Chemical Availability and Reactivity of Functional Groups grafted to Magnetic Nanoparticles monitored In situ by ATR-IR Spectroscopy

Characterization of organic functional groups grafted to magnetic nanoparticles often causes considerable difficulty because of lack of information concerning their accessibility and chemical reactivity. Classical solid-state techniques only allow gaining qualitative information and the determination of upper bounds of the total loading of organic moieties. To overcome this limitation we applied attenuated total reflection (ATR) infrared spectroscopy in a specially designed mechanically stirred in situ batch reactor-cell. Using the example of iron oxide magnetic nanoparticles embedded in a silica matrix that have been functionalized by grafting aminopropyl groups, we elucidated the accessibility and reactivity of amino groups. Reactivity tests using the transformation of the amine to benzaldimine revealed that less than 40% of the introduced amino groups reacted to the corresponding benzaldimine indicating that the theoretical loading provides only an upper bound of active functional groups. The reactivity of these groups has been followed by observing the transformation of the amine to the corresponding benzaldimine, revealing population density, accessibility, and reactivity of these groups. The present in situ technique facilitates the monitoring of the transformation of grafted functional groups and simultaneously provides quantitative and structural information when the material is suspended in a solvent system.