Development of a hydrogel-based colorimetric sensor using Cu(II)- Bathocuproine disulfonate reagent for total antioxidant capacity determination.

Oxidative stress leads to the accumulation of reactive oxygen species, initiating radical reactions that damage biomolecules and contribute to various diseases. Consuming antioxidant-rich foods is an effective strategy to mitigate these effects, increasing the importance of assessing antioxidant capacity in food extracts. Optical sensors are particularly attractive for this purpose due to their rapid, cost-effective, miniaturizable, and on-site analysis capabilities. In this study, a hydrogel bead-based optical sensor was developed for the measurement of total antioxidant capacity (TAC) using the Cu(II)-bathocuproine disulfonate (BCDS) system. Acrylamide-based hydrogels were employed due to their low cost, ease of preparation, and ability to absorb large amounts of water while maintaining structural integrity. The Cu(II)-BCDS complex was immobilized onto the beads, enabling sensitive response to antioxidants. Upon antioxidant exposure, Cu(II)-BCDS was reduced to Cu(I)-BCDS, resulting in a visible color change that allowed TAC determination with small sample volumes under mild conditions. The linear equation of the calibration graph obtained for trolox (TR) was found to be ∆E = 0.0499c + 9.8359, r = 0.9993 (ΔE: the total color difference, c: concentration (μM)). The LOD value for TR was found to be 67.2 μM. Unlike solution-based TAC assays, the sensor works even in turbid solutions since color develops in the hydrogel. TAC values were determined for spiked black tea infusions with acceptable recovery and precision. The colorimetric response was further evaluated using a smartphone-based application. This sensor bears the future potential of integration with portable readers after storage stability tests, and provides a practical and low-cost measurement platform for routine antioxidant capacity analysis.