Organic and inorganic nanoparticle hybrids

Icosahedral plant viral nanoparticles - bioinspired synthesis of nanomaterials/nanostructures

Viral nanotechnology utilizes virus nanoparticles (VNPs) and virus-like nanoparticles (VLPs) of plant viruses as highly versatile platforms for materials synthesis and molecular entrapment that can be used in the nanotechnological fields, such as in next-generation nanoelectronics, nanocatalysis, biosensing and optics, and biomedical applications, such as for targeting, therapeutic delivery, and non-invasive in vivo imaging with high specificity and selectivity. In particular, plant virus capsids provide biotemplates for the production of novel nanostructured materials with organic/inorganic moieties incorporated in a very precise and controlled manner. Interestingly, capsid proteins of spherical plant viruses can self-assemble into well-organized icosahedral three-dimensional (3D) nanoscale multivalent architectures with high monodispersity and structural symmetry. Using viral genetic and protein engineering of icosahedral viruses with a variety of sizes, the interior, exterior and the interfaces between coat protein (CP) subunits can be manipulated to fabricate materials with a wide range of desirable properties allowing for biomineralization, encapsulation, infusion, controlled self-assembly, and multivalent ligand display of nanoparticles or molecules for varied applications. In this review, we discuss the various functional nanomaterials/nanostructures developed using the VNPs and VLPs of different icosahedral plant viruses and their nano(bio)technological and nanomedical applications.

Quantum dot tailored to single wall carbon nanotubes: a multifunctional hybrid nanoconstruct for cellular imaging and targeted photothermal therapy

Hybrid nanomaterial based on quantum dots and SWCNTs is used for cellular imaging and photothermal therapy. Furthermore, the ligand conjugated hybrid system (FaQd@CNT) enables selective targeting in cancer cells. The imaging capability of quantum dots and the therapeutic potential of SWCNT are available in a single system with cancer targeting property. Heat generated by the system is found to be high enough to destroy cancer cells.

Efficient CD44-targeted magnetic resonance imaging (MRI) of breast cancer cells using hyaluronic acid (HA)-modified MnFe2O4 nanocrystals

Targeted molecular imaging with hyaluronic acid (HA) has been highlighted in the diagnosis and treatment of CD44-overexpressing cancer. CD44, a receptor for HA, is closely related to the growth of cancer including proliferation, metastasis, invasion, and angiogenesis. For the efficient detection of CD44, we fabricated a few kinds of HA-modified MnFe2O4 nanocrystals (MNCs) to serve as specific magnetic resonance (MR) contrast agents (HA-MRCAs) and compared physicochemical properties, biocompatibility, and the CD44 targeting efficiency. Hydrophobic MNCs were efficiently phase-transferred using aminated polysorbate 80 (P80) synthesized by introducing spermine molecules on the hydroxyl groups of P80. Subsequently, a few kinds of HA-MRCAs were fabricated, conjugating different ratios of HA on the equal amount of phase-transferred MNCs. The optimized conjugation ratio of HA against magnetic content was identified to exhibit not only effective CD44 finding ability but also high cell viability through in vitro experiments. The results of this study demonstrate that the suggested HA-MRCA shows strong potential to be used for accurate tumor diagnosis.

Differential uptake of chemically modified cowpea mosaic virus nanoparticles in macrophage subpopulations present in inflammatory and tumor microenvironments

There remains a tremendous need to develop targeted therapeutics that can both image and localize the toxic effects of chemotherapeutics and antagonists on diseased tissue while reducing adverse systemic effects. These needs have fostered the development of a nanotechnology-based approach that can combine targeting and toxicity potential. In this study, CPMV nanoparticles were chemically modified with the dye Alexa Flour 488 and were also tandemly modified with PEG1000 followed by AF488; and the derivatized nanoparticles were subsequently added to macrophages stimulated with either LPS (M1) or IL-4 (M2). Previously published studies have shown that M1/M2 macrophages are both present in an inflammatory microenvironment (such as a tumor microenvironment and atherosclerosis) and play opposing yet balancing roles; M2 macrophages have a delayed and progressive onset in the tumor microenvironment (concomitant with an immunosuppression of M1 macrophages). In this study, we show higher uptake of CPMV-AF488 and CPMV-PEG-AF488 by M2 macrophages compared to M1 macrophages. M1 macrophages showed no uptake of CPMV-PEG-AF488. More specifically, M2 macrophages are known to be up-regulated in early atherosclerosis plaque. Indeed, previous work showed that M2 macrophages in plaque also correlate with CPMV internalization. These studies emphasize the potential effectiveness of CPMV as a tailored vehicle for targeting tumor macrophages involved in cancer metastasis or vascular inflammation and further highlight the potential of CPMV in targeted therapeutics against other diseases.

