This is dynamic (collisional) quenching: a diffusing quencher molecule Q collides with an already-excited fluorophore F* and deactivates it non-radiatively, competing with the fluorophore's own spontaneous emission.
F + hν → F* (excitation, rate ∝ pump)
F* → F + hν' (emission, rate 1/τ₀)
F* + Q → F + Q (collisional quenching)
Stern-Volmer equation:
I₀/I = 1 + Ksv[Q] = 1 + kq·τ₀·[Q]
- I₀ is the fluorescence intensity with no quencher present; I is the intensity at quencher concentration [Q]. Their ratio grows linearly with [Q] — this simulator measures both directly from the actual emission events in the scene.
- [Q] slider sets how many quencher molecules diffuse through the cuvette.
- τ₀ slider sets the excited-state lifetime: a longer-lived F* has more time to be intercepted by a quencher, so quenching grows with τ₀ too.
- Diffusion mobility sets how fast quenchers random-walk, which sets the collision (encounter) frequency kq — this is why dynamic quenching is diffusion-controlled.
- Each collision between an excited sphere and a quencher sphere is resolved in real time: it either emits a green photon (spontaneous decay) or gets silently deactivated on contact with Q, exactly as the kinetic scheme above describes. Ksv is fit live from the measured I₀/I and [Q], not hardcoded.
Real-world relevance: Stern-Volmer analysis is how chemists measure oxygen concentration with luminescent sensors, probe protein conformational dynamics, and detect analytes that quench a reporter dye — the intrinsic ns-scale lifetime is slowed here by many orders of magnitude purely so the collisions are visible.