Integration of 3D printed Mg²⁺ potentiometric sensors into microfluidic devices for bioanalysis

Electrochemical sensors provide an affordable and reliable approach towards the detection and monitoring of important biological species ranging from simple ions to complex biomolecules. The ability to miniaturize electrochemical sensors, coupled with their affordability and simple equipment requirements for signal readout, permits the use of these sensors at the point-of-care where analysis using non-invasively obtainable biofluids is receiving growing interest by the research community. This paper describes the design, fabrication, and integration of a 3D printed Mg²⁺ potentiometric sensor into a 3D printed microfluidic device for the quantification of Mg²⁺ in low-sample volume biological fluids. The sensor employs a functionalized 3D printable photocurable methacrylate-based ion-selective membrane affixed to a carbon-mesh/epoxy solid-contact transducer for the selective determination of Mg²⁺ in sweat, saliva and urine. The 3D printed Mg²⁺ ion-selective electrode (3Dp-Mg²⁺-ISE) provided a Nernstian response of 27.5 mV per decade with a linear range of 10 mM to 39 μM, covering the normal physiological and clinically relevant levels of Mg²⁺ in biofluids. 3Dp-Mg²⁺-ISEs selectively measure Mg²⁺ over other biologically present cations - sodium, potassium, calcium, ammonium - as well as provide high stability in the analytical signal with a drift of just 13 μV h^-1 over 10 hours. Comparison with poly(vinylchloride)-based Mg²⁺-ISEs showed distinct advantages to the use of 3Dp-Mg²⁺-ISEs, with respect to stability, resilience towards biofouling and importantly providing a streamlined and rapid approach towards mass production of selective and reliable sensors. The miniaturization capabilities of 3D printing coupled with the benefits of microfluidic analysis (i.e., low sample volumes, minimal reagent consumption, automation of multiple assays, etc.), provides exciting opportunities for the realization of the next-generation of point-of-care diagnostic devices.