Smart dual-mode fluorescent label from rhodamine–alginate and fluorescein–chitosan for real-time shrimp freshness monitoring

Abstract

A dual-mode pH-responsive label was developed for real-time visual monitoring of shrimp freshness. The system integrates oxidized sodium alginate grafted with rhodamine B ethylenediamine (ORE) and chitosan grafted with fluorescein (CSF) into screen-printable inks, forming a dual-color platform with complementary colorimetric and inverse ratiometric fluorescence responses, where ORE emission decreases while CSF emission increases with rising pH. This inverse behavior enhances visual contrast and enables intuitive color discrimination. The label exhibited reversible color transitions—from pink to yellow (daylight) and from bright orange to green (UV) in response to pH and volatile amines showing high sensitivity, selectivity, and storage stability. Applied to shrimp packaging, its optical response correlated with total volatile base nitrogen (TVB-N) levels, effectively distinguishing fresh, sub-fresh, and spoiled samples. RGB analysis using a smartphone enabled rapid, on-site freshness evaluation. This dual-mode, printable sensing platform offers a practical and scalable approach for seafood freshness monitoring and intelligent food packaging.