Use of the photoacoustic spectroscopy in the investigation of the dilution process in surface-coated nanoparticles

In this study photoacoustic spectroscopy was used to investigate the effect of water dilution of a biocompatible magnetic fluid sample upon the nanoparticle surface-coating. The observed spectral features, in the 300–1000 nm wavelength region, are analyzed in terms of three distinct bands (L, S, and C). Band-L, in the spectral range of 650–900 nm, is related to the characteristic coating layer of the magnetic nanoparticle. Band-S, occurring around 470 nm, is related to the metal–polyoxy-hydroxy surface layer built up at the surface during the nanoparticle synthesis. Changes of band-L and band-S features upon water dilution of the stock magnetic fluid sample are discussed in terms of surface reorganization of the DMSA (dimercaptosuccinic acid) surface-coating agent with simultaneous surface release of hydroxyl-groups. The model picture of the surface reorganization is consistent with the picture of disruption of longer chain-like structures into shorter ones as a result of the sample dilution.