Engineering Biological Electron Transfer and Redox Pathways for Nanoparticle Synthesis

Abstract

Many species of bacteria are naturally capable of types of electron transport not observed in eukaryotic cells. Some species live in environments containing heavy metals not typically encountered by cells of multicellular organisms, such as arsenic, cadmium, and mercury, leading to the evolution of enzymes to deal with these environmental toxins. Bacteria also inhabit a variety of extreme environments, and are capable of respiration even in the absence of oxygen as a terminal electron acceptor. Over the years, several of these exotic redox and electron transport pathways have been discovered and characterized in molecular-level detail, and more recently synthetic biology has begun to utilize these pathways to engineer cells capable of detecting and processing a variety of metals and semimetals. One such application is the biologically controlled synthesis of nanoparticles. This review will introduce the basic concepts of bacterial metal reduction, summarize recent work in engineering bacteria for nanoparticle production, and highlight the most cutting-edge work in the characterization and application of bacterial electron transport pathways.

Bacterial Metal Reduction

Bacteria have evolved pathways to detect, sequester, and change the oxidation state of a variety of metal compounds and ions.¹ The reason for this is simple; bacteria live in environments with chemical conditions not typically encountered by cells within multicellular organisms.^2,3 There are bacteria thriving in environments with high concentrations of arsenic, mercury, copper, lead, and zinc. Bacterial proteins have been discovered capable of reducing metals including iron, manganese, vanadium, chromium, molybdenum, cobalt, palladium, gold, silver, mercury, arsenic, selenium, uranium, and polonium.⁴ This wide range of redox activity is a main reason why whole bacteria and bacterial enzymes have been engineered for metal reduction and synthesis of nanoparticles (Fig. 1A).

FIG. 1.

Engineering bacteria for nanoparticle synthesis utilizes native pathways for metal reduction. (A) Using techniques from synthetic biology, bacteria have been engineered to express genes related to nanoparticle formation, including redox enzymes, electron transfer pathways, and biomolecules that control the nucleation and growth of nanoparticles. Examples of native bacterial metal reduction pathways are shown in (B, C). (B) The arsenic reduction pathways in Shewanella sp. ANA-3 include both cytosolic and periplasmic redox enzymes. (C) The Mtr Pathway from Shewanella oneidensis is used for extracellular electron transport. Electrons from metabolic processes in the cell are passed through CymA on the IM to periplasmic electron shuttles and leave the cell through the Mtr cytochrome complex in the OM. From the Mtr complex, electrons move to extracellular electron acceptors, including redox shuttle molecules, such as flavins and metals. IM, inner membrane; OM, outer membrane.

Nanoparticles are now in widespread use in a variety of applications, including additives to textiles and building materials, electronics, medicine, filtration technologies, environment remediation, and energy production.⁵ Biogenic synthesis of nanomaterials offers a, more environmentally friendly route of synthesis as compared with current chemical processes.^6,7 Given the recent expansion of the tools for engineering biological systems, future developments within biogenic nanomaterial production may reveal additional benefits over chemical processing, such as the ability to produce novel nanomaterials. In this study, we review the ways in which bacteria interact with and process metals and semimetals, and the efforts to adopt these pathways for nanomaterial synthesis.

There are three main reasons why bacteria have evolved pathways to interact with these heavy metals. The first reason is that many metals are required for cells to perform basic metabolic and biochemical processes in the cell. Single metal atoms are often incorporated into proteins to help with electron transport and act as sensors of redox conditions.⁸ Many are essential components of enzymes and protein cofactors. Metals such as selenium, cobalt, and copper are present in trace amounts within the environment, and pathways have evolved to sequester and chaperone the movement of these atoms into the cell and facilitate their incorporation into enzymes.⁹ Iron concentrations are also tightly regulated, with several mechanisms that sense iron availability and regulate genes related to sequestration, transport, and storage.^10–12

The second reason to bind and reduce metals is to prevent their toxic effects. If not tightly regulated, high concentrations of many metals can have adverse effects on the biochemistry of a cell, including displacing metal centers of enzymes and binding to and disrupting the conformations and normal activity of many biomolecules. Bacteria have evolved mechanisms to interact with metals such as arsenic, mercury, lead, and silver to reduce their toxic effects.^4,13,14 In the case of Ag(I), resistant cells have been found to use a combination of efflux pumps and silver chelating proteins. In other contexts, a family of bacterial proteins called metallothionein chelate metals such as Cd²⁺ and Zn²⁺ to increase tolerance to these ions in the environment.¹⁵ In the case of arsenic, some bacteria are able to reduce As(V) to As(III) to better clear the toxic metal from the cell¹⁶ (Fig. 1B). A study on Klebsiella aerogenes found that microbial regulated precipitation of metals increased tolerance to mercury, lead, and cadmium.¹⁷

