SHORT COMMUNICATION Conducting polymer-based impedimetric aptamer biosensor for in situ detection Wei Liao & Brad A. Randall & Nicolas A. Alba & Xinyan Tracy Cui Received: 23 June 2008 / Revised: 12 August 2008 / Accepted: 15 August 2008 / Published online: 12 September 2008 # Springer-Verlag 2008 Abstract We have successfully developed the first proto- type of an electrochemical protein biosensor using poly- pyrrole as the substrate and doped aptamer as the probe. The sensitivity for a specific target is 10 ng/ml. Two targets, platelet-derived growth factor and immunoglobulin E, have been tested. In both cases the sensor exhibited high specificity and the signal was found to increase as the target concentration increased. Keywords Aptamer biosensor . Platelet-derived growth factor . Immunoglobulin E . In situ monitoring. Electrochemical impedance spectroscopy Label-free biosensors for in situ measurement with high sensitivity and high specificity are of significant interest for the development of point-of-care medical diagnostics devices [1–3]. Electrochemical detection using electro- chemical impedance spectroscopy (EIS) is advantageous because of its label-free aspect and high sensitivity. Label- free and reagentless aptamer biosensors based on EIS have been reported recently, demonstrating the proof of principle [4, 5]. Nevertheless, the detection limit (1–10 μg/ml) is still too high for many clinical applications (nanograms per milliliter) [4, 5]. Regarding impedimetric detection, sensi- tivity can be greatly improved by increasing the surface density of the immobilized probe and by introducing an electroactive supporting film. In the procedures described below, electropolymerized polypyrrole was used as a supporting film, and an electro- chemical mediator [Fe(CN) 6 ] 3-/4- was applied during detec- tion, both of which allow the sensor to achieve much higher sensitivity. Polypyrrole, a biocompatible and electrically conductive polymer, can be electropolymerized into a porous film with negatively charged molecules, known as dopants, immobilized along the positively charged polypyrrole backbone. In biosensing applications, polypyrrole has been used to immobilize biorecognition components as dopants in the conductive matrix for the electrochemical detection of glucose, proteins, and DNA [3, 6, 7]. Incorporating proteins such as enzymes, antigens, and antibodies within the polypyrrole film without signal amplification typically results in protein denaturation and steric hindrance for binding, resulting in low detection specificity and sensitiv- ity [3, 7]. In contrast, doping polypyrrole with a DNA- detecting oligonucleotide resulted in improved specificity and excellent correlation between signal and concentration, because of its smaller size, higher biostability, and unique electrical properties [6, 8]. Aptamers, or molecules consist- ing of fragments of DNA or RNA, are selected from a random oligonucleotide library to bind specific targets such as small molecules or proteins. Thus, aptamers will also be incorporated into the polypyrrole matrix to a significant degree. Our previous experiments indicated that aptamers will maintain their binding-ready conformation with proper charge density under the appropriate electric potential [4]. With the addition of [Fe(CN) 6 ] 3-/4- , it is expected that the Anal Bioanal Chem (2008) 392:861–864 DOI 10.1007/s00216-008-2354-8 W. Liao : B. A. Randall : N. A. Alba : X. T. Cui (*) Department of Bioengineering, University of Pittsburgh, 5063 Bioscience Tower 3, 3501 Fifth Ave, Pittsburgh, PA 15260, USA e-mail: xic11@pitt.edu X. T. Cui Center for Neural Basis of Cognition, University of Pittsburgh, 5063 Bioscience Tower 3, 3501 Fifth Ave, Pittsburgh, PA 15260, USA X. T. Cui McGowan Institute for Regenerative Medicine, University of Pittsburgh, Bioscience Tower 3, 3501 Fifth Ave, Pittsburgh, PA 15260, USA