Chemical modification of PVA-based Fricke gel dosimeters: the effect of salicylic acid derivatives on dose sensitivity and diffusion control

Fricke gels are chemical dosimeters that may enable the measurements of three-dimensional (3D) dose distributions due to the exposure to ionizing radiation. One limitation of Fricke Gels is the loss of spatial and temporal dosimetric information due to ferric (Fe3+) ions diffusion and ferrous (Fe2+) ions oxidation. Although chelating agents were employed to slow diffusion by forming coordination complexes with Fe3+ ions, the issue remains unresolved. To further reduce this phenomenon, this work aims to investigate Fe3+ ligands that allow covalent functionalization of the polyvinyl alcohol (PVA) hydrogel matrix in a few steps. Starting from 5-Sulfosalicylic acid (SSA), two salicylic acid derivatives, i.e. Acetyl-4-aminosalicylic acid (Acetyl-4-ASA) and Acetyl-5-aminosalicylic acid (Acetyl-5-ASA), were used as ligands to form Fe3+ complexes. Their optical properties and the complex stability over the time were investigated and compared to those of SSA. In all three metal complexes, the optical response increased linearly with the concentration of Fe3+ ions. However, Acetyl-5-ASA Fe3+-complex showed very low stability over time compared to Acetyl-4-ASA and SSA. Based on these results, PVA was chemically functionalized with 4-ASA via a two-step reaction and then jellified using glutaraldehyde. The functionalized matrix showed effective dosimetric behavior, characterized by a linear response up to at least 21 Gy and a 25% decrease in Fe3+ diffusion relative to the non-functionalized counterpart. Overall, this study demonstrates that polymer functionalization via chelating ligands is a viable strategy for enhancing the spatial stability of Fricke gel dosimeters, providing a rational pathway toward high-resolution, clinically relevant 3D dosimetry.