Another purpose for bacterial metal reduction is related to respiration.¹⁸ An important step in respiration involves the transfer of electrons down an electron transport chain, removing small amounts of energy to create a proton gradient later used mainly for ATP generation. As initially high-energy electrons progress down the chain, they lose energy to become electrons with a large positive redox potential. The cell needs to get rid of these low-energy electrons, otherwise the pathway will become clogged and high-energy electron from metabolism will not be able to continue generating the proton gradient. In aerobic settings, oxygen is the terminal electron acceptor that reacts with low-energy electrons at the end of the respiratory chain. Oxygen has a high redox potential of 0.8 V, capable of being reduced by even low-energy electrons.¹⁹ However, many bacterial species live in environments where oxygen is not present. It is useful to remember that life on Earth existed for a billion years before the Great Oxidation Event that resulted in an atmosphere capable of supporting aerobic metabolism. Therefore, many bacteria past and present utilize alternative terminal electron acceptors. A look at the redox tower, a chart listing the reduction potential associated with common oxidation and reduction reactions, reveals that many metals also have high redox potentials, not quite as low as oxygen but sufficient to remove the low-energy electrons from the cell. Therefore, bacteria in anaerobic environments have evolved pathways to utilize terminal electron acceptors such as iron, manganese, and even arsenic,²⁰ a process called dissimilatory metal reduction. These pathways often perform extracellular electron transport (EET) to use these low-energy electrons to reduce external metals in the environment.

Dissimilatory metal reduction is carried out by many bacterial species, including Shewanella oneidensis, which contains one of the best-understood electron transfer pathways in bacteria. Originally isolated from Lake Oneida in upstate New York, when these bacteria are grown without oxygen gas, they couple the incomplete oxidation of carbon sources to the reduction of a wide range of electron acceptors, including insoluble iron and manganese oxide minerals.²¹ EET to these insoluble substrates requires a series of redox active proteins and protein complexes known collectively as the Mtr Pathway, as depicted in Figure 1C, which traverses two distinct membranes (cytoplasmic or inner, and the outer membrane) and the space between them, known as the periplasm.²² Electrons must flow from the cytoplasmic membrane of the bacterium across which the proton gradient described previously is generated. The tetraheme c-type cytochrome, CymA, is localized within the cytoplasmic membrane where it oxidizes reduced, membrane-soluble electron carriers called menaquinones,²³ which become reduced through a variety of metabolic processes.²⁴ CymA moves electrons to soluble c-type cytochromes, CctA or FccA, located in the periplasm of S. oneidensis.²⁵ These periplasmic electron carrying proteins appear to have overlapping functional roles in delivering electrons to the MtrCAB complex, which spans the outer membrane. MtrA and MtrC are both decaheme c-type cytochromes, though they have highly specialized structures. MtrA seems to be a simple wire, and nearly fully embedded inside the integral outer membrane protein MtrB, with a small part of the protein accessible to CctA or FccA in the periplasm. MtrC sits atop the MtrAB complex, exhibiting a “staggered-cross” organization of its ten heme groups where electrons received from MtrA enter the protein and can exit three different ways.²⁶ The novel configuration of MtrC may explain the ability of S. oneidensis to not only reduce substrates that it directly contacts, but also substrates at a distance through a process termed electron shuttling.²⁷ The general configuration and function of the Mtr pathway has been genetically validated in two related species, Vibrio and Aeromonas, with variations observed in the cytoplasmic membrane and periplasmic components.^28,29 Many of components of the S. oneidensis pathway are well characterized genetically, biochemically, and structurally, as summarized in Shi et al.,³⁰ and have been successfully reconstructed in Escherichia coli.³¹

As you can see, biology has evolved many mechanisms to specifically interact with many elements of the periodic table, including metals and semimetals that are the building blocks of nanoscale materials. The past two decades have seen the synthetic biology community adapt and repurpose these metal-reducing and metal-binding molecules for biogenic nanoparticle synthesis.

