Research & Technology News

RESEARCH & TECHNOLOGY NEWS
Impedometric Detection of Antigens: Towards the Development of a Bioimpedance Lab-on-a-Chip
Sensitive optics based immunomethods for antigen detection, such as enzyme-linked immunosorbent assay (ELISA), have been widely employed in laboratories over the past few decades. Their operation is multistage and complex and time consuming since their analysis time is a distinct entity separate from the capture detection system. These techniques require highly skilled technicians, well equipped laboratories and are of large size and cost and are non-portable, thus preventing point-of-care monitoring. Laser surface plasmon resonance (SPR) devices and surface acoustic wave (SAW) devices and the variant of this the quartz crystal microbalance (QCMB) have been used for antigen detection. However, these techniques do not compare in robustness or price to classic immunoassays. In recent years there has been an increased interest in the development of electrochemical biosensors. Such biosensors offer the advantages of simplicity and low cost and high miniaturization, making them disposable and portable. They are precise; they provide analytical time reduction and offer a completely label-free detection. In general, biosensors are based on modifying surfaces with specific biomolecules for detecting a specific antigen. Reported biosensors use stable antibody-antigen systems [1], self assembled monolayers (SAM) [2] and aptamers [3]. More complicated structures have been investigated, even employing gold nanoparticles [6]. Fig. 1 illustrates an example of two surface architectures for the detection of hCGβ, described and analyzed in [5] and [7]. Monitoring hCGβ levels provides valuable information concerning the progress of pregnancy and the health of a fetus (Downs syndrome screening and other), it may be used as a cancer biomarker since it is associated with gestational trophoblastic disease (GTD), germ cell tumours (testicular cancer) and some non-trophoblastic gynaecological cancers and common epithelial tumours. Levels of hCG, and in particular free hCGβ, are central to patient treatment monitoring and relapse detection in GTD. It has to be noted that according to [6], “utilization of hCGβ as a tumour marker is hampered by a limited availability of sensitive and specific assays”.
Electrochemical impedance spectroscopy (EIS) has been proven [1]-[6] to provide important information regarding the electrical properties of such biologically and chemically modified systems. In EIS a small amplitude periodic AC signal within a range of frequencies is applied to a system. The applied signal can either be a voltage or a current and accordingly a voltage or a current is measured. Using Ohm’s law, the impedance can then be calculated, providing valuable insight into the processes taking place at a sensors interface. This technique can be used for the detection of any antigen, provided that specific antigen immobilization is possible. This technique may also be used for cultured cell population monitoring and bacterial detection and DNA detection. Previous EIS analysis of such devices was conducted, using commercially available or custom-made electrodes and impedance analyzers. These studies satisfied simplicity, label-free detection and analytical time reduction, but not the rest of the requirements especially due to the actuating and measurement system. This highlights the need for a custom-made integrated circuit solution, consisting of the actuating, measuring and sensing elements all together on a single chip. An artistic impression of such a system is shown in Fig. 2, illustrating a petri dish (with an opening at the bottom), for solution and sample containment, and a four-gold-electrode bioimpedance sensor located within the dish bottom-opening. The biosensor would be constructed on top of these electrodes. This solution would satisfy portability and disposability requirements and would remove all cabling, thus reducing measurement errors. Finally, a sensor specifically designed for this application would greatly increase sensor sensitivity and hence, antigen detection. New analog circuit architectures, especially for the front-end of such a system and electromagnetic modeling, MEMS and microfluidics are being combined in this area.
References
[1] Wang M, Wang L, Wang G et al. (2004) Application of impedance spectroscopy for monitoring colloid Au-enhanced antibody immobilization and antibody-antigen reactions. Biosensors and Bioelectronics 19:575-582 DOI 10.1016/S0956-5663(0300252-5). [2] Dijksma M, Kamp B, Hoogvliet J C et al. (2000) Formation and electrochemical characterization of self-assembled monolayers of thioctic acid on polycrystalline gold electrodes in phosphate buffer pH 7.4. Langmuir 16:3852-3857 DOI 10.1021/1a991387q. [3] Cai H, Lee T M-H, Hsing I-M (2006) Label-free protein recognition using an aptamer-based impedance measurement assay. Sensors and Actuators B 114:433-437. [4] Lasseter T L, Cai W and Hamers R J (2004) Frequency-dependent electrical detection of protein binding events. The Analyst 128:3-8 DOI 10.1039/b307591e. [5] Kassanos P, Iles R K, Bayford R H and Demosthenous A (2007) Development of a biosensor for hCG detection Proc ICEBI XIII-VIII EIT (Graz, Austria 29 August-2 September 2007) (IFMBE Proc. vol 17) ed H Scharfetter and R Merwa (Germany, Springer) pp 620-623 DOI 10.1007/978-3-540-73841-1_160 [6] Feng J J, Zhao G, Xu J J and Hong-Yuan C (2005) Direct electrochemistry and electrocatalysis of heme proteins immobilized on gold nanoparticles stabilized by chitosan Analytical Biochemistry 342: 280-286 DOI 10.1016/j.ab.2005.04.040. [7] Neubert H, Jacoby E S, Bansal S S, Iles R K et al. (2002) Enhanced affinity capture MALDI-TOF MS: orientation of an immunoglobulin G using recombinant protein G. Anal. Chem. 74:3677-3683.
Panagiotis Kassanos and Andreas Demosthenous, Dept. of Electronic and Electrical Engineering, University College London, UK.
Richard H. Bayford and Ray K. Iles, Department of Natural Sciences, Middlesex University, UK (Email: p.kassanos@ee.ucl.ac.uk)

This article was recommended by Michael Tse