoparticles are capped by proteins secreted by the fungus in the reaction mixture, which makes them water dispersible and provides stability in solution by preventing their agglomeration. These nanoparticles also showed antibacterial activity against Gram-positive and Gram-negative bacteria. This study demonstrates that fungus based approach provides a novel, rational and environment friendly synthesis protocol for nanomaterials synthesis. This is the ﬁrst demonstration of a fungal-mediated approach for the synthesis of CdTe quantom dots (Syed and Ahmad 2013). smtiquia@umich.edu 20 1.3 1 Nanoparticles Synthesized by Microorganisms Alloy Nanoparticles Alloy nanoparticles or bimetallic nanoparticles exhibit unique electronic, optical, and catalytic properties that are different from those of the corresponding individual metal particles (Harada et al. 1993). For instance gold nanoparticles supported on metal oxide or gold-containing bimetallic nanoparticles are found to exhibit enhanced catalytic activity (Bond 2002). 1.3.1 Gold—Silver (Au–Ag) Nanoparticles In many functional properties, the performances of Au–Ag alloy nanoparticles are superior to the corresponding monometallic ones, such as in surface enhanced Raman spectroscopy (SERS), sensors, and catalysis. Therefore, many synthesis approaches of bimetallic Au–Ag nanoparticles have been developed, such as digestive ripening, laser synthesis method, seed growth method, ligand binding method, and galvanic reaction. Since the seminal discovery of catalytic activity by gold nanoparticles, supported Au–Ag alloy nanoparticles have been receiving increasing attentions for possible enhancement in catalytic activity (Liu et al. 2013). In 2005, Liu et al. reported that Au and Ag showed obvious synergetic effect in CO oxidation reaction over an alloy nanocatalyst Au–Ag@MCM-41 catalyst. Since then, the Au–Ag alloy system has been applied to various reactions including oxidation of alcohols, acetylene hydrogenation, and photocatalytic reaction. Sandoval et al. (2011) investigated the Au–Ag alloy nanoparticles supported on TiO2 by a sequential precipitation-deposition method, where they deposited Ag ﬁrst and Au at the second step, and found that Au–Ag alloy nanoparticles are also very stable under high temperature pretreatment. However, the effect of the deposition sequence on the formation of Au–Ag bimetallic nanoparticles supported on SiO2 and their catalytic performances have not been investigated yet. Since Sun and Xia (2002) found the replacement reaction method could synthesize Au–Ag bimetallic systems, various Au–Ag nanostructures have been developed including hollow cubes, porous surfaces and alloy nanoparticles. Senapati et al. (2005) reported the synthesis of bimetallic Au–Ag alloy by Fusarium oxysporum and argued that the secreted cofactor NADH plays an important role in determining the composition of Au–Ag alloy nanoparticles. Using TEM, well-separated nanoparticles with occasional aggregation in the size range 8–14 nm are clearly observed (Fig. 1.12). The amount of cofactor NADH plays an important role in determining the nanoalloy composition (Senapati et al. 2005). Zheng et al. (2010) studied Au–Ag alloy nanoparticles biosynthesized by yeast cells (Fig. 1.13). Fluorescence microscopic and transmission electron microscopic characterizations indicated that the Au–Ag alloy nanoparticles were mainly synthesized via an extracellular approach and generally existed in the form of irregular smtiquia@umich.edu 1.3 Alloy Nanoparticles 21 Fig. 1.12 TEM images of Au–Ag nanoparticles formed by reaction of a mixture of 1 mm HAuCl4 and 1 mm AgNO3 with 60 g Fusarium oxysporum wet biomass for 96 h. Source Senapati et al. (2005). Copyright © 2013, Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission Fig. 1.13 Yeast cell solutions containing HAuCl4 (a and b), AgNO3 (c and d) and the mixture of HAuCl4 and AgNO3 (e and f) before (a, c and e) and after (b, d and f) standing for 24 h, fluorescence microscopy bright ﬁeld images of the surface of yeast cells’ ﬁlm after reacting with AgNO3 (g), HAuCl4 (h) and the mixture of AgNO3 and HAuCl4 (i), and TEM images of Au–Ag alloy nanoparticles (j). Source Zheng et al. (2010). Copyright © 2010, Elsevier. Reproduced with permission polygonal nanoparticles. Electrochemical investigations revealed that the vanillin sensor based on Au–Ag alloy nanoparticles modiﬁed glassy carbon electrode was able to enhance the electrochemical response of vanillin for at least ﬁve times. Sawle et al. (2008) demonstrated the synthesis of core-shell Au–Ag alloy nanoparticles from fungal strains Fusarium semitectum and showed that the smtiquia@umich.edu 22 1 Nanoparticles Synthesized by Microorganisms nanoparticle suspensions are quite stable for many weeks. As Fusarium. oxysporum is known to synthesize technologically important metal sulﬁde quantum dots extracellularly, this procedure can further be extended for the synthesis of other composite nanoparticles such as Au–CdS, Ag–CdS, and CdS–PbS. 1.4 Oxide Nanoparticles The industrial use of metallic oxide nanoparticles in a wide variety of applications has been rapidly expanding in the last decade. Such applications include the use of silicon, titanium, iron, and other metallic oxide nanoparticles, thereby increasing the occupational and other environmental exposure of these nanoparticles to humans and other species (Lai et al. 2007a). Nevertheless, the health effects of exposure of humans and other species to metallic oxide nanoparticles have not been systematically investigated as their impact on the environment has not been under the scrutiny of regulatory control (Lai et al. 2007b). Oxide nanoparticle is an important type of compound nanoparticle synthesized by microbes. In this section, Li et al. (2011) reviewed the biosynthesis of oxide nanoparticles. Most of the examples of the magnetotactic bacteria used for the production of magnetic oxide nanoparticles and biological systems for the formation of nonmagnetic oxide nanoparticles (Table 1.2). 1.4.1 Cerium Oxide Nanoparticles Cerium, which is the ﬁrst element in the lanthanide group with 4f electrons, has attracted much attention from researchers in physics, chemistry, biology and materials science. It is the most abundant of rare-earth metals found in the Earth’s crust. Several Ce-carbonate, -phosphate, -silicate, and -(hydr)oxide minerals have been historically mined and processed for pharmaceutical uses and industrial applications. Of all Ce minerals, cerium dioxide has received much attention in the global nanotechnology market due to their useful applications for catalysts, fuel cells, and fuel additives. When combined with oxygen in a nanoparticle formulation, cerium oxide adopts a fluorite crystalline structure that emerges as a fascinating material. This cerium oxide nanoparticle (CeONP) has been used proliﬁcally in various engineering and biological applications, such as solid-oxide fuel cells, high-temperature oxidation protection materials, catalytic materials,, solar cells and potential pharmacological agents (Xu and Qu 2014). Although useful because of its various properties and applications, the main application of CeONPs is in the ﬁeld of catalysis, and stems from their unique structure and atomic properties compared with other materials. Cerium nanoparticles have found numerous applications in the biomedical industry due to their strong antioxidant properties. Industrial