Bioconjugation strategies for microtoroidal optical resonators

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.

Keywords: bioconjugation; high quality factor; optical resonators; sensors.

Figure 1.

Optical resonant cavity. (a) An image of a microtoroid resonant cavity. (b) Finite element method simulation of the intensity of the optical field at 633 nm for a microtoroid cavity. As can be seen, the optical field is primarily confined inside the silica cavity, but a small portion evanesces into the environment. (c) The intensity profile of the optical field inside (blue) and outside (red) of the cavity. The black line indicates the air/silica interface. It can be clearly seen that the evanescent tail is approximately 100 nm.

Figure 2.

Overall reaction scheme for the biotinylation of silica microtoroids. (a) Hydroxylation of the silica surface. (b) Amination of the hydroxylated surface via silane coupling agent. (c) Biotinylation of the aminated surface via NHS ester chemistry.

Figure 3.

(a) Scanning electron micrograph of as-fabricated microtoroid. (b) Side view optical micrograph of microtoroid during device characterization. The tapered fiber can be clearly seen. (c) PovRay rendering of the microtoroid resonator and tapered optical fiber. The optical field is primarily confined inside the silica cavity, but a small portion evanesces into the environment.

Figure 4.

Optical micrographs of silica microtoroids. (a) As-fabricated microtoroid. (b) Microtoroid after piranha etch. (c) Microtoroid after piranha etch and 24 h drying at 100 °C. The white circles indicate regions of structural damage. (d) Microtoroid after O₂ plasma treatment.

Figure 5.

Ellipsometry data showing film thicknesses after various reaction times for organic solvent deposition and vapor deposition.

Figure 6.

Fluorescent micrographs of microtoroids after labeling. (a) FITC-labeled, aminated microtoroid after piranha treatment, followed by organic solvent deposition. (b) Texas Red-labeled, biotinylated microtoroid after piranha treatment, followed by vapor deposition. (c) Texas Red-labeled, biotinylated microtoroid after O₂ plasma treatment, followed by organic solvent deposition. (d) Texas Red-labeled, biotinylated microtoroid after O₂ plasma treatment, followed by vapor deposition.

Figure 7.

Chemical composition of control surfaces after each reaction step (using O₂ plasma etching and vapor deposition conditions).

Figure 8.

Fluorescent labeling intensities of biotinylated devices for given storage times, normalized according to device surface area. Note that the x-axis is plotted on a log-scale.

Figure 9.

Effect of surface functionalization on the quality factor of the microtoroids at each reaction step.

Figure 10.

Transmission spectra of biotinylated toroid, showing a single, high-Q resonance (data connected by a solid black line) with the corresponding Lorentz fit (solid red line)