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<p>A dual-mode optical freshness indicator based on a copper metal–organic framework (Cu-MOF) integrated with soy protein isolate (SPI) and immobilized on a cellulose matrix (Cu-MOF/SPI@CM) was developed for real-time monitoring of fish spoilage. The sensing mechanism relies on selective coordination interactions between copper (Cu<sup>2</sup>⁺) active sites and volatile spoilage biomarkers, namely ammonia (NH₃) and hydrogen sulfide (H₂S), leading to distinct colorimetric transitions and fluorescence quenching. The indicator exhibited multistage visible color changes from blue (fresh) to yellow–brown tones (spoiled), while fluorescence intensity decreased without emission peak shift, consistent with a static quenching mechanism supported by Stern–Volmer analysis and ultraviolet–visible (UV–Vis) spectroscopy. Colorimetric limits of detection (LOD) were 1.05 μM for H₂S and 1.51 μM for NH₃, while fluorometric LOD values reached 0.63 μM and 1.14 μM, respectively. Adsorption kinetics followed a pseudo-second-order model, indicating chemisorption-dominated interactions, with faster response toward H₂S due to stronger Cu–S coordination. Fourier transform infrared (FT-IR) spectroscopy confirmed analyte-induced coordination changes, and pH-dependent UV–Vis and fluorescence analyses demonstrated enhanced structural stability imparted by the SPI matrix. Application to real fish samples stored at room temperature (60 h) and under refrigeration (10 days) showed progressive, temperature-dependent optical responses. Strong linear correlations were obtained between total color difference (ΔE) and key spoilage indicators, including total volatile basic nitrogen (TVB-N), pH, and total viable count (TVC), confirming predictive reliability. Compared with previously reported metal–organic framework-based sensors, the proposed system provides a balanced combination of broad visual detectability, quantitative fluorescence response, and improved environmental stability. This dual-mode biodegradable platform offers a practical strategy for intelligent seafood packaging and real-time freshness assessment.</p>

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A Cu-MOF–soy Protein Isolate Integrated on Cellulose Matrix for Dual-mode Colorimetric and Fluorescence Tracking of Fish Freshness

  • 期刊论文

  • 发表日期：

    2026年4月14日

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  • Zahra Teymouri,
  • Hajar Shekarchizadeh

摘要

A dual-mode optical freshness indicator based on a copper metal–organic framework (Cu-MOF) integrated with soy protein isolate (SPI) and immobilized on a cellulose matrix (Cu-MOF/SPI@CM) was developed for real-time monitoring of fish spoilage. The sensing mechanism relies on selective coordination interactions between copper (Cu2⁺) active sites and volatile spoilage biomarkers, namely ammonia (NH₃) and hydrogen sulfide (H₂S), leading to distinct colorimetric transitions and fluorescence quenching. The indicator exhibited multistage visible color changes from blue (fresh) to yellow–brown tones (spoiled), while fluorescence intensity decreased without emission peak shift, consistent with a static quenching mechanism supported by Stern–Volmer analysis and ultraviolet–visible (UV–Vis) spectroscopy. Colorimetric limits of detection (LOD) were 1.05 μM for H₂S and 1.51 μM for NH₃, while fluorometric LOD values reached 0.63 μM and 1.14 μM, respectively. Adsorption kinetics followed a pseudo-second-order model, indicating chemisorption-dominated interactions, with faster response toward H₂S due to stronger Cu–S coordination. Fourier transform infrared (FT-IR) spectroscopy confirmed analyte-induced coordination changes, and pH-dependent UV–Vis and fluorescence analyses demonstrated enhanced structural stability imparted by the SPI matrix. Application to real fish samples stored at room temperature (60 h) and under refrigeration (10 days) showed progressive, temperature-dependent optical responses. Strong linear correlations were obtained between total color difference (ΔE) and key spoilage indicators, including total volatile basic nitrogen (TVB-N), pH, and total viable count (TVC), confirming predictive reliability. Compared with previously reported metal–organic framework-based sensors, the proposed system provides a balanced combination of broad visual detectability, quantitative fluorescence response, and improved environmental stability. This dual-mode biodegradable platform offers a practical strategy for intelligent seafood packaging and real-time freshness assessment.