Layer-by-Layer Growth of an Oriented Tetraphenylethylene-Based MOF Thin Film for Fluorescence Sensing

The development of the orientation-growth metal-organic framework (MOF) materials is of significant importance for fluorescence sensing. Herein, a new oriented tetraphenylethylene-based MOF thin film of the Cu²⁺ fluorescence sensor is reported. The resulting MOF ZnTCBPE (TCBPE, 4',4″,4‴,4‴'-(ethene-1,1,2,2-tetrayl)tetrabiphenyl-4-carboxylic acid) thin film prepared by using a liquid-phase epitaxial layer-by-layer growth method has high growth orientation, homogeneous surface, and intense photoluminescence. In addition, the grown ZnTCBPE thin film has only two layers with a quantum yield of 33.51% due to the aggregation-induced emission (AIE) activity and exhibits excellent fluorescence sensing performance for Cu²⁺ detection (benefitting from the large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) overlap region) with a K_SV value of 1.14 × 10⁵. The K_SV value of the 2-layer thin film is greater than that of the 5-layer thin film (3.19 × 10⁴), which arises from the thinner structure, facilitating a more efficient Cu²⁺ diffusion and coordination with the uncoordinated O in MOF, thereby promoting stronger fluorescence quenching. Further, compared to the nonoriented ZnTCBPE thin film, the oriented thin film with its ordered structure growing along the [002] direction allows for a more uniform Cu²⁺ transport and coordination during the detection process, thus enabling more accurate sensing. This study provides a novel oriented AIE MOF thin film, offering new perspectives for the development of fluorescence sensors with advanced applications.