applications include its use as a polishing agent, ultraviolet absorbing compound in smtiquia@umich.edu 1.4 Oxide Nanoparticles 23 Table 1.2 Oxide nanoparticles synthesized by microorganisms Microorganism Type nanoparticle produce Size (nm) Shape Reference Curvularia lunata Fusarium oxysporum Fusarium oxysporum Fusarium oxysporum Lactobacillus sp. Lactobacillus sp. Saccharomyces cerevisiae Shewanella oneidensis Shewanella oneidensis MR-1 Yeast cells CeO2 5 Spherical TiO2 6–13 Spherical BaTiO3 4–5 Spherical ZrO2 3–11 Spherical BaTiO3 20–80 Tetragonal T1O2 8–35 Spherical Sb2O3 2–10 Spherical Fe3O4 40–50 Fe3O3 30–43 Rectangular, rhombic, hexagonal Pseudohexagonal/irregular or rhombohedral Munusamy et al. (2016) Bansal et al. (2005) Bansal et al. (2006) Bansal et al. (2004) Jha and Prasad (2010) Jha et al. (2009a) Jha et al. (2009b) Perez-Gonzalez et al. (2010) Bose et al. (2009) Fe3O4 Spherical Spherical Penicillium sp. CuO Fusarium oxysporum Stereum hirsutum CuO Not available Not available 2–5 CuO 5–20 Wormhole- like Spherical Zhou et al. (2009) Honary et al. (2012) Hosseini et al. (2012) Cuevas et al. (2015) sunscreen, solid electrolytes in solid oxide fuel cells, as a fuel additive to promote combustion, and in automotive exhaust catalysts (Sindhu et al. 2015). CeONPs have also been used in ﬁghting inflammation and cancer, and in radiation protection of cells (Shah et al. 2012). A recent mass flow modeling study predicted that a major source of CeO2 nanoparticles from industrial processing plants (e.g., electronics and optics manufactures) is likely to reach the terrestrial environment such as landﬁlls and soils. The environmental fate of CeO2 nanoparticles is highly dependent on its physicochemical properties in low temperature geochemical environment. Though there are needs in improving the analytical method in detecting/quantifying CeO2 nanoparticles in different environmental media, it is clear that aquatic and terrestrial organisms have been exposed to CeO2 NPs, potentially yielding in negative impact on human and ecosystem health. Interestingly, there has been contradicting reports about the toxicological effects of CeO2 nanoparticles, acting as either an antioxidant smtiquia@umich.edu 24 1 Nanoparticles Synthesized by Microorganisms or reactive oxygen species production-inducing agent) (Dahle and Arai 2015). This poses a challenge in future regulations for the CeO2 nanoparticle application and the risk assessment in the environment. Arumugam et al. (2015) successfully synthesize CeO2 nanoparticles using Gloriosa superba L. leaf extract. The synthesized nanoparticles retained the cubic structure, which was conﬁrmed by X-ray diffraction studies. The oxidation states of the elements (C [1s], O [1s] and Ce [3d]) were conﬁrmed by XPS studies. TEM images showed that the CeO2 nanoparticles possessed spherical shape and particle size of 5 nm. The Ce–O stretching bands were observed at 451 cm−1 and 457 cm−1 from the FT-IR and Raman spectra respectively. The band gap of the CeO2 NPs was estimated as 3.78 eV from the UV–visible spectrum. From the photoluminescence measurements, the broad emission composed of eight different bands were found. The antibacterial studies performed against a set of bacterial strains showed that Gram-positive bacteria were relatively more susceptible to the NPs than Gram-negative bacteria. The toxicological behavior of CeO2 NPs was found due to the synthesized NPs with uneven ridges and oxygen defects in CeO2 NPs. CeO2 NPs have been successfully synthesized using the fungus Curvularia lunata (Munusamy et al. 2016). The XRD patterns, Micro Raman spectra and SAED pattern studies suggest the formation CeO2 NPs cubic fluorite structure. The TEM images showed spherical morphology with the average size of 5 nm. Synthesized CeO2 NPs were investigated by antibacterial activity. The perusal results observed at 100 µg CeO2 NPs had most signiﬁcant effect of antibacterial activity due to the strong electrostatic forces to binding the bacterial cell membrane to inhibit the bacterial growth. 1.4.2 Silica Dioxide Nanoparticles Silica, or silicon dioxide, is the same material used to make glass. In nature, silica makes up quartz and the sand you walk on at the beach. Unlike metals such as gold and iron, silica is a poor conductor of both electrons and heat. Despite these limitations, silica (silicon oxide) nanoparticles form the framework of silica aerogels. Silica aerogels are composed of silica nanoparticles interspersed with nanopores ﬁlled with air. As a result, this substance is mostly made up of air. Because air has very low thermal conductivity and silica has low thermal conductivity, they are great materials to use in insulators. These properties make nano aerogels one of the best thermal insulators known to man. Silica nanoparticles can also be functionalized by bonding molecules to a nanoparticle that also is able to bond to another surface, such as a cotton ﬁber. The functionalized silica nanoparticles attach to the cotton ﬁber and form a rough surface that is hydrophobic (water repellent), giving an effect similar to the water repellency of lotus leaves. Another type of silica nanoparticle is riddled with nanoscale pores. Researchers are developing drug delivery methods where therapeutic molecules stored inside the pores are slowly released in a diseased region of the body, such as near a cancer tumor (Argyo et al. 2014). smtiquia@umich.edu 1.4 Oxide Nanoparticles 25 Application of silica nanoparticles as ﬁllers in the preparation of nanocomposite of polymers has drawn much attention, due to the increased demand for new materials with improved thermal, mechanical, physical, and chemical properties. The chemical synthesis of silica-based materials are relatively expensive and exo-hazardous, often requiring extreme temperature, pressure and pH. Singh et al. (2008) demonstrated the synthesis of silicon/silica nanoparticle composites by Acinetobacter sp. The formation of silicon/silica nanocomposite is shown to occur when the bacterium is exposed to K2SiF6 precursor under ambient conditions. This bacterium has been shown to synthesize iron oxide and iron sulﬁde nanoparticles. It is hypothesized that this bacterium secretes reductases and oxidizing enzymes which lead to Si/SiO2 nanocomposite synthesis. The synthesis of silica nanoparticles by bacteria demonstrates the versatility of the organism, and the formation of elemental silicon by this environmentally friendly process expands further the scope of microorganism-based nanomaterial synthesis. 1.4.3 Titatium Oxide Nanoparticles Titanium dioxide (TiO2) has become part of our everyday lives. It is found in various consumer goods and products of daily use such as cosmetics, paints, dyes and varnishes, textiles, paper and plastics, food and drugs, and even paving stones. 