Table 3. Comparative analysis of MQD-based fluorescence sensing platforms for high-valence ion detection under experimentally relevant conditions.

Target ion	MQD design/modification	Dominant mechanism	LOD/Linear range	pH/buffer (expert inference)	Probe concentration	Response time	Detection matrix/Optical notes	Key functionality	Ref.
Fe3+	Pristine Ti3C2 MQDs (∼1.75 nm)	Electron transfer + inner filter effect	LOD: 310 nM	∼pH 7–7.4 (PBS or similar buffer likely used for serum compatibility)	∼0.02–0.05 mg mL−1 (typical MQD fluorescence assay)	<5 min (fast coordination quenching typical for Fe3+ sensors)	Tested in serum and seawater; excitation-dependent fluorescence typical for MQDs	Selective detection in complex matrices	69
Fe3+	Amino-functionalized Ti3C2Tx MQDs	Redox interaction + surface coordination	LOD: 2 nM; nM–µM	Likely pH 6–7.5 (Fe3+ hydrolysis avoided; weakly buffered medium)	Not reported	Rapid (<3 min typical for Fe3+ binding to amine groups)	Environmental water analysis	Ultrahigh sensitivity; pH-responsive fluorescence	70
Cr(vi)	N-doped Ti3C2 MQDs	Inner filter effect + static quenching; redox recovery with AA	LOD: 0.012 µM; linear: 0.1–500 µM	Typically pH 4–6 (Cr(vi) stable as HCrO4−/Cr2O72− in slightly acidic media)	Not reported	∼2–5 min typical for IFE-based quenching	Aqueous detection system	“On–off–on” sensing of Cr(vi)/AA	71
Cr2O72−	N,B co-doped MQDs on PEI-paper	Fluorescence quenching + adsorption	LOD: 3.8–17 nM	Wide pH tolerance reported (likely 3–10 typical for paper sensors)	Immobilized MQDs on a paper substrate	∼10 s (reported rapid response)	Paper-based device; filtration and immersion modes	Simultaneous detection and adsorption	72
Mn(vii)	Ti3C2 MQDs (intrinsic reducibility)	Direct redox reaction with permanganate	LOD: 5.2 nM	Likely acidic to neutral (∼pH 4–7) to maintain MnO4− stability	Not reported	Very rapid (<1–2 min typical redox quenching)	Applied to plant leaf extracts	Dual function: sensing + scavenging	73
Ag+	Pristine Ti3C2 MQDs	Reduction of Ag+ → Ag nanoparticles	LOD: 0.45 µM	Likely near neutral (∼pH 6–7) to allow controlled AgNP formation	Not reported	Minutes (NP nucleation kinetics)	Dual fluorescence + colorimetric signal	Label-free dual-mode detection	74
Fe3+/Cu2+	Covalently N-doped MQDs (APTES)	Coordination-induced fluorescence quenching	LOD: 0.17 µM (Fe3+), 0.15 µM (Cu2+); linear: 0.5–500 µM	Likely buffered near neutral (∼pH 7) due to SHPP masking chemistry	Not reported	∼2–5 min typical coordination quenching	Real water samples	Improved stability via covalent functionalization	75
Fe3+	BSA-functionalized MQDs	Coordinative quenching via protein binding sites	Linear: 0–150 µM	Physiological pH (∼7.4 PBS) due to protein stability	Biomolecule-stabilized probe (µg mL−1 range typical)	Few minutes typical protein-ion interaction	Tested in real samples and cytotoxicity assays	High biocompatibility sensing platform	76