A Ratiometric Fluorescent Sensor Based on Silicon Quantum Dots and Silver Nanoclusters for Beef Freshness Monitoring

Abstract

A ratiometric fluorescent sensor with hydrogen sulfide (H₂S) and methanthiol (CH₃SH) sensitivity was developed to real-time monitor beef freshness. A silicon quantum dots (SiQD) and silver nanoclusters (AgNC) complex, namely SiQD-AgNC, was used as the dual emission fluorescence materials. Due to the fluorescence resonance energy transfer (FRET) effect between SiQD and AgNC, when the fluorescence of AgNC (610 nm) was quenched by H₂S or CH₃SH, the fluorescence of SiQD (468 nm) recovered, resulting in an increase of the fluorescent intensity ratio (I₄₆₈/I₆₁₀). I₄₆₈/I₆₁₀ showed a linear relationship with the H₂S concentration within the concentration range of 1.125-17 μM, with a limit of detection (LOD) value of 53.6 nM. Meanwhile, I₄₆₈/I₆₁₀ presented two linear relationships with the CH₃SH concentration within the concentration range of 1.125-17 μM and 23.375-38.25 μM, respectively, with a LOD value of 56.5 nM. The SiQD-AgNC complex was coated on a polyvinylidene fluoride (PVDF) film to form a portable SiQD-AgNC/PVDF film sensor. This film showed purplish red-to-cyan color changes in response to H₂S and CH₃SH, with LOD values of 224 nM and 233 nM to H₂S and CH₃SH, respectively. When the film was used to monitor beef freshness at 4 °C, its fluorescent color gradually changed from purplish red to cyan. Hence, this study presented a new ratiometric fluorescent sensor for intelligent food packaging.

Keywords: beef freshness; fluorescent sensor; intelligent packaging; silicon quantum dots; silver nanoclusters.

Figure 1

(A) The fluorescent spectrum of SiQD with different synthesis time. (B) TEM of SiQD (scale bar 20 nm). (C) Size distribution of SiQD. (D) The fluorescent spectrum of AgNC with different synthesis time. (E) TEM of AgNC (scale bar 20 nm). (F) Size distribution of AgNC.

Figure 2

(A) The emission spectra of SiQD at different excitation light. (B) The emission spectra of AgNC at different excitation light. (C) The maximum emission spectrum of SiQD and the maximum excitation spectrum of AgNC. (D) The emission spectrum of the SiQD-AgNC complex, and the fluorescent photos (insets) of the SiQD, AgNC, and SiQD-AgNC complex.

Figure 3

The detection principle of the SiQDs-AgNCs complex to hydrogen sulfide and mercaptan.

Figure 4

The changes of I₄₆₈/I₆₁₀ of SiQD-AgNC reacting with (A) H₂S and (B) CH₃SH under different pH. The changes of I₄₆₈/I₆₁₀ of SiQD-AgNC reacting with (C) H₂S and (D) CH₃SH under different ion strength.

Figure 5

The fluorescent spectra of SiQD-AgNC reacting with different concentration of (A) H₂S and (C) CH₃SH. The relation between I₄₆₈/I₆₁₀ and the concentrations of (B) H₂S and (D) CH₃SH.

Figure 6

(A) The ΔI values of SiQD-AgNC reacting with 340 μM of other volatile gases or 17 μM of H₂S and CH₃SH.(B) The ΔI values of SiQD-AgNC simultaneously reacting with 340 μM of other volatile gases in the presence of 17 μM of H₂S. (C) The ΔI values of SiQD-AgNC simultaneously reacting with 340 μM of other volatile gases in the presence of 17 μM of CH₃SH.

Figure 7

The fluorescence change of the SiQD-AgNC/PVDF film in response to (A) H₂S and (B) CH₃SH. The relation between ∆C of SiQD-AgNC/PVDF film and the concentrations of (C) H₂S and (D) CH₃SH.

Figure 8

(A) The visible light photos and (B) fluorescent light photos of a polypropylene packaging box integrated with a SiQD-AgNC/PVDF film, during storage at 4 °C. (C) The fluorescent light photos of the packaging system with beef sample, during storage at 4 °C. (D) The change of TVC values of beef during storage at 4 °C. (E) The relation between ∆C of SiQD-AgNC/PVDF film and TVC value of beef.