Long-Term Physicochemical Stability of Silver and Selenium Nanoparticles Designed for Caries Arrest: A 12-Month Evaluation

Purpose: To evaluate the 12-month physicochemical stability of silver and selenium nanoparticles (NPs) fabricated initially for topical caries arrest. Materials and Methods: The nanoparticles were synthesised via chemical reduction and capped with naturally derived agents. Post-synthesis suspensions were washed and stored at 4 °C in the dark for 12 months. Stability was assessed by comparing the physicochemical properties at baseline and 12 months. The characterisation approaches included Tyndall scattering and UV–visible spectroscopy, transmission electron microscopy, and inductively coupled plasma optical emission spectroscopy. Results: As formulations aged, they showed differences in stability. Citrate-capped silver NPs (AgL1) retained a defined surface plasmon resonance and showed a 6.7% reduction in elemental concentration. Silver NPs capped with casein (Ag1) and bovine serum albumin (Ag2) showed moderate reductions in elemental concentration (26.9% and 54.8%, respectively), accompanied by the formation of larger particle populations (~20–40 nm). Their initial mean particle sizes were 3.6 ± 3.4 nm for Ag1 and 2.5 ± 0.8 nm for Ag2. In contrast, silver NPs capped with citrate and tannic acid (AgL2) exhibited a substantial reduction in elemental concentration (88.6%), accompanied by pronounced particle aggregation. Selenium nanoparticles were the least stable: UV–Vis absorption features collapsed, TEM showed agglomeration (>70–100 nm), and ICP–OES indicated a significant reduction in the measured elemental concentration in the analysed suspension especially in Se1 (99.9%) and Se2 (99.96%). Conclusions: Under refrigerated, dark storage for 12 months, citrate-capped silver nanoparticles showed preserved integrity, whereas others showed moderate to severe issues with colloidal and elemental stability.