Exploring Tb3+-Mediated Interactions with Glutathione-Capped Gold Nanoclusters to Develop a Fluorophore-Modified Ratiometric Probe toward Lactoferrin.

A ratiometric fluorescent probe for lactoferrin (Lf) was developed by conjugating fluorescent BDP-FL molecules onto glutathione (GSH)-capped gold nanoclusters, Au29-43(GSH)27-37, followed by terbium ion (Tb3+)-induced aggregation-induced emission enhancement (AIEE) of the clusters. The resultant BDP-FL-conjugated AIEE dots exhibit characteristic emissions at 517 and 606 nm. Conjugation of BDP-FL to the Au29-43(GSH)27-37 clusters provides a green-emissive internal reference, enabling ratiometric signal output. Upon addition of Lf, competitive binding with Tb3+ disrupts the aggregates, leading to selective attenuation of the red emission from Au29-43(GSH)27-37 while preserving the green BDP-FL emission. This ratiometric design affords a wide linear range (0.01-4.0 mg/mL), a low limit of detection (3.4 μg/mL), and excellent reproducibility (relative standard deviation < 1.2%). Importantly, the probe remains effective in 10-fold-diluted human tear samples, achieving recovery rates of 99.98-101.8% and providing results consistent with capillary electrophoresis. Mechanistic studies reveal that Tb3+ reduces electrostatic repulsion and enhances van der Waals and bridging interactions, thereby promoting aggregation of Au29-43(GSH)27-37. Using the Derjaguin-Landau-Verwey-Overbeek theoretical model, the relative contributions of van der Waals, electrostatic, and bridging interaction energies were quantified, offering deeper insight into the aggregation mechanism. This ratiometric AIEE probe demonstrates practicality, sensitivity, and reliability for Lf determination in clinical samples and may provide guidance for the rational design of nanomaterials with tailored optical properties.