Herein, we have developed a label-free and homogeneous fluorescence resonance energy transfer (FRET) immunoassay
for the detection of neopterin (NPT), which is an early and valuable biochemical marker of cellular immunity. Owing to
intrinsic fluorescence properties of antibody and NPT, anti-NPT antibody (anti-NPT) and analyte played roles as the
respective donor and acceptor in the FRET immunoassay. As the concentration of NPT increases, the fluorescence
intensity at ~350 nm decreases owing to the formation of increasing amounts of the anti-NPT/NPT complex in which
FRET takes place. The assay system was found to display a high specificity and a low detection limit (0.14 ng mL-1) for
NPT. A practical application of the FRET immunoassay system was demonstrated by its use in the detection of NPT in
spiked human serum samples. The observations made in these efforts show that the homogeneous FRET immunoassay
strategy, which requires a simple sample preparation procedure, serves as a powerful tool for the rapid and sensitive
quantitative determination of NPT.