Bidirectionally regulated catalytic performance of Hemin/oCNT based aptasensor via salt-induced charge screening for smartphone-assisted colorimetric detection of ATP

The existing aptamer-regulated nanozymes for colorimetric detection primarily rely on unidirectional activity modulation. The role of salt ions in regulating nanozyme activity and the underlying mechanisms remain poorly understood, limiting their broader application. Herein, we report a novel bidirectional regulation strategy using a hemin-functionalized oxidized carbon nanotubes (Hemin/oCNT) nanozyme. Contrary to convertional approaches that seek to mitigate salt interference, we here leverage the salt-induced charge screening effect that high salt suppresses the peroxidase-like activity of Hemin/oCNT, yet paradoxically promotes aptamer binding to Hemin/oCNT. The formed aptamer/Hemin/oCNT complex exhibits restored and enhanced activity in high salt, attributed to improved dispersion, facilitated substrate (3,3′,5,5′-tetramethylbenzidine, TMB) adsorption and accelerated electron transfer. This bidirectional mechanism was harnessed to develop a one-step ‘mix-and-detect’ aptasensor for adenosine triphosphate (ATP). The assay achieves a wide linear range of 0.625–50 μM with a low detection limit of 300 nM (UV–vis). Furthermore, a smartphone-assisted dual mode (RGB/CMY, Red-Green-Blue/Cyan-Magenta-Yellow) platform was established, where the R channel achieved a superior detection limit of 190 nM, and the CMY mode possessed excellent precision analysis (RSD ≤ 2 %) owing to its signal stability. This sensor showed high selectivity for ATP and successful application in spiked diluted serum. This work represents a paradigm for designing high-performance biosensors by exploiting, rather than avoiding salt effects.