1. Introduction
Nanoscale materials offer precise tuning of material properties through atomistic control of matter and energy interactions at increasingly small length scales. This precision in material properties enables vast new opportunities in sensing, data storage, energy storage, and catalysis, among other areas [1–5]. Key to this process is the synthesis of nanoscale materials with well-defined and uniform architectures [4, 6]. Traditional chemical and physical synthesis technologies rely on purely material- and energy-intensive processes that are difficult to control and scale, rely on toxic or otherwise non-green chemicals, have non-uniform outputs, and limited control of atomistic features [7–10]. In contrast, biology synthesizes uniform nanoscale biomolecules via well-defined design rules, which may be engineered and serve as biotemplates for the synthesis of metallic nanomaterials [11, 12]. Biomolecules, such as nucleic acids, microtubules, amyloid fibers, and viruses, have been used as scaffolds for the construction of hierarchical complex nanomaterials [13–17]. Their surfaces present diverse biochemical functionalities that are used to organize nanoparticle synthesis, and may be modified via conjugation with organic or inorganic materials to create novel devices and control metal mineralization [18, 19]. Finally, biomolecules possess well-defined nanoscale architectures, are structurally stable across a wide range of conditions, and can be easily manipulated via genetic engineering. All these features make biomolecules attractive biotemplates for bottom-up nanomaterial assembly.
Viruses possess many advantages over other types of biomolecules for nanoparticle synthesis as they occur in a wide range of shapes and sizes, and present diverse chemical functionalities for synthesis. Plant viruses are widely used because they are harmless to human beings [11]. For instance, Cowpea chlorotic mottle virus, Cowpea mosaic virus, and Brome mosaic virus form icosahedral structures that range in size from 18-30 nm while Tobacco mosaic virus and Barley stripe mosaic virus assume rod-shaped structures up to 300 nm in length [11]. This diversity enables biotemplating of diverse nanomaterials for incorporation as catalysts, sensors, battery anodes, and semiconductor digital memory devices [1–5]. Viral particles consist of self-assembled capsid proteins (CPs) and nucleic acids that genetically encode the CPs. The CPs present diverse biochemical functionalities via amino acid residues on the particle surface that interact with metals in solution and drive nanoparticle synthesis. These residues may be conjugated to other compounds to enable synthesis of different nanomaterials and create novel functional properties [18, 19]. Similarly, the presented protein functionalities and dimensions can be directly modified via engineering the encoding nucleic acid sequence without dramatically altering viral structure to enable synthesis of new materials [1, 20, 21].
Non-infectious virus-like particles (VLPs) may be generated via heterologous expression of CPs in non-native species without using the complete viral genome [22]. Expressed CPs spontaneously self-assemble into VLPs that possess the same rich chemical diversity on their surfaces to drive nanoparticle synthesis. Plant VLPs also offer several compelling features over real viruses for VLP engineering and industrial-scale production. First, VLPs are more tolerant of mutations than live virus enabling more engineering opportunities to enhance function. For example, genetic modifications that enhance particle structural stability to improve nanomaterial synthesis yields enable the formation of nucleic acid-free VLPs that are unable to infect host cells [23]. VLP production does not rely on infection for production and may be stably produced with this enhancement in a heterologous host. Second, heterologous microbial hosts replicate and produce CPs much more rapidly than plants, which need several weeks to grow and mature before infection with virus for production [24]. Moreover, live viruses are infectious agents and must be grown in a Biosafety Level 2 greenhouse by plant virologists to contain potential environmental contamination, adding to their costs [25]. Third, bacterial VLP production also leverages a wealth of bioprocessing infrastructure that has been developed for large scale production of food, pharmaceuticals, and chemicals [26]. Thus, VLPs are more compelling platforms for the development of viral biotemplates.
Tobacco mosaic virus (TMV) is widely used for biotemplating due to its architecture and physicochemical properties (Table 1). The dimensions of TMV are well suited to biotemplating applications such as the production of batteries and sensors [1, 6, 27]. TMV is self-assembled from over a thousand copies of a single CP into a 300 nm long nanotube whose inner and outer diameters are 4 nm and 18 nm, respectively [28]. This aspect ratio maximizes the available surface area in compact volumes enabling more efficient battery electrodes with higher charge densities, and increased sensitivity to chemical analytes as sensors. Moreover, the biochemical/physicochemical properties of TMV enable reduction of metal ions and nanoparticle synthesis on the template under ambient conditions [3, 29]. Finally, TMV and its VLPs consist of a single CP that is amenable to genetic and chemical modifications that expand the types of nanomaterials that may be synthesized, enhance morphological uniformity, and increase particle density [30]. While TMV is the most commonly used in bionanotechnology, the evolutionarily-related Barley stripe mosaic virus (BSMV) provides a promising alternative biotemplate [16]. BSMV has similar architecture to TMV (Table 1) but presents distinct surface functional groups that accelerate nanoparticle synthesis and increase nanoparticle density for increased electrical and thermal conductivities and analyte sensitivity as sensors [16]. Moreover, the evolutionary similarities between TMV and BSMV may allow successful engineering strategies from TMV to be applied in BSMV to expand and enhance the properties of BSMV-derived biotemplates. Thus, BSMV is emerging as an attractive virus biotemplate for nanomaterial synthesis.
The convergence of advances from materials science, structural biology, molecular biology, chemistry, machine learning, engineering, and synthetic biology, now enable the rapid engineering and development of TMV, BSMV, and their VLPs for biotemplating. Their physicochemical properties make them well-suited for the synthesis of diverse nanomaterials for applications such as catalysis, energy storage, and sensing. In this review, we provide an overview of the properties and use of TMV, BSMV and their VLPs for nanoparticle synthesis, and focus on emerging technologies and approaches to engineer VLPs to enhance their function and broaden the nanomaterials that may be synthesized.