Synthetic Biology Control Knobs for Nanoparticle Synthesis

Given the multiple ways in which bacterial naturally interact with metal ions, it is not surprising that bacteria have been reported to synthesize nanoscale, metallic structures. In some of the earliest reports, the formation of metallic nanomaterials was induced by naturally occurring biomolecules that bind to metals. A short peptide found in the yeast species Candida glabrata and Schizosaccharomyces pombe were found to enable the nucleation and growth of CdS nanoparticles.³² A later study showed that a specific protein in the periplasm of Pseudomonas stutzeri led to the formation of silver nanostructures inside of cells, presumably through a protein with high affinity for silver ions.³³ Since then, approaches have utilized a variety of strategies to influence biogenic synthesis of nanomaterials. Before diving into how cells can be engineered for nanomaterial synthesis, it is first helpful to review the general physics of nanoparticle formation.

Nucleation and growth of nanoparticles

Taking a step back, nanomaterial formation classically involves three processes: chemical transformation of reactants into an insoluble precursor, nucleation of metastable nuclei composed of said precursors, and growth of now stable nuclei.³⁴ The first step is often reaction of a precursor to change redox state, one more likely to form a crystalline material. Next, nucleation is a process whereby transiently stable assemblies of insoluble atoms must overcome a free energy threshold to form more stable nuclei. In the absence of a mediating interface, nucleation occurs homogeneously throughout the parent phase, driven by an oversaturated concentration of precursor in the environment. Heterogeneous nucleation can occur as nucleus formation is aided by surfaces or other molecules that interact with the precursor. Nucleation of either type often occurs over relatively short time scale compared with the chemical or growth steps. Lastly, growth occurs, driven by the availability of precursor not consumed during nucleation. Nanomaterial growth depends on surface reaction at the material interface and diffusion of precursor to the surface. Changes in nanomaterial size are also influenced by processes such as particle aggregation and Ostwald ripening, a mechanism by which smaller particles dissolve, yielding their material to larger particle. All told, abundant opportunities exist for microbes to orchestrate nanomaterial assembly at every step and scale. Next, we discuss work to date that has used live cells or biological components to modulate various aspects of nanoparticle formation.

Electron transfer and redox enzymes

As noted above, a key aspect of bacterial cells that initially led to the interest in applying them to the formation of metal and semiconductor nanoparticles is their ability to reduce a wide variety of materials. Transforming reactants to the correct oxidation state is an important first step of nanoparticle synthesis. For example, extracellular sulfate reductase from Fusarium oxysporum can convert sulfate to sulfide to enable the formation of CdS nanoparticles. In another study, electrons liberated from hydrogen through a hydrogenase were suspected of reducing gold and leading to the formation of periplasmic gold nanoparticles.³⁵ NADPH and glutathione reductases in yeast have also been used for CdSe nanoparticle synthesis.^36,37 Reductase activity within metabolically active S. oneidensis cells were also capable of copper, sulfate, and palladium reduction initiating nanoparticle formation.^38–40 In other studies, arsenic sulfide nanomaterials were formed following arsenic reduction by Shewanella species.^41,42 In more synthetic biology contexts, regulation of redox enzyme production can also be used to specify the change in the concentration of reactant ions over time. Inducible expression of the mtr pathway, as shown in Figure 2A, controlled the level of manganese doping into zinc sulfide nanoparticles⁴³ Potentially in future applications, different redox pathways can also be expressed at different points in time to dynamically change the composition of a particle as it grows. The ability to heterologously express different enzymes in a dynamic fashion gives synthetic biologists a useful toolbox to regulate how the concentration and redox state of reactants changes over time, an important factor that strongly influences nanoparticle properties.

FIG. 2.

Examples of nanoparticle synthesis in engineered bacteria. (A) Inducible expression of the mtr pathway, Shewanella MR-1, controlled the amount of doping in Mn:ZnS nanoparticles. Reproduced from Chellamuthu et al..⁴³ (B) The location of palladium nanoparticle synthesis by Shewanella oneidensis could be controlled by changing the redox pathways. Expression of periplasmic hydrogenases hydA and hydB led to particle formation in the periplasm, whereas expression of only the mtr pathway formed extracellular nanoparticles. Scale bar 100 nm. Adapted from Dundas et al.⁴⁹

