Facile synthesis and ozonation of carbon dots using mango pulps for dual-mode colorimetry and ratiometric fluorescence detection of sarcosine

We synthesized excitation-dependent fluorescent carbon dots (CDs) through a one-pot hydrothermal process using mango pulps as precursors and discovered that the ozone-treated CDs (O3-CDs) could stabilize the diimine form of oxidized 3,3′,5,5′-tetramethylbenzidine (diimine oxTMB). A dual-mode colorimetry and ratiometric fluorescence method using O3-CDs and TMB as probes was developed for detecting sarcosine. During measurement, sarcosine was selectively oxidized by sarcosine oxidase, and the resulting hydrogen peroxide was catalyzed by horseradish peroxidase to convert TMB into a blue-colored oxidized product. Under acidic conditions, this product was further oxidized to diimine oxTMB, which exhibited an absorbance band at 450 nm. Due to the inner filter effect, the fluorescence emission of O3-CDs at 450 nm was significantly quenched by the diimine oxTMB, while the emission at 550 nm was utilized as a reference signal. The dual-mode method selectively detected sarcosine, with detection limits of 0.034 µM and 0.096 µM for colorimetry and ratiometric fluorescence detection, respectively. The ratiometric fluorescence sensing strategy provided a built-in correction to prevent errors originating from environmental variabilities. Moreover, the stabilization of diimine oxTMB mediated by O3-CDs improved the detection performance of the method. This study enriches the library of luminescent CDs through the rational utilization of natural biomass and introduces a novel approach to improve the analytical performance of dual-mode detection.