Development of an Electrochemical Cholesterol Sensor System for Food Analysis

Topics: Cholesterol, Electrode, Sensor Pages: 10 (3330 words) Published: September 28, 2012
ANALYTICAL SCIENCES FEBRUARY 2012, VOL. 28 2012 © The Japan Society for Analytical Chemistry

187

Advancements in Instrumentation

Development of an Electrochemical Cholesterol Sensor System for Food Analysis Tsutomu NAGAOKA,*† Shiho TOKONAMI,** Hiroshi SHIIGI,* Hiroaki MATSUMOTO,* Yasuhiro TAKAGI,*** and Yasunori TAKAHASHI*** *Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-2 Gakuen-cho, Naka, Sakai 599–8570, Japan **Nanoscience and Nanotechnology Research Center, Research Organization for the 21th Century, Osaka Prefecture University, 1-2 Gakuen-cho, Naka, Sakai 599–8570, Japan ***JSK Co. Ltd., 1-5-1 Higashi-amakawa, Takatsuki, Osaka 560–0012, Japan

In this article, we report on a food-cholesterol monitoring sensor based on a non-enzymatic approach. Amorphous and single-crystal gold electrodes were modified with an alkanethiol self-assembled monolayer to quantify it by voltammetry. We first discuss the basic characteristics of these sensors and provide more information about the instrument developed by JSK Co. This instrument is a battery-operated handheld voltammetric analyzer, which mounts a sensor chip to monitor cholesterol contents in food samples. The sensor showed excellent linearity with the cholesterol concentration; egg-yolk samples were analyzed to give an excellent agreement between the values obtained by the sensor (1.4 mM) and chromatography (1.1 mM). (Received October 7, 2011; Accepted December 19, 2011; Published February 10, 2012)

Introduction
Currently, there is a surge in demand for the real-time monitoring of chemical compounds, which is required for various types of critical operations, such as detecting food poisoning and chemical/biological threat agents. Many chemical sensors have been developed for these purposes as well as for avoiding labor-intensive analytical operations. However, developing a sensor device is made very difficult by the requirement that, based on its selectivity scheme, the device must discriminate one particular molecule or certain types of molecules of interest out of the large number of other molecules existing in a sample. Due to this difficulty, highly selective sensors are mostly fabricated using biologically acquired ligands, such as enzymes and antibodies.1–7 However, these ligands are usually expensive and prone to lose their activities for prolonged use. It is therefore desired to make use of artificial receptors possible, especially in low-cost sensing applications as well as in harsh However, such chemical and/or physical environments.8 receptors often have serious problems with insufficient selectivities against target molecules. To circumvent this dilemma, we have recently developed sensing devices based on molecularly imprinted polymers using overoxidized conducting polymers, which can discriminate amino acid enantiomers, structural isomers, etc. in highly efficient manners.9–14 We have also developed self-assembled monolayer (SAM) based sensors for the detection of vitamin K1 and skin cholesterol.15–17 Cholesterol is essential for many metabolic processes, but †

high levels are often associated with heart disease. To know of its level in food as well as in our body, a number of papers related to electrochemical cholesterol sensors have been published based on enzymes,18–22 molecularly imprinted polymers,16,17,23,24 and other non-enzymatic approaches.25,26 This article presents a SAM-based sensor optimized for food cholesterol. The monolayer, structured on a gold substrate, has high selectivity for cholesterol to provide a convenient means for developing cost-effective sensors especially suited for disposable use. Based on these results examined in this study, JSK Co. has recently marketed a handheld sensor system for food cholesterol analysis; the performance of this system is discussed in this article.

Experimental
Chemicals and apparatus Cholesterol and its derivatives were obtained from Wako...


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Fig. 4 chip.
Photographic images of (A) the instrument and (B) sensor
(2  working electrodes, and 1 quasi-reference and counter electrodes) on a glass strip, which is highly polished until it reaches a surface roughness of 10 nm, considering the higher selectivities observed for the single-crystal surface. Vapor deposition on the glass strip results in the formation of amorphous gold electrodes; the pads are then coated with a plastic film to expose the minimum area of the electrodes. The instrument has also been designed to work as a versatile voltammetric analyzer by inserting an unmodified electrode chip. To determine total cholesterol in food samples, many techniques, including spectrophotometry and amperometry based on enzymatic reactions, and gas and liquid chromatographic techniques, have been reported.19,28–37 Cholesterol in a yolk mostly exists as free cholesterol, although the exact ratio of free cholesterol to total cholesterol seems to depend on the pretreatment procedure of a yolk sample and the quantification technique employed.19,30,35 Although the responses of cholesterol esters showed 18 – 49% as high as the response of free cholesterol (see Table S1, Supporting Information), the influence of the esters on the quantized value could not be important due to the small fraction of the esters in the yolk samples. The total cholesterol concentration of 1.4 mM for an egg-yolk sample determined by this system agreed well with the value of 1.1 mM obtained by gas chromatography. In conclusion, we have succeeded in developing a sensor instrument, along with a robustly designed bio-free disposable sensor chip, to provide an excellent analytical solution, which
ANALYTICAL SCIENCES FEBRUARY 2012, VOL. 28
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