Thinning shell thickness of CuInS₂@ZnS quantum dots to boost detection sensitivity

Quantum dots (QDs), drawing large attention during the past three decades, have been extensively applied in lighting, display, and biodetection. However, the mechanism for their ability in biodetection, especially in recognizing toxic metal ions, has scarcely been explored. In this work, three sets of CuInS₂@ZnS QDs systems with inert shell thickness varying from 1.1 to 4.1 nm have been performed. As the shrinkage of inert shell, QDs not only show red-shift emission but also demonstrate more sensitive and higher response to the added Cd²⁺. The thin-shell CuInS₂@ZnS QDs could detect 0.91 nM Cd²⁺, and could further detect 4.36 nM Cd²⁺ when integrated with paper-based platform. Importantly, thin-shell CuInS₂@ZnS QDs combined with paper-based platform can detect 105.86 nM Cd²⁺ even just applying mobile phone as detector and hand-held UV lamp as excitation resource. The mechanism is further proposed based on the energy transfer routes. The thin inert shell can not completely protect the emissive core away from the surface defects, but it can neither exclude the energy transfer from the surface to the emissive core. The added Cd²⁺ would facilitate the formation of CdS on the surface of QDs, which not only can alleviate the surface defects but also can transfer energy to emissive CuInS₂, thus thinning the thickness of inert shell greatly boost the detection sensitivity.