Session X39: Focus Session: Materials and Functional Structures for Biological Interfaces - Nanoscale Materials

Designing super-selectivity in multivalent nano-particle binding

A key challenge in nano-science is to design ligand-coated nano-particles that can bind selectively to surfaces that display the cognate receptors above a threshold (surface) concentration. Nano-particles that bind monovalently to a target surface do not discriminate sharply between surfaces with high and low receptor coverage. In contrast, ``multivalent'' nano-particles that can bind to a larger number of ligands simultaneously, display regimes of ``super-selectivity'' where the fraction of bound particles varies sharply with the receptor concentration. We present numerical simulations that show that multivalent nano-particles can be designed such that they approach the ``on-off'' binding behavior ideal for receptor-concentration selective targeting. We propose a simple analytical model that accounts for the super-selective behavior of multi-valent nano-particles. We propose a simple rule of thumb to predict the conditions under which super-selectivity can be achieved. We validate our model predictions against the Monte Carlo simulations. Finally, we investigate the role of multi-component ligand-receptor interactions in the enhancement of targeting selectivity.   

Nano-Engineered Cubic Zirconia for Orthopaedic Implant Applications

Osseointegration failure of the prosthesis prevents long-term stability, which contributes to pain, implant loosening, and infection that usually necessitates revision surgery. Cell attachment and spreading in vitro is generally mediated by adhesive proteins such as fibronectin and vitronectin. We designed and produced pure cubic zirconia (ZrO2) ceramic coatings by ion beam assisted deposition (IBAD) with nanostructures comparable to the size of proteins. Our ceramic coatings exhibit high hardness and a zero contact angle with serum. In contrast to Hydroxyapatite (HA), nano-engineered zirconia films possess excellent adhesion to all orthopaedic materials. Adhesion and proliferation experiments were performed with a bona fide mesenchymal stromal cells cell line (OMA-AD). Our experimental results indicated that nano-engineered cubic zirconia is superior in supporting growth, adhesion, and proliferation. We performed a comparative analysis of adsorption energies of the FN fragment using quantum mechanical calculations and Monte Carlo simulation on both types of surfaces: smooth and nanostructured. We have found that the initial FN fragment adsorbs significantly stronger on the nanostructured surface than on the smooth surface.   

Computational characterization of DNA/peptide/nanotube self assembly for bioenergy applications

Multi-enzyme pathways have become a subject of increasing interest for their role in the engineering of biomimetic systems for applications including biosensors, bioelectronics, and bioenergy. The efficiencies found in natural metabolic pathways partially arise from biomolecular self-assembly of the component enzymes in an effort to avoid transport limitations. The ultimate goal of this effort is to design and build biofuel cells with efficiencies similar to those of native systems by introducing biomimetic structures that immobilize multiple enzymes in specific orientations on a bioelectrode. To achieve site-specific immobilization, the specificity of DNA-binding domains is exploited with an approach that allows any redox enzyme to be modified to site-specifically bind to double stranded (ds) DNA while retaining activity. Because of its many desirable properties, the bioelectrode of choice is single-wall carbon nanotubes (SWNTs), but little is known about dsDNA/SWNT assembly and how this might affect the activity of the DNA-binding domains. Here we evaluate the feasibility of the proposed assembly by performing atomistic molecular dynamics simulations to look at the stability and conformations adopted by dsDNA when bound to a SWNT. We also evaluate the effects of the presence of a SWNT on the stability of the complex formed by a DNA-binding domain and DNA.   

Engineering upconverting nanophosphors as biosensors and biotherapeutic agents

Contrast agents play an important role in the study of biological tissues and whole organisms, since they enable visualization of functional structures. Fluorescent contrast agents also enable specific targeting in therapeutic approaches. Developing optimized contrast agents is central to optimizing the performance of both imaging and therapy. Upconversion nanophosphors (UCNPs) are a class of nanoparticles which enable efficient 2-photon fluorescence. Their intrinsic properties of low toxicity, low excitation intensity, narrow fluorescence line width, multiplexing capability, zero fluorescent background, and absence of bleaching and blinking make them strong candidates as a contrast agent with wide applicability. I am working to develop these nanomaterials with biological significance as contrast agents in biolabels , in biosensors and as therapeutic agents in photodynamic therapy. This will be combined with the enhancement of brightness of the UCNPs through coupling to resonant gold nanofeatures. The study is necessary in order to bring about a break through in upconversion luminescence enhancement, particularly at ever decreasing nanoparticle sizes necessary for biological applications.   

Optically Trappable Single Wall Carbon Nanotubes

We are developing a single walled carbon nanotube (NT) force probe which would utilize an optical trap to manipulate NTs fitted with micron scale dielectric handles. With its nanometer diameter and micron length a NT harnessed in this way may eventually allow us to probe a cell's membrane and interior. In pursuit of this goal we have developed a method to create parallel arrays of aligned NT cantilevers. In this process we transfer highly aligned NTs to a substrate composed of alternating regions of Si separated by SiO$_{2}$. Patterning the NTs and etching away the oxide leaves behind ridges supporting arrays of NT cantilevers 0.7nm-2nm in diameter and up to 700nm in length with densities of over one cantilever per micron. We will discuss our work modifying this technique to pattern and release micron scale NT cantilever probes into solution. We have designed and optically tested lithographically patterned SiO$_{2}$ and SU-8 handles, shaped such that they can be manipulated with an optical trap in predictable orientations. We will focus on our current efforts in attaching these optically trappable dielectric handles to individual nanotubes so they can be implemented to make direct force measurements.   

