Label-free biosensing leverages the intrinsic optical properties of silicon photonic devices to detect biomolecular interactions without the need for fluorescent or radioactive labels. Silicon photonic microring resonators and interferometers are among the most sensitive platforms for this purpose, offering high detection limits, scalability, and compatibility with CMOS fabrication processes. These devices operate by monitoring shifts in resonant wavelengths or phase changes induced by the binding of target analytes to functionalized surfaces.
Silicon microring resonators confine light in a circular waveguide, creating a resonant condition at specific wavelengths. When biomolecules bind to the sensor surface, the effective refractive index near the waveguide changes, causing a measurable shift in the resonant wavelength. The sensitivity of these devices depends on the overlap between the evanescent field and the analyte, typically achieving detection limits in the picomolar to femtomolar range for proteins and nucleic acids. Interferometers, such as Mach-Zehnder interferometers, split light into two arms, one of which serves as a sensing path while the other acts as a reference. Biomolecular binding alters the phase of light in the sensing arm, enabling detection through interference patterns.
Surface functionalization is critical for specificity and sensitivity. Silane chemistry, such as (3-aminopropyl)triethoxysilane (APTES), is commonly used to create amine-terminated surfaces for subsequent conjugation with biorecognition elements like antibodies, aptamers, or DNA probes. Carbodiimide crosslinking (EDC/NHS) facilitates covalent attachment of proteins, while thiol-based chemistry is employed for gold-coated surfaces. Passivation with polyethylene glycol (PEG) reduces nonspecific binding, enhancing signal-to-noise ratios. The choice of functionalization strategy depends on the target analyte and the required stability of the sensor surface.
Detection limits are influenced by the device design and the strength of the evanescent field interaction. Microring resonators typically achieve limits of detection (LOD) between 1 pg/mm² and 10 pg/mm² for proteins, while interferometers can reach sub-pg/mm² levels due to their higher phase sensitivity. Multiplexing is enabled by integrating arrays of resonators or interferometers on a single chip, each functionalized with different capture molecules. Wavelength-division multiplexing (WDM) allows simultaneous monitoring of multiple resonances, making these platforms suitable for high-throughput screening in diagnostics.
Evanescent field sensing relies on the decay of light intensity outside the waveguide, typically extending 100–200 nm into the surrounding medium. While this provides sufficient sensitivity for many applications, plasmonic enhancements can further improve detection by localizing light at metal-dielectric interfaces. Surface plasmon resonance (SPR) and localized surface plasmon resonance (LSPR) amplify the electromagnetic field, increasing sensitivity for small molecules and low-abundance targets. However, plasmonic structures introduce higher optical losses and fabrication complexity compared to all-silicon photonic devices.
For point-of-care diagnostics, microring resonators and interferometers offer advantages in miniaturization and integration with microfluidics. Their label-free operation eliminates the need for secondary detection steps, simplifying workflows. Compared to plasmonic sensors, silicon photonic devices provide better long-term stability and lower manufacturing costs, though plasmonics may outperform in scenarios requiring ultra-high sensitivity for small molecules.
Recent advancements include the use of high-Q resonators to enhance sensitivity and the development of hybrid plasmonic-photonic structures to combine the benefits of both approaches. Future directions focus on improving multiplexing capabilities, reducing sample volumes, and integrating these sensors with portable readout systems for real-world applications.
In summary, silicon photonic microring resonators and interferometers are powerful tools for label-free biosensing, offering high sensitivity, multiplexing potential, and compatibility with scalable fabrication. While evanescent field sensing provides robust performance for many diagnostic applications, plasmonic enhancements can further push detection limits for specialized use cases. The choice between these approaches depends on the specific requirements of sensitivity, cost, and system integration.