Capillary-coupled flow for tunable electrochemical sensing and rapid analysis in a paper-based platform

Continuous-flow paper-based electrochemical platforms offer attractive opportunities for portable chemical analysis owing to their simplicity, low cost, and pump-free operation. However, passive regulation of capillary-driven fluid transport remains challenging, limiting the simultaneous achievement of rapid analysis and high electrochemical sensing performance. Herein, we introduce a capillary-coupled flow (CCF) strategy in which a laminated barrier-free filter-paper channel is coupled with an underlying secondary porous pathway to passively regulate the capillary driving force without external pumps or electrical power. By simply varying the number of tissue layers beneath the primary paper channel, the carrier flow can be systematically regulated, enabling tuning of FIA-like peak characteristics and electrochemical response. Under the optimized four-layer configuration, the platform achieved a flow rate of 23.6 ± 2.3 μL min^-1, an analysis time of 35 s, and a sample throughput of 103 analyses h^-1, while exhibiting high fabrication reproducibility (1.9 % RSD), analytical precision (2.8-3.0 % RSD), and operational durability (260 injections). Compared with a conventional single-layer paper channel, the capillary-coupled architecture generated substantially higher amperometric peak currents while maintaining rapid analysis. The incorporation of a replaceable high-capacity cotton-wool waste sink further enabled sustained continuous operation for up to 1 h 40 min, with uninterrupted operation readily restored by replacing the saturated sink. Fabricated by a simple paper-stacking and hot-lamination process at an estimated material cost of approximately USD 0.04 per device, the proposed CCF strategy provides a simple and versatile approach for passively regulating capillary-driven transport in continuous-flow paper-based electrochemical systems.