4.68 million tons of titanium dioxide were produced worldwide in 2009. Titanium dioxide (TiO2) is a material of great signiﬁcance in many ﬁelds, e.g., photocatalysis, solar cell devices, gas sensors, and biomaterials (Gong and Selloni 2005). The nontoxic and biocompatible properties of Titania ﬁnd its applications in biomedical sciences such as bone tissue engineering as well as in pharmaceutical industries (Jha et al. 2009a). TiO2 catalysts have been conﬁrmed to be excellent and efﬁcient photocatalysts for the degradation and inhibition of numerous toxic environmental contaminants. Various applications of titanium dioxide include air and water cleaning and surface cleaning (Pan et al. 2010). Titanium is recommended for desalinization plants because of its strong resistance to corrosion from seawater. In medical applications the titanium pins are due to because of their non-reactive nature when contacting bone and flesh (Prasad et al. 2007). The TiO2 nanoparticles are synthesized using various methods such as sol gel, hydrothermal, flame combustion, solvothermal, fungal mediated biosynthesis Titanium oxide nanoparticles can be synthesized from Bacillus subtilis (Kirthi et al. 2011). The TiO2 nanoparticles have shown spherical clusters of the nanoparticles. Nanoparticles were spherical, oval in shape, individual as well as a few aggregates having the size of 66–77 nm. The particle size distribution reveals the morphological homogeneity with the grain size falling mostly in submicron range. The energy yielding material—glucose, which controls the value of oxidation—reduction potential (rH2), the ionic status of the medium pH and overall rH2, which is partially controlled by the bicarbonate, negotiate the synthesis of TiO2 nanoparticles in the presence of B. subtilis. The synthesis of n-TiO2 might have smtiquia@umich.edu 26 1 Nanoparticles Synthesized by Microorganisms Fig. 1.14 Schematics for the biosynthesis of n-TiO2. Source Jha et al. (2009a). Copyright © 2009, Elsevier. Reproduced with permission resulted due to pH-sensitive membrane bound oxidoreductases and carbon source dependent rH2 in the culture solution (Kirthi et al. 2011). A low-cost green and reproducible microbes (Lactobacillus sp. and Saccharomyces cerevisiae) mediated biosynthesis of TiO2 nanoparticles was carried out by Jha et al. (2009a). The synthesis was performed akin to room temperature in the laboratory ambience. X-ray and transmission electron microscopy analyses were performed to ascertain the formation of TiO2 nanoparticles. Individual nanoparticles as well as a few aggregate having the size of 8–35 nm are found. Concentric Scherrer rings in the selected area electron diffraction pattern indicated that the nanoparticles are having all possible orientations. A possible involved mechanism for the biosynthesis of nano-TiO2 has also been proposed in which pH as well as partial pressure of gaseous hydrogen (rH2) or redox potential of the culture solution seems to play an important role in the process (Fig. 1.14). 1.4.4 Iron Oxide Nanoparticles Eight iron oxides are known (Cornell and Schwertmann 2003), among these iron oxides, hematite (α-Fe2O3), magnetite (Fe3O4) and maghemite (γ-Fe2O3) are very promising and popular candidates due to their polymorphism involving temperatureinduced phase transition. Each of these three iron oxides has unique biochemical, magnetic, catalytic, and other properties which provide suitability for speciﬁc technical and biomedical applications. As the most stable iron oxide and n-type semiconductor under ambient conditions, hematite (α-Fe2O3) is widely used in catalysts, pigments and gas sensors due to its low cost and high resistance to corrosion. It can also be used as a starting material for the synthesis of magnetite (Fe3O4) and maghemite (γ-Fe2O3), which have been intensively pursued for both fundamental scientiﬁc interests and technological applications in the last few decades (Wu et al. 2010a). As shown in Fig. 1.15b, Fe3O4 has the face centered cubic spinel structure, based on 32 O2− ions and close-packed along the direction. Fe3O4 differs from most smtiquia@umich.edu 1.4 Oxide Nanoparticles 27 Fig. 1.15 Crystal structure and crystallographic data of the hematite, magnetite and maghemite (the black ball is Fe2+, the green ball is Fe3+ and the red ball is O2. Source Wu et al. (2015). Copyright © 2015, Taylor and Francis. Reproduced with permission other iron oxides in that it contains both divalent and trivalent iron. Fe3O4 has a cubic inverse spinel structure that consists of a cubic close packed array of oxide ions, where all of the Fe2+ ions occupy half of the octahedral sites and the Fe3+ are split evenly across the remaining octahedral sites and the tetrahedral sites. Fe3O4 has the lowest resistivity among iron oxides due to its small bandgap (0.1 eV). As shown in Fig. 1.15c, the structure of γ-Fe2O3 is cubic; each unit of maghemite contains 32 O2− ions, 211/3 Fe3+ ions and 21/3 vacancies. Oxygen anions give rise to a cubic close-packed array while ferric ions are distributed over tetrahedral sites (eight Fe ions per unit cell) and octahedral sites (the remaining Fe ions and vacancies). Therefore, the maghemite can be considered as fully oxidized magnetite, and it is an n-type semiconductor with a bandgap of 2.0 eV. The bacterium Actinobacter sp. has been shown to be capable of extracellularly synthesizing iron based magnetic nanoparticles, namely maghemite (γ-Fe2O3) and greigite (Fe3S4) under ambient conditions depending on the nature of precursors used (Bharde et al. 2008). More precisely, the bacterium synthesized maghemite when reacted with ferric chloride and iron sulﬁde when exposed to the aqueous solution of ferric chloride-ferrous sulfate. Challenging the bacterium with different metal ions resulted in induction of different proteins, which bring about the speciﬁc biochemical transformations in each case leading to the observed products. Maghemite and iron sulﬁde nanoparticles show superparamagnetic characteristics as expected. Compared to the earlier reports of magnetite and greigite synthesis by magnetotactic bacteria and iron reducing bacteria, which take place strictly under anaerobic conditions, the present procedure offers signiﬁcant advancement since the reaction occurs under aerobic condition. Moreover, reaction end products can be tuned by the choice of precursors used. The process of magnetic nanoparticles mineralization can be divided into four steps (Faramazi and Sadighi 2013): (1) vesicle formation and iron transport from outside of the bacterial membrane into the cell; (2) magnetosomes alignment in chain; (3) initiation of crystallization; and (4) crystal maturation (Fig. 1.16). smtiquia@umich.edu 28 1 Nanoparticles Synthesized by Microorganisms Fig. 1.16 Magnetosome biomineralization in magnetotactic bacteria (MTB). (I) MamI, MamL, MamB, and MamQ proteins initiate the membrane invagination and form a vesicular membrane around the magnetosome structure. (II) The protease-independent function of MamE recruits other proteins such as MamK, MamJ and MamA to align magnetosomes in a chaib. (III) Iron uptake occurs via MagA, a transmembrane protein, and initiation of magnetic crystal biomineralization occurs through MamM, MamNm and MamO proteins. (IV) Finally, MamR, MamS, MamT, MamP, MamC, MamD, MamF, MamG, the protease-dependent of MamE and Mms6, a membrane tightly bounded GTPase, regulate crystal growth and determine morphology of the produced magnetic nanoparticles. Source Faramazi and Sadighi (2013). Copyright © 2013, Elsevier. Reproduced with permission 1.4.5 Zirconium Dioxide (ZrO2) Nanoparticles Zirconium is a strong transition metal that resembles titanium. Because of its strong resistance to corrosion, it is used as an alloying agent in materials that are exposed to corrosive agents such as surgical appliances, explosive primers, vacuum tube getters and ﬁlaments. Since it has a very negative reduction potential (−1.55 V), it is never found as the native metal. It is obtained mainly from the mineral