Molecular-Counting-Free and Electrochemiluminescent Single-Molecule Immunoassay with Dual-Stabilizers-Capped CdSe Nanocrystals as Labels.

Biorelated single-molecule detection (SMD) has been achieved typically by imaging the redox fluorescent labels and then determining each label one by one. Herein, we demonstrated that the capping agents (i.e., mercaptopropionic acid and sodium hexametaphosphate) can facilitate the electrochemical involved hole (or electron) injecting process and improve the stability of the dual-stabilizers-capped CdSe nanocrystals (NCs), so that the CdSe NCs could be electrochemically and repeatedly inspired to excited states by giving off electrochemiluminescence (ECL) in a cyclic pattern. With the CdSe NCs as ECL label and carcinoembryonic antigen (CEA) as target molecule, a convenient single-molecule immunoassay was proposed by simply detecting the ECL intensity of the dual-stabilizers-capped CdSe NCs in a sandwich-typed immune complex. The limit of detection is 0.10 fg/mL at S/N = 3, which corresponds to about 6-8 CEA molecules in 20 μL of serum sample. Importantly, the ECL spectra of both CdSe NCs and its conjugate with probe antigen in the immune complex were almost identical to the photoluminescence spectrum of bare CdSe NCs, indicating that all emissions were originated from the same excited species. The molecular-counting-free and ECL-based SMD might be a promising alternative to the fluorescent SMD.