Scientists have unveiled a compact optical sensor using differential guided-mode resonance that achieves record sensitivity in detecting refractive index changes. Their sensor opens new possibilities for real-time medical and environmental monitoring.
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A new optical sensor has surpassed sensitivity records by detecting minute changes in refractive index with unparalleled precision. The device, described in Nature Communications, uses a technique called differential guided-mode resonance (dGMR) and could lead to more portable, real-time diagnostic and environmental monitoring tools.
The Importance of Refractive Index Sensors
Refractive index sensors can detect subtle shifts in optical properties, and this sensitivity makes them indispensable in fields ranging from medical diagnostics to environmental monitoring. Techniques such as immunofluorescence and mass spectrometry are highly reliable but require bulky hardware and elaborate sample preparation. Even optical refractive index sensors, while sensitive, are often hampered by intricate setups and limited portability.
How the dGMR Sensor Works
The dGMR sensor addresses these limitations by engineering nanometre-scale thickness variations in a planar waveguide atop a metal substrate, using the well-known Kretschmann configuration.
Incident light excites surface plasmon polaritons at the metal surface, which couple into guided-mode resonances within the waveguide. Introducing two slightly different thicknesses in the waveguide produces two resonant modes that shift relative to one another as the surrounding refractive index changes.
These shifts appear as ring-shaped dark stripes in reflected light, easily captured by a standard Complementary Metal Oxide Semiconductor camera. Tracking the position of these rings allows for extremely precise measurements: the team achieved a sensitivity approaching one million pixels per refractive index unit (RIU), about three orders of magnitude better than previous imaging-based approaches.
Unlike conventional resonance sensors, whose performance depends heavily on the resonance quality factor (Q-factor), the dGMR’s sensitivity is predominantly governed by the difference in thicknesses in the waveguide. This design choice enables high performance without overly complicated optical setups.
Fabrication and Chip Designs
To demonstrate the concept, the researchers fabricated two types of sensor chips: one featuring pixelated arrays patterned by electron beam lithography, and another with continuous thickness gradients formed by plasma-enhanced chemical vapor deposition. Both methods provided nanoscale precision in controlling waveguide thickness.
The sensor proved highly versatile in tests. On continuous gradient chips, even small refractive index changes produced spatial shifts of hundreds of pixels in the resonance rings. The response remained linear, enabling accurate, quantitative measurements.
By adjusting the incident illumination angle, the team extended the dynamic range to cover wider refractive index intervals. This is useful, for example, in tracking glucose concentrations.
The figure of merit, defined as sensitivity divided by the full width at half the maximum of the resonance peak, reached 104 RIU-1. This result demonstrates this sensor's superior performance compared to conventional surface plasmon resonance sensors.
Practical demonstrations included measuring glucose solutions with concentration errors below 0.5 %, monitoring polydopamine film deposition, and detecting nanomolar biotin via streptavidin-functionalised chips. Both pixelated and gradient designs produced clear, interpretable patterns tied to analyte concentration.
A Compact, Portable Prototype
To showcase its portability, the team built a compact prototype measuring just 20 x 14 x 8 cm, integrating a dGMR chip with a smartphone camera. The device successfully mapped humidity patterns by detecting local refractive index changes, illustrating its potential for field-based environmental monitoring.
Microfluidic channels expanded the platform’s capabilities, enabling multi-channel sensing arrays for simultaneous biochemical assays. The sensor's high sensitivity at low concentrations supports early disease detection. At the same time, the ability to encode refractive index data into QR code-like patterns shows potential for secure optical data storage and transmission.
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Looking Ahead
For now, the team's focus is on refining fabrication for scalable production, miniaturizing the system further, and developing intuitive software for pattern analysis. Expanding the platform to support multi-parameter sensing could make it even more versatile for real-time monitoring in medical, environmental, and biochemical applications.
With its combination of ultra-high sensitivity, wide dynamic range, and compact design, the dGMR sensor establishes a new benchmark in optical sensing.
Journal Reference
Liu, Z., et al. (2025). Ultrasensitive imaging-based sensor unlocked by differential guided-mode resonance. Nat Commun 16, 6113. DOI: 10.1038/s41467-025-60947-3, https://www.nature.com/articles/s41467-025-60947-3.
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