We demonstrate herein a newly developed lable-free photoelectrochemical immunosensor using a CdS quantum dots (QDs) multilayer film coupled with a biospecific interaction. The CdS QDs multilayer film was prepared by layer-by-layer assembling positively charged poly(dimethyldiallylammonium chloride) (PDDA) and thioglycolic acid (TGA)-capped water-soluble CdS QDs with negative charges on the surface of an indium−tin oxide (ITO) electrode. Ascorbic acid (AA) was exploited as an efficient and nontoxic electron donor for scavenging photogenerated holes under mild solution medium. The photoexcitation of CdS QDs modified electrode potentiostated at 0 V (vs. Ag/AgCl) in the presence of 0.1 M AA led to a stable anodic photocurrent. To perform the immunoassay, goat antimouse IgG was conjugated onto CdS QDs modified electrode by using the classic EDC coupling reactions between COOH groups on the surfaces of the TGA capped CdS QDs and NH2 groups of the antibody. The concentrations of mouse IgG were measured through t...