Mediator-Less Catalytic Oxidation of NADH Using Oxygen Plasma Treated Screen Printed Carbon Electrodes
Authors: Sankararao Mutyala , Mathiyarasu Jayaraman 
Published: February 2020

β-Nicotinamide adenine dinucleotide (NADH) is a vital cofactor in redox enzymes and plays a key role in the production of energy through redox reactions [1]. Thus, detection of NADH has attracted attention and widely employed in biosensor and bioconversion processes. Direct electrochemical oxidation of NADH at a platinum or carbon electrodes has resulted in high overpotential [2]. The high potential cause strong adsorption of NADH or its oxidized products lead to surface fouling, lack of sensitivity and reproducibility. Hence, the task is to minimize the electrode fouling and improve electron-transfer kinetics without using “redox mediators”. Despite advantages of numerous carbon nanomaterials, there is still significant interest in the development of novel carbonaceous materials to accelerate the development of electroanalytical procedures. The screen-printing technology has widely performed and it provides attractive opportunity for the development of miniaturized low cost disposable sensors strips. However, screen printed carbon electrodes (SPCE) without any treatment display meager performance due to non-electroactive components originate from ink formulations and relatively low graphitic content imposed by restrictions of screen-printing. The performance of SPCE depends on the surface properties and nature of ink. It could be improved by treating carbon electrodes using chemical / physical processes for e.g. acid / alkali, oxygen / argon plasma treatments. Oxygen plasma treatment is an efficient method in the field of surface modification that effectively grafts oxygen atoms leads to enhanced oxygen functionality onto the carbon surface. Compared to other chemical modification, oxygen plasma treatment has advantages of shorter reaction time and non-deprave process.
Here, a method is reported for NADH oxidation using oxygen plasma treated screen printed carbon electrodes (OPSPCE) without employing “redox mediator”. The introduction of oxygen-containing functional sites on carbon electrodes during plasma treatment is confirmed by Infrared and Raman spectroscopy (Figure not included) techniques. Fig.IA&B shows scanning electron microscopic (SEM) images of SPCE and OPSPCE respectively. On viewing, it clearly showed that after plasma treatment, surface roughness is marginally augmented than the untreated one. To confirm the changes and biocompatibility of carbon surface, contact angle (CA) measurement was performed. Fig.IC&D shows CA measurement of SPCE and OPSPCE, which are measured to be 107.1o and 39.1orespectively. The higher CA value attributed to hydrophobic nature of SPCE whereas OPSPCE showed a lower CA value indicated hydrophilic nature of the surface (i.e. oxygen plasma treatment introduced the hydrophilic groups on SPCE) that may greatly enhance the biocompatibility. Fig.IE showed the cyclic voltammetric response of respective materials carried-out in 5 mM NADH containing phosphate buffer solution (PBS) at a scan rate of 0.05 V/s. NADH oxidation peak potential was observed at 370 mV at OPSCE with a threefold increase in peak current, whereas oxidation occur at 500 mV in SPCE. Therefore a decrease in overpotential of 130 mV with enhanced peak current is visualized due to the benefit of oxygen treatment. It is proposed that new functional groups are created on SPCE surface after breaking -C=C bonds that generates more edge plane sites during the treatment process [3].
Fig. II A shows steady state current response, which is obtained for electro-oxidation of NADH at OPSPCE, with a successive addition of 3 μM of NADH in under strirred 0.1 MPBS. Steady state current response for NADH oxidation is examined by fixing the potential at 0.37 V. The resulted current response is directly proportional to NADH concentration and calibration plot of current vs concentration is shown in Fig. II B. The plot shows a good linear fit for a wide range of concentration 3to27 μM, with regression coefficient (R2) of 0.99 and a lower detection limit of 0.6 μM (S/N=3), which is better than other reports [4]. The NADH sensor showed remarkable analytical characteristics such as lower detection limit, higher sensitivity and fast response time (<3s), suggests that OPSPCE may hold great potential application in designing NAD+ biosensors.

Figure caption

Figure (I) shows SEM images of (A) SPCE (B) OPSPCE, Contact angle photograph of (C) SPCE (D) OPSPCE and (E) Cyclic-voltammograms of SPCE and OPSPCE in 5 mM NADH contain PBS (pH-7.4, 0.1M) scan rate 50 mV/s. (II) (A) Amperometric response of OPSPCE to successive addition of 3 μM NADH in 0.1 M PBS (B) corresponding calibration plot.

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