Hybrid nanoparticles for detection and treatment of cancer

There is currently considerable effort to incorporate both diagnostic and therapeutic functions into a single nanoscale system for the more effective treatment of cancer. Nanoparticles have great potential to achieve such dual functions, particularly if more than one type of nanostructure can be incorporated in a nanoassembly, referred to in this review as a hybrid nanoparticle. Here we review recent developments in the synthesis and evaluation of such hybrid nanoparticles based on two design strategies (barge vs. tanker), in which liposomal, micellar, porous silica, polymeric, viral, noble metal, and nanotube systems are incorporated either within (barge) or at the surface of (tanker) a nanoparticle. We highlight the design factors that should be considered to obtain effective nanodevices for cancer detection and treatment.

Liposomes: from a clinically established drug delivery system to a nanoparticle platform for theranostic nanomedicine

For decades, clinicians have used liposomes, self-assembled lipid vesicles, as nanoscale systems to deliver encapsulated anthracycline molecules for cancer treatment. The more recent proposition to combine liposomes with nanoparticles remains at the preclinical development stages; however, such hybrid constructs present great opportunities to engineer theranostic nanoscale delivery systems, which can combine simultaneous therapeutic and imaging functions. Many novel nanoparticles of varying chemical compositions are being developed in nanotechnology laboratories, but further chemical modification is often required to make these structures compatible with the biological milieu in vitro and in vivo. Such nanoparticles have shown promise as diagnostic and therapeutic tools and generally offer a large surface area that allows covalent and non-covalent surface functionalization with hydrophilic polymers, therapeutic moieties, and targeting ligands. In most cases, such surface manipulation diminishes the theranostic properties of nanoparticles and makes them less stable. From our perspective, liposomes offer structural features that can make nanoparticles biocompatible and present a clinically proven, versatile platform for further enhancement of the pharmacological and diagnostic efficacy of nanoparticles. In this Account, we describe two examples of liposome-nanoparticle hybrids developed as theranostics: liposome-quantum dot hybrids loaded with a cytotoxic drug (doxorubicin) and artificially enveloped adenoviruses. We incorporated quantum dots into lipid bilayers, which rendered them dispersible in physiological conditions. This overall vesicular structure allowed them to be loaded with doxorubicin molecules. These structures exhibited cytotoxic activity and labeled cells both in vitro and in vivo. In an alternative design, lipid bilayers assembled around non-enveloped viral nanoparticles and altered their infection tropism in vitro and in vivo with no chemical or genetic capsid modifications. Overall, we have attempted to illustrate how alternative strategies to incorporate nanoparticles into liposomal nanostructures can overcome some of the shortcomings of nanoparticles. Such hybrid structures could offer diagnostic and therapeutic combinations suitable for biomedical and even clinical applications.

The application of nanoparticles in gene therapy and magnetic resonance imaging

The combination of nanoparticles, gene therapy, and medical imaging has given rise to a new field known as gene theranostics, in which a nanobioconjugate is used to diagnose and treat the disease. The process generally involves binding between a vector carrying the genetic information and a nanoparticle, which provides the signal for imaging. The synthesis of this probe generates a synergic effect, enhancing the efficiency of gene transduction and imaging contrast. We discuss the latest approaches in the synthesis of nanoparticles for magnetic resonance imaging, gene therapy strategies, and their conjugation and in vivo application.

Controlling forces and pathways in self-assembly using viruses and DNA

The ability of both viruses and DNA to self-assemble in solution has continues to enable numerous applications at the nanoscale. Here we review the relevant interactions dictating the assembly of these structures, as well as discussing how they can be exploited experimentally. Because self-assembly is a process, we discuss various strategies for achieving spatial and temporal control. Finally, we highlight a few examples of recent advances that exploit the features of these nanostructures.