zircon, which can be puriﬁed with chlorine (Eshed et al. 2011). Because of its intrinsic physicochemical properties such as hardness, shock wear, strong acid and alkali resistance, low frictional resistance, and high melting temperature, zirconia can be used as an abrasive, as a hard, resistant coating for cutting tools, and in high temperature engine components. For these reasons, it is often called ceramic steel. Zirconia nanoparticles are of great interest due to their improved optical and electronic properties with application as a piezoelectric, electro-optic and dielectric material. Zirconia is also emerging as an important class of catalyst. The synthesis of zirconia has been realized by physico-chemical methods such as sol–gel synthesis, aqueous precipitation, smtiquia@umich.edu 1.4 Oxide Nanoparticles 29 thermal decomposition and hydrothermal synthesis (Bansal et al. 2004). However, all these methods require extremes of temperature (in the case of thermal synthesis) and pressure (hydrothermal synthesis). Bansal et al. (2004) have shown that the fungus Fusarium oxysporum secretes proteins capable of hydrolyzing aqueous ZrF62− ions extracellularly to form zirconia at room temperature. Particularly interesting is the fact that the fungus is capable of hydrolyzing tough metal halide precursors under acidic conditions. While the hydrolytic proteins secreted by Fusarium oxysporum are yet to be sequenced and studied for their role in the fungus metabolic pathways, our studies indicate that they are cationic proteins of molecular weight centered around 24 and 28 kDa and thus, similar to silicatein. The regenerative capability of biological systems coupled with the fact that that fungi such as Fusarium oxysporum are capable of hydrolyzing metal complexes that they never encounter during their growth cycle shows enormous promise for development, particularly in large-scale synthesis of metal oxides. 1.4.6 Antimony Trioxide (Sb2O3) Nanoparticles Antimony trioxide (Sb2O3) is a good semiconducting material and an excellent catalyst for the production of PET plastic used in the packaging of mineral water and soft drinks, which has been conﬁrmed by the World Health Organization and the European Food Safety Authority. In addition, Sb2O3/Sb2O5 is a potential compound for the synthesis of antimony gluconate, which is considered to be an effective medicine against Kala azar (visceral leishmaniasis). Common salts of antimony are irritants, thus an oral administration produces nausea, vomiting and diarrhea; they are therefore administered parenterally. It is also a cumulative drug. Antimony compounds are avoided in cases of pulmonary tuberculosis, jaundice, nephrites and dysentery (Acharya 1972). Nanoscale antimony compounds could prove to be less toxic to the body because they can cross the renal barrier. Further, Sb2O3 greatly increases flame retardant effectiveness when used as a synergist in combination with halogenated flame retardants in plastics, paints, adhesives, sealants, rubber and textile back coatings (Ye et al. 2006). Sb2O3 also has several applications, such as a ﬁning agent or as a degasser (to remove bubbles) in glass manufacturing, an opaciﬁer in porcelain and enameling services, and a white pigment in paints. Sb2O3 nanoparticles have been synthesized using different techniques by different groups of researchers (Guo et al. 2000; Friedrichs et al. 2001; Ye et al. 2002). A microbe (Lactobacillus sp.)-mediated biosynthesis of Sb2O3 nanoparticles was reported by Jha et al. (2009b). The synthesis was performed at around room temperature. X-ray and transmission electron microscopy analyses were performed to ascertain the formation of Sb2O3 nanoparticles. X-ray analysis indicated that Sb2O3 nanoparticles had a face-centered cubic unit cell structure. Individual nanoparticles as well as a few aggregate of 3–12 nm were found. smtiquia@umich.edu 30 1.4.7 1 Nanoparticles Synthesized by Microorganisms Copper Oxide (CuO) Nanoparticles Copper as a metal or copper oxides exhibit broad-spectrum biocidal activity, and several studies during the last two years found that copper demonstrates remarkable antibacterial activity at the nanoscale (Cuevas et al. 2015). In contrast to silver nanoparticles, which have been studied extensively for antibacterial application, copper is an essential element for living organisms and may be suitable for biomedical applications (Rubilar et al. 2013). It is important to note that copper is approximately 10-fold cheaper than silver in the market, and therefore, a method utilizing copper would prove to be quite cost-effective. On the other hand, it has been reported that copper nanoparticles are less toxic than silver nanoparticles (Bondarenko et al. 2013). Microorganisms such as Fusarium oxysporum are able to leach copper from integrated circuits present on electronic boards under ambient conditions (Cuevas et al. 2015). The analysis of the biogenic synthesis of copper oxides from CuSO4 has been observed in by Penicillium aurantiogriseum, P. citrinum, P. waksmanii, and F. oxysporum showed no large polydispersity in the pH range of 5–9 (Honary et al. 2012; Hosseini et al. 2012). A limited number of studies have been published, and these evaluated different fungal strains for the biosynthesis of copper nanoparticles. Fungi, such as Penicillium sp. and F. oxysporum strains, have been reported to biosynthesize copper oxide and Cu2S nanoparticles (Honary et al. 2012; Hosseini et al. 2012). Cuevas et al. (2015) evaluated the ability to synthesize copper and copper oxide nanoparticles using a mycelium-free extract produced by Stereum hirsutum, a white-rot fungus, in the presence of three different copper salts and to characterize and assess the involvement of proteins in the formation of the nanoparticles. The nanoparticles biosynthesis in presence of all copper salts demonstrated higher formation with 5 mM CuCl2 under alkaline conditions. TEM analysis conﬁrmed that the nanoparticles were mainly spherical (5–20 nm). The presence of amine groups attached to nanoparticles was conﬁrmed by FTIR, which suggests that extracellular protein of fungus is responsible for the formation of the nanoparticles. 1.4.8 Zinc Oxide (ZnO) Zinc oxide (ZnO) NPs have unique optical and electrical properties, and as a wide band gap semiconductor, they have found more uses in biosensors, nanoelectronics, and solar cells. These NPs are being used in the cosmetic and sunscreen industry due to their transparency and ability to reflect, scatter, and absorb UV radiation and as food additives. Furthermore, zinc oxide NPs are also being considered for use in next-generation biological applications including antimicrobial agents, drug delivery, and bioimaging probes (Jayaseelan et al. 2012). A low-cost and simple procedure for synthesis of zinc oxide NPs using reproducible bacterium, Aeromonas hydrophila, was reported. X-ray diffraction (XRD) conﬁrmed the crystalline nature smtiquia@umich.edu 1.4 Oxide Nanoparticles 31 of the NPs, and atomic force microscopy (AFM) showed the morphology of the nanoparticle to be spherical, oval with an average size of 57.72 nm. The antibacterial and antifungal activity was ended with corresponding well diffusion and minimum inhibitory concentration. The maximum zone of inhibition was observed in the ZnO NPs (25 μg/mL) against Pseudomonas aeruginosa (*22 ± 1.8 mm) and Aspergillus flavus (*19 ± 1.0 mm) (Jayaseelan et al. 2012). 