Biogenic nanoparticles

Transcription

1 Biogenic nanoparticles Dr. K. Uma Maheswari Professor, School of Chemical & Biotechnology SASTRA University Joint Initiative of IITs and IISc Funded by MHRD Page 1 of 10

2 Table of Contents 1 WHAT ARE BIOGENIC NANOPARTICLES? SYNTHESIS OF BIOGENIC METAL NANOPARTICLES MECHANISM INVOLVED IN THE SYNTHESIS OF BIOGENIC METAL NANOPARTICLES REFERENCE ADDITIONAL READING Joint Initiative of IITs and IISc Funded by MHRD Page 2 of 10

3 Module objective This module aims to familiarize the learner with the concepts involved in the synthesis of nanoparticles by microorganism, plants and higher organisms. An insight in to the probable mechanisms involved in the synthesis is also discussed. Preface Synthesis of nanoparticles using physical and chemical methods has been found to possess several difficulties in maintaining the size, shape and monodispersivity. However, several microorganisms have been found to synthesize nearly monodisperse nanoparticles with great control over size and shape. This module discusses the significance of such biogenic synthesis and the mechanisms involved. This lecture aims to introduce the learners to the concept of biogenic nanoparticles and some of the probable mechanisms involving peptides in the synthesis of these biogenic nanoparticles. 1 What are biogenic nanoparticles? Nanoparticles synthesized using biological systems (eg. Microbes, fishes, plants etc.) are referred to as biogenic nanoparticles. The process of synthesis of the nanostructures by organisms is also known as biomineralization. As the process involves no harsh chemicals and solvent systems, this method of nanoparticle synthesis is classed as a Green synthesis. These nanoparticles possess the advantage of having uniform size and shape. Moreover, they have also been found to possess better stability owing to stabilization by proteins and other biomolecules from the organism. The biogenic nanoparticles may be sequestered in separate intracellular compartments inside the cells and are referred to as intracellular biogenic particles. Separation of these nanoparticles requires disruption of the cells. On the other hand, some organisms synthesize these nanoparticles outside the cell or send them outside post-synthesis. Such nanoparticles are referred to as extracellular biogenic nanoparticles. Why do organisms synthesize nanoparticles? One of the major reasons involved in such synthesis is detoxification. The major route to shield the cell(s) from the toxicity of certain soluble ionic species such as metal ions is to convert them into insoluble forms through reduction or precipitation. In general, four major detoxification strategies have been identified in organisms. Joint Initiative of IITs and IISc Funded by MHRD Page 3 of 10

4 (a) Modifications in the cellular transport mechanisms to restrict entry of the toxic ions into the cell (b) Sequestration of the toxic species within the cell (intracellular sequestration) or outside the cell (extracellular sequestration) (c) Activation of energy-dependent efflux pathways to eliminate the toxic species (d) Enzyme catalyzed oxidation or reduction of the toxic species to a less toxic form In certain cases, the synthesis of nanoparticles is initiated to meet the cell s requirements for a functional component such as iron oxide or silica. Such synthesis occurs via oxidation and condensation-association routes. Some of these nanostructures have exquisite and complex morphology that is too difficult to be recreated using chemical routes of synthesis. Moreover, large-scale production of these biogenic nanoparticles can be easily accomplished and hence this area has been among the most widely explored areas in nanoscience and technology. 2 Synthesis of biogenic metal nanoparticles The synthesis of metal nanoparticles by biological systems is mainly a strategy employed to protect the system against the toxic effects of the soluble metal ions. Several species of microbes and plants exhibit tolerance to metal ions through this strategy. Both bacterial and fungal species have been employed for synthesizing metal nanoparticles. The most widely synthesized metal nanoparticles are silver and gold due to their widespread applications in many fields of science and technology. The metal nanoparticles synthesized using fungal species is referred to as mycogenic nanoparticles while those synthesized using bacterial species is known as bacterioform nanoparticles. Even higher plants have been shown to be effective in synthesizing silver and gold nanoparticles and the amount of reports available in this topic is too large to be summarized. Did you know? The huge number of fungal species that have been used to synthesize different nanoparticles with excellent stability and size control has led to the genesis of a new field known as Myconanotechnology. The cost-effective large-scale production of metallic, semiconductor and metal oxide nanoparticles is being developed for various applications in this rapidly expanding field. Some of the major organisms that have been employed to synthesize nanoparticles are listed in the following Table.(Note:This Table is not meant to intimidate the learners with its length but is given here mainly to highlight the enormous quantity of work that has been carried out in this area!) Joint Initiative of IITs and IISc Funded by MHRD Page 4 of 10

