Fabrication of self-supporting nitrogen-doped graphene microelectrodes for in situ analysis of salicylic acid in plants

Abstract A needle electrochemical sensor was developed based on a nitrogen-doped graphene (NDG) microelectrode for the analysis of salicylic acid (SA) in plants. Self-supporting NDG microelectrodes were synthesized by electron-assisted hot filament chemical vapor deposition using ethanol and nitrogen as the C and N sources, respectively. The formation mechanism of the NDG microelectrodes is discussed by analyzing the ethanol decomposition and nitrogen-doping processes. The microelectrodes were composed of few-layered NDG nanosheets, and the N content of the nanosheets varied in the 0.96–12.5 at% range. Furthermore, a novel needle sensor was constructed based on a cylindrical three-electrode system consisting of a Pt wire, optimized NDG microelectrode, and Ti tube with diameters of approximately 0.3, 0.9, and 1.5 mm, respectively, arranged concentrically. The needle sensors were also assembled into a parallel array consisting of four sensors. At a pH range of 4.5–7.5, the NDG-microelectrode-based needle sensor array exhibited a low detection limit (0.32–0.14 μA μM−1, S/N = 3), wide linear range (1–500 μM), and excellent reproducibility, selectivity, and stability for the detection of SA. The NDG-based needle sensors and sensor arrays are reliable electrochemical platforms for the detection of SA in plant tissue both in vivo and in vitro.