1.5 Sulﬁde Nanoparticles In addition to oxide nanoparticles, sulﬁde nanoparticles have also attracted great attention in both fundamental research and technical applications as quantum-dot fluorescent biomarkers and cell labeling agents because of their interesting and novel electronic and optical properties (Yang et al. 2005). Examples of sulﬁdeproducing nanoparticles are listed in Table 1.3. Table 1.3 Sulﬁde nanoparticles synthesized by microorganisms Microorganism Type nanoparticle produce Size (nm) Shape Reference Coriolus versicolor CdS 100–200 Spherical Desulfobacteraceae CdS 2–5 Escherichia coli CdS 2–5 Fusarium oxysporum CdS 5–20 Hexagonal lattice Wurtzite crystal Spherical Lactobacillus CdS 4.7–5.1 Spherical Rhodopseudomonas palustris Schizosaccharomyces pombe Yeast CdS 8 Cubic Sanghi and Verma (2009) Labrenz et al. (2000) Sweeney et al. (2004) Ahmad et al. (2002) Prasad and Jha (2010) Bai et al. (2009) CdS 1–1.5 CdS 3.4–3.8 Hexagonal lattice Spherical Rhodopseudomonas sphaeroides Sulfate-reducing bacteria Fusarium oxysporum PbS 10.35–10.65 Spherical FeS 2 Spherical CuS 10–40 Rhodopseudomonas sphaeroides ZnS 8 Not available Hexagonal lattice smtiquia@umich.edu Kowshik et al. (2002) Prasad and Jha (2010) Bai and Zhang (2009) Watson et al. (1999) Schafﬁe and Hosseini (2014) Bai et al. (2006) 32 1.5.1 1 Nanoparticles Synthesized by Microorganisms Cadmium Sulﬁde (CdS) Nanoparticles CdS nanocrystal is one typical type of sulﬁde nanoparticle and has been synthesized by microorganisms (Prasad and Jha 2010; Kowshik et al. 2002). Cunningham and Lundie found that Clostridium thermoaceticum could precipitate CdS on the cell surface as well as in the medium from CdCl2 in the presence of cysteine hydrochloride in the growth medium where cysteine most probably acts as the source of sulﬁde (Cunningham and Lundie 1993). Dameron et al. (1989) have used Schizosaccharomyces pombe and Candida glabrata (yeasts) to produce intracellular CdS nanoparticles with cadmium salt solution. ZnS and PbS nanoparticles were successfully synthesized by biological systems. Rhodobacter sphaeroides and Desulfobacteraceae have been used to obtain ZnS nanoparticles intracellularly with 8 nm and 2–5 nm in average diameter, respectively (Bai et al. 2006; Labrenz et al. 2000). Kang et al. (2008) reported phytochelatin-mediated intracellular synthesis of CdS nanocrystals in engineered E. coli. By controlling the population of the phytochelatin (PCs), E. coli cells were engineered as an eco-friendly biofactory to produce uniformly sized PC-coated CdS nano-crystals. This is the ﬁrst systematic approach toward tunable synthesis of semiconductor nano-crystals by genetically engineered bacteria. The ﬁrst report on the production of semiconductor nano-crystal synthesis in bacteria was published by Sweeney et al. (2004). The study revealed that E. coli has the endogenous ability to direct the growth of nano-crystals. Parameters such as growth phase and strain type are essential for initiating nano-crystal growth. El-Shanshoury et al. (2012) reported a rapid and low-cost biosynthesis of CdS using culture supernatants of Escherichia coli ATCC 8739, Bacillus subtilis ATCC 6633, and Lactobacillus acidophilus DSMZ 20079T. The CdS nanoparticles synthesis were performed at room temperature and were formed within 24 h. The process of extracellular and fast biosynthesis may help in the development of an easy and eco-friendly route for synthesis of CdS nanoparticles. Mousavi et al. (2012) reported the synthesis of CdS nanoparticles using Escherichia coli PTTC 1533 and Klebsiella pneumoniae PTTC 1053. The synthesis of 5-200 nm nanoparticles occurred after 96 h of incubation at 30 °C and pH 9. A plausible mechanism has been made to understand the synthesis of CdS nanoparticles (Fig. 1.17). At the initial phase of the reaction, CdCl2 dissociates into Cd2+ and accumulates around the bacterial membrane because of the negative potential of the bacterial membrane (Triphati et al. 2014). Cd2+ being a heavy metal ion creates stress conditions for bacteria, thus leading bacterial biomass to initiate a defense mechanism. This leads bacteria to secrete certain enzymes/proteins in order to detoxify the metal ions that created the metal stress condition. The secreted protein by bacterial biomass binds up with Cd2+. Subsequently, Na2S being added dissociates into S2− in the solution and it also binds with the protein. The CdS nuclei then grow following the process of Ostwald ripening leading to the formation of CdS nanoparticles. Thus, protein secreted in this process becomes incorporated as it serves a capping layer for synthesis of CdS nanoparticles (Triphati et al. 2014). smtiquia@umich.edu 1.5 Sulﬁde Nanoparticles 33 Fig. 1.17 Biosynthesis mechanism of CdS nanoparticles. Source Tripathi et al. (2014). Copyright © 2014, IOP Science 1.5.2 Lead Sulﬁde (PbS) Nanoparticles Semiconductor PbS nanoparticles have attracted great attention in recent decades as a result of their interesting optical and electronic properties, and some of them are used for the fabrication of devices size (Bai and Zhang 2009). Some new chemical methods for the preparation of lead sulﬁde nanoparticles require special container, high temperature or long time for initiating the reaction. Biological synthesis method is one of the promising methods due to requiring a relatively milder condition, one step synthetic procedure, clean, and controllable size distribution. An earlier study found that Torulopsis sp. is capable of synthesizing PbS nanocrystals intracellularly when challenged with Pb2+. PbS nanoparticles were also synthesized by using Rhodobacter sphaeroides, whose diameters were controlled by the culture time (Bai and Zhang 2009). 1.5.3 Iron Sulﬁde Nanoparticles Ahmad et al. (2002) have found eukaryotic organisms such as fungi to be a good candidate for the synthesis of metal sulﬁde nanoparticles extracellularly. Another kind of sulﬁde nanoparticle was magnetic Fe3S4 or FeS nanoparticle. Bazylinski et al. (1995) reported the formation of Fe3S4 by uncultured magnetotactic bacteria. They examined a sediment sample that contained approximately 1 × 105 magnetotactic bacteria per cm3, and approximately 105 cells were obtained after puriﬁcation by the racetrack method. Magnetosomes in the uncultured cells exhibited elongated rectangular shape. The average magnetosome number per cell was approximately 40, and they were mainly located as a large cluster within the cell. Aligned magnetosomes forming a chainlike structure were also observed beside the large cluster. Sulfate-reducing bacteria were capable of producing magnetic FeS nanoparticles (Watson et al. 1999). smtiquia@umich.edu 34 1.5.4 1 Nanoparticles Synthesized by Microorganisms Copper Sulﬁde Nanoparticles Copper sulﬁde (CuS) nanoparticles have attracted increasing attention from biomedical researchers across the globe, because of their intriguing properties which have been mainly explored for energy- and catalysis-related applications to date. Recently, CuS nanoparticles are gradually emerging as a promising platform for sensing, molecular imaging, photothermal therapy, drug delivery, as well as multifunctional agents that can integrate both imaging and therapy (Goel et al. 2014). Although copper sulﬁde nanoparticles have been previously synthesized by several chemical, electrochemical, organic and enzymatic methods, the ﬁrst report on a biosynthesis approach was published in 2012 