Flexible Laser-Induced Graphene for Nitrogen Sensing in Soil

Flexible graphene electronics are rapidly gaining interest, but their widespread implementation has been impeded by challenges with ink preparation, ink printing, and postprint annealing processes. Laser-induced graphene (LIG) promises a facile alternative by creating flexible graphene electronics on polyimide substrates through the one-step laser writing fabrication method. Herein, we demonstrate the use of LIG, created with a low-cost UV laser, for electrochemical ion-selective sensing of plant-available nitrogen (i.e., both ammonium and nitrate ions: NH4+ and NO3-) in soil samples. The laser used to create the LIG was operated at distinct pulse widths (10, 20, 30, 40, and 50 ms) to maximize the LIG electrochemical reactivity. Results illustrated that a laser pulse width of 20 ms led to a high percentage of sp2 carbon (77%) and optimal peak oxidation current of 120 μA during cyclic voltammetry of ferro/ferricyanide. Therefore, LIG electrodes created with a 20 ms pulse width were consequently functionalized with distinct ionophores specific to NH4+ (nonactin) or NO3- (tridodecylmethylammonium nitrate) within poly(vinyl chloride)-based membranes to create distinct solid contact ion-selective electrodes (SC-ISEs) for NH4+ and NO3- ion sensing, respectively. The LIG SC-ISEs displayed near Nernstian sensitivities of 51.7 ± 7.8 mV/dec (NH4+) and -54.8 ± 2.5 mV/dec (NO3-), detection limits of 28.2 ± 25.0 μM (NH4+) and 20.6 ± 14.8 μM (NO3-), low long-term drift of 0.93 mV/h (NH4+ sensors) and -5.3 μV/h (NO3- sensors), and linear sensing ranges of 10-5-10-1 M for both sensors. Moreover, soil slurry sensing was performed, and recovery percentages of 96% and 95% were obtained for added NH4+ and NO3-, respectively. These results, combined with a facile fabrication that does not require metallic nanoparticle decoration, make these LIG electrochemical sensors appealing for a wide range of in-field or point-of-service applications for soil health management.