Unlocking room-temperature bistable spin transition at the nanoscale: the synthesis of core@shell [Fe(NH 2 trz) 3 (NO 3 ) 2 ]@SiO 2 nanoparticles.

In this work, we address the synthesis of stable spin-crossover nanoparticles capable of undergoing a hysteretic spin transition at room temperature. For this purpose, we use the reverse-micelle protocol to prepare naked [Fe(NH 2 trz) 3 ](NO 3 ) 2 and core@shell [Fe(NH 2 trz) 3 ](NO 3 ) 2 @SiO 2 nanoparticles. Through meticulous adjustment of synthetic parameters, we achieved nanoparticle sizes ranging from approximately 40 nm to 60 nm. Our findings highlight that [Fe(NH 2 trz) 3 ](NO 3 ) 2 presents a modest thermal hysteresis of 7 K, which decreases by downsizing. Conversely, silica-coated nanoparticles with sizes of ca. 60 and 40 nm demonstrate a remarkable hysteretic response of approximately 30 K, switching their spin state around room temperature. Moreover, the presence of a SiO 2 shell substantially enhances the nanoparticles' stability against oxidation. In this context, the larger 60 nm [Fe(NH 2 trz) 3 ](NO 3 ) 2 @SiO 2 hybrid remains stable in water for up to two hours, enabling the observation of an unreported water-induced spin transition after 30 min. Therefore, this work also introduces an intriguing avenue for inducing spin transitions through solvent exchange, underscoring the versatility and potential of these nanoparticles.