M13 Bacteriophage-Based Self-Assembly Structures and Their Functional Capabilities.

Controlling the assembly of basic structural building blocks in a systematic and orderly fashion is an emerging issue in various areas of science and engineering such as physics, chemistry, material science, biological engineering, and electrical engineering. The self-assembly technique, among many other kinds of ordering techniques, has several unique advantages and the M13 bacteriophage can be utilized as part of this technique. The M13 bacteriophage (Phage) can easily be modified genetically and chemically to demonstrate specific functions. This allows for its use as a template to determine the homogeneous distribution and percolated network structures of inorganic nanostructures under ambient conditions. Inexpensive and environmentally friendly synthesis can be achieved by using the M13 bacteriophage as a novel functional building block. Here, we discuss recent advances in the application of M13 bacteriophage self-assembly structures and the future of this technology.

INTRODUCTION

Controlling the assembly of basic structural building blocks in a systematic and orderly fashion is an emerging issue in various areas of science and engineering including physics, chemistry, material science, biological engineering, and electrical engineering. The self-assembly technique can be used to develop functional nanostructures associated with organic-, inorganic-nanoparticles, copolymers, and proteins, and has several unique advantages of which the most important is its easy processing [1-5]. Functional nano-structures can be achieved at low cost since the solvent-based procedure utilized in their creation can proceed without additional manipulation of the physical, chemical, and vapor pressure of the solvent of the basic structural building blocks. The regular formation of highly packed building blocks increases device performance by increasing the density of building blocks per unit volume [6, 7].

The construction of building blocks using self-assembly techniques is inspired by biological systems [8-11]. Although mimicking the biological process of assembly is still in its infancy due to the uncertainty and complexity of the structure, using natural building blocks can be a solution for overcoming this obstacle. Of the natural materials including DNA, collagen, yeast, and viruses, viruses were recently highlighted as unique natural building blocks for the self-assembly process [12-15]. Furthermore, the M13 bacteriophage has attracted significant attention in the field because of its easy growth and handling properties. The use of biological materials as templates enables nontoxic synthesis at low cost.

The M13 bacteriophage (Phage) is composed of 2, 700 copies of the helically arranged pVIII major coat protein on its body. Five to seven copies each of pIII, pVI, pIX, and pVII are located at each end of the M13 phage. It is a virus nanofiber of approximately 880 nm in length and 6.6 nm in diameter and is safe in humans. It can be easily modified genetically and chemically to reveal designated functions. It can also be used as a template to reveal homogeneous distribution and the percolated network structure of inorganic nanostructures under ambient conditions [16-19]. Inexpensive and environmentally friendly synthesis is possible through M13 bacteriophage engineering. In this review, we introduce the recent advances in the application of the M13 bacteriophage self-assembly structure and discuss the future of this technology.

FABRICATION OF THE M13 BACTERIOPHAGE SELF-ASSEMBLY (M13SA) BUILDING BLOCK

The traditional preparation of M13SA is based on the layer-by-layer (LBL) assembly technique. By using linear polyethyleneimine and polyacrylic acid, this controlled molecular interaction leads to regular orientation of the bacteriophage [20-22]. Recently, novel techniques for two-dimensional (2-D) and three-dimensional (3-D) assembly of the M13 bacteriophage were introduced.

2-D Structure Fabrication

Low permeability has been a significant issue in 2-D fabrication because of the thickness of traditional membranes at the active area. Current research efforts are therefore primarily focused on making thin membranes that maintain separation efficiency with high permeability [23-26]. Interconnected basic structural building blocks can provide structural integrity and large-scale production with excellent permeability [27, 28]. However, a range in pore size distribution due to the non-uniformed integration of the building blocks limits the application of this system however using the M13 bacteriophage could be a solution to this problem. Lee et al. generated a unidirectionally aligned 2-D M13 bacteriophage structure on a graphene oxide (GO) surface using a simple fabrication method [29]. The genetically programmed pIII protein placed at the end of the M13 bacteriophage had specific binding interactions with the carboxylate-functionalized edges of the GO. The relatively neutral body of virus had a weaker connection with the GO surface, which was less chemically active than the end of the virus with the GO edge. The genetically modified pIII protein made strong salt-bridge type interactions with the GO edge via hydrogen bonding. The pIII protein, displayed with a disulfide-bond-constrained peptide made a strong cyclic structure with the edge of the GO and the aligned virus in the same direction owing to the energetic affinity of peptide geometry [30]. In contrast, the pVIII protein on the virus body demonstrated slight electrostatic repulsion due to the inherent negative charge. The fabricated M13 virus on the GO surface can be unidirectionally aligned by external shear force by water stream. This technique, involving orienting the direction of M13 by the dipping and sweeping procedure, was previously reported [31]. Fig. () shows the cyclic binding interaction between the modified pIII protein and the functionalized edge of GO. Fig. () shows a representative sketch of the alignment of the M13 virus by employing external shear force. Actual alignment of the M13 virus was observed using transmission electron microscopy (TEM). Through this approach, it is anticipated that highly orientated 2-D viral structures can be produced on both a large scale and at low cost, while maintaining high performance [30].

