Abstract
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.
Highlights
The development of biosensors with high sensitivity and specificity is of significant interest to scientific communities, such as medical diagnostics and environmental monitoring, where rapid and real-time detection of antigens, bacteria, viruses, etc., is necessary
Four different routes to biotinylation were explored for their suitability towards silica microtoroid resonators, based on combinations of hydroxylation and amination conditions
We require the maintenance of the Q above 106 after biotinylation for these devices to be used as sensors in aqueous environments, for such applications as medical diagnostics or environmental monitoring
Summary
The development of biosensors with high sensitivity and specificity is of significant interest to scientific communities, such as medical diagnostics and environmental monitoring, where rapid and real-time detection of antigens, bacteria, viruses, etc., is necessary. While optical resonant cavities can be fabricated in many geometries, and from many different materials, the motivation to maximize the Q factor and the photon lifetime across a wide range of operating frequencies has led to silica-based, optical resonator devices that are circular in nature, such as microspheres, microrings, microdisks, and microtoroids [20,49,60,61] The advantage of the latter three shapes is that they may be fabricated on a planar substrate via lithographic techniques, increasing ease of use and allowing potential integration with on-chip microfluidics. Label-free whispering gallery mode optical resonators, especially those fabricated on a planar substrate, represent an intriguing platform for high sensitivity detection in complex environments. They must first be bioconjugated to add specificity to the device for optimal performance in these environments. This strategy could accelerate the development of label-free sensors for rapid diagnostics
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