Our group is actively involved in multiple areas of research. Below is a brief description of each area.
Polymer Electronics and Molecular Wires for Chemical Sensors
In the 1990’s our group was the first to demonstrate the amplification possible with molecular wires for chemical sensors. We have designed many sensory systems since then and many other groups throughout the world use these methods/concepts. Chemoresistivity-based sensory systems have been designed wherein molecular structures can be designed to have large resistance changes in response to a chemical of interest (analyte). Arranging these materials into structures that approximate the schematic structure wherein a single molecule spans two electrodes is an ongoing research effort. The definitive study that proved that molecular wires (electronic polymers) amplify was accomplished on isolated molecules in solution. These methods have been extended to detect the high explosive TNT and are the basis of the most sensitive explosives detectors ever produced. These detectors are called Fido™ (Nomadics Inc.) in reference to the fact that like dogs they detect explosives vapors (smells) rather than particles as is required by the much less sensitive conventional ion mobility spectrometers that are used at airports.
We have recently demonstrated a new sensory concept wherein we achieve even greater sensory enhancements by attenuated lasing. Specifically we find that a lasing fluorescent signal is much more sensitive than the spontaneous emission to the presence of quenchers like TNT. This is a general concept and we are extending it to a number of different analytes.
Novel Organization Methods
We have developed a new paradigm for the organization of polymers and small molecules based upon the interest in materials to minimize their free volume. In general materials try to gain as many favorable intermolecular forces as possible by packing closely. However particular molecular scaffolds can prevent efficient packing and if properly designed can be used to create degrees of order not possible by conventional methods. We have shown the broad implications of this method for the organization of conjugated polymers and the modulation of energy transfer within these structures. We have further shown in collaboration with Professor Edwin Thomas (Materials Science and Engineering at MIT) that these methods can give rise to materials with increased strength and ductile properties. Extensions of this work promise to make for materials with unusual mechanical properties.
Conducting Polymer Actuators
Mammalian muscle is an amazing system of systems with organization elements from the molecular scale to the macroscale. We seek to create novel electrically activated polymers that deliver superior performance to muscle in terms of force, speed, and power efficiency. Our approach is to create molecular mechanical mechanisms that translate electrical events (oxidation and reduction) into large dimensional changes. Conducting polymers are natural materials for this process because they can in principle effectively transport the required charge rapidly throughout a material with low resistive losses. The formation of true materials that display the performance that we aspire to will likely require great complexity similar to natural muscle. We are developing nanostructures to limit diffusion times, mixtures of conductors to minimize resistive losses, and matrix materials to give the proper mechanical stiffness and strength. We maintain active collaborations with Professor Ian Hunter (Mechanical Engineering at MIT) and Professor Nicola Marzari (Materials Science and Engineering at MIT) in this area.
Conjugated Polymer Monolayers
We have developed a number of designs for the organization of conjugated polymers at the air-water and solid-water interface. Our methods can be used to create nanofibrils and interesting phase transitions that have revealed the nature of the electronic interactions both intrapolymer and interpolymer. Ongoing studies are directed at creating films and dynamic fluid membranes for biological sensing. Ongoing projects include the detection of proteases, DNA and multivalent protein ligand interactions.
We are engaged in active collaborations directed at optical and NMR imaging. In the case of optical systems we have a collaboration with the Alzheimer's Disease Research Unit at the Massachusetts General Hospital (Professors Brian Bacskai and Bradley Hyman). We are focusing on the development of far red dyes that can be imaged in vivo and selectively bind to and fluoresce when bound to amyloids in the brain ( “In Vivo Optical Imaging of Amyloid Aggregates in Brain: Design of Fluorescent Markers”Angew. Chem. Int. Ed. 2005, 44, 5452-5456). In NMR imaging we are collaborating with Professor Robert Griffin on the synthesis of diradicals that can be used in Dynamic Nuclear Polarization to create population inversions that give locally enhance 13C NMR emissions (“Dynamic Nuclear Polarization with Biradicals” J. Am. Chem. Soc. 2004, 126, 10844-10845)