Carbon Dots: Advanced Fluorescent Platforms for Sensitive Metal‑Ion Detection

Abstract

Fluorescent carbon dots (CDs), including carbon quantum dots (CQDs) and graphene quantum dots (GQDs), are emerging nanomaterials prized for their low cost, minimal toxicity, and tunable optical properties. Surface passivation and functionalization allow precise control over fluorescence, enabling their use as versatile nanoplatforms for sensing, imaging, and delivery. This review surveys synthetic strategies for CQDs and GQDs, their structural and photophysical characteristics, and recent advances in using CDs for heavy‑metal ion detection.

Introduction

Since their discovery, CDs—fluorescent carbon nanostructures below 10 nm—have attracted attention for optoelectronic, biomedical, and sensing applications. Derived from fullerenes, graphite, carbon nanotubes, or graphene, CDs offer superior photostability and quantum efficiency compared to organic dyes and semiconductor quantum dots, while avoiding the heavy‑metal toxicity of the latter. Their biocompatibility, ease of synthesis from abundant carbon sources, and straightforward surface functionalization make CDs ideal for detecting toxic metal ions such as Hg²⁺, Pb²⁺, and Cd²⁺, which accumulate in the body and disrupt biological processes.

Synthesis of Carbon Quantum Dots

Top‑Down Approaches

Top‑down methods cleave bulk carbon materials via arc discharge, laser ablation, or electrochemical oxidation. Arc‑discharge‑derived CDs were first identified as a byproduct of single‑walled nanotube synthesis. Laser ablation of graphite in the presence of PEG (e.g., PEG₁₅₀₀N) yields fluorescent particles that require subsequent polymeric passivation to enhance photoluminescence. Electrochemical exfoliation of multi‑wall carbon nanotubes or graphite under controlled potentials generates uniformly spherical CDs (~2–4 nm) with oxygenated surface groups, enabling blue emission under UV excitation.

Bottom‑Up Approaches

Bottom‑up synthesis constructs CDs from small organic precursors—citric acid, carbohydrates, amino acids—via hydrothermal, microwave, or pyrolysis routes. This route offers precise size and surface‑group control. For example, citric acid and ethylenediamine, subjected to microwave‑assisted hydrothermal treatment, produce amine‑functionalized CDs with quantum yields up to 60 % and selective Fe³⁺ quenching (LOD = 1 ppm). One‑step hydrothermal reduction of oxidized carbon inks with heteroatom dopants (N, S, Se) yields 1–6 nm CQDs with tunable emission and heightened sensitivity to Cu²⁺ and Hg²⁺.

Physical Properties of Carbon Dots

Structures

CDs possess graphitic lattice spacings of 0.18–0.24 nm in‑plane and 0.32 nm interlayer, with a core of sp²‑hybridized carbon and an amorphous periphery. Raman spectra display D (≈1350 cm⁻¹) and G (≈1600 cm⁻¹) peaks, confirming mixed sp²/sp³ domains. Surface functional groups (C=O, C–N, C=N) introduced via passivation or doping significantly influence photoluminescence.

Fluorescence

CD emission originates from quantum‑confinement, surface state, and heteroatom‑induced electronic transitions. Temperature and reaction time in microwave‑hydrothermal synthesis produce either full‑color or blue‑emitting CDs, attributed to size‑dependent band gaps and surface‑functional‑group density. Full‑color CDs exhibit abundant C=N/C=O and C–N groups, whereas blue CDs are smaller with fewer functional groups.

Surface Passivation and Doping

Polymeric passivation (PEG₁₅₀₀N, branched PEI, oligomeric PEG) protects surface defects, enhances quantum yields, and modulates solubility. Heteroatom doping (N, S, Se, B) shifts emission: N‑doping causes blue‑shift, S/Se‑doping red‑shift, and O‑functionalization widens the band gap. N‑doped CQDs (N‑CQDs) and GQDs show selective detection of Fe³⁺ and Cu²⁺ with nanomolar limits of detection.

Decoration of CDs for Heavy‑Metal Detection

Organic Molecules

Functionalization with amines, carboxyls, or hydroxyls tailors selectivity. For instance, citric‑acid/ethylenediamine CDs detect Fe³⁺ (LOD = 1 ppm), while amine‑functionalized GQDs respond to Cu²⁺. N‑ or S‑co‑doped CDs provide sub‑nanomolar LODs for Hg²⁺ (0.05 nM) and Cu²⁺ (32.5 nM). Boron‑doped CDs (B‑CQDs) exhibit strong quenching by Cu²⁺ and Pb²⁺, enabling dual‑ion sensing.

Biomolecules and Natural Materials

Biomolecule‑derived CDs (e.g., BSA‑lysine‑coated CQDs, valine‑functionalized GQDs, dopamine‑conjugated GQDs) combine biocompatibility with high fluorescence and metal‑ion selectivity. BSA‑lysine CQDs detect Cu²⁺ at 2 nM; valine‑GQDs achieve Hg²⁺ LOD of 0.4 nM; dopamine‑GQDs provide 7.6 nM Fe³⁺ detection via FRET. Natural precursors such as chitosan and Ocimum sanctum leaves yield N‑doped CDs and lead‑responsive G‑CNQDs with LODs of 80 nM and 0.59 nM, respectively.

Metal Nanoparticles

Hybridizing CDs with noble metal nanoparticles (AuNPs, AgNPs, PtNPs) leverages surface plasmon resonance to enhance sensitivity. Ag‑NP‑decorated GQDs offer rapid Ag⁺ detection; AuNP‑GQD composites achieve Hg²⁺ and Cu²⁺ limits of 0.02 nM and 0.05 nM. Gd(III)‑doped CDs provide dual fluorescence/MRI contrast, while CQD‑AuNC hybrids enable ratiometric Cd²⁺ sensing with an “on‑off‑on” response to ascorbic acid.

Conclusion

CDs’ low cost, biocompatibility, and tunable photophysics position them as powerful platforms for on‑site heavy‑metal monitoring. While challenges remain—particularly aqueous solubility and mechanistic understanding—ongoing research is rapidly advancing eco‑friendly, cost‑effective synthesis and practical sensor integration.