(Differential) Refractive Index Measurement with low Uncertainties and Dimensions using an Asymmetric Nanofluidic Grating Detector

Refractive index measurement is essential for understanding the physical and chemical properties of substances. Existing technologies include various microfluidic solutions, but challenges remain in reducing uncertainties and device dimensions. The pharmaceutical industry, environmental monitoring, adulteration detection, and biosensing extensively use refractive index measurements. The push towards further miniaturizations and enhanced sensitivity necessitates the development of advanced technologies, such as the proposed asymmetric nanofluidic grating detector, to meet the increasing demands for precise and reliable measurements.

The technology features an asymmetric nanofluidic diffraction grating device where reference and detection nanochannels are arranged asymmetrically (Fig. 1A). This design creates a non-mirror-symmetric diffraction pattern (I−_m ≠ I_m), which is instrumental in detecting changes in the refracting index. The asymmetric pattern only emerges when the refractive indices of the reference (yellow) and the detection channel (green) differ (Δn = n_Ref − n_Det ≠ 0).

When a collimated laser beam (λ=635 nm, beam diameter 360 μm, for example) impinges on this nanofluidic gratin, the resulting diffraction pattern is reflected onto a mirror and then directed into a CCD camera that records the intensity (Fig. 2). This mirror, crafted from a glass wafer coated with a reflective gold layer, includes a central slit that permits the passage of the incoming laser beam while reflecting the diffraction pattern towards the camera. The design strategically excludes the 0^th maximum, which also passes through this slit, to avoid interference with other maxima, enhancing the accuracy of the signal processing.
This setup exploits the asymmetric distribution of the diffraction pattern to precisely measure the refractive index within the detection nanochannels.  By measuring the difference in intensity ΔI_m = I_m – I_-m between the reference and detection channels, it enables accurate detection of refractive index variations (Fig. 1B).^[2] This capability provides a robust solution for application requiring precise and noncontact measurements.

• Low uncertainties: Precise measurements with minimal error margins.
• Compact dimensions: Reduced size facilitates integration into various systems.
• Contactless detection: Ensures sample integrity and reduces contamination risks.
• High sensitivity: Capable of detecting minute differences in refracting index.
• Real-time measurement: Provides dynamic updates on refractive index changes.

• Concentration measurement: Accurate determination of substance concentrations in solutions.
• Dynamic monitoring: Real-time tracking f refractive index variations.
• Biosensing: Detection of biomolecules and biological processes.
• Environmental monitoring: Assessment of water quality and pollutant levels.
• Pharmaceutical industry: Quality control and formulation development.

[1] Purr, F., Bassu, M., Lowe, R. D., Thürmann, B., Dietzel, A., & Burg, T. P. (2017). Asymmetric nanofluidic grating detector for differential refractive index measurement and biosensing. Lab on a Chip, 17(24), 4265–4272. https://doi.org/10.1039/C7LC00929A

[2] EP3698123B1, US11340162B2 and CN111226107B active.