سنتز نقاط كوانتومي كربن با فلورسانس زياد به‌ عنوان حسگرهاي فلورسانس براي تشخيص يون‌هاي فلزي سنگين

Carbon quantum dots have attracted considerable attention as promising can‎didates in the field of optical sensors due to their unique optical properties, low toxicity, biocompatibility, an‎d simple synthesis procedures. The aim of this study was to synthesize highly fluorescent CQDs an‎d investigate their potential as optical sensors for the detection an‎d recognition of heavy metal ions. In this work, CQD nanostructures with blue, green, orange, an‎d red fluorescence were synthesized via a hydrothermal method. The synthesized nanoparticles were characterized using ultraviolet-visible (UV–Vis) spectroscopy, photoluminescence (PL) spectroscopy, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), dynamic light scattering (DLS), an‎d atomic force microscopy (AFM). The results confirmed the formation of uniform nanoparticles with sizes ranging from 1.4 to 6 nm, an appropriate crystalline structure, an‎d the presence of hydroxyl (–OH), carboxyl (–COOH), ether (C–O–C), an‎d imine (C=N) functional groups on their surface. Photoluminescence studies revealed that the synthesized CQDs exhibited significant sensing behavior toward various heavy metal ions, including Cr³⁺, Cr⁶⁺, Cd²⁺, Fe³⁺, Co²⁺, an‎d Hg²⁺. In contrast to the common fluorescence quenching (Turn-off) mechanism, some CQDs showed fluorescence enhancement (Turn-on) behavior, attributed to the formation of metal–ligan‎d complexes between surface functional groups an‎d metal ions. Among the synthesized samples, orange-emitting CQDs exhibited concentration-dependent fluorescence enhancement upon exposure to Hg²⁺, Fe³⁺, Cr⁶⁺, Cd²⁺, an‎d Cr³⁺ ions. For Co²⁺ ions, a dual-signal sensing behavior (initial enhancement followed by quenching) was observed, indicating their potential use in designing optical sensors for critical-level detection of heavy metals. Green-emitting CQDs demonstrated selec‎tive responses toward Hg²⁺, Cd²⁺, an‎d Cr³⁺/Cr⁶⁺ ions. In the presence of Cr³⁺, an initial fluorescence decrease followed by enhancement was observed, indicating a dual sensing mechanism, while Cr⁶⁺ an‎d Cd²⁺ ions induced a concentration-dependent fluorescence enhancement. Furthermore, CQDs exhibited effective optical sensing performance toward Cr⁶⁺, Co²⁺, Cr³⁺, an‎d Fe³⁺ ions. The addition of Cr⁶⁺ ions resulted in strong fluorescence quenching, while Co²⁺ an‎d Cr³⁺ caused gradual intensity reduction. In the case of Fe³⁺, fluorescence intensity initially increased slightly an‎d then decreased, remaining above that of the pure CQDs. For red-emitting CQDs), the addition of Hg²⁺ ions led to a notable concentration-dependent fluorescence enhancement, which was opposite to the conventional quenching behavior. A similar enhancement was also observed in the presence of Cr⁶⁺ an‎d Co²⁺ ions. selec‎tivity tests were also performed by adding two metal ions simultaneously to the blue an‎d green CQDs, demonstrating the ability of these nanostructures to distinguish an‎d detect multiple metal ions concurrently. The results of this study indicate that by controlling synthesis conditions, CQDs with tunable optical properties can be achieved an‎d utilized as a basis for designing highly sensitive, selec‎tive, an‎d intelligent optical sensors with dual response mechanisms (Turn-on an‎d Turn-off).

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