Sensitive detection of Staphylococcus aureus enterotoxin B (SEB) is of importance for preventing food poisoning from threatening human health. In this work, an electrochemical and colorimetric dual-signal detection assay of SEB was developed. The probe (Ab2/AuPt@Fe-N-C) was bound to SEB captured by Ab1, where the Ab2/AuPt@Fe-N-C triggered methylene blue degradation and resulted in the decrease of electrochemical signal. Furthermore, the probe catalyzed the oxidation of 3, 3’, 5, 5’-tetramethyl biphenyl to generate a colorimetric absorbance at 652 nm. Once the target was captured and formed a sandwich-like complex, the color changed from colorless to blue. SEB detection by colorimetric and electrochemical methods showed a linear relationship in the concentration ranges of 0.0002–10.0000 and 0.0005–10.0000 ng/mL, with limits of detection of 0.0667 and 0.1670 pg/mL, respectively. The dual-signal biosensor was successfully used to detect SEB in milk and water samples, which has great potential in toxin detection in food and the environment.

Chymosin is one of the critical enzymes in cheese making. Herein, we proposed a novel fluorometric assay for chymosin determination. Firstly, covalent organic frameworks (COF) were synthesized and exfoliated to 2-dimensional COF nanosheets (COF NS) by ultrasound treatment. Gold nanoparticles (Au NPs) were loaded with COF NS to prepare AuNPs/COF NS (Au@COF NS). Secondly, rhodamine B (RhB) modified substrate peptide (Pep) for chymosin was linked with Au@COF NS to construct a Pep-Au@COF NS nanocomposite. For the sensing principle, fluorescence of RhB was quenched by Au@COF NS and the fluorescence intensity was weak due to the fluorescence resonance energy transfer between COF NS and RhB of Pep. However, in the presence of chymosin, the RhB was released by specific cleavage of the substrate peptide by chymosin and resulted in the recovery of fluorescence. The increased fluorescence intensity was proportional to the increase of chymosin concentration and thus a "turn on" fluorescent sensor for chymosin was constructed. The sensor showed a linear range in the concentration of 0.05-60 μg/mL for the detection of chymosin with a detection limit of 20 ng/mL. The sensor was used to quantify chymosin in rennet product with good selectivity, which has the potential applications in cheese manufacturing.

Norovirus (NoV) is regarded as one of the most common causes of foodborne diarrhea in the world. It is urgent to identify the pathogenic microorganism of the diarrhea in short time. In this work, we developed an electrochemical and colorimetric dual-mode detection for NoV based on the excellent dual catalytic properties of copper peroxide/COF-NH₂ nanocomposite (CuO₂@COF-NH₂). For the colorimetric detection, NoV can be directly detected by the naked eye based on CuO₂@COF-NH₂ as a laccase-like nonazyme using “peptide-NoV-antibody” recognition mode. The colorimetric assay displayed a wide and quality linear detection range from 1 copy/mL to 5000 copies/mL of NoV with a low limit of detection (LOD) of 0.125 copy/mL. For the electrochemical detection of NoV, CuO₂@COF-NH₂ showed an oxidation peak of copper ion from Cu⁺ to Cu²⁺ using “peptide-NoV-antibody” recognition mode. The electrochemical assay showed a linear detection range was 1‒5000 copies/mL with a LOD of 0.152 copy/mL. It’s worthy to note that this assay does not need other electrical signal molecule, which provide the stable and sensitive electrochemial detection for NoV. The electrochemical and colorimetric dual-mode detection was used to detect NoV in foods and faceal samples, which has the potential for improving food safety and diagnosing of NoV-infected diarrhea.