Interactions of Common Biological Buffers with Iron Oxide Nanoparticles.

Iron oxide nanoparticles (IONPs) have been studied extensively for biomedical applications, which require that they be aqueous-stable at physiological pH. The structures of some of these buffers, however, may also allow for binding to surface iron, thus potentially exchanging with functionally relevant ligands, and altering the desired properties of the nanoparticles. We report here on the interactions of five common biologically relevant buffers (MES, MOPS, phosphate, HEPES, and Tris) with iron oxide nanoparticles through spectroscopic studies. The IONPs in this study are capped with 3,4-dihydroxybenzoic acid (3,4-DHBA) to serve as models for IONP functionalized with catechol ligands. Unlike previous studies, which relied exclusively on dynamic light scattering (DLS) and ζ-potential measurements to characterize buffer interactions with IONPs, we use Fourier transform infrared (FTIR) and ultraviolet-visible (UV-visible) spectroscopic techniques to characterize the IONP surface to demonstrate binding of buffers and etching of the IONP surface. Our findings establish that phosphate and Tris bind to the IONP surface, even in the presence of strongly bound catechol ligands. We further observe significant etching of IONPs in Tris buffer, with the release of surface Fe into solution. Minor etching is noted in HEPES, and to a lesser degree, in MOPS, while no etching is observed in MES. Our findings suggest that, while morpholino buffers, such as MES and MOPS, may be more appropriate for use with IONPs, proper buffer selection should always be considered on a case-by-case basis.

Biomolecular Self-Assembly of Nanorings on a Viral Protein Template.

Although there have been many advances in synthesizing nanoparticles, their assembly into deterministic and controllable patterns remains a major challenge. Biological systems operate at the nanoscale, building structural components with great chemical specificity that enable the processes of life. By adapting them to our needs, it is possible to utilize well-defined and well-controlled scaffolds to produce materials with novel properties resulting from precise ordering on the nanoscale. This approach uses spatial arrangement instead of nanoparticle size, shape, or composition to control material properties through the collective interactions between neighboring nanoparticles. Here, we demonstrate the use of tobacco mosaic virus (TMV) coat protein as a template to self-assemble plasmonic nanoparticles. Surface plasmons are resonant oscillations in the free electrons of a metal that are excited through interaction with light. These plasmonic oscillations can couple together, giving rise to more complex modes like plasmonic ring resonances that can be used to tune the response to incident light. By exploiting the self-assembling properties and chemical addressability of TMV coat protein, we can utilize site-directed mutagenesis and bioconjugation strategies to produce highly symmetrical plasmonic nanorings, as evidenced by transmission electron microscopy (TEM). Thus, we show the utility of viral proteins in designing and assembling nanostructured building blocks for advanced materials.

Alcohol-perturbed self-assembly of the tobacco mosaic virus coat protein.

The self-assembly of the tobacco mosaic virus coat protein is significantly altered in alcohol-water mixtures. Alcohol cosolvents stabilize the disk aggregate and prevent the formation of helical rods at low pH. A high alcohol content favours stacked disk assemblies and large rafts, while a low alcohol concentration favours individual disks and short stacks. These effects appear to be caused by the hydrophobicity of the alcohol additive, with isopropyl alcohol having the strongest effect and methanol the weakest. We discuss several effects that may contribute to preventing the protein-protein interactions between disks that are necessary to form helical rods.

Tunable Assembly of Protein Enables Fabrication of Platinum Nanostructures with Different Catalytic Activity.

Proteins are promising biofunctional units for the construction of nanomaterials (NMs) due to their abundant binding sites, intriguing self-assembly properties, and mild NM synthetic conditions. Tobacco mosaic virus coat protein (TMVCP) is a protein capable of self-assembly into distinct morphologies depending on the solution pH and ionic strength. Herein, we report the use of TMVCP as a building block to organize nanosized platinum into discrete nanorings and isolated nanoparticles by varying the solution pH to modulate the protein assembly state. Compared with a commercial Pt/C catalyst, the TMVCP-templated platinum materials exhibited significant promotion of the catalytic activity and stability toward methanol electrooxidation in both neutral and alkaline conditions. The enhanced catalytic performance is likely facilitated by the protein support. Additionally, Pt nanorings outperformed isolated nanoparticles, although they are both synthesized on TMVCP templates. This could be due to the higher mechanical stability of the protein disk structure and possible cooperative effects between adjacent nanoparticles in the ring with narrow interparticle spacing.

