The response and recovery times of Pt NP assembly sensor, defined as the time to reach 90% of the total signal, were ~7 s at 2 H C > 1% and extended to ~1 min at 0.05% (Fig. ). Such negative dependence of sensor response and recovery times on C is associated with a slow reaction rate between H2 and the sensing material. (4, 5, 7) Even though the response time of Pt NP assembly sensor does not meet the DOE target of 1 s, it is still comparable to, or better than, the performance of existing catalytic H2 sensors on the market (~10-30 s). It is also important to point out that the Pt NP assembly sensor is capable of discriminating different H2 concentrations in 1 s based on its transient temperature change (fig. ). Due to the self-heating effect of Pt NP assembly during catalytic H2 sensing, the sensor is operational over a wide ambient temperature range from -30°C to 80°C and a humidity range from 0 to 98% (Fig. , fig. ). In particular, when 2 H C > 2%, the sensor response, response and recovery times are not affected by the ambient temperature and humidity.