Laser-Induced Graphene Microfluidic Integrated Sensors (LIGMIS) for In-Field Ion-Selective Electrochemical Sensing

Herein, we introduce a novel category of microfluidic sensors, termed Laser Induced Graphene Microfluidic Integrated Sensors (LIGMIS), which seamlessly integrate both the electrical and microfluidic components of electrochemical systems to develop multiplexed electrochemical sensors including solid contact ion selective electrode (SC-ISEs) sensors. With LIGMIS, both the microfluidics and connecting electrochemical electrodes are made of the same material, laser-induced graphene (LIG) fabricated simultaneously from the same standard carbon dioxide laser in ambient conditions on a single polyimide substrate. In relation to ion sensor development, we demonstrate that the surface wettability of the LIG substrate can be adjusted by tuning the lasing parameters. This adjustment enables the surface to exhibit hydrophobic (contact angle > 90°) and highly hydrophobic characteristics (contact angle ≈ 130°). Such modifications enhance the adhesion of the PVC-based ionophore membrane to the LIG surface and reduce the water layer at the LIG-membrane interface, thereby minimizing ion-selective sensor drift. Overall, the wettability, surface chemistry, and morphology of the LIG surface properties are measured and correlated with resultant low-frequency capacitance and water layer formation to pinpoint their effects on the ion selective sensor sensitivity (Nernstian response), reproducibility (E°’ variation), and potential stability of the LIG-based SC-ISEs. Finally, we demonstrate how LIGMIS can be used to selectively measure plant nutrient ions for on-site soil/water quality measurements. The ion-selective sensors exhibited near-Nernstian sensitivity across a broad concentration range (micromolar to millimolar) for nitrate, ammonium, calcium, magnesium, potassium, and sodium ions. These capabilities make LIGMIS a versatile and powerful tool for advancing in-situ environmental ion monitoring and analysis.