by Hosseini et al. in which the copper sulﬁde nanoparticles were synthesized from a pure copper sulfate solution by F. oxysporum. Industrially, the heavy metals in wastewaters are precipitated in the sulﬁde forms via dissimilatory reduction of sulfate that is performed by anaerobes. However, the performance of these sulfate-reducing bacteria is limited to anaerobic environments. Metal sulﬁdes are also formed from sulfate by assimilatory sulfate reduction (Tiquia 2008; Tiquia et al. 2006) performed via aerobic pathways by overproducing two unique enzymes called serine acetyltransferase (SAT) and cysteine desulfhydrase. The precursor of cysteine biosynthesis (O-acetylserine) is produced by the acetylation of serine that is catalyzed by SAT, but a single amino acid change renders the SAT insensitive to feedback inhibition by cysteine that results in cysteine overproduction by the microorganism. Then the excess cysteine is converted into pyruvate, ammonia and sulﬁde ions by cysteine desulfhydrase. Finally, the secreted sulﬁde precipitates metal ions like copper and removes it from the solution as copper sulﬁde nanoparticles. Schafﬁe and Hosseini (2014) demonstrated in their study that is feasible to produce copper sulﬁde nanoparticles from acid mine drainage (AMD) through a biological approach. The properties of the produced nanoparticles were the same as the nanoparticles synthesized from the pure copper sulfate solution. These nanoparticles have the same composition like covelite and an average size of 10–40 nm. 1.5.5 Silver Sulﬁde Nanoparticles Silver sulﬁde nanoparticles possess unique semi-conducting, optical, and electrical properties and are highly stable. Owing to these features, they are broadly used in solar cell batteries (Tubtimtae et al. 2010), thermoelectric sensors, etc. (Yan et al. 2011). The recently obtained Ag2S/graphene nanocomposite is promising for the development of super capacitors (Mo et al. 2012). The great potential of practical applications of Ag2S nanoparticles brought into existence numerous protocols for their preparation. The thermolysis of silver xanthates with long aliphatic chains at 200 °C brings about egg shaped particles with a narrow range of sizes (Zhang et al. 2012). Rod shaped Ag2S nanocrystals have been obtained from silver nitrate and smtiquia@umich.edu 1.5 Sulﬁde Nanoparticles 35 thioacetamide (Zhao et al. 2007). Lea fshaped Ag2S nanolayers can be produced by autoclaving an ethanol solution of silver nitrate and carbon disulﬁde at 160 °C (Chen et al. 2008). Bacteria of the genus Shewanella are commonly used in the preparation of nanoparticles of metals, oxides, and sulﬁdes (Perez-Gonzalez et al. 2010). These bacteria can reduce many substances, including metal oxides, nitrates, sulfates, etc. They have been employed in the synthesis of gold nanoparticles (Suresh et al. 2011), arsenic sulﬁde nanotubes (Jiang et al. 2009), and uranium dioxide nanoparticles (Burgos et al. 2008). Debabova et al. (2013) synthesize Ag2S nanoparticles using the metal-reducing bacterium Shewanella oneidensis MR1 an aqueous solution of AgNO3 and Na2S2O3 at an ordinary temperature and pressure. The nanoparticles vary in size within 2–16 nm, and the fraction 6–12 nm in size constitutes about 70 %. The maximum yield of nanoparticles in silver equivalent is 53 %. Being visualized by transmission electron microscopy, the particles look like spheres with average diameters varying from 7 ± 2 to 9 ± 2 nm. The elemental composition of synthesized nanoparticles has been analyzed by energy dispersive X-ray spectroscopy, and the estimated silver to sulfur atomic ratio is 2:1. The presence of living bacterial cells is mandatory for the formation of Ag2S nanoparticles in the aqueous salt solution. Changes in the reaction conditions (reagent concentrations, temperature, and cell incubation time in the reaction mixture) influence the yield of nanoparticles dramatically, but have little influence on their size. 1.5.6 Zinc Sulﬁde (ZnS) Nanoparticles Zinc sulﬁde (ZnS), are the most attractive materials for applications in areas such as IR optical devices and fast optical switching devices. Zinc sulﬁde nanoparticles can be prepared by different methods, such as colloidal aqueous and micellar solution synthesis method (Khiewa et al. 2005), using ultrasonic waves (Behboudnia et al. 2005), microwaves (Ni et al. 2004), and gamma-irradiation (Qiao et al. 2000). In most cases, particles prepared by these methods have some problems including poor reproducibility, control of particle size, distribution and shape. Some reactions require high temperature, and/or high pressure for initiating the reaction, and/or inert atmosphere protection, and/or using toxic matters such as H2S, toxic template and stabilizer, and metallic precursors. When zinc sulﬁde nanoparticles are used as biological probes in clinic examinations, the synthesis of zinc sulﬁde is expected to be clean (Dubertret et al. 2002). Consequently, researchers in nanoparticles synthesis have turned to biological systems for inspiration. Spherical aggregates of 2–5 nm sphalerite ZnS particles were formed by sulfate-reducing bacteria under anaerobic conditions (Mandal et al. 2006). A novel, clean biological transformation reaction by immobilized Rhodobacter sphaeroides has been developed for the synthesis of zinc sulﬁde (ZnS) nanoparticles was developed by Bai et al. (2006). Rhodobacter sphaeroides is a purple, non-sulphur, photosynthetic bacterium. It can grow not only aerobically smtiquia@umich.edu 36 1 Nanoparticles Synthesized by Microorganisms Fig. 1.18 Sulfate assimilation action of Rhodobacter sphaeroides. The enzymes present in Rhodobacter sphaeroides are indicated by the corresponding genes: cysP, sulfate permease; ylnB, ATP-sulfurylase; cysH, phosphoadenosine phosphosulfate reductase; cysJI, sulﬁte reductase; cysK, o-acetylserine synthase. Source Bai et al. (2006). Copyright © 2013, Springer. Reproduced with permission in the dark but also anaerobically in the light and has tolerance to heavy metals (Giotta et al. 2006). In the biological synthetic process for ZnS nanoparticles by Rhodobacter sphaeroides, soluble sulfate acts as the source of sulfur. The formation mechanism of ZnS nanoparticles by biological transformation reaction of Rhodobacter sphaeroides can be explained in (Fig. 1.18). First, the soluble sulfate enters into immobilized beads via diffusion, and later is carried to the interior membrane of Rhodobacter sphaeroides cell facilitated by sulfate permease Then, the sulfate is reduced to sulﬁte by ATP sulfurylase and phosphoadenosine phosphosulfate reductase, and next sulﬁte is reduced to sulﬁde by sulﬁte reductase. The sulﬁde reacts with O-acetylserine to synthesize cysteine via O-acetylserine thiolyase (Holmes et al. 1997; Auger et al. 2005), and then cysteine produces S2− by a cysteine desulfhydrase in presence of zinc. After this process, S2− reacts with the soluble zinc salt and the ZnS nanoparticles are synthesized. Finally, ZnS nanoparticles are discharged from immobilized cells to the solution. In the synthetic process, the particle size is controlled by the culture time of the Rhodobacter sphaeroides, and simultaneously the immobilized beads act on separating the ZnS nanoparticles from the Rhodobacter sphaeroides. Although the detailed mechanism study of this process is in progress, it suggests that many other high grade binary metal sulﬁdes can also be produced using