Current research in the Blum Group
Self-assembly on a Tobacco mosaic virus coat protein template
While current nanoscale production is dominated by top-down methods such as ion, e-beam, or photo- lithography, the significant increase in cost involved as feature sizes decrease below 50 nm makes alternative techniques more attractive. Furthermore, it has beome clear that producing nano-based materials with properties not found in nature requires breakthroughs in the ability to position materials with nanometer precision. Overcoming these obstacles has led to a growing interest in bottom-up, self-assembling systems. One approach is to use biomolecules as scaffolds because of the specificity and versatility they provide. In particular, the use of viruses as nanoscale scaffolds offers the promise of exquisite control for positioning on the nanoscale, using a particle that can undergo further self-assembly into extended structures, and allowing the simultaneous creation of many identical complex submicron geometrical structures.
As platforms for templated self-assembly, plant viruses have the advantage of having no pathogenicity towards humans and can often be utilized without their infectious genome. This is because most of the chemical functionality that assembles the capsid is incorporated into the proteins themselves. As a result, they can often assemble independently or around a simulant of the genetic material. These Virus-Like Particles (VLPs) assemble into a variety of geometries and can also respond to chemical and physical changes in their environment such as pH and temperature. In the Blum Group, we use Tobacco mosaic virus coat protein as a scaffold for self-assembly.
Controlling the surface chemistry of superparamagnetic iron oxide nanoparticles
Aqueous stable superparamagnetic iron oxide nanoparticles (IONPs) are a key component in many current technologies, such as MRI contrast agents, magnetic separations, drug targeting, and hybrid inorganic-organic nanomaterials. As IONP properties depend on particle size, stability, and ligand shell identity, these parameters must be strictly controlled for their successful use as functional materials. Ultrasmall (<15 nm) nanoparticles are required to facilitate downstream nanomaterials applications (that require templation on a small size regime) or to maintain desired superparamagnetic properties. As MRI contrast agents, smaller IONPs possess a longer half-life in the bloodstream. Particles with a diameter <10 nm can penetrate the endothelium and show increased bioavailabilty. Synthesis of ultrasmall IONPs that are stable in aqueous solution is critical for bio-medical applications.
This project works to develop a systematic understanding of the factors controlling surface structure, aqueous solubility, stability and degradation pathways of IONPs, which will have an immediate impact on biomedical imaging. IONPs are composed of an inorganic iron oxide (typically Fe3O4) core protected by surface capping ligands and, due to their unique combination of magnetic properties, low biological toxicity and low cost, have been recognized as ideal components for advanced technologies including magnetic resonance imaging (MRI) contrast agents, magnetic separations, and drug targeting. Unfortunately, the inability to readily and reliably generate IONPs with different capping ligands using conventional, solution-based techniques has severely hindered the development of IONP technologies and has largely prevented the development of a systematic understanding of how the key properties of IONPs, particularly surface structure and stability, can be modified and optimized by the choice and structure of capping ligands.
Catechols are amongst the most widely employed capping groups for IONPs, owing to their high-affinity for Fe3+ on the nanoparticle surface. Surface functionalization with catechols is known to enable the stabilization of IONPs, modification of their aqueous solubility, as well as the modification of their magnetic properties. Thus, catecholate functionalized nanoparticles exhibit saturation magnetization approaching that of bulk magnetite (~80 emu/g for catechol/IONP vs 92 emu/g for magnetite), which is significantly improved from the corresponding carboxylate coated particles. Due to their high affinity for IONP surfaces, several catechols have been employed as anchoring groups with derivatized tails intended for biomedical applications, with particular emphasis on MRI contrast agents. Despite the widespread use of catechols as IONP anchoring groups, there is an alarming lack of data surrounding structure-activity relationships between the catechol capping ligand and IONP surface structure and stability.
In the Blum group, we have developed a general, low temperature, and simple protocol for exchanging the oleic acid ligands used in nanoparticle synthesis to generate monodisperse particles with catechol-based ligands containing a variety of functionalities. We explore new surface chemistries, and seek to understand the principles which govern surface interactions between iron oxide and organic ligands to control nanoparticle properties.
