Hydrogen sensing measurements. The sensors' LoD determination was performed in a custommade reactor chamber (effective volume ca. 1.5 mL) equipped with two fused silica viewports (1.33‖ CF Flange, Accu-Glass) that enabled transmission-mode optical monitoring. The detail of the chamber is reported in ref. . The transmission measurements were carried out through fibercoupled, unpolarized halogen light source (AvaLight-HAL-S-Mini) and a high-resolution visible range spectrophotometer (Avantes Sensline Avaspec-HS-TEC). The H 2 gas concentration was controlled by adjusting the flow rate ( [mL/min]) ratio of 1000 ppm H 2 (diluted in Ar) and 100% Ar using mass flow controllers (MFCs, Bronkhorst El-Flow Select series), see Supplementary Table . All experiments were carried out at constant 30°C, regulated via a PID controller (Eurotherm 3216) in a feedback loop manner, where the sample surface temperature inside the chamber was continuously used as input. As readout, the LSPR peak descriptors (λ peak ) were obtained following the method we established earlier. In detail, a Lorentzian fit was applied to the wavelength range at ±60 nm around the LSPR peak in the measured optical extinction spectra. Despite the asymmetry of the global LSPR peak, a good fit (R 2 > 0.97) was obtained, and thus the fit is appropriate to determine the λ peak (Supplementary Figures and).