Tuning Photoluminescence and Magnetic Properties of Ag–Ga–S and Zn–Ag–Ga–S Quantum Dots via Mn²⁺ Doping

Abstract

Transition metal-doped colloidal quantum dots (QDs) are promising candidates for dual-mode imaging due to their tunable optical properties and potential magnetic functionalities. Here, we report the synthesis and characterization of Mn²⁺-doped AgGaS₂ (AGS) and ZnS-alloyed Ag–Ga–S (AGZS) QDs via a one-pot heating-up method. Upon Mn²⁺ incorporation, both AGS and AGZS QDs exhibited two distinct photoluminescence (PL) peaks originating from the host QDs and doped Mn²⁺ ions, with the relative intensities tunable by varying Mn²⁺-to-total metal cation ratio in the precursor (Mn²⁺/cation_pre). The AGZS QDs prepared with Mn²⁺/cation_pre ratio of 1.1 % exhibited a high PL quantum yield of 45 %, with 40 % of the emission attributed to the d-d transition of Mn²⁺ ions (⁴T₁ → ⁶A₁). Time-resolved PL measurements revealed efficient energy transfer from excitons in the host QDs to doped Mn²⁺ ions, contributing to enhanced Mn²⁺-related emissions. Surface passivation with a ZnS shell followed by ligand exchange with 3-mercaptopropionic acid allowed stable dispersion of AGZS QDs in aqueous media while retaining optical properties. Since the Mn²⁺-doped QDs also exhibited paramagnetic behavior, we investigated the performance of Mn²⁺-doped AGZS QDs as a contrast agent for magnetic resonance imaging (MRI). The intensity of the T₁-weighted MRI signal increased with an increase in the content of doped Mn²⁺. These results demonstrate that Mn²⁺-doped AGZS QDs are promising single-component dual-mode (PL/MRI) nanoprobes for biomedical imaging applications.