- Open Access
Development of flagella bio-templated nanomaterials for electronics
© Jo et al.; licensee Springer 2014
Received: 18 November 2013
Accepted: 24 January 2014
Published: 21 March 2014
Bacterial flagella with their unique structural properties have proven to be promising bio-templates and can be exploited for the creation of nanomaterial with very high aspect ratio and surface area. Their chemically modifiable surfaces allow the flagella be modified to possess electrical/electronic properties. Their extraordinary physical properties along with the many possibilities for manipulation make them ideal systems to study for the purpose of developing nanoelectronics. First, this article reviews the characteristics of bacterial flagella and their utilization as biologically inspired templates. Next, the use of bio-templates for electronic systems such as dye-sensitized solar cell and lithium ion battery is discussed. Finally, we show the future directions for the use of flagella biotemplatednanomaterials for applications in electrical engineering fields.
The transition to renewable energy is met with the growing demand to develop new generations of electronic devices with improved efficiency and environmental performance in order to address the concerns regarding limited energy supplies and environmental issues. Moreover, the challenges in manufacturing extremely small features on mass-produced multifunctional devices incite the need to search for novel methods for the synthesis of the small scaled material integrated with high performance of functionality. As a result, the interface between nanosystems and biosystems is emerging as one of the largest and most active areas of technology to address the requirements and specifications for developing renewable energy [1, 2]. The combination of bio and nano hold the promise to yield advances in the creation of new and powerful methods that enable direct, sensitive, and rapid energy generation [3, 4]. In this respect, devices using on nanomaterials synthesized by biologically inspired templates can be one of the most promising platforms for direct ultrasensitive electrical detection and for building functional interfaces of core components . Biological templates have self-assembled hierarchical structures and can be massively obtained in nature, allowing the massive fabrication of nanostructured materials with precise dimensions and structures. Furthermore, biologically templated nanomaterial can be easily modified to change their natural functions and properties to create new types of highly efficient electronic devices, such as dye-sensitized solar cells (DSSCs) or lithium ion batteries (LIBs), with lightweight and high performance.
Using bacterial flagella as biologically inspired templates is a relatively new approach to assemble nanoscale architectures, which can revolutionize the fabrication process and the materials used in ultra-densely integrated electronic devices. Of particular significance is the finding that it is possible to selectively metallize the flagella templates or flagella based templates to obtain electrical properties through interelement positioning at the nanometer scale [6, 7]. However, even though their natural tubular nanostructure and the ease manipulation of their surface properties have already been well demonstrated, there is a lack of progress in the field of flagella-based device fabrication. To stimulate advances in using flagella as bio-templates in these forward-looking fields, this article will examine three main sections. The first section explains the characteristics of bacterial flagella to highlight their attractive properties that can be exploited for nanofabrication. Next, this article will introduce current electronic devices, including DSSCs and LIBs, built using other bio-templates such as virus or diatom. In the last section, we will discuss recent developments in the flagella based bio-templates and present the future prospects of applying them in the electrical areas.
Characteristics of bacterial flagella
Bacterial flagella, such as those from Escherichia coli (E. coli) or Salmonella typhimurium (S. typhimurium), are organelles used for propulsion. Bacterial flagella are self-assembled helical nanostructures, approximately 20 nm in diameter and 10 μm in length. The flagella have extraordinary mechanical properties; they are extremely stiff and have an elastic modulus estimated to be around 1010 N/m2[8–10]. As a result, they have remarkable durability and stability. In addition, they have the properties to be active nanostructures. They are able to actively adapt to the environment by changing helical handedness and pitch in response to various external stimuli; this is known as polymorphic transformation. Specifically, bacterial flagella can undergo polymorphic transformations both in loaded and unloaded conditions due to chemical, electrical, thermal, mechanical, or optical stimulation. Furthermore, flagellar filaments’ remarkable durability and stability allow them to withstand high temperature (up to 60°C) and extreme pH (7 ± 4) [11–13].
