FRET-Based Dual-Color Carbon Dot Ratiometric Fluorescent Sensor Enables the Smartphone-Integrated Device for Noninvasive and Portable Diagnosis of Chronic Kidney Disease

Cystatin C (Cys C), a crucial renal disease marker for chronic kidney disease (CKD), plays a vital role in early diagnosis and treatment guidance. However, most current methods for detecting Cys C rely on a single signal and find it difficult to perform noninvasive and portable diagnosis. Here, we developed a ratiometric fluorescent carbon dot (CD) detection system for point-of-care testing (POCT) of Cys C through fluorescence resonance energy transfer (FRET). The detection is based on the hydrolysis effect of papain on a bovine serum albumin (BSA) scaffold and the specific inhibitory effect of Cys C on papain, endowing high-resolution color variance. Moreover, a low-cost, portable, yet reliable smartphone-assisted miniaturized device for real-time quantitative POCT of Cys C has been developed with a limit of detection (LOD) as low as 0.4 μg/mL. This sensing platform can effectively differentiate patients from healthy volunteers, which facilitates self-screening for healthy individuals and home monitoring for CKD patients.