Anodic electrogenerated chemiluminescence of quantum dots: size and stabilizer matter

The electrogenerated chemiluminescence (ECL) of semiconductor quantum dots (QDs) is generally believed to be independent of particle sizes or the capping agents used. Herein, we demonstrate that CdTe QDs with different sizes and stabilizers evidently exhibit different ECL behavior in aqueous solution. The ECL of CdTe QDs stabilized by 3-mercaptopropionic acid (MPA) displays two waves at potentials of about +1.17 V and +1.74 V vs. Ag/AgCl, respectively. ECL spectra confirm that the ECL of QDs is attributed to their band gap luminescence, in which the peak positions are changed with QD sizes. The ECL mechanism of CdTe QDs involves superoxide radical generation by reduction of dissolved oxygen at lower potential or water splitting at higher potential. Direct evidence for superoxide radicals in this medium was obtained via electron spin resonance (ESR) experiments. In comparison, the 2-mercaptoethylamine (MEA)-capped CdTe QDs did not exhibit any ECL in air-saturated pH 7.4 PBS. Both ESR and X-ray photon spectroscopy (XPS) experiments revealed that amine groups in MEA-capped QDs were responsible for the absence of ECL. The reaction of an amine group with a superoxide radical leads to the quenching of ECL. The ECL quenching of MPA-capped CdTe QDs was further used to detect melamine. Under the optimum conditions, the inhibited ECL was linear with the logarithm of concentration of melamine within the concentration range of 10⁻⁹ to 10⁻⁵ M and the detection limit was found to be 6.74 × 10⁻¹⁰ M, which was 100-100,000 times lower than that of the most previous methods.