New insights into non-contact reflectance IR mapping of teeth.

Teeth are made of highly mineralized tissues that withstand years of daily usage. Their mechanical properties have been studied at all length scales with ageing, but mainly small areas have been the focus of high-sensitivity chemical property quantification. A comprehensive, spatially resolved examination across centimeter sized tooth specimens is needed to expand our comprehension of young versus aged teeth. Specular Reflectance Fourier Transform Infrared Spectroscopy (srFTIR) is a nondestructive microscopy technique that can be used to characterize the chemical composition of mineralized surfaces. This technique provides spatially resolved chemical absorption information, making it useful to study both organics and mineral in visibly accessible tooth tissues. Highly polished dehydrated cross-sectional surfaces of bovine and human teeth were srFTIR imaged, and spectra were processed using a Kramers-Kronig transformation to generate absorbance-like chemical maps. Results are comparable with the well-established FTIR Transmission and Attenuated Total Reflection (ATR) methods. Maps of multiple ν₃-phosphate vibrations, ν₂-carbonate, and amide I reveal insights into structural and chemical variations across dentine. Young bovine teeth exhibited many chemical and structural similarities to old, partially sclerotic human teeth, affirming their use as proxies in dental research. This work highlights the capability of specular reflection infrared spectroscopy imaging to reveal spatial chemical information and expand our understanding of tooth microstructure and composition variations.