5 Table 1: Biological systems used for synthesizing nanoparticles Name of the biological species Nanoparticle Size (nm) Acalypha indica Silver Actinobacter spp. Magnetite Aspergillus clavatus Silver Aspergillus flavus Silver 8.92±1.61 Aspergillus fumigatus Silver 5-25 Aspergillus niger Silver 20 Apiin extracted from henna leaves Silver & Gold 39; (respectively) Avena sativa (oat) Gold 5-20 (ph 3-4) & (ph 2) Azadirachta indica Silver 50 Brassica juncea (mustard) Silver 2-35 Bacillus licheniformis Silver 50 Bacillus licheniformis (culture supernatant) Silver 50 Bacillus megaterium Silver 46.9 Bacillus sp. Silver 5-15 Bacillus subtilis Silver 5-60 Bacillus subtilis (culture supernatant) Silver 5-60 Brevibacterium casei Silver 50 Candida glabrata CdS Not available Carica Papaya Silver Cinnamomum camphora leaf Silver Citrus limon (lemon) Silver <50 Cladosporium cladosporioides Silver Clostridium thermoaceticum CdS Not available Cochlospermum gossypium Silver 3 Coriandrum satiyum leaf extract Silver 26 Coriandrum sativum Gold Corynebacterium sp. Silver Cymbopogon flexuosus (lemongrass) Gold Cycas sp. (cycas) Silver 2-6 Datura metel Silver Desmodium triflorum Silver 5-20 Eclipta sp. Silver 2-6 Enhydra fluctuans Silver Escherichia coli Silver Escherichia coli CdS Not available Eucalyptus camaldulensis (river red gum) Gold Eucalyptus citriodora (neelagiri) Silver 20 Eucalyptus hybrida (safeda) Silver Euphorbia hirta Silver Ficus bengalensis (marri) Silver 20 Joint Initiative of IITs and IISc Funded by MHRD Page 5 of 10

6 Fusarium acuminatum Silver 5-40 Fusarium oxysporum Silver 5-50 Fusarium oxysporum and Verticillium sp. Magnetite Not available Fusarium semitectum Silver Fusarium solani Silver 5-35 Garcinia mangostana (mangosteen) Silver 35 Geobacter sulfurreducens Silver 200 Gliricidia sepium Silver Glycine max (soybean) leaf extract Silver Honey Silver 4 Ipomoea aquatic Silver Jatropha curcas (seed extract) Silver Klebsiella pneumonia (culture supernatant) Silver 50 Lactic acid bacteria Silver 11.2 Lactobacillus Strains Silver 500 Ludwigia adscendeous (ludwigia) Silver Magnetospirillium magneticum Fe 3 O 4, Fe 3 S 4 Not available Mentha piperita (peppermint) Silver & Gold 5-30, 90; 150 Moringa oleifera Silver 57 Morganella sp. Silver 20±5 Murraya koenigii Silver & Gold 10; 20 Nelumbo nucifera (lotus) Silver Ocimum sanctum (tulsi; root extract) Silver 10 Ocimum sanctum (tulsi; leaf extract) Silver Penicillium brevicompactum WA 2315 Silver Penicillium fellutanum Silver Phanerochaete chrysosporium Silver 100 Phyllanthus amarus Silver Proteus mirabilis Silver Psidium guajava (guava) Gold Pseudomonas stutzeri AG259 Silver 200 Scutellaria barbata D. Don (Barbated Gold 5-30 skullcup) Schizosaccharomyces pombe CdS Not available Sesbania drummondii (leguminous) Gold 6-20 Stapphylococcus aurens Silver Syzygium aromaticum (clove) Gold Syzygium cumini (jambul) Silver Terminalia catappa (almond) Gold Trichoderma asperellum Silver Trichoderma viride Silver 5-40 Verticillium sp. Silver 25±12 Joint Initiative of IITs and IISc Funded by MHRD Page 6 of 10

