Quantitative analysis of adhesive resin in the hybrid layer using Raman spectroscopy

Abstract

The objective was to determine absolute molar concentration of adhesive resin components in the hybrid layer by establishing methods based on Raman spectroscopy fundamentals. The hybrid layer was treated as a three-component system consisting of collagen and an adhesive resin containing two monomers. Adhesive standard specimens and Raman peak area ratios obtained with a 785 nm excitation wavelength were used to construct separate calibration curves for comonomer relative molar concentration and Bis-GMA absolute molar concentration. As collagen and water had no measurable peaks in the fingerprint region, a dilution coefficient K(j) was defined to describe their impact on Raman peak area and to calculate HEMA absolute molar concentration. Methodology was validated using an analogous system containing acetone/ethanol/water. The absolute molar concentration of Bis-GMA and HEMA decreased 87% and 83%, respectively, from the top quarter to the middle of the hybrid layer. Additionally, less Bis-GMA penetrated the hybrid layer than HEMA, as indicated by the approximately 20% decrease in comonomer molar concentration ratio between the adhesive resin layer and the top half of the hybrid layer. Lack of complete monomer infiltration will further challenge dentin-adhesive bond longevity. (c) 2010 Wiley Periodicals, Inc. J Biomed Mater Res, 2010.

Figure 1

Characteristic peaks in Raman spectrum of HEMA and Bis-GMA: internal reference peak at 605 cm^-1, C=C double bond reactive peaks at 1410 and 1640 cm^-1, Bis-GMA benzene ring at 1610 cm^-1, and carbonyl peak at 1720 cm^-1. No characteristic peaks were identified in the Raman spectrum of the demineralized dentin. (Figure 2 in Ref 25)

Figure 2

Molar concentration calibration curves for photopolymerized system (Bis-GMA and HEMA with CQ/EDMAB photoinitiator system) based on the Raman peak area ratio of the Bis-GMA aromatic ring peak at 1610 cm^-1 and the methacrylate CCO peak at 605 cm^-1.

Figure 3

Absolute molar concentration calibration curve for Bis-GMA. The peak area of the Bis-GMA aromatic ring peak at 1610 cm^-1 was normalized by the standard with the highest Bis-GMA concentration (98 wt % Bis-GMA, 1 wt % CQ, 1 wt % EDMAB). Note: linear best fit is shown as dashed line.

Figure 4

Transmission electron micrograph of the resin-dentin hybrid layer depicting where Raman spectral data were analyzed. Note the electron-lucent spaces around cross-sectioned collagen fibrils that contain adhesive resin. AR: adhesive resin, THL: top of hybrid layer, TQHL: top quarter of hybrid layer, MHL: middle of hybrid layer, BQHL: bottom quarter of hybrid layer, BHL: bottom of hybrid layer. Resin composite not shown. (Original magnification 16,000 ×) (Modified from Figure 2 in Ref 22)

Figure A1

Raman spectrum of acetone, ethanol, and water: acetone characteristic peak at 780 cm^-1 (CCC stretching) and ethanol characteristic peak at 880 cm^-1 (CCO symmetric stretching). No characteristic peaks were identified in the Raman spectrum for water.

Figure A2

Molar concentration calibration curve for homogeneous acetone/ethanol system based on the Raman peak intensity ratio of acetone peak intensity at 780 cm^-1 and the sum of the ethanol peak intensity at 880 cm^-1 and acetone peak intensity at 780 cm^-1.

Figure A3

Absolute molar concentration calibration curve for ethanol. The peak intensity of the ethanol peak at 880 cm^-1 was normalized to the peak intensity of pure ethanol. Note: linear best fit is shown as dashed line.

Figure A4

Comparison of the molar concentration of ethanol calculated from the calibration curves in Figures A2 and A3 and the true molar concentration of ethanol in the acetone/ethanol/water three-component system. Note: linear best fit is shown as dashed line.

Figure A5

Comparison of the true molar concentration of acetone in the acetone/ethanol/water three-component system with the molar concentration of acetone calculated from the calibration curves in Figures A2 and A3 and the dilution coefficient K_j. Note: linear best fit is shown as dashed line.