3-D Structure Fabrication

While 2-D bacteriophage formation is important for the application of thin structures such as ultrathin membranes, the 3-D structure of bacteriophage is also essential for engineering batteries [32], piezoelectric generators [33], and photovoltaics [34]. Chen et al. reported the generation of a polyaniline (PANI) and single walled carbon nanotube (SWNT) composite 3-D structure using the M13 bacteriophage as a template [35]. PANI is an excellent conductive polymer and exhibits enhanced performance when mixed with SWNT. PANI and SWNT composites are studied extensively through different assembly methods such as colloidal mixtures [36], electrostatic deposition [37], electropolymerization [38-40], radical polymerization in solution [41], and direct polymerization on SWNT supports [42-45]. However, poor dispersion and aggregation of SWNTs in composites limit the progress of these studies. Chemical modification of SWNTs [41, 43] by adding surfactants [42, 45] or binders [38, 39] was applied to overcome this hurdle. Although fabrication of composites was successful in this approach, heavy loading of SWNTs to achieve efficiency creates another obstacle for further application due to the high associated costs [38, 39]. The M13 bacteriophage was introduced as a template to generate a better PANI/SWNT composite because of its efficient dispersion ability. It was genetically engineered to bind SWNTs along the length of the bacteriophage, thereby allowing SWNTs to be clustered without aggregation. The M13/SWNTs composite was cross-linked to form a hydrogel type of scaffold. The continuous 3-D porous M13/SWNTs scaffold was successfully combined with PANI using a direct polymerization method. Fig. () shows a schematic illustration of the fabrication process used to create the M13/SWNTs/PANI composite. Fig. () shows the resulting 3-D porous film possessing 48.4% average porosity with an average pore size of 350 nm. The interconnected pore structure provides maximum interfacial contact with the electrolytes. A maximal specific capacity of 668 Fg-1 with 4.8% w/w of SWNT was achieved. This result represents one of the best cost effective solutions, since the aqueous-based synthetic process allows for reduced cost and easy large-scale production.

VARIOUS APPLICATION FIELDS OF M13 BACTERIOPHAGE SELF-ASSEMBLY (M13SA)

Rechargeable battery

Despite the excellent energy density (~1, 000 Whkg-1) of nonaqueous Li-O2 batteries, there are currently several limitations that restrict their effectiveness, such as a large voltage hysteresis, low power, poor cycle life, and poor selectivity for oxygen molecules [46]. Single metal oxide cathodes were employed to overcome these limitations [47-50]. The single metal oxide cathode from the previous synthetic method produced random particle sizes, plates, and spheres [51-53]. Therefore, a systematic synthetic approach is required to compare different methodologies. Oh et al. reported metal oxide synthesis on a M13 bacteriophage template [54]. The viral templates provided homogeneous distribution due to the length and diameter of the M13 bacteriophage (approximately 50 nm in diameter and 880 nm in length). E3/E4 virus clones were used as templates for metal oxides due to their capacity for effective nucleation of single metal and bimetallic oxides [55-58]. The E3/E4 virus clone contains either EEAE (E3) or EEEE (E4), a sequence of triple or quadruple glutamates at pVIII. Fig. () shows a schematic procedure of metal oxide synthesis on the viral template and the subsequent results. Two-step synthesis produced highly porous nanowire on viral templates with a mass that was 3 times greater than nanowires without M13 bacteriophage templates. The high porosity and homogeneity of the final product led to 83, 78, and 78% Coulombic efficiency (Co3O4 (OH), MnCo2O, and CoMn2O, respectively) in the first galvanostatic cycling, tested at 400 mA g-1. However carbon electrodes demonstrated 51% efficiency. The smaller true electrode surface area of bio-metal oxides can suppress surface driven parasitic reactions. Therefore, bio-metal oxides possess higher oxygen evolution reaction and oxygen reduction reaction activity in media [59, 60]. The final products also showed superior discharge capacities, specific capacities, and longer cycle life than bare carbon electrodes.

