Biosensors and Bioelectronics 22 (2007) 3126–3131 Investigations of the steric effect on electrochemical transduction in a quinone-based DNA sensor B. Piro, S. Reisberg, V. Noel, M.C. Pham ∗ Laboratoire Interfaces-Traitements-Organisation et Dynamique des Syst` emes (ITODYS), Universit´ e Paris 7-Denis Diderot, associ´ e au CNRS, UMR 7086, 1 rue Guy de la Brosse, 75005 Paris, France Received 10 July 2006; received in revised form 19 January 2007; accepted 6 February 2007 Available online 16 February 2007 Abstract Electrochemical biosensors for DNA hybridization are receiving increasing interest. A key point for their efficiency is to obtain a high signal level for low DNA concentration. This implies the design of an efficient transducing surface. Conducting polymers are interesting for this purpose but the great majority of conducting polymer-based electrodes present a signal decrease upon hybridization (a “signal-off” behavior), which impedes their response and makes them sensitive to false positive ones. The sensor described here presents a “signal-on” behavior, due to the use of a quinone group as the transducing agent. The specific aim of this work is to study the steric effect on transduction. To this end, the electrochemical response was monitored versus the DNA target length, for a constant DNA probe length. The results indicate that the current depends on the length of the double strand. A model which can explain the electrochemical behavior takes into account the steric hindrance of the ODN strands. © 2007 Elsevier B.V. All rights reserved. Keywords: Oligonucleotide; Electrochemical hybridization sensor; Reagentless detection; Modified electrode 1. Introduction There is a great interest in the development of easy-to-use DNA sensors. At the moment, hybridization transduction sys- tems require the use of a fluorescent or chemiluminescent label (Xu and Bard, 1995; Epstein et al., 2002; Calvo-Munoz et al., 2005), or without a label, mainly using surface plasmon res- onance (Heaton et al., 2001; Livache et al., 2001). Although widely used and sensitive, these methods may be lengthy and costly. For this reason there is much interest in electrochemical biosensors (Palecek, 1988, 2002; Millan and Mikkelsen, 1993; Souteyrand et al., 1997; Steel et al., 1998; Marrazza et al., 1999; Yang and Thorp, 2001; Wang et al., 2001, 2004; Wang, 2005; Ozkan et al., 2002; Oliveira-Brett et al., 2002; Liu and Anzai, 2004; Dominguez et al., 2004; Katz et al., 2004; Del Pozo et al., 2005), as the electrochemical transduction requires less time and instrumentation. However, the electroactive transduction of DNA–DNA hybridization, which does not imply any redox reac- tion, remains a challenge. Electronically conducting polymer ∗ Corresponding author. Tel.: +33 1 44276961; fax: +33 1 44276814. E-mail address: minh-chau.pham@univ-paris-diderot.fr (M.C. Pham). (ECP)-modified electrodes can be used as transducers because their redox properties are modulated following hybridization. Nice examples exist in the literature (Korri-Youssoufi et al., 1997; Wang et al., 1999; Emge and Bauerle, 1999; Lassalle et al., 2001; Thompson et al., 2003; Cha et al., 2003; Peng et al., 2005). These studies often report a “signal-off” detection and use p-doping polymers, which exchange anions to compensate their charge during oxidation. Interpretations of the detection process are based upon changes in the ion-exchange process, or polymer structure reorganization. We recently described a new ECP-based sensor using a cation-exchange redox group (quinone) as the immobi- lized redox-active label. A “signal-on” is observed following hybridization (Pham et al., 2003; Piro et al., 2005; Reisberg et al., 2005, 2006). The main differences between conventional p-doped ECP and our quinone-based redox polymer are the charge transport mechanism inside the film and the nature of the ionic flux at the polymer/solution interface (anionic and cationic, respectively). When ssDNA probes are grafted onto the ECP surface after electropolymerization, the main sens- ing element of the modified electrode is the polymer/solution interface. To increase the sensitivity of ECP-based biosen- sors, the interactions between the electrochemical transducer 0956-5663/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.bios.2007.02.007