Magnetic composite – Based dual nanozymes derived from coffee ground waste: A sustainable approach for colorimetric assay and prolonged reuse

Nanozymes serve as an excellent substitute for traditional enzymes across various scientific domains, particularly in the colorimetric sensor array. Nonetheless, their tendency to aggregate into larger clusters and the challenges associated with recovery and reuse have limited their applicability, especially in the context of sustainable development. Based on the 3R principle “Reduce-Recycle-Reuse”, this study introduces a magnetic composite in which two typical nanozymes composed of silver nanoparticles and magnetite nanoparticles are immobilized on the activated carbon derived from spent coffee grounds using one-pot in-situ synthetic approach for easy recovery and long-term reuse. The successful synthesis of the targeted composite was verified through its characterization analysis techniques, including XRD, FT-IR, SEM-EDS, element mapping, HR-TEM, VMS, BET, and TGA. The experimental results revealed that the coexistence of these nanoparticles not only guarantees the magnetic property of the composite but also enhances its peroxide-mimicking activity in the catalytic oxidation of a reliable colorimetric substrate (3,3,5,5-tetramethylbenzidine) through the predominant of •OH radical. The conditions for optimizing the catalytic efficiency of the composite were also presented. Under this condition, the targeted composite showed high accuracy, selectivity, and anti-interference ability in colorimetrically quantifying the ascorbic acid concentration, as confirmed by high-performance liquid chromatography measurement. Notably, the catalytic activity, magnetic property, and fundamental characteristics of the proposed composite remained almost unchanged even after a series of at least 6 consecutive assays. This observation substantiates its remarkable capacity for prolonged reuse, a unique attribute not present in recently developed nanozymes.