Table 1. Minimum experimental evidence for distinguishing fluorescence-response mechanisms in MQD-based sensing systems.

Mechanism	Primary origin	Lifetime behavior	Spectral overlap requirement	Surface/electronic evidence	Reversibility behavior	Typical diagnostic experiments
Inner filter effect (IFE)	Optical absorption of excitation/emission light	No change	Strong overlap with MQD excitation or emission	None	Usually reversible	UV-vis absorption analysis; dilution correction
Dynamic quenching	Collisional interaction in excited state	Lifetime decreases	Not required	None	Reversible	Time-resolved fluorescence; Stern–Volmer analysis
Static quenching	Ground-state complex formation	Lifetime unchanged	Often weak	Evidence of complex formation	Often reversible	UV-vis spectral shift; binding constant analysis
Electron transfer (ET)	Charge transfer between MQD and analyte	Lifetime decreases	Not essential	Redox compatibility; XPS or electrochemistry	Often reversible	Cyclic voltammetry; transient spectroscopy
Redox-triggered recovery	Chemical reduction/oxidation restoring emission	Signal restored after redox step	System-dependent	Oxidation-state change (XPS/EPR)	Reversible after reduction/oxidation	Chemical reduction tests; electrochemical cycling