Photoacoustic spectroscopy: a promising technique to investigate magnetic fluids

In this paper, photoacoustic spectroscopy is used as a promising experimental technique to investigate biocompatible magnetic fluids. The observed spectral features, in the 0.3 to 1.0 μm wavelength region, are discussed in terms of three distinct bands (L, S, and C). Band-L, in the spectral range of 0.70 to 0.90 μm, is related to the characteristic coating layer of the magnetic nanoparticle. Band-S, occurring around 0.55 μm, is related to the metal-polyoxy-hydroxy surface layer built up at the surface during the nanoparticle synthesis. Band-C (0.30-0.50 μm) is associated to the optical absorption of the semiconductor magnetic nanoparticle. Features in band-C are discussed in terms of energy levels in the quantum-confined structure. Distinct Fe₃O₄ nanoparticles (average diameters of 3.0 and 8.5 nm) are used as core materials to address the nanoparticle size effect.