this method (Bai et al. 2006). smtiquia@umich.edu 1.5 Sulﬁde Nanoparticles 1.5.7 37 Antimony Sulphide (Sb2S3) Nanoparticles Sb2S3 exhibits important applications in photovoltaic, photosensors, optical nanodevices, and photoelectronics. It has been are used in electronics as poor conductors of heat and electricity. Very pure antimony is used to make certain types of semiconductor devices, such as diodes and infrared detectors (Grigorescu and Stradling 2001; Wei et al. 2006). Antimony, as a metallic form, is not soluble in body fluids and, as reported in old literature, cannot produce any effect on human system (Filella et al. 2002a, b). In contrast, organic or inorganic salts of antimony can be decomposed by the fluids and have been used for therapeutic purposes (Filella et al. 2002a, b; Johnson et al. 2005). Antimony salts are currently chosen for the treatment of leishmaniasis, a disease that affects 12 million people annually around the world (Berman 1997; Haldar et al. 2011). Moreover, trivalent antimony compounds have been used in treating bilharzia, trypanosomiasis and kala-azar for more than a century (Berman 1997; Isago et al. 2008). Bahrami et al. (2012) explored the biological synthesis of antimony sulﬁde using Serratia marcescens. The bacterium was isolated from the Caspian Sea in northern Iran and was used for intracellular biosynthesis of antimony sulﬁde nanoparticles. This isolate was identiﬁed as nonpigmented Serratia marcescens using conventional identiﬁcation assays and the 16S rDNA fragment ampliﬁcation method, and was used to prepare inorganic antimony nanoparticles. Antimony-supplemented nutrient agar (NA) plates (SbCl3, 1 % w/v) were inoculated with the bacterial isolate. These inoculated NA media were incubated aerobically at 30 °C. After 72 h, bacterial cells were harvested from the surface of culture media. The biogenic nanoparticles were released by liquid nitrogen and extracted using two sequential solvent extraction systems. The energy-dispersive x-ray demonstrated that the extracted nanoparticles consisted of antimony and sulfur atoms. The transmission electron micrograph showed the small and regular non-aggregated nanoparticles ranging in size less than 35 nm. Although the chemical synthesis of antimony sulﬁde nanoparticles has been reported in the literature, the biological synthesis of antimony sulﬁde nanoparticles has not previously been published. This is the ﬁrst report to demonstrate a biological method for synthesizing inorganic nanoparticles composed of antimony (Bahrami et al. 2012). 1.6 Palladium and Platinum Nanoparticles Palladium is an excellent hydrogenation and dehydrogenation catalyst available in organo-metallic forms. Palladium nanoparticles have found to be effective catalysts in a number of chemical reactions due to their increased surface area over the bulk metal. The sulfate-reducing bacterium, Desulfovibrio desulfuricans and metal iron-reducing bacterium, Shewanella oneidensis, were capable of reducing soluble smtiquia@umich.edu 38 1 Nanoparticles Synthesized by Microorganisms palladium (II) into insoluble palladium (0) with formate, lactate, pyruvate, or H2 as the electron donor (Lloyd et al. 1998; Yong et al. 2002a; de Windt et al. 2005). Coker et al. (2010) demonstrated a novel biotechnological route for the synthesis of heterogeneous catalyst, consisting of reactive palladium nanoparticles arrayed on a nanoscale biomagnetite support. The magnetic support was synthesized at ambient temperature by the Fe (III)-reducing bacterium Geobacter sulfurreducens. The palladium nanoparticles were deposited on the nanomagnetite using simple one-step method to an organic coating priming the surface for Pd adsorption, which was produced by the bacterial culture during the formation of the nanoparticles. A recent biological method used to produce palladium nanoparticles is the precipitation of Pd on a bacterium. These palladium nanoparticles can be applied as catalyst in dehalogenation reactions. Large amounts of hydrogen are required as electron donors in these reactions, resulting in considerable cost. A study carried out by Hennebel et al. (2011) demonstrates how bacteria is cultivated under anaerobic conditions and can be used to reductively precipitate the palladium catalysts and generate the hydrogen (electron donor). This avoids the cost coupled to hydrogen supply. Batch reactors with nanoparticles formed by Citrobacter braakii showed the highest diatrizoate dehalogenation (Hennebel et al. 2011). Konishi et al. (2007b) demonstrated that resting cells of Shewanella algae were able to deposit platinum NPs by reducing PtCl62− ions within 60 min at pH 7 and 25 °C. Biogenic platinum NPs of about 5 nm were located in the periplasmic space. In this case, the cell suspension changed the color from pale yellow to black in 10 min. The black appearance provided a convenient visible signature for the microbial formation of metallic platinum NPs. The observed decrease in aqueous platinum concentration was presumably caused by the rapid reduction of PtCl62− ions into insoluble platinum. In the absence of lactate, however, S. algae cells were not able to reduce the PtCl62− ions. They reported that the PtCl62− ions were not chemically reduced by lactate. Yong et al. (2002b) also reported that the sulfate-reducing bacterium Desulfovibrio desulfuricans was able to adsorb only 12 % of platinum (IV) ions on the bacterial cells from 2 mM platinum chloride solution. In another study, Gram-negative cyanobacterium, Pediastrum boryanum UTEX 485, extracellularly produced Pt (II)-organics and metallic platinum NPs at 25–100 °C for up to 28 days and 180 °C for 1 day with different morphologies of spherical, bead-like chains and dendritic in the size range of 30 nm–0.3 μm (Lengke et al. 2006c). 1.7 Selenium Tellurium Nanoparticles Selenium is of considerable environmental importance as it is essential at low concentrations but toxic at high concentrations for animals and humans, with a relatively small difference between these values (Fordyce 2005). Selenium occurs in different oxidation states as reduced form (selenide, Se2−), least mobile elemental selenium (Se0) and water soluble selenite (SeO23)/selenate (SeO24) oxyanions. smtiquia@umich.edu 1.7 Selenium Tellurium Nanoparticles 39 Selenium possesses several applications in medicine, chemistry and electronics. Selenium (Se), as a functional material, is an important semiconductor and photoelectric element due to its special physical properties (Zhang et al. 2011). Therefore, Se is used in many applications ranging from photocells, photographic exposure meters and solar cells to semiconductor rectiﬁers. Recently, there has been increasing interest in the synthesis of nanoparticles using microorganisms, leading to the development of various biomimetic approaches (Mohampuria et al. 2008). However, most methods used to synthesize SeNPs are characterized by elevated temperatures and high pressures and are hazardous to the environment (Zhang et al. 2011). Stenotrophomonas maltophilia SELTE02 showed promising transformation of 0 selenite (SeO−2 3 ) to elemental selenium (Se ) accumulating selenium granules either in the cell cytoplasm or in the extracellular space. In addition, Enterobacter cloacae SLD1a-1, Rhodospirillum rubrum, and Desulfovibrio desulfuricans have also been found to bioreduce selenite to selenium both inside and outside the cell with various morphologies