Sensors and Actuators B 166–167 (2012) 833–836 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical journa l h o mepage: www.elsevier.com/locate/snb Short communication Detection of oxidized LDL using a carbon nanotube electrode Seiji Takeda a , Shu-Ping Hui a , Keisuke Fukuda a , Hirotoshi Fuda a , Shigeki Jin a , Toshihiro Sakurai a , Atsushi Ishii a,b , Koichi Mukasa a,b , Kazuhisa Sueoka c , Hitoshi Chiba a,∗ a Faculty of Health Sciences, Hokkaido University, Sapporo 060-0812, Japan b Biosensor Inc., Sapporo 060-0051, Japan c Graduate School of Information Science and Technology, Hokkaido University, Sapporo 060-0814, Japan a r t i c l e i n f o Article history: Received 28 December 2011 Received in revised form 15 February 2012 Accepted 21 February 2012 Available online 3 March 2012 Keywords: Sensor Lipid Lipoprotein Oxidation Nanotube a b s t r a c t Oxidized low-density lipoproteins (LDLs) play a key role in cardiovascular disease development, but no convenient measurement method is available for them. Using a carbon nanotube (CNT) electrode, we measured oxidized LDL using amperometric detection. Treating SWCNT with a mixture of acids produced a CNT dispersion that yielded nanotube-based electrodes after deposition on a gold electrode and drying. Current was monitored in the nanotube electrode before and after adding LDL or oxidization of LDL. Oxidized LDL changed the current more than 10 nA, although LDL addition induced no significant change. Our CNT electrode enables simple detection of oxidized LDL. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Plasma lipoproteins are soluble spheroidal complexes of lipids and proteins that deliver hydrophobic lipids from the liver and intestine to other tissues. Lipoproteins are taken up by the liver and various peripheral tissues via lipoprotein receptors. Low-density lipoproteins (LDLs) are the major lipoprotein species in human. They contain lipids such as cholesterol, triacylglycerols, cholesterol esters, phospholipids, and apolipoprotein B-100 (apoB-100) as the major apolipoprotein [1,2]. Once unsaturated lipids in LDL are oxi- dized to lipid peroxides by radical oxygen species, lipid peroxides can accumulate in LDL by a radical chain reaction or auto-oxidation, and might modify the apoB-100 molecule. Consequently, modified LDLs are called oxidized LDLs, which can change macrophages in vascular walls into the lipid-loaded form, or foam cell, which is probably an initial process of atherosclerosis. Several methods for measuring oxidized LDL have been reported, such as the measurement of optical absorbance for the conjugated diene in lipid peroxides or their alcohol deriva- tives, the measurement of thiobarbituric acid reactive substances (TBARS) including lipid hydroperoxides, aldehydes and alcohols, enzyme-linked immunosorbent assay (ELISA) using antibody to detect modified apoB-100 or short-chained phospholipids [2,3], ∗ Corresponding author. Tel.: +81 11 706 3698; fax: +81 11 706 3698. E-mail address: chibahit@med.hokudai.ac.jp (H. Chiba). and high-performance liquid chromatography/mass spectrometry (LC/MS) for lipid peroxides [4]. Principles of these methods are well known, and manipulation of the sample is not complicated. However, they require laborious and time-consuming procedures in addition to expensive apparatus. Simple and quick detection of oxidized LDL is necessary in hospitals, particularly in operat- ing rooms, where lipid oxidation occurs in patients with ischemic reperfusion during surgical operations such as organ transplanta- tion. Electrochemical detection is expected to be an appropriate method to detect a redox molecule in a solution because minia- turization of the apparatus is possible. For over a decade, carbon nanotube (CNT)-based electrodes have been investigated. Many research groups have investigated their application because of their greater sensitivity and lesser overpotential [5–7]. The CNT electrode shows specific catalytic activities in an electrochemi- cal reaction of hydrogen peroxide. Actually, CNT electrodes are known to show a response to hydrogen peroxide with high sen- sitivity. We inferred that lipid–peroxide groups on the surface of oxidized LDL also might be measured using an electrode because lipids have affinity to CNT [8,9]. Antibodies or enzymes or medi- ators are usually introduced to the working electrode to increase specificity and efficiency [10–14]. However, in this study, ox-LDL including lipid peroxides were chosen as target materials with- out enzymes or mediators. This report is the first to describe a demonstration of oxidized LDL detection using a CNT-based elec- trode. 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2012.02.064