Development of a High-Sensitivity Electrochemical Immunoassay Using a Fully 3D-Printed Electrocatalytic Microelectrode Probe Platform

Electrochemical biosensors are a promising route to point-of-care diagnostics, yet their translation is hindered by the need for electrode surface functionalization, which introduces variability, increases cost and production complexity, and limits scalability and stability. Additive manufacturing using conductive filaments for rapid fabrication of three-dimensional (3D) electrodes overcomes these limitations. This study evaluates composite filaments comprising polylactic acid (PLA), carbon black (CB), and multiwalled carbon nanotubes (MWCNTs) for fused filament fabrication (FFF) of electrochemical electrodes for indirect detection via TMB⁺ measurement. Both the filament composition and electrode size were directly compared to determine the most suitable electrode architecture for sensitive measurements in complex samples. As a key novelty, the electrode diameter was reduced from 1 mm to the microscale to investigate the influence of electrode size on electron transfer efficiency. To improve measurement consistency and throughput, a 3D-printed assay accessory ("The Consistent Dipper") was developed to guide electrode immersion and reduce movement during amperometric measurements. PLA/MWCNT microelectrodes exhibited increased current density and reduced background noise compared to carbon black filament electrodes. The MWCNT microelectrodes were subsequently applied to a cardiac troponin I (cTnI) electrochemical immunoassay and in undiluted human serum, a cTnI detection limit of 7.4 pg mL^-1 was achieved, representing an approximately 19-fold improvement compared to PLA/MWCNT macroelectrodes (140 pg mL^-1). Following optimization to reduce incubation times, a clinically relevant detection limit of 85 pg mL^-1 was obtained within a total assay time of 1 h. By combining enhanced electrochemical performance with low-cost, flexible FFF-printed microelectrodes, this platform provides a scalable route to rapid immunodiagnostics. This study represents the first application of microscale 3D-printed PLA/MWCNT electrodes for clinical biomarker detection using a readily manufacturable sensor system.