Biosensors and Bioelectronics 22 (2006) 233–240 A comparative study of capacitive immunosensors based on self-assembled monolayers formed from thiourea, thioctic acid, and 3-mercaptopropionic acid Warakorn Limbut a,b , Proespichaya Kanatharana a,b , Bo Mattiasson c , Punnee Asawatreratanakul a,d , Panote Thavarungkul a,e,∗ a Biophysics Research Unit, Biosensors and Biocurrents, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand b Department of Chemistry, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand c Department of Biotechnology, Center for Chemistry and Chemical Engineering, Lund University, Box 124, 221 00 Lund, Sweden d Department of Biochemistry, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand e Department of Physics, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand Received 23 October 2005; received in revised form 13 December 2005; accepted 20 December 2005 Available online 7 February 2006 Abstract A procedure was developed for the covalent coupling of anti-alpha-fetoprotein antibody (anti-AFP) to a gold surface modified with a self- assembled monolayer (SAM) of thiourea (TU). The performance of the SAM-antibody layer was compared to those of similar layers based on thioctic acid (TA) and 3-mercaptopropionic acid (MPA) by using flow injection capacitive immunosensor system. Covalent coupling of anti-AFP on self-assembled thiourea monolayer (SATUM) modified gold electrode can be used to detect alpha-fetoprotein with high efficiency, similar sensitivity, the same linear range (0.01–10 gl -1 ) and detection limit (10 ng l -1 ) as those obtained from sensors based on self-assembled thioctic acid monolayer (SATAM) and self-assembled 3-mercaptopropionic acid monolayer (SAMPAM). The system is specific for alpha-fetoprotein and can be regenerated and reused up to 48 times. Therefore, self-assembled monolayer using thiourea which is cheaper than thioctic acid and 3-mercaptopropionic acid is a good alternative for biosensor applications when SAMs are used. © 2006 Elsevier B.V. All rights reserved. Keywords: Thiourea; Capacitive immunosensor; Thioctic acid; 3-Mercaptopropionic acid; Self-assembled monolayer; Alpha-fetoprotein 1. Introduction Immunosensors are based on binding interactions between immobilized biomolecules and the analyte of interest and their subsequent detection by appropriate detector (Mattiasson, 1984; Taylor, 1991). Several electrochemical detection principles have been used, such as potentiometric (Tang et al., 2004a; Taylor et al., 1991), amperometric (Ramanaviciene and Ramanavocous, 2004), conductimetric (Yagiuda et al., 1996), and impedemet- ric (Tang et al., 2004b). Capacitive measurement has also been investigated as a highly sensitive approach (Berggren et al., 1998, 2001; Berggren and Johansson, 1997; Bontidean et al., 1998; Hedstr ¨ om et al., 2005; Hu et al., 2002, 2005). ∗ Corresponding author. Tel.: +66 74 288753; fax: +66 74 212817. E-mail address: panote.t@psu.ac.th (P. Thavarungkul). Capacitive immunosensor is based on the principle that for an electrolytic capacitor the capacitance depends on the thickness and dielectric behavior of a dielectric layer on the surface of a metal (Gebbert et al., 1992). It can be constructed by immobiliz- ing biorecognition elements in a thin layer on an electrode and measuring changes in the dielectric properties when an analyte binds to the biorecognition elements on the electrode, causing capacitance to decrease. Immobilization is an important part in capacitive immunosen- sor since the electrode surface has to be electrically insulated. Different immobilization techniques have been developed and biorecognition elements can be immobilized on capacitive sen- sors via modified semiconductor surfaces (Barraud et al., 1993; Bataillard et al., 1988), metal oxides surfaces (Gebbert et al., 1992, 1994), and self-assembled monolayers (SAMs) of sul- fur compounds on gold (Berggren et al., 1998; Berggren and Johansson, 1997; Hedstr ¨ om et al., 2005). 0956-5663/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.bios.2005.12.025