The effect of the oxidation degree of multiwalled carbon nanotubes (MWCNTs) for the detection of NADH was evaluated in this paper. MWCNTs were oxidized by microwave‐assisted sulfonitic treatment at different times (5, 10, 15, and 30 min) and deposited onto a graphite screen printed electrodes. Oxidized MWCNTs were characterized and the electrochemical performance evaluated. The best sensor in terms of sensitivity and stability was obtained after 15 minutes of oxidation (SPE/CNT15). A significant reduction of the NADH oxidation potential was recorded for oxidized MWCNTs compared with unmodified MWCNTs (0.270 V and 0.500 V, respectively vs. Ag/AgCl pseudo reference electrode), increasing the selectivity of the system. Chronoamperometric calibration curves carried out applying a potential of 0.3 V for 1 min were linear in the 4–35 μM range of NADH. A limit of detection of 1 μM was achieved with negligible surface fouling (three consecutive calibration curves, 30 total measurements: slope decrease 5.9 %). Inter electrode reproducibility (n=13) was good resulting in RSD of 15.2 % and 5.0 % for the peak intensity and the oxidation potential, respectively. Quantification of glucose in white wine samples was carried out to demonstrate the ability of the NADH sensor to work in real samples. A good correlation with a spectrophotometric kit for the glucose quantification was achieved.
Hybrid nanomaterials have outstanding properties that are superior to the corresponding constituents working alone. This work reports on the electroanalysis of a hybrid material‐decorated screen‐printed carbon electrode (SPCE) that consists of iron nanoparticles supported at multi‐wall carbon nanotubes (MWCNT), coated with graphene layers, named Fe@G‐MWCNT. Electrochemical and morphological characterizations were carried out by cyclic voltammetry, electrochemical impedance spectroscopy and high‐resolution transmission electron microscopy, respectively. After optimizing the amount of hybrid material to be drop casted at the SPCE, its electrochemical activation in sulphuric acid produced an enhanced response. The resultant electrochemically reduced Fe@G‐MWCNT‐e‐modified electrode exhibited a diffusion‐controlled redox process with an enhanced heterogeneous electron‐transfer rate constant of 3.21×10−2 cm⋅s−1, which was superior to that from the MWCNT counterpart. However, it was slightly lower than that from a Fe‐MWCNT‐decorated electrode. The graphene coating limited slightly the electron‐transfer process, but works as a protective layer that prevent the loss of Fe catalytic activity. The electrochemical response of the hybrid with graphene coated Fe decreased only a 24.3 % after one week, respect to 51.9 % of the uncoated one. In addition, the hybrid material‐modified electrode exhibited electrocatalytic activity towards the reduction of H2O2 in a linear range of 0.5 mM to 9.8 mM, with sensitivity of 7.97 μA⋅mM−1 and LOD of 0.65 mM, thereby opening an avenue for the development of more specific and highly sensitive Fe@G‐MWCNT hybrid‐based (bio) sensors.
Nanomaterials and nanocomposites have gained relevance in science and technology due to their excellent properties. Therefore, the characterization of these materials is important. Thermogravimetric analysis is a powerful technique for the characterization of iron-carbon nanotubes (Fe/MWCNT) as hybrid nanomaterials, which may be prepared by impregnation step (alkaline or microwave-assisted precipitation). High-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD and in situ XRD), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA) were the instrumental techniques used to characterize these hybrid materials. Through TGA, it was possible to determine the quantity of effective impregnated iron on the MWCNT. Further, in a TGA, nitrogen atmosphere reveals a thermal event reflecting the iron reduction by C from MWCNT and the shape of the signal reflects the dispersion and size of the iron particles on the surface. This thermal event is related to the particle sizes and chemical nature of iron oxides present. Thermal events from TGA may be correlated with the results obtained from XRD, XPS, and HR-TEM. The presence of smaller and well-distributed iron nanoparticles impacts the shape of the reducing event in the TGA. The reduction temperature as observed in TGA curves is related to the nature of metal compounds present, such as nitrates or oxides. These results suggest that TGA can be used as a rapid and economical technique for the evaluation of different Fe/MWCNT hybrid material properties. These results may facilitate the estimation of the structural and chemical nature of the Fe/MWCNT nanohybrid materials and permit the projections of potential applications.
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