Amino Acid-Capped Water-Soluble Near-Infrared Region CuInS2/ZnS Quantum Dots for Selective Cadmium Ion Determination and Multicolor Cell Imaging

Abstract

Although attractive for their low toxicity, CuInS₂/ZnS core/shell quantum dots (CIS/ZnS QDs) still suffer from poor luminescence efficiency and poor water solubility. Herein, two amino acids (AAs), i.e., cysteine (Cys) and threonine (Thr), are used to tune the properties of CIS/ZnS QDs by capping them in both core and shell. It is found that Thr can regulate the density of Cys on the surface of QDs, thus causing a synergistic effect on the enhancement of photoluminescence (PL) intensity. Capping in the shell mainly leads to the enhancement of PL intensity, and capping in the core results in a red-shift of PL wavelength. Accordingly, a new kind of near-infrared region CIS/ZnS QDs with improved optical properties has been prepared. In addition, the Cys- and Thr-capped CIS/ZnS QDs possess outstanding water solubility and biocompatibility. In this work, the QDs are further employed in Cd²⁺ determination and multicolor imaging, indicating their potential applications. Relying on the enhancement of PL intensity via cation exchange, the Cys- and Thr-capped CIS/ZnS QDs can sense Cd²⁺ sensitively. Notably, because ZnS shells of the QDs will not be affected by Zn²⁺, the analytical method can discriminate Cd²⁺ from Zn²⁺ depending on the inherent characteristics of QDs. Moreover, intercellular Cd²⁺ can also be evaluated by the bright PL from the QDs, and the QDs can achieve multicolor imaging. Overall, this work demonstrates that various properties of QDs may be tuned by capping with AAs, and AA-capped QDs are of great value in advanced biosensing and bioimaging.