Platinum nanoparticle modified carbon paste electrode for ethanol sensing . 5

especially in the control of drunk driving. Recently, a variety of methods and strategies had been reported for the determination of ethanol including gas chromatography [2,3], liquid chromatography [4,5], refractometry [6], and spectrophotometry [7]. These methods which are relatively expensive and complex to perform; therefore, an amperometric biosensor may be the most promising method due to their convenience, simplicity and accuracy.

The fabrication of an electrochemical alcohol biosensor has been achieved using either alcohol dehydrogenase (ADH) [8-12] or alcohol oxidase (AOD) [8,13,14], by immobilising these enzymes onto suitable transducers based on the detection of β-nicotinamide adenine dinucleotide (NADH) or hydrogen peroxide (H2O2). The biosensors which are based on alcohol dehydrogenase require the presence of NAD⁺ as cofactor which increases the overall cost of manufacture [15]. In addition, the cofactor needs to be close to the electrode surface where the enzyme is deposited, without becoming irreversibly entrapped, or linked, onto the electrode surface [16]. However, such problems lead to a decrease in sensitivity and life-time of sensors. In addition to enzymes, other materials for use in electrochemical ethanol sensors have been developed. The carbon nanotubes (CNTs) [17-19], sputtered complex nano-metals [20], and rhodium oxide (RuO2) [21] modified sensor for determination of ethanol were developed. The results of measurement for ethanol were showed a good performance with sensitivity and novel fabrication process.

An amperometric ethanol biosensor modified with Pt nanoparticles and based on carbon paste substrates was reported. Ethanol undergoes catalytic splitting on platinum (Pt)

electrodes and presents a high potentiality for future applications on technological systems, such as fuel cells [22-24]. Due to the high electrooxidation ability of ethanol, the Pt was applied as the modifier based on the biosensor. Pt nanoparticles with precisely controlled shape exhibit unique catalytic properties of ethanol. The faceted platinum nanoparticles exhibit a higher catalytic activity as compared to spherical particles. To prevent the pollution

during the sensing procedure, the property of disposable is very important in biosensor.

Because the Pt modifier was in the nano-scale and the sensor substrates was a mass-printed film, the cost of this Pt nanoparticles modified biosensor was as low as disposable.

The performance of this present modified biosensor for determination of real samples was used to measure the ethanol in alcoholic beverages and serum. CV and chronoamperometry were employed to analyze its electrochemical behavior, while different electrode modified conditions were investigated to optimize the sensing characteristics. The characteristics and performance of this ethanol sensor such as its response time, linear range, sensitivity, reproducibility, and long-term stability were also performed.

I.1.2.2 Carbon nanotubes modified ITO electrode for catecholamines sensing

Since their initial discovery by Iijima in 1991 and subsequent report of their synthesis by Ebbesen and Ajayan in 1992, carbon nanotubes (CNTs) have been the subject of many

experimental and theoretical investigations [25-27], the structure of CNTs is shown as in Fig.

I-4. Two kinds of CNTs are known: single-wall carbon nanotubes (SWNTs) and multiwall carbon nanotubes (MWNTs). They have attracted much attention as a result of their unique structures and chemical and physical properties, which make them suitable for use in a range of potential applications [25,28-31], such as high sensitivity microbalances [32], gas detectors [33,34], catalyst supports [35,36], electron sources in field emission-mode displays [37], tiny tweezers for nanoscale manipulation [38], and probe tips for scanning probe microscopy [39].

Theoretical calculations have indicated that, depending on their symmetry and diameter, CNTs can exhibit metallic or semiconductive behavior [40-42].

Figure I-4. (A) A column with hexagon structure of CNT, the hybrid orbital between carbon atoms is sp² type. (B) The closed-shell of Fullerene with the pentagon topological defect [43].

Furthermore, CNTs are more highly conductive than graphite [44]. The large surface areas and abundances of functional groups presented on nanomaterials make them suitable for specific electroanalytical reactions of certain substances when incorporated into electrodes.

