Using G-quadruplex/hemin to "switch-on" the cathodic photocurrent of p-type PbS quantum dots: toward a versatile platform for photoelectrochemical aptasensing.

We present a novel photoelectrochemical (PEC) biosensing platform by taking advantage of the phenomenon that hemin intercalated in G-quadruplex "switched-on" the cathode photocurrent of p-type PbS quantum dots (QDs). Photoinduced electron transfer between PbS QDs and G-quadruplex/hemin(III) complexes with the subsequent catalytic oxygen reduction by the reduced G-quadruplex/hemin(II) led to an obvious enhancement in the cathodic photocurrent of the PbS QDs. For the detection process, in the presence of hemin, the specific recognition of the targets with the sensing sequence would trigger the formation of a stable G-quadruplex/hemin complex, which will result in reduced charge recombination and hence amplified photocurrent intensity of the PbS QDs. By using different target sequences, the developed system made possible a novel, label-free "switch-on" PEC aptasensor toward versatile biomolecular targets such as DNA and thrombin. Especially, with ambient oxygen to regenerate G-quadruplex/hemin(II) to G-quadruplex/hemin(III), this substrate-free strategy not only promoted the photoelectric effect and thus the enhanced sensitivity of the system, but also avoided the addition of supplementary substrates of G-quadruplex/hemin such as H2O2 and organic substances.