Eu³⁺ Complex-Protein Co-Crystals as Smart Sensors of Biologically Relevant Cations in Blood

This study aimed to develop a novel fluorescent sensor based on Eu³⁺ complex-doped protein crystal (EC-PC) for the efficient detection of metal ions in blood. By meticulously controlling the crystallization and annealing conditions in the co-crystallization strategy, the crystal growth processes were optimized to obtain doped Eu³⁺ complex-co-protein crystalline (EC-PC) structures. Thus, through co-crystallization of hen egg white lysozyme (HEWL) as a model protein and Eu³⁺ complex as fluorescent center, we successfully prepared Eu³⁺ complex-doped-HEWL co-crystals (EC-HC) with excellent fluorescent properties. Further treatment with 4% glutaraldehyde cross-linking enhanced the structural stability of the co-crystals. Moreover, the characteristic of sensitive, selective quenching of EC-PC fluorescence by biologically relevant cations, such as Cu²⁺, Zn²⁺, Mg²⁺, Ca²⁺ and Fe³⁺ ions, set up a smart sensing system in blood. For example, the fluorescence intensity of the crystals at 610 nm, as measured by a UV-visible spectrophotometer, decreases dose-dependently with the concentration of copper ions, thereby validating the sensor's high sensitivity to copper ion detection. Significantly, we also found that this hybrid protein-based sensor did not induce hemolysis, at various volume concentrations, confirming good anticoagulation in blood. This research not only provides a new perspective on the application of Eu³⁺ complex-doped protein crystals in the field of biosensing but also offers a new strategy for the detection of biologically relevant cations in blood. Future work will focus on further optimizing the sensor's performance and exploring its potential applications in clinical sample analysis.