et al., 1996). Immunosensor based on non-faradaic process with
Currently, the most sensitive and specific assay diagnosis for no addition of reagent has also been studied and shown that this
infectious disease is thelaboratory-based polymerase chain reac- scheme was somewhat more amenable to point-of-care applica-
tion (PCR) (Mackay, 2004) or Enzyme-Linked ImmunoSorbent tions (Berggren et al., 2001; Rickert et al., 1996).
Assay (ELISA) methods. However, these methods are both time Barium strontium titanate (BST) thin films have long been
consuming and costly, hence notsuitable for clinical screening or receiving intensive research attentions for various applications.
point-of-care molecular diagnosis in the field. The immunosen- It has been widely used in electronic devices, such as tunable
sor based on antigen/antibody interaction has the potential of microwave filters and radio-frequency microelectromechanical
simple, rapid,label-free and low-cost molecular diagnosis and capacitive switches (Morales-Cruz et al., 2005), due to their
has attracted great interest in research and development (Daniels high permittivity, low loss and electric tunable feature. They are
and Pourmand, 2007; Luppa et al., 2001). A small, disposable also used as transducers of various microsensors, i.e. bolome-immunosensor device for dengue infection diagnosis, with sim- ters (Hashimoto et al., 2001; Noda, 2005), gas sensor (Deng et
ple interdigitated electrode design and suitable for point-of-use by al., 2004; Tan et al., 1999, 2003; Zhu et al., 2000), humidity sen-
unskilled users in the field is highly desirable. sors (Agarwal and Sharma, 2002; Agarwal et al.,2001). These
The conventional electrochemical impedance spectroscopy is microsensors employ BST thin films as the middle dielectric layers
mainly based on the faradaic process where charge is transferred in sandwich structure capacitors, with a measurable capacitance
across the interface. Redox-couple, most often [Fe(CN)6 ]4−/3− , is change when the films adsorb radiantheat, gas, or water vapor. The
added to the electrolyte solution. However, it was observed by sol–gel derived BST thin film was exploited in the area of biosen-
some researchers that the long term presence of the redox-couple sors by Tan and co-workers (Fang et al., 2006; Tan et al., 2005,
2006–2007).They demonstrated the successful immobilization of
biomolecules (i.e. bovine serum albumin (BSA)) on BST thin film
that deposited on Pt coated Si substrate. Three-electrode system
∗ Corresponding author. Tel.: +65 6790 5411; fax: +65 6791 2687.was applied for all the measurements with an Ag/AgCl reference
E-mail address: firstname.lastname@example.org (O.K. Tan).
0956-5663/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
1138 X. Fang et al. / Biosensors and Bioelectronics 25 (2010)1137–1142
electrode, a platinum coated wafer as a counter electrode and the 2.2. Device fabrication
biomolecules immobilized BST film as a working electrode. Elec-
trochemical impedance spectroscopy and cyclic voltammetry were Three major steps were involved in device fabrication, namely
applied to investigate the immune reaction between BSA and anti-...