Dual Luminescent Mn(II)-Doped Cu-In-Zn-S Quantum Dots as Temperature Sensors in Water

CuInS₂ quantum dots have emerged in the last years as non-toxic alternative to traditional Pb and Cd based quantum dots, especially for biological applications. In this work, the hydrothermal synthesis of alloyed Cu-In-Zn-S quantum dots (CIZS) doped with manganese(II) is explored, with different metal ratios (Mn-CIZSy). The doped quantum dots show the sensitized emission of Mn²⁺ (approximately ms lifetime), together with the emission of the CIZS structure (approximately µs lifetime). The relative contribution of Mn²⁺ emission is highly dependent on the composition of the CIZS hosting structure (In:Cu ratio). In addition to that, it is shown that Mn²⁺ sensitization requires a threshold energy, which suggests the involvement of an intermediate state in the sensitization mechanism. The long-lived emission intensity decay of Mn²⁺ shows a stable and reversible temperature response in physiological conditions (25-45 °C, pH = 7.4). Mn-CIZSy quantum dots are thus interesting candidates as biological luminescent temperature probe thanks to their easy synthesis, high colloidal stability, insensitivity to dioxygen quenching and quantitative time-gated detection.