DNA Molecular Beacon Biosensor

Assay Conditions

Target DNA concentration 50 nM

Hybridization temperature 50 °C

Ionic strength (NaCl) 150 mM

Live Readout

Relative fluorescence0%

Target-bound (signal)

0 / 0

Thermally open (bg)

0 / 0

Assay time

0.0 s

How it works

A molecular beacon is a hairpin-shaped DNA probe with a fluorescent donor dye on one end and a dark quencher on the other. When the stem is closed the two dyes sit only a couple of nanometres apart and Förster resonance energy transfer (FRET) drains almost all of the donor's excited-state energy into the quencher, non-radiatively:

E(r) = 1 / (1 + (r / R0)^6)
Fluorescence F(r) = Fmax * (1 - E(r))
R0 (Förster radius) ≈ 5 nm for a typical dye/quencher pair

The stem itself is in thermal equilibrium between closed and open, set by its melting temperature T_m (here 55 °C) via a sigmoid:

P_open(T) = 1 / (1 + exp(-(T - Tm) / w))

A probe can only capture a complementary target strand from solution while its stem is transiently open (the loop must be exposed). Once a target hybridizes, the duplex is far more stable than the stem and the beacon is locked open — permanently fluorescent — for the rest of the assay:

Target concentration — sets the per-second capture probability while open (∝ k_on·[target]); more target means faster signal accumulation.
Temperature — controls P_open(T). Too cold and the stem barely breathes, so even abundant target is captured slowly; too hot and probes flap open on their own, raising background fluorescence from unbound beacons — real assays run near T_m to balance the two.
Ionic strength — screens the negatively-charged phosphate backbone, which speeds duplex formation at higher salt (faster hybridization kinetics), matching real PCR/hybridization buffer design.

This is the exact readout principle behind real-time PCR beacons, COVID-19 nucleic-acid tests and many DNA/RNA point-of-care diagnostics: an "off" background with a target-triggered fluorescent "on" signal that grows with analyte concentration.

Donor dye (fluoresces) Dark quencher Free target DNA

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