Single-Particle Enumeration-Based Sensitive Glutathione S-Transferase Assay with Fluorescent Conjugated Polymer Nanoparticle.

Glutathione S-transferase (GST) is a group of multifunctional enzyme and participates in many physiological processes, such as xenobiotic biotransformation, drug metabolism, and degradation of toxic products. Herein, we demonstrate a label-free fluorescent conjugated polymer nanoparticle (FCPNPs)-based single-particle enumeration (SPE) method for the sensitive GST assay. Fluorescence resonance energy transfer (FRET) is formed between the glutathione-modified FCPNPs (FCPNPs-GSH) and polyethylenimine-capped gold nanoparticles (GNPs@PEI). Therefore, the fluorescence of FCPNPs-GSH is quenched remarkably. In the presence of GST, GNPs@PEI stay away from FCPNPs-GSH due to the specific interaction between FCPNPs-GSH and GST, leading to the inhibition of FRET. As a result, the fluorescence emission of FCPNPs-GSH is restored, which is reflected as the increase of the number of fluorescent particles in the microscopic image. By statistically counting the target concentration-dependent fluorescent particle number, accurate quantification of GST is achieved. The linear range from 0.01 to 6 μg/mL is obtained for GST assay and the limit-of-detection (LOD) is 1.03 ng/mL, which is much lower than the ensemble fluorescence spectra measurements in bulk solution. In urine sample assay, satisfactory recoveries in the range of 97.5-106.5.0% are achieved. Because of the high sensitivity and excellent specificity, this method can be extended to the detection of other disease-related biomolecules in the future.