Electronic Supporting Material
Low-potential amperometric determination of NADH using a disposable
indium-tin-oxide electrode modified with carbon nanotubes
Qiuhong Wang, Weibo Li, Ning Bao, Chunmei Yu*, Haiying Gu*
School of Public Health, Nantong University, Nantong 226019, P. R. China
Electrochemical characterization of the MWNTs/ITO
Fig. S1. (A) Cyclic voltammograms of MWNTs/ITO electrode in 0.1 M KCl
containing 1 mM K3Fe(CN)6 at different scan rate (a−h: 50, 100, 150, 200, 250, 300,
350 and 400 mV s-1). (B) Plots of oxidation and reduction peak currents vs. square
root of scan rate.
*
Corresponding author. Tel/Fax: +86 513 85012916
E-mail address: hygu@ntu.edu.cn, cmyu@ntu.edu.cn
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Reaction mechanism of NADH at the surface of MWNTs/ITO
Fig. S2. Schematic illustration of the oxidation mechanism of NADH at the surface of
MWNTs/ITO.
Optimization of experimental conditions
Fig. S3. Effects of (A) the amount of MWNTs and (B) working electrode areas on the
oxidation peak current for 1mM NADH at MWNTs/ITO.
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The amount of the MWNTs used to modify the working electrode can affect the
response of the sensor to NADH. To study this effect, we have deposited different
amount of MWNTs suspension on the surface of the ITO electrode and let it to dry
according to the procedure described above. The sensors were tested by differential
pulse voltammograms and the peak currents for 1 mM NADH were compared. As can
be observed from Fig. S3A, with the increase of the amount of MWNTs from 3 to 9
µL, the anodic response signal of NADH increased accordingly. The maximum
current was obtained when the quantity of MWNTs was 9 µL, and the peak current of
NADH was about 2.7 times than that at 3 µL. When the MWNTs amount further
increased to be 12 µL, the peak current began to decrease. Previous study reported
that the working electrode modified by certain amount of MWNTs possessed good
electrical conductivity [1]. Such results indicated that 9 µL MWNTs (0.4 wt.‰) was
better for the detection of NADH.
It is noteworthy that the proposed approach presented here allows us to change the
surface area of the working electrode conveniently, which can be realized by changing
the diameter of the hole punched on the plastic adhesive tape. Fig. S3B shows the
influence of different working areas on the measurement of 1 mM NADH. The peak
current of NADH increased with increasing of the electrode diameter from 2 mm to 4
mm. With further increase in the diameter, there was a plateau for the peak current.
For the analytical cost, reducing the pipetting volume of electrolyte solution and the
size of working zone would decrease the analytical cost remarkably by saving the
reagents and materials. Therefore, considering the analytical cost and sensitivity, the
working electrode diameter of 4 mm was applied for the detection of NADH.
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Fig. S4. (A) Effect of pH of phosphate buffer on the oxidation peak current for 1 mM
NADH at MWNTs/ITO. (B) Linear relationship between pH and peak potentials.
The effect of pH of PBS on the oxidation of NADH at the disposable electrode
was also investigated by cyclic voltammetry. As indicated in Fig. S4A, the oxidation
peak current increased gradually from pH 5.5 to 7.4, and then the current decreased
slightly when the pH value further increased to 8.5. NADH is known to be unstable in
highly alkaline and acidic solutions due to its instant degradation [2]. Therefore, the
optimum pH to study NADH oxidation is determined to be 7.4 in this work. The
relationship between the oxidation potential and pH was also constructed. A linear
shift of Epa toward negative potential with an increasing pH can be obtained (Fig.
S4B). The regression equation was expressed as Epa(V) = −0.0579 pH + 0.8813
(R2=0.9992). A slope of 57.9 mV/pH suggests that the number of electron transfer is
equal to the proton number involved in the NADH oxidation.
Reference
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electrochemical CS-MWCNTs/Au-NPs composite DNA biosensor for staphylococcus
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2. Salimi A, Hallaj R, Ghadermazi M (2005) Modification of carbon ceramic
electrode prepared with sol-gel technique by a thin film of chlorogenic acid:
application to amperometric detection of NADH. Talanta 65:888−894
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