Revealing acid-induced enamel damage: Deep UV Raman and multivariate analysis of hydroxyapatite loss

Deep ultraviolet (DUV) Raman spectroscopy was employed to investigate molecular alterations in human enamel after orthophosphoric acid etching. This etching process served as an in vitro model for hydroxyapatite loss and surface conditioning. By leveraging resonance enhancement and effective fluorescence suppression, the technique enabled the acquisition of high-quality spectra directly from intact tooth surfaces. Spectral analysis revealed a consistent reduction in the phosphate band (∼950 cm^-1) and a relative increase in the protein-associated amide band (∼1620 cm^-1) in treated regions, resulting in a decreased mineral-to-organic ratio (I₉₅₀/I₁₆₂₀). These changes are indicative of localized hydroxyapatite demineralization and exposure of the organic matrix. The study underscores the importance of intra-tooth comparisons to minimize inter-sample variability and accurately characterize localized chemical changes. Partial Least Squares Discriminant Analysis (PLS-DA) was employed as chemometric tool to highlight the most discriminant spectral regions and guide band selection for ratio metrics, paving the way for a future formal validation of predictive performance. These findings support DUV Raman spectroscopy as a non-destructive, label-free approach for sensitive assessment of early structural changes in dental enamel, with potential implications for diagnostic and restorative dentistry.