Electrogenerated chemiluminescence determination of C-reactive protein with carboxyl CdSe/ZnS core/shell quantum dots

Electrogenerated chemiluminescence (ECL) of water-soluble core/shell CdSe/ZnS quantum dots (QDs) coated with carboxylated polyethylene glycol polymers (“Qdot 625”) was investigated in aqueous solutions using 2-(dibutylamino)ethanol (DBAE) and tri-n-propylamine (TPrA) as ECL coreactants. In both cases, ECL emissions at glassy carbon (GC) electrode appeared at the same potential of ∼0.80 V vs. Ag/AgCl (3.0 M KCl), which was ∼200 and ∼150 mV more positive compared with the oxidation potentials for DBAE (∼+0.60 V vs. Ag/AgCl) and TPrA (∼+0.65 V vs. Ag/AgCl), respectively. The ECL intensity, however, was significantly affected by the type and the concentration of the ECL coreactant used as well as the nature of the working electrode. Under the present experimental conditions, ECL from DBAE was ∼17 times stronger than that from TPrA. The maximum ECL was obtained at GC electrode when [DBAE] ≈ 53 mM, where a ratio of 11 : 3 : 1 in ECL intensity was evaluated for GC, Au, and Pt electrodes, respectively. The ECL emission of the Qdot 625/DBAE system had an apparent peak value of ∼625 nm that matched well the fluorescence data. The QD as a label for ECL-based immunoassays of C-reactive protein (CRP) was realized by covalent binding of avidin on its surface, which allowed biotinylated anti-CRP to be attached and interacted with solution-phase CRP and the anti-CRP linked to micro-sized magnetic beads. The newly formed sandwich type aggregates were separated magnetically from the solution matrix, followed by the ECL generation at partially transparent Au nanoparticle-coated ITO electrode or Au/CD electrode in the presence of DBAE. Much stronger ECL responses were observed from the Au/CD electrode, at which a dynamic range of 1.0–10.0 μg mL−1 CRP and a limit of detection of 1.0 μg mL−1 CRP were obtained, respectively.