Screen-Printed Biochar-Derived Graphene Solid-Contact Ion-Selective Electrodes for Nutrient Monitoring in Hydroponic Solutions

Scalable manufacturing of graphene-based solid-contact ion-selective electrodes (SC-ISEs) is constrained by reliance on mined graphite and on patterning methods that are costly, energy-intensive, or poorly suited to high-throughput production. Here, we present a sustainable, manufacturing-ready approach in which graphene derived from hardwood biochar is formulated into printable inks and patterned by high-throughput screen printing to produce bioderived graphene solid-contact ion-selective electrodes (BioG-SC-ISEs). Unlike prior biomass-derived electrodes based on amorphous or activated carbons, this work demonstrates the conversion of biochar into graphene-like nanosheets compatible with scalable printing. By functionalizing the screen-printed electrodes with poly(vinyl chloride) (PVC)-based ion-selective membranes doped with distinct ionophores, we produced sensors capable of selectively monitoring six plant nutrients (K+, Na+, NH4+, Ca2+, Mg2+, and NO3-). The resulting sensors exhibited near-Nernstian sensitivities with detection limits between 0.064 and 0.526 mg·L-1 with wide sensing ranges (10-6 to 10-1 M for monovalent ions, 10-5 to 10-1 M for Mg2+, and 10-5 to 10-2 M for Ca2+). As an application-relevant demonstration, K+ BioG-SC-ISEs tracked progressive potassium depletion across modified Hoagland formulations (100%, 50%, and 25% K+), achieving a sensitivity of 57.4 ± 0.7 mV·dec-1 and a detection limit nearly three orders of magnitude below typical hydroponic potassium levels, with measurements in close agreement with inductively coupled plasma optical emission spectrometry (ICP-OES). Overall, this work establishes a sustainable strategy for scalable SC-ISE manufacturing and provides a versatile platform for high-performance ion sensing with direct implications for hydroponic agriculture, environmental monitoring, and other electrochemical sensing applications requiring selective ion detection.