H⁺ Exchange‐Driven ppb‐Level and High‐Selective Formaldehyde Detection at Room Temperature for Environmental and Clinical Applications

Formaldehyde is both a pervasive air pollutant and a critical breath biomarker for tumor‐related diseases, yet its reliable detection remains difficult due to ultralow concentrations and interference from ubiquitous volatile organic compounds (VOCs). Here, an H⁺‐exchange strategy is reported that markedly enhances the sensing performance of sodium titanate (Na 2 Ti 3 O 7 , NTO) by introducing abundant surface hydroxyl groups and tuning conduction pathways. In H⁺‐exchanged NTO (H‐NTO), hydroxyl groups act as selective adsorption sites for formaldehyde, while formaldehyde adsorption simultaneously suppresses surface‐proton and internal‐electron conduction by increasing the activation energy for proton hopping and generating electron‐trapping states. This dual modulation effectively eliminates cross‐sensitivity to other VOCs (e.g., methanol), enabling H‐NTO to achieve an ultralow detection limit of 2 ppb, a wide dynamic range up to 100 ppm, and stable operation over two months—contrasting with the negligible response of pristine NTO. To demonstrate practical utility, we developed a handheld H‐NTO prototype for wireless indoor air‐quality monitoring and non‐invasive breath‐based breast cancer screening. Coupled with machine learning, the system achieved high diagnostic accuracy, establishing H⁺‐exchange as a powerful route toward next‐generation intelligent formaldehyde sensors.