Amperometric nitrite sensor based on free-standing carbon nanotube/methylene blue composite paper

Free-standing paper-like electrochemical sensors have many advantages such as high flexibility, thickness adjustability, and large electroactive surface area, when compared to traditional metal electrodes. Generally, carbon nanomaterials, e.g., graphene and carbon nanotubes (CNTs), are main materials of paper-like sensors, and these materials are supported by different species such as metal nanoparticles, dyes, and polymers. We present the fabrication procedure of free-standing composite paper electrode including CNTs and methylene blue (MB) with a simple vacuum filtration process of a suspension including oxidized CNTs (CNTs-ox) and MB through a suitable membrane. CNTs-ox/MB paper was peeled from membrane, and this paper was electrochemically reduced to improve its electrical conductivity. Characterization of this paper-like electrode was performed by using scanning electron microscopy, scanning tunneling microscopy, electron-dispersive X-ray analysis, powder X-ray diffraction spectroscopy, Raman spectroscopy, UV-vis absorption spectroscopy, four-point probe electrical conductivity, and cyclic voltammetry techniques. Amperometric response of the composite paper electrode for determination of nitrite was linearly proportional to nitrite amount in the range of 12-2500 mu M, with a detection limit of 3.6 mu M. The sensitivity of the free-standing CNTs/MB paper sensor was calculated as 0.40 mu A mu M-1 cm(-2).