Bioconjugation Strategies for Microtoroidal Optical Resonators

The development of label-free biosensors with high sensitivity and specificity is of significant interest for medical diagnostics and environmental monitoring, where rapid and real-time detection of antigens, bacteria, viruses, etc., is necessary. Optical resonant devices, which have very high sensitivity resulting from their low optical loss, are uniquely suited to sensing applications. However, previous research efforts in this area have focused on the development of the sensor itself. While device sensitivity is an important feature of a sensor, specificity is an equally, if not more, important performance parameter. Therefore, it is crucial to develop a covalent surface functionalization process, which also maintains the device’s sensing capabilities or optical qualities. Here, we demonstrate a facile method to impart specificity to optical microcavities, without adversely impacting their optical performance. In this approach, we selectively functionalize the surface of the silica microtoroids with biotin, using amine-terminated silane coupling agents as linkers. The surface chemistry of these devices is demonstrated using X-ray photoelectron spectroscopy, and fluorescent and optical microscopy. The quality factors of the surface functionalized devices are also characterized to determine the impact of the chemistry methods on the device sensitivity. The resulting devices show uniform surface coverage, with no microstructural damage. This work represents one of the first examples of non-physisorption-based bioconjugation of microtoroidal optical resonators

Tailoring the Protein Adsorption Properties of Whispering Gallery Mode Optical Biosensors

Label-free biosensor technologies have the potential to revolutionize environmental monitoring, medical diagnostics, and food safety evaluation processes due to their unique combinations of high-sensitivity signal transducers and high-specificity recognition elements. This enables their ability to perform real-time detection of deleterious compounds at extremely low concentrations. However, to further improve the biosensors’ performance in complex environments, such as wastewater, blood, and urine, it is necessary to minimize nonspecific binding, which in turn will increase their specificity, and decrease the rate of false positives. In the present work, we illustrate the potential of combining emerging high-sensitivity optical signal transducers, such as whispering gallery mode (WGM) microcavities, with covalently bound poly­(ethylene glycol) (PEG) coatings of varying thickness, as an effective treatment for the prevention of nonspecific protein adsorption onto the biosensor surface. We monitor the sensitivity of the coated biosensor, and investigate the effect of PEG chain length on minimizing nonspecific adsorption via protein adsorption studies. Experimental results confirm not only that PEG-functionalization reduces nonspecific protein adsorption to the surface of the sensor by as much as a factor of 4 compared to an initialized control surface, but also that chain length significantly impacts the nonfouling character of the microcavity surface. Surprisingly, it is the short chain PEG surfaces that experience the best improvement in specificity, unlike many other systems where longer PEG chains are preferred. The combination of WGM microcavities with PEG coatings tuned specifically to the device will significantly improve the overall performance of biosensor platforms, and enable their wider application in complex, real-world monitoring scenarios