Portable 3D-Printed Paper Microfluidic System with a Smartphone Reader for Fast and Reliable Copper Ion Monitoring

Copper ions (Cu2+) are the third most essential transition metal ions critical to human health. Rapid detection of Cu2+ in water and biological fluids is of significant importance. In this study, we develop a sensitive multi-channel paper microfluidic device integrated with a 3D-printed smartphone-based colorimetric reader for the rapid detection of Cu2+. A novel rhodamine derivative, 1-(N,N-dichloromethine) amino-4-rhodamine B hydrazine-benzimide (RBCl), exhibiting high selectivity and sensitivity to Cu2+, was synthesized and applied as the detection reagent. The interaction mechanism between RBCl and Cu2+ was investigated, revealing a structural transition from a colorless spirolactam (closed-ring) to an open-ring amide structure, resulting in a pink color upon Cu2+ binding. A multi-channel paper microfluidic device with eight detection zones was fabricated, enabling the simultaneous analysis of eight samples. To enhance portability and quantification, a 3D-printed smartphone colorimetric reader was integrated, providing a rapid and efficient detection platform. The system achieved highly specific Cu2+ detection within 2 min, with a detection limit as low as 1.51 ng/mL, meeting water monitoring standards in most countries. Excellent recoveries were demonstrated in real samples, including tap water, river water, blood serum, and urine diluent. This integrated paper microfluidic system is highly sensitive and specific, offering a promising solution for water quality monitoring and health assessment through its rapid sample-to-answer capability.