Researchers have developed an eye-shaped photonic crystal biosensor with excellent sensitivity and precision, providing a faster, non-invasive method to detect cancer cells early.
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Why Photonic Crystals are so Effective
Photonic crystal-based biosensors manipulate and confine light at microscopic scales with remarkable precision. These devices use periodic arrangements of dielectric materials, such as silicon, to create photonic bandgaps that enable highly effective resonant sensing.
Until now, most designs have explored circular or hexagonal defect geometries, whilst other shapes, like elliptical or eye-shaped cavities, have largely been ignored. These unconventional geometries offer certain advantages, such as better light confinement, higher Q-factors, and improved sensitivity. These properties are essential for detecting subtle biomolecular changes.
The sensors can detect differences in refractive index between healthy and cancerous cells, which have measurable shifts in resonance wavelength. In this study, published in Scientific Reports, the team proposed a new structural configuration featuring an eye-shaped cavity designed to strengthen light-matter interaction and better distinguish cancerous cells.
Designing the Biosensor
The researchers designed and evaluated the biosensor using detailed simulations based on the finite element method (FEM). Its core structure features a 2D square lattice of silicon rods arranged periodically within an air medium; each silicon rod has a radius of 0.1 μm, and the lattice constant is optimized at 540 nm.
The entire design integrates two linear defect waveguides, serving as channels for input and output optical signals, with an eye-shaped cavity that holds the analyte. This cavity’s distinctive shape is intended to improve light confinement and the resonance behavior of the sensor.
Key design parameters, including the cavity’s dimensions, the placement of rods, and the nature of the defects, were carefully optimized to maximize sensitivity and Q-factor. The team analysed how changes to these parameters influenced the resonance wavelength, transmission spectra, and the ability to detect biomolecular variations.
The sensor's performance was assessed using standard metrics: resonance wavelength shift per refractive index unit (RIU), Q-factor indicating resonance sharpness, and figure of merit (FoM), which combines sensitivity and Q-factor.
To assess its practicality, the researchers also tested the sensor’s durability against fabrication imperfections and operational temperature fluctuations, evaluating how minor geometric and material changes affected performance.
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What the Simulations Revealed
The results from the simulation demonstrated the biosensor's excellent performance. Resonance wavelength shifted linearly with changes in the analyte’s refractive index, enabling precise quantification of biomolecular concentrations.
The sensor achieved a sensitivity of up to 243 nm/RIU and a Q-factor approaching 87,070, both surpassing previously reported designs.
The high Q-factor indicates the sharply defined resonance peaks, allowing the detection of even subtle refractive index differences. The FoM reached 11,800 RIU-1, further demonstrating the sensor’s ability to distinguish between cellular states with fine resolution.
These precise and accurate results were found to be directly correlated to the unusual eye-shaped cavity. It enhanced light confinement and concentrated optical modes within the sensing region, and the measurable resonance shifts were amplified when the refractive index varied.
Tests of fabrication tolerance showed that the sensor maintained high sensitivity and Q-factor even with deviations of ±2 nm in cavity dimensions and ±20 nm in lattice constant. Thermal stability trials demonstrated the sensor's consistent performance between 25 °C and 75 °C, with minimal influence from material absorption.
Why This Could be a Practical Diagnostic Tool
This novel design marks a step forward in photonic biosensing, pairing an unconventional eye-shaped cavity with robust engineering to deliver precise, label-free cancer detection.
Beyond its impressive sensitivity and stability, the sensor’s resilience to fabrication and environmental variation makes it a promising candidate for practical, noninvasive diagnostics. It may help clinicians spot malignant cells earlier and more reliably.
Journal Reference
Rizk S. et al. (2025). Photonic crystal biosensor featuring an eye-shaped cavity for precise identification of cancerous cells. Scientific Reports 15, 23926. DOI: 10.1038/s41598-025-07938-y, https://www.nature.com/articles/s41598-025-07938-y