Chemical characterization of the dentin/adhesive interface by Fourier transform infrared photoacoustic spectroscopy

Irreversible bonding of composite materials to tooth structure depends on chemical as well as mechanical adhesion. The proposed bonding mechanism for several commercial dental adhesives is chemical adhesion to the dentin surface. The purpose of this in vitro investigation was to characterize the chemical nature of the surface interaction between dentin and two commercial adhesives by use of Fourier transform infrared photoacoustic spectroscopy (FTIR/PAS). The occlusal thirds of the crowns of freshly extracted, non-carious, unerupted human molars were sectioned perpendicular to the long axis. Dentin disks, 6 mm x 2 mm, were prepared from these sectioned teeth. The exposed dentin surface was treated with either Scotchbond 2, a BIS-GMA resin, or Dentin-Adhesit, a polyurethane resin. All spectra were recorded from 4000 to 400 cm-1 by use of an Analect RFX-65 FTIR spectrometer equipped with an MTEC Photoacoustics Model 200 photoacoustic cell. An initial spectrum of the dentin surface was collected. This surface was primed according to manufacturer's instructions and spectra recorded of the primed surface plus one to three layers of adhesive. By comparison of these spectra, it was possible for us to record changes in the phosphate and amide I and II bands due to surface interactions between the adhesive and the dentin. Although early results do not indicate covalent bonding between the dentin and these adhesives, this technique presents several advantages for spectroscopic evaluation of the dentin/adhesive interface.