Question

Outline how ECL may be applied for biosensing applications

Answer 1

Electrochemiluminescence (ECL) technology has been developed rapidly due to its low background, high sensitivity, and simple format and may become a powerful tool for the detection of intracellular Pb²⁺. In addition, to broaden the application of ECL in biological analysis, several kinds of novel luminescent materials with excellent biocompatibilities have been introduced into an ECL system, such as fluorescent dyes, noble-metal nanoclusters, iridium complexes and carbon dots (CDs). Among these luminescent materials, CDs are easily prepared, size-controllable and have a superior biological compatibility. However, the luminescence efficiency of traditional CDs is very low. Moreover, the water solubility of CDs makes it difficult to immobilize them on an electrode, which further limits their application in an ECL system. Therefore, the key points in using CDs as signal probes effectively in an ECL system may lie in enhancing the ECL intensity of CDs and immobilizing the CDs well. On these accounts, some of the previous literature has reported that nitrogen (N) doped CDs have a higher luminescence efficiency than traditional CDs, and o-phenylenediamine (OPD), a well-known precursor of CDs, which contains carbon and nitrogen, can be utilized as a carbon source and a nitrogen source to synthesize N doped CDs (N-CDs). To further enhance the ECL signal of N-CDs, core-shell Pd–Au hexoctahedrons (Pd@Au HOHs) with special catalytic activity for the ECL reaction are introduced into the fabrication of an ECL biosensor based on N-CDs.

An ECL biosensor using N-CDs in situ electro-polymerized on a glassy carbon electrode (GCE) as luminophores and Pd–Au hexoctahedrons (Pd@Au HOHs) as enhancers were developed for the detection of intracellular Pb²⁺. OPD was in situ electro-polymerized onto a glassy carbon electrode to form N-CDs. Due to the presence of amino groups on the N-CDs, Ag nanoparticles (AgNPs) were immobilized on the electrode. In turn, amino-modified capture DNA (T1) was immobilized on the AgNPs through Ag–N bonds. Afterward, complementary DNA (T2) and ssDNA1 (S1) labeled Pd@Au HOHs (Pd@Au HOHs–T2–S1) were modified on the electrode through chain hybridization between T1 and T2. Simultaneously, ssDNA2 (S2) labeled Pd@Au HOHs (Pd@Au HOHs–S2) were introduced by hybridization between S1 and S2, with the formation of Pd@Au HOHs–DNA dendrimers on the electrode. Owing to the high catalytic activity of Pd@Au HOHs, the ECL reaction of the N-CDs was promoted and the signal of the N-CDs was significantly enhanced. Finally, intracellular Pb²⁺ was used to couple with numerous repeated DNA sequences of S1 and S2 to form a Pb²⁺ stabilized G-quadruplex (G4) structure. With the formation of the Pb²⁺ stabilized G4 structure, the ECL intensity of the N-CDs was quenched by Pb²⁺. Therefore, the ECL biosensor based on N-CDs was successfully fabricated for the detection of intracellular Pb²⁺. In addition, Pd@Au HOHs–DNA dendrimers not only greatly enhanced the ECL signal of the N-CDs, but also realized the effective capture of Pb²⁺, which was beneficial to improve the sensitivity of the signal-off biosensor for the detection of intracellular Pb²⁺. Therefore, this work provided a novel and effective detection strategy for intracellular Pb²⁺.

In summary, a sensitive ECL biosensor using in situ electropolymerized N-CDs as luminophores and Pd@Au HOHs as enhancers was developed for the detection of intracellular Pb²⁺. The proposed biosensor had good sensitivity and accuracy for the detection of intracellular Pb²⁺. Furthermore, the proposed ECL biosensor presented a novel way to apply N-CDs in the ECL system in which the ECL signal of N-CDs could be effectively enhanced by Pd@Au HOHs. This method holds great potential for further application in the research of intracellular heavy metal ion related biological processes.