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Here, we report a ligand\u2010driven strategy to modulate the\n T<\/jats:italic>\n 2<\/jats:sub>\n relaxivity (\n r<\/jats:italic>\n 2<\/jats:sub>\n ) of monodisperse 12\u00a0nm iron oxide\u2010based contrast agents synthesized by thermal decomposition. Five surface chemistries\u2013polyacrylic acid (PAA), poly(isobutylene\u2010alt\u2010maleic anhydride) (PMA), poly(maleic anhydride\u2010alt\u20101\u2010octadecene) (PMAO), citric acid (CA), and silica (SiO\n 2<\/jats:sub>\n )\u2500 were investigated under physiological conditions and in vivo using relaxometry (1.4\u00a0T), clinical (3\u00a0T), and UHF (9.4\u00a0T) MRI, achieving up to a 333\u00a0m\n m<\/jats:sc>\n \u2212<\/jats:sup>\n 1<\/jats:sup>\n \u00a0s\n \u2212<\/jats:sup>\n 1<\/jats:sup>\n increase in\n r<\/jats:italic>\n 2<\/jats:sub>\n . CA\u2010coated\n T<\/jats:italic>\n 2<\/jats:sub>\n contrast agents exhibited record\u2010high\n r<\/jats:italic>\n 2<\/jats:sub>\n values (522\u00a0m\n m<\/jats:sc>\n \u2212<\/jats:sup>\n 1<\/jats:sup>\n \u00a0s\n \u2212<\/jats:sup>\n 1<\/jats:sup>\n at 3\u00a0T; 381\u00a0m\n m<\/jats:sc>\n \u2212<\/jats:sup>\n 1<\/jats:sup>\n \u00a0s\n \u2212<\/jats:sup>\n 1<\/jats:sup>\n at 9.4\u00a0T) in spherical iron oxide MNPs within the superparamagnetic size range (\n d<\/jats:italic>\n \u00a0<\u00a020\u00a0nm). Correlations of\n r<\/jats:italic>\n 2<\/jats:sub>\n with hydrodynamic size, \u03b6\u2010potential, and coating thickness revealed that ligand chemistry\u2013specifically hydrophilicity and anionic surface charge\u2013dominates over physical shell dimensions in governing water accessibility and magnetic dephasing. This scalable ligand\u2010exchange strategy enables precise\n T<\/jats:italic>\n 2<\/jats:sub>\n tuning at UHF, with phantom results reliably predicting in vivo UHF\u2010MRI performance in rat brain models, advancing the design of neuroimaging nanoprobes.\n <\/jats:p>","DOI":"10.1002\/smll.202509792","type":"journal-article","created":{"date-parts":[[2026,2,3]],"date-time":"2026-02-03T10:35:08Z","timestamp":1770114908000},"update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":1,"title":["Ligand\u2010Driven Optimization of Iron Oxide Nanoprobes for In Vivo MRI Enhancement at Ultra\u2010High Field"],"prefix":"10.1002","volume":"22","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-0103-6772","authenticated-orcid":false,"given":"Pelayo","family":"Garc\u00eda\u2010Acevedo","sequence":"first","affiliation":[{"name":"NANOMAG Laboratory Applied Physics Department iMATUS Materials Institute and Health Research Institute of Santiago de Compostela (IDIS) Universidade de Santiago de Compostela Santiago de Compostela Spain"},{"name":"Neuroimaging and Biotechnology Laboratory (NOBEL) Clinical Neurosciences Research Laboratory (LINC) Health Research Institute of Santiago de Compostela (IDIS) Santiago de Compostela Spain"},{"name":"Advanced (magnetic) Theranostic Nanostructures Lab International Iberian Nanotechnology Laboratory Braga Portugal"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Mar\u00eda Luz","family":"Alonso\u2010Alonso","sequence":"additional","affiliation":[{"name":"Neuroimaging and Biotechnology Laboratory (NOBEL) Clinical Neurosciences Research Laboratory (LINC) Health Research Institute of Santiago de Compostela (IDIS) Santiago de Compostela Spain"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Sara","family":"Ortega\u2010Espina","sequence":"additional","affiliation":[{"name":"Neuroimaging and Biotechnology Laboratory (NOBEL) Clinical Neurosciences Research Laboratory (LINC) Health Research Institute of Santiago de Compostela (IDIS) Santiago de Compostela Spain"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Manuel","family":"Ba\u00f1obre\u2010L\u00f3pez","sequence":"additional","affiliation":[{"name":"Advanced (magnetic) Theranostic Nanostructures Lab International Iberian Nanotechnology Laboratory Braga Portugal"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Yolanda","family":"Pi\u00f1eiro","sequence":"additional","affiliation":[{"name":"NANOMAG Laboratory Applied Physics Department iMATUS Materials Institute and Health Research Institute of Santiago de Compostela (IDIS) Universidade de Santiago de Compostela Santiago de Compostela Spain"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Ram\u00f3n","family":"Iglesias\u2010Rey","sequence":"additional","affiliation":[{"name":"Neuroimaging and Biotechnology Laboratory (NOBEL) Clinical Neurosciences Research Laboratory (LINC) Health Research Institute of Santiago de Compostela (IDIS) Santiago de Compostela Spain"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jos\u00e9","family":"Rivas","sequence":"additional","affiliation":[{"name":"NANOMAG Laboratory Applied Physics Department iMATUS Materials Institute and Health Research Institute of Santiago de Compostela (IDIS) Universidade de Santiago de Compostela Santiago de Compostela Spain"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"311","published-online":{"date-parts":[[2026,2,3]]},"reference":[{"key":"e_1_2_8_2_1","doi-asserted-by":"publisher","DOI":"10.1002\/adma.201804567"},{"key":"e_1_2_8_3_1","doi-asserted-by":"publisher","DOI":"10.1002\/adma.201906539"},{"key":"e_1_2_8_4_1","doi-asserted-by":"publisher","DOI":"10.1021\/cr100025t"},{"key":"e_1_2_8_5_1","first-page":"1727","article-title":"Nanoparticles in Magnetic Resonance Imaging: From Simple to Dual Contrast Agents","volume":"10","author":"Estelrich 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