Spectroscopic Investigation of 5-FU Association at Chitosan-Stabilized Au Nanoparticle Interfaces

Chitosan-stabilized gold nanoparticles (Au NPs-Chi) are candidate carriers for combined chemo-photothermal therapy because of their tunable surface properties and ability to incorporate therapeutic molecules. Understanding how formulation parameters influence interfacial organization and the chemical environment of the polymer-rich interfaces is important for controlling drug-nanocarrier interactions and for designing Au NPs-Chi as chemo-photothermal drug carriers. In this study, Au NPs-Chi loaded with 5-fluorouracil (5-FU) were prepared using different gold precursor and chitosan concentrations to investigate how formulation composition affects nanoparticle morphology, interfacial chemistry, and 5-FU association and retention. The systems were characterized by STEM, UV–vis, ATR-FTIR, and XPS. Mean STEM diameters ranged from 9.1 ± 0.2 nm (15Au-1Chi) to 22.0 ± 7.0 nm (10Au-0.5Chi), the LSPR band appeared at 527–534 nm, and all suspensions exhibited positive zeta potentials of +48 to +62 mV. Formulations prepared with the intermediate precursor concentration showed the most uniform nanoparticle morphology. Spectroscopic investigations indicated that 5-FU-associated changes occurred primarily within the chitosan-rich interfacial layer, while no clear evidence of a dominant direct interaction with the metallic Au core was observed. Fluorine from 5-FU (F 1s, 0.6–1.5 at %) was detected in all loaded samples and was absent from the unloaded formulations. End-point free-drug measurements under physiological saline conditions revealed formulation- and loading-time-dependent variations in the detectable free 5-FU fraction, with normalized ratios ranging from 0.5 to 1.8 relative to the 1 h value. After 1 h of incubation, 8–25% of the added 5-FU was detected in the medium. For the 15Au-1Chi formulation loaded for 4 h, the normalized free-drug ratio remained between approximately 1.0 and 1.2 over the investigated incubation period. These results relate formulation composition to the chemical environment of 5-FU in the chitosan-rich interfacial layer and provide physicochemical information relevant to the further development of Au NPs-Chi systems for combined chemo-photothermal applications.