Piezoelectric Nanogenerator

At the end of the fossil fuel-based energy era, renewable energy harvesting has become one of the most important issues [61]. The piezoelectric system is a highlighted source for energy due to the direct conversion from mechanical to electrical energy [62]. Owing to the purity issues with polymeric residues and surfactants for traditional perovskite-structured chemicals such as BaTiO3 (BTO), biological templates have attracted attention as a means of fabricating distinguished properties in ambient conditions [63, 64]. The genetically programmable M13 bacteriophage was introduced as a template in a recent study by Jeong et al. [65]. Their previous findings revealed that the homogeneous distribution of the piezoelectric nanostructure was the key point for high electrical output [66, 67]. They introduced Ba and Ti precursors into the glutamate trimer (E3)-M13 viral template to form virus template-BTO. A further calcination process at 1, 000°C resulted in high crystalline BTO. Fig. () shows a schematic diagram and TEM images of virus template-BTO formation. The overall procedure and simulated piezopotential are shown in Fig. ( and ), respectively. The detailed scheme and TEM images of each step are shown in Fig. (-). After mixing virus template- BTO with polydimethylsioxane (PDMS), the product was coated in between a flexible indium tin oxide (ITO)-coated polyethylene terephthalate (PET) substrate. BTO is well dispersed in PDMS due to the virus template process. The final flexible device showed excellent performance of approximately 300 nA and 6 V over 21, 000 cycles at fast frequency.

The M13 bacteriophage is however an excellent piezoelectric source. In nature, hierarchically ordered tissues such as bones, collagen fibrils, and peptide nanotubes exhibit piezoelectric properties [68-72]. The strong dipole moment of the M13 bacteriophage possesses intrinsic piezoelectric property as a result of the lack of inversion symmetry. Lee et al. reported a piezoelectric response of genetically engineered M13 bacteriophage film on substrates. Fig. () shows the device structure and experimental set up. The pVIII coat protein of M13 bacteriophage was engineered to have four negatively charged glutamates (4E; EEEE), resulting in a higher dipole moment than the wild type. The single phage generator exhibited a current of 6 nA and potential of 400 mV by mechanical strain which was higher in order of magnitude than the performance of natural collagen. The combination phage film demonstrated an even better performance [33]. It is anticipated that these devices could be applied in miniature-scale actuators.

Solar Cell

The increasingly growing requirement of green energy accelerates the development of solar energy technology. The dye-sensitized solar cell (DSSC) takes center stage on account of its relatively easy process and low cost [73]. The power conversion efficiency (PCE) of the solar cell is highly dependent on the ability of photoanodes to collect electrons [74]. TiO2 photoanodes constructed with high aspect ratio material such as nanorods, nanotubes, and nanofibers showed better performance than nanoparticles (NPs). However, a one-dimensional composite structure provides less surface area than three-dimensional structures for dye and light absorption. Therefore, a novel pathway is required to improve device performance. Belcher et al. suggested a 3-D viral network composed of TiO2 nanowire coated Au NPs to maximize localized surface plasmon resonance (LSPR) [75]. Fig. () shows a schematic of the mechanism of a TiO2/Au NPs structure with 3-D virus template enhancing light absorption and electron collection. Photoanodes with a 3-D plasmon-enhanced virus-template exhibited 13.72 mAcm-2 short-circuit current density and 8.46% PCE. Easy large-scale production can be anticipated with the use of the aqueous virus template process. This 3-D network morphology can also be applied to other energy-related studies.

Extracellular Matrix (ECM)

Modification of the physical and chemical properties of artificial extracellular matrix (ECM) is an important key to maintaining or controlling the proliferation or differentiation of stem cells [76-80]. Wang et al. reported that specific fibronectin peptides (RGD and PHSRN) displayed on the bacteriophage matrix with unique topology served as an ECM for differentiating mesenchymal stem cells (MSC) into osteoblasts [81]. Fig. () shows the process of bacteriophage fabrication with specific peptides as well as the ECM fabrication process using the LBL method. The phage-based film showed a unique rigid/groove nanostructure, and the ECM topology had a significant influence on cell behavior such as cell differentiation by cell shape elongation [82-84]. This demonstrates that the rigid/groove structure formed through self-assembly of the M13 bacteriophage significantly induced the elongation and parallel alignment of rat MSC. In addition, insertion of RGD and PHSRN peptides into the M13 bacteriophage increased cell adhesion and vitality [80]. The simultaneous functionality of ECM topology and peptides via the M13 bacteriophage could be a novel strategy in stem cell research.