Viral nanoparticles as platforms for next-generation therapeutics and imaging devices

Nanomaterials have been developed for potential applications in biomedicine, such as tissue-specific imaging and drug delivery. There are many different platforms under development, each with advantages and disadvantages, but viral nanoparticles (VNPs) are particularly attractive because they are naturally occurring nanomaterials, and as such they are both biocompatible and biodegradable. VNPs can be designed and engineered using both genetic and chemical protocols. The use of VNPs has evolved rapidly since their introduction 20 years ago, encompassing numerous chemistries and modification strategies that allow the functionalization of VNPs with imaging reagents, targeting ligands, and therapeutic molecules. This review discusses recent advances in the design of "smart" targeted VNPs for therapeutic and imaging applications.

FROM THE CLINICAL EDITOR

This review focuses on viral nanoparticles, which are considered attractive naturally occurring nanomaterials due to their inherent biocompatibility and biodegradability. These can be used as imaging reagents, targeting ligands and therapeutic molecules.

Modified natural nanoparticles as contrast agents for medical imaging

The development of novel and effective contrast agents is one of the drivers of the ongoing improvement in medical imaging. Many of the new agents reported are nanoparticle-based. There are a variety of natural nanoparticles known, e.g. lipoproteins, viruses or ferritin. Natural nanoparticles have advantages as delivery platforms such as biodegradability. In addition, our understanding of natural nanoparticles is quite advanced, allowing their adaptation as contrast agents. They can be labeled with small molecules or ions such as Gd(3+) to act as contrast agents for magnetic resonance imaging, (18)F to act as positron emission tomography contrast agents or fluorophores to act as contrast agents for fluorescence techniques. Additionally, inorganic nanoparticles such as iron oxide, gold nanoparticles or quantum dots can be incorporated to add further contrast functionality. Furthermore, these natural nanoparticle contrast agents can be re-routed from their natural targets via the attachment of targeting molecules. In this review, we discuss the various modified natural nanoparticles that have been exploited as contrast agents.

Buckyballs meet viral nanoparticles: candidates for biomedicine

Fullerenes such as C(60) show promise as functional components in several emerging technologies. For biomedical applications, C(60) has been used in gene- and drug-delivery vectors, as imaging agents, and as photosensitizers in cancer therapy. A major drawback of C(60) for bioapplications is its insolubility in water. To overcome this limitation, we covalently attached C(60) derivatives to Cowpea mosaic virus and bacteriophage Qbeta virus-like particles, which are examples of naturally occurring viral nanoparticle (VNP) structures that have been shown to be promising candidates for biomedicine. Two different labeling strategies were employed, giving rise to water-soluble, stable VNP-C(60) and VNP-PEG-C(60) conjugates. Samples were characterized using a combination of transmission electron microscopy, scanning transmission electron microscopy (STEM), gel electrophoresis, size-exclusion chromatography, dynamic light scattering, and Western blotting. "Click" chemistry bioconjugation using a poly(ethylene glycol) (PEG)-modified propargyl-O-PEG-C(60) derivative gave rise to high loadings of fullerene on the VNP surface, as indicated by the imaging of individual C(60) units using STEM. The cellular uptake of dye-labeled VNP-PEG-C(60) complexes in a human cancer cell line was found by confocal microscopy to be robust, showing that cell internalization was not inhibited by the attached C(60) units. These results open the door for the development of novel therapeutic devices with potential applications in photoactivated tumor therapy.

Facile and General Method for Synthesis of Sugar Coated Gold Nanoparticles

This letter describes a general method for the preparation of carbohydrate coated gold nanoparticles. The generality of this method has been demonstrated by surface coating AuNPs with the following sugars: glucose (monosaccharide); sucrose, maltose, or lactose (disaccharides); raffinose (trisaccharide); and starch (polysaccharide). The non-toxic, water-soluble phosphino aminoacid P(CH(2)NHCH(CH(3)-)COOH)(3), THPAL, has been used as a reducing agent in this process. The sizes of sugar coated AuNPs that have been generated in this study are: 30 ± 8 nm (Glucose), 10 ± 6 nm (sucrose), 8 ± 2 nm (maltose), 3 ± 1 nm (lactose), 6 ± 2 nm (raffinose), and 39 ± 9 nm (starch).

Chemical modification of viruses and virus-like particles

Protein capsids derived from viruses may be modified by methods, generated, isolated, and purified on large scales with relative ease. In recent years, methods for their chemical derivatization have been employed to broaden the properties and functions accessible to investigators desiring monodisperse, atomic-resolution structures on the nanometer scale. Here we review the reactions and methods used in these endeavors, including the modification of lysine, cysteine, and tyrosine side chains, as well as the installation of unnatural amino acids, with particular attention to the special challenges imposed by the polyvalency and size of virus-based scaffolds.

