Making sense of polymer-based biosensors

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Abstract
In this issue of PNAS, Chen et al. (1) show feasibility for producing a highly sensitive biosensor, capable of detecting nanomolar quantities of biologically interesting molecules in “real time.” If the phenomenon can be generalized to other systems, it could have significant implications for a wide variety of fields, including toxicology and medical diagnostics. The last two decades have witnessed the emergence of conjugated polymers as an intriguing class of organic macromolecules that have the electrical and optical properties of metals and semiconductors and, in addition, have the processing advantages and mechanical properties of polymers (2). The study of conjugated (semiconducting) polymers has resulted in fundamental insights into the understanding of the chemistry and physics of this novel class of materials (3, 4), and it has stimulated the development of a number of applications (5, 6). Included among these are all-polymer integrated electronic circuits (7), photodetectors and solar cells (8), and flat-panel emissive displays fabricated from polymer light emitting diodes (9). Chen et al. (1) have now demonstrated that the luminescent properties of such polymers can be further exploited to develop a new form of biosensor. Conjugated polymers such as poly(phenylene vinylene) (PPV) and its soluble derivatives are known to exhibit photoluminescence with high quantum efficiency (6). This luminescence can be described in terms of semiconductor band theory. On photoexcitation, an electron is excited from the highest occupied energy band (the π-band) to the lowest unoccupied energy band (the π*-band). The excited electron and the oppositely charged “hole” (the empty state in the π-band) attract one another. When the excited electron recombines with the hole, a photon is emitted (luminescence or fluorescence). The wavelength of the absorbed light is determined by the π-π* energy gap and can be manipulated by altering the molecular structure of the polymer. Conjugated …