We investigate phenomena in the length scale of tens to thousands of angstroms, with a goal of gaining insight into the fundamental equation of the transition from molecular to macroscopic. At these length scales, the number of surface and bulk molecules are comparable, and interesting changes in such systems, such as growth and loss, occur via interaction with another phase through an interface. Hence, central to the understanding of such phenomena is the study of interfaces. Thus, there are two main components to our studies: small structures and interfaces.
We use atomic force microscopy for the elucidation of complex structures at the nanometer scale. Two projects underway concern the mechanism of assembly of collagen fibres, and of paired helical filaments (PHF), which are an obligate feature of Alzheimer's disease. Our work on PHF in the past year has called into question the accepted structure of these filaments.
With regard to direct studies of interfaces, we utilize the surface sensitivity of second order nonlinear effects, (second harmonic and sum frequency generation), which are symmetry forbidden in isotropic bulk media. We are concerned with the relationship between structure and dynamics at the liquid/air, liquid/liquid, and liquid/solid interfaces. Current projects include studies of water near charged interfaces, and cooperative and competitive absorption at model membranes.
Since interfaces and finite systems are ubiquitous, our approach is interdisciplinary: we have examined biological systems, polymers, colloids and minerals, and used techniques of physics, chemistry, and biology as needed.
Diffraction sensing is explored as a possible means to improve upon the limit of detection and detection time in measuring antibiotic susceptibility. The goal of this research area is twofold: 1) to decode the unique light-cell diffractive interactions in the search for new, more powerful biosensor technologies and 2) to develop new tools to better understand bacterial-surface interactions and the complex thermodynamics that govern them. Up until now, work has mainly focused on immobilizing bacterial cells in diffractive patterns with the help of electrostatics, micro contact printing, and micro molding. These patterns produce a detectable diffraction pattern when illuminated with a laser, and diffraction intensity changes are monitored as cells grow or die in the presence of antibiotics. To further understand bacterial-surface preferences and optimize cell capture, bulk and surface cell concentrations are compared using adsorption isotherms to estimate the free energy of adsorption on various surfaces. Experiments are currently focusing on studying these interactions on controllable, charged polyelectrolyte multilayer surfaces in micro-fluidic channels.
Researcher: Nicholas K. Kotoulas
Tissue stiffening has been recently associated with the accumulation of sugar-derived cross-links in collagen constructs. In a process known as glycation, reducing sugars such as D-glucose or D-ribose react with amino functional groups of select amino acids along the collagen monomer backbone, commonly lysine and arginine residues. These reactions eventually produce stable and highly irreversible species called advanced glycation end-products (AGEs), some of which include effective cross-linkers such as pentosidine and glucosepane. In this study, we developed an approach by which we were able to extract measurable changes in the stiffness of native-type collagen fibrils with prolonged exposure to sugar. The atomic force microscope (AFM) was used to measure the Young’s modulus through the nanoindentation method.
Researcher: Nicholas K. Kotoulas
Our work focuses on the applications of visible light photocatalytic materials in waste water treatment applications. To date, research has been focused on TiO2-graphene composites and BiOX materials to tackle anthropogenic sources of pollution in our water. The bulk of this work involves the synthesis and characterization of photoactive nano materials. New techniques have been developed using Q-NMR to more accurately understand the complex interactions between photocatalytic materials and the degradation process. Future work will continue to study and optimize material properties and begin applying the materials to solve environmental issues including waste water treatment, air purification and antibacterial self cleaning.
Researchers: Reece Lawrence & Mark Croxall
Photogenerated charge separation is critical for photocatalytic applications of graphitic carbon nitride (g-C3N4). Graphene carbon dots deposited on g-C3N4 are known to promote photogenerated charge separation at the interface by building heterojunctions. The research involves the fabrication a novel lateral composite by growing g-C3N4 around nitrogen-doped graphene quantum dots (NGQD) through carbon-nitrogen bonds on a tertiary nitrogen. The electronic coupling between NGQDs and g-C3N4 generates new bands localized at the interface to form an electronic gradient driving charges moving laterally along the π-p-π network and extending the photosensitive region. This results in improved light harvesting for photocatalytic applications. Our work focuses on using this novel carbon nanomaterial for multiple photocatalytic applications, such as: the degradation of harmful NOx gases and the generation of H2 gas as a renewable energy source.
Researcher: Kevin Yu
Carbon nitride is a promising material for photocatalysis due to its ability to absorb in the visible region and its cheap producibility. Unfortunately, it is disadvantaged by its high recombination rates which limits the amount of free carriers it can produce. This research focuses on lowering the recombination rate of carbon nitride through doping the material with transition metals, which could potentially induce a polarization effect that increases the separation of the material electron-hole pairs. A potentiostat has been designed and built to measure the resulting photocurrent response. We plan on further modifying our instrument to characterize our materials through cyclic voltammetry, electrochemical impedance spectroscopy, and chronoamperometry.
Researcher: Fa-Yuan (Lawrence) Wang
Natural, biodegradable polyelectrolytes, such as the glycosaminoglycan hyaluronic acid, are currently studied to determine their suitability as polymeric nanoparticle active ingredient delivery systems. The phenomena of counter-ion condensation describes exposure to high ionic strength conditions masks the charges distributed along the polymer’s structure, reducing interchain repulsion and changing conformation from linear to collapsed. By optimizing conditions of polymer collapse, the chemical environment of the interior of the polymer nanoparticle may be described, and encapsulation of suitable active ingredients follows. Nanoparticle structure is maintained by irreversible chemical cross-linking utilizing carbodiimide chemistry. Characterization includes kinematic viscosity, dynamic light scattering, zeta potential, atomic force microscopy, FTIR and absorbance spectroscopies.
Researcher: Charlotte Wallace
Several nanoparticle materials can be synthesized using the polymer collapse method invented in our group. Research in this area has spurred a number of projects and a successful company Vive Crop Protection. To date we have synthesized metallic nanoparticles (Ag, Au, etc), quantum dots (CdSe, CdTe, etc) transition metal oxides (TiO2, WO3) and lanthanum oxides. Current work is continuing to better understand factors involved in the collapse and expand the 'menu' of particles which can be synthesized, starting with copper alloys.
Researcher: Mark Croxall
Light Up the World
In celebration of UNESCO’s declaration of 2015 as the International Year of Light, the Impact Centre set out on a challenge to light up the world. This challenge was sparked by Professor Cynthia Goh who grew up in a remote part of the Philippines without electricity. Many such communities in developing countries do not have access to the electric grid. This means that after sundown, the light goes out. To tackle this ‘light poverty’, Prof. Goh mobilized her team at the Impact Centre, and the rest of U of T to design and create a practical, cost-effective lighting system for low-resource settings to help ensure access to a reliable overhead light source. For more information, click here.