Wang et al. also suggested a 3-D structure of M13 bacteriophage-based ECM for vascularized osteogenesis of MSC [85]. It is known that new bone formation is promoted by proper angiogenesis [86]. Thus, to induce bone tissue angiogenesis in artificial ECM, RGD peptides that contain the blood vessel regeneration factor, α group integrin, were introduced through the M13 bacteriophage. RGD peptides are highly unstable when mixed physically or chemically with ECM or media. Therefore, an RGD-displayed virus matrix could be a possible solution for maintaining RGD peptides in ECM for a long period in order to achieve successful bone regeneration in vivo. Fig. () demonstrates RGD display to the pVIII major coat protein on the M13 bacteriophage and illustrates scaffold fabrication. RGD-displayed virus base matrix was implanted into a rat radial bone defect. The angiogenic growth factor VEGF and wild type M13 were used as positive and negative controls, respectively, for comparison. Quantification of bone volume of the RGD-displayed virus base matrix sample showed competitive results in comparison to the normal bone tissue sample. The number of blood vessels formed was approximately 50% greater than the positive control. The RGD-displayed virus base matrix induced formation of vascularized bone without VEGF [85]. Based on these results, the application of various peptide-displayed bacteriophages in nanomedicine and regenerative medicine is highly anticipated.

Colorimetric Sensors

The majority of developing colorimetric sensors is inspired by nature through structurally colored biomaterials such as the butterfly wing, beetle exocuticle, cephalopod skin, mammalian skin, and avian skin and feathers [87-95].

Due to the complex designs and synthetic process of incorporating analyte-responsive elements into current sensors (defined as type pattern recognition and also called ‘artificial nose’), an alternative approach is challenging but undoubtedly needed [96-99]. Oh et al. suggested a tunable M13 bacteriophage-based structure inspired by a turkey’s skin color change due to coherent scattering of light from collagen bundle structures [91, 100]. Fig. () demonstrates the fabrication of phage film by a simple pulling method and its associated color changes in response to changes in humidity. The hierarchically structure morphology of phage film can be generated by controlling pulling speed. The quasi-ordered phage fiber bundle exhibited angle-independent color change by polarity when exposing solvents to film.

Due to the selectivity and sensitivity of the peptides displayed on phage surfaces, genetically engineered M13 bacteriophage can be applied to target-specific sensors. Oh et al. reported the creation of a TNT detective M13 bacteriophage color sensor [100]. M13 bacteriophage was genetically engineered to display a TNT-binding peptide on the major coat protein. Fig. () shows the phage display of identifying the peptide, gas phase TNT detection down to 300 ppb and the ability of selective sensing between TNT, DNT, and MNT by the identified peptide displaying phage. This demonstrates that the TNT sensitive virus film shows highly selective TNT sensing. Virus-based color sensors can be applied to simple solvent or humidity sensors, and they can also be used for the detection of specific chemicals and toxicants.

CONCLUSION

The M13 bacteriophage has demonstrated its capacity to fabricate various noble structures with evident functionality. The characteristic self-assembly property, the aqueous mild synthesis condition, and easy functionality of the M13 bacteriophage structure allows for its significant contribution to a variety of scientific areas. Recently innovative 2-D and 3-D structures were successfully constructed using the M13 bacteriophage. Its unique properties have been exploited in several studies relating to energy storage and harvesting area such as rechargeable batteries, piezoelectric devices, and solar cells. The M13 bacteriophage was bioengineered with specific peptides and showed excellent biocompatibility and feasibility for use as a novel ECM. The structural color, one of the unique properties of the M13 bacteriophage, facilitated an innovative approach to fabricate color sensors. The M13 bacteriophage was also bioengineered with target chemical selective peptides to generate highly selective and sensitive color sensors. Furthermore, intensive studies are now actively in progress in the areas of drug delivery, gene transport, and bio imaging. A fundamental understanding of the novel assembly technique and functionality of the M13 bacteriophage is dependent on further extensive studies.

CONFLICT OF INTEREST

The authors confirm that this article content has no conflict of interest.