S-Doped Fe–N–C Single-Atom Nanozyme with Enhanced Oxidase-Like Activity for Colorimetric Detection of Serotonin

Serotonin (5-hydroxytryptamine, 5-HT) is a crucial neurotransmitter, the abnormal levels of which are closely associated with various neuropsychiatric disorders and male reproductive dysfunctions. In this study, a sulfur-doped Fe-N-C single-atom nanozyme (S@Fe-CN_<i>x</i>) was successfully synthesized, and its oxidase-like activity and catalytic mechanism were systematically investigated. The experimental results indicate that sulfur doping significantly enhanced Fe-CN_<i>x</i> oxygen activation capacity and catalytic kinetics by effectively optimizing the electronic structure of the FeN_<i>x</i> active centers. Leveraging its superior oxidase-like activity, we developed a H₂O₂-free colorimetric sensing method for the highly sensitive and selective detection of 5-HT. The method exhibited a good linear relationship between absorbance and 5-HT concentration in the range of 40 ng/mL to 10 μg/mL, achieving a low limit of detection of 6.0 ng/mL. The successful application of this method for detecting 5-HT in human serum samples demonstrates its great potential for practical use. Furthermore, we established a smartphone-based colorimetric detection platform that quantified 5-HT by correlating its concentration with the grayscale intensity of standard solution images captured by a smartphone. This approach provides a facile strategy for developing point-of-care testing platforms.