Structure of flagellar filaments
Polymorphic transformation of flagellar filaments
Given their structural makeup, filaments of different polymorphic forms can be obtained by using different flagellin types from different bacterial strains or by mixing different quantities of flagellin obtained from L-type and R-type straight filaments . However, a filament’s polymorphic configuration depends not only on the flagellin type, it also depends upon environmental conditions (i.e. pH, ionic strength, and temperature) as established in a series of experiments by Kamiya and Asakura [11, 12, 24]. Most of these experiments were performed with wild-type strains of Salmonella, but identical results were obtained with an E. coli K12 strain . As an example, filaments in 0.1 M KCl buffer expose to a raise in pH from 7 to 9 changes from the normal form to the coil form, which consequentially reduces their end-to-end length by about a factor of nearly 3. The flagellar filament also have an induced dipole moment of 5 × 10-24C⋅m in an electric field of E = 106 V/m . For example, straight polymorphic filaments align along the field, but close-coiled forms align with the helical axis perpendicular to the field . Under the appropriate condition, the filaments can even go so far as to change the handedness of the flagellar helix. Such astonishing degree of actuation opens up the possibility to utilize flagellar filaments for a broad range of applications.
Flagella depolymerization and repolymerization
Biomaterials as templates for the application of electronics
Recently, there is a fast growing interest in biological template technology, such as the use of bacteria [28, 29], virus [30–32], diatoms [33, 34], butterfly wings [35, 36], and so on. Most biological species possess a variety of unique characteristics such as distinctive nano or microstructures. Thus, it is very desirable to utilize these naturally formed unique structures as templates to create complex inorganic structures for different applications, such as nanoelectronics. Biotemplating is an eloquent and economic way to massively synthesize hierarchically periodic micro and nanostructures for electrical parts without using complex and expensive processes, making the biotemplate technology ideal for the synthesis of high surface area nanomaterials. In this section, we will review some recent works on DSSC and LIB devices built using biotemplate based nanomaterials.
Dye-Sensitized Solar Cell (DSSC)
The negative impacts from existing energy source, such as environmental impacts, and the increasing scarcity of energy resources have prompted the need for cheap, renewable, and clean energy technologies. Solar energy has been a major focus of modern science and engineering, especially with the threat of global warming and fossil fuel depletion. Each year, 3 × 1024 Joules of energy radiates from the sun to the earth ; this manifested the idea to harness such large amount of energy through the use of solar technologies. However, existing solar technologies are expensive when compared with hydrocarbon fuels, which their limits widespread commercialization. In order to increase the appeal of solar cells, their efficient must be increased and their cost must be decreased. Over the last decade, a type of photovoltaic device called the dye-sensitized solar cell (DSSC) had emerged as a technology that can achieve reasonably high efficiency and low manufacturing cost. Unlike conventional p-n junction diode based photovoltaics, the DSSCs create photovoltaic effect by photoexcitation of dye molecules that then inject electrons into wide band gap semiconductors that are not themselves able to be photoexcited. The performance of the DSSCs can be enhanced by optimizing the structure of the nanoparticle layers (TiO2 or ZnO) in the photoanode, the photosensitizing dye, and the electrolyte. In a typical particle based DSSC, the solar conversion efficiency is directly related to the electron percolation through the nanoparticle layers. However, the structural disorder at the contact between two crystalline NPs leads to enhanced scattering of free electrons. Therefore, it reduces electron mobility resulting in slow percolation which may lead to a decrease in efficiency .
Recently, much work had been performed to improve the optical path length of photoanodes by modifying surface structure or properties of photoanode . Since the light harvesting efficiency is directly affected by the photoanode properties, the optimization and improvement of photoanode plays an essential role in the performance of DSSCs . One approach was to replace the two dimensional flat surface with a three dimensional structure consisting of a dense array of aligned nanoscale features, such as the hierarchically self-assembled nanomaterial. The array of nanostructures created a larger surface areas compared with the flat surface; as a result, the optical path length was increased which in turn enabled rapid collection of carriers and achieved better light-accumulation effects . Furthermore, the increase of the optical path length inside DSSCs increases the probability of interaction between dye molecules which is essential to yield high light absorption and to facilitate the transfer of photo-generated electrons.
