Ultra Stable Molecular Sensors by Submicron Referencing and Why They Should Be Interrogated by Optical Diffraction—Part II. Experimental Demonstration

Frutiger et al. show experimentally that sensors using the spatial affinity lock-in and diffraction readout overcome drawbacks of refractometric biosensors. They compare an unstabilized diffractometric biosensor directly with a Biacore 8k instrument.

The Challenge: Cross-Sensitivity of Refractometric Biosensors

Surface plasmon resonance is the standard in label-free interaction analysis, despite a significant cross-reactivity to environmental and experimental noise. Part I proposed the spatial affinity lock-in as the answer. This paper tests the idea in the lab.

The Approach: Direct Comparison and a Combined Sensor

The authors compared the sensitivity to refractive index jumps and the surface mass resolution of both sensor types. A combined refractometric and diffractometric biosensor then measured both signals in the same experiments. A conceptual and quantitative discussion explains the physical reasons and defines a figure of merit.

Key Results Against Refractometric Detection

• Refractive index jumps: a 0.03 RIU discontinuity lowered the diffractometric signal by 2 to 3 pg/mm². That is more than three orders of magnitude below the refractometric response in the same experiment.
• Cross-sensitivity: calculated at 69.3 pg/mm²/RIU in water for the diffractometric sensor, against a measured 760 pg/mm²/RIU for the referenced Biacore 8k output.
• Drift: below 100 fg/mm² for the unstabilized diffractometric sensor, with some of the best Biacore flow cells on par.
• Combined sensor: baseline noise of 0.1 pg/mm² rms diffractometric against 100 pg/mm² rms refractometric.
• Precision: for the same mass resolution, a refractometric sensor needs a relative precision up to four orders of magnitude higher.

Why It Matters for Label-Free Sensing

Low-precision, unstabilized diffractometric devices reached the resolution of large stabilized refractometric sensors. The authors believe label-free optical sensors could therefore move beyond the drug discovery lab. They point to miniaturized, mass-produced environmental and medical sensors as possible uses.