Self-assembled peptide nanowires on single-layer graphene and MoS$_{2}$ with biomolecular doping effect

Developing elegant hybrid systems of biological molecules on atomic single layers (ASLs), such as graphene, is a key in creating novel bio-nanoelectronic devices, where versatile biological functions are integrated with electronics of ASLs. Biomolecules self-assembling into ordered structures on ASLs offer a novel bottom-up approach, where organized supramolecular architectures spatially govern the ASL electronics. Despite the potential in bridging nano- and bio-worlds at the molecular scale, no work has yet realized a way to control electronic and optical properties of ASLs by the biomolecular structures. Here, we demonstrate that engineered dodecapeptides self-assemble into supramolecular networks of peptide nanowires on single-layer graphene and MoS$_{2}$. Peptide nanowires introduce electric charge into these ASLs \textit{via} biomolecular doping. Abrupt boundaries of nanowires create electronic junctions in graphene, which manifest themselves within the single-layer as a self-assembled electronic network. Furthermore, the designed peptides modify both conductivity and photoluminescence of single-layer MoS$_{2}$. Controlling nano-electronics through engineered peptides potentially opens up new avenues in self-assembly of nanodevices for future bioelectronics.   

{\it {Ab initio}} Calculations of Electronic Fingerprints of DNA bases on Graphene

We have carried out first principles DFT calculations of the electronic local density of states (LDOS) of DNA nucleotide bases (A,C,G,T) adsorbed on graphene using LDA with ultra-soft pseudo-potentials. We have also calculated the longitudinal transmission currents $T(E)$ through graphene nano-pores as an individual DNA base passes through it, using a non-equilibrium Green's function (NEGF) formalism. We observe several dominant base-dependent features in the LDOS and $T(E)$ in an energy range within a few eV of the Fermi level. These features can serve as electronic fingerprints for the identification of individual bases from $dI/dV$ measurements in scanning tunneling spectroscopy (STS) and nano-pore experiments. Thus these electronic signatures can provide an alternative approach to DNA sequencing.   

Controlled Chemical Patterns with ThermoChemical NanoLithography (TCNL)

Many research areas, both fundamental and applied, rely upon the ability to organize non-trivial assemblies of molecules on surfaces. In this work, we introduce a significant extension of ThermoChemical NanoLithography (TCNL), a high throughput chemical patterning technique that uses temperature-driven chemical reactions localized near the tip of a thermal cantilever. By combining a chemical kinetics based model with experiments, we have developed a protocol for varying the concentration of surface bound molecules. The result is an unprecedented ability to fabricate extremely complex patterns comprised of varying chemical concentrations, as demonstrated by sinusoidal patterns of amine groups with varying pitches ($\sim $5-15 $\mu $m) and the replication of Leonardo da Vinci's \textit{Mona Lisa} with dimensions of $\sim $30 x 40 $\mu$m$^2$. Programmed control of the chemical reaction rate should have widespread applications for a technique which has already been shown to nanopattern various substrates including graphene nanowires, piezoelectric crystals, and optoelectronic materials.   

Interface-Limited Spherulitic Growth of Hydroxyapatite/Chondroitin Sulfate Composite Enamel-like Films

Understanding and mimicking the growth of hard tissues such as tooth enamel may lead to innovative approaches toward engineering novel functional materials and providing new therapeutics. Up to now, in vitro growth of enamel-like materials is still a great challenge, and the microscopic formation mechanisms are far from well understood. Here we report synthesis of large-scale hydroxyapatite (HAP) and chondroitin sulfate (ChS) composite films by an efficient solution-air interface growth method. The products have the characteristic hierarchical prism structures of enamel and the mechanical properties comparable to dentin. We demonstrate that the films are assembled by spherulites nucleated at the solution surface. The growth of the spherulites is limited by the interfaces between them as well as between the solution and air, leading to the ordered prism structure. The results are beneficial for a clearer understanding of the fundamentals of tooth enamel formation.   

Biomimetic Calcium Phosphate Crystallization: Synchrotron X-ray Studies

The nucleation and growth of calcium phosphate by organic templates attract great attention due to its relevance to bone biomineralization. In spite of the vast studies in the field, the role of the organic templates in the process is still not well understood. One reason for this drawback is the lack of experimental tools to probe the organic template structure during the process. We studied the nucleation and growth of calcium phosphate under floating Langmuir monolayers, at the air/water interface, using two complementary X-ray scattering methods. We show that Grazing Incidence X-ray Diffraction (GID) and Grazing Incidence X-ray off-Specular Scattering (GIXOS) can reveal the organic-inorganic interface properties \textit{in situ}. By using GID and GIXOS together, we can simultaneously determine the lateral interface structure and the electron density profile normal to the interface. Combined with \textit{ex situ} methods, these techniques can improve our understanding of the role of the organic template during biomineralization.