Highly Sensitive Photoelectrochemical Immunosensors Based On Quantum Dots And Their Nanocomposites

Photoelectrochemical (PEC) immunoassay is a newly emerged detection method, which possesses the advantages of high sensitivity, good selectivity, easily minimized instrument, and great potential for infield analysis. Due to these merits, it has attracted considerable attentions in th analytical chemistry field. During the past two decades, a variety of novel PEC immunoassay methods have been developed and showed great potential in the fields of clinical diagnosis, environmental monitor, and food safety. However, in comparison to their electrochemical and photochemical counterparts, the study of PEC immunoassay is still in the eaily stage. Nowadays, only a few kinds of PEC active nanomaterials could be applied for PEC immunoassay, the detection model which could effectively improve the analytical performance of the immunosensor is limited, and the sensitivity of the PEC immunoassay method still has big room to be improved. To solve these problems, this thesis developed two novel PEC immuassay methods, based on a newly synthesized graphene and CdTe (GR-CdTe) nanocomposite and a novel exciton-exciton interaction mechanism. Under optimal conditions, both PEC immunosensors showed high sensitivity, good stability, and acceptable accuracy for the detection of carcinoembryonic antigen (CEA), an important tumor marker involved in many malignant cancers. The detailed information is as follows:1. One-pot synthesis of GR-CdTe nanocomposites and their application for highly sensitive PEC immunoassay.A one-pot synthesis procedure was developed for the preparation of GR-CdTe nanocomposites, which was simple, fast, and could lead to the formation of a high quality GR-CdTe nanocomposite with good water dispersibility, excellent conductivity, high CdTe quantum dots (QDs) loading density and good PEC activity. The synthesis mechanism was investigated by a series of methods, including TEM, UV-vis spectrometry, and PEC method, which indicated the presence of a linker molecule,p-aminophenol was the key factor determing the quanlity of the as-synthesized GR-CdTe nanocomposites. The role of p-aminophenol could be ascribed to its strong interaction with both graphene and CdTe QDs. The as-synthesized GR-CdTe nanocomposites could quench more than 90% of the PEC signal of the Mn:CdS/TiO2/FTO electrode. Based on this interaction and employing GR-CdTe as a signal amplification tag, a highly sensitive PEC immunoassay method was developed for the detection of CEA. Under optimal conditions, the biosensor showed good analytical performances, with a wide linear range of 5x10-12 to 10-9g/mL and a low limit of detection of 5 pg/mL.2. The exciton-exciton interaction between CdS:Mn and CdTe QDs and its application for PEC immunoassay of CEA.The interaction mechanism between CdTe and CdS.Mn QDs was investigated by a series of methods, including UV-vis absorption spectra, fluorescence emission spectra, and PEC methods, and a possible exciton-exciton interaction mechanism was proposed. In detail, under irradiation, both CdTe and CdS:Mn QDs could be excited to form excitons, which both could generate local electrical fields around each other. Under the local electrical field of CdTe QDs, the CdS:Mn excitons would be quenched significantly and loss more than 90% of its photo-to-current conversion efficiency. Based on this strong interaction and employing CdTe as a signal amplification tag, a novel PEC immunoassay method was developed for the highly sensitive detection of CEA. Under optimal conditions, the as-fabricated PEC immunosensor showed good analytical performances, with a wide linear range of 10-12to 10-8 g/mL and the detection limit of CEA is 1 pg/mL.