Construction of photoelectrochemical thrombin aptasensor via assembling multilayer of graphene-CdS nanocomposites

A photoelectrochemical (PEC) aptasensor for highly sensitive and specific detection of thrombin was developed by using graphene-CdS nanocomposites multilayer as photoactive species and electroactive mediator hexaammineruthenium(III) chloride (RuNH363+) as signal enhancer. Graphene-CdS nanocomposites (G-CdS) were synthesized by one-pot reduction of oxide graphene and CdCl2 with thioacetamide. The photoactive multilayer was prepared by alternative assembly of the negatively charged 3-mercaptopropionic acid modified graphene-CdS nanocomposites (MPA-G-CdS) and the positively charged polyethylenimine (PEI) on ITO electrode. This layer-by-layer assembly method enhanced the stability and homogeneity of the photocurrent readout of G-CdS. Thrombin aptamer was covalently bound to the multilayer by using glutaraldehyde as cross-linking. Electroactive mediator RuNH363+ could interact with the DNA phosphate backbone and thus facilitated the electron transfer between G-CdS multilayer and electrode and enhanced the photocurrent. Hybridizing of a long complementary DNA with thrombin aptamer could increase the adsorption amount of RuNH363+, which in turn boosted the signal readout. In the presence of target thrombin, the affinity interaction between thrombin and its aptamer resulted in the long complementary DNA releasing from the G-CdS multilayer and decreasing of photocurrent signal. On the basis of G-CdS multilayer as the photoactive species, RuNH363+ as an electroactive mediator, and aptamer as a recognition module, a high sensitive PEC aptasensor for thrombin detection was proposed. The thrombin aptasensor displayed a linear range from 2.0 pM to 600.0 pM and a detection limit of 1.0 pM. The present strategy provided a promising ideology for the future development of PEC biosensor.