Quantum Dots For Novel Photoelectrochemical Biosensing Strategies

Photoelectrochemical biosensing is a newly developed research area derived from solar energy conversion. With the advantages such as low detection potential and good biocompatibility, photoelectrochemical methods have been used in immunoassay, cytosensing, nuclear acid analysis, life related small molecules and etc. Compared with traditional electrochemiluminescence method, the light excitation in photoelectrochemical method is more compatible to biosamples than electrical excitation, which would make less damage. Quantum dots as the quasi-zero-dimensional material is controlled by quantum confinement effect which lead to good controllable photoelectrochemical response. By using analyte-induced formation of exciton trapping, the metal ion sensor was developed. Further, the phenomenon of directly capturing of the photoelectrons by dye to visualize the photo-to-current process was discovered. Moreover, the visualization process was controlled by quantum photoelectric effect. Based on the visualization reaction, the visual method to detect trace copper and protein was developed. Details are as follows.1. Cathode photoelectrochemical sensing of copper (Ⅱ) based on analyte-induced formation of exciton trappingThe analyte induced exciton trapping upon quantum dots was proposed and applied in the cathode photoelectrochemical detection of copper ion. The copper ion was reduced after the interaction with thiol groups on the surface of the quantum dots. The reduced copper led to the formation of exciton-trapping sites and thus leading to the decreased cathode photocurrent under cathode applied potential. As such, the DMSA-CdTe quantum dots was coated on the indium-tin oxide glass electrode and the sensor to detect Cu2+ was thus gained, with good selectivity and sensitivity. The detection range towards Cu2+ concentration was from 0.06 to 100 μM, and the detection limit was 5.9 nM. Since the good anti-interference ability, the sensor was successfully applied in the detection of copper element in human hair sample.2. Highly efficient visual detection of trace copper (Ⅱ) and protein by visualization of quantum photoelectric effectThis work presented a photocurrent response mechanism of quantum dots (QDs) under illumination with a concept of quantum photoelectric effect. Upon irradiation the photoelectron could directly escape from QDs. By using nitro blue tetrazolium (NBT) to capture the photoelectron, a new visual system was proposed due to the formation of insoluble reduction product, purple formazan, which could be used to visualize the quantum photoelectric effect. The interaction of copper (Ⅱ) with QDs could form trapping sites to interfere with the quantum confinement and thus blocked the escape of photoelectron, leading to a "signal off" visual method for sensitive copper (Ⅱ) detection. Meanwhile, by using QDs as a signal tag to label antibody, a "signal on" visual method was also proposed for immunoassay of corresponding protein. With meso-2,3-dimercaptosuccinic capped CdTe QDs and carcino-embryonic antigen as models, the proposed visual detection methods showed high sensitivity, low detection limit and wide detectable concentration ranges. The visualization of quantum photoelectric effect could be simply extended for the detection of other targets. This work opens a new visual detection way and provides a highly efficient tool for bioanalysis.