[Fe:1].[CH2:2]1[C:7](=[O:8])[N:6]([O:9][C:10]([CH2:12][I:13])=[O:11])[C:4](=[O:5])[CH2:3]1>OP([O-])(O)=O.OP([O-])([O-])=O.[Na+].[Na+].[Na+].[Cl-].[Cl-].[K+].[K+].CS(C)=O>[O-2:5].[Fe+2:1].[CH2:3]1[C:4](=[O:5])[N:6]([O:9][C:10]([CH2:12][I:13])=[O:11])[C:7](=[O:8])[CH2:2]140 nm dextran-coated iron oxide nanoparticles (NP; 115,000 g/mole per iron core) were dissolved in 1× phosphate buffered saline (PBS; 137 mM NaCl, 10 mM Phosphate, 2.7 mM KCl, pH 7.4) at a concentration of 2 mg/mL. Vivotag-750 fluorophore was labeled on the NPs such that each NP has around 2 VT-750 fluorophores. The linker N-succinimidyl iodoacetate (SIA) was dissolved in DMSO at 20 mg/mL. The two solutions were mixed to obtain a 1-to-7 mass ratio between iron oxide NPs and SIA for 2 hr at room temperature with shaking. Size exclusion chromatography (column diameter×height=1 cm×30 cm; media: Sephadex G-50-coarse) was used to separate out the excess SIA and to exchange NPs into borate buffer (50 mM sodium borate, 5 mM EDTA, pH 8.3).
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