Lithium Ion Battery (LIB)
To meet the demand for a clean and secure electrical energy storage unit, extensive research efforts have been made to develop rechargeable batteries with large capacity, high power, and reasonable life cycle. The most notable are the LIBs due to a number of significant advantages over competing technologies . The use of lithium intercalation/alloying compounds had successfully enhanced the cycling life, safety, capacity, charge/discharge rates capability of the battery, as well as other battery characteristics such as high energy density and flexibility in shape and size. The three primary components of a LIB are the cathode, the anode, and the electrolyte. The operation of the LIB involves a electrochemical process where lithium ions migrate between the cathode and the anode while electrons flow through a close external circuit. Electrical energy is provided to the connected device via discharge where lithium ions move out of the anode (extraction) and into the cathode (intercalation). These reactions take place continuously while electrons continue to flow. Electrical energy can be restored to the battery through recharge, which is simply a reversal of the discharge process.
Though LIBs are generally smaller and lighter than other rechargeable batteries, the growing demand for portable electronics, such as cell phones and laptops, have facilitated the need to develop even smaller batteries for the propose of scaling down portable devices. However, it is still a challenge to breach the limitations of LIBs to create smaller batteries without sacrificing performance. Fortunately, the emerging technologies to fabricate and pattern nanostructures have incited a promising solution. The high surface area offered by patterned nanostructures can significantly increase the electrode/electrolyte contact areas, improved mechanical stability, and reduced distances for electron and ion transport to facilitate faster reaction kinetics. Furthermore, biogenic, bio-enabled, and bio-inspired hierarchical nanomaterials can be used to further increase reactive surface area. For instance, biological template such as viruses have proven to be extremely effect in the mass production of self-assembled hierarchically ordered structures with high surface to volume ratio. The advantages of using biotemplate technology may hold the key to a new class of electrode materials.
Flagella biological templating technology
Flagella bio-templated nanomaterials and metallization
Compared to the viral bio-templates, bacterial flagella templates have not been demonstrated to be used to create nanomaterials for electronic devices. However, recent promising studies had shown that flagella can be a good candidate as bio-templates. In their natural state, flagella exhibit self-assembled spiral and tubular nanostructures with extremely small outer diameter (20 nm) and controllable length. Aside from having attractive structural properties, flagella are extremely stiff and highly durable; they are stable under harsh conditions such as high temperature and extreme pH. Additionally, the solvent-exposed domains (D2 and D3) of the flagellin subunits are genetically modified, enabling simple interactions with external materials via chemical or genetic processes. In the context of fabricating biologically inspired flagella templates, some studies were also focused on using genetically engineered flagella for the well-defined functional groups on their surfaces, which led to the generation of ordered arrays of nanoparticles or monodisperse nanotubes. The peptide loop modified flagella nanotubes have been demonstrated to be useful scaffolds with high affinities for metal ions including Cu(II), Au(I), Co(II), Cd(II), and Pd(II)  and for the immobilization of Pd and Au nanoparticles [40, 58]. Genetically modified flagella have favorable in that their surfaces can be selectively functionalized to have strong affinities with different types of nanoparticles; however, the process can be complex and require expertise.
Characterization of electrical properties of flagella-templated nanotubes
Flagella templated dye-sensitized solar cell
The previous results suggest the possibility that the bacterial flagella can be selected for use in the DSSCs due to their unique properties; in particular, they have controllable length, they can change their helical form through polymorphic transformation, they can be genetically modified, and their surface can be functionalized. The functionalization of the flagella also makes the positioning of flagella nanotubes on a substrate with high precision; possibly more so than positioning nanowire arrays in current DSSCs devices. The poor control over the placement of the nanowires causes the cells to not perform as well as possible. Using bottom-up and top-down nanofabrication techniques , it is possible to place the flagella exactly where they are desired, and highly ordered arrays can be created, forming a “flagellar forest”. The flagella of S. typhimurium will be used in the DSSC design due to the well-defined and studied polymorphic forms.
