Surface engineering of a triboelectric nanogenerator for room temperature high-performance self-powered formaldehyde sensors

The development of miniaturized self-powered gas sensors to achieve real-time respiratory monitoring and analyses has tremendous clinical significance for early diagnosis. In this work, room temperature high-performance self-powered formaldehyde (FA) sensors based on a monolithic triboelectric nanogenerator (TENG) are demonstrated through surface modification strategies, involving the exploitation of novel 4,4 '-bipyridine (bpy)-functionalized phosphomolybdic acid (bpy-PMA) that can act as both a triboelectric layer and an active sensing layer, and incorporation of phenothiazine (PTZ) as the seed layer for the Ag nanoparticle (NP) electrode. Our results indicate that PMA can be an excellent candidate to act as both triboelectric and active sensing layers owing to its strong oxidizing properties. However, the hygroscopic nature of PMA inevitably imposes poor stability, thereby limiting its practical applications. To address this issue, novel 4,4 '-bipyridine (bpy)-functionalized phosphomolybdic acid (bpy-PMA) is exploited. The newly-developed bpy-PMA layer possesses excellent moisture resistance while maintaining an appropriate surface work-function (WF) value, enabling the resulting TENG to exhibit superior stability without compromising output performance and FA sensing ability. On the other hand, the use of PTZ/Ag NPs as the electrode layer exerts several advantages including an increase in effective contact area, improvement of triboelectric charge transfer, and excellent air stability. With all the advantages, the resulting TENG manifests a high power density of up to 8.93 W m(-2) and outstanding stability with open-circuit voltage nearly unchanged after 200 000 cycles of continuous operation. Meanwhile, such a TENG also delivers remarkable sensing properties for FA detection at room temperature. To the best of our knowledge, this is the first demonstration of TENG-based self-powered FA sensors, and the remarkable selectivity (130) and ultra-fast response time (approximate to 5 s) to a sub-ppm level achieved herein are superior to those of state-of-the-art FA sensors. More encouragingly, the detection of the FA concentration in exhaled breath is also demonstrated by integrating the large-area flexible TENG (contact area = 40 cm(2)) with a facemask. These promising results surpass those of the currently dominant gas sensing technologies and are believed to be at the forefront of self-powered room-temperature gas sensing technology. This work provides new insights and guidance in the development of next-generation self-powered wearable gas sensors.