7 The differences in the reducing agents, stabilizing moieties as well as templating molecules between different species cause variations in the size, size distribution as well as crystal type of the nanoparticles. 3 Mechanism involved in the synthesis of biogenic metal nanoparticles Though different biogenic sources have been identified for synthesis of nanoparticles, the mechanism of synthesis is yet to be deciphered clearly. Many theories have been suggested to explain the mechanism of synthesis, especially for metal nanoparticles. As biogenic metal nanoparticles have been found to be stable for three months and more, it is evident that there should be a reducing agent as well as a stabilizing agent in the system that would facilitate formation and stabilization of the metal nanoparticles. The identification of these components from the complex cellular environment is extremely challenging. The nature of the reducing agent and stabilizing agent also varies with the species employed for the synthesis. In general, peptides, proteins, polyols and heterocyclic compounds have been suggested to have an important role in the synthesis of metal nanoparticles through an electron shuttle or charge capping mechanism. Peptides have been investigated for their role as possible reducing agents as well as stabilizing agents in many microbial species. Interestingly, several reports have indicated that a single amino acid might not be as effective as a polypeptide sequence containing the same amino acid residue for the synthesis. The metal ion is first reduced to form metal nanoparticle, which acts as a nucleus for further growth of the metal crystal. The peptide is initially thought to adsorb to the surface of the metal nanoparticle clusters causing a localized reducing environment that results in the reduction of more metal ions at the interface between the peptide and the metal nuclei. As a result, nanoparticles with large size distributions are formed. Figure 1 depicts a cartoon on the formation of a silver nanocrystal in the presence of peptide. Joint Initiative of IITs and IISc Funded by MHRD Page 7 of 10

8 Fig. 1: Formation of silver nanocrystals in the presence of peptide Formation of twinned crystals could also occur in this process. A twinned crystal forms when two crystals have some similar lattice points in a symmetrical manner. A similar kind of mechanism has been proposed with proteins in plants and is referred to as the recognition-reduction-limited nucleation. The positively charged metal ions interact electrostatically with negatively charged residues in the proteins followed by reduction resulting in nucleation at specific sites. The protein template further directs the growth of the crystals in a specific orientation. Several amino acid residuess have been implicated in the reduction and stabilization of silver and gold nanoparticles. These include arginine, cysteine, lysine, methionine, tyrosine and tryptophan. Tyrosine residue has been shown to reduce silver and gold ions under alkaline conditions. Figure 2 gives the possible reaction involved in the formation of metal nanoparticles in the presence of tyrosine residue. Fig. 2: Role of tyrosine residue in the reduction of silver ions to form silver nanoparticles Joint Initiative of IITs and IISc Funded by MHRD Page 8 of 10

9 The phenolic group of tyrosine gets converted into a semi-quinone during this process. The N-terminus of the peptide will interact with the metal nanoparticle conferring stability. Is there any evidence for this theory? Yes! It was found that an engineered viral protein template developed from the capsid of cowpea chlorotic mottle virus (CCMV) containing tyrosine residue was found to effectively synthesize gold nanoparticles. Removal of the tyrosine residues resulted in no reduction reaction and hence no gold nanoparticle formation occurred.this observation strongly suggests that the presence of tyrosine residue is a key factor in the formation of metal nanoparticles. Figure 3 shows a schematic representation of a viral protein containing tyrosine involved in the synthesis of metal nanoparticles. Fig. 3: Viral capsid involved in the synthesis of gold nanoparticles Tryptophan residue in peptides also has been shown to play a key role in the formation of metal nanoparticles. The tryptophan residue forms a transient tryptophyl radical after donating an electron to the metal ion during the reduction reaction. The highly unstable radical forms ditryptophan, a fluorescent dimer and kynurenine, another fluorescent molecule. This scheme of reactions is represented in Figure4. Joint Initiative of IITs and IISc Funded by MHRD Page 9 of 10

10 Fig. 4: Role of tryptophan residue in the formation of metal nanoparticles 4 Reference Encyclopedia of Nanoscience& Nanotechnology, Volume 1. Edited by: American Scientific Publishers, 2005 H.S. Nalwa, 5 Additional Reading Mechanistic aspects in the biogenic synthesis of extracellular metal nanoparticles by peptide, bacteria, fungi and plants, Nelson Duran, Priscyla D. Marcata, Marcela Duran, Alka Yadav, Aniket Gade, Mahendra Rai, Appl. Microbiol. Biotechnol. 90, (2011), Joint Initiative of IITs and IISc Funded by MHRD Page 10 of 10