Engineering a pH-responsive fluorometric system: Dual-emission carbon dots and BTB for selective urea sensing

Abstract

Reliable detection of urea levels is critical for tracking kidney health, identifying diseases, and studying essential physiological functions. Here, we present a novel, straightforward, and selective fluorometric method for urea detection based on a dual-emission carbon dots system coupled with bromothymol blue (BTB) as a pH-sensitive indicator. The sensing platform consists of uniquely engineered carbon dots exhibiting dual emission peaks at 442 nm and 612 nm, working in conjunction with BTB in the presence of urease. The detection mechanism relies on urease-catalyzed hydrolysis of urea, which generates ammonia and subsequently increases the local pH. This pH elevation triggers a distinct color transformation of BTB from yellow (430 nm) to blue (615 nm), simultaneously causing a drop in fluorescence intensity at 612 nm and a rise at 442 nm, enabling ratiometric detection of urea. The developed method demonstrates excellent analytical performance with good linearity (R² = 0.9937) across a concentration range of 10.0–100.0 μM and achieves a detection limit of 2.86 μM. Notably, the method exhibits remarkable selectivity in complex biological matrices, successfully quantifying urea in human serum samples with high recovery values despite the presence of potential interferents. Furthermore, we extended the application of this sensing system to develop a smartphone-based colorimetric detection platform, enhancing its accessibility for point-of-care diagnostics. This dual-mode detection method provides a fast and dependable tool for urea monitoring in clinical and biological applications.