A smartphone-integrated colorimetric sensor of total volatile basic nitrogen (TVB-N) based on Au@MnO2 core-shell nanocomposites incorporated into hydrogel and its application in fish spoilage monitoring

Total volatile basic nitrogen (TVB-N) content is the most widely used index for the assessment of fish freshness. Herein, a smartphone-integrated colorimetric sensor of TVB-N is developed based on a functional hydrogel loaded with [email protected]2 core-shell nanocomposites and β-d-glucose pentaacetate (β-d-GP). In the sensing hydrogel, [email protected]2 serves as a colorimetric substance and β-d-GP is a precursor of reducing agent. TVB-N can permeate the sensing hydrogel, causing an alkaline environment in which β-d-GP is hydrolyzed to produce β-d-glucose. A redox reaction between MnO2 and β-d-glucose can occur and thus the TVB-N can induce the etching of MnO2 layer of the [email protected]2. Owing to the change of localized surface plasma resonance (LSPR) effect, the color of AuNPs changing from purple to red is easily distinguished by the naked eye. To quantify the variation conveniently, the color information is digitized by a smartphone combined with RGB analysis. The hydrogel sensing platform is further used for real-time monitoring of fish spoilage successfully. The introduction of smartphone avoids the usage of expensive instrument and opens a practical way for cost-effective, non-invasive, and portable monitoring of foodstuff freshness.