The effect of CQDs' particle size on its fluorescence behavior and Cu²⁺ detection

Carbon quantum dots (CQDs) have emerged as promising fluorescent sensors for ion detection due to their tunable optical properties. In this study, CQDs with systematically controlled particle sizes were synthesized via a hydrothermal method to investigate the effect of size on their fluorescence properties and Cu²⁺ detection capabilities. Characterization techniques, including transmission electron microscopy (TEM), X-ray diffraction (XRD), and spectroscopic analysis, revealed that the smaller CQDs particles (<3 nm) exhibited the weaker fluorescence intensity and shorter emission wavelength compared to those of the larger CQDs particles (>3 nm). These phenomena were attributed to quantum confinement effects and surface state modulation, where reduced particle size enhanced quantum confinement, leading to discrete energy levels and blue-shifted emission. However, CQDs with smaller particle size was easy to aggregate in solution because of its high surface energy, which made it difficult to achieve system stability, thus interfering with the detection accuracy. The optimized CQDs demonstrated high sensitivity for Cu²⁺ detection with a linear response in the concentration range of 0-3 mM and a detection limit as low as 0.09 nM. Mechanism studies indicated that Cu²⁺-induced fluorescence quenching followed a dynamic quenching process, where size-dependent surface charge and functional groups (e.g., hydroxyl, carboxyl) influenced the binding affinity between CQDs and Cu²⁺. Smaller CQDs, despite their aggregation tendency, exhibited stronger electrostatic interactions with Cu²⁺ due to their higher surface-to-volume ratio, enhancing detection specificity. This work highlighted the critical role of particle size in tailoring CQDs' optical properties and sensing performance, providing insights for designing efficient fluorescent probes for environmental monitoring and ion analysis.