Facile hydrothermal synthesis of Gd-doped carbon quantum dots for dual-mode colorimetric and photoluminescence sensing of Hg2+ ions with potential real samples and bioimaging applications

Abstract

Carbon quantum dots (CQDs) are luminous, nearly spherical nanoparticles known for their water solubility, excellent biocompatibility, and adjustable chemical and physical characteristics, making them suitable for diverse applications. In this work, Gadolinium-doped carbon quantum dots (Gd-CQDs) were synthesized by hydrothermal method with gadolinium used as metal source, citric acid and melamine precursor used as a carbon and nitrogen source. The as-prepared Gd-CQDs were characterized by using UV–Vis, photoluminescence (PL), FTIR, XRD, HRTEM, and XPS analysis. The Gd-CQDs exhibited strong UV–Vis absorption peaks at 293 nm and 367 nm, with a photoluminescence (PL) emission at 526 nm and a fluorescence quantum yield of 19.7 % with an average size was 3.21 ± 0.3 nm. A novel dual-readout sensor, based on Gd-CQDs were also tested as colorimetric and PL sensors for the highly selective and sensitive detection of mercury ions (Hg²⁺). Upon interaction with Hg²⁺ ions, the sensing Gd-CQDs solution transitions from yellow to colorless is observable by the naked eye, while its PL remains quenched. The colorimetric and PL response to Hg²⁺ ions is good linear in the ranges between 0 and 50 μM, and the limit of detection (LOD) are 104.4 nM in case of colorimetry and 60.2 nM in case of PL. The reversibility of the sensor was also evaluated, revealing a retention of 90 % efficiency after 10 cycles. In biological assays, the Gd-CQDs exhibited negligible cytotoxicity, with cell viability exceeding 94 %, and were successfully used for bioimaging of Hg²⁺ ions in HCT-116 cells. Moreover, real water sample analysis demonstrated high accuracy in Hg²⁺ detection, with recovery rates ranging from 98.9 % to 103.8 % and relative standard deviations (RSD) between 0.36 % and 2.32 %. These results highlight the Gd-CQDs as a highly effective, stable, and selective probe for Hg²⁺ ions detection in environmental and biological applications.