Sub-micrometer chemical imaging of dental adhesive/dentin interfaces via mid-infrared photothermal microscopy

Dental adhesive bonding plays a critical role in restorative dentistry and ensures durable restoration. Despite advancements, bond failure remains a challenge due to polymerization heterogeneity at the adhesive-dentin interface, which is influenced by various factors such as micro-voids. Conventional analytical methods used for dental materials, such as micro-Raman and FTIR spectroscopy, have limitations, such as the interference of autofluorescence and lack of spatial resolution. This study employed mid-infrared photothermal (MIP) microscopy to visualize the molecular distribution and polymerization degree of dental adhesives at the sub-micrometer scale. MIP imaging enables high-resolution chemical mapping without interfering with autofluorescence. The present results revealed heterogeneous polymerization across the adhesive-dentin interface, with lower degree of conversion (DC) rates near the dentin owing to the presence of hybrid layers. Additionally, localized reductions in the DC rate were observed, likely caused by micro-voids that hindered polymerization due to oxygen-inhibited layers. These findings provide direct evidence of molecular heterogeneity in dental adhesives that was previously unattainable using conventional techniques. MIP microscopy offers enhanced spatial resolution and chemical specificity, making it a promising tool for studying biomaterial interfaces and improving adhesive formulations for reliable dental treatments.