Freeze-thaw-induced interfacial deactivation of CeO2 nanozymes enables cascade colorimetric readout for cold-chain monitoring.

Tracking freeze-thaw (FT) history remains challenging once temperature-sensitive foods or specimens return to an apparently normal state, creating a need for reliable visual indicators. Herein, we report an interfacial deactivation-driven CeO2 nanozyme cascade for quantitative, intelligent, and visual discrimination of FT exposure. Ultrasmall CeO2 nanozymes synthesized through an L-carnosine-assisted low-temperature aqueous route possessed mixed Ce3+/Ce4+ valence states, oxygen-vacancy-related sites, and a hydrated interface. In neutral aqueous solution, the nanozymes catalyzed phosphoester cleavage, and ATP hydrolysis further triggered downstream chromogenic oxidation, establishing a neutral phosphatase-/oxidase-like cascade. Crucially, FT treatment markedly suppressed this upstream phosphatase-like initiation step, blocking ATP hydrolysis and switching off the colorimetric output. Control experiments ruled out low temperature alone, simple aggregation, Ce valence variation, and oxygen-defect changes as the dominant causes; instead, FT-induced perturbation of the hydroxylated and hydrated interface was identified as the main factor impairing the hydrolytic microenvironment. Leveraging this FT-sensitive cascade, multimodal readout strategies were developed for cold-chain sample monitoring. UV-vis analysis using TMB and ABTS as dual indicators enabled threshold-based discrimination between non-FT and FT-exposed samples. Smartphone imaging coupled with a convolutional neural network (CNN) achieved 95.0% accuracy in the verification set and 90.0% accuracy for real sample matrices. Finally, an equipment-free bend-to-mix CeO2-ATP-TMB label was fabricated for on-demand visual readout. This work converts FT-induced interfacial deactivation into an amplified colorimetric signal, providing a portable strategy for cold-chain monitoring.