Micro-wheels about 7 μm average diameter and 2.5 μm thick were formed by the self-assembly of tungsten oxide nanorods about 1.5 μm long and 10 nm in diameter. The micro-wheels were hydrothermally synthesized for the ppt level monitoring of highly toxic chlorine (Cl2) gas. The gas sensor based on the fabricated micro-wheels showed excellent performance within a wide range of Cl2 concentrations (100 ppb to 500 ppb), having a high response of ∼10- to 25-fold. This sensor also had an ultra-low detection limit of ∼18 ppt, which was significantly lower than its permissive environmental concentration. In addition, the transient stability of the sensor even after five cycles of switching on/off from air-to-gas demonstrated effective reusability of the device.
