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.