Spectroscopic Study of Maghemite Nanoparticles Surface-Grafted with DMSA

Nanosized maghemite (below 10 nm average diameter), surface-functionalized with meso-2,3-dimercapto-succinic 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 mu m) 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.