Nanocrystal core high-density lipoproteins: a multimodality contrast agent platform

High density lipoprotein (HDL) is an important natural nanoparticle that may be modified for biomedical imaging purposes. Here we developed a novel technique to create unique multimodality HDL mimicking nanoparticles by incorporation of gold, iron oxide, or quantum dot nanocrystals for computed tomography, magnetic resonance, and fluorescence imaging, respectively. By including additional labels in the corona of the particles, they were made multifunctional. The characteristics of these nanoparticles, as well as their in vitro and in vivo behavior, revealed that they closely mimic native HDL.

Chemical addressability of ultraviolet-inactivated viral nanoparticles (VNPs)

Background

Cowpea Mosaic Virus (CPMV) is increasingly being used as a nanoparticle platform for multivalent display of molecules via chemical bioconjugation to the capsid surface. A growing variety of applications have employed the CPMV multivalent display technology including nanoblock chemistry, in vivo imaging, and materials science. CPMV nanoparticles can be inexpensively produced from experimentally infected cowpea plants at high yields and are extremely stable. Although CPMV has not been shown to replicate in mammalian cells, uptake in mammalian cells does occur in vitro and in vivo. Thus, inactivation of the virus RNA genome is important for biosafety considerations, however the surface characteristics and chemical reactivity of the particles must be maintained in order to preserve chemical and structural functionality.

Methodology/Principal Findings

Short wave (254 nm) UV irradiation was used to crosslink the RNA genome within intact particles. Lower doses of UV previously reported to inactivate CPMV infectivity inhibited symptoms on inoculated leaves but did not prohibit systemic virus spread in plants, whereas higher doses caused aggregation of the particles and an increase in chemical reactivity further indicating broken particles. Intermediate doses of 2.0–2.5 J/cm² were shown to maintain particle structure and chemical reactivity, and cellular binding properties were similar to CPMV-WT.

Conclusions

These studies demonstrate that it is possible to inactivate CPMV infectivity while maintaining particle structure and function, thus paving the way for further development of CPMV nanoparticles for in vivo applications.

Polyvalent display of heme on hepatitis B virus capsid protein through coordination to hexahistidine tags

The addition of a hexahistidine tag to the N terminus of the hepatitis B capsid protein gives rise to a self-assembled particle with 80 sites of high local density of histidine side chains. Iron protoporphyrin IX has been found to bind tightly at each of these sites, making a polyvalent system of well-defined spacing between metalloporphyrin complexes. The spectroscopic and redox properties of the resulting particle are consistent with the presence of 80 site-isolated bis(histidine)-bound heme centers, comprising a polyvalent b-type cytochrome mimic.

Photoluminescent, transparent and flexible di-ureasil hybrids containing CdSe/ZnS quantum dots

We describe, in this paper, the sol-gel synthesis of di-ureasil based nanocomposites prepared in situ in the presence of organically capped CdSe quantum dots (QDs) or CdSe QDs which have been coated with a ZnS shell. For the latter a new chemical route to coat the CdSe QDs with ZnS shells was investigated and is now reported. The QDs became well dispersed in the final nanocomposites, whose microstructural homogeneity was evaluated by atomic force microscopy (AFM) and transmission electron microscopy (TEM) analyses. In order to understand the optical behaviour of di-ureasil containing QDs, a detailed photoluminescent study was undertaken for a selected particle size distribution of ZnS coated CdSe QDs (d∼4.5 nm). Emission quantum yields up to 0.11 were measured in the final nanocomposites that present a huge (between 3 and 6 orders of magnitude) increase in the lifetime of the QDs (relative to that of isolated ones), as a result of energy transfer occurring between the intimately mixed di-ureasil host and the QDs.

Nanoscale memory characterization of virus-templated semiconducting quantum dots

We have developed a substrate-based bottom-up approach to assemble two different color emitting quantum dots (CdSe/ZnS core/shell QDs) on the surface of a novel virus mutant, CPMV-T184C. Electrical characteristics of individual hybrids were investigated by conductive atomic force microscopy for potential digital memory applications (i.e., RAM). These individual 40 nm CPMV-QD(1,2) hybrids exhibited reversible bistable electrical behavior during repeatable writing-reading-erasing processes at the nanoscale.

