Free-space-coupled, frequency-locked microtoroid sensor for part-per-trillion detection of mustard gas simulant

Sulfur mustard remains a serious chemical warfare threat, but its extreme toxicity complicates sensor development. As a chemically and physically similar simulant, 2-chloroethyl ethyl sulfide (2-CEES) is widely used in laboratory studies owing to its volatility and thioether reactivity. Whispering-gallery-mode (WGM) microtoroid resonators combined with frequency locking offer single-molecule sensitivity, yet their reliance on fragile fiber tapers has restricted their use to controlled laboratory environments. Here, we overcome this limitation by integrating frequency-locked WGM microtoroids with a robust free-space optical coupling scheme previously demonstrated only for temperature sensing. Polyvinylamine coatings synthesized via reversible addition–fragmentation chain-transfer (RAFT) polymerization could be immobilized to the microtoroid to provide selective and stable binding to 2-CEES. Housed in a compact, coin-sized gas chamber, the sensor achieves a room-temperature detection limit of 25 parts per trillion, surpassing the previous best of 10 parts per billion, and maintains mechanical resilience under ambient conditions. Continuous frequency locking enables unattended, long-duration operation. This first demonstration of free-space–coupled WGM microtoroids for gas-phase chemical sensing establishes a scalable path toward ultra-sensitive, rugged detectors for defense, security, and environmental monitoring.