Portable dual-mode detection and visual quantification of terminal deoxynucleotidyl transferase activity in cells using smartphone

Terminal deoxynucleotidyl transferase (TdT) plays a pivotal role in lymphocyte development, antigen receptor diversity, and leukemia progression. In this study, a smartphone-assisted dual-mode detection system is introduced for the precise and visual quantification of TdT activity in cells. The system leverages the Ag(I)-o-phenylenediamine (OPD) reaction, where fluorescence intensity, UV–vis absorption and colorimetric changes generate multimodal signals. The inhibition of Ag(I)-catalyzed OPD oxidation results from the formation of stable C-Ag-C complexes, driven by strong interactions between Ag(I) and cytosine-rich DNA, which sequester free Ag(I) and shield its catalytic sites. Unlike other ions, Ag(I) specifically catalyzes OPD oxidation due to its elevated redox potential and noble metal properties, as evidenced by significant signal reduction and a distinct color transition. Furthermore, the integration of smartphone technology enables real-time RGB value analysis, creating a portable and cost-effective platform suitable for point-of-care diagnostics. The system demonstrates exceptional sensitivity, with detection limits of 0.00034 U/mL (fluorescence) and 0.0049 U/mL (UV-Vis), making it be conducive to the early detection of acute lymphoblastic leukemia. Owing to the intrinsic specificity of TdT elongation activity, the method exhibits high selectivity, accurately quantifying TdT activity in Molt-4 cells and distinguishing leukemia cells from normal cells. Additionally, the platform extends to inhibitor screening through the use of positive and negative small-molecule controls, demonstrating its practical utility in biochemical research. Compared to traditional TdT assay kits, the system offers superior sensitivity, enhanced diagnostic accuracy, and broader accessibility, representing a significant advancement in clinical diagnostics and personalized medicine.