Plant viruses as biotemplates for materials and their use in nanotechnology

In recent years, plant virus capsids, the protein shells that form the surface of a typical plant virus particle, have emerged as useful biotemplates for material synthesis. All virus capsids are assembled from virus-coded protein subunits. Many plant viruses assemble capsids with precise 3D structures providing nanoscale architectures that are highly homogeneous and can be produced in large quantities. Capsids are amenable to both genetic and chemical modifications allowing new functions to be incorporated into their structure by design. The three capsid surfaces, the interior surface, the exterior surface, or the interface between coat protein subunits, can be independently functionalized to produce multifunctional biotemplates. In this review, we examine the recent advances in using plant virus capsids as biotemplates for nanomaterials and their potential for applications in nanotechnology, especially medicine.

Functionalized carbon nanotubes for detecting viral proteins

We investigated the biocompatibility, specificity, and activity of a ligand-receptor-protein system covalently bound to oxidized single-walled carbon nanotubes (SWNTs) as a model proof-of-concept for employing such SWNTs as biosensors. SWNTs were functionalized under ambient conditions with either the Knob protein domain from adenovirus serotype 12 (Ad 12 Knob) or its human cellular receptor, the CAR protein, via diimide-activated amidation. We confirmed the biological activity of Knob protein immobilized on the nanotube surfaces by using its labeled conjugate antibody and evaluated the activity and specificity of bound CAR on SWNTs, first, in the presence of fluorescently labeled Knob, which interacts specifically with CAR, and second, with a negative control protein, YieF, which is not recognized by biologically active CAR proteins. In addition, current-gate voltage (I-V(g)) measurements on a dozen nanotube devices explored the effect of protein binding on the intrinsic electronic properties of the SWNTs, and also demonstrated the devices' high sensitivity in detecting protein activity. All data showed that both Knob and CAR immobilized on SWNT surfaces fully retained their biological activities, suggesting that SWNT-CAR complexes can serve as biosensors for detecting environmental adenoviruses.

Formation of protein-metal oxide nanostructures by the sonochemical method: observation of nanofibers and nanoneedles

The sonochemical reaction of iron pentacarbonyl is explored in water and in water with the protein BSA (bovine serum albumen). In water, the reaction is found to produce spherical nanoparticles of magnetite (Fe3O4) with a particle size distribution of <10 to approximately 60 nm. In water with BSA, the reaction produces either nanofibers or nanoneedles, depending on the concentration of BSA. The nanofiber and nanoneedle samples are found to be mixtures of goethite, lepidocrocite, and hematite (alpha-FeOOH, gamma-FeOOH, and alpha-Fe2O3, respectively). The sonochemical reaction of iron pentacarbonyl with BSA in water is thought to proceed through the thermal decomposition mechanism for iron pentacarbonyl with BSA acting as a templating agent.

Localized delivery of DNA to the cells by viral collagen-loaded silica colloidal crystals

Low-molecular-weight colloidal crystals with enhanced biocompatibility and ordered porous structure are used in drug-delivery systems. The objective of our study is to demonstrate the use of silica nanoscale colloid particles for localized recombinant DNA release. The colloids were coated with collagen-containing viral vector constructs of lentiviral green fluorescent protein (GFP), and solidified at 37°C. The colloid-collagen-viral vector platform (CCP) was transferred to cell monolayer cultures of human lung fibroblasts. Results show specific infection of cells directly beneath the platform, as evidenced by positive GFP in their cytoplasm, while neighboring cells show no cytoplasmic GFP. The infection of specific cells is probably due to the gradual release of viral particles from the collagen matrix by cell-secreted collagenase, which avoids overdosing the cells with viral particles, resulting from the cytopathic effect often seen with high-titer viral infection. Cells infected with the lentiviral-GFP or lentivirus alone, not incorporated into the colloid-collagen device, show caspase 3-associated apoptotic cell death. This suggests that colloidal crystal-coated collagen may be used as a powerful platform to deliver genes of choice to localized subgroups of specific cells of interest. This specificity in the delivery mode is beneficial for functional studies of gene-directed impact on a particular cell population of interest in a heterogeneous cell culture.

