Sensing nitrite with a glassy carbon electrode modified with a three-dimensional network consisting of Ni7S6 and multi-walled carbon nanotubes

We describe an amperometric sensor for nitrite that is based on a glassy carbon electrode modified with a 3-dimensional network consisting of Ni7S6 and multi-walled carbon nanotubes. The nickel sulfide was prepared by a hydrothermal method starting from nickel chloride and thiourea. The morphology and catalytic properties of the sensor material were characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, cyclic and linear sweep voltammetry, electrochemical impedance spectroscopy, and chronoamperometry. The results showed the Ni7S6/MWCNTs to possess improved catalytic activity towards the oxidation of nitrite when compared to plain Ni7S6. The sensor is best operated at 0.425 V (vs. Ag/AgCl) in 0.1 M NaOH solution where it shows a linear response in the 1.0 μM to 4.2 mM nitrite concentration range, with a sensitivity as high as 185.0 μA·mM−1·cm−2 and a 0.3 μM detection limit (at a signal-to-noise ratio of 3). These features are mainly attributed to the large specific surface area of Ni7S6, the good electrical conductivity of the MWCNTs, and the synergy between Ni7S6 and the MWCNTs. The method was applied to the determination of nitrite in (spiked) water samples where it gave recoveries that ranged between 98.6 and 100.1 %.