Colorimetric Gold-Nanoparticle Aptasensor

Assay Conditions

Target concentration 40 nM

Ionic strength (salt trigger) 70%

Nanoparticle diameter 20 nm

Aptamer affinity K_d 30 nM

Solution Color

Perceived color is the complement of the plasmon absorption band — dispersed AuNPs absorb green (~520 nm) and look red; aggregated AuNPs absorb further into orange/red and look purple-blue.

Live Readouts

Plasmon peak λ

520 nm

A650/A520 ratio

0.10

Binding fraction θ

57%

Aggregation

40%

How it works

Each gold nanoparticle (AuNP) is coated with a target-specific aptamer, which sterically and electrostatically protects the particle from salt-induced aggregation. When the target analyte binds, the aptamer changes conformation or desorbs, removing that protection so dissolved salt screens the surface charge and the particles clump together.

Binding follows a Langmuir isotherm; the resulting inter-particle gap sets the plasmon coupling shift via the empirical "plasmon ruler" equation (Jain, Huang & El-Sayed, 2007):

θ = C / (C + Kd)                 fraction of aptamers with target bound
Aggregation ≈ θ · (salt / 100)   both binding AND salt are needed to clump
Δλ/λ0 = A·exp[-(gap/D)/τ]        A ≈ 0.18, τ ≈ 0.23 (plasmon ruler)
λ_peak = λ0 + λ0·Δλ/λ0           gap shrinks as aggregation increases

A Gaussian extinction curve is built around λ_peak (broadening as clusters form), and its values at 520 nm and 650 nm give the A650/A520 ratio real assays use as their colorimetric readout — the higher the ratio, the more aggregation, and the further the solution color shifts from red toward blue.

Target concentration — more analyte raises the Langmuir binding fraction θ.
Ionic strength — the salt that actually drives clumping once protection is lost; with no salt, nothing aggregates even at full binding.
Nanoparticle diameter — sets the baseline (unaggregated) plasmon wavelength and scales the plasmon-ruler gap.
Aptamer affinity Kd — lower Kd means the sensor saturates (and aggregates) at lower target concentration, i.e. a more sensitive assay.

Real-world relevance: this is the working principle behind naked-eye colorimetric AuNP aptasensors used for rapid field detection of ions, small molecules, and pathogens without any instrument beyond the human eye or a simple absorbance reader.

Gold nanoparticles Cuvette (solution)

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