A portable sensing platform enabled by permanganate-peroxide coupling for time-resolved analysis of xanthine oxidase...

Early cancer diagnosis demands rapid, sensitive and portable approaches for detecting cellular metabolic activities. Herein, we construct a portable time-resolved biosensing platform for visual and quantitative detection of xanthine oxidase (XOD) activity in circulating tumor cells (CTCs) based on a permanganate-peroxide coupling mechanism. In this strategy, XOD catalyzes the oxidation of xanthine to uric acid, accompanied by the generation of hydrogen peroxide (H₂O₂). The as-produced H₂O₂ subsequently reduces potassium permanganate (KMnO₄) under mildly acidic conditions, resulting in a rapid decrease in the characteristic absorption of Mn(VII) species and a visually distinguishable color transition from purplish red to light yellow within 60 s. Since the yield of H₂O₂ generated is directly related to enzymatic turnover, XOD activity can be quantitatively translated into optical and digital colorimetric signals. The reaction can be monitored spectroscopically through absorbance changes at 526 nm or analyzed using smartphone-based RGB extraction for instrument-free quantification. Under optimized conditions, the optical response shows a good linear correlation with the logarithm of XOD activity with a detection limit of 0.0039 U/L (S/N = 3). Cellular studies revealed elevated XOD activity in cancer-derived CTCs compared with normal cells, as well as significant responses to XOD inhibition and oxidative stress. This platform provides a simple and portable tool for cellular enzyme profiling and point-of-care cancer diagnostics.