The robust and practical nature of this electrosynthetic method was demonstrated by the gramscale preparation of Pt NP assembly (fig. ). After synthesis, Pt NP assembly was subjected to critical point drying to prepare a dry powder (fig. ). The electrochemically active surface area measured by hydrogen underpotential deposition of Pt NP assembly powder was 70 ± 1 cm 2 /mg, ~80% lower than that of Pt NPs (340 ± 25 cm 2 /mg, fig. ), due to the loss of surface area when NPs are assembled. H2 sensors were prepared by drop-casting a mixture of Pt NP assembly powder and ethanol onto a commercial thermocouple, followed by drying in air (Fig. , insert). The sensor response is the temperature change (ΔT) of the catalyst-coated thermocouple upon exposure to H2. We found that the Pt NP assembly was highly active for catalyzing H2 oxidation in air. Fig. shows that the sensor temperature rapidly increased from room temperature of ~22°C to ~360°C in ~10 s, after being exposed to a stream of 4% H2 in air (i.e., the lower explosive limit of H2). Upon the removal of H2, the sensor temperature quickly returned to room temperature within 10 s. ΔT decreases in response to stepwise decreasing H2 concentrations ( C at the logarithmic scale (Fig. ). In contrast, when the same loading (~1 mg) of Pt NPs was used to prepare a catalytic H2 sensor, the sensor response is nearly negligible (< 1°C) even at 2 H C = 4% (Fig. , insert, and Fig. ). The low catalytic activity of Pt NPs is not caused by their surface ligands blocking the active Pt sites. We ran control experiments using various Pt catalysts, including ligand-free Pt black with a surface area of 144 ± 5 cm 2 /mg (fig. ), Pt powder, Pt NPs after ligand removal, and Adams' catalyst (PtO2) (fig. ). None of them shows similar or even close catalytic activity to Pt NP assembly, suggesting the high catalytic activity of Pt NP assembly originates from its unique structure.