Rapid detection of procalcitonin using dual-color carbon quantum dots as fluorescent probes in FRET assay

FRET, known as Förster Resonance Energy Transfer, serves as a versatile technique enabling precise quantitative assessment of protein concentrations with minimal disruption to biological processes. In this study, we have developed two types of fluorescent carbon quantum dots, B-CDs and G-CDs, serving as donor and acceptor probes, respectively, for FRET-based procalcitonin (PCT) detection. The B-CDs exhibit strong blue fluorescence (excitation/emission peaks at 360/442 nm) as donors, while the G-CDs display faint green fluorescence (excitation/emission peaks at 449/520 nm) as acceptors. These CDs are conjugated with PCT antibodies to form immune probes. Upon antigen binding, the antibody quickly binds and reacts with the antigen. Concurrently, the fluorescence energy from the donor is transferred to the acceptor, leading to a decrease in the blue fluorescence intensity emitted by B-CDs. The ratio of the fluorescence intensity between the green probe (G-CDs-Ab2) and the blue probe (B-CDs-Ab1) exhibited a positive correlation with the concentration of PCT antigen in the low concentration range of 0.5 to 5 ng/mL. Consistent results were observed in both phosphate-buffered saline (PBS) and diluted human serum. In the concentration range of 0.5-5 ng/mL in PBS buffer, the linear equation is expressed as F_A/F_D = -0.0045 × c [PCT]^2 + 0.0021 × c [PCT] + 0.3232, with a limit of detection (LOD) of 0.1 ng/mL based on 3σ K^-1. For concentrations spanning from 5-80 ng/mL, the equation is F_A/F_D = 0.0021 × c [PCT] + 0.4212. These findings suggest that carbon dot-labeled immune probes could offer a novel approach for sensitive detection of disease biomarkers.