Nucleating agents

Biological nucleation agents are another tool that can be used to control nanoparticle formation. As discussed above, nucleation can occur homogeneously in solution or associated with a surface. Such heterogeneous nucleation, on the cells' surface or on a macromolecule, will make nucleation energetically favorable by lowering the interfacial energy. Thus, the addition of nucleating agents to a system encourages nucleation and enables nucleation to occur even at a low reactant concentration. Many proteins naturally have strong binding to metals, and in some cases these proteins also facilitate particle nucleation. As discussed above, some of the early studies of biogenic nanoparticle formation used cells with naturally occurring metal-binding peptides and proteins.³² A putative cystathionine lyase from Stenotrophomonas maltophilia was also shown to both nucleate CdS nanoparticles and generate the required sulfide.⁴⁴ Another strategy to incorporate nucleating agents into cells is to use high-throughput screening. Phage display and cell surface display libraries have been used to identify peptide sequences capable of nucleating metal sulfide nanocrystals.^45–47 Such screens have identified peptide multiple amino acid sequences around 10 residues in length capable of nucleating nanomaterials, including CdS, ZnS, ZnO, Cu₂O, and Au. Intriguingly, both chemical reduction and nucleation may be facilitated by a single short peptide.⁴⁸

Capping agents

Another strategy to regulate nanoparticle synthesis is the use of capping agents to control the rate of growth and stability of particles. Capping agents, or molecules that coat the outside of a particle during growth, modulate the rate at which reactants from solution are delivered to and react with the particle surface.⁵⁰ Capping agents play a role in determining nanoparticle geometry and also prevent particle aggregation. Thus, particle size and polydispersity can be set by capping agents. A variety of compounds can serve as capping agents, including mercaptoethanol and triphenylphosphine. Many biomolecules such as surfactin, dextran, and glutathione have also been used as capping agents,⁵¹ and early work explored the ability of phytochelatin to regulate metal nanoparticle synthesis.⁵² A recent study compared a CdS nanocrystal formation in the presence of glutathione, l-cysteine, and a sulfide-producing enzyme made by S. maltophilia that served a surprising dual role of generating reactants for synthesis and regulating particle growth as a capping agent.⁴⁴ Heterologous expression of phytochelatin by E. coli also served as a capping agent for CdS.⁵³ Amino acids have been added during synthesis of gold nanoparticles and were found to regulate particle size.⁵⁴ Many molecules in the cell have the physiochemical properties that would make them attracted to nanoparticle services and therefore likely act as capping agents during particle growth in the vicinity of cell culture. Future work should continue the development of cellular systems to produce capping agents to explore to what extent this can be used as a new control knob to regulate the properties of biogenic nanoparticles.

Changing the location of synthesis

One aspect that cells brings to nanoparticle synthesis is the ability to sequester the reactions into compartments. The idea of using compartments to run chemical reactions requiring specialized conditions has naturally evolved and more recently been incorporated into synthetic biology efforts. For example, carboxysomes are a protein cage, which photosynthetic cells use to run the reactions needed for carbon fixation.⁵⁵ Magnetosomes are an example of natural cellular compartments used for magnetic nanoparticle synthesis, where the concentrations of reactants and oxygen is tightly regulated.⁵⁶ The advantage of running such reactions in compartments is that compartments enable the creation of physical and chemical conditions that favor the reactions of interest but may not be suitable for the more basic biochemical reactions in the cell.

Second, compartments can be used to concentrate reactants, using both transporters to bring reactants into the compartment and by preventing the escape of intermediates from the compartment.⁵⁷ Synthetic biology has utilized microcompartments to control a variety of reaction pathways. By gathering the enzymes and reactants required for biosynthesis, synthetic microcompartments have been engineered for a multistep reaction that produces 1,2-propanediol from glycerol.⁵⁸ This strategy has even been incorporated into nanoparticle synthesis. CdS nanoparticles were synthesized inside a cage-like protein from Listeria innocua and virus-like particles.^59,60

More generally, a cell itself can be seen as a microcompartment, with separate chemical and physical conditions than the surrounding environment. In Gram-negative bacteria, the periplasm is an additional compartment which would be used for synthesis. In a recent study, the synthesis of palladium nanoparticles could be moved from the cell exterior to the periplasmic space by changing the redox pathway from the externally reducing Mtr pathway to periplasmic hydrogenases,⁴⁹ as shown in Figure 2B. As a result, particle nucleation and growth occurred in a different cellular location and changed particle size. Moving forward, treating cells as compartments whose chemical and physical properties can be manipulated may prove a useful general strategy for engineering cells for nanoparticle synthesis.