like spherical, ﬁbrillar, and granular structure or with small atomic aggregates. E. coli also deposited elemental selenium both in periplasmic space and cytoplasm, and P. stutzeri also aerobically reduced selenite to elemental selenium (Narayanan and Sakthivel 2010). Under aerobic conditions, Hunter and Manter (2008) reported that Tetrathiobacter kashmirensis bioreduced selenite to elemental red selenium. A 90-kDa protein present in the cell-free extract was believed to be responsible for this bioreduction. Moreover, Yadav et al. (2008) showed that P. aeruginosa SNT1 biosynthesized nanostructured selenium by biotransforming selenium oxyanions to spherical amorphous allotropic elemental red selenium both intracellularly and extracellularly. Oremland et al. (2004) reported the biogenesis of SeNPs under anaerobic conditions. Se0 nanoparticles (SeNPs) formed by the Se-respiring bacteria, such as Sulfurospirillum barnesii, Bacillus selenitireducens, and Selenihalanaerobacter shriftii, are structurally unique when compared to Se0 formed by chemical synthesis. However, anaerobic conditions have limitations, such as culture conditions and isolate characteristics that make biosynthesis processes tedious and challenging (Prakash et al. 2009). The generation of SeNPs by soil bacteria Pseudomonas aeruginosa and Bacillus sp. under aerobic conditions has been reported; however, these studies only include the partial characterization of selenium nanospheres (Yadav et al. 2008; Prakash et al. 2009). The characterization of the nanospheres in relation to size is of great importance, both in industrial and biologic activities. Recent reports describe that Se0 nanoparticles with a size under 100 nm have a greater bioavailability (Thakkar et al. 2009; Dhanjal and Cameotra 2010). In addition, other studies mention that a smaller size increases the ability to trap free radicals with greater antioxidant effect (Huang et al. 2003). Peng et al. (2007) mentioned that the size of Se0 nanoparticles plays an important role in their biologic activity and, as expected, 5–200 nm nano-Se can directly scavenge free radicals in vitro in a size dependent fashion. The bio-reduction of selenite (Se [IV]) by Pantoea agglomerans generates nanoparticles with sizes ranging between 30 and 300 nm was reported by Torres et al. (2012). Their study demonstrated that, smtiquia@umich.edu 40 1 Nanoparticles Synthesized by Microorganisms Fig. 1.19 Proposed mechanism for biogenesis of selenium nanospheres at different time intervals: a selenite reduction at 0 h; b formation of red elemental selenium in membrane fraction after 3–4 h of incubation; and c insoluble fraction after 12 h of incubation. Source Dhanjal and Cameotra (2010). Copyright © 2010, BioMed Central. Reproduced with permission Pantoea agglomerans strain UC-32 produce amorphous SeNPs under aerobic conditions with a size optimal for biotechnological use (such as trapping free radicals in the induction of selenoenzymes) after at least 20 h of incubation. These results are of great importance due to the low culture requirements of UC-32 strain with the subsequent low cost of biologically active SeNPs production. Synthesis of S0 under aerobic conditions by Bacillus cereus was investigated by Dhanjal and Cameota (2010). The aerobic reduction of selenate (SeO42−) and selenite (SeO32−) to Se0 is depicted in Fig. 1.19. The electron transfer was initiated from NADPH/NADH by NADPH/NADH-dependent electron carrier. The results show: (1) selenite reduction in 0 h; (2) formation of red elemental selenium in membrane fraction after 3–4 h of incubation; and (3) prolonged incubation for 12 h resulted in formation of red elemental selenium in soluble fraction. Tellurium (Te) has been reduced from tellurite to elemental tellurium by two anaerobic bacteria, Bacillus selenitireducens and Sulfurospirillum barnesii. B. selenitireducens initially formed nanorods of 10 nm in diameter and 200 nm in length were clustered together to form larger rosettes of *1000 nm but with S. barnesii small irregularly shaped extracellular nanospheres of diameter <50 nm were formed (Baesman et al. 2007). In another study, tellurium-transforming Bacillus sp. BZ isolated from the Caspian Sea in northern Iran was used for the intracellular biosynthesis of elemental tellurium NPs. The biogenic NPs were released by liquid nitrogen and puriﬁed by an n-octyl alcohol water extraction system. TEM analysis showed rod-shaped NPs with dimensions of about 20 × 180 nm. The produced NPs had a hexagonal crystal structure (Zare et al. 2012). smtiquia@umich.edu 1.8 Bismuth Nanoparticles 1.8 41 Bismuth Nanoparticles The bismuth nanoparticles (BiNPs) has attracted a great deal of interest because of its potential applications in X-ray radiation therapy, catalysts, thermoelectricity, and optical uses (Hossain and Su 2012; Wang and Buhro 2010; Carotenuto et al. 2009; Lin et al. 2011). BiNPs electrodes have been applied in the detection of heavy metal ions as a substitute for bismuth ﬁlm (Sahoo et al. 2013) In addition, bismuth compounds, such as BiPO4 (Pan et al. 2010), BiVO4 (Qu et al. 2013), Bi2O3, (Zhou et al. 2009) and Bi2S3 nanoparticles (Wu et al. 2010b) were also reported over the past decades as novel catalysts for photodegradation of environmental pollutants. Several approaches have been employed to fabricate BiNPs including thermal plasma (Wang et al. 2007) an electrochemical method (Reim et al. 2013) a gas condensation method (Lee et al. 2007) and solution phase chemical methods. The latter is the most popular method, which often involves the reduction of relevant metal salts with various reductants in the presence of morphology-controlling surfactants. The biological synthesis of BiNPs was explored using Serratia marcescens by Nazari et al. (2012). The biogenic bismuth NPs were released by liquid nitrogen and puriﬁed using an n-octanol water two-phase extraction system. The energydispersive X-ray and X-ray diffraction (XRD) patterns demonstrated that the puriﬁed NPs consisted of only bismuth and are amorphous. In addition, the transmission electron micrograph showed that the small NPs formed larger aggregated NPs around 150 nm. 1.9 Conclusions and Future Perspectives Biological systems; bacteria, fungi, actinomycetes, and algae have many opportunities for utilization in nanotechnology, especially in the development of a reliable and eco-friendly processes for the synthesis of metallic nanoparticles. The rich microbial diversity points to their innate potential for acting as potential biofactories for nanoparticles synthesis. Despite some related reports, many aspects of nanoparticles biosynthesis remain unclear especially with regards to why and how the size and shapes of the synthesized nanoparticles are influenced by the biological systems. The biochemical and molecular mechanisms of biosynthesis of metallic nanoparticles need to be better understood to improve the rate of synthesis and monodispersity of the product. The properties of NPs can be controlled by optimization of important parameters which control the growth condition of organisms, cellular activities, and enzymatic processes (optimization of growth and reaction conditions). The large-scale synthesis of NPs using microorganisms is interesting because it does not need any hazardous, toxic, and expensive chemical materials for synthesis and stabilization processes. 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