Fabrication of photoresist SU-8 pyrolyzed carbon electrodes and their integration in a microfluidic hematocrit detection system

Abstract

Carbon microelectrodes are attractive for microfluidic biosensing due to their wide potential window, low overpotentials, chemical stability, and compatibility with low-cost microfabrication. Pyrolysis of photopatterned SU-8 offers a scalable route to carbon electrode fabrication; however, conventional two-step pyrolysis often leads to adhesion issues, geometric distortion, and limited surface area in low-aspect-ratio planar designs. These constraints restrict performance and broader adoption in analytical microdevices. Hematocrit determination, a clinically essential diagnostic measurement, requires electrodes with high reproducibility and robust electrochemical response. A simplified fabrication strategy that improves electrode surface area and performance while maintaining low-aspect-ratio planar geometries remains unresolved. We developed a three-step pyrolysis protocol that enables fabrication of low-aspect-ratio planar SU-8–derived carbon electrodes with both micro- and millimeter-scale features while improving structural integrity and surface morphology. The electrodes were integrated into a PDMS-based microfluidic device for hematocrit detection by measuring current responses of red blood cells suspended in phosphate-buffered saline at 100 V for 30 s. A linear correlation was observed between current and red blood cell concentration. The device achieved 3.6% precision and 3.8% accuracy for hematocrit determination, comparable to previously reported platinum-based systems (2.8% precision, 2.6% accuracy). Surface characterization via field-emission scanning electron microscopy and atomic force microscopy revealed a 25% increase in surface area relative to platinum electrodes, resulting from 2.7 times greater roughness and 58 times greater thickness. These structural enhancements reduced charge-transfer and concentration overpotentials, improving electrochemical performance. This work introduces a reproducible three-step pyrolysis method that overcomes long-standing limitations in the fabrication of low-aspect-ratio pyrolytic carbon electrodes. By increasing electroactive surface area without complex coatings or noble metals, the approach enables high-performance, low-cost microelectrodes suitable for integrated biosensing. The demonstrated equivalence to platinum electrodes in hematocrit detection highlights its translational potential for scalable diagnostic microdevices.