Using graphene quantum dots as photoluminescent probes for protein kinase sensing

A simple and sensitive photoluminescence (PL) assay for the activity of a protein kinase based on the selective aggregation of phosphorylated peptide-graphene quantum dot (GQD) conjugates triggered by Zr(4+) ion coordination has been established. With more sophisticated design of the peptide substrate sequences, detecting other enzymes could also be possible. Under optimal conditions, a linear relationship between the decreased PL intensity of peptide-GQD conjugates and the concentration of casein kinase II (CK2) in the range from 0.1 to 1.0 unit mL(-1) with a detection limit of 0.03 unit mL(-1) (3σ) was obtained. The EC50 value (i.e., the enzyme concentration producing 50% substrate conversion) for CK2 was evaluated to be 0.34 unit mL(-1). The proposed method showed potential applications in kinase inhibitor screening. To demonstrate the potential of this GQD-based platform for screening of kinase inhibitors in real biological systems, the inhibition of CK2 phosphorylation activity by four different inhibitors (ellagic acid, 5,6-dichlorobenzimidazole-l-β-d-ribofuranoside, emodin, and quercetin) was tested in human serum by comparing signals from samples incubated with the inhibitors against that without any inhibitor. As expected, in the presence of inhibitors, the PL intensity increased with increasing inhibitor efficiency. The IC50 value (inhibitor concentration producing 50% inhibition) for ellagic acid was estimated to be 0.041 μM. The developed protocol provides a new and promising tool for the analysis of both the enzyme and its inhibitors with low cost and excellent performance.