Plasmonic Enhancement of Two-Photon Excitation Fluorescence by Colloidal Assemblies of Very Small AuNPs Templated on M13 Phage.

In this study, an engineered M13 bacteriophage was examined as a biological template to create a well-defined spacing between very small gold nanoparticles (AuNPs 3-13 nm). The effect of the AuNP particle size on the enhancement of the nonlinear process of two-photon excitation fluorescence (2PEF) was investigated. Compared to conventional (one-photon) microscopy techniques, such nonlinear processes are less susceptible to scattering given that the density of background-scattered photons is too low to generate a detectable signal. Besides this, the use of very small AuNPs in 2PEF microscopy becomes more advantageous because individual "isolated" AuNPs of this size do not sufficiently enhance 2PEF to produce a detectable signal, resulting in even less background signal. To investigate the 2PEF of the AuNP-M13 assemblies, a variety of sample preparation approaches are tested, and surface-enhanced Raman spectroscopy (SERS) is employed to study the strength of plasmon coupling within the gaps of AuNPs assembled on the M13 template. Results indicate that assemblies prepared with 9-13 nm AuNP were able to clearly label Escherichia coli cells and produce a 2PEF signal that was orders of magnitude higher than the isolated AuNP (below the threshold of detection). This study thus provides a better understanding of the opportunities and limitations relevant to the use of such small AuNPs within colloidal plasmonic assemblies, for applications in biodetection or as imaging contrast agents.

Correction: Biosynthesized silver nanorings as a highly efficient and selective electrocatalysts for CO2 reduction.

Correction for 'Biosynthesized silver nanorings as a highly efficient and selective electrocatalysts for CO2 reduction' by Yani Pan et al., Nanoscale, 2019, 11, 18595-18603.

Biosynthesized silver nanorings as a highly efficient and selective electrocatalysts for CO2 reduction.

Inspiration from nature has driven the development and applications of greener inorganic nanomaterials prepared using biotemplates in the field of nanoscience. In this study, we report the superiority of using a biosynthesized silver nanoring material for CO formation in CO2 saturated KHCO3. Compared to bulk silver and free silver nanoparticles prepared by pure chemical reduction, this silver nanoring (assembled on tobacco mosaic virus coat protein) exhibits significantly enhanced activity and selectivity for the conversion of CO2 to CO. The highest CO faradaic efficiency reaches 95.0% at an overpotential of 910 mV. Additionally, the CO partial current density is 2.7-fold higher than that of the free silver nanoparticles. We believe that the improved catalytic performance is related to the structuring ligand effect of the protein. The numerous functional groups on the protein may tune the reaction activity by influencing the binding energies of the intermediate species from CO2 reduction or hydrogen evolution.

Efficient One-Step PEG-Silane Passivation of Glass Surfaces for Single-Molecule Fluorescence Studies.

Surface passivation to inhibit nonspecific interactions is a key requirement for in vitro single-molecule fluorescent studies. Although the standard passivation methods involve the covalent attachment of poly(ethylene glycol) (PEG) in two steps preferably over quartz surfaces, this protocol and improvements thereon require extensive labor and chemicals. Herein, we report an efficient one-step surface grafting of PEG-silane that yields enhanced passivation, as evidenced by reduced nonspecific interactions, over the conventional method at a minimal time and reagent cost and on glass surfaces. Our method is rooted in a mechanistic understanding of the silane reaction with the silanol groups on the glass surface. Single-molecule fluorescence studies with fluorescently tagged proteins and DNA on PEG-silane-functionalized glass surfaces validate the enhanced performance of the method. Combined with atomic force microscopy surface characterization, our study further illustrates that few remaining pinhole defects, plausibly from defects on the glass, on PEG-silane glass-coated surfaces account for the minimal background, where typically no more than one molecule is nonspecifically attached in a given diffraction-limited spot on the surface.

TMV Disk Scaffolds for Making sub-30 nm Silver Nanorings.

Nanosized bioscaffolds can be utilized to tackle the challenge of size reduction of metallic rings owing to their miniature features as well as their well-known biomineralization capacity. The tobacco mosaic virus coat protein is used as a command surface to grow and assemble silver nanoparticles into sub-30 nm rings. The versatility of TMV allows the formation of both solid silver rings and rings consisting of discrete silver nanoparticles. The pH-dependent coulombic surface map along with the annular geometry of the protein aggregate allow the generation of rings with or without a central nanoparticle. Our silver rings are believed to be the smallest to date, and they can offer a test material for existing theories on metallic nanorings of this heretofore unreached size scale.

