Portable silver-doped prussian blue nanoparticle hydrogels for colorimetric and photothermal monitoring of shrimp and fish freshness

Herein, we explored a colorimetric and photothermal readout strategy based on silver-doped prussian blue nanoparticles (SPB NPs) hydrogel for detection of trimethylamine (TMA) and monitoring of shrimp/fish freshness. The functional hydrogel is fabricated via the addition of SPB NPs to agarose hydrogel. TMA vapor can permeate into the functional hydrogel, thereby causing the decomposition of SPB NPs along with the color evolution (blue to colorless) and the disappearance of photothermal effects. The color information was digitized by a smartphone combined with RGB (red/green/blue) analysis. The photothermal effects was recorded by a handheld thermal imager. The ∆R gray value and temperature evolutions of the functional hydrogel were all closely related to the concentration of TMA, making it successful in the assessment of shrimp/fish freshness. Both colorimetric and photothermal signals are linear response to TMA concentration from 0.21 to 0.54 ppm. The application of smartphone and handheld thermal imager greatly improves the portability and practicality of monitoring on-spot. More significantly, the photothermal signal can still be applied to assess food freshness, when the hydrogel is contaminated by the colored food substrates and the colorimetric signal cannot be used to assessment. To our knowledge, this is the first time that a photothermal strategy has been developed to monitor food freshness. Therefore, this simple, cost-effective, nondestructive and sensitive dual-mode functional hydrogel will open up a new prospect for monitoring of food spoilage.