Recyclable chloride trapping and sustained nicotinate release monitored by photoluminescence of Eu3+-doped layered yttrium hydroxide†

Nicotinic acid (NA; also known as niacin or vitamin B₃), an essential nutrient for humans, was inserted into the interlayer spaces of Eu³⁺-doped layered yttrium hydroxide (LYH:Eu). This hybrid system, NA-LYH:Eu, demonstrated remarkable efficiency in the storage of NA, offering long-term stability, reusability, and controlled, sustained release capabilities. Notably, the intercalated NA sensitized the luminescence of Eu³⁺ ions via the transfer of UV-energy absorbed for its π → π* transition from the S₀ ground state to the S₁ excited state, leading to the characteristic ⁵D₀ → ⁷F_J (J = 1–4) transitions. The resulting bright red emission could be reversibly quenched and restored through repeated NA release (chloride uptake) and re-inclusion cycles within the LYH:Eu interlayers, demonstrating the potential application as a chloride-induced corrosion inhibitor. The release rate of NA in a phosphate buffer solution could be modulated by tuning the crystallinity and particle size of the LYH:Eu host. When highly crystalline NA-LYH:Eu was employed, the in vitro release of NA continued beyond 5 days without reaching equilibrium. Concurrently, the red emission from a colloidal solution containing NA-LYH:Eu particles gradually diminished as the release progressed, providing a visually observable indicator. This luminescence ‘on/off’ behavior offers a practical method for in situ visual monitoring of NA inclusion and release.