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 (Na₂Ti₃O₇, 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.