Luminescent metal halides doped with ns²-metal ions such as 6s²-metal Bi³⁺ have aroused reviving interest owing to their outstanding optical properties; however, the origin of the photoluminescence (PL) remains controversial and unclear. Herein, we report a strategy for the controlled synthesis of Bi³⁺-doped vacancy-ordered double perovskite Cs₂SnCl₆ nanocrystals (NCs) and unravel the triplet excited-state dynamics of Bi³⁺ through temperature-dependent PL and ultrafast femtosecond transient absorption spectroscopies. Owing to the aliovalent Bi³⁺ doping in the spatially confined zero-dimensional (0D) structure of Cs₂SnCl₆, Bi³⁺ ions experience an enhancive Jahn-Teller distortion in the excited state, which results in intense broadband blue PL originating from the inter-configurational ³P_0,1 → ¹S₀ transitions of Bi³⁺ at 450 nm, with a large Stokes shift and a quantum yield of 35.2%. Specifically, an unusual thermal-enhanced Jahn-Teller splitting of the excitation band and a remarkable transition of the PL lifetime from ms at 10 K to μs at 300 K were observed, as solid evidence for the isolated Bi³⁺ emission. These findings clarify the controversy about the PL origin in ns²-metal ion-doped lead-free luminescent metal halides, thereby paving the way for exploring their optoelectronic applications.

Inorganic luminescent nanocrystals (NCs) doped with main-group ns²-metal ions have evoked tremendous interest in many technological fields owing to their superior optical properties. Herein, we report a new class of luminescent nanoprobes based on 5s²-metal Sb³⁺-doped CaS NCs that are excitable by using a near ultraviolet light-emitting diode. The optical properties and excited-state dynamics of Sb³⁺ in CaS NCs are comprehensively surveyed through temperature-dependent steady-state and transient photoluminescence (PL) spectroscopies. Owing to the strong electron–phonon coupling of Sb³⁺ in CaS NCs, Sb³⁺ ions experience a dynamic Jahn-Taller distortion on the excited state, which results in bright green PL of Sb³⁺ with a broad emission band, a large Stokes shift, and a high PL quantum yield up to 17.3%. By taking advantage of the intense PL of Sb³⁺, we show in proof-of-concept experiments the application of biotinylated CaS: Sb³⁺ NCs as sensitive luminescent nanoprobes for biotin receptor-targeted cancer cell imaging and zebrafish imaging with a high imaging contrast. These findings provide fundamental insights into the excited-state dynamics of Sb³⁺ in CaS NCs, thus laying a foundation for future design of novel and versatile luminescent nanoprobes via main-group ns²-metal doping.

The accurate detection of blood glucose is of critical importance in the diagnosis and management of diabetes and its complications. Herein, we report a novel strategy based on an upconversion nanoparticles-polydopamine (UCNPs-PDA) nanosystem for the accurate detection of glucose in human serum and whole blood through a simple blending of test samples with ligand-free UCNPs, dopamine, and glucose oxidase (GOx). Owing to the high affinity of lanthanide ions exposed on the surface of ligand-free UCNPs, dopamine monomers could spontaneously attach to the UCNPs and further polymerize to form a PDA shell, resulting in a remarkable upconversion luminescence (UCL) quenching (97.4%) of UCNPs under 980-nm excitation. Such UCL quenching can be effectively inhibited by H₂O₂ produced from the GOx/glucose enzymatic reaction, thus enabling the detection of H₂O₂ or glucose based on the UCL quenching/inhibition bioassay. Owing to the highly sensitive UCL response and background-free interference of the UCNPs-PDA nanosystem, we achieved a sensitive, selective, and high-throughput bioassay for glucose in human serum and whole blood, thereby revealing the great potential of the UCNPs-PDA nanosystem for the accurate detection of blood glucose or other H₂O₂-generated biomolecules in clinical bioassays.