Understanding the Selective Detection of Fe³⁺ Based on Graphene Quantum Dots as Fluorescent Probes: The K_sp of a Metal Hydroxide-Assisted Mechanism

Abstract

Graphene quantum dots (GQDs) have been widely used as fluorescence probes to detect metal ions with satisfactory selectivity. However, the diverse chemical structures of GQDs lead to selectivity for multiple metal ions, and this can lead to trouble in the interpretation of selectivity due to the lack of an in depth and systematic analysis. Herein, bare GQDs were synthesized by oxidizing carbon black with nitric acid and used as fluorescent probes to detect metal ions. We found that the specific ability of GQDs to recognize ferric ions relates to the acidity of the medium. Specifically, we demonstrated that the coordination between GQDs and Fe is regulated by the pH of the aqueous GQDs solution. Dissociative Fe can coordinate with the hydroxyl groups on the surface of the GQDs to form aggregates (such as iron hydroxide), which induces fluorescence quenching. A satisfactory selectivity for Fe ions was achieved under relatively acidic conditions; this is because of the extremely small K of ferric hydroxide compared to those of other common metal hydroxides. To directly survey the key parameter for Fe ion specificity, we performed the detection experiment in an environment free of interference from the buffer solution, noninherent groups, and other complex factors. This study will help researchers understand the selectivity mechanisms of GQDs as fluorescence probes for metal ions, which could guide the design of other GQD-based sensor platforms.

Graphene Quantum Dots as Flourishing Nanomaterials for Bio-Imaging, Therapy Development, and Micro-Supercapacitors

“…However, the increasing pH value showed the interference from other ions and selectivity for Fe 3+ , which was quite low at pH 1.4. Moreover, this study showed similar results in terms of quenching performed by Fe 3+ [ 141 ].…”

The Fluorescent Quenching Mechanism of N and S Co-Doped Graphene Quantum Dots with Fe3+ and Hg2+ Ions and Their Application as a Novel Fluorescent Sensor

“…Some reports have shown that the good affinity between quencher cations and functional groups on the surface of quantum dots relates to the good selectivity of GQDs’ fluorescence probe [47,48,49]. XPS and FTIR analysis show that there are many carboxyl, hydroxyl, amino, and hydrosulfonyl groups on the surface of N, S-GQDs.…”

Graphene Quantum Dot Embedded Hydrogel for Dissolved Iron Sensing

“…For each ion, sensing has been reported through a variation in GQD fluorescence due to a quenching mechanism. The quenching of GQD fluorescence by dissolved ions has been attributed to both the formation of GQD‐ion complexes, resulting in non‐radiative electron transfer from the GQD excited state to the ion, and to disruption of the surface passivation of the GQD, which decreases the GQDs photoluminescent quantum yield …”