Unexpected influences of biology on particle synthesis

Although many clever tricks have been used to integrate biological components into the process of nanomaterial synthesis, it is worth noting that many impacts of the biology on particle synthesis are poorly understood. Cells naturally contain a variety of biomolecules with redox activity and some ability to nonspecifically nucleate particle formation. Cells also have a long list of biomolecules that can coat or even be incorporated into particles during growth. Previous studies in biogenic nanoparticle synthesis have revealed unexpected surprises in the ability of cells to manipulate the synthetic process. For example, the host strain used to express an arsenic reduction system modulated the shape of arsenic sulfide nanomaterials from wires to spheres.⁶¹ A similar size dependence on host was observed for tellurium nanoparticle biosynthesis.⁶² In another example, the growth phase of the cell was shown to strongly influence the yield of CdS nanoparticles formed by E. coli.⁶³ It will be challenging to predict and understand all the unexpected ways in which living systems can modulate nanoparticle synthesis, and perhaps such unexpected findings could be leveraged as additional control mechanisms within synthetic schemes.

Frontiers in Bacterial EET

Moving forward, there are many ways in which advances in our understanding of bacterial redox activity and electron transfer would potentially benefit the ability to engineer cells for nanoparticle synthesis. Some of these advances might come in the form of new enzymes or pathways for metal reduction as well as the development of new host strains.

New host strains for biogenic synthesis, ideally ones that naturally contain metal-reducing redox and EET pathways, could be a great benefit to future approaches to biogenic nanoparticle system. Host strains that naturally have a high tolerance for heavy metals, such as Klebsiella planticola, could also be advantageous for synthesis.⁶⁴ Recent advances in synthetic biology tools have been applied to iron oxide production in host strain Magnetospirillum magneticum.⁶⁵ Still, limited genetic tools are available for bacteria capable of extracellular electron transfer as compared with more typical bacterial host strains.⁶⁶ Geobacter are also well-known electrically active bacterial species, and although genetic tools have been developed, this remains a challenging strain for genetic manipulation.^67,68 EET and redox pathways have been reported in Aeromonas hydrophila,²⁸ and the potential for these strains to serve as hosts for heterologous expression of enzymes for nanoparticle synthesis should be explored. Synthetic biology tools have also been expanded for strains such as S. oneidensis.^49,69–71 One challenge when introducing EET pathways to a new host, is both the proper expression and assembly of cytochrome complexes and also pairing these complexes with the right partner molecules to create a complete electron transfer pathway. When the mtr pathway was first introduced into E. coli, electron flux was low until inner membrane cytochromes and flavins were introduced.³¹ Similar problems would need to be identified and addressed as new host systems are introduced in future work.

Another exciting recent development is the finding that bacterial electron conduits, including the abovementioned multiheme cytochromes that facilitate metal reduction, can also facilitate long-distance electron transport (LDET) across neighboring cells. This multicellular redox conduction mechanism has been shown to be responsible for the conductivity of Geobacter and Shewanella biofilms.^72,73 In addition, multiheme cytochromes have been implicated in direct interspecies electron transfer within microbial partnerships, including one of the most environmentally significant metabolic interactions in nature: syntrophic consortia of anaerobic methanotrophic archaea and sulfate-reducing bacteria, various lineages of which carry out anaerobic oxidation of methane in ocean sediments.⁷⁴

Perhaps the most stunning example of LDET has been the recent discovery of centimeter-scale electron conduction in Candidatus Electrothrix and Candidatus Electronema cable bacteria, found in marine and freshwater sediments, respectively.^78,79 These multicellular cables, as shown in Figure 3A, composed of thousands of end-to-end cells, gain energy from LDET by coupling sulfide oxidation in deeper sediment layers to oxygen reduction near the sediment/water interface. By monitoring oxygen reduction and corresponding sulfide oxidation rates in sediments to arrive at the electric current density,^80,81 and estimating the cable bacteria filament density,^78,80,82 it is possible to arrive at per cable electron transport rates as high as 10⁹–10¹⁰ electrons/s. Such astonishing electron transport rates over centimeter distances were previously thought impossible in biological systems. While details of the underlying molecular pathway for cable bacteria LDET and its transport mechanism remain unclear, recent solid-state measurements revealed a conductive periplasmic network of nanofibers^75,83 running along the unusual cell envelope of cable bacteria⁸⁴ underneath an outer membrane shared by all cable cells. The identity of these conductive structures remains unknown, but c-type cytochromes are thought to be abundant in the cell envelope and show a redox gradient along the cable.⁸⁵ More recently, nickel-containing proteins were more directly implicated in the conductive nanofibers.⁸⁶ In addition to its fundamental physiological relevance, this unique “electric metabolism” may offer additional pathways for the formation of nanomaterials over macroscopic length scales, as evidenced by recent observations of different biominerals associated with cable bacteria, including clay (nano)particles and iron minerals encrusting the cell envelope.⁸⁷

FIG. 3.

