A glassy carbon electrode modified with in-situ generated chromium-loaded CdS nanoprobes and heparin for ultrasensitive electrochemiluminescent determination of thrombin

We describe the in-situ electrochemical synthesis of a chromium-loaded CdS nanoprobe (CdS-Cr) for use in ultrasensitive electrochemiluminescence (ECL) detection of thrombin. A glassy carbon electrode was covered with (a) a film of electro-polymerized polyaniline nanofibers (PANI-NF), (b) the in-situ synthesized CdS-Cr nanoprobe, (c) heparin, and (d) a coating of BSA to prevent unspecific adsorption. Atomic force microscopy and scanning electron microscopy images showed the size of nanoprobe to have increased to an average size of 50 +/- 5 nm, which was larger than pure CdS nanocrystals (NCs) with their typical size of 10 +/- 2 nm. The modified glassy carbon electrode was electrochemically characterized by cyclic voltammetry and impedance spectroscopy. The thrombin-heparin interaction served as a new recognition tool for thrombin and represented an attractive alternative to respective aptamers. About 6-fold enhancement of ECL intensity (compared to pure CdS NCs) was observed in presence of persulfate. This ECL assay has a wide dynamic range (from 10 fM to 100 pM concentrations of thrombin), and a lower detection limit of 6.8 fM at 3 sigma. The technique for in-situ electrochemical preparation of the nanoprobe is simple, facile, and yields a sensor surface with favorable space structure, positive charge and stability.