Designer variable repeat length polypeptides as scaffolds for surface immobilization of quantum dots

We demonstrate the use of a series of engineered, variable-length de novo polypeptides to discretely immobilize luminescent semiconductor nanocrystals or quantum dots (QDs) onto functional surfaces. The polypeptides express N-terminal dicysteine and C-terminal hexahistidine residues that flank a variable number (1, 3, 5, 7, 14, 21, 28, or 35) of core beta-strand repeats, with tyrosine, glutamic acid, histidine, and lysine residues located at the turns. Polypeptides have molecular weights ranging from 4 to 83 kDa and retain a rigid structure based on the antiparallel beta-sheet motif. We first use a series of dye-labeled polypeptides to test and characterize their self-assembly onto hydrophilic CdSe-ZnS QDs using fluorescence resonance energy transfer (FRET). Results indicate that peptides maintain their beta-sheet conformation after self-assembly onto the QD surfaces, regardless of their length. We then immobilize biotinylated derivatives of these polypeptides on a NeutrAvidin-functionalized substrate and use them to capture QDs via specific interactions between the peptides' polyhistidine residues and the nanocrystal surface. We found that each of the polypeptides was able to efficiently capture QDs, with a clear correlation between the density of the surface-tethered peptide and the capacity for nanocrystal capture. The versatility of this capture strategy is highlighted by the creation of a variety of one- and two-dimensional polypeptide-QD structures as well as a self-assembled surface-immobilized FRET-based nutrient sensor.

Thermal scalpel to target cancer

Nanotechnology is increasingly applied to the field of medicine, particularly for the treatment of cancer. In this regard, gold nanoparticles can mediate hyperthermia induction and kill tumor cells upon laser irradiation, thereby functioning as a 'thermal scalpel'. Recent developments in gold nanoparticle design have resulted in their absorption of energy in the near-infrared wavelength spectrum, which is best suited to tissue penetration and, thus, clinical application. Furthermore, to ensure accumulation of nanoparticles in neoplastic tissue, targeting ligands are being incorporated into the thermal scalpel schema. Examples of targeting ligands include antibodies and targeted gene therapy vectors. Therapeutic efficacy has been established in cell culture models for several developed thermal scalpel systems and a small number have demonstrated a therapeutic effect in animal models of cancer. Future considerations include analysis of the biodistribution and therapeutic efficacy of thermal scalpels using stringent models of cancer. Furthermore, the immunogenicity and toxicity of thermal scalpels must be established before clinical translation can be achieved.

Interaction between a 54-kilodalton mammalian cell surface protein and cowpea mosaic virus

Cowpea mosaic virus (CPMV), a plant virus that is a member of the picornavirus superfamily, is increasingly being used for nanotechnology applications, including material science, vascular imaging, vaccine development, and targeted drug delivery. For these applications, it is critical to understand the in vivo interactions of CPMV within the mammalian system. Although the bioavailability of CPMV in the mouse has been demonstrated, the specific interactions between CPMV and mammalian cells need to be characterized further. Here we demonstrate that although the host range for replication of CPMV is confined to plants, mammalian cells nevertheless bind and internalize CPMV in significant amounts. This binding is mediated by a conserved 54-kDa protein found on the plasma membranes of both human and murine cell lines. Studies using a deficient cell line, deglycosidases, and glycosylation inhibitors showed that the CPMV binding protein (CPMV-BP) is not glycosylated. A possible 47-kDa isoform of the CPMV-BP was also detected in the organelle and nuclear subcellular fraction prepared from murine fibroblasts. Further characterization of CPMV-BP is important to understand how CPMV is trafficked through the mammalian system and may shed light on how picornaviruses may have evolved between plant and animal hosts.

Utilisation of plant viruses in bionanotechnology

Bionanoscience/technology sits at the interface of chemistry, biology, physics, materials science, engineering and medicine and involves the exploitation of biomaterials, devices or methodologies on the nanoscale. One sub-field of bionanoscience/technology is concerned with the exploitation of biomaterials in the fabrication of new nano-materials and/or -devices. In this Perspective we describe examples of how plant viruses, focusing particularly on cowpea mosaic virus, a naturally occurring pre-formed sphere-like nanoparticle, are being used as templates and/or building blocks in bionanoscience and indicate their potential for future application.

Plant viral capsids as nanobuilding blocks: construction of arrays on solid supports

The virions of Cowpea mosaic virus (CPMV) can be regarded as programmable nanobuilding blocks with a diameter of approximately 28 nm. The particles display a number of features that can be exploited for nanoscale material fabrication. In this study we use the virus-derived building blocks for construction of arrays on solid supports. Biotin-modified CPMV particles are used with Streptavidin as a linker molecule in order to enable self-assembly of arrays from the surface up by a layer-by-layer approach. CPMV particles with different fluorescent labels, which enable differential detection of each layer, have been immobilized on surfaces and arranged in defined layers. This approach provides novel structured arrays which have the potential for development as functional devices at the nanoscale.