Electron transport and nanoparticle synthesis over multiple length scales. (A) SEM image of an individual cable bacterium filament across a gold electrode. Scale bar 5 μm. Reproduced from Meysman et al.⁷⁵ (B) Arrangement of gold nanoparticles along curli fibrils produced by Escherichia coli. Scale bar 100 nm. Reproduced from Chen et al.⁷⁶ (C) SEM images of a Shewanella biofilm. Cells are covered with iron sulfide nanoparticles, which resulted in increased electron transport through the biofilm. Scale bar are 100 and 1 μm. Reproduced from Jiang et al.⁷⁷ SEM, scanning electron microscopy.

Another area of interest in the expansion of synthetic applications for bioelectronic systems is the coupling of electrically active bacteria with more traditional chemical synthesis of organic molecules. Shewanella electron transfer was used to assist in the polymerization of an organic compound through interactions with a copper-centered catalyst.⁸⁸ Although engineering cells, including bacteria, for synthesis of organic molecules is well established, electron transfer in living systems has not been previously shown to participate in a metal-catalyzed reaction scheme. Much of modern synthetic chemistry relies on the incredible diversity and specificity of metal-based catalysts. Applying our detailed understanding of biological electron transfer beyond applications in inorganic synthesis is an exciting new direction.

Future Perspectives

As the field of biogenic nanoparticle systems moves forward, there are several key questions that should guide developments in this field. What are the best applications of biogenic nanoparticle synthesis? One answer to this could be large-scale synthesis under “greener” conditions of lower temperature, atmospheric pressures, and without the use of harsh solvents and reagents.⁶ Scale-up to a 24 L process was demonstrated for biogenic synthesis of ZnS particles,⁸⁹ but more work is needed to determine if scale-up different synthetic schemes will be equally promising. Another underexplored application is the use of biogenic particles as a reporter of biological activity. Fluorescent and luciferase-based reporters have changed biology and medicine, and the development of nanoparticle-based reporters could have a similar effect in the future. Many nanoparticles have novel electrochemical and electrooptical properties. In fact, chemically synthesized nanoparticles are already important aspects of many new biotechnologies, including medical imaging.^90,91

A second important question is what are the advantages of biogenic synthesis over chemical synthesis? Unlike chemical systems, cells have the ability to create order over multiple spatial scales. Biogenic synthesis enables a multiscale approach, to arrange nanoparticles within larger arrangements of cells and biopolymers. This idea has been demonstrated previously, as the protein polymer curli has been shown to template the arrangement of gold nanoparticles into wires,⁷⁶ as shown in Figure 3B. Nanoparticles have also been incorporated into bacterial biofilms, leading to increased conductance of these films,⁷⁷ as shown in Figure 3C. There have been advances recently in the ability to pattern bacteria on surfaces and as clusters free in solution using the latest tools in synthetic biology.^92–94 The combination of approaches for particle synthesis and cellular-level patterning could lead to novel platform and applications in synthetic biology.^76,95 Biological systems also have the advantage of having built-in mechanisms to generate chemical diversity and evolve their components. Work in the space of directed evolution has proven that even naturally occurring enzymes are only a few mutations away from incredible new functionality, such as the ability to form carbon to silicon bonds.⁹⁶ Perhaps in the realm of biogenic synthesis a few minor changes in an enzyme, nucleation site, or capping agent, or the use of a new host that enables these components to act in a slightly different way, may lead to the breakthroughs needed to demonstrate the enormous range and potential of biogenic synthesis of nanomaterials.

Authors' Contributions

The article was conceived, written, and edited by J.Q.B., M.G., K.N., F.Z., J.A.G., and M.Y.E. All coauthors have reviewed and approved the article before submission. The article has not been published, in press, or submitted elsewhere.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This work was supported by the Office of Naval Research Multidisciplinary University Research Initiative Grant number N00014-18-1-2632.