Tuning the optical properties of silver nanoparticles
There is a great deal of interest in the controlled synthesis of noble metal nanoparticles that can be largely attributed to their versatile applications in various fields including electronics, catalysis, optics, biological labeling and surface-enhanced Raman spectroscopy (SERS). Nobel metal nanoparticles can support localized surface plasmon resonances (LSPRs) - coherent oscillations of conduction band electrons in response to specific wavelengths of light - which can result in an extraordinary enhancement of local electromagnetic fields in the junction between two adjacent metal particles. These junctions or “hot spots,” have a focusing effect on electromagnetic field intensity. Although silver nanoparticles are of great interest due to their properties as SERS enhancers and due to their plasmon resonance in the blue wavelengths, not much is known about reliably controlling their assembly into extended structures to tune their optical properties. The controlled organization of plasmonic nanoparticles into highly anisotropic 1D arrays and networks offers a platform to attune the flux of surface plasmons. Extended planar nanoparticle assemblies are capable of subwavelength optical guiding, which can result in the miniaturization of integrated optical, photonic and biosensor devices.
This project is based on our new green and facile aqueous synthesis of silver nanoparticles by chemical reduction and their stabilization by four short molecules: glycine, cysteine, cysteamine, and dithiothreitol (DTT). We can generate extensive 1D self-assembled superstructures of metallic silver nanoparticles by controlling the degree of nanoparticle surface passivation and polarization by either of the three ditopic ligands: cysteine, cysteamine and dithiothrietol. UV-Vis reveals the characteristic surface plasmon band of silver colloids (SP) centered at 390 nm as well as a low-energy longitudinal plasmon band (LP) that is suggested to arise from uniaxial coupling of the isotropic surface plasmons. The plasmon coupling is reinforced by the hydrogen-bonding offered by the end moieties of the ligands chosen. The spontaneous formation of the chains is due to the alignment of dipoles within short inter-dipole distances to minimize the enthalpy cost as well as the disorder provided by the branched domains. Transmission Electron Microscopy (TEM) is also used to unveil the morphologies of the superstructures that self-assembled in the form of discrete chains, branched and looped chains as well as interconnected chain networks. The stability of the assembled structures colloids seems to depend on the degree of aggregation as revealed by TEM and LP: SP magnitude ratio. Simulations of the extinction spectra of random chains of silver nanoparticles suspended in water were performed using discrete dipole approximation (DDA) and Comsol methods. Simulation data agreed with the experimentally observed persistence of the transverse plasmon band and the appearance of a longitudinal band whose position and broadness are influenced by the number of chain particles, the polarizability of the particles and the inter-particle distance.
Molecular electronics based nanosensors
Molecular electronics has attracted great interest since Aviram and Ratner’s innovative paper launching the field in 1974. The concept is appealing for many reasons, such as the chance to create transistors with molecular dimensions and the ability to synthesize many identical functional molecules on a large scale. In using molecules as devices, it becomes possible to tailor molecular properties and therefore device properties through synthetic approaches leading to the development of new functions for molecular circuits. This type of molecular engineering holds great promise in the area of chemical sensing, where specific interactions can be engineered into conductive molecules, as it is known that the conductivity of a molecular wire is highly sensitive to its local environment.
The Blum group has demonstrated that the conductivity of oligophenylene-vinylene (OPV) molecules decreases when exposed to aromatic molecules containing electron withdrawing groups. It is believed that since these same molecules also quench the fluorescence intensity of OPV, that the decrease in conductance is through π donor-acceptor interactions. Furthermore, both fluorescence and conductance experiments demonstrated that the greater the degree of substitution, the greater the effect on the fluorescence intensity and conductivity, as expected given the greater number of electron-withdrawing groups. Thus, we have shown that molecular conductors can act as potential sensors for electron-accepting compounds.
However, controlling inter-particle spacing remains an important issue that needs to be addressed is molecule-based sensors are to be practical. Metallic nanoparticle networks are attracting a great deal of interest for this purpose, since their properties are widely studied. Biological scaffolds have been one proposed solution as they offer the ability to work under mild, aqueous conditions. Peptides are one such example and have been shown to participate in the synthesis, binding and self-assembly of inorganic materials. Their functionality can be further enhanced by fusing two different domains together, creating a fusion peptide that is multifunctional in nature. Viral scaffolds are another useful template for self-assembly due to their organized structure and high degree of monodispersity with the tobacco mosaic virus (TMV) being one well-studied example. This project combines affinity peptides with virus templated assemblies to generate nanoparticle networks that can be interlinked with conducting molecules.
Current efforts take advantage of fusion peptides and viruses to form large scale gold nanoparticle assemblies for binding to silicon dioxide surfaces for potential applications in molecular electronics. Peptide-stabilized gold nanoparticles have previously been covalently bound to TMV disks. Due to the peptide stabilizing the gold nanoparticle these rings could potentially form aggregates in solution via divalent metal ion interactions. These aggregates can then be tethered to silicon dioxide substrates through another fusion peptide and readily characterized with AFM. Ultimately, the conductance of such nanoparticle measurements can be measured to demonstrate the usefulness of such self-assembled networks in molecular electronics based sensors.