We studied the effect of hydrogen in yttrium nanoparticles on a quartz substrate, using optical spectroscopy and electrical resistance measurements. Pulsed laser deposition is used to obtain the Y clusters in an UHV environment. We show, that these clusters are highly sensitive to monoatomic Hi produced from ambient hydrogen gas pressures, ranging fron 10(-5) to 50 mbar with our experimental arrangement. The changes of optical and electrical properties due to the chemical reaction within the particles are sufficient to consider this material as a possible sensor for low concentrations of hydrogen.