Abstract
Biomarkers are commonly used in both basic research and clinical practice as diagnostic tools. Particularly, cancer biomarkers can be applied to the early detection and diagnosis or screening the tumors. Of special note, malignant tumors can release serotonin (5-hydroxytryptamine, 5-HT). In fact, 5-HT is an excellent biomarker for carcinoid tumors since its level in blood plasma is hardly affected by other parameters. Moreover, secretion of 5-HT gives rise to elevated levels of 5-hydroxyindole-3-acetic acid (HIAA), the major metabolite of 5-HT, in urine samples of the patients with carcinoid tumors. 5-HT and HIAA are regarded as eligible biomarkers, of which concentrations in blood plasma and urine samples can be utilized for prognosis, and early diagnosis of carcinoid syndrome. Here, we unveil the facile synthesis and characterization of new Tb(III)-doped carbon dots (CDs) in the present work. Strikingly, CDs exhibited superior features for sensing 5-HT and HIAA. It is of worth to note that the Tb(III)-doped CDs induce phosphorescent response to both 5-HT and HIAA in water. Limit of detections (LODs) for 5-HT and HIAA were found to be 0.44 nM and 0.49 nM, respectively. Remarkably, these are the lowest reported values for luminescent systems to date. Moreover, CDs were utilized as chemosensors for 5-HT and HIAA in simulated blood plasma and synthetic urine samples, respectively. To this purpose, extensive luminescence measurements were performed. Accordingly, the obtained results attested that CDs were promising chemosensors for the detection of both 5-HT and HIAA in simulated blood plasma and synthetic urine samples, respectively. Last but not least, we successfully demonstrate that Tb(III)-doped CDs can be used for the detection of 5-HT in artificial cerebrospinal fluid as well as in conditions mimicking the environment within secretory vesicles where 5-HT is stored. To the best of our knowledge, this is the first example of CDs which can induce remarkable phosphorescent response to 5-HT and HIAA in biological fluids.
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