Interfacing Whispering Gallery Mode Optical Microresonator Biosensors with the Plant Defense Elicitor Chitin

The biomaterial class of chitooligosaccharides (chitin), commonly found in insects and fungi, is one of the most abundant on earth. Substantial evidence implicates chitin in mediating a diverse array of plant cellular signaling events, including the induction of plant defense mechanisms against invading pests. However, these recognition and mediation mechanisms, including the binding kinetics between chitin and their plant recognition receptors, are not fully understood. Therefore, the creation of a platform capable of both interfacing with chitin and plant cell receptors, and monitoring their interactions, would significantly advance our understanding of this plant defense elicitor. Recently, a label-free, highly sensitive biosensor platform, based on Whispering Gallery Mode optical microresonators, has been developed to study such biomolecular interactions. Here, we demonstrate how this unique platform can be interfaced with chitin using simple carbohydrate chemistry. The surface chemistry is demonstrated using X-ray photoelectron spectroscopy, fluorescence microscopy, optical profilometry, ellipsometry, and contact angle measurements. The resulting surface is uniform, with an average surface roughness of 1.25 nm, and is active toward chitin recognition elements. Optical loss measurements using standard quantitative cavity analysis techniques demonstrate that the bioconjugated platforms maintain the high performance (Q > 106) required to track binding interactions in this system. The platform is able to detect lectin, which binds COs, at 10 μg/mL concentration. This biosensor platformʹs unique capabilities for label-free, high sensitivity biodetection, when properly interfaced with the biomaterials of interest, could provide the basis for a robust analytical technique to probe the binding dynamics of chitin–plant cell receptors.