Selective Detection of Cholesterol Using Carbon Nanotube Based Biochip
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Selective Detection of Cholesterol using Carbon Nanotube Based Biochip Somenath Roy, Harindra Vedala, and Wonbong Choi* Mechanical and Materials Engineering Department, Florida International University, Miami, FL 33174, USA (* Corresponding author: [email protected]) ABSTRACT A novel type of working electrode based on MWCNT is fabricated on silicon substrate for detection of total cholesterol. Multiwall carbon nanotubes (MWNT), which are vertically grown on a silicon platform, promote heterogeneous electron transfer between the enzyme and the working electrode. Immobilization of cholesterol oxidase (Pseudomonas fluorescens.) on the electrode is done by entrapment of the enzyme in a bio-friendly, water soluble polymer, polyvinyl alcohol (PVA), which also increases the hydrophilicity of the carbon nanotube (CNT) surface (with experimental evidence) and thereby facilitating the intimate attachment of the protein molecules. Cyclic voltammograms exhibit efficient detection of cholesterol in the range of 100-300 mg/dl, which covers the entire range of possible cholesterol concentration in human blood. The influence of interferents, like glucose, ascorbic acid and uric acid, present in the physiological fluids, is also examined. Owing to its compatibility with standard silicon microfabrication technology, the signal processing circuitry can also be integrated onto the same silicon substrate yielding a compact miniaturized biochip. INTRODUCTION Detection of cholesterol is imperative for the risk assessment of cardiovascular conditions, such as atherosclerosis and hypertension, which can lead to coronary heart disease, myocardial and cerebral infarction (commonly known as stroke). The desired total plasma cholesterol for an individual is less than 5.2 mM (200 mg/dl), and it poses potential threat when the level is greater than 6.2 mM (240 mg/dl) [1]. Cholesterol is carried in plasma by a series of protein-containing micelles known as lipoproteins. The total cholesterol present in all lipoprotein fractions can be determined using a colorimetric assay specific for cholesterol [2]. Cholesterol esters are acted upon by cholesterol esterase to release free cholesterol and then cholesterol oxidase catalyzes the reaction to generate H2O2, which is reacted to yield quinoneimine dye. The absorbance of the dye is proportional to H2O2, hence the cholesterol concentration. Electrochemical detection techniques imitate the colorimetric assay by immobilizing cholesterol esterase and cholesterol oxidase onto electrode surfaces to liberate cholesterol and subsequently generate H2O2, which is measured amperometrically [3]. Advantages to these biosensors include rapid analysis, reusability, thermal stability, and linearity. There have been consistent efforts to improve the performance of the electrochemical biosensors which include functionalization of the working electrode surface by polymer or nanomaterials. Recently Li et al. [4] reported modification of screen-printed carbon electrodes (on a polycarbonate substrate) with carbon
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