Pinilla Cienfuegos, ElenaMascaró Burguera, LucasTorres Cavanillas, RamónEchavarría, J. IgnacioRegueiro, AlejandroCoronado, EugenioHernández Rueda, Francisco Javier2026-03-132026-03-132026-02-03E. Pinilla-Cienfuegos, L. Mascaró-Burguera, R. Torres-Cavanillas, J.I. Echavarría, A. Regueiro, E. Coronado, and J. Hernandez-Rueda, “Unveiling Spin Transition at Single-Particle Level in Levitating Spin Crossover Nanoparticles,” ACS Nano 20(6), 5044–5053 (2026).1936-085110.1021/acsnano.5c18794https://hdl.handle.net/20.500.14352/133996Copyright © 2026 The Authors. SEJIGENT/2021/039. PRTR-C17.I1. MFA/2022/050. MFA/2022/025. PROMETEO Program CIPROM/2024/51. TSI-069100-2023-0012.The ability to control and understand phase transitions of individual nanoscale building blocks is key to advancing the next generation of low-power reconfigurable nanophotonic devices. To address this critical challenge, molecular nanoparticles (NPs) exhibiting spin crossover (SCO) phenomenon are trapped by coupling a quadrupole Paul trap to a multispectral polarization-resolved scattering microscope. This contact-free platform simultaneously confines, optically excites, and monitors the spin transition in Fe(II)–triazole NPs in a pressure-tunable environment, eliminating substrate artifacts. Thus, we demonstrate light-driven manipulation of the spin transition in levitating NPs, enabled by laser heating and free of substrate-induced effects. Using the robust spin bistability near room temperature of our SCO system, we quantify reversible optovolumetric changes of up to 10%, revealing precise switching thresholds at the single-particle level. Independent pressure modulation produces a comparable volume increase, confirming mechanical control over the same bistable transition. These results constitute full real-time control and readout of spin states in levitating SCO NPs, with operating conditions compatible with ultralow-power optical switching, data storage, and nanoscale sensing.engAttribution 4.0 Internationalhttp://creativecommons.org/licenses/by/4.0/journal article1936-086Xhttps://dx.doi.org/10.1021/acsnano.5c18794https://pubs.acs.org/doi/10.1021/acsnano.5c18794open access535538.9Laser-induced spin transitionMolecular materialsMultispectral scattering microscopyQuadrupole trapSingle-nanoparticle sensingSingle-particle photonicsSpin crossoverÓptica (Física)Física de materiales2209 Óptica33 Ciencias Tecnológicas