Hindawi Publishing Corporation Journal of Sensors Volume 2012, Article ID 217958, 6 pages doi:10.1155/2012/217958 Research Article Measurement of Hepatitis B Surface Antigen Concentrations Using a Piezoelectric Microcantilever as a Mass Sensor Sangkyu Lee, 1 Jongyun Cho, 1 Yeolho Lee, 2 Sangmin Jeon, 3 Hyung Joon Cha, 3 and Wonkyu Moon 1 1 Department of Mechanical Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea 2 Corporate Technology Operations SAIT, Samsung Electronics Co., Ltd., Yongin 446-712, Republic of Korea 3 Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea Correspondence should be addressed to Wonkyu Moon, wkmoon@postech.ac.kr Received 15 June 2011; Accepted 28 July 2011 Academic Editor: Martin Hegner Copyright © 2012 Sangkyu Lee et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Hepatitis B surface antigen (HBsAg) concentrations were measured using a piezoelectric microcantilever sensor (PEMS) developed by the authors. The developed PEMS is label-free and detects the sensing signal electrically. It was designed to measure the mass of biomolecules attached to it using an accurate mass-microbalancing technique; its probe area is confined to the end of the cantilever, and its equivalent spring constant is relatively high to minimize the effect of changes in the surface stress when the biomolecules are attached to it. The “dip- and-dry” technique was used to enable the probe area of the sensor to react with reagents in controlled environmental conditions. HBsAg was detected by an immunoreaction whereas the reaction time, antibody density, and its area on the probe were kept at a constant level. The mass of the detected HBsAg was measured in the range of 0.1–100 ng/mL. 1. Introduction Hepatitis B virus (HBV) infection causes the disease hepatitis B and may also lead to cirrhosis and hepatocellular carci- noma [1]. An estimated two billion people worldwide have been infected by HBV, and of these, 350 million are chron- ically infected. Hepatitis B surface antigen (HBsAg) forms part of the surface of the virus and is used as a biomarker for the HBV infection [1]. We need to detect HBsAg in very low concentrations to accurately diagnose HBV. In hospitals, chemiluminescence immunoassay is widely used to detect HBsAg, and its detection limit is approximately 0.05 ng/mL [2]. A piezoelectric microcantilever sensor (PEMS) offers many advantages as a biosensor and is suitable for the detec- tion of HBsAg. A PEMS is a highly sensitive label-free sensor that is sufficiently small to be developed as a portable device; multiplexed detection and electrical readout are also available [3]. Many studies have been conducted using a PEMS to detect various biomarkers [4]. The principle of detection in a PEMS is based on changes in the resonant frequency of the PEMS before and after a target protein is attached to it; target proteins are captured on the probe area of the PEMS by an immunoreaction. The frequency changes depend on changes in the surface stress and mass loading due to the attached biomolecules. The influence of the surface stress on the resonant frequency decreases as the effective stiffness (spring constant) of the PEMS increases [5, 6]. If we use a PEMS as a mass sensor based on a mass-microbalancing technique [7], the effective stiffness should be sufficiently large, and then, the resonant frequency will vary only in response to the mass loading effects. The experimental setup consists of a part that measures the resonant frequency and a part that enables the probe area of the sensor to react with reagents. An impedance analyzer is usually used to measure the resonant frequency of the PEMS by detecting the peak point of the phase angle [8], dielectric loss [6], and so forth. The “dip- and-dry” technique is widely used in detection experiments [9]. The PEMS is dipped into reagents to either immobilize the antibody or bind the antigen, and the resonance frequencies are detected in air. The quality factor of the PEMS should be large because it is related to the accuracy of the detection of the resonant