Rational design of CuCo-ZIF@HHTP heterojunction nanozyme via MOF-on-MOF strategy for efficient colorimetric sensing of quercetin

Abstract

Quercetin (QR) is a natural flavonoid widely found in fruits, with notable antioxidant, anti-inflammatory, and antiviral activities. However, excessive intake of QR may cause adverse effects, so it is essential to develop an accurate, sensitive and efficient detection method. In this work, a novel CuCo-ZIF@HHTP heterostructured nanozyme was rationally constructed by in situ growth of conductive HHTP-based metal-organic framework (MOF) on bimetallic CuCo-ZIF via a MOF-on-MOF strategy. This hierarchical architecture combines the high surface area and metal diversity of zeolitic imidazolate frameworks (ZIFs) with the excellent π-conjugation and electrical conductivity of 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP), resulting in enhanced peroxidase-like activity due to synergistic interactions between Cu/Co centers and π-conjugated ligands. The increased surface roughness leads to greater exposure of catalytic active sites, while the presence of mixed-valence metal species (Cu⁺/Cu²⁺ and Co²⁺/Co³⁺) promotes efficient redox cycling and facilitates the generation of reactive oxygen species (ROS). Based on these merits, a robust colorimetric sensing platform was developed using the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) by H₂O₂, which was significantly inhibited by QR due to its reductive nature. The system achieved a low detection limit of 0.17 μM and demonstrated high selectivity, reproducibility (the relative standard deviation (RSD) < 3 %), and long-term catalytic stability. Moreover, the platform was successfully applied to the detection of QR in real food samples with satisfactory recoveries ranging from 95.5 % to 104.6 %, confirming its analytical reliability and practical applicability. This work provides an effective strategy for designing MOF-based nanozymes and a promising platform for practical colorimetric biosensing applications.