Sensitive detection of meat spoilage VOCs and visual monitoring of freshness via a UiO-66-NH2@MR nanocomposite-based colorimetric paper sensor

Abstract

Ensuring food freshness is essential for public health, highlighting the need for advanced food safety detection technologies. Recently, pH colorimetric sensors based on metal–organic frameworks (MOFs) have attracted increasing attention. However, challenges remain regarding their sensitivity and practical applicability. In this work, methyl red (MR) is employed as a colorimetric indicator and immobilized onto UiO-66-NH₂ to fabricate UiO-66-NH₂@MR nanocomposites, which exhibits broad pH-responsive performance across the pH range of 2.0–11.0. To enable portable detection, these nanocomposites are integrated with filter paper to develop a paper-based sensor (denoted as UNMPS). Using ammonia (NH₃), dimethylamine (DMA), and trimethylamine (TMA) as representative spoilage-related volatile organic compounds (VOCs), the UNMPS demonstrates high sensitivity, a wide detection range, and good specificity, attributable to the high surface area and porosity of UiO-66-NH₂ coupled with the pH sensitivity of MR. In the concentration range of 1–1000 ppb, the sensor shows excellent linear responses to NH₃, DMA, and TMA, with detection limits of 0.826, 0.546, and 0.926 ppb, respectively. Furthermore, the color difference (ΔE) of UNMPS shows a strong correlation with total volatile basic nitrogen (TVB-N) contents in shrimp and beef, yielding R² of 0.9446 and 0.9245, respectively. The proposed UNMPS exhibits high sensitivity, satisfactory repeatability, reproducibility, and stability. Hierarchical cluster and principal component analysis further confirm its reliability for real-time monitoring of meat freshness. Overall, UNMPS provides a convenient and efficient visual platform for food freshness assessment.