Spectroscopic Study of Maghemite Nanoparticles Surface-Grafted with DMSA

Nanosized maghemite (below 10 nm average diameter), surface-functionalized with meso-2,3-dimercaptosuccinic acid (DMSA), was investigated with respect to the content of DMSA molecules attached onto its surface and the onset of S−S bridges due to oxidation of neighboring S−H groups. To support our investigation, we introduced the use of photoacoustic spectroscopy to monitor thiol groups (S−H) conjugated with Raman spectroscopy to monitor the disulfide bridges (S−S). The normalized intensity (N R ) of the Raman feature peaking at 500 cm−1 was used to probe the S−S bridge whereas the normalized intensity (N P) of the photoacoustic band-S (0.42−0.65 μm) was used to probe the S−H moiety. The perfect linearity observed in the N R versus (1 − N P) plot strongly supports the oxidation process involving neighboring S−H groups as the DMSA surface grafting coefficient increases whereas the approach used in this report allows the evaluation of the [S−H]/[S−S] ratio. The observation of the reduction of the hydrodynamic diameter as the nominal DMSA-grafting increases supports the proposed model picture, in which the intraparticle (interparticle) S−S bridging takes place at higher (lower) DMSA-grafting values.