For example, CNTs have the ability to mediate electron transfer reactions of electroactive species in solution, exhibiting catalytic effects on the electrochemical behavior of dopamine [45], proteins [46], and oxygen [47].

A conventional approach toward fixing CNTs onto the surfaces of working electrodes is through spin-coating of a film of, for example, Nafion mixed with CNTs [48]. Nafion, a cation exchange polymer, forms films that are highly permeable to cations but almost impermeable to anions. CNTs lacking any additional functional groups can be dissolved completely in Nafion solutions. Using glassy carbon electrodes (GCEs) modified with CNTs increases the sensitivity of CV toward the detection of analytes [48].

Modified GCEs are, however, still susceptible to poor reproducibility because of the need to clean the GCE surface. One approach to solving this problem is the use of a

disposable material as an electrode base; this material must have low cost and be readily available and easily modified. Carbon paste is a widely used material for fabricating disposable electrodes [49]; screen-printing techniques can be employed for their mass production. Indium tin oxide (ITO) is another possible material that has recently become widely used industrially for thin display panels; thus, it is easy to purchase at standard

specifications [50]. ITO is an excellent photoelectric material because of its high conductivity and photo-penetrability. Although it can decrease electrical resistance when used as an

electrode substrate, the electroanalytical activity of ITO is relatively low when compared with most other types of detectors. Surface modification of ITO glass is one means to increasing its electroanalytical activity. We suspected that modifying ITO with CNTs would provide a substrate exhibiting the pure properties of CNTs without any interference from graphite. The images of CNTs modified ITO are shown in Fig. I-5.

Figure I-5. SEM images of (a) Nafion and (b) CNT/Nafion films on ITO electrodes;

magnified by a factor of 10,000 [51].

Because catecholamines are very important neurotransmitters in mammalian central

nervous systems [52], intensive efforts have been made to determine their levels in vivo and in vitro using selective electrochemical techniques [53–55]. Ideally, we would like to establish simple, rapid, and selective methods for the routine analysis of catecholamines.

The electroanalytical performance of CNT/Nafion-modified ITO electrodes for the detection of catecholamines, including dopamine, epinephrine, and

3,4-dihydroxyphenylalanine (DOPA) was described. Nafion is an effective solubilizing agent for CNTs, and the resulting CNT/Nafion films exhibited very good adhesion on the ITO surface. Because various activation procedures influence the electrochemical reactivity of electrodes, the effects of chemical pretreatment of the CNTs and the pH of the buffer on the electroanalytical behavior of CNT/Nafion-modified ITO electrodes were studied. The electrooxidation toward catecholamines at CNT/Nafion-modified ITO electrodes was investigated.

Table I-1. Figures of merit for the determination of catecholamines [51]

Catechol Dopamine Epinephrine DOPA Coefficient of

Determination (r²) 0.997 0.995 0.998 0.997 Linear Range (μM) 1 – 10³ 1 – 10³ 10^-1 – 10³ 1 – 10³

a. Amplification factor was calculated for the detection current, relative to that from a bare ITO electrode, at 0.5 mM of the analyte.

CNT/Nafion-modified ITO electrodes exhibit excellent electrocatalytic activity towards the redox reactions of catecholamines, the results of detection are summarized in table I-1.

The modified electrode enhanced the peak currents and lowered the overpotentials. Nafion

was a useful solubilizing agent for preparing the CNT-modified ITO electrodes. Such CNT/Nafion-modified ITO electrodes are easy to prepare and exhibit good reproducibility, remarkable electrocatalytic properties, and low detection limits for catecholamines.

Consequently, these modified ITO electrode could be employed as electrochemical sensors exhibiting high selectivity and sensitivity toward catecholamines, without interference from the presence of ascorbic acid. Furthermore, photolithography can be used to fabricate ITO glass into specific electrode patterns that are readily integrated into biosensors.