Antiswelling Charge-Transfer Hydrogels for Highly Sensitive Underwater Temperature Sensing

Underwater temperature monitoring is needed for marine exploration, early warning systems in aqueous environments, and temperature tracking during aquatic activities, yet it remains challenging for hydrogel-based sensors due to swelling-induced signal drift and limited underwater sensing performance. To address this, an antiswelling charge-transfer hydrogel incorporating a viologen/pyranine charge-transfer (VP-CT) complex was developed. The VP-CT complex was formed by charge-transfer interactions and equipped with a long alkyl chain, increasing the cross-linking density and hydrophobicity of the hydrogel to improve its water resistance. The charge-transfer interactions and hydrophobic interaction from VP-CT complex, along with hydrogen bonding from poly(vinyl alcohol) (PVA) and tannic acid (TA), endowed the prepared hydrogel with exceptional swelling resistance (equilibrium swelling ratios of −4.5% in water and −6.7% in artificial seawater after 15 days). The multiple ionic groups within the VP-CT complex facilitated high ionic conductivity (0.031 S cm–1 at 25 °C) of the hydrogel. Additionally, the hydrogel-based underwater temperature sensor displayed ultrahigh temperature coefficient of resistance (TCR) of −9.86% °C1– and stable underwater temperature monitoring. This sensor also successfully detected water gradient variations when attached to human skin and served as a key element in an early warning system for sudden temperature changes in aqueous environments. This work provides a robust strategy for high-performance underwater temperature sensing in marine, sports, and biomedical applications.