Engineered flagella forest
Flagellar nanotube forest as a dye-sensitized solar cell
Once the flagellar nanotubes are harness into a nanotube forest device, they can be readily utilized to enhance DSSCs. A schematic of the flagellar nanotube enhanced DSSC is shown on Figure 12b. This device works by photoexciting dye molecules which inject electrons into semiconducting scaffolds which then transport the electrons to the transparent conducting electrode (TCO), upon which incident light enters the device. The dye chosen for this photovoltaic device should be a ruthenium-based dye, as demonstrated in a similar device . The appropriate TCO to be used on this device is Fluorine doped Tin Oxide (FTO). The dye scaffolds used in this device will be silica nanotubes. The use of flagella as templates allows for high aspect ratio and high surface area of the dye scaffolds. This charge transfer mechanism shown in Figure 12b requires that the redox electrolyte be oxidized so that an electron can be added to the photoexcited dye in order to bring the dye back to the ground state. In the case of this solar cell, the mediator is the I-/I3 - redox couple, which has been shown to be the best known redox couple for this application . The load to be driven by the solar cell is placed between the TCO and the counter electrode, which is on the opposite side of the solar cell from the TCO, and is generally covered by platinum. Platinum is considered due to its high electrocatalytic activity, which is necessary to reduce the redox electrolyte. This reduction of the redox electrolyte is necessary to return the oxidized redox electrolyte back to its normal state. The electrons that pass through the load and then end up on the platinum counter electrode are then transferred to the redox electrolyte which reduces the oxidized electrolyte .
The solar cell is assembled by dye sensitizing the TCO with the nanotube forest in a solution of the (Bu4N)2Ru(dcbpyH)2(NCS)2(N719) dye in dry ethanol. The platinum coated counter electrode, which is fabricated by evaporation onto an FTO electrode, and the dye-sensitized electrode are spaced apart with the use of hot melt spacers. The redox electrolyte (0.5 M LiI, 50 mM I2, 0.5 M 4-tertbutylpyridine in 3-methoxypropionitrile) is then added to the space between the two electrodes.
In order to optimize the performance of the nanotube forest as a DSSCs, the efficiency (η) and the fill factor (FF) should be well understood. The overall efficiency is important in determining the percentage of energy from the incident light that is able to be converted into useful electrical energy. The fill factor is used in determining how ideally the solar cell works as a diode, which can change depending on how well the counter electrode acts as an electrochemical catalyst, or how changing the electrolyte of the photovoltaic can affect the fill factor and be an indicator of how scalable the system is.
The current research effort and developments made towards electronic device applications have been heavily influenced by the utilization of nanomaterials synthesized from biologically inspired templates. These fascinating developments were the direct result of the advantages of using biotemplates; specifically the capability to mass produce biomaterials of uniform size, precise structure, complex morphology, and surface modification functionality. Many of these properties enhanced the fabrication method of nanomaterials and their functionalities in electronic device components. Especially, it have been said that the current technological limitations of solar cells and batteries can be amended through the use of nanostructures due to their high surface area to volume ratio. Many biotemplates had successfully been used to fabricate nanomaterials for electronic devices. The favorable results from these works have inspired the possibility to use bacterial flagella as biotemplates. The bacterial flagella can be a viable choice for biotemplates because of their unique natural properties such as tubular structures with controllable length and their polymorphic transformation. Although comprehensive studies had been done on the synthesis of high quality nanomaterials using bacterial flagella biotemplates, research on using these flagella templated nanomaterial in practical application is still in an early stage. We believe that future direction of this work should be focused on applying them to electronic devices. To move towards to realization of this work, we must optimize the manufacturing process to generate large quantity of nanomaterials with high quality in more controllable, accruable, and simple ways. Therefore, the flagella biological properties combined with synthetic chemistry have promising prospect in the fabrication of useful materials for electronic devices with high yield and functionality.
This work was funded by Army Research Office Young Investigator Award (W911NF-10-1-0173) and National Science Foundation (CMMI-0745019).
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