A dual-mode sensing platform based on bifunctional iron-doped carbon dots for sarcosine detection

Sarcosine (SA) serves as a crucial biomarker for the diagnosis of prostate cancer, and its concentration in the body fluids of patients is strongly correlated with disease progression. Therefore, it is highly significant to develop a sensitive method for SA detection. In this study, iron-doped carbon dots (Fe-CDs) exhibiting peroxidase-like activity and bright yellow-green fluorescence were synthesized. A dual-mode sensing platform for the detection of SA was subsequently constructed based on the as-prepared Fe-CDs. Under the catalysis of sarcosine oxidase, SA underwent an oxidative reaction with molecular oxygen to produce formaldehyde and hydrogen peroxide (H₂O₂). The H₂O₂ was then decomposed by Fe-CDs with peroxidase-like activity, generating reactive oxygen species (ROS), which subsequently oxidized the colorless substrate N, N-diethyl-p-phenylenediamine sulfate (DPD) into pink-colored oxidized DPD (oxDPD). The resulting oxDPD displayed characteristic absorption peaks at 512 nm and 551 nm and concurrently quenched the fluorescence of Fe-CDs at 525 nm via fluorescence resonance energy transfer. Consequently, sensitive and quantitative detection of SA can be achieved by monitoring the reduction in fluorescence intensity and the corresponding increase in absorbance. This method demonstrates a detection limit of 0.023 μM and a linear range of 0.05-120 μM in the fluorescence mode, and a detection limit of 0.17 μM with a linear range of 1-50 μM in the colorimetric mode. Furthermore, this sensing platform was successfully applied to the quantitative detection of SA in human urine samples, with spiked recovery rates of 96.0%-109.2% for fluorescence and 91.0%-107.4% for colorimetry, highlighting its considerable potential for applications in disease diagnosis.