Cowpea mosaic virus for material fabrication: addressable carboxylate groups on a programmable nanoscaffold

For the first time, decoration of surface-exposed carboxylate groups on Cowpea mosaic virus particles is reported, thus increasing the number and types of addressable surface groups on this nanoscaffold. First, the addressabilty of carboxylates was demonstrated using a carboxylate-selective fluorescent dye, N-cyclohexyl-N'-(4-(dimethylamino)naphthyl)carbodiimide. Second, it was shown that the virions can be decorated with approximately 180 redox active, methyl(aminopropyl)viologen moieties by coupling to the surface carboxylates. The display of multiple redox centers on the virus particle surface may lead to the development of novel electron-transfer mediators in redox catalysis, to biosensors, and to nanoelectronic devices such as molecular batteries.

Canine parvovirus-like particles, a novel nanomaterial for tumor targeting

Specific targeting of tumor cells is an important goal for the design of nanotherapeutics for the treatment of cancer. Recently, viruses have been explored as nano-containers for specific targeting applications, however these systems typically require modification of the virus surface using chemical or genetic means to achieve tumor-specific delivery. Interestingly, there exists a subset of viruses with natural affinity for receptors on tumor cells that could be exploited for nanotechnology applications. For example, the canine parvovirus (CPV) utilizes transferrin receptors (TfRs) for binding and cell entry into canine as well as human cells. TfRs are over-expressed by a variety of tumor cells and are widely being investigated for tumor-targeted drug delivery. We explored whether the natural tropism of CPV to TfRs could be harnessed for targeting tumor cells. Towards this goal, CPV virus-like particles (VLPs) produced by expression of the CPV-VP2 capsid protein in a baculovirus expression system were examined for attachment of small molecules and delivery to tumor cells. Structural modeling suggested that six lysines per VP2 subunit are presumably addressable for bioconjugation on the CPV capsid exterior. Between 45 and 100 of the possible 360 lysines/particle could be routinely derivatized with dye molecules depending on the conjugation conditions. Dye conjugation also demonstrated that the CPV-VLPs could withstand conditions for chemical modification on lysines. Attachment of fluorescent dyes neither impaired binding to the TfRs nor affected internalization of the 26 nm-sized VLPs into several human tumor cell lines. CPV-VLPs therefore exhibit highly favorable characteristics for development as a novel nanomaterial for tumor targeting.

Nano-oncology: drug delivery, imaging, and sensing

Innovation in the last decade has endowed nanotechnology with an assortment of tools for delivery, imaging, and sensing in cancer research-stealthy nanoparticle vectors circulating in vivo, assembled with exquisite molecular control, capable of selective tumor targeting and potent delivery of therapeutics; intense and photostable quantum dot-based tumor imaging, enabling multicolor detection of cell receptors with a single optical excitation source; arrays of semiconducting nanowire and carbon nanotube sensor elements for selective multiplexed sensing of cancer markers without the need for probe labeling. These rapidly emerging tools are indicative of a burgeoning field ready to expand into medical applications. This review attempts to outline most of the current nanoparticle toolset for therapeutic release by liposomes, dendrimers, smart polymers, and virus-based systems. Advantages of nanoparticle-based imaging and targeting by use of nanoshells and quantum dots are also explored. Finally, emerging nanoelectronics-based sensing and a global discussion on the utility of each nanoparticle system addresses their fundamental advantages and shortcomings in cancer research.

A cowpea mosaic virus nanoscaffold for multiplexed antibody conjugation: application as an immunoassay tracer

Cowpea mosaic virus (CPMV), an icosahedral 30 nm virus, offers a uniquely programmable biological nanoscaffold. This study reports initial optimization of the simultaneous modification of two CPMV mutants with AlexaFluor 647 fluorescent dyes and either IgG proteins or antibodies at specific sites on the virus scaffold. The capacity of CPMV as a simultaneous carrier for different types of molecules was demonstrated, specifically, when applied as a tracer in direct and sandwich immunoassays. The ability to label the virus capsid with antibody and up to 60 fluorescent dyes resulted in an improved limit of detection in SEB sandwich immunoassays, when used as a tracer, relative to a mole equivalent of dye-labeled antibody.