Viral-based nanomaterials for plasmonic and photonic materials and devices.

Over the last decade, viruses have established themselves as a powerful tool in nanotechnology. Their proteinaceous capsids benefit from biocompatibility, chemical addressability, and a variety of sizes and geometries, while their ability to encapsulate, scaffold, and self-assemble enables their use for a wide array of purposes. Moreover, the scaling up of viral-based nanotechnologies is facilitated by high capsid production yield and speed, which is particularly advantageous when compared with slower and costlier lithographic techniques. These features enable the bottom-up fabrication of photonic and plasmonic materials, which relies on the precise arrangement of photoactive material at the nanoscale to control phenomena such as electromagnetic wave propagation and energy transfer. The interdisciplinary approach required for the fabrication of such materials combines techniques from the life sciences and device engineering, thus promoting innovative research. Materials with applications spanning the fields of sensing (biological, chemical, and physical sensors), nanomedicine (cellular imaging, drug delivery, phototherapy), energy transfer and conversion (solar cells, light harvesting, photocatalysis), metamaterials (negative refraction, artificial magnetism, near-field amplification), and nanoparticle synthesis are considered with exclusive emphasis on viral capsids and protein cages. This article is categorized under: Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Sensing of heavy metal ions by intrinsic TMV coat protein fluorescence.

We propose the use of a cysteine mutant of TMV coat protein as a signal transducer for the selective sensing and quantification of the heavy metal ions, Cd2+, Pb2+, Zn2+ and Ni2+ based on intrinsic tryptophan quenching. TMV coat protein is inexpensive, can be mass-produced since it is expressed and extracted from E-coli. It also displays several different functional groups, enabling a wide repertoire of bioconjugation chemistries; thus it can be easily integrated into functional devices. In addition, TMV-ion interactions have been widely reported and utilized for metallization to generate organic-inorganic hybrid composite novel materials. Building on these previous observations, we herein determine, for the first time, the TMV-ion binding constants assuming the static fluorescence quenching model. We also show that by comparing TMV-ion interactions between native and denatured coat protein, we can distinguish between chemically similar heavy metal ions such as cadmium and zinc ions.

Iron Oxide Surface Chemistry: Effect of Chemical Structure on Binding in Benzoic Acid and Catechol Derivatives.

The excellent performance of functionalized iron oxide nanoparticles (IONPs) in nanomaterial and biomedical applications often relies on achieving the attachment of ligands to the iron oxide surface both in sufficient number and with proper orientation. Toward this end, we determine relationships between the ligand chemical structure and surface binding on magnetic IONPs for a series of related benzoic acid and catechol derivatives. Ligand exchange was used to introduce the model ligands, and the resultant nanoparticles were characterized using Fourier transform infrared-attenuated internal reflectance spectroscopy, transmission electron microscopy, and nanoparticle solubility behavior. An in-depth analysis of ligand electronic effects and reaction conditions reveals that the nature of ligand binding does not solely depend on the presence of functional groups known to bind to IONPs. The structure of the resultant ligand-surface complex was primarily influenced by the relative positioning of hydroxyl and carboxylic acid groups within the ligand and whether or not HCl(aq) was added to the ligand-exchange reaction. Overall, this study will help guide future ligand-design and ligand-exchange strategies toward realizing truly custom-built IONPs.

Tunable longitudinal modes in extended silver nanoparticle assemblies.

Nanostructured materials with tunable properties are of great interest for a wide range of applications. The self-assembly of simple nanoparticle building blocks could provide an inexpensive means to achieve this goal. Here, we generate extended anisotropic silver nanoparticle assemblies in solution using controlled amounts of one of three inexpensive, widely available, and environmentally benign short ditopic ligands: cysteamine, dithiothreitol and cysteine in aqueous solution. The self-assembly of our extended structures is enforced by hydrogen bonding. Varying the ligand concentration modulates the extent and density of these unprecedented anisotropic structures. Our results show a correlation between the chain nature of the assembly and the generation of spectral anisotropy. Deuterating the ligand further enhances the anisotropic signal by triggering more compact aggregates and reveals the importance of solvent interactions in assembly size and morphology. Spectral and morphological evolutions of the AgNPs assemblies are followed via UV-visible spectroscopy and transmission electron microscopy (TEM). Spectroscopic measurements are compared to calculations of the absorption spectra of randomly assembled silver chains and aggregates based on the discrete dipole approximation. The models support the experimental findings and reveal the importance of aggregate size and shape as well as particle polarizability in the plasmon coupling between nanoparticles.

Nanoring formation via in situ photoreduction of silver on a virus scaffold.

The fabrication of plasmonic nanorings remains of substantial interest by virtue of their enhanced electric and magnetic response to light fields which can be subsequently exploited in diverse applications. Scaling down the size of nanorings holds promise in creating artificial magnetism at wavelengths matching the solar spectrum. Nanosized bioscaffolds can be utilized to tackle the challenge of size reduction of metallic rings owing to their miniature features as well as their well-known biomineralization capacity. Herein, we use the tobacco mosaic virus coat protein as a command surface to grow and assemble silver nanoparticles into sub-30 nm rings. The versatility of TMV allows the formation of both solid rings and rings consisting of discrete nanoparticles that are characterized by UV-vis and TEM. The pH-dependent coulombic surface map along with the annular geometry of the protein aggregate allow the generation of rings with or without a central nanoparticle. Our silver rings are believed to be the smallest to date, and they can offer a test material for existing theories on metallic nanorings of this heretofore unreached size scale.

One-step ligand exchange and switching from hydrophobic to water-stable hydrophilic superparamagnetic iron oxide nanoparticles by mechanochemical milling.

We describe a simple, rapid methodology for the synthesis of water-stable iron oxide nanoparticles (IONPs) compatible with a variety of aqueous buffers, based on mechanochemical milling exchange of covalently bound surface ligands on pre-fabricated oleic acid-protected IONPs. Application of milling for IONP ligand exchange eliminates steps required for transforming hydrophobic into negatively charged, water-soluble superparamagnetic IONPs.

Tobacco Mosaic Virus capsid protein as targets for the self-assembly of gold nanoparticles.

Bottom-up self-assembly techniques are a powerful method of building nanoscale structures in an energy efficient and cost effective manner. The use of biological templates, such as proteins, takes advantage of the monodispersity and precision of naturally evolved systems to produce highly organized assemblies of small molecules and nanoparticles. Here we describe a method whereby arginine residues on a viral coat protein (Tobacco Mosaic Virus) are targeted by bis(p-sulfonatophenyl)phenylphosphine (BSPP)-passivated gold nanoparticles with high specificity to create 22 nm rings.

Molecular sensing: modulating molecular conduction through intermolecular interactions.

We observe changes in the molecular conductivity of individual oligophenylene-vinylene (OPV) molecules due to interactions with small aromatic molecules. Fluorescence experiments were correlated with scanning tunneling microscopy measurements in order to determine the origin of the observed effect. Both nitrobenzene and 1,4-dinitrobenzene decreased fluorescence intensity and molecular conductivity, while toluene had no effect. The observed changes in the fluorescence and conduction of OPV correlate well with the electron withdrawing ability of the interacting aromatic molecules. These results demonstrate the potential usefulness of OPV as a sensor for aromatic compounds containing electron withdrawing groups.

Conductance switching in the photoswitchable protein Dronpa.

Dronpa, a photoswitchable GFP-like protein, was self-assembled onto gold substrates, and its conductance was measured using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS).

Solution phase gold nanorings on a viral protein template.

Current studies on materials that exhibit metamaterial properties are mainly focused on lithography-generated 2D substrates. Here we report the successful fabrication of 22 nm gold nanoparticle rings with and without a central nanoparticle assembled on Tobacco Mosaic Virus coat protein disks. These structures are one of the first examples of nanorings produced independently of a substrate and represent the first steps toward the realization of a solution-phase or coatings-based metamaterial.

Role of hexahistidine in directed nanoassemblies of tobacco mosaic virus coat protein.

A common challenge in nanotechnology is the fabrication of materials with well-defined nanoscale structure and properties. Here we report that a genetically engineered tobacco mosaic virus (TMV) coat protein (CP), to which a hexahistidine (His) tag was incorporated, can self-assemble into disks, hexagonally packed arrays of disks, stacked disks, helical rods, fibers, and elongated rafts. The insertion of a His tag to the C-terminus of TMV-CP was shown to significantly affect the self-assembly in comparison to the wild type, WT-TMV-CP. Furthermore, the His tag interactions attributed to the alternative self-assembly of His-TMV-CP can be controlled through ethanol and nickel-nitrilotriacetic acid (Ni-NTA) additions as monitored with atomic force microscopy.