The detection of heavy metals in aqueous solutions has always attracted much attention from all over the world. A fluorescent probe of CdSe/CdS core-shell quantum dots (QDs) was designed to detect trace Cd²⁺ in aqueous solutions using the OFF–ON mode rapidly and efficiently, likely based on adsorption and desorption reactions between ethylenediaminetetraacetic acid disodium salt (EDTA) and CdSe/CdS QDs. In the OFF mode, the optical shielding function of EDTA results in fluorescence quenching owing to the strong adsorption ability of EDTA with Cd²⁺ on the sites of CdSe/CdS QDs surface. In the ON mode, the introduction of Cd²⁺ promotes the desorption of EDTA from the EDTA-CdSe/CdS QDs and restores the fluorescence intensity. There were two linear response ranges which were 0.1–20 µmol/L and 20–90 µmol/L for the EDTA-CdSe/CdS system to detect Cd²⁺. The detection limit was 6 nmol/L, and the standard deviation was below 4% for the detection of Cd²⁺ concentration in tap water.

A series of novel Li⁺ doped Bi_3.46Ho_0.04Yb_0.5Ti₃O₁₂ (BHYTO: xLi, 0 ≤ x ≤ 0.15) upconversion phosphors were prepared through a sol-gel-sintering method. There exist three emission bands centered at 545 nm, 658 nm, and 756 nm in the upconversion emission spectra at 980 nm excitation, corresponding to energy transitions of ⁵F₄/⁵S₂ → ⁵I₈, ⁵F₅ → ⁵I₈ and ⁵F₄/⁵S₂ → ⁵I₇ of Ho³⁺, and the upconversion emission intensity of BHYTO: 0.05Li is about 2.2 times stronger than that of BHYTO samples. The luminescent lifetime of the strongest emission (545 nm) is in the range of 45.25 to 65.99 μs for the different BHYTO: xLi phosphors. The energy transfers during the upconversion pumping process from Yb³⁺ to Ho³⁺ are mainly responsible for all the emissions, each belonging to a double-photon process. Li⁺ mainly entered into the interspace sites or occupied Bi³⁺ sites in Bi₄Ti₃O₁₂ host during the fabrication process according to its dosage, and the possibility is very low for Li⁺ to take part in the energy transfer process directly due to its lack of matching levels with 4f of Ho³⁺ and Yb³⁺. However, Li⁺ doping can not only increase the size of crystal grains to improve crystallinity through XRD analysis, but also reduced oxygen vacancies to decrease the number of quenching centers through XPS analysis. The improved crystallinity and reduced quenching centers are proposed to be the main causes for the enhanced upconversion luminescence of the Li⁺ doped BHYTO phosphor.