Functionalized Regioregular Polyalkylthiophenes for Biosensing Applications
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Functionalized regioregular polyalkylthiophene for biosensing applications. Simon J. Higgins, Fouzi Mouffouk, Stewart J. Brown, Naser Sedghi1, Bill Eccleston1 and Stuart Reeman2 Department of Chemistry, University of Liverpool, Crown Street, LIVERPOOL L69 7ZD, U.K. 1. Department of Electrical and Electronic Engineering, University of Liverpool, Brownlow Hill, LIVERPOOL L69 3GJ, U.K. 2. Dstl Chemical and Biological Sciences, SALISBURY, SP4 0JQ, U.K. ABSTRACT A regioregular copolymer of 3–hexylthiophene and 3–(6–hydroxyhexan–1yl)thiophene has been functionalised with biotin hydrazide; binding of avidin to the biotin moieties causes drastic changes to the absorption spectrum of the polymer in solution, and to the electrochemistry and conductivity of the polymer in thin films. INTRODUCTION Polythiophenes bearing covalently–attached receptor sites have properties that make them particularly desirable as sensor materials. Changes in optical, electrical and redox behaviours, induced by binding events at the receptors, are often conveniently detected owing to the comparatively small bandgap, and the modest potentials needed for p–doping [1]. Head–to–tail regioregular polyalkylthiophenes [2] are of particular interest. They have been shown to have particularly high conductivity when doped, they are highly solvato– and thermochromic [3] and they give unusually high field effect mobility when used as the semiconductor in field effect transistors [4]. This has been ascribed to the observed self–assembly of the polymers, during solvent casting, into a 2-dimensional structure in which the regularly–placed alkyl groups of neighbouring polymer chains are able to intercalate, and efficient π– π stacking between neighbouring polythiophene chains occurs in the remaining dimension, facilitating carrier transport. The synthesis of functionalised versions for sensing is therefore of great interest. Examples published to date either involve post–polymerisation functionalization of poly(ω– bromoalkyl)thiophenes [5], or Stille coupling of appropriate functionalised (and protected) thiophenes or 2,2’–bithiophenes [6]. Avidin is a highly stable tetrameric glycoprotein (RMM ca. 68 kD), each subunit containing a binding site for biotin; its complex with biotin is exceedingly strong for a noncovalent interaction (Ka 1015 M–1) and it has been widely used in the life sciences, both as a model system and in many practical applications, for instance, isolation (affinity chromatography), localization (affinity cytochemistry, cell cytometry, and blotting technology), diagnostics (immunoassay, histopathology, and gene probes), hybridoma technology, bioaffinity sensing, affinity targeting, and drug delivery [7]. Several reports of the use of conjugated polymers in avidin sensing have been published, including a biotinylated poly(3–alkyl–4–alkoxy)thiophene [8] and a biotinylated PPE [9]. We were intrigued by the possibility of using the unusually favourable optical and electronic properties of regioregular polyalkylthiophenes in biosensing, parti
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