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Direct Nanoparticle Sensing in Liquids with Free-Space Excited Optical Whispering-Gallery-Mode Microresonators
Davide D'Ambrosio1, Saverio Avino1, Gianluca Gagliardi1
1Consiglio Nazionale delle Ricerche, Istituto Nazionale di Ottica (INO), Via Campi Flegrei, 34-Comprensorio A. Olivetti, 80078 Pozzuoli, Italy.
Sensors (Basel, Switzerland)
|August 28, 2025
Summary
This study demonstrates a simplified optical sensor using whispering-gallery-mode (WGM) microresonators for real-time nanoparticle detection in water. The novel method achieves sub-nanometer sensitivity without complex equipment, paving the way for environmental monitoring.
Area of Science:
- Optical Sensing
- Nanotechnology
- Biophysics
Background:
- Whispering-gallery-mode (WGM) microresonators offer high sensitivity for bio-chemical sensing.
- Existing WGM sensing methods often require complex coupling devices and signal processing.
- Practical application of WGM sensors is limited by system complexity.
Purpose of the Study:
- To develop a simplified, efficient optical sensing method using WGM microresonators.
- To enable real-time detection of nanoparticles in aqueous environments.
- To overcome the limitations of complex coupling and signal processing in WGM sensing.
Main Methods:
- Utilized a silica microsphere in an aqueous environment.
- Excited optical WGMs using a free-space visible laser (640-nm diode laser).
- Collected sensing information from transmitted and back-scattered light without optical couplers.
Main Results:
- Achieved real-time, fast sensing of dielectric nanoparticles with direct analog readout.
- Demonstrated sensor operation in water for extended periods without degradation.
- Determined a minimum detectable particle size below 1 nm, limited by laser jitter.
- Identified an ultimate detection bound of 0.3 nm due to laser frequency instability.
Conclusions:
- The developed free-space WGM sensing scheme simplifies optical sensing for nanoparticles.
- The method offers high sensitivity and stability in aqueous environments.
- This approach holds potential for detecting nanoplastics in natural water without complex laboratory setups.
