An Ion-Activated Molecular Electronic Device R. B. Dabke, † G. D. Singh, ‡ A. Dhanabalan, † R. Lal, ‡ and A. Q. Contractor* ,† Departments of Chemistry and Electrical Engineering, Indian Institute of Technology, Powai, Bombay 400 076, India Several molecular devices based on the electronic switch- ing properties of conducting polymers have been de- scribed previously and involve an addition/ removal of a proton/ electron to/ from the polymer chain. This “dop- ing” process creates charged topological defects such as polarons or bipolarons, which act as charge carriers. In the device presented here, we have attempted to produce a topological defect without effecting an electron/ proton transfer. Such a topological defect has been produced by incorporating an ion-binding cavity in the polymer phase, which undergoes a conformational change on occupation of the cavity by the appropriate ion. Thus, anchoring 1 8 - crown-6 in polyaniline results in the electronic switching of the polymer device in the presence of as low as 10 -7 M K + ion concentration. That this switching is ac- companied by a conformational change was confirmed by measuring the mean molecular area of a Langmuir mono- layer of polyaniline-crown film in the presence and absence of K + . It was found that there is an increase and then a saturation in the mean molecular area in response to increasing concentrations of K + ions in the subphase, a trend which is similar to the electronic conductivity changes in the polymer film. This mechanism of switching makes it possible to design molecular devices that re- spond to a wide class of ionic species which need not undergo electron transfer to trigger the device. One of the goals in the area of molecular devices is to realize an electronic gate that can be activated by a specific chemical species. 1 A class of molecular devices based on the electronic switching properties of conducting polymers was first described by Wrighton et al. 2 A number of devices have since been described, 3-5 in which the device responds to the presence of a specific chemical species. The switching, in these cases, is a consequence of the production/ consumption of protons or elec- trons. We describe here an electronic switch which is activated by the presence of a specific metal ion. The switching in the present case is a consequence of binding of the metal ion to its specific receptor, which is anchored in the polymer phase, and the resultant conformational changes in the polymer system. It is now well known that the electronic conductivity of “conducting polymers” is sensitive to the chemical state of the polymer. The electronic properties of such a polymer chain can be described in terms of a band structure analogous to that of a solid. 6 Changes in electronic conductivity can, in general, be explained in terms of changes in the band structure caused by local conformational defects in the polymer backbone. These may be associated with a charge (polarons or bipolarons) or may be uncharged (e.g., neutral solitons, in polymers with a degenerate ground state). Charged distortions, polarons or bipolarons, may be caused by the removal/ addition of one or two electrons, respectively, from/ to the polymer chain (oxidation/ reduction), resulting in so-called p-type/ n-type conductivity. It may also be possible to create a conformational change in the polymer backbone by causing a local perturbation by polarizing the π-electron system without effecting a complete removal/ addition of an electron. One way of doing this could be to bring a charged species (a cation or an anion) in close proximity to the polymer chain without actually effecting electron transfer from or to the polymer chain. This can be done by incorporating within the polymer matrix an ion-binding species such as, for example, a crown ether. If the crown ether has a preferentially high binding constant for a certain ionic species as compared to others, the polymer would respond preferentially to that ion. In this context, the recent reports on “secondary”, doping by MacDiarmid et al. 7,8 are relevant and will be discussed later. There have been attempts at imparting ion specificity by covalent linking of azacrown ethers to polypyrrole, 9 pseudorotax- anes to polythiophene, 10 and, very recently, functionalization of polythiophenes with calix[4]arene-based ion receptors. 11 These systems do show an ion-specific response but at concentrations as high as 10 -1 M of the metal ion in nonaqueous media. We describe here a system that shows a response at concentrations as low as 10 -8 M of the concerned metal ion in aqueous media. In the present case, the system 18-crown-6/ K + has been chosen for study in polyaniline. The occupation of the crown cavity by the metal ion results in a local electrostatic field, which causes a perturbation in the polymer backbone and a change in the density of states in the band gap. These changes are observed even at concentrations as low as 1 × 10 -8 M K + . This is manifested at the macroscopic level as a change in the electronic † Department of Chemistry. ‡ Department of Electrical Engineering. (1) Lehn, J.-M. Supramolecular ChemistrysConcepts and Perspectives; VCH: Weinheim, Germany, 1995. (2) Kittlesen, G. P.; White, H. S.; Wrighton, M. S. J. Am. Chem. Soc. 1984 , 106, 7389-7396. (3) (a) Hoa, D. T.; Suresh Kumar, T. N.; Punekar, N. S.; Srinivasa, R. S.; Lal, R.; Contractor, A. Q. Anal. Chem. 1992 , 64, 2645-2646. (b) Contractor, A. Q.; Suresh Kumar, T. N.; Narayanan, R.; Sukeerthi, S.; Lal, R.; Srinivasa, R. S. Electrochim. Acta 1994 , 39, 1321-1324. (4) Nishizawa, M.; Matsue, T.; Uchida, I. Anal. Chem. 1992 , 64, 2642-2644. (5) Bartlett, P. N.; Birkin, P. R. Anal. Chem. 1993 , 65, 1118-1119. (6) (a) Bredas, J. L.; Street, G. B. Acc. Chem. Res. 1985 , 18, 309-315. (b) Heeger, A. J.; Kivelson, S.; Schrieffer, J. R.; Su, W.-P. Rev. Mod. Phys. 1988 , 60, 781-850. (7) MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1994 , 65, 103-116. (8) Avlyanov, J. K.; Min, Y.; MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1995 , 72, 65-71. (9) Youssoufi, H. K.; Hmyene, M.; Garnier, F.; Delabouglise, D. J. Chem. Soc., Chem. Commun. 1993 , 1550-1552. (10) Marsella, M. J.; Carroll, P. J.; Swager, T. M. J. Am. Chem. Soc. 1994 , 116, 9347-9348. (11) Marsella, M. J.; Newland, R. J.; Carroll, P. J.; Swager, T. M. J. Am. Chem. Soc. 1995 , 117, 9842-9848. Anal. Chem. 1997, 69, 724-727 724 Analytical Chemistry, Vol. 69, No. 4, February 15, 1997 S0003-2700(96)00487-8 CCC: $14.00 © 1997 American Chemical Society