SPR Links
Research Interest:
Design and fabrication of novel sensors and sensor arrays for biological molecules and agents, in particular bacterial protein toxins, peptides, and microbes. The multidisciplinary research carried out in our group encompasses aspects of molecular recognition, supramolecular assembly, charge transfer, optical and impedance spectroscopy, voltammetry, and microfluidics. Major areas of interest include biosensor/sensor arrays based on self-assembled monolayer or supported lipid bilayers which mimic cell surface interactions, functional materials for analytical purposes synthesized by using principles of supramolecular chemistry, and surface plasmon resonance (SPR) spectroscopy for label-free detection of protein toxins.
Membrane-Mimicking Biosensors. Biosensors are molecule-based devices that transduce a biochemical process or binding event into a measurable signal. We are interested in making sensors or sensor arrays that mimic Nature's own biosensing mechanisms, which are widely observed in cells. Current effort has focused on detection of bacterial protein toxins that can bind to cell surface receptors and form channels or pores in the membrane. Many toxins are major determinants of bacterial virulence, and thus responsible for toxic activity and infection even without infestation by the microbes. Therefore, toxin sensors can offer effective tools for use in clinical diagnostics, food safety monitoring, epidemic control, and even counter-terrorism campaign. We study a variety of bacterial protein toxins and their interactions with receptors on bilayer membrane of vesicles. A recent project used lipid vesicles to encapsulate redox species to form a surface bound sensing interface through self-organizing process. The target protein toxin, streptolysin O (SLO) from Streptococcus pyogenes, interacts with membrane and form transmembrane pores of nanometer size, leading to release of redox molecules that enable amperometric signaling. We also investigate the use of PDMS microfluidic devices to build miniaturized chip sensors for multichannel analysis. The surface hydrophilicity of PDMS, which is critical to protein-based applications, has been optimized under aggressive study.
Additional interests include quartz crystal microbalance sensing technique and spatially addressable micropatterned toxin sensors fabricated with supported bilayer membranes (SBMs). SBMs are formed by attaching a single lipid bilayer to a solid substrate either by physical interactions or by chemical bonds. They offer excellent film stability while a great degree of membrane fluidity can still be retained. Typical methods of fabrication involve fusion of bilayer vesicles, microcontact printing, or the Langmuir-Blodgett technique. We are exploring the building of SBMs with both the natural and synthetic lipids capable of inner crosslinking that further strengthens the membrane's robustness.
Functional Materials for Analytical Purposes. Lack of appropriate optical and electronic materials has been a severe bottleneck for development of new sensing technology. The functional materials project is aimed at design and test new materials of analytical significance. The current emphasis is on conjugated polymers with desirable optical properties and unusual morphological structures. Two systems under study are polydiacetylenes (PDAs) and polythiophenes (PTs). PDAs are conjugated polymer materials with interesting optical properties. Formation of PDA supramolecular aggregates is an entropically driven, self-assembling process. The polymerization of diacetylene aggregates can be conveniently achieved by UV irradiation where the morphology of the aggregates remains intact. PDAs have been known to undergo color transitions from blue to red in response to thermal and mechanical effects. This property has earned PDA an attractive platform in development of colorimetric biosensors for pathogenic agents that allow for direct detection in a "litmus test" fashion.
In addition to PDA colorimetric sensors, we also explore new methodologies for fabricating nanofiber and other microstructured materials using PDA aggregates. It was demonstrated previously that under mild conditions, diacetylene lipids could self-assemble to form microstructures of varied morphologies including tubules, helices, ribbons, braided fibers and flat sheets. These materials can have potential applications as field emitting cathodes and in controlled drug release. For bolaamphiphilic diacetylene lipids which are terminated with carboxylic group on one end and glutamic acid on the other, nanofibers can be obtained at mild conditions through electrostatic repulsive forces that overcome the van der Waals force to fray the ribbon aggregates into fibrous structures. Analytical implications of lipid-based fibers in chiral separation, sensing and redox conductivity are being examined.
Surface Plasmon Resonance Spectroscopy. SPR is a surface-sensitive analytical technique that measures small changes in the refractive index of molecular layer adjacent to a metal film. A surface plasmon can be regarded as a bound evanescent wave propagating along at the metal-dielectric interface. The electric field decays exponentially normal to the plane, providing a high surface sensitivity to changes in its vicinity. SPR spectroscopy has become widely used in the fields of chemistry and biochemistry. Sensitive and capable of real time measurement, SPR is viewed as one of the foremost sensor types for direct, label-free observation of biomolecular interactions.
Our lab has two operating SPR units, a commercial unit (Biosuplar) and a home-built setup. The home-built SPR device has dual capability: spectroscopic analysis and imaging. Surface plasmon imaging/microscopy is performed at a fixed angle with a variable area under investigation. The resulting 2-D image is captured with a cooled CCD camera, which allows imaging over the entire visible and near infrared spectrum. The system allows measurements with a temporal resolution of ca. 1 tenth of a second possible. The open flow cell architecture allows investigations into the influence of magnetic, electric or thermal effects on the studied systems.
We are currently investigating the interactions of protein toxins with their natural receptor on various sensing interfaces using SPR, especially the pore-forming functionality on artificial membranes. New chip sensors based on SBMs are being investigated as well. Additional interest in SPR spectroscopy includes development of new analytical methodologies by combining SPR with other instrumental methods. These include, certainly are not limited to, electrochemical-SPR (E·SPR), impedance-SPR (I·SPR) and SPR-microfluidics.
