Self-Amplifying Semiconducting Polymers for Chemical Sensors
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Self-Amplifying
Semiconducting Polymers for Chemical Sensors
Timothy M. Swager and Jordan H. Wosnick Abstract The ability of excited states (excitons) to migrate rapidly and efficiently through conjugated polymers makes these materials ideal for use in sensors based on fluorescence quenching or amplification of fluorescence signals. The structural features we are able to introduce into these polymers have allowed us both to design highly sensitive fluorescent sensors for specific analytes, such as the explosive trinitrotoluene (TNT), and to create assemblies that control energy transfer along a predetermined path. The principles involved have broad utility in the design of sensory materials as well as of electronic devices and display components based on electronic polymers. Keywords: electroactive organic materials, energy migration, fluorescence, polymers.
Carrier mobility is a critical performance determinant in most electronic materials applications—for example, the mobility of electrons and holes in semiconductor devices controls the amplification afforded by field-effect transistors. Similarly, carrier transport has been used in conducting polymers to create highly sensitive sensors in which molecular interactions are used to trap charge carriers or inject new ones. Gain (amplification) in these systems is optimized by enhancing carrier mobility. By introducing receptor units capable of interrupting charge-carrier transport on binding a target species (analyte), we have demonstrated several polymeric, reversible, and chemoresistive sensors.1,2 However, the amplifying ability of electronic polymers extends beyond the use of conventional charge carriers, and we have further introduced the concept of energy migration (exciton transport) as a mechanism for amplifying a sensory signal.3 Exciton transport has of late been studied in the context of electroluminescence, but it has long been important in the study of molecular crystals.4 When considering exciton migration in organic polymeric semiconductors, one must consider that the disorder inherent in these materials creates energy gradients that the excitons follow 446
to give a reduction in their energy.5 The deliberate introduction of low-energy trapping sites into electroluminescent displays has been used to provide specific colors. As a result of exciton transport, these sites need only be very dilute for their effects to dominate the emission of the material. The same phenomenon can be used in conjugated polymers to amplify a chemical signal if the analyte produces trapping sites. In this article, we discuss how exciton transport can be used to create selfamplifying, ultrasensitive sensory polymers and how exciton mobility can be optimized through molecular design. This technology has broad applicability and can be extended to many different types of processes commonly used in fluorescent sensing. There have been a number of innovative extensions of our amplification methods by other groups, and the interested reader is directed to a recent comprehensive
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