In situ formation of p–n junction: A novel principle for photoelectrochemical sensor and its application for mercury(II) ion detection

The discovery and development of photoelectrochemical sensors with novel principles are of great significance to realize sensitive and low-cost detection. In this paper, a new photoelectrochemial sensor based on the in situ formation of p–n junction was designed and used for the accurate determination of mercury(II) ions. Cysteine-capped ZnS quantum dots (QDs) was assembled on the surface of indium tin oxide (ITO) electrode based on the electrostatic interaction between Poly(diallyldimethylammonium chloride) (PDDA) and Cys-capped ZnS QDs. The in situ formation of HgS, a p-type semiconductor, on the surface of ZnS facilitated the charge carrier transport and promoted electron-hole separation, triggered an obviously enhanced anodic photocurrent of Cys-capped ZnS QDs. The formation of p–n junction was confirmed by P–N conductive type discriminator measurements and current–voltage (I–V) curves. The photoelectrochemical method was used for the sensing of trace mercuric (II) ions with a linear concentration of 0.01 to 10.0 µM and a detection limit of 4.6 × 10−9 mol/L. It is expected that the present study can serve as a foundation to the application of p–n heterojunction to photoelectrochemical sensors and it